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Photocatalytic Reduction of Artificial and Natural Nucleotide Cofactors with a Chlorophyll-Like Tin-Dihydroporphyrin Sensitizer K. T. Oppelt, E. Wöß, M. Stiftinger, W. Schöfberger, W. Buchberger and G. Knör *
Materials and Methods All Chemicals used in the synthesis were purchased from Aldrich and used without any further purification, if not otherwise stated. 4-Pyridyl-aldehyde was purchased from ABCR (97%), pyridine came from Acros Organics (+99%, extra pure). Tin chloride dihydrate was provided by VWR (technical). The disodium salt of EDTA was purchased from Merck (Titriplex III, p.a.). The sodium salt of NAD+ and ADH (yeast alcohol dehydrogenase, EC 1.1.1.1, > 300 U/mg protein) were obtained from Sigma-Aldrich. UV-Vis-spectra were recorded with a Varian Cary 50 diode array spectrophotometer and photoluminescence measurements were performed with a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. For NMR analysis, a Bruker Biospin 200 MHz spectrometer and a Bruker DRX 500 MHz spectrometer were used. Chemical shifts of the tin NMR spectra reported relative to the external reference Sn(Me)4 (δ=0 ppm) and were measured with Sn(Ph)4 (δ(119Sn)= - 121.1 ppm) used as a secondary standard (solid state NMR).1,2 HPLC experiments were carried out with an Agilent 1100 system. High-resolution MS measurements were performed with an Agilent 6510 quadrupole time-offlight (Q-TOF) instrument and an ESI source. The photochemical and photocatalytic experiments are described in more detail below.
Synthesis of Compounds Tin(IV)-meso-tetrakis-(N-methylpyridinium)-porphyrin complexes (SnP) The free base 5,10,15,20-tetrakis-(4-pyridyl)-porphyrin ligand H2(TPyP) was prepared according to a modified Adler-Longo-procedure.3 3.16 g of freshly distilled pyrrole were added portionwise to a hot solution (120°C) of 4.84 g of 4-pyridyl-aldehyde in 135 ml of propionic acid and 15 ml acetic anhydride. Afterwards, the mixture was heated to reflux for 90 min. The reaction mixture was cooled down and the acidic solvent was removed on the rotary evaporator until only 30 ml were left. By addition of aqueous sodium hydroxide solution, the pH of the residue was adjusted to 12. The solvent was removed via 1
vacuum filtration and the brown raw product (6.88 g) was purified by column chromatography on silica gel 60 with chloroform/methanol (1000:75) as eluent to yield the purple product. In one representative run of the purification procedure, the yield from 0.66 g raw product was 9.5 %. Table S1:
1
H-NMR of H2(TPyP)
δ(ppm)
Mult.
J (Hz)
Intensity
Assignment
8.99
d
4.4
8H
3,3’,3’’, 3’’’,5,5’, 5’’, 5’’’-Pyridyl protons
8.80
s
8H
Pyrrole protons
8.09
d
8H
2,2’,2’’,2’’’,6,6’,6’’,6’’’-Pyridyl protons
4.4
Dichloro-(5,10,15,20-tetrakis-(4-pyridyl)-porphyrinato)tin(IV) Sn(TPyP)Cl2 was prepared according to modified literature procedures in pyridine solution.4 The free base porphyrin H2(TPyP) (0.292 g) was heated to reflux in 35 ml pyridine. After addition of 0.9 g of SnCl2·2H2O in 20 ml pyridine the reaction solution was refluxed and stirred for 3 h, then another 0.78 g of SnCl2·2H2O were added and refluxed overnight. The reaction progress was observed by UV-Vis. After evaporation of the solvent, the raw product was purified by column chromatography on neutral Al2O3 with chloroform/pyridine (4:1) as eluent to yield 0.24 g of Sn(TPyP)Cl2 . Table S2:
1
H-NMR of Sn(TPyP)Cl2
δ(ppm)
Mult.
9.30
s
9.12
d
8.37
d
J (Hz)
Intensity
Assignment
8H
Pyrrole protons
6.0
8H
3,3’,3’’,3’’’,5,5’,5’’,5’’’-Pyridyl protons
6.0
8H
2,2’,2’’,2’’’,6,6’,6’’,6’’’-Pyridyl protons
The water soluble complex dichloro-(5,10,15,20-tetrakis-(N-methyl-pyridinium-4-yl)-porphyrinato)tin(IV) tetrachloride [(TMPyP)Sn(Cl)2]4+ (Cl-)4 was synthesized according to Harriman et.al.4 The product of the former reaction step (0.239 g) was stirred with an excess of iodomethane at room temperature for two days. The purple precipitate was filtered off and washed with dichloromethane. The raw product was dissolved in water and treated with freshly precipitated AgCl for 20 min to remove the iodide anion. After filtration, the filtrate was subjected to ion-exchange chromatography over DOWEX 1x 8 ion exchange resin. Water was removed under reduced pressure and after vacuum drying the violet solid was washed twice with acetone and sonicated during this process. Afterwards the purple powder was dried again in vacuum (yield: 0.20 g).
2
1
4+
Table S3: H-NMR of the [(TMPyP)Sn(Cl)2] cation in D2O
δ(ppm)
Mult.
9.57
s
9.37
d
9.07
d
4.8
s
J (Hz)
Intensity
Assignment
8H
Pyrrole protons
4.6
8H
3,3’,3’’,3’’’,5,5’,5’’,5’’’-Pyridinium protons
4.6
8H
2,2’,2’’,2’’’,6,6’,6’’,6’’’-Pyridinium protons
12H
Methyl protons
4,0 422 nm
3,5
Absorbance
3,0 2,5 2,0 1,5 1,0
555 nm
0,5 594 nm
0,0 200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
4+
Figure S1 UV-Vis absorption spectrum of the chloride salt of the [(TMPyP)Sn(Cl)2] cation in water at neutral pH
3
The pH-dependence of the position of the Soret band maximum in the UV-visible absorption spectrum of SnP was followed in 0.1 M phosphoric acid solution upon KOH-addition. The titration results indicate a stability range of the unprotonated dichloro species [(TMPyP)Sn(Cl)2]4+ between pH 6 and pH 10 as given below (Figure S2).
Figure S2 Position of the Soret band maximum of tin(IV)-meso-tetrakis-(N-methylpyridinium)-porphyrin hexachloride in 0.1 M sodium phosphate solution, indicating the exchange of axial chloride by hydroxide around pH 12. The reversible spectral shift between pH 4 and pH 6 according to the literature originates from the protonation of one of the pyrrole nitrogens of the porphyrin [Delmarre, D.; Veret-Lemarinier, A.-V.; Bied-Charreton, C. J. Luminesc. 1999, 82, 57-67].
4
Figure S3 ESI Q-TOF mass spectrum for [(TMPyP)Sn(X)2]
4+
(Agilent 6510 Q-TOF, LC-MS, positive mode) giving
clear evidence for a possible hydrolytic exchange of X = Cl in aqueous solution. The peak pattern shown corresponds to the deprotonated doubly-charged species [(TMPyP)Sn(O)2]
5
2+
2+
with C44H36N8O2Sn .
[Cp*Rh(bpy)Cl]Cl [Cp*Rh(bpy)Cl]Cl was prepared according to a literature procedure5 using standard Schlenk-techniques. 0.15g of dichloro(pentamethylcyclopentadienyl)rhodium(III)dimer were dissolved in absolute methanol and 2,2’-bipyridine (0.098 g) was added until no unreacted red crystals of the rhodium salt were visible anymore, and a yellow solution was formed. Methanol was partly evaporated and the product was precipitated upon addition 3 ml of diethyl ether. The orange precipitate was dried in vacuum overnight and characterized by 1H-NMR-spectroscopy and elemental analysis. The chloride ligand may undergo substitution processes in solution. 1
2+
Table S4 H-NMR data of [Cp*Rh(bpy)D2O] in D2O
δ (ppm)
Mult.
1.65
S
8.92
D
7.81
J (Hz)
Intensity
Assignment
15 H
Cp*
8.06
2H
H 6,6’
T
7.03
2H
H 5,5’
8.26
Td
7.87 0.97
2H
H4,4’
8.85
D
5.47
2H
H3,3’
2+
Table S5 Elemental Analysis of [Cp*Rh(bpy)Cl]Cl
Calculated
C%
51.63
H%
4.95
N%
5.98
Found
C%
50.20
H%
5.34
N%
5.89
N-Benzyl-3-carbamoyl-pyridinium-chloride Benzyl nicotine amide dinucleotide bromide was synthesized according to a modified literature procedure6 from benzyl bromide and nicotine amide in absolute ethanol by stirring overnight at 40°C. After filtration and washing of the white crystals with ether, the product was dried in high vacuum. The anion exchange to chloride was performed by stirring the aqueous solution of benzyl nicotine amide dinucleotide bromide with freshly precipitated silver chloride7. After 30 min the pale yellow precipitate of silver bromide was filtered off and the residual solvent was removed by vacuum to yield the white product N-benzyl-3-carbamoyl-pyridinium chloride (BNAD+). 6
N-Benzyl-1,4-dihydronicotineamide BNAD+ was reduced to the 1,4-dihydronicotineamide derivative by reduction with sodium dithionite in basic sodium carbonate solution under inert conditions8 using standard Schlenk-techniques with nitrogen as inert gas. N-benzyl-1,4-dihydronicotineamide (BNADH) crystallized from the reaction solution as pale yellow solid. The solvent was removed by inert filtration. The precipitate was washed twice with degassed water and then dried in vacuum.
1
+
Table S6 H-NMR Data for BNAD (N-Benzyl-3-carbamoyl-pyridinium-chloride) in CDCl3
δ (ppm)
J (Hz)
9.32
s
9.04
d
8.88
Intensity Assignment
1H
2
5.77
1H
6
d
8.03
1H
4
8.17
t
6.97
1H
5
7.48
dt
7.98; 3.46
1H
8, 8’, 9, 9’, 10
5.87
dd
3.47; 1.60
2H
4,4’
O
4 5
NH2 + N
6 8 9
2 – Cl 7,7'
8'
10 9'
1
Table S7 H-NMR Data for BNADH (N-benzyl-1,4-dihydronicotineamide) in CDCl3
δ (ppm)
J (Hz)
7.29
m
7.15
d
5.74
Intensity Assignment
5H
8, 8’, 9, 9’, 10
1H
2
ddd 8.24; 3.98; 1.92
1H
6
5.15
bs
2H
-NH2
4.75
dt
1H
5
11
4.30
s
2H
7,7’
12
1.65
7.98; 3.46
4 5
10 9,9' 10' 11'
7
2
6
Setup used for Photoreaction Quantum Yield Determinations A home-built optical bench mounted system producing an electrical signal proportional to the photon flux absorbed by the investigated samples was used for exact conversion quantum yield determinations. The technical details of this setup are following the quantum counter setup reported and tested recently by Riedle et. al.9, which allows for a convenient absolute radiative power detection with a calibrated solar cell detector. Based on the construction plans generously offered by these authors, our system was slightly modified with a turning wheel for rapid switching between four different permanently installed high-power LED light sources covering the visible spectral range for photolysis experiments. The spectral output of the actually built-in high-power LEDs (470, 525, 592 and 623 nm)10 used for visible-light illumination experiments is shown below:
Fig S4 Emission spectra of the high-power LEDs used
Calibration of the home-built system was carried out by applying classical chemical actinometry with Reinecke´s salt in dilute acidic solution according to the recommended standard procedures 11,12 (0.01 M: φ = 0.31 at 470nm; 0.04M: φ = 0.29 at 525nm; 0.01M: φ = 0.27 at 592nm; 0.025M: φ = 0.29 at 623nm).
8
Photogeneration of Hydroporphyrin Derivatives Irradiation experiments to generate SnC in solution were performed in 1cm quartz glass cuvettes sealed with a septum in aqueous solutions containing 0.1 mol l-1 sodium phosphate, 10 mmol l-1 EDTA and 1.5·10-5 mol l-1 SnP. First the chlorin species was formed upon irradiation for 8 min at room temperature on an optical bench equipped with a 150 W short arc Xe lamp, a quartz condensor lens, a liquid water filter (AMKO) to eliminate IR-radiation, and a Schott glass cut off filter (OG 530). The sample was saturated with air by repeated shaking and stirring. The reaction progress was followed by UV-VIS spectroscopy. It is important to note that upon prolonged irradiation also tetrahydroporphyrins (tinbacteriochlorin-derivatives SnBC) could be detected by their characteristic Q-band patterns in the far-red spectral region (see also Figure S7). For the determination of the corresponding quantum yield, the same procedure was used, but the light source was a green LED with a maximum at 525 nm (see Figure S4).
-5
Figure S5 Typical spectral changes observed during monochromatic irradiation of 1.5·10 M SnP in aqueous 0.1 M phosphate buffer solution (pH 8.8) containing EDTA with an 3 W LED at 525 nm under ambient air.
9
A photochemical quantum yield value of φ = 4•10-3 was obtained for the chlorin formation under these conditions (EDTA donor, 525nm). Using alternatively sodium sulphite as the electron donor, a much faster conversion from SnP to SnC was observed (typically within a few seconds under similar conditions with φ > 0.3). Sulfate was identified as a permanent product in the latter systems. Since an excess of other sulfur compounds than sulphate remaining in the system might negatively influence the performance of the rhodium-based redox-mediators added under catalytic conditions, we decided to completely remove these reducing compounds for all of our tests with sulphite-photogenerated SnC as the sensitizer. Therefore, the pH of the sulfite containing tin chlorin solutions was adjusted to a value below 1 by adding HCl. Subsequent purging of the reaction mixture with air was carried out in order to remove the evolving SO2 gas. The photosensitizer solution thus obtained was directly used for the following irradiation experiments at various defined pH-values in buffered aqueous solution.
Figure S6 UV-Vis absorption spectrum of the tin chlorin photocatalyst SnC in water. Inset: Spectral features of 15 native chlorophyll b for comparison.
10
Photocatalytic Formation and Detection of Reduced Nucleotide Cofactor Analogues After in situ generation of the SnC photocatalyst, 0.3 ml of a 1mM [Cp*Rh(bpy)Cl]Cl-solution (3·10-6 mol) and 1.3 mg BNAD+Cl- (5.2 10-6 mol) were added, and the solution was purged with Ar for 30 min. Then the second phase of irradiation was done with a 3W LED at 623 nm (selective excitation of SnC in the Qband region corresponding to the lowest excited singlet state of the sensitizer). UV-Vis spectra were recorded to follow the reaction progress. After 900 min, the sample was transferred under inert conditions into a HPLC vial and the run was started immediately, because of the air sensitivity of the reduced cofactor model (see also Figure S9 in next section!). After 4 h the fluorescence spectrum (excitation at 340 nm) and the excitation spectrum (at 470 nm) of the sample were recorded (see Figure S7). For the determination of the exact quantum yield data, the increase in absorbance at 355 nm due to BNADH formation was used (ε= 7240) 13.
Excitation (470 nm)
Luminescence
Emission (340 nm)
250
300
350
400
450
500
550
600
650
Wavelength (nm)
Figure S7 Photoluminescence (excitation wavelength 340 nm) and excitation spectra (measurement wavelength 470 nm) of BNADH formed from an irradiated solution (3 W LED 623 nm) of SnC in aqueous 0.1 M phosphate buffer -5 + solution (pH 8.8) with 8.6·10 M [Cp*Rh(bpy)Cl]Cl, 0.01 M EDTA and 1.5 mM BNAD after 4h of irradiation time.
Without the rhodium catalyst in the system, no increase in absorbance in the range between 320 and 360 nm could be observed under similar conditions (Figure S8).
11
-5
Figure S8 Irradiation of 1.4·10 M SnC in aqueous 0.1 M phosphate buffer solution (pH 8.8) containing 0.01 M EDTA -1 + and 1.5 mmol l BNAD with an 3 W LED at 623 nm (note the peak at 766nm due to the four electron reduced tetrahydroporphyrin derivative tin bacteriochlorin SnBC present under these conditions of selective SnC excitation).
Detection of BNADH by HPLC/MS The setup used was an Agilent 1100 HPLC. The injection volume was 10µl on a Phenomenex Kinetex 2.6 µm C18 50x4.6 mm column. For chromatographic separation, a water: acetonitrile gradient from 0 to 100% acetonitrile within 20 min was used with a flow of 0.9 ml min-1. Then 10 min 100% acetonitrile and 10 min 100% water followed for conditioning the column after each run. 0.1 M sodium phosphate buffer solutions containing either 200 ppm BNAD+Cl- or 230 ppm BNADH were used as references for the HPLC. MS detection was performed on an Agilent 6510 Q-TOF mass spectrometer with an ESI source and an Agilent G1607A coaxial sprayer. Prior to the ESI ion source of the Q-TOF, a 0.1 ml min-1 make up flow of 1% formic acid of was used to enhance the ionization.
12
Figure S9 Mass spectra for the retention times 6.2 min and 8.2 min in the HPLC for the irradiated samples and the BNADH reference molecule.
Comparison of the positive ion traces collected in Fig. S10 clearly shows that BNADH detected at m/z= 215.1179 as the mono-protonated cation BNADH/H+ is generated upon excitation of the tin chlorin and rhodium complex containing reaction mixture. No such reduction of the cofactor analogue occurs in the absence of one of the essential components required.
13
+
+
Figure S10. HPLC/MS-detection of protonated BNADH/H (m/z= 215.1179) starting from BNAD (m/z= 213.1022). Extracted ion traces for the irradiated samples and corresponding reference solutions are compared.
Photocatalytic Performance under Long-Term Steady-State Irradiation with Red-Light Depending on the relative concentrations of the functional components (photosensitizer, rhodium complex, nucleotide cofactor, excitation wavelength), a photostationary level of photoproducts is reached upon longer irradiation times (Fig. S11 and S12). Under the conditions chosen for the present paper, which were guided by the requirement of following the spectral changes of all relevant components in situ, typically a saturation concentration of 0.4-0.5 mM NADH was reached (absorbance at 340nm > 2.5 in 1cm cell). In natural photosynthesis, NADH is an intermediate and not a long-term chemical storage medium of solar energy, which is immediately coupled to synthetic follow up processes. Therefore it is no surprise that in our artificial photosynthetic model system reported here after several hours the decomposition of accumulated NADH in solution becomes evident. In Figure S12, the spectral variations observed during the red-light driven NADH formation with SnC as a photosensitizer are shown. In the initial phase of the process, the reduced nucleotide cofactor NADH is 14
continuously accumulated as shown by the increasing UV-absorption band at 340nm. At the same time, the steady-state concentration of the tin chlorin species SnC becomes very low, and the reaction mixture shows additional spectral signatures in the NIR-region, which indicate the presence of tetrapyrrole ligand . π-radical species SnC - as well as other reduction products.14 The radical species presumably underly a disproportionation and protonation equilibrium to yield both SnC2- and SnC under the catalytic conditions. While the regenerated chlorin SnC is again irradiated, the two-electron reduced π-dianion complex SnC2- undergoes pH-dependent protonation steps leading to hydroporphyrins such as SnCHwith a chlorin-phlorin-anion type spectrum. 14
Figure S11. UV/Vis spectral changes of a solution containing 7.5 μM SnC; 1 mM NAD+; 15 w/v% TEOA and 7.7·10-5 M [Cp*Rh(bpy)Cl]+ under Ar in 0.1 M phosphate buffer pH 8.8; 150 W Xe lamp with 590 nm CutOff-Filter; Numbers of %Conversion of NAD+ into NADH are given relative to the total initial amount of NAD+. TON (SnC) = 56.
15
+
-5
Figure S12 UV-Vis spectral changes of a solution containing 7.5 μM SnC; 1 mM NAD ; 15 w/v% TEOA and 7.7·10 M + [Cp*Rh(bpy)Cl] under Ar in 0.1 M phosphate buffer pH 8.8; 150 W Xe lamp irradiation with 610 nm cut-off-filter. + + Numbers of % conversion of NAD into NADH are given relative to the total initial amount of NAD . TON (SnC): 56. Note that a beginning decomposition of the accumulated NADH becomes evident after about 10-15 hours in solution.
Interactions and Dark Reactions between SnP and other Components of the System Without product separation, cross reactions of energy-rich compounds may influence the net performance of any artificial photosynthetic system. For example, NADH instead of TEOA may serve as an alternative electron donor for the reductive quenching of photoexcited SnP. This process can be avoided by wavelength-selective excitation. In order to study also potential interactions between the ground state tin complexes SnP or SnC and other reducing components formed in the mixture, the dark reaction with the two-electron reduced rhodium co-catalyst was investigated. For this purpose, the rhodium hydride complex [Cp*Rh(bpy)H]+ thought to be capable of NAD+ cofactor reduction was chemically generated in a dark reaction with sodium formate as the reductant.8 As shown in Figure S13, SnP added to such a mixture undergoes a thermal reduction and a rapid bleaching of the Soret band under these conditions. According to the principle of microscopic reversibility this type of thermal reactivity supports the postulated interaction between photogenerated chlorin-phlorin-type tinhydroporphyrin SnCH- acting as a “hydride source” (transfer of 2e- and H+) and the rhodium-hydride 16
species [Cp*Rh(bpy)H]+ in the catalytic key-steps finally responsible for NADH generation (see also next section).
2+
Figure S13 Dark reaction of SnP with [Cp*Rh(bpy)H2O] and sodium formate in 0.1 M sodium phosphate buffer, 8 measurement interval: 2 min. The reaction of formate with the rhodium complex is well-known to generate the + hydride complex [Cp*Rh(bpy)H] , which obviously interacts with the SnP complex to form an equilibrium with obviously meso-protonated non-aromatic phlorin-type tin-hydroporphyrin derivatives giving rise to the 14,16 characteristic broad NIR-absorption bands.
17
Comparison of Formate Anion and Photogenerated Tin(IV) Chlorin-Phlorin Anion as Hydride Sources
CO2
NADH
Rh
N N
SnC
N
H
NADH
Rh
N
H
hν
? 2eN
H+
N
Rh
N
SnC*
Rh
N
SnCH
OCOH 2eH+
HCOO-
SnCH-
8
Figure S14 Some analogies and differences between chemical and artificial photosynthetic co-catalyst activation mechanisms suitable for regioselective 1,4-NADH production.
17
References:
(1)
Charissé, M., Zickgraf, A.; Stenger, H.; Bräu, E.; Desmarquet, C.; Dräger, M.; Gerstmann, S.; Dakternieks, D.; Hook, J. Polyhedron 1998, 17, 4497-4506.
(2)
Marsmann, H. C.; Uhlig, F. in Rappoport, Z. (Ed.) The Chemistry of Organic Germanium, Tin and Lead Compounds, Vol. 2, Wiley 2002; Chpt. 6, p. 423.
(3)
Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. Journal of Organic Chemistry 1967, 32, 476.
(4)
Harriman, A.; Porter, G.; Walters, P. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1983, 79, 1335-1350. 18
(5)
Kölle, U.; Grätzel, M. Angewandte Chemie 1987, 99, 572-574.
(6)
Marcinek, A.; Zielonka, J.; Gȩbicki, J.; Gordon, C. M.; Dunkin, I. R. The Journal of Physical Chemistry A 2001, 105, 9305-9309.
(7)
Karrer, P.; Schwarzenbach, G.; Benz, F.; Solmssen, U. Helvetica Chimica Acta 1936, 19, 811-828.
(8)
Lo, H. C.; Leiva, C.; Buriez, O.; Kerr, J. B.; Olmstead, M. M.; Fish, R. H. Inorganic Chemistry 2001, 40, 6705-6716.
(9)
Megerle, U.; Lechner, R.; König,B.; Riedle, E. Photochem. Photobiol. Sci., 2010, 9, 1400–1406 (see also: http://www.rsc.org/suppdata/pp/c0/c0pp00195c/c0pp00195c.pdf)
(10)
ALUSTAR 3W LED-Modules (LEDxON) commercially available (Conrad electronics) mounted with precise lens optics into an Al-case with 10° output angle were used. The ready-to-use system is based on the EDISON Opto® Edixeon® S single color high-power LED series (http://www.edisonopto.com.tw).
(11)
Kuhn, H- J.; Braslavsky, S. E.; Schmidt, R. Chemical Actinometry, IUPAC Technical Report 2004 (http://www.iupac.org/publications/pac/76/12/2105/pdf/)
(12)
Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of Photochemistry, 3rd Ed. , CRC Taylor & Francis, 2006, p. 609, and references cited therein
(13)
Mauzerall, D.; Westheimer, F. H. Journal of the American Chemical Society 1955, 77, 2261-2264.
(14)
Stolzenberg, A. M.; Stershic, M. T. J. Am. Chem. Soc. 1988, 110, 6391-6402. (An additional participation of low-valent tin(II) chlorin derivatives which are formally redox-isomeric forms of the two-electron ring reduced tin(IV) π-dianion species SnC2- can also not be excluded at the present stage of our studies: Wang, X.; Gray, S. D.; Chen, J.; Woo, K. L. Inorg. Chem 1998, 37, 5-9.)
(15)
Data for Chorophyll b adopted from: Dixon, J. M., M. Taniguchi and J. S. Lindsey (2005), "PhotochemCAD 2. A Refined Program with Accompanying Spectral Databases for Photochemical Calculations, Photochem. Photobiol., 81, 212-213.
(16)
Baral, S.; Hambright, P.; Neta, P. J. Phys. Chem. 1984, 88, 1595-1600.
(17)
This work.
19
Photocatalytic Reduction of Artificial and Natural Nucleotide Cofactors with a Chlorophyll-Like Tin-Dihydroporphyrin Sensitizer K. T. Oppelt, E. Wöß, M. Stiftinger, W. Schöfberger, W. Buchberger and G. Knör *
Materials and Methods All Chemicals used in the synthesis were purchased from Aldrich and used without any further purification, if not otherwise stated. 4-Pyridyl-aldehyde was purchased from ABCR (97%), pyridine came from Acros Organics (+99%, extra pure). Tin chloride dihydrate was provided by VWR (technical). The disodium salt of EDTA was purchased from Merck (Titriplex III, p.a.). The sodium salt of NAD+ and ADH (yeast alcohol dehydrogenase, EC 1.1.1.1, > 300 U/mg protein) were obtained from Sigma-Aldrich. UV-Vis-spectra were recorded with a Varian Cary 50 diode array spectrophotometer and photoluminescence measurements were performed with a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. For NMR analysis, a Bruker Biospin 200 MHz spectrometer and a Bruker DRX 500 MHz spectrometer were used. Chemical shifts of the tin NMR spectra reported relative to the external reference Sn(Me)4 (δ=0 ppm) and were measured with Sn(Ph)4 (δ(119Sn)= - 121.1 ppm) used as a secondary standard (solid state NMR).1,2 HPLC experiments were carried out with an Agilent 1100 system. High-resolution MS measurements were performed with an Agilent 6510 quadrupole time-offlight (Q-TOF) instrument and an ESI source. The photochemical and photocatalytic experiments are described in more detail below.
Synthesis of Compounds Tin(IV)-meso-tetrakis-(N-methylpyridinium)-porphyrin complexes (SnP) The free base 5,10,15,20-tetrakis-(4-pyridyl)-porphyrin ligand H2(TPyP) was prepared according to a modified Adler-Longo-procedure.3 3.16 g of freshly distilled pyrrole were added portionwise to a hot solution (120°C) of 4.84 g of 4-pyridyl-aldehyde in 135 ml of propionic acid and 15 ml acetic anhydride. Afterwards, the mixture was heated to reflux for 90 min. The reaction mixture was cooled down and the acidic solvent was removed on the rotary evaporator until only 30 ml were left. By addition of aqueous sodium hydroxide solution, the pH of the residue was adjusted to 12. The solvent was removed via 1
vacuum filtration and the brown raw product (6.88 g) was purified by column chromatography on silica gel 60 with chloroform/methanol (1000:75) as eluent to yield the purple product. In one representative run of the purification procedure, the yield from 0.66 g raw product was 9.5 %. Table S1:
1
H-NMR of H2(TPyP)
δ(ppm)
Mult.
J (Hz)
Intensity
Assignment
8.99
d
4.4
8H
3,3’,3’’, 3’’’,5,5’, 5’’, 5’’’-Pyridyl protons
8.80
s
8H
Pyrrole protons
8.09
d
8H
2,2’,2’’,2’’’,6,6’,6’’,6’’’-Pyridyl protons
4.4
Dichloro-(5,10,15,20-tetrakis-(4-pyridyl)-porphyrinato)tin(IV) Sn(TPyP)Cl2 was prepared according to modified literature procedures in pyridine solution.4 The free base porphyrin H2(TPyP) (0.292 g) was heated to reflux in 35 ml pyridine. After addition of 0.9 g of SnCl2·2H2O in 20 ml pyridine the reaction solution was refluxed and stirred for 3 h, then another 0.78 g of SnCl2·2H2O were added and refluxed overnight. The reaction progress was observed by UV-Vis. After evaporation of the solvent, the raw product was purified by column chromatography on neutral Al2O3 with chloroform/pyridine (4:1) as eluent to yield 0.24 g of Sn(TPyP)Cl2 . Table S2:
1
H-NMR of Sn(TPyP)Cl2
δ(ppm)
Mult.
9.30
s
9.12
d
8.37
d
J (Hz)
Intensity
Assignment
8H
Pyrrole protons
6.0
8H
3,3’,3’’,3’’’,5,5’,5’’,5’’’-Pyridyl protons
6.0
8H
2,2’,2’’,2’’’,6,6’,6’’,6’’’-Pyridyl protons
The water soluble complex dichloro-(5,10,15,20-tetrakis-(N-methyl-pyridinium-4-yl)-porphyrinato)tin(IV) tetrachloride [(TMPyP)Sn(Cl)2]4+ (Cl-)4 was synthesized according to Harriman et.al.4 The product of the former reaction step (0.239 g) was stirred with an excess of iodomethane at room temperature for two days. The purple precipitate was filtered off and washed with dichloromethane. The raw product was dissolved in water and treated with freshly precipitated AgCl for 20 min to remove the iodide anion. After filtration, the filtrate was subjected to ion-exchange chromatography over DOWEX 1x 8 ion exchange resin. Water was removed under reduced pressure and after vacuum drying the violet solid was washed twice with acetone and sonicated during this process. Afterwards the purple powder was dried again in vacuum (yield: 0.20 g).
2
1
4+
Table S3: H-NMR of the [(TMPyP)Sn(Cl)2] cation in D2O
δ(ppm)
Mult.
9.57
s
9.37
d
9.07
d
4.8
s
J (Hz)
Intensity
Assignment
8H
Pyrrole protons
4.6
8H
3,3’,3’’,3’’’,5,5’,5’’,5’’’-Pyridinium protons
4.6
8H
2,2’,2’’,2’’’,6,6’,6’’,6’’’-Pyridinium protons
12H
Methyl protons
4,0 422 nm
3,5
Absorbance
3,0 2,5 2,0 1,5 1,0
555 nm
0,5 594 nm
0,0 200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
4+
Figure S1 UV-Vis absorption spectrum of the chloride salt of the [(TMPyP)Sn(Cl)2] cation in water at neutral pH
3
The pH-dependence of the position of the Soret band maximum in the UV-visible absorption spectrum of SnP was followed in 0.1 M phosphoric acid solution upon KOH-addition. The titration results indicate a stability range of the unprotonated dichloro species [(TMPyP)Sn(Cl)2]4+ between pH 6 and pH 10 as given below (Figure S2).
Figure S2 Position of the Soret band maximum of tin(IV)-meso-tetrakis-(N-methylpyridinium)-porphyrin hexachloride in 0.1 M sodium phosphate solution, indicating the exchange of axial chloride by hydroxide around pH 12. The reversible spectral shift between pH 4 and pH 6 according to the literature originates from the protonation of one of the pyrrole nitrogens of the porphyrin [Delmarre, D.; Veret-Lemarinier, A.-V.; Bied-Charreton, C. J. Luminesc. 1999, 82, 57-67].
4
Figure S3 ESI Q-TOF mass spectrum for [(TMPyP)Sn(X)2]
4+
(Agilent 6510 Q-TOF, LC-MS, positive mode) giving
clear evidence for a possible hydrolytic exchange of X = Cl in aqueous solution. The peak pattern shown corresponds to the deprotonated doubly-charged species [(TMPyP)Sn(O)2]
5
2+
2+
with C44H36N8O2Sn .
[Cp*Rh(bpy)Cl]Cl [Cp*Rh(bpy)Cl]Cl was prepared according to a literature procedure5 using standard Schlenk-techniques. 0.15g of dichloro(pentamethylcyclopentadienyl)rhodium(III)dimer were dissolved in absolute methanol and 2,2’-bipyridine (0.098 g) was added until no unreacted red crystals of the rhodium salt were visible anymore, and a yellow solution was formed. Methanol was partly evaporated and the product was precipitated upon addition 3 ml of diethyl ether. The orange precipitate was dried in vacuum overnight and characterized by 1H-NMR-spectroscopy and elemental analysis. The chloride ligand may undergo substitution processes in solution. 1
2+
Table S4 H-NMR data of [Cp*Rh(bpy)D2O] in D2O
δ (ppm)
Mult.
1.65
S
8.92
D
7.81
J (Hz)
Intensity
Assignment
15 H
Cp*
8.06
2H
H 6,6’
T
7.03
2H
H 5,5’
8.26
Td
7.87 0.97
2H
H4,4’
8.85
D
5.47
2H
H3,3’
2+
Table S5 Elemental Analysis of [Cp*Rh(bpy)Cl]Cl
Calculated
C%
51.63
H%
4.95
N%
5.98
Found
C%
50.20
H%
5.34
N%
5.89
N-Benzyl-3-carbamoyl-pyridinium-chloride Benzyl nicotine amide dinucleotide bromide was synthesized according to a modified literature procedure6 from benzyl bromide and nicotine amide in absolute ethanol by stirring overnight at 40°C. After filtration and washing of the white crystals with ether, the product was dried in high vacuum. The anion exchange to chloride was performed by stirring the aqueous solution of benzyl nicotine amide dinucleotide bromide with freshly precipitated silver chloride7. After 30 min the pale yellow precipitate of silver bromide was filtered off and the residual solvent was removed by vacuum to yield the white product N-benzyl-3-carbamoyl-pyridinium chloride (BNAD+). 6
N-Benzyl-1,4-dihydronicotineamide BNAD+ was reduced to the 1,4-dihydronicotineamide derivative by reduction with sodium dithionite in basic sodium carbonate solution under inert conditions8 using standard Schlenk-techniques with nitrogen as inert gas. N-benzyl-1,4-dihydronicotineamide (BNADH) crystallized from the reaction solution as pale yellow solid. The solvent was removed by inert filtration. The precipitate was washed twice with degassed water and then dried in vacuum.
1
+
Table S6 H-NMR Data for BNAD (N-Benzyl-3-carbamoyl-pyridinium-chloride) in CDCl3
δ (ppm)
J (Hz)
9.32
s
9.04
d
8.88
Intensity Assignment
1H
2
5.77
1H
6
d
8.03
1H
4
8.17
t
6.97
1H
5
7.48
dt
7.98; 3.46
1H
8, 8’, 9, 9’, 10
5.87
dd
3.47; 1.60
2H
4,4’
O
4 5
NH2 + N
6 8 9
2 – Cl 7,7'
8'
10 9'
1
Table S7 H-NMR Data for BNADH (N-benzyl-1,4-dihydronicotineamide) in CDCl3
δ (ppm)
J (Hz)
7.29
m
7.15
d
5.74
Intensity Assignment
5H
8, 8’, 9, 9’, 10
1H
2
ddd 8.24; 3.98; 1.92
1H
6
5.15
bs
2H
-NH2
4.75
dt
1H
5
11
4.30
s
2H
7,7’
12
1.65
7.98; 3.46
4 5
10 9,9' 10' 11'
7
2
6
Setup used for Photoreaction Quantum Yield Determinations A home-built optical bench mounted system producing an electrical signal proportional to the photon flux absorbed by the investigated samples was used for exact conversion quantum yield determinations. The technical details of this setup are following the quantum counter setup reported and tested recently by Riedle et. al.9, which allows for a convenient absolute radiative power detection with a calibrated solar cell detector. Based on the construction plans generously offered by these authors, our system was slightly modified with a turning wheel for rapid switching between four different permanently installed high-power LED light sources covering the visible spectral range for photolysis experiments. The spectral output of the actually built-in high-power LEDs (470, 525, 592 and 623 nm)10 used for visible-light illumination experiments is shown below:
Fig S4 Emission spectra of the high-power LEDs used
Calibration of the home-built system was carried out by applying classical chemical actinometry with Reinecke´s salt in dilute acidic solution according to the recommended standard procedures 11,12 (0.01 M: φ = 0.31 at 470nm; 0.04M: φ = 0.29 at 525nm; 0.01M: φ = 0.27 at 592nm; 0.025M: φ = 0.29 at 623nm).
8
Photogeneration of Hydroporphyrin Derivatives Irradiation experiments to generate SnC in solution were performed in 1cm quartz glass cuvettes sealed with a septum in aqueous solutions containing 0.1 mol l-1 sodium phosphate, 10 mmol l-1 EDTA and 1.5·10-5 mol l-1 SnP. First the chlorin species was formed upon irradiation for 8 min at room temperature on an optical bench equipped with a 150 W short arc Xe lamp, a quartz condensor lens, a liquid water filter (AMKO) to eliminate IR-radiation, and a Schott glass cut off filter (OG 530). The sample was saturated with air by repeated shaking and stirring. The reaction progress was followed by UV-VIS spectroscopy. It is important to note that upon prolonged irradiation also tetrahydroporphyrins (tinbacteriochlorin-derivatives SnBC) could be detected by their characteristic Q-band patterns in the far-red spectral region (see also Figure S7). For the determination of the corresponding quantum yield, the same procedure was used, but the light source was a green LED with a maximum at 525 nm (see Figure S4).
-5
Figure S5 Typical spectral changes observed during monochromatic irradiation of 1.5·10 M SnP in aqueous 0.1 M phosphate buffer solution (pH 8.8) containing EDTA with an 3 W LED at 525 nm under ambient air.
9
A photochemical quantum yield value of φ = 4•10-3 was obtained for the chlorin formation under these conditions (EDTA donor, 525nm). Using alternatively sodium sulphite as the electron donor, a much faster conversion from SnP to SnC was observed (typically within a few seconds under similar conditions with φ > 0.3). Sulfate was identified as a permanent product in the latter systems. Since an excess of other sulfur compounds than sulphate remaining in the system might negatively influence the performance of the rhodium-based redox-mediators added under catalytic conditions, we decided to completely remove these reducing compounds for all of our tests with sulphite-photogenerated SnC as the sensitizer. Therefore, the pH of the sulfite containing tin chlorin solutions was adjusted to a value below 1 by adding HCl. Subsequent purging of the reaction mixture with air was carried out in order to remove the evolving SO2 gas. The photosensitizer solution thus obtained was directly used for the following irradiation experiments at various defined pH-values in buffered aqueous solution.
Figure S6 UV-Vis absorption spectrum of the tin chlorin photocatalyst SnC in water. Inset: Spectral features of 15 native chlorophyll b for comparison.
10
Photocatalytic Formation and Detection of Reduced Nucleotide Cofactor Analogues After in situ generation of the SnC photocatalyst, 0.3 ml of a 1mM [Cp*Rh(bpy)Cl]Cl-solution (3·10-6 mol) and 1.3 mg BNAD+Cl- (5.2 10-6 mol) were added, and the solution was purged with Ar for 30 min. Then the second phase of irradiation was done with a 3W LED at 623 nm (selective excitation of SnC in the Qband region corresponding to the lowest excited singlet state of the sensitizer). UV-Vis spectra were recorded to follow the reaction progress. After 900 min, the sample was transferred under inert conditions into a HPLC vial and the run was started immediately, because of the air sensitivity of the reduced cofactor model (see also Figure S9 in next section!). After 4 h the fluorescence spectrum (excitation at 340 nm) and the excitation spectrum (at 470 nm) of the sample were recorded (see Figure S7). For the determination of the exact quantum yield data, the increase in absorbance at 355 nm due to BNADH formation was used (ε= 7240) 13.
Excitation (470 nm)
Luminescence
Emission (340 nm)
250
300
350
400
450
500
550
600
650
Wavelength (nm)
Figure S7 Photoluminescence (excitation wavelength 340 nm) and excitation spectra (measurement wavelength 470 nm) of BNADH formed from an irradiated solution (3 W LED 623 nm) of SnC in aqueous 0.1 M phosphate buffer -5 + solution (pH 8.8) with 8.6·10 M [Cp*Rh(bpy)Cl]Cl, 0.01 M EDTA and 1.5 mM BNAD after 4h of irradiation time.
Without the rhodium catalyst in the system, no increase in absorbance in the range between 320 and 360 nm could be observed under similar conditions (Figure S8).
11
-5
Figure S8 Irradiation of 1.4·10 M SnC in aqueous 0.1 M phosphate buffer solution (pH 8.8) containing 0.01 M EDTA -1 + and 1.5 mmol l BNAD with an 3 W LED at 623 nm (note the peak at 766nm due to the four electron reduced tetrahydroporphyrin derivative tin bacteriochlorin SnBC present under these conditions of selective SnC excitation).
Detection of BNADH by HPLC/MS The setup used was an Agilent 1100 HPLC. The injection volume was 10µl on a Phenomenex Kinetex 2.6 µm C18 50x4.6 mm column. For chromatographic separation, a water: acetonitrile gradient from 0 to 100% acetonitrile within 20 min was used with a flow of 0.9 ml min-1. Then 10 min 100% acetonitrile and 10 min 100% water followed for conditioning the column after each run. 0.1 M sodium phosphate buffer solutions containing either 200 ppm BNAD+Cl- or 230 ppm BNADH were used as references for the HPLC. MS detection was performed on an Agilent 6510 Q-TOF mass spectrometer with an ESI source and an Agilent G1607A coaxial sprayer. Prior to the ESI ion source of the Q-TOF, a 0.1 ml min-1 make up flow of 1% formic acid of was used to enhance the ionization.
12
Figure S9 Mass spectra for the retention times 6.2 min and 8.2 min in the HPLC for the irradiated samples and the BNADH reference molecule.
Comparison of the positive ion traces collected in Fig. S10 clearly shows that BNADH detected at m/z= 215.1179 as the mono-protonated cation BNADH/H+ is generated upon excitation of the tin chlorin and rhodium complex containing reaction mixture. No such reduction of the cofactor analogue occurs in the absence of one of the essential components required.
13
+
+
Figure S10. HPLC/MS-detection of protonated BNADH/H (m/z= 215.1179) starting from BNAD (m/z= 213.1022). Extracted ion traces for the irradiated samples and corresponding reference solutions are compared.
Photocatalytic Performance under Long-Term Steady-State Irradiation with Red-Light Depending on the relative concentrations of the functional components (photosensitizer, rhodium complex, nucleotide cofactor, excitation wavelength), a photostationary level of photoproducts is reached upon longer irradiation times (Fig. S11 and S12). Under the conditions chosen for the present paper, which were guided by the requirement of following the spectral changes of all relevant components in situ, typically a saturation concentration of 0.4-0.5 mM NADH was reached (absorbance at 340nm > 2.5 in 1cm cell). In natural photosynthesis, NADH is an intermediate and not a long-term chemical storage medium of solar energy, which is immediately coupled to synthetic follow up processes. Therefore it is no surprise that in our artificial photosynthetic model system reported here after several hours the decomposition of accumulated NADH in solution becomes evident. In Figure S12, the spectral variations observed during the red-light driven NADH formation with SnC as a photosensitizer are shown. In the initial phase of the process, the reduced nucleotide cofactor NADH is 14
continuously accumulated as shown by the increasing UV-absorption band at 340nm. At the same time, the steady-state concentration of the tin chlorin species SnC becomes very low, and the reaction mixture shows additional spectral signatures in the NIR-region, which indicate the presence of tetrapyrrole ligand . π-radical species SnC - as well as other reduction products.14 The radical species presumably underly a disproportionation and protonation equilibrium to yield both SnC2- and SnC under the catalytic conditions. While the regenerated chlorin SnC is again irradiated, the two-electron reduced π-dianion complex SnC2- undergoes pH-dependent protonation steps leading to hydroporphyrins such as SnCHwith a chlorin-phlorin-anion type spectrum. 14
Figure S11. UV/Vis spectral changes of a solution containing 7.5 μM SnC; 1 mM NAD+; 15 w/v% TEOA and 7.7·10-5 M [Cp*Rh(bpy)Cl]+ under Ar in 0.1 M phosphate buffer pH 8.8; 150 W Xe lamp with 590 nm CutOff-Filter; Numbers of %Conversion of NAD+ into NADH are given relative to the total initial amount of NAD+. TON (SnC) = 56.
15
+
-5
Figure S12 UV-Vis spectral changes of a solution containing 7.5 μM SnC; 1 mM NAD ; 15 w/v% TEOA and 7.7·10 M + [Cp*Rh(bpy)Cl] under Ar in 0.1 M phosphate buffer pH 8.8; 150 W Xe lamp irradiation with 610 nm cut-off-filter. + + Numbers of % conversion of NAD into NADH are given relative to the total initial amount of NAD . TON (SnC): 56. Note that a beginning decomposition of the accumulated NADH becomes evident after about 10-15 hours in solution.
Interactions and Dark Reactions between SnP and other Components of the System Without product separation, cross reactions of energy-rich compounds may influence the net performance of any artificial photosynthetic system. For example, NADH instead of TEOA may serve as an alternative electron donor for the reductive quenching of photoexcited SnP. This process can be avoided by wavelength-selective excitation. In order to study also potential interactions between the ground state tin complexes SnP or SnC and other reducing components formed in the mixture, the dark reaction with the two-electron reduced rhodium co-catalyst was investigated. For this purpose, the rhodium hydride complex [Cp*Rh(bpy)H]+ thought to be capable of NAD+ cofactor reduction was chemically generated in a dark reaction with sodium formate as the reductant.8 As shown in Figure S13, SnP added to such a mixture undergoes a thermal reduction and a rapid bleaching of the Soret band under these conditions. According to the principle of microscopic reversibility this type of thermal reactivity supports the postulated interaction between photogenerated chlorin-phlorin-type tinhydroporphyrin SnCH- acting as a “hydride source” (transfer of 2e- and H+) and the rhodium-hydride 16
species [Cp*Rh(bpy)H]+ in the catalytic key-steps finally responsible for NADH generation (see also next section).
2+
Figure S13 Dark reaction of SnP with [Cp*Rh(bpy)H2O] and sodium formate in 0.1 M sodium phosphate buffer, 8 measurement interval: 2 min. The reaction of formate with the rhodium complex is well-known to generate the + hydride complex [Cp*Rh(bpy)H] , which obviously interacts with the SnP complex to form an equilibrium with obviously meso-protonated non-aromatic phlorin-type tin-hydroporphyrin derivatives giving rise to the 14,16 characteristic broad NIR-absorption bands.
17
Comparison of Formate Anion and Photogenerated Tin(IV) Chlorin-Phlorin Anion as Hydride Sources
CO2
NADH
Rh
N N
SnC
N
H
NADH
Rh
N
H
hν
? 2eN
H+
N
Rh
N
SnC*
Rh
N
SnCH
OCOH 2eH+
HCOO-
SnCH-
8
Figure S14 Some analogies and differences between chemical and artificial photosynthetic co-catalyst activation mechanisms suitable for regioselective 1,4-NADH production.
17
References:
(1)
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(2)
Marsmann, H. C.; Uhlig, F. in Rappoport, Z. (Ed.) The Chemistry of Organic Germanium, Tin and Lead Compounds, Vol. 2, Wiley 2002; Chpt. 6, p. 423.
(3)
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(4)
Harriman, A.; Porter, G.; Walters, P. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1983, 79, 1335-1350. 18
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Megerle, U.; Lechner, R.; König,B.; Riedle, E. Photochem. Photobiol. Sci., 2010, 9, 1400–1406 (see also: http://www.rsc.org/suppdata/pp/c0/c0pp00195c/c0pp00195c.pdf)
(10)
ALUSTAR 3W LED-Modules (LEDxON) commercially available (Conrad electronics) mounted with precise lens optics into an Al-case with 10° output angle were used. The ready-to-use system is based on the EDISON Opto® Edixeon® S single color high-power LED series (http://www.edisonopto.com.tw).
(11)
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(14)
Stolzenberg, A. M.; Stershic, M. T. J. Am. Chem. Soc. 1988, 110, 6391-6402. (An additional participation of low-valent tin(II) chlorin derivatives which are formally redox-isomeric forms of the two-electron ring reduced tin(IV) π-dianion species SnC2- can also not be excluded at the present stage of our studies: Wang, X.; Gray, S. D.; Chen, J.; Woo, K. L. Inorg. Chem 1998, 37, 5-9.)
(15)
Data for Chorophyll b adopted from: Dixon, J. M., M. Taniguchi and J. S. Lindsey (2005), "PhotochemCAD 2. A Refined Program with Accompanying Spectral Databases for Photochemical Calculations, Photochem. Photobiol., 81, 212-213.
(16)
Baral, S.; Hambright, P.; Neta, P. J. Phys. Chem. 1984, 88, 1595-1600.
(17)
This work.
19
