Direct Control of Spin Distribution and Anisotropy in Cu ... - MDPI
inorganics Article
Direct Control of Spin Distribution and Anisotropy in Cu-Dithiolene Complex Anions by Light Hiroki Noma 1 , Keishi Ohara 1 and Toshio Naito 1,2, * 1 2
*
Graduate School of Science and Engineering, Ehime University, 2-5, Bunkyo-cho, Matsuyama 790-8577, Japan; [email protected] (H.N.); [email protected] (K.O.) Division of Material Science, Advanced Research Support Center (ADRES), Ehime University, 2-5, Bunkyo-cho, Matsuyama 790-8577, Japan Correspondence: [email protected]; Tel.: +81-89-927-9604
Academic Editor: Duncan H. Gregory Received: 15 January 2016; Accepted: 21 March 2016; Published: 30 March 2016
Abstract: Electrical and magnetic properties are dominated by the (de)localization and the anisotropy in the distribution of unpaired electrons in solids. In molecular materials, these properties have been indirectly controlled through crystal structures using various chemical modifications to affect molecular structures and arrangements. In the molecular crystals, since the energy band structures can be semi-quantitatively known using band calculations and solid state spectra, one can anticipate the (de)localization of unpaired electrons in particular bands/levels, as well as interactions with other electrons. Thus, direct control of anisotropy and localization of unpaired electrons by locating them in selected energy bands/levels would realize more efficient control of electrical and magnetic properties. In this work, it has been found that the unpaired electrons on Cu(II)-complex anions can be optically controlled to behave as anisotropically-delocalized electrons (under dark) or isotropically-localized electrons like free electrons (under UV), the latter of which has hardly been observed in the ground states of Cu(II)-complexes by any chemical modifications. Although the compounds examined in this work did not switch between conductors and magnets, these findings indicate that optical excitation in the [Cu(dmit)2 ]2´ salts should be an effective method to control spin distribution and anisotropy. Keywords: Cu(II)-dithiolene complex; electron spin resonance; π–d interaction; quantum chemical calculation; molecular crystal
1. Introduction Electrical and magnetic properties are based on unpaired electrons in solids. Delocalized unpaired electrons can exhibit electrical conduction, while localized unpaired electrons can exhibit magnetism. This trend is true for any solid, including molecular materials. In addition, the (an)isotropy in the interactions among unpaired electrons also matters in these properties. Thus, the control of conduction and/or magnetism requires the control of the (de)localization and anisotropy of the unpaired electrons. If one can switch unpaired electrons between localized and delocalized states, then one may switch the solids between conductive and magnetic materials. This point of view is different from that of light-induced excited spin-state trapping (LIESST), where the spin states of transition-metal complexes are switched between high- and low-spins by photoexcitations at low temperature [1–9]. Additionally, the abovementioned point of view is also different from that of photo-induced phase transitions (PIPTs), where phase transitions, such as metal-insulator and ionic-neutral transitions at certain temperatures (TC s), are brought about by photo-irradiation near the TC s [10,11]. The purpose of the present work is the control of spin distribution and anisotropy independent of thermodynamic conditions, unlike PIPTs. Thus far, most of the studies to control the electrical and magnetic properties have utilized chemical modifications to affect molecular structures and arrangements. However, Inorganics 2016, 4, 7; doi:10.3390/inorganics4020007
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after tens of years of worldwide extensive research, this strategy has turned out to be challenging for the fine and precise control of conduction and magnetism [12–14]. In both organic and inorganic materials, the slightest chemical modifications often result in unexpectedly marked differences in crystal structures [15,16]. In addition, the slightest differences in crystal structures would sometimes make qualitative differences in the physical properties. In particular, in molecular crystals, there are many kinds of weak intermolecular interactions with keeping a subtle balance to form complicated electronic and crystal structures. Because of this characteristic situation, molecular crystals containing unpaired electrons often have several metastable states, as well as unique excited states with peculiar relaxation mechanisms [17] and are sensitive to perturbation, such as photo-irradiation [18]. All of these properties are interesting in the development of electrical and magnetic materials. However, a precise control of atomic/molecular arrangements in crystals remains impossible. Accordingly, it is interesting, as well as important to develop new methods to control the conductive and magnetic properties of the molecular crystals. Paying attention to such unique features of molecular crystals, the control of conduction [19,20], or magnetism [21,22], or other physical properties [23–28] was carried out using photochemical redox reactions in solid states. In the reactions, photo-induced electron transfer occurs between two different kinds of molecular species in the crystal, e.g., the cations and the anions; one species is responsible for conduction and/or magnetism, and the other serves as the charge reservoir in high-TC cuprates [29]. The latter species is not involved in either conductive or magnetic properties. As photochemical reactions generally have high spatial resolutions, this method is utilized to make junction structures of a molecular single crystal for electronic devices when the electron transfer is irreversible [19,30]. Furthermore, reversible and simultaneous control of conduction and magnetism has been recently realized using charge transfer (CT) transition between two different molecular species in salts [31,32], instead of chemical reactions. In this case, one of the molecular species accommodates localized spins for magnetism, in addition to serving as the charge reservoir. The remaining species is optically doped to have excited carriers, like photoconductors. “Giant photoconductivity”, which possibly originates from reversible melting of charge order, has been observed by UV irradiation upon a related molecular crystalline salt [33]. The advantage of this photochemical or optical method lies in the fact that control of the crystal structure is not required at any level. Now that a crystal with an appropriate structure and physical properties, which are known under dark conditions, is selected, nothing but photo-irradiation is required to excite and produce localized and/or delocalized unpaired electrons in the crystal. As long as the electronic band structure and the solid state spectra are known, selected photoexcitation with appropriate wavelength and intensity can take place with accidental or uncontrollable factors being minimized in the control of conduction and magnetism. In addition to the abovementioned sensitivity to light and unique photo-physics of molecular crystals, this kind of photo-control is based on the fact that molecular crystals often contain different kinds of molecules for different roles and also the fact that they have well-defined crystal and band structures, which are clarified by standard experimental and calculation techniques. This idea will make a step forward when a single kind of molecule is responsible for both magnetism and conduction. This will be realized when the distribution of unpaired electrons on the molecule can be controlled by irradiation. In other words, the desired molecular orbitals for unpaired electrons should be selected by appropriate photoexcitation. If the unpaired electrons are excited to be localized, the magnetic properties dominate, and vice versa. Based on this idea, a unique Cu(II)-dithiolene complex anion has been found to be suitable for this purpose, after a number of transition-metal complexes were examined. Metal-dithiolene complex molecules, M(dmit)2 (M = Ni, Pd, Pt, Au, etc., dmit2´ = 1,3-dithiole-2-thione-4,5-dithiolate, Figure 1), have attracted attention for a long time as building blocks for conducting, magnetic and optical materials [34–48]. Among a great number of molecular species, M(dmit)2 and their derivatives have always shown us new possibilities of states of matter, such as single-component metals/superconductors [49–59] and spin liquids [60]. This is because M(dmit)2 have uniquely many degrees of freedom, i.e., spin, charge and orbital degrees of freedom,
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M(dmit)2 have uniquely many degrees of freedom, i.e., spin, charge and orbital degrees of freedom, originating from d-orbitals d-orbitals at at the the metal metal centers centers and and π-orbitals π-orbitals of ofthe thedmit-ligands. dmit-ligands. The The M(dmit) M(dmit)22 originating from species often fractional formal charges in solid states. In theInsolid species often become becomestable stableradical radicalanions anionswith with fractional formal charges in solid states. the states, they closely interact with each other through the overlap of the molecular orbitals (MOs) solid states, they closely interact with each other through the overlap of the molecular orbitals accommodating the unpaired electrons. Among the M(dmit) and related complexes, the Cu(dmit)2 (MOs) accommodating the unpaired electrons. Among the2 M(dmit) 2 and related complexes, the anions [61–67] are unique in that the number and distribution of the unpaired electronselectrons may be Cu(dmit) anions [61–67] are unique in that the number and distribution of the unpaired 2 reversibly controlled with UV radiation [68]. In this work, the electronic and spin densities of may be reversibly controlled with UV radiation [68]. In this work, the electronic and spin densities of 2− 2 ´ [Cu(dmit)22]] anions [Cu(dmit) anionsare arecompared comparedinindetail detailin insalts saltswith with various various cations. cations. Among Among them, them, three three salts, salts, (nBu 4 N) 2 [Cu(dmit) 2 ] (1), [(DABCO)H] 2 [Cu(dmit) 2 ]CH 3 CN (DABCO = 1,4-diazabicyclo[2.2.2]octane) (nBu4 N)2 [Cu(dmit)2 ] (1), [(DABCO)H]2 [Cu(dmit)2 ]CH3 CN (DABCO = 1,4-diazabicyclo[2.2.2]octane) 2+ = dibenzofuran-2,2′-bis(N-methylene-4,4′-bipyridinium)) (3) (2) and andBP BP22DBF[Cu(dmit) DBF[Cu(dmit)22]] (BP (BP22DBF DBF2+ (2) = dibenzofuran-2,2’-bis(N-methylene-4,4’-bipyridinium)) (3) (Figure 1), 1), exhibited exhibited systematic systematic differences differences in in structural structural and and physical physical properties. properties. The The following following are are (Figure discussed in this paper for 1–3: the crystal and molecular structures, the temperature-dependent discussed in this paper for 1–3: the crystal and molecular structures, the temperature-dependent electrical resistivities, resistivities, the electrical the calculated calculated charge charge and and spin spin densities, densities, the the electron electron spin spin resonance resonance (ESR) (ESR) spectra and and the the temperature-dependent temperature-dependent magnetic magnetic susceptibilities. susceptibilities. spectra
1. Molecular structures of abbreviated chemical species. dmit2− = Figure 1. Molecular structures of abbreviated chemical species. dmit2´ = 1,3-dithiole-2-thione-4,5-dithiolate; 2+ 2+ DABCO, 1,4-diazabicyclo[2.2.2]octane; BP 1,3-dithiole-2-thione-4,5-dithiolate; DABCO, 1,4-diazabicyclo[2.2.2]octane; BP2DBF = 2 DBF = dibenzofuran-2,2’-bis(N-methylene-4,4’-bipyridinium). dibenzofuran-2,2′-bis(N-methylene-4,4′-bipyridinium).
2. Results and Discussion 2. Results and Discussion 2.1. Crystal and Molecular Structures 2.1. Crystal and Molecular Structures The crystal and molecular structures of 1–3 are shown in Figure 2, and the crystal data are summarized in Table in the Supplementary The crystal and 2, molecular of 1–3 The crystal and S1 molecular structures of Materials. 1–3 are shown in Figure and the structures crystal data are remain unchanged from low temperature, except for a few percentage of thermal shrinkage summarized in Table S1room in thetoSupplementary Materials. The crystal and molecular structures of in unit cell volumes V (V(100 K)/V(296 = 96.6%, 97.5% andfor 97.9% for percentage 1–3, respectively). The 1–3the remain unchanged from room to low K) temperature, except a few of thermal electrical are cooperative phenomena are mainly based intermolecular shrinkageand in magnetic the unit properties cell volumes V (V(100 K)/V(296 K)and = 96.6%, 97.5% andon97.9% for 1–3, interactions through sulfur–sulfur (S–S) interatomic contacts in 1–3. With respect to the respectively). The electrical and magnetic properties are cooperative phenomena andarrangement are mainly 2´ anions, based intermolecular interactions through sulfur–sulfurS–S (S–S) interatomic contactsKinin 1–3. of the on [Cu(dmit) the shortest intermolecular distances at 296/100 1–3With are 2] 2− respect to the arrangement of the [Cu(dmit)2Å ] inanions, shortest intermolecular S–S distances 6.138(1)/5.8082(7) Å in 1, 3.499(1)/3.429(1) 2 and the 3.921(3)/3.848(5) Å in 3. Those in 1 and at 3 296/100 K than in 1–3 are the 6.138(1)/5.8082(7) in 1, of 3.499(1)/3.429(1) Å in ˆ 2 2and 3.921(3)/3.848(5) Å inthe 3. are larger twice van der Waals Å radius a sulfur atom (1.85 = 3.70 Å; simply called Thoseder in Waals 1 and 3distance” are larger than twice vanshould der Waals radius of ainteraction sulfur atom (1.85 ×the 2 =overlap 3.70 Å; “van below). Thus,the there hardly be any through 2 ´ simply theadjacent “van der[Cu(dmit) Waals distance” below). Thus, should hardly be any of MOs called between anions in 1 and 3. there This suggests that both saltsinteraction should be 2] through thewhich overlap of MOs between adjacent [Cu(dmit)2]2− anions and 3. This suggests that both insulators, is supported by electrical measurements below. in A 1tight-binding band calculation salts should be insulators, is 3). supported by electrical below. A tight-binding (TBBC) was carried out for 2which (Figure The calculated band ismeasurements slightly dispersive along such lines as band calculation (TBBC) was carried out 2 (Figure 3). The calculated band is slightly dispersive Г(0,0,0)–X(0.5,0,0), Y(0,0.5,0)–Z(0,0,0.5) andfor Z(0,0,0.5)–R(0.5,0.5,0.5). There are no partially-filled bands, along such as gap Г(0,0,0)–X(0.5,0,0), Y(0,0.5,0)–Z(0,0,0.5) andEFZ(0,0,0.5)–R(0.5,0.5,0.5). are no and there is lines a band of ~0.07 eV (~800 K) at the Fermi level . These results mean thatThere 2 should be partially-filled and there a band gaproom of ~0.07 eV (~800 K) at the Fermi level semiconductive. EF. These results an insulator in bands, its ground state, is but around temperature (RT), it can become mean 2 should besalts, an insulator in its ground around room temperature (RT), it can As for that the remaining a TBBC suggests that 1state, and 3but should also be either semiconductors or become semiconductive. As for the remaining salts, a TBBC suggests that 1 and 3 should also be insulators, since the calculated bands are too narrow to exhibit metallic conduction (1, Figure S1c in the either semiconductors or insulators, since the calculated bands are too narrow to exhibit metallic
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conduction (1, Figure S1c in the Supplementary Materials) or do not include partially-filled bands (3,
Supplementary Materials) or do not include partially-filled bands (3, Figure S1o in the Supplementary Figure S1o in the Supplementary Materials). These results are consistent with the observed electrical Materials). These properties (see results below). are consistent with the observed electrical properties (see below).
Figure 2. Crystal and molecular structures of 1–3 at 297 K: (a,b) 1; (c,d) 2; and (e) 3. Yellow lines
Figure 2. Crystal and molecular structures of 1–3 at 297 K: (a,b) 1; (c,d) 2; and (e) 3. Yellow lines designate the edges of the unit cells. Hydrogen atoms are omitted for clarity. Yellow, blue, brown, designate the edges of the unit cells. Hydrogen atoms are omitted for clarity. Yellow, blue, brown, pink pink and red octants designate S, C, Cu, N and O atoms, respectively. In (d), broken lines designate and S–S red short octants designate C, Cu, N and2]2−O anions atoms,are respectively. In (d),tobroken lines designate S–S contacts, and S, only [Cu(dmit) drawn in order show the conduction 2´ anions are drawn shortpathways contacts,inand only [Cu(dmit) ] in order to show the conduction 2− 2+ 2 In (e), [Cu(dmit)2] and BP2DBF are drawn separately inpathways the ac-planes (upper). the 2+ are drawn separately in the upper and in the ac-planes (upper). In (e), [Cu(dmit) ]2´ and upper and middle parts, respectively, for 2clarity. In BP (b,d,e), the directions of the magnetic fields (H), 2 DBF middle parts, forand clarity. In (b,d,e), directions of the relative magnetic fields (H),arrows those of the those of therespectively, UV irradiation the rotation axesthe of the single crystals to H (circular around the a-axis in 1,rotation around the in single 2 and around therelative axis perpendicular to both the band the UV irradiation and the axesb-axis of the crystals to H (circular arrows around c-axes 3) in angle-dependent are perpendicular shown. a-axis in 1,inaround the b-axis in 2ESR andmeasurements around the axis to both the b- and c-axes in 3) in angle-dependent ESR measurements are shown.
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Figure Figure 3. 3. Calculated Calculated band band structure structure of of 22 using using aa tight-binding tight-binding approximation. approximation. The The broken broken line line (E (EFF)) designates Г, X, X, M, M, Y, Y, Z Z and and R R designate points in in reciprocal reciprocal space space located located at at (0,0,0), (0,0,0), designates the the Fermi Fermi level. level. Г, designate points (0.5,0,0), (0.5,0.5,0), (0,0.5,0), (0,0,0.5) and (0.5,0.5,0.5), respectively. Band structures of other salts (0.5,0,0), (0.5,0.5,0), (0,0.5,0), (0,0,0.5) and (0.5,0.5,0.5), respectively. Band structures of other salts and and those those of of wider wider energy energy ranges ranges are are shown shown in in Figure Figure S1 S1 in in the the Supplementary SupplementaryMaterials. Materials.
2.2. Molecular Structures 2.2. Molecular Structures and and Spin Spin Delocalization Delocalization Unlike other kinds of metal-dithiolene-complex anions, not only the crystal structures, but also 2´ anions the molecular molecular structures structures of of the the [Cu(dmit) [Cu(dmit)22]]2− anions vary with the the counter-ion counter-ion (Figure (Figure 2). 2). The geometries at the metal center (CuS4 ) are almost square-planar, slightly distorted square-planar and distorted-tetrahedral in 1–3, respectively. Such Such flexibility flexibility of the CuS4 geometry geometry has been known for long [56–59,61–68], yet yet the thereason reasonfor fortheir theirthermodynamical thermodynamicalstabilities stabilities has never been discussed has never been discussed by 2´ anions in 2− by considering both crystal and molecular structures. The structure of the [Cu(dmit) ] considering both crystal and molecular structures. The structure of the [Cu(dmit)2] 2anions in 2 is 2unique is unique in that is also distorted alongthe thelong longmolecular molecularaxis, axis, which which is is disadvantageous for in that it isit also distorted along delocalization energy of the dmit ligands. In order to elucidate whether the crystal structures affect the delocalization energy of the dmit ligands. In order to elucidate whether the crystal structures affect 2 ´ 2− coordination geometries or not, thethe structural optimization of of thethe [Cu(dmit) the coordination geometries or not, structural optimization [Cu(dmit) anions as as isolated isolated 2 ] 2] anions molecules was carried out using using Gaussian Gaussian 09 09 [69]. [69]. The results indicate that both the planar and non-planar coordination geometries are stable and that the stable structure depends on the initial structure (Figure S2). Assuming the initial structures to exist as observed in 1 and 3, the the optimized optimized structures do not significantly differ from the initial structures, respectively. This means that the significantly differ from the initial structures, respectively. 2 ´ 2− molecular structures of the anionsare are governed governed by by their their own own stability stability and are not the [Cu(dmit) [Cu(dmit)22] anions governed by intermolecular interactions in the solid solid state. state. The calculation results indicate that the differences 1)1) and distorted-tetrahedral (like that in differences in in stability stabilitybetween betweenthe thesquare-planar square-planar(like (likethat thatinin and distorted-tetrahedral (like that 3) geometries are small and are to subtle conditions of crystallization. in coordination 3) coordination geometries are (~0.1 smalleV)(~0.1 eV)subject and are subject to subtle conditions of This situation realizes the flexibility of the i.e.,geometries, the degreesi.e., of crystallization. This situation realizesand thevariety flexibility andcoordination variety of thegeometries, coordination 2´ anions. However, 2− freedom in the molecular structures of the [Cu(dmit) ] the same calculation the degrees of freedom in the molecular structures of 2the [Cu(dmit)2] anions. However, the same for 2 gave afor rather different molecular structure fromstructure the initial,from i.e., the structure in the calculation 2 gave a rather different molecular the observed initial, i.e., the observed crystal (Figure S2). The optimized structure is close to that for 3, a distorted-tetrahedral geometry witha structure in the crystal (Figure S2). The optimized structure is close to that for 3, planar ligands. Thus, geometry the molecular in 2 is affected surrounding chemical in the distorted-tetrahedral withstructure planar ligands. Thus, thebymolecular structure in 2 species is affected by solid state, resulting inspecies a uniquely-distorted structure. Such molecular structure cana surrounding chemical in the solid state, resulting in aa “frustrated” uniquely-distorted structure. Such be related to molecular unusuallystructure small g-values 2 discussed below. Figure 4 shows SOMOs “frustrated” can beobserved related tofor unusually small g-values observed for the 2 discussed (SOMO = the singly-occupied MO)(SOMO and spin densities calculatedMO) for and 1–3 using Gaussian 09. The below. Figure 4 shows the SOMOs = the singly-occupied spin densities calculated electron densities are distributed with high symmetry over the entire molecule in all over threethe salts 1–3. for 1–3 using Gaussian 09. The electron densities are distributed with high symmetry entire Similarly, the spin distributions do not differ from each other in 1–3. The extent of delocalization of the molecule in all three salts 1–3. Similarly, the spin distributions do not differ from each other in 1–3. 2+ unpaired at the Cuof the ionsunpaired was experimentally by was measurement of theexamined electrical The extentelectrons of delocalization electrons at examined the Cu2+ ions experimentally resistivity, the magnetic susceptibility and the electron spin resonance (ESR), as discussed the by measurement of the electrical resistivity, the magnetic susceptibility and the electroninspin next section. resonance (ESR), as discussed in the next section.
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4. Calculated singly-occupied molecularorbitals orbitals (SOMO) 2; 2; and (c) (c) 3) via extended FigureFigure 4. Calculated singly-occupied molecular (SOMO)((a) ((a)1;1; 1;(b) (b) and 3) via via extended Figure 4. Calculated singly-occupied molecular orbitals (SOMO) ((a) (b) 2; and (c) 3) extended anions via Gaussian 09 Hückel calculation and spin densities ((d) 1; (e) 2; and (f) 3) for [Cu(dmit)2]2−2´ Hückel calculation and spin densities ((d) 1; (e) 2; and (f) 3) for [Cu(dmit) ] anions via Gaussian 09 2− 2 2] anions via Gaussian 09 Hückel calculation and spin densities ((d) 1; (e) 2; and (f) 3) for [Cu(dmit) (B3LYP/6-31+G(3d)). Red and blue in (a)–(c) and green and blue in (d)–(f) designate different orbital (B3LYP/6-31+G(3d)). Red and blue in (a)–(c) and green and blue in (d)–(f) designate different orbital (B3LYP/6-31+G(3d)). and blue in (a)–(c) and green and blue in (d)–(f) designate orbital lobes. Green linesRed in (a)–(c) and grey lines in (d)–(f) designate chemical bonds. In (d)–(f)different yellow and lobes. Green Green lines lines in in (a)–(c) (a)–(c) and and grey grey lines in (d)–(f) designate chemical bonds. In (d)–(f) (d)–(f) yellow yellow and lobes. lines in (d)–(f) designate chemical bonds. In grey spheres designate S and C atoms, respectively. For each molecular structure, see Figure 2. MOs and grey spheres designate Sshown and Cinatoms, respectively. For each molecular structure, see Figure 2. MOs of of exciteddesignate states are S Figure S9 in the Supplementary grey spheres and C atoms, respectively. For each Materials. molecular structure, see Figure 2. MOs excited states are shown in Figure S9 in the Supplementary Materials. of excited states are shown in Figure S9 in the Supplementary Materials. 2.3. Electrical Properties under Dark Conditions
2.3. Electrical Electrical Propertiesofunder under Dark Conditions Conditions 2.3. Properties Dark Irrespective the relative direction of the applied electric field, salts 1 and 3 are insulators (>10 ohm·cmof at the 300 K). On thedirection other hand, measurements along the b-axis that 2 Irrespective relative of resistivity the applied electric field, field, salts andindicate Irrespective of the relative direction of the applied electric salts 11 and 33 are are insulators insulators RT of = (6.5 ± 3.5) × 105 ohm·cm is a semiconductor at room temperature (298 K), with a resistivity ρ 7 (>10 ohm¨ cmatat300 300K). K).On Onthe theother otherhand, hand, resistivity resistivity measurements measurements along along the the b-axis b-axis indicate indicate that that 22 (>107 ohm·cm and an activation energy Ea of = 0.15 ± 0.02 eV (Figure 5a). These observations are qualitatively 5 ohm¨ cm is a semiconductor at room temperature (298 K), with a resistivity ρ of = (6.5 ˘ 3.5) ˆ 10 5 RT of = (6.5energy ± 3.5) gap × 10(~0.06 ohm·cm is a semiconductor temperature (298 K), above. with aHowever, resistivitythe ρRT consistent with at theroom crystal structures described calculated and an activation energy EEaa of == 0.15 ˘ 0.02 eV (Figure 5a). These observations observations are are qualitatively and an activation energy of 0.15 ± 0.02 eV (Figure 5a). These qualitatively eV) is significantly smaller than that estimated from the observed Ea, i.e., the gap is estimated to be consistent with crystal structures described However, theeffects calculated energy (~0.06 eV) is consistent with the crystal structures described above. However, the calculated energy gapthe (~0.06 2Ea~0.30 eV.the This inconsistency suggests thatabove. electron correlation are strong ingap 2, since significantly smaller than that estimated from the observed E , i.e., the gap is estimated take them intothe consideration. correlation originateto eV) istight-binding significantlyapproximation smaller than does that not estimated from observeda EThe a, i.e., the gapeffects is estimated to be be 2Eaa~0.30 ~0.30 eV. This inconsistency inconsistency suggests thatand electron correlation effects are strong in in 2, 2, since since the [70].strong fromeV. electron–electron Coulombic repulsion often open a band gap at EFare 2E This suggests that electron correlation effects the tight-binding approximation approximation does does not The correlation correlation effects effects originate tight-binding not take take them them into into consideration. consideration. The originate from electron–electron Coulombic repulsion and often open a band gap at E [70]. from electron–electron Coulombic repulsion and often open a band gap at EFF [70]. 7
Figure 5. Cont.
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Figure 5. Temperature dependence of electrical resistivity along the b-axis of 2. (a) Resistivity ρ in the
Figure 5. Temperature dependence of electrical resistivity along the b-axis of 2. (a) Resistivity ρ in the dark; (inset) Arrhenius plot of the same data; (b) surface resistivity Rsq in the dark (crosses) and dark;under (inset) UV Arrhenius plot(triangles); of the same(inset) data; (b) surface plot resistivity in thedata; dark(c) (crosses) andofunder radiation Arrhenius of theRsqsame response UV radiation (triangles); (inset) Arrhenius ploton/off. of the same data; (c) response of photoconduction to photoconduction to switching the light source switching the light source on/off. 2.4. Electrical Properties under UV Irradiation
2.4. Electrical Properties under (250–450 UV Irradiation Upon UV irradiation nm; 3 Wcm−2), only 2 exhibited a response; the resistivity clearly decreased. The temperature dependence of the´resistivity continuous UV irradiation is shown 2 ), only 2under Upon UV irradiation (250–450 nm; 3 Wcm exhibited a response; the resistivity clearly in Figure 5b. As shown in Figure 5c, the change in the resistivity was quick and reversible upon the decreased. The temperature dependence of the resistivity under continuous UV irradiation is shown commencement/cessation of UV irradiation; the resistivity instantly increased/decreased. The in Figure 5b. As shown in Figure 5c, the change in the resistivity was quick and reversible upon observed current ratio between dark and UV irradiated conditions IUV/Idark gradually increased with the commencement/cessation UVK, irradiation; the resistivity instantly increased/decreased. decreasing temperature: ~1.7 of at 297 ~1.8 at 293 K, ~1.9 at 285 K, ~2.0 at 270 K and ~2.1 at 262 K. This The observed current ratioconsistent between with dark the andtemperature UV irradiated conditions dark gradually is quantitatively dependency ofIUV the/Iresistivity underincreased dark and with decreasing temperature: ~1.7 at 297 K, ~1.8 at 293 K, ~1.9 at 285 K, ~2.0 at 270 K and ~2.1 at 262 K. UV irradiated conditions (Figure 5b). Since the activation energy remains fundamentally unchanged during irradiation consistent and since the resistivity change upon irradiation of is small, the thermally-excited This is quantitatively with the temperature dependency the resistivity under dark and carriers and the photoexcited are considered to coexist, and the former should be dominant UV irradiated conditions (Figurecarriers 5b). Since the activation energy remains fundamentally unchanged in 2. This observation suggests that the molecular crystal structures in the photoexcited states during irradiation and since the resistivity change and upon irradiation is small, the thermally-excited remain practically unchanged, since both activation energy and conductivity remain practically carriers and the photoexcited carriers are considered to coexist, and the former should be dominant in unchanged under irradiation. This interpretation is experimentally corroborated by the ESR 2. This observation suggests that the molecular and crystal structures in the photoexcited states remain measurements discussed below. practically unchanged, since both activation energy and conductivity remain practically unchanged under2.5. irradiation. This interpretation is experimentally corroborated by the ESR measurements Magnetic Susceptibilities under Dark Conditions discussed below. The temperature (T) dependence of the magnetic susceptibilities (χ) of 1–3 clearly shows that all are diamagnetic (Figure 6). Both the zero-field-cooling and field-cooling processes were examined 2.5. Magnetic Susceptibilities under Dark Conditions and were confirmed to yield identical results within experimental error. The increases in χ at low The temperature (T) dependence magnetic (χ) ofthat 1–31clearly that all temperature (T ≤ 50–60 K), as well of as the small jumps insusceptibilities χ at ~50 K suggest and 2 shows contained impurities, such as oxygen adsorbed on the sample. Curve-fitting analyses using Equations (1) examined (for χ are diamagnetic (Figure 6). Both the zero-field-cooling and field-cooling processes were
and were confirmed to yield identical results within experimental error. The increases in χ at low temperature (T ď 50–60 K), as well as small jumps in χ at ~50 K suggest that 1 and 2 contained
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impurities, (1) Inorganics such 2016, 4,as 7 oxygen adsorbed on the sample. Curve-fitting analyses using Equations 8 of 21 (for χ vs. T) and (2) (for χT vs. T) consistently and unambiguously distinguished the intrinsic behavior T) and (2) (for χT vs. T) consistently and unambiguously distinguished the intrinsic behavior from vs. those of impurities: from those of impurities: C χobs “ ` χdia (1) T =
(1)
=
(2)
χobs T “ χdia T ` C
(2)
where χobs , χdia and C designate the observed χ, the diamagnetic contribution from the samples and where , contribution and C designate the observed χ, the diamagnetic contribution from the samples the paramagnetic from impurities, respectively. The diamagnetic susceptibilities obtained and the paramagnetic contribution from impurities, respectively. The diamagnetic susceptibilities 2 ´ 1 (χdia ˆ 10 (emu mol )) for 1–3 −1are ´1.06 ˘ 0.05, ´1.24 ˘ 0.03 and ´0.80 ˘ 0.08, respectively. The obtained ( 10 (emu mol )) for 1–3 are −1.06 ± 0.05, −1.24 ± 0.03 and −0.80 ± 0.08, respectively. observed diamagnetism of 1–3 means that the unpaired electrons on the [Cu(dmit)2 ]2´ anions interact The observed diamagnetism of 1–3 means that the unpaired electrons on the [Cu(dmit)2]2− anions with interact each other rather strongly in antiferromagnetic ways in all three of the salts and that there with each other rather strongly in antiferromagnetic ways in all three of the salts and thatare no phasethere transitions at 2–300 K in any of them. is a rather unexpected result, since all are no phase transitions at 2–300 K inThe anydiamagnetism of them. The diamagnetism is a rather unexpected 2´ of theresult, intermolecular in the [Cu(dmit) of 1–3 are long. Such long-range since all ofdistances the intermolecular distances the [Cu(dmit) 2]2− anions of 1–3 are long. spin–spin Such 2 ] inanions long-range are known as “through-space” interactions [71]. The observed interactions are spin–spin known asinteractions “through-space” interactions [71]. The observed diamagnetism for the three for the three salts is consistent withattheir properties at the ground states. salts diamagnetism is consistent with their insulating properties the insulating ground states.
Figure 6. Temperature dependence themagnetic magnetic susceptibility susceptibility (χ) 2 (crosses) andand 3 Figure 6. Temperature (T) (T) dependence ofofthe (χ)ofof1 1(circles), (circles), 2 (crosses) 3 (triangles). Both field-cooling (blue) and zero-field-cooling (red) processes are shown. (Inset) χT vs. vs. T (triangles). Both field-cooling (blue) and zero-field-cooling (red) processes are shown. (Inset) χT T of 1–3 for the same field-cooling data as those in the main panel; the small jumps around 50 K in 1 of 1–3 for the same field-cooling data as those in the main panel; the small jumps around 50 K in 1 and and 3 are extrinsic, possibly due to residual adsorbed oxygen. 3 are extrinsic, possibly due to residual adsorbed oxygen.
2.6. Electron Spin Resonance under Dark Conditions
2.6. Electron Spin Resonance under Dark Conditions
Next, we should discuss the ESR spectra under dark conditions. In the following discussion,
Next, we should discuss theinESR spectra under conditions. In crystal the following g-values and their anisotropies this work mean thosedark averaged in an entire instead ofdiscussion, those g-values and their anisotropies in thisproperties work mean those averaged in an entire crystal instead for isolated molecules. The latter of [Cu(dmit) 2]2− are reported using solution[63]of orthose 2´(θ)-dependence magnetically-diluted single-crystalline samples The angle of the g-values for for isolated molecules. The latter properties of [62]. [Cu(dmit) ] are reported using solution[63] or 2 1–3 is shown in Figure 7a–c. Since all of the spectra under dark conditions were symmetric without magnetically-diluted single-crystalline samples [62]. The angle (θ)-dependence of the g-values for structures, the g-values wereall approximately estimated from conditions the maxima were of thesymmetric integrated ESR 1–3 isfine shown in Figure 7a–c. Since of the spectra under dark without spectra. The θ-dependence indicates that the g-values are in the range of ~2.01–2.06, depending on fine structures, the g-values were approximately estimated from the maxima of the integrated ESR the relative angle of the applied field and the salts. For comparison, the calculated g-values, i.e., the spectra. The θ-dependence indicates that the g-values are in the range of ~2.01–2.06, depending on principal values of g-tensors for the isolated [Cu(dmit)2]2− anions, using Gaussian 09 based on the the relative angle of the applied field and the salts. For comparison, the calculated g-values, i.e., the optimized molecular structures are gxx = 2.0403, 2.0314, gyy = 2.0644, 2.0407 and gzz = 2.0919, 2.1210 for 2 ´ principal of g-tensors for the isolated anions, Gaussian 09 based on the 2] 1 andvalues 3, respectively. The observed g-values[Cu(dmit) for 1 (2.024–2.028) and 3using (~2.02–2.06) are smaller than optimized molecular structures are g = 2.0403, 2.0314, g = 2.0644, 2.0407 and g = 2.0919, 2.1210 xx yy zz the calculated values and those generally observed for Cu(II)-complexes (for the spectra measured for 1 andwith 3, respectively. The observed g-values 1 (2.024–2.028) and 3 (~2.02–2.06) smallerwere than the internal standards, see Figure S7 infor the Supplementary Materials). Largerare g-values observed in other under dark conditions (2.02–2.04 for 1, 2.02–2.09 forspectra 2 and 2.02–2.05 for with calculated values anddirections those generally observed for Cu(II)-complexes (for the measured 3; Figure S8 in see the Figure Supplementary Thus, the observedLarger g-values indicatewere rather large internal standards, S7 in theMaterials). Supplementary Materials). g-values observed in other directions under dark conditions (2.02–2.04 for 1, 2.02–2.09 for 2 and 2.02–2.05 for 3; Figure S8 in the Supplementary Materials). Thus, the observed g-values indicate rather large anisotropy of the
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three salts under the dark conditions. Both observed g-values and their θ-dependence are smaller in Inorganics 2016, 4, 7 9 of 21 2 than in 1 and 3, because the single crystals were rotated in such a way that the magnetic field was 2´ anions in 2, while the magnetic field always approximately the molecular plane the [Cu(dmit) anisotropy of the inthree salts under the ofdark conditions.2 ] Both observed g-values and their θ-dependence arenearly smallerparallel in 2 thanand in 1nearly and 3, because the single were rotated in such a way was rotated between perpendicular tocrystals the molecular planes in 1 and 3. The ˝ (minimum) thatand the magnetic field alwaysare approximately in thethe molecular plane of theFor [Cu(dmit) 2]2− anions minima maxima of thewas g-values consistent with crystal structures. 1, θ~45 ˝ (maximum) in 2, while the magnetic field was parallel nearly perpendicular and θ~135 coincide withrotated where between the longnearly molecular axisand of all of the [Cu(dmit)2 to ]2´the anions molecular planes in 1 and 3. The minima and maxima of the g-values are consistent with the crystal ˝ becomes nearly parallel with and perpendicular to H, respectively. For 2, θ~25 (minimum) and θ~115˝ structures. For 1, θ~45° (minimum) and θ~135° (maximum) coincide with where the long molecular (maximum) also coincide with where the long molecular axis of [Cu(dmit)2 ]2´ becomes nearly parallel axis of all of the [Cu(dmit)2]2− anions becomes nearly parallel˝ with and perpendicular to H, with and perpendicular to H, respectively. Similarly, for 3, θ~50 (minimum) coincides with where respectively. For 2, θ~25° (minimum) and θ~115° (maximum) also coincide with where the long 2´ becomes nearly parallel with H. ESR signals were observed the long molecular axis of [Cu(dmit) 2] molecular axis of [Cu(dmit) 2]2− becomes nearly parallel with and perpendicular to H, respectively. only Similarly, for ~15˝ ď ď θ~50°(minimum) ~90˝ in 3. The signal almost in other directions S32]2−in the forθ 3, coincides with disappeared where the long molecular axis of (Figure [Cu(dmit) Supplementary Materials). ThisH. is ESR consistent the following explanation. In 3, observed becomes nearly parallel with signals with were observed only for ~15° ≤ θ ≤ ~90° inthe 3. The signal S–S almost disappeared in other (Figure S3 in the Supplementary Materials). Thisof is consistent distances (
Direct Control of Spin Distribution and Anisotropy in Cu-Dithiolene Complex Anions by Light Hiroki Noma 1 , Keishi Ohara 1 and Toshio Naito 1,2, * 1 2
*
Graduate School of Science and Engineering, Ehime University, 2-5, Bunkyo-cho, Matsuyama 790-8577, Japan; [email protected] (H.N.); [email protected] (K.O.) Division of Material Science, Advanced Research Support Center (ADRES), Ehime University, 2-5, Bunkyo-cho, Matsuyama 790-8577, Japan Correspondence: [email protected]; Tel.: +81-89-927-9604
Academic Editor: Duncan H. Gregory Received: 15 January 2016; Accepted: 21 March 2016; Published: 30 March 2016
Abstract: Electrical and magnetic properties are dominated by the (de)localization and the anisotropy in the distribution of unpaired electrons in solids. In molecular materials, these properties have been indirectly controlled through crystal structures using various chemical modifications to affect molecular structures and arrangements. In the molecular crystals, since the energy band structures can be semi-quantitatively known using band calculations and solid state spectra, one can anticipate the (de)localization of unpaired electrons in particular bands/levels, as well as interactions with other electrons. Thus, direct control of anisotropy and localization of unpaired electrons by locating them in selected energy bands/levels would realize more efficient control of electrical and magnetic properties. In this work, it has been found that the unpaired electrons on Cu(II)-complex anions can be optically controlled to behave as anisotropically-delocalized electrons (under dark) or isotropically-localized electrons like free electrons (under UV), the latter of which has hardly been observed in the ground states of Cu(II)-complexes by any chemical modifications. Although the compounds examined in this work did not switch between conductors and magnets, these findings indicate that optical excitation in the [Cu(dmit)2 ]2´ salts should be an effective method to control spin distribution and anisotropy. Keywords: Cu(II)-dithiolene complex; electron spin resonance; π–d interaction; quantum chemical calculation; molecular crystal
1. Introduction Electrical and magnetic properties are based on unpaired electrons in solids. Delocalized unpaired electrons can exhibit electrical conduction, while localized unpaired electrons can exhibit magnetism. This trend is true for any solid, including molecular materials. In addition, the (an)isotropy in the interactions among unpaired electrons also matters in these properties. Thus, the control of conduction and/or magnetism requires the control of the (de)localization and anisotropy of the unpaired electrons. If one can switch unpaired electrons between localized and delocalized states, then one may switch the solids between conductive and magnetic materials. This point of view is different from that of light-induced excited spin-state trapping (LIESST), where the spin states of transition-metal complexes are switched between high- and low-spins by photoexcitations at low temperature [1–9]. Additionally, the abovementioned point of view is also different from that of photo-induced phase transitions (PIPTs), where phase transitions, such as metal-insulator and ionic-neutral transitions at certain temperatures (TC s), are brought about by photo-irradiation near the TC s [10,11]. The purpose of the present work is the control of spin distribution and anisotropy independent of thermodynamic conditions, unlike PIPTs. Thus far, most of the studies to control the electrical and magnetic properties have utilized chemical modifications to affect molecular structures and arrangements. However, Inorganics 2016, 4, 7; doi:10.3390/inorganics4020007
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after tens of years of worldwide extensive research, this strategy has turned out to be challenging for the fine and precise control of conduction and magnetism [12–14]. In both organic and inorganic materials, the slightest chemical modifications often result in unexpectedly marked differences in crystal structures [15,16]. In addition, the slightest differences in crystal structures would sometimes make qualitative differences in the physical properties. In particular, in molecular crystals, there are many kinds of weak intermolecular interactions with keeping a subtle balance to form complicated electronic and crystal structures. Because of this characteristic situation, molecular crystals containing unpaired electrons often have several metastable states, as well as unique excited states with peculiar relaxation mechanisms [17] and are sensitive to perturbation, such as photo-irradiation [18]. All of these properties are interesting in the development of electrical and magnetic materials. However, a precise control of atomic/molecular arrangements in crystals remains impossible. Accordingly, it is interesting, as well as important to develop new methods to control the conductive and magnetic properties of the molecular crystals. Paying attention to such unique features of molecular crystals, the control of conduction [19,20], or magnetism [21,22], or other physical properties [23–28] was carried out using photochemical redox reactions in solid states. In the reactions, photo-induced electron transfer occurs between two different kinds of molecular species in the crystal, e.g., the cations and the anions; one species is responsible for conduction and/or magnetism, and the other serves as the charge reservoir in high-TC cuprates [29]. The latter species is not involved in either conductive or magnetic properties. As photochemical reactions generally have high spatial resolutions, this method is utilized to make junction structures of a molecular single crystal for electronic devices when the electron transfer is irreversible [19,30]. Furthermore, reversible and simultaneous control of conduction and magnetism has been recently realized using charge transfer (CT) transition between two different molecular species in salts [31,32], instead of chemical reactions. In this case, one of the molecular species accommodates localized spins for magnetism, in addition to serving as the charge reservoir. The remaining species is optically doped to have excited carriers, like photoconductors. “Giant photoconductivity”, which possibly originates from reversible melting of charge order, has been observed by UV irradiation upon a related molecular crystalline salt [33]. The advantage of this photochemical or optical method lies in the fact that control of the crystal structure is not required at any level. Now that a crystal with an appropriate structure and physical properties, which are known under dark conditions, is selected, nothing but photo-irradiation is required to excite and produce localized and/or delocalized unpaired electrons in the crystal. As long as the electronic band structure and the solid state spectra are known, selected photoexcitation with appropriate wavelength and intensity can take place with accidental or uncontrollable factors being minimized in the control of conduction and magnetism. In addition to the abovementioned sensitivity to light and unique photo-physics of molecular crystals, this kind of photo-control is based on the fact that molecular crystals often contain different kinds of molecules for different roles and also the fact that they have well-defined crystal and band structures, which are clarified by standard experimental and calculation techniques. This idea will make a step forward when a single kind of molecule is responsible for both magnetism and conduction. This will be realized when the distribution of unpaired electrons on the molecule can be controlled by irradiation. In other words, the desired molecular orbitals for unpaired electrons should be selected by appropriate photoexcitation. If the unpaired electrons are excited to be localized, the magnetic properties dominate, and vice versa. Based on this idea, a unique Cu(II)-dithiolene complex anion has been found to be suitable for this purpose, after a number of transition-metal complexes were examined. Metal-dithiolene complex molecules, M(dmit)2 (M = Ni, Pd, Pt, Au, etc., dmit2´ = 1,3-dithiole-2-thione-4,5-dithiolate, Figure 1), have attracted attention for a long time as building blocks for conducting, magnetic and optical materials [34–48]. Among a great number of molecular species, M(dmit)2 and their derivatives have always shown us new possibilities of states of matter, such as single-component metals/superconductors [49–59] and spin liquids [60]. This is because M(dmit)2 have uniquely many degrees of freedom, i.e., spin, charge and orbital degrees of freedom,
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M(dmit)2 have uniquely many degrees of freedom, i.e., spin, charge and orbital degrees of freedom, originating from d-orbitals d-orbitals at at the the metal metal centers centers and and π-orbitals π-orbitals of ofthe thedmit-ligands. dmit-ligands. The The M(dmit) M(dmit)22 originating from species often fractional formal charges in solid states. In theInsolid species often become becomestable stableradical radicalanions anionswith with fractional formal charges in solid states. the states, they closely interact with each other through the overlap of the molecular orbitals (MOs) solid states, they closely interact with each other through the overlap of the molecular orbitals accommodating the unpaired electrons. Among the M(dmit) and related complexes, the Cu(dmit)2 (MOs) accommodating the unpaired electrons. Among the2 M(dmit) 2 and related complexes, the anions [61–67] are unique in that the number and distribution of the unpaired electronselectrons may be Cu(dmit) anions [61–67] are unique in that the number and distribution of the unpaired 2 reversibly controlled with UV radiation [68]. In this work, the electronic and spin densities of may be reversibly controlled with UV radiation [68]. In this work, the electronic and spin densities of 2− 2 ´ [Cu(dmit)22]] anions [Cu(dmit) anionsare arecompared comparedinindetail detailin insalts saltswith with various various cations. cations. Among Among them, them, three three salts, salts, (nBu 4 N) 2 [Cu(dmit) 2 ] (1), [(DABCO)H] 2 [Cu(dmit) 2 ]CH 3 CN (DABCO = 1,4-diazabicyclo[2.2.2]octane) (nBu4 N)2 [Cu(dmit)2 ] (1), [(DABCO)H]2 [Cu(dmit)2 ]CH3 CN (DABCO = 1,4-diazabicyclo[2.2.2]octane) 2+ = dibenzofuran-2,2′-bis(N-methylene-4,4′-bipyridinium)) (3) (2) and andBP BP22DBF[Cu(dmit) DBF[Cu(dmit)22]] (BP (BP22DBF DBF2+ (2) = dibenzofuran-2,2’-bis(N-methylene-4,4’-bipyridinium)) (3) (Figure 1), 1), exhibited exhibited systematic systematic differences differences in in structural structural and and physical physical properties. properties. The The following following are are (Figure discussed in this paper for 1–3: the crystal and molecular structures, the temperature-dependent discussed in this paper for 1–3: the crystal and molecular structures, the temperature-dependent electrical resistivities, resistivities, the electrical the calculated calculated charge charge and and spin spin densities, densities, the the electron electron spin spin resonance resonance (ESR) (ESR) spectra and and the the temperature-dependent temperature-dependent magnetic magnetic susceptibilities. susceptibilities. spectra
1. Molecular structures of abbreviated chemical species. dmit2− = Figure 1. Molecular structures of abbreviated chemical species. dmit2´ = 1,3-dithiole-2-thione-4,5-dithiolate; 2+ 2+ DABCO, 1,4-diazabicyclo[2.2.2]octane; BP 1,3-dithiole-2-thione-4,5-dithiolate; DABCO, 1,4-diazabicyclo[2.2.2]octane; BP2DBF = 2 DBF = dibenzofuran-2,2’-bis(N-methylene-4,4’-bipyridinium). dibenzofuran-2,2′-bis(N-methylene-4,4′-bipyridinium).
2. Results and Discussion 2. Results and Discussion 2.1. Crystal and Molecular Structures 2.1. Crystal and Molecular Structures The crystal and molecular structures of 1–3 are shown in Figure 2, and the crystal data are summarized in Table in the Supplementary The crystal and 2, molecular of 1–3 The crystal and S1 molecular structures of Materials. 1–3 are shown in Figure and the structures crystal data are remain unchanged from low temperature, except for a few percentage of thermal shrinkage summarized in Table S1room in thetoSupplementary Materials. The crystal and molecular structures of in unit cell volumes V (V(100 K)/V(296 = 96.6%, 97.5% andfor 97.9% for percentage 1–3, respectively). The 1–3the remain unchanged from room to low K) temperature, except a few of thermal electrical are cooperative phenomena are mainly based intermolecular shrinkageand in magnetic the unit properties cell volumes V (V(100 K)/V(296 K)and = 96.6%, 97.5% andon97.9% for 1–3, interactions through sulfur–sulfur (S–S) interatomic contacts in 1–3. With respect to the respectively). The electrical and magnetic properties are cooperative phenomena andarrangement are mainly 2´ anions, based intermolecular interactions through sulfur–sulfurS–S (S–S) interatomic contactsKinin 1–3. of the on [Cu(dmit) the shortest intermolecular distances at 296/100 1–3With are 2] 2− respect to the arrangement of the [Cu(dmit)2Å ] inanions, shortest intermolecular S–S distances 6.138(1)/5.8082(7) Å in 1, 3.499(1)/3.429(1) 2 and the 3.921(3)/3.848(5) Å in 3. Those in 1 and at 3 296/100 K than in 1–3 are the 6.138(1)/5.8082(7) in 1, of 3.499(1)/3.429(1) Å in ˆ 2 2and 3.921(3)/3.848(5) Å inthe 3. are larger twice van der Waals Å radius a sulfur atom (1.85 = 3.70 Å; simply called Thoseder in Waals 1 and 3distance” are larger than twice vanshould der Waals radius of ainteraction sulfur atom (1.85 ×the 2 =overlap 3.70 Å; “van below). Thus,the there hardly be any through 2 ´ simply theadjacent “van der[Cu(dmit) Waals distance” below). Thus, should hardly be any of MOs called between anions in 1 and 3. there This suggests that both saltsinteraction should be 2] through thewhich overlap of MOs between adjacent [Cu(dmit)2]2− anions and 3. This suggests that both insulators, is supported by electrical measurements below. in A 1tight-binding band calculation salts should be insulators, is 3). supported by electrical below. A tight-binding (TBBC) was carried out for 2which (Figure The calculated band ismeasurements slightly dispersive along such lines as band calculation (TBBC) was carried out 2 (Figure 3). The calculated band is slightly dispersive Г(0,0,0)–X(0.5,0,0), Y(0,0.5,0)–Z(0,0,0.5) andfor Z(0,0,0.5)–R(0.5,0.5,0.5). There are no partially-filled bands, along such as gap Г(0,0,0)–X(0.5,0,0), Y(0,0.5,0)–Z(0,0,0.5) andEFZ(0,0,0.5)–R(0.5,0.5,0.5). are no and there is lines a band of ~0.07 eV (~800 K) at the Fermi level . These results mean thatThere 2 should be partially-filled and there a band gaproom of ~0.07 eV (~800 K) at the Fermi level semiconductive. EF. These results an insulator in bands, its ground state, is but around temperature (RT), it can become mean 2 should besalts, an insulator in its ground around room temperature (RT), it can As for that the remaining a TBBC suggests that 1state, and 3but should also be either semiconductors or become semiconductive. As for the remaining salts, a TBBC suggests that 1 and 3 should also be insulators, since the calculated bands are too narrow to exhibit metallic conduction (1, Figure S1c in the either semiconductors or insulators, since the calculated bands are too narrow to exhibit metallic
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conduction (1, Figure S1c in the Supplementary Materials) or do not include partially-filled bands (3,
Supplementary Materials) or do not include partially-filled bands (3, Figure S1o in the Supplementary Figure S1o in the Supplementary Materials). These results are consistent with the observed electrical Materials). These properties (see results below). are consistent with the observed electrical properties (see below).
Figure 2. Crystal and molecular structures of 1–3 at 297 K: (a,b) 1; (c,d) 2; and (e) 3. Yellow lines
Figure 2. Crystal and molecular structures of 1–3 at 297 K: (a,b) 1; (c,d) 2; and (e) 3. Yellow lines designate the edges of the unit cells. Hydrogen atoms are omitted for clarity. Yellow, blue, brown, designate the edges of the unit cells. Hydrogen atoms are omitted for clarity. Yellow, blue, brown, pink pink and red octants designate S, C, Cu, N and O atoms, respectively. In (d), broken lines designate and S–S red short octants designate C, Cu, N and2]2−O anions atoms,are respectively. In (d),tobroken lines designate S–S contacts, and S, only [Cu(dmit) drawn in order show the conduction 2´ anions are drawn shortpathways contacts,inand only [Cu(dmit) ] in order to show the conduction 2− 2+ 2 In (e), [Cu(dmit)2] and BP2DBF are drawn separately inpathways the ac-planes (upper). the 2+ are drawn separately in the upper and in the ac-planes (upper). In (e), [Cu(dmit) ]2´ and upper and middle parts, respectively, for 2clarity. In BP (b,d,e), the directions of the magnetic fields (H), 2 DBF middle parts, forand clarity. In (b,d,e), directions of the relative magnetic fields (H),arrows those of the those of therespectively, UV irradiation the rotation axesthe of the single crystals to H (circular around the a-axis in 1,rotation around the in single 2 and around therelative axis perpendicular to both the band the UV irradiation and the axesb-axis of the crystals to H (circular arrows around c-axes 3) in angle-dependent are perpendicular shown. a-axis in 1,inaround the b-axis in 2ESR andmeasurements around the axis to both the b- and c-axes in 3) in angle-dependent ESR measurements are shown.
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Figure Figure 3. 3. Calculated Calculated band band structure structure of of 22 using using aa tight-binding tight-binding approximation. approximation. The The broken broken line line (E (EFF)) designates Г, X, X, M, M, Y, Y, Z Z and and R R designate points in in reciprocal reciprocal space space located located at at (0,0,0), (0,0,0), designates the the Fermi Fermi level. level. Г, designate points (0.5,0,0), (0.5,0.5,0), (0,0.5,0), (0,0,0.5) and (0.5,0.5,0.5), respectively. Band structures of other salts (0.5,0,0), (0.5,0.5,0), (0,0.5,0), (0,0,0.5) and (0.5,0.5,0.5), respectively. Band structures of other salts and and those those of of wider wider energy energy ranges ranges are are shown shown in in Figure Figure S1 S1 in in the the Supplementary SupplementaryMaterials. Materials.
2.2. Molecular Structures 2.2. Molecular Structures and and Spin Spin Delocalization Delocalization Unlike other kinds of metal-dithiolene-complex anions, not only the crystal structures, but also 2´ anions the molecular molecular structures structures of of the the [Cu(dmit) [Cu(dmit)22]]2− anions vary with the the counter-ion counter-ion (Figure (Figure 2). 2). The geometries at the metal center (CuS4 ) are almost square-planar, slightly distorted square-planar and distorted-tetrahedral in 1–3, respectively. Such Such flexibility flexibility of the CuS4 geometry geometry has been known for long [56–59,61–68], yet yet the thereason reasonfor fortheir theirthermodynamical thermodynamicalstabilities stabilities has never been discussed has never been discussed by 2´ anions in 2− by considering both crystal and molecular structures. The structure of the [Cu(dmit) ] considering both crystal and molecular structures. The structure of the [Cu(dmit)2] 2anions in 2 is 2unique is unique in that is also distorted alongthe thelong longmolecular molecularaxis, axis, which which is is disadvantageous for in that it isit also distorted along delocalization energy of the dmit ligands. In order to elucidate whether the crystal structures affect the delocalization energy of the dmit ligands. In order to elucidate whether the crystal structures affect 2 ´ 2− coordination geometries or not, thethe structural optimization of of thethe [Cu(dmit) the coordination geometries or not, structural optimization [Cu(dmit) anions as as isolated isolated 2 ] 2] anions molecules was carried out using using Gaussian Gaussian 09 09 [69]. [69]. The results indicate that both the planar and non-planar coordination geometries are stable and that the stable structure depends on the initial structure (Figure S2). Assuming the initial structures to exist as observed in 1 and 3, the the optimized optimized structures do not significantly differ from the initial structures, respectively. This means that the significantly differ from the initial structures, respectively. 2 ´ 2− molecular structures of the anionsare are governed governed by by their their own own stability stability and are not the [Cu(dmit) [Cu(dmit)22] anions governed by intermolecular interactions in the solid solid state. state. The calculation results indicate that the differences 1)1) and distorted-tetrahedral (like that in differences in in stability stabilitybetween betweenthe thesquare-planar square-planar(like (likethat thatinin and distorted-tetrahedral (like that 3) geometries are small and are to subtle conditions of crystallization. in coordination 3) coordination geometries are (~0.1 smalleV)(~0.1 eV)subject and are subject to subtle conditions of This situation realizes the flexibility of the i.e.,geometries, the degreesi.e., of crystallization. This situation realizesand thevariety flexibility andcoordination variety of thegeometries, coordination 2´ anions. However, 2− freedom in the molecular structures of the [Cu(dmit) ] the same calculation the degrees of freedom in the molecular structures of 2the [Cu(dmit)2] anions. However, the same for 2 gave afor rather different molecular structure fromstructure the initial,from i.e., the structure in the calculation 2 gave a rather different molecular the observed initial, i.e., the observed crystal (Figure S2). The optimized structure is close to that for 3, a distorted-tetrahedral geometry witha structure in the crystal (Figure S2). The optimized structure is close to that for 3, planar ligands. Thus, geometry the molecular in 2 is affected surrounding chemical in the distorted-tetrahedral withstructure planar ligands. Thus, thebymolecular structure in 2 species is affected by solid state, resulting inspecies a uniquely-distorted structure. Such molecular structure cana surrounding chemical in the solid state, resulting in aa “frustrated” uniquely-distorted structure. Such be related to molecular unusuallystructure small g-values 2 discussed below. Figure 4 shows SOMOs “frustrated” can beobserved related tofor unusually small g-values observed for the 2 discussed (SOMO = the singly-occupied MO)(SOMO and spin densities calculatedMO) for and 1–3 using Gaussian 09. The below. Figure 4 shows the SOMOs = the singly-occupied spin densities calculated electron densities are distributed with high symmetry over the entire molecule in all over threethe salts 1–3. for 1–3 using Gaussian 09. The electron densities are distributed with high symmetry entire Similarly, the spin distributions do not differ from each other in 1–3. The extent of delocalization of the molecule in all three salts 1–3. Similarly, the spin distributions do not differ from each other in 1–3. 2+ unpaired at the Cuof the ionsunpaired was experimentally by was measurement of theexamined electrical The extentelectrons of delocalization electrons at examined the Cu2+ ions experimentally resistivity, the magnetic susceptibility and the electron spin resonance (ESR), as discussed the by measurement of the electrical resistivity, the magnetic susceptibility and the electroninspin next section. resonance (ESR), as discussed in the next section.
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4. Calculated singly-occupied molecularorbitals orbitals (SOMO) 2; 2; and (c) (c) 3) via extended FigureFigure 4. Calculated singly-occupied molecular (SOMO)((a) ((a)1;1; 1;(b) (b) and 3) via via extended Figure 4. Calculated singly-occupied molecular orbitals (SOMO) ((a) (b) 2; and (c) 3) extended anions via Gaussian 09 Hückel calculation and spin densities ((d) 1; (e) 2; and (f) 3) for [Cu(dmit)2]2−2´ Hückel calculation and spin densities ((d) 1; (e) 2; and (f) 3) for [Cu(dmit) ] anions via Gaussian 09 2− 2 2] anions via Gaussian 09 Hückel calculation and spin densities ((d) 1; (e) 2; and (f) 3) for [Cu(dmit) (B3LYP/6-31+G(3d)). Red and blue in (a)–(c) and green and blue in (d)–(f) designate different orbital (B3LYP/6-31+G(3d)). Red and blue in (a)–(c) and green and blue in (d)–(f) designate different orbital (B3LYP/6-31+G(3d)). and blue in (a)–(c) and green and blue in (d)–(f) designate orbital lobes. Green linesRed in (a)–(c) and grey lines in (d)–(f) designate chemical bonds. In (d)–(f)different yellow and lobes. Green Green lines lines in in (a)–(c) (a)–(c) and and grey grey lines in (d)–(f) designate chemical bonds. In (d)–(f) (d)–(f) yellow yellow and lobes. lines in (d)–(f) designate chemical bonds. In grey spheres designate S and C atoms, respectively. For each molecular structure, see Figure 2. MOs and grey spheres designate Sshown and Cinatoms, respectively. For each molecular structure, see Figure 2. MOs of of exciteddesignate states are S Figure S9 in the Supplementary grey spheres and C atoms, respectively. For each Materials. molecular structure, see Figure 2. MOs excited states are shown in Figure S9 in the Supplementary Materials. of excited states are shown in Figure S9 in the Supplementary Materials. 2.3. Electrical Properties under Dark Conditions
2.3. Electrical Electrical Propertiesofunder under Dark Conditions Conditions 2.3. Properties Dark Irrespective the relative direction of the applied electric field, salts 1 and 3 are insulators (>10 ohm·cmof at the 300 K). On thedirection other hand, measurements along the b-axis that 2 Irrespective relative of resistivity the applied electric field, field, salts andindicate Irrespective of the relative direction of the applied electric salts 11 and 33 are are insulators insulators RT of = (6.5 ± 3.5) × 105 ohm·cm is a semiconductor at room temperature (298 K), with a resistivity ρ 7 (>10 ohm¨ cmatat300 300K). K).On Onthe theother otherhand, hand, resistivity resistivity measurements measurements along along the the b-axis b-axis indicate indicate that that 22 (>107 ohm·cm and an activation energy Ea of = 0.15 ± 0.02 eV (Figure 5a). These observations are qualitatively 5 ohm¨ cm is a semiconductor at room temperature (298 K), with a resistivity ρ of = (6.5 ˘ 3.5) ˆ 10 5 RT of = (6.5energy ± 3.5) gap × 10(~0.06 ohm·cm is a semiconductor temperature (298 K), above. with aHowever, resistivitythe ρRT consistent with at theroom crystal structures described calculated and an activation energy EEaa of == 0.15 ˘ 0.02 eV (Figure 5a). These observations observations are are qualitatively and an activation energy of 0.15 ± 0.02 eV (Figure 5a). These qualitatively eV) is significantly smaller than that estimated from the observed Ea, i.e., the gap is estimated to be consistent with crystal structures described However, theeffects calculated energy (~0.06 eV) is consistent with the crystal structures described above. However, the calculated energy gapthe (~0.06 2Ea~0.30 eV.the This inconsistency suggests thatabove. electron correlation are strong ingap 2, since significantly smaller than that estimated from the observed E , i.e., the gap is estimated take them intothe consideration. correlation originateto eV) istight-binding significantlyapproximation smaller than does that not estimated from observeda EThe a, i.e., the gapeffects is estimated to be be 2Eaa~0.30 ~0.30 eV. This inconsistency inconsistency suggests thatand electron correlation effects are strong in in 2, 2, since since the [70].strong fromeV. electron–electron Coulombic repulsion often open a band gap at EFare 2E This suggests that electron correlation effects the tight-binding approximation approximation does does not The correlation correlation effects effects originate tight-binding not take take them them into into consideration. consideration. The originate from electron–electron Coulombic repulsion and often open a band gap at E [70]. from electron–electron Coulombic repulsion and often open a band gap at EFF [70]. 7
Figure 5. Cont.
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Figure 5. Temperature dependence of electrical resistivity along the b-axis of 2. (a) Resistivity ρ in the
Figure 5. Temperature dependence of electrical resistivity along the b-axis of 2. (a) Resistivity ρ in the dark; (inset) Arrhenius plot of the same data; (b) surface resistivity Rsq in the dark (crosses) and dark;under (inset) UV Arrhenius plot(triangles); of the same(inset) data; (b) surface plot resistivity in thedata; dark(c) (crosses) andofunder radiation Arrhenius of theRsqsame response UV radiation (triangles); (inset) Arrhenius ploton/off. of the same data; (c) response of photoconduction to photoconduction to switching the light source switching the light source on/off. 2.4. Electrical Properties under UV Irradiation
2.4. Electrical Properties under (250–450 UV Irradiation Upon UV irradiation nm; 3 Wcm−2), only 2 exhibited a response; the resistivity clearly decreased. The temperature dependence of the´resistivity continuous UV irradiation is shown 2 ), only 2under Upon UV irradiation (250–450 nm; 3 Wcm exhibited a response; the resistivity clearly in Figure 5b. As shown in Figure 5c, the change in the resistivity was quick and reversible upon the decreased. The temperature dependence of the resistivity under continuous UV irradiation is shown commencement/cessation of UV irradiation; the resistivity instantly increased/decreased. The in Figure 5b. As shown in Figure 5c, the change in the resistivity was quick and reversible upon observed current ratio between dark and UV irradiated conditions IUV/Idark gradually increased with the commencement/cessation UVK, irradiation; the resistivity instantly increased/decreased. decreasing temperature: ~1.7 of at 297 ~1.8 at 293 K, ~1.9 at 285 K, ~2.0 at 270 K and ~2.1 at 262 K. This The observed current ratioconsistent between with dark the andtemperature UV irradiated conditions dark gradually is quantitatively dependency ofIUV the/Iresistivity underincreased dark and with decreasing temperature: ~1.7 at 297 K, ~1.8 at 293 K, ~1.9 at 285 K, ~2.0 at 270 K and ~2.1 at 262 K. UV irradiated conditions (Figure 5b). Since the activation energy remains fundamentally unchanged during irradiation consistent and since the resistivity change upon irradiation of is small, the thermally-excited This is quantitatively with the temperature dependency the resistivity under dark and carriers and the photoexcited are considered to coexist, and the former should be dominant UV irradiated conditions (Figurecarriers 5b). Since the activation energy remains fundamentally unchanged in 2. This observation suggests that the molecular crystal structures in the photoexcited states during irradiation and since the resistivity change and upon irradiation is small, the thermally-excited remain practically unchanged, since both activation energy and conductivity remain practically carriers and the photoexcited carriers are considered to coexist, and the former should be dominant in unchanged under irradiation. This interpretation is experimentally corroborated by the ESR 2. This observation suggests that the molecular and crystal structures in the photoexcited states remain measurements discussed below. practically unchanged, since both activation energy and conductivity remain practically unchanged under2.5. irradiation. This interpretation is experimentally corroborated by the ESR measurements Magnetic Susceptibilities under Dark Conditions discussed below. The temperature (T) dependence of the magnetic susceptibilities (χ) of 1–3 clearly shows that all are diamagnetic (Figure 6). Both the zero-field-cooling and field-cooling processes were examined 2.5. Magnetic Susceptibilities under Dark Conditions and were confirmed to yield identical results within experimental error. The increases in χ at low The temperature (T) dependence magnetic (χ) ofthat 1–31clearly that all temperature (T ≤ 50–60 K), as well of as the small jumps insusceptibilities χ at ~50 K suggest and 2 shows contained impurities, such as oxygen adsorbed on the sample. Curve-fitting analyses using Equations (1) examined (for χ are diamagnetic (Figure 6). Both the zero-field-cooling and field-cooling processes were
and were confirmed to yield identical results within experimental error. The increases in χ at low temperature (T ď 50–60 K), as well as small jumps in χ at ~50 K suggest that 1 and 2 contained
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impurities, (1) Inorganics such 2016, 4,as 7 oxygen adsorbed on the sample. Curve-fitting analyses using Equations 8 of 21 (for χ vs. T) and (2) (for χT vs. T) consistently and unambiguously distinguished the intrinsic behavior T) and (2) (for χT vs. T) consistently and unambiguously distinguished the intrinsic behavior from vs. those of impurities: from those of impurities: C χobs “ ` χdia (1) T =
(1)
=
(2)
χobs T “ χdia T ` C
(2)
where χobs , χdia and C designate the observed χ, the diamagnetic contribution from the samples and where , contribution and C designate the observed χ, the diamagnetic contribution from the samples the paramagnetic from impurities, respectively. The diamagnetic susceptibilities obtained and the paramagnetic contribution from impurities, respectively. The diamagnetic susceptibilities 2 ´ 1 (χdia ˆ 10 (emu mol )) for 1–3 −1are ´1.06 ˘ 0.05, ´1.24 ˘ 0.03 and ´0.80 ˘ 0.08, respectively. The obtained ( 10 (emu mol )) for 1–3 are −1.06 ± 0.05, −1.24 ± 0.03 and −0.80 ± 0.08, respectively. observed diamagnetism of 1–3 means that the unpaired electrons on the [Cu(dmit)2 ]2´ anions interact The observed diamagnetism of 1–3 means that the unpaired electrons on the [Cu(dmit)2]2− anions with interact each other rather strongly in antiferromagnetic ways in all three of the salts and that there with each other rather strongly in antiferromagnetic ways in all three of the salts and thatare no phasethere transitions at 2–300 K in any of them. is a rather unexpected result, since all are no phase transitions at 2–300 K inThe anydiamagnetism of them. The diamagnetism is a rather unexpected 2´ of theresult, intermolecular in the [Cu(dmit) of 1–3 are long. Such long-range since all ofdistances the intermolecular distances the [Cu(dmit) 2]2− anions of 1–3 are long. spin–spin Such 2 ] inanions long-range are known as “through-space” interactions [71]. The observed interactions are spin–spin known asinteractions “through-space” interactions [71]. The observed diamagnetism for the three for the three salts is consistent withattheir properties at the ground states. salts diamagnetism is consistent with their insulating properties the insulating ground states.
Figure 6. Temperature dependence themagnetic magnetic susceptibility susceptibility (χ) 2 (crosses) andand 3 Figure 6. Temperature (T) (T) dependence ofofthe (χ)ofof1 1(circles), (circles), 2 (crosses) 3 (triangles). Both field-cooling (blue) and zero-field-cooling (red) processes are shown. (Inset) χT vs. vs. T (triangles). Both field-cooling (blue) and zero-field-cooling (red) processes are shown. (Inset) χT T of 1–3 for the same field-cooling data as those in the main panel; the small jumps around 50 K in 1 of 1–3 for the same field-cooling data as those in the main panel; the small jumps around 50 K in 1 and and 3 are extrinsic, possibly due to residual adsorbed oxygen. 3 are extrinsic, possibly due to residual adsorbed oxygen.
2.6. Electron Spin Resonance under Dark Conditions
2.6. Electron Spin Resonance under Dark Conditions
Next, we should discuss the ESR spectra under dark conditions. In the following discussion,
Next, we should discuss theinESR spectra under conditions. In crystal the following g-values and their anisotropies this work mean thosedark averaged in an entire instead ofdiscussion, those g-values and their anisotropies in thisproperties work mean those averaged in an entire crystal instead for isolated molecules. The latter of [Cu(dmit) 2]2− are reported using solution[63]of orthose 2´(θ)-dependence magnetically-diluted single-crystalline samples The angle of the g-values for for isolated molecules. The latter properties of [62]. [Cu(dmit) ] are reported using solution[63] or 2 1–3 is shown in Figure 7a–c. Since all of the spectra under dark conditions were symmetric without magnetically-diluted single-crystalline samples [62]. The angle (θ)-dependence of the g-values for structures, the g-values wereall approximately estimated from conditions the maxima were of thesymmetric integrated ESR 1–3 isfine shown in Figure 7a–c. Since of the spectra under dark without spectra. The θ-dependence indicates that the g-values are in the range of ~2.01–2.06, depending on fine structures, the g-values were approximately estimated from the maxima of the integrated ESR the relative angle of the applied field and the salts. For comparison, the calculated g-values, i.e., the spectra. The θ-dependence indicates that the g-values are in the range of ~2.01–2.06, depending on principal values of g-tensors for the isolated [Cu(dmit)2]2− anions, using Gaussian 09 based on the the relative angle of the applied field and the salts. For comparison, the calculated g-values, i.e., the optimized molecular structures are gxx = 2.0403, 2.0314, gyy = 2.0644, 2.0407 and gzz = 2.0919, 2.1210 for 2 ´ principal of g-tensors for the isolated anions, Gaussian 09 based on the 2] 1 andvalues 3, respectively. The observed g-values[Cu(dmit) for 1 (2.024–2.028) and 3using (~2.02–2.06) are smaller than optimized molecular structures are g = 2.0403, 2.0314, g = 2.0644, 2.0407 and g = 2.0919, 2.1210 xx yy zz the calculated values and those generally observed for Cu(II)-complexes (for the spectra measured for 1 andwith 3, respectively. The observed g-values 1 (2.024–2.028) and 3 (~2.02–2.06) smallerwere than the internal standards, see Figure S7 infor the Supplementary Materials). Largerare g-values observed in other under dark conditions (2.02–2.04 for 1, 2.02–2.09 forspectra 2 and 2.02–2.05 for with calculated values anddirections those generally observed for Cu(II)-complexes (for the measured 3; Figure S8 in see the Figure Supplementary Thus, the observedLarger g-values indicatewere rather large internal standards, S7 in theMaterials). Supplementary Materials). g-values observed in other directions under dark conditions (2.02–2.04 for 1, 2.02–2.09 for 2 and 2.02–2.05 for 3; Figure S8 in the Supplementary Materials). Thus, the observed g-values indicate rather large anisotropy of the
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three salts under the dark conditions. Both observed g-values and their θ-dependence are smaller in Inorganics 2016, 4, 7 9 of 21 2 than in 1 and 3, because the single crystals were rotated in such a way that the magnetic field was 2´ anions in 2, while the magnetic field always approximately the molecular plane the [Cu(dmit) anisotropy of the inthree salts under the ofdark conditions.2 ] Both observed g-values and their θ-dependence arenearly smallerparallel in 2 thanand in 1nearly and 3, because the single were rotated in such a way was rotated between perpendicular tocrystals the molecular planes in 1 and 3. The ˝ (minimum) thatand the magnetic field alwaysare approximately in thethe molecular plane of theFor [Cu(dmit) 2]2− anions minima maxima of thewas g-values consistent with crystal structures. 1, θ~45 ˝ (maximum) in 2, while the magnetic field was parallel nearly perpendicular and θ~135 coincide withrotated where between the longnearly molecular axisand of all of the [Cu(dmit)2 to ]2´the anions molecular planes in 1 and 3. The minima and maxima of the g-values are consistent with the crystal ˝ becomes nearly parallel with and perpendicular to H, respectively. For 2, θ~25 (minimum) and θ~115˝ structures. For 1, θ~45° (minimum) and θ~135° (maximum) coincide with where the long molecular (maximum) also coincide with where the long molecular axis of [Cu(dmit)2 ]2´ becomes nearly parallel axis of all of the [Cu(dmit)2]2− anions becomes nearly parallel˝ with and perpendicular to H, with and perpendicular to H, respectively. Similarly, for 3, θ~50 (minimum) coincides with where respectively. For 2, θ~25° (minimum) and θ~115° (maximum) also coincide with where the long 2´ becomes nearly parallel with H. ESR signals were observed the long molecular axis of [Cu(dmit) 2] molecular axis of [Cu(dmit) 2]2− becomes nearly parallel with and perpendicular to H, respectively. only Similarly, for ~15˝ ď ď θ~50°(minimum) ~90˝ in 3. The signal almost in other directions S32]2−in the forθ 3, coincides with disappeared where the long molecular axis of (Figure [Cu(dmit) Supplementary Materials). ThisH. is ESR consistent the following explanation. In 3, observed becomes nearly parallel with signals with were observed only for ~15° ≤ θ ≤ ~90° inthe 3. The signal S–S almost disappeared in other (Figure S3 in the Supplementary Materials). Thisof is consistent distances (
