Supporting Information for Photon Upconversion in Molecular ...
Supporting Information for Photon Upconversion in Molecular Lanthanide Complex in Anhydrous Solution at Room Temperature Iko Hyppänen,*,† Satu Lahtinen,† Timo Ääritalo,† Joonas Mäkelä,† Jouko Kankare,‡ and Tero Soukka† †
Department of Biotechnology, University of Turku, Tykistökatu 6 A, FI-20520 Turku, Finland
‡
Laboratory of Materials Chemistry and Chemical Analysis, Department of Chemistry, University of
Turku, Vatselankatu 2, FI-20014 Turku, Finland
Number of pages 8 Number of figures 5 Contents 1. Synthesis details 2. Measurement details 3. Comparison of upconversion emission spectra of Er(TTA)4(IR-806) and NaYF4:Yb,Er 4. Spectral features of upconversion spectrum 5. Upconversion emission spectra as a function of the excitation power 6. Proposed mechanism of upconversion process 7. Photobleaching of IR-806 dye 8. References
S1
1. Synthesis details The erbium complex Er(TTA)4K was synthesized according the published procedure.1,2 The IR-806 iodide salt was synthesized from a commercially available iodide salt of cyanine dye IR-780 by nucleophilic substitution reaction.3 The product was characterized by 1H and
13
C NMR and mass
spectrometry. The ion-associated complex Er(TTA)4(IR-806) was prepared by dissolving Er(TTA)4K and IR-806 dye iodide salt separately in small amounts of DMF. The dissolved compounds were mixed with 1 to 1 molar ratio and the resulting solution was evaporated to dryness under vacuum. The erbium complex without the organic chromophore Er(TTA)4K was treated similarly, i.e. dissolved in DMF and dried under vacuum but IR-806 dye was not added. Samples for measurement were prepared by dissolving Er(TTA)4(IR-806), Er(TTA)4K or IR-806 in deuterated chloroform. The inorganic reference material NaYF4:Yb,Er was synthesized with oleic acid method.4 2. Measurement details The absorbance measurements were performed on a Varian Cary 300 Bio UV-Visible spectrophotometer. The emission (up to 750 nm) and excitation spectra of IR-806 were recorded on a Varian Cary Eclipse fluorescence spectrometer operated in the Fluorescence mode. The fluorescence emission spectrum of IR-806 dye up to 1100 nm was recorded on an Ocean Optics QE65 Pro spectrometer using a separate infrared laser diode module RLDB808-350-3 (Roithner Lasertechnik GmbH) with 808 nm operational wavelength as excitation source. Upconversion measurements were performed on a Varian Cary Eclipse fluorescence spectrometer operated in the Bio/Chemiluminescence mode using 775FW82-50S hot mirror (Andover Corporation) emission filter and a custom cuvette holder with mounting for the laser diode module as the excitation source. The excitation source for molecular upconversion experiments was an 808 nm laser diode moledule RLDB808-350-3 (Roithner Lasertechnik GmbH) equipped with RG695 and RG780 (Coherent) long-pass filters. The power of the excitation source was measured with a LaserPADTM Laser Power Analysis Display System (PC windows software) and SmartSensor Interface Module with an Air-Cooled Thermopile Sensor (model LM-10 HTD, Coherent, Inc.). The measured excitation power was 95 mW. The laser beam was focused in the middle of the cuvette to a spot of 0.01 cm2, hence providing 9500 mW cm–2. The infrared laser power was adjusted with neutral density filters placed between the excitation source and the sample. The effect of neutral density filters on the laser power was determined with LaserPAD system. Another excitation source, a 968 nm laser diode module C2021-F1 (Roithner Lasertechnik GmbH) equipped with two RG850 (Andover Corporation) long-pass filters was used to excite the reference upconverting nanoparticle (UCNP) sample NaYF4:Yb,Er. The measured power of the 968 nm laser was 69 mW. It
S2
was focused to a spot of 0.03 cm2, hence providing 2300 mW cm–2. All measurements were performed at room temperature. 3. Comparison of upconversion emission spectra of Er(TTA)4(IR-806) and NaYF4:Yb,Er Two emission bands at 510–540 nm and 540–565 nm in the upconversion spectrum of Er(TTA)4(IR-806) were associated with erbium ion transitions. In Figure S1 the upconversion spectrum is shown together with the upconversion spectrum of an inorganic crystalline upconverting nanoparticle material (NaYF4:Yb,Er). The equivalence of the two upconversion emission spectra is not perfect but the differences in the relative strengths of emission peaks are expected due to the effect of different crystal fields on the Er3+ transition probabilities.5-7 The red emission at 650 nm from the 4F9/2→4I15/2 transition of Er3+ ion is visible in the spectrum of NaYF4:Yb,Er, but is not observed in the spectrum of the Er(TTA)4(IR-806) complex. For the present it is unknown whether the red emission is masked by the IR-806 fluorescence emission or the excitation route to the red emitting 4F9/2 level is not efficient in the Er(TTA)4(IR-806) complex. The latter could be easily explained in case the excitation route is strongly dependent on the Er-Er cross-relaxations as earlier suggested,8 thus requiring the presence of two Er3+ ions in close proximity. In a recent publication, however, a new mechanism involving Er-to-Yb back energy transfer was shown to be the main route to the red emitting level of erbium in
β-NaYF4:Yb3+,Er3+ material, while the contribution of the Er-Er cross-relaxation and the nonradiative relaxation 4S3/2→4F9/2 were estimated to be negligible.9 This would result in that the excitation route to red emitting level is not effective in erbium complex lacking ytterbium ion.
Figure S1. Emission spectra of Er(TTA)4(IR-806) (black) in CDCl3 (concentration 100 µM) excited at 808 nm (emission slit 5.0 nm) and NaYF4:Yb,Er nanoparticles (red) in DMSO (1 mg/ml) excited at 968 nm (emission slit 2.5 nm). S3
4. Spectral features of upconversion spectrum The upconversion spectrum of Er(TTA)4(IR-806) is presented in Figure S2. The two emission bands within 510–565 nm range were associated with erbium ion emission. In addition to these emission bands the upconversion emission spectrum has two other features. These are a shoulder at 560–640 nm and a steep rise of the emission intensity starting at 650 nm. When the 808 nm excitation is used the shoulder is present only on the spectrum of the Er(TTA)4(IR-806). However, when the IR-806 dye sample is excited at wavelength 525 nm a spectrum with an emission band with a maximum at 570 nm is produced (Figure S2). Based on these results we concluded that the origin of the shoulder in the spectrum of the Er(TTA)4(IR-806) is due to an energy transfer from the excited erbium ion back to the IR-806 dye or to some impurity responsible of the emission at 570 nm. The steep rise of the luminescence intensity is observed when the samples containing IR-806 dye are excited with the 808 nm laser. This emission is due to the high energy tail of the IR-806 emission band with a maximum at 828 nm.
Figure S2. Emission spectra of Er(TTA)4(IR-806) (black) and IR-806 dye salt (red) dissolved in CDCl3 (100 µM) excited at 808 nm (emission slit 10 nm). Emission spectra of IR-806 dye salt (blue) dissolved in CDCl3 (100 µM) excited at 525 nm (emission slit 5 nm).
S4
5. Upconversion emission spectra as a function of the excitation power The upconversion emission spectra of Er(TTA)4(IR-806) obtained with different excitation powers are presented in Figure S3. The excitation intensity was controlled by placing neutral density filters between the laser and the sample cell.
Figure S3. Emission spectra of Er(TTA)4(IR-806) dissolved in CDCl3 (100 µM) with different excitation powers at 808 nm. Excitation intensities used: 9500, 7100, 5700 and 3400 mW cm–2 in black, red, green and blue curves, respectively. Emission slit 5 nm.
6. Proposed mechanism of upconversion process It is generally considered that the energy transfer between organic antenna dyes and chelated lanthanide ions occurs from the triplet state of the antenna chromophore.10-12 Unfortunately due to the instrumental problems we had no possibility to measure the phosphorescence spectrum of the Gd(TTA)4(IR-806) complex. Hence, the energy the IR-806 dye’s first triplet state T1 compared to the energy of the transitions of Er3+ is unknown. However, another NIR dye IR140 has absorbance and fluorescence emission bands close to those of IR-806 and there is a reasonable overlap between the phosphorescence emission of the IR140 dye and the 4I15/2 → 4I11/2 transition of Er3+.1 Our hypothesis is that the IR-806 dye cation absorbs the excitation light, is excited to its first singlet state S1 and undergoes an intersystem crossing to the T1 level (Figure S4). The IR-806 dye then returns to the ground state S0 by resonance energy transfer (RET) to the Er3+ ion at ground state 4I15/2 and Er3+ ion is excited to the 4I11/2 level. The whole process is repeated except that the Er3+ ion transition involved is now 4I11/2 → 4F7/2. A fast nonradiative multi-phonon relaxation of 4F7/2 level leads to the population of the 2H11/2 and 4S3/2 levels.
S5
Finally the green upconversion emission around 530 nm and 550 nm is due to the 2H11/2 → 4I15/2 and 4
S3/2 → 4I15/2 transitions, respectively.
Figure S4. Schematic energy level diagram of the proposed excitation-emission system of Er(TTA)4(IR-806). Vertical solid lines: absorption and emission transitions, ISC: inter system crossing, ET: resonance energy transfer, tilted solid lines of Er3+: multiphonon relaxation. The energies of Er3+ energy levels are taken from literature.13 The energy of S1 level of IR-806 dye is the measured fluorescence maxima of the dye and the error bar marks the half-wide of the fluorescence spectrum. The energy of the T1 level is an estimate.
S6
7. Photobleaching of IR-806 dye Photobleaching of IR-806 dye was studied by following the change in absorbance as the IR-806 sample was exposed to NIR laser. Absorption spectra of IR-806 dye salt in CDCl3 are presented in Figure S5. The absorbance at 808 nm is decreased as the exposure time to NIR laser is increased. Absorbance is decreased about 50% in 30 seconds. This seems quite a large change, but the used laser power was also high and 30 second is more than adequate time for most applications.
Figure S5. Absorbance spectra of IR-806 dye salt in CDCl3 (100 µM) exposed to a 808 nm NIR laser (9500 mW cm–2). Exposure times 0 s (black), 30 s (red), 5 min (green) and 10 min (blue).
S7
8. References (1) Wang, H. S.; Yang, Y.; Cui, Y. J.; Wang, Z. Y.; Qian, G. D. Sensitized Near-Infrared Luminescence from Erbium Ion-Associated Complex with IR140 Dye. Dyes Pigm. 2012, 95, 69-73. (2) Wang, H. S.; Qian, G. D.; Wang, M. Q.; Zhang, J. H.; Luo, Y. S. Enhanced Luminescence of an Erbium(III) Ion-Association Ternary Complex with a Near-Infrared Dye. J. Phys. Chem. B 2004, 108, 8084-8088. (3) Zou, W. Q.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Broadband DyeSensitized Upconversion of Near-Infrared Light. Nat. Photonics 2012, 6, 560-564. (4) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y. H.; Wang, J.; Xu, J.; Chen, H. Y.; Zhang, C.; Hong, M. H.; Liu, X. G. Simultaneous Phase and Size Control of Upconversion Nanocrystals Through Lanthanide Doping. Nature 2010, 463, 1061-1065. (5) Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W. The Emission Spectrum and the Radiative Lifetime of Eu3+ in Luminescent Lanthanide Complexes. Phys. Chem. Chem. Phys. 2002, 4, 1542-1548. (6) Li, M.; Selvin, P. R. Luminescent Polyaminocarboxylate Chelates of Terbium and Europium - the Effect of Chelate Structure. J Am Chem Soc 1995, 117, 8132-8138. (7) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media. Chem. Mater. 2003, 15, 46044616. (8) Solis, D.; De la Rosa, E.; Meza, O.; Diaz-Torres, L. A.; Salas, P.; Angeles-Chavez, C. Role of Yb3+ and Er3+ Concentration on the Tunability of Green-Yellow-Red Upconversion Emission of Codoped ZrO2:Yb3+-Er3+ Nanocrystals. J. Appl. Phys. 2010, 108. (9) Anderson, R. B.; Smith, S. J.; May, P. S.; Berry, M. T. Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF4:Yb3+, Er3+. J. Phys. Chem. Lett. 2014, 5, 36-42. (10) Crosby, G. A.; Whan, R. E.; Alire, R. M. Intramolecular Energy Transfer in Rare Earth Chelates Role of Triplet State. J. Chem. Phys. 1961, 34, 743-748. (11) Hayes, A. V.; Drickamer, H. G. High-Pressure Luminescence Studies of Energy-Transfer in RareEarth Chelates. J. Chem. Phys. 1982, 76, 114-125. (12) Latva, M.; Takalo, H.; Mukkala, V. M.; Matachescu, C.; RodriguezUbis, J. C.; Kankare, J. Correlation Between the Lowest Triplet State Energy Level of the Ligand and Lanthanide(III) Luminescence Quantum Yield. J. Lumin. 1997, 75, 149-169. (13) Carnall, W. T.; Fields, P. R.; Rajnak, K. Electronic Energy Levels in Trivalent Lanthanide Aquo Ions. I. Pr3+ Nd3+ Pm3+ Sm3+ Dy3+ Ho3+ Er3+ and Tm3+. J. Chem. Phys. 1968, 49, 4424-4442.
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Department of Biotechnology, University of Turku, Tykistökatu 6 A, FI-20520 Turku, Finland
‡
Laboratory of Materials Chemistry and Chemical Analysis, Department of Chemistry, University of
Turku, Vatselankatu 2, FI-20014 Turku, Finland
Number of pages 8 Number of figures 5 Contents 1. Synthesis details 2. Measurement details 3. Comparison of upconversion emission spectra of Er(TTA)4(IR-806) and NaYF4:Yb,Er 4. Spectral features of upconversion spectrum 5. Upconversion emission spectra as a function of the excitation power 6. Proposed mechanism of upconversion process 7. Photobleaching of IR-806 dye 8. References
S1
1. Synthesis details The erbium complex Er(TTA)4K was synthesized according the published procedure.1,2 The IR-806 iodide salt was synthesized from a commercially available iodide salt of cyanine dye IR-780 by nucleophilic substitution reaction.3 The product was characterized by 1H and
13
C NMR and mass
spectrometry. The ion-associated complex Er(TTA)4(IR-806) was prepared by dissolving Er(TTA)4K and IR-806 dye iodide salt separately in small amounts of DMF. The dissolved compounds were mixed with 1 to 1 molar ratio and the resulting solution was evaporated to dryness under vacuum. The erbium complex without the organic chromophore Er(TTA)4K was treated similarly, i.e. dissolved in DMF and dried under vacuum but IR-806 dye was not added. Samples for measurement were prepared by dissolving Er(TTA)4(IR-806), Er(TTA)4K or IR-806 in deuterated chloroform. The inorganic reference material NaYF4:Yb,Er was synthesized with oleic acid method.4 2. Measurement details The absorbance measurements were performed on a Varian Cary 300 Bio UV-Visible spectrophotometer. The emission (up to 750 nm) and excitation spectra of IR-806 were recorded on a Varian Cary Eclipse fluorescence spectrometer operated in the Fluorescence mode. The fluorescence emission spectrum of IR-806 dye up to 1100 nm was recorded on an Ocean Optics QE65 Pro spectrometer using a separate infrared laser diode module RLDB808-350-3 (Roithner Lasertechnik GmbH) with 808 nm operational wavelength as excitation source. Upconversion measurements were performed on a Varian Cary Eclipse fluorescence spectrometer operated in the Bio/Chemiluminescence mode using 775FW82-50S hot mirror (Andover Corporation) emission filter and a custom cuvette holder with mounting for the laser diode module as the excitation source. The excitation source for molecular upconversion experiments was an 808 nm laser diode moledule RLDB808-350-3 (Roithner Lasertechnik GmbH) equipped with RG695 and RG780 (Coherent) long-pass filters. The power of the excitation source was measured with a LaserPADTM Laser Power Analysis Display System (PC windows software) and SmartSensor Interface Module with an Air-Cooled Thermopile Sensor (model LM-10 HTD, Coherent, Inc.). The measured excitation power was 95 mW. The laser beam was focused in the middle of the cuvette to a spot of 0.01 cm2, hence providing 9500 mW cm–2. The infrared laser power was adjusted with neutral density filters placed between the excitation source and the sample. The effect of neutral density filters on the laser power was determined with LaserPAD system. Another excitation source, a 968 nm laser diode module C2021-F1 (Roithner Lasertechnik GmbH) equipped with two RG850 (Andover Corporation) long-pass filters was used to excite the reference upconverting nanoparticle (UCNP) sample NaYF4:Yb,Er. The measured power of the 968 nm laser was 69 mW. It
S2
was focused to a spot of 0.03 cm2, hence providing 2300 mW cm–2. All measurements were performed at room temperature. 3. Comparison of upconversion emission spectra of Er(TTA)4(IR-806) and NaYF4:Yb,Er Two emission bands at 510–540 nm and 540–565 nm in the upconversion spectrum of Er(TTA)4(IR-806) were associated with erbium ion transitions. In Figure S1 the upconversion spectrum is shown together with the upconversion spectrum of an inorganic crystalline upconverting nanoparticle material (NaYF4:Yb,Er). The equivalence of the two upconversion emission spectra is not perfect but the differences in the relative strengths of emission peaks are expected due to the effect of different crystal fields on the Er3+ transition probabilities.5-7 The red emission at 650 nm from the 4F9/2→4I15/2 transition of Er3+ ion is visible in the spectrum of NaYF4:Yb,Er, but is not observed in the spectrum of the Er(TTA)4(IR-806) complex. For the present it is unknown whether the red emission is masked by the IR-806 fluorescence emission or the excitation route to the red emitting 4F9/2 level is not efficient in the Er(TTA)4(IR-806) complex. The latter could be easily explained in case the excitation route is strongly dependent on the Er-Er cross-relaxations as earlier suggested,8 thus requiring the presence of two Er3+ ions in close proximity. In a recent publication, however, a new mechanism involving Er-to-Yb back energy transfer was shown to be the main route to the red emitting level of erbium in
β-NaYF4:Yb3+,Er3+ material, while the contribution of the Er-Er cross-relaxation and the nonradiative relaxation 4S3/2→4F9/2 were estimated to be negligible.9 This would result in that the excitation route to red emitting level is not effective in erbium complex lacking ytterbium ion.
Figure S1. Emission spectra of Er(TTA)4(IR-806) (black) in CDCl3 (concentration 100 µM) excited at 808 nm (emission slit 5.0 nm) and NaYF4:Yb,Er nanoparticles (red) in DMSO (1 mg/ml) excited at 968 nm (emission slit 2.5 nm). S3
4. Spectral features of upconversion spectrum The upconversion spectrum of Er(TTA)4(IR-806) is presented in Figure S2. The two emission bands within 510–565 nm range were associated with erbium ion emission. In addition to these emission bands the upconversion emission spectrum has two other features. These are a shoulder at 560–640 nm and a steep rise of the emission intensity starting at 650 nm. When the 808 nm excitation is used the shoulder is present only on the spectrum of the Er(TTA)4(IR-806). However, when the IR-806 dye sample is excited at wavelength 525 nm a spectrum with an emission band with a maximum at 570 nm is produced (Figure S2). Based on these results we concluded that the origin of the shoulder in the spectrum of the Er(TTA)4(IR-806) is due to an energy transfer from the excited erbium ion back to the IR-806 dye or to some impurity responsible of the emission at 570 nm. The steep rise of the luminescence intensity is observed when the samples containing IR-806 dye are excited with the 808 nm laser. This emission is due to the high energy tail of the IR-806 emission band with a maximum at 828 nm.
Figure S2. Emission spectra of Er(TTA)4(IR-806) (black) and IR-806 dye salt (red) dissolved in CDCl3 (100 µM) excited at 808 nm (emission slit 10 nm). Emission spectra of IR-806 dye salt (blue) dissolved in CDCl3 (100 µM) excited at 525 nm (emission slit 5 nm).
S4
5. Upconversion emission spectra as a function of the excitation power The upconversion emission spectra of Er(TTA)4(IR-806) obtained with different excitation powers are presented in Figure S3. The excitation intensity was controlled by placing neutral density filters between the laser and the sample cell.
Figure S3. Emission spectra of Er(TTA)4(IR-806) dissolved in CDCl3 (100 µM) with different excitation powers at 808 nm. Excitation intensities used: 9500, 7100, 5700 and 3400 mW cm–2 in black, red, green and blue curves, respectively. Emission slit 5 nm.
6. Proposed mechanism of upconversion process It is generally considered that the energy transfer between organic antenna dyes and chelated lanthanide ions occurs from the triplet state of the antenna chromophore.10-12 Unfortunately due to the instrumental problems we had no possibility to measure the phosphorescence spectrum of the Gd(TTA)4(IR-806) complex. Hence, the energy the IR-806 dye’s first triplet state T1 compared to the energy of the transitions of Er3+ is unknown. However, another NIR dye IR140 has absorbance and fluorescence emission bands close to those of IR-806 and there is a reasonable overlap between the phosphorescence emission of the IR140 dye and the 4I15/2 → 4I11/2 transition of Er3+.1 Our hypothesis is that the IR-806 dye cation absorbs the excitation light, is excited to its first singlet state S1 and undergoes an intersystem crossing to the T1 level (Figure S4). The IR-806 dye then returns to the ground state S0 by resonance energy transfer (RET) to the Er3+ ion at ground state 4I15/2 and Er3+ ion is excited to the 4I11/2 level. The whole process is repeated except that the Er3+ ion transition involved is now 4I11/2 → 4F7/2. A fast nonradiative multi-phonon relaxation of 4F7/2 level leads to the population of the 2H11/2 and 4S3/2 levels.
S5
Finally the green upconversion emission around 530 nm and 550 nm is due to the 2H11/2 → 4I15/2 and 4
S3/2 → 4I15/2 transitions, respectively.
Figure S4. Schematic energy level diagram of the proposed excitation-emission system of Er(TTA)4(IR-806). Vertical solid lines: absorption and emission transitions, ISC: inter system crossing, ET: resonance energy transfer, tilted solid lines of Er3+: multiphonon relaxation. The energies of Er3+ energy levels are taken from literature.13 The energy of S1 level of IR-806 dye is the measured fluorescence maxima of the dye and the error bar marks the half-wide of the fluorescence spectrum. The energy of the T1 level is an estimate.
S6
7. Photobleaching of IR-806 dye Photobleaching of IR-806 dye was studied by following the change in absorbance as the IR-806 sample was exposed to NIR laser. Absorption spectra of IR-806 dye salt in CDCl3 are presented in Figure S5. The absorbance at 808 nm is decreased as the exposure time to NIR laser is increased. Absorbance is decreased about 50% in 30 seconds. This seems quite a large change, but the used laser power was also high and 30 second is more than adequate time for most applications.
Figure S5. Absorbance spectra of IR-806 dye salt in CDCl3 (100 µM) exposed to a 808 nm NIR laser (9500 mW cm–2). Exposure times 0 s (black), 30 s (red), 5 min (green) and 10 min (blue).
S7
8. References (1) Wang, H. S.; Yang, Y.; Cui, Y. J.; Wang, Z. Y.; Qian, G. D. Sensitized Near-Infrared Luminescence from Erbium Ion-Associated Complex with IR140 Dye. Dyes Pigm. 2012, 95, 69-73. (2) Wang, H. S.; Qian, G. D.; Wang, M. Q.; Zhang, J. H.; Luo, Y. S. Enhanced Luminescence of an Erbium(III) Ion-Association Ternary Complex with a Near-Infrared Dye. J. Phys. Chem. B 2004, 108, 8084-8088. (3) Zou, W. Q.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Broadband DyeSensitized Upconversion of Near-Infrared Light. Nat. Photonics 2012, 6, 560-564. (4) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y. H.; Wang, J.; Xu, J.; Chen, H. Y.; Zhang, C.; Hong, M. H.; Liu, X. G. Simultaneous Phase and Size Control of Upconversion Nanocrystals Through Lanthanide Doping. Nature 2010, 463, 1061-1065. (5) Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W. The Emission Spectrum and the Radiative Lifetime of Eu3+ in Luminescent Lanthanide Complexes. Phys. Chem. Chem. Phys. 2002, 4, 1542-1548. (6) Li, M.; Selvin, P. R. Luminescent Polyaminocarboxylate Chelates of Terbium and Europium - the Effect of Chelate Structure. J Am Chem Soc 1995, 117, 8132-8138. (7) Stouwdam, J. W.; Hebbink, G. A.; Huskens, J.; van Veggel, F. C. J. M. Lanthanide-Doped Nanoparticles with Excellent Luminescent Properties in Organic Media. Chem. Mater. 2003, 15, 46044616. (8) Solis, D.; De la Rosa, E.; Meza, O.; Diaz-Torres, L. A.; Salas, P.; Angeles-Chavez, C. Role of Yb3+ and Er3+ Concentration on the Tunability of Green-Yellow-Red Upconversion Emission of Codoped ZrO2:Yb3+-Er3+ Nanocrystals. J. Appl. Phys. 2010, 108. (9) Anderson, R. B.; Smith, S. J.; May, P. S.; Berry, M. T. Revisiting the NIR-to-Visible Upconversion Mechanism in β-NaYF4:Yb3+, Er3+. J. Phys. Chem. Lett. 2014, 5, 36-42. (10) Crosby, G. A.; Whan, R. E.; Alire, R. M. Intramolecular Energy Transfer in Rare Earth Chelates Role of Triplet State. J. Chem. Phys. 1961, 34, 743-748. (11) Hayes, A. V.; Drickamer, H. G. High-Pressure Luminescence Studies of Energy-Transfer in RareEarth Chelates. J. Chem. Phys. 1982, 76, 114-125. (12) Latva, M.; Takalo, H.; Mukkala, V. M.; Matachescu, C.; RodriguezUbis, J. C.; Kankare, J. Correlation Between the Lowest Triplet State Energy Level of the Ligand and Lanthanide(III) Luminescence Quantum Yield. J. Lumin. 1997, 75, 149-169. (13) Carnall, W. T.; Fields, P. R.; Rajnak, K. Electronic Energy Levels in Trivalent Lanthanide Aquo Ions. I. Pr3+ Nd3+ Pm3+ Sm3+ Dy3+ Ho3+ Er3+ and Tm3+. J. Chem. Phys. 1968, 49, 4424-4442.
S8
