Triplet Sensitization in an Anionic Poly(phenyleneethynylene

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Triplet Sensitization in an Anionic Poly(phenylene ethynylene) Conjugated Polyelectrolyte by Cationic Iridium Complexes Jarrett H. Vella, Anand Parthasarathy, and Kirk S. Schanze J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp405577z • Publication Date (Web): 19 Jul 2013 Downloaded from http://pubs.acs.org on July 22, 2013

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The Journal of Physical Chemistry

Triplet Sensitization in an Anionic Poly(phenylene ethynylene) Conjugated Polyelectrolyte by Cationic Iridium Complexes Jarrett H. Vella, Anand Parthasarathy and Kirk S. Schanze* Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida, 32611-7200 *Corresponding author. E-mail: [email protected]. Telephone: 352-392-9133. Fax: 352-392-2395 RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) Abstract We describe a systematic study of triplet sensitization in a poly(phenylene ethynylene) conjugated polyelectrolyte (CPE) in methanol solution by using a series of three cationic iridium complexes with varying triplet energy. The cationic iridium complexes bind to the anionic CPE by ion-pairing, leading to singlet state quenching of the polymer, and allowing for efficient back transfer of triplet excitation energy to the polymer. Efficient (amplified quenching) of the polymer’s fluorescence is observed for each iridium complex, with SternVolmer quenching constants in excess of 105 M-1. Triplet sensitization is confirmed for two of the iridium complexes by monitoring the relative yield of the CPE triplet state by transient absorption spectroscopy. One of the iridium complexes does not sensitize the CPE triplet, and consideration of the energies of the three complexes allows us to bracket the triplet energy of the CPE within the range 1.95 - 2.26 eV. Keywords: Conjugated polyelectrolyte, conjugated polymer, triplet sensitization, energy transfer 1 ACS Paragon Plus Environment


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Introduction The study of triplet excitons in π-conjugated polymers has attracted considerable attention in part due to their role in applications including organic and polymer light emitting diodes (LED) and photovoltaic devices (OPV).1-4

Triplet excitons have a longer lifetime than singlets, and this difference can influence device

efficiency in both LED and OPV applications.1-4 Despite their strong absorption and fluorescence in the visible region of the spectrum, π-conjugated polymers based on poly-(p-phenylene), poly-(phenylene ethynylene) and poly-(phenylene vinylene) backbones have comparatively low intersystem crossing (ISC) yields. As a result, direct photoexcitation of these polymers leads only to low yields of triplet exciton states, both in solution or in the solid (film). In general, the ISC efficiency of a conjugated polymer can be increased by doping or covalent modification with a heavy-metal containing chromophore.5 It is generally assumed that this process involves a “back and forth” energy transfer process, whereby the heavy metal complex serves to sensitize the polymer triplet state by acting as a singlet energy acceptor, intersystem crossing center, and triplet energy donor.5-10 However, despite this assumption, few studies have been carried out to explore the mechanism of the triplet sensitization, in part due to the complexity of studying polymer/sensitizer systems in the solid state or as covalently linked arrays.4,5,7,9,11,12 Triplet energy transfer occurs by Dexter exchange, a process that requires direct orbital overlap and hence a small separation distance (< 10 Å) between donor and acceptor.13 Thus, in order to facilitate the study of triplet sensitization in a polymer-metal chromophore system, it is necessary to constrain the two components so they remain in close spatial proximity. This has been accomplished in several ways, including the formation of π-acid complexes,14 inclusion of the donor and acceptor chromophores into zeolite matrices,15 via ion-pair interactions,16 or by covalently attaching metals to a polymer backbone.7 Evans et al. reported that the triplet energy back transfer could be observed in thin films by covalently attaching iridium based dyes to a

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polyfluorene backbone; they further observed the efficiency of triplet energy back transfer could be controlled by controlling the length of the spacer that separates the polymer backbone and the dye.7 Conjugated polyelectrolytes (CPEs) feature a π-conjugated backbone with pendant ionic side groups that makes them soluble in water and polar organic solvents.17 The amplified fluorescence quenching effect of CPEs by oppositely charged quencher ions has been well documented.17-19 The effect arises in part due to the formation of ion-pair complexes between the CPE chains and the quencher ions, keeping them in close proximity and leading to efficient (static) quenching. A few studies have explored the properties of the triplet exciton in CPEs, primarily through the use of transient absorption spectroscopy.20,21

Here we present a

systematic study of triplet sensitization in the poly-(phenylene ethynylene) (PPE) based CPE, PPESO3-22 using the series of cationic Ir(III) complexes 1 - 3 as triplet sensitizers (Chart 1). This study takes advantage of the electrostatic interaction between the anionic PPESO3- chains and the cationic Ir(III) complexes to hold the polymer/sensitizer pair together in an ion-pair complex. By using this approach we are able to directly observe triplet sensitization of PPESO3- by the iridium complexes. In addition, we are able to establish the energetic criteria necessary for the sensitization process to occur by varying the triplet energy in the iridium complex sensitizer. Results and Discussion We initially used the series of iridium complexes of the type Ir(C^N)2(bpy)+ (C^N = an orthometalling ligand and bpy = 2,2’-bipyridine) as quenchers to study their effect on the fluorescence (singlet state quenching) of the anionic conjugated polyelectrolyte, PPESO3-. These complexes were selected for this study because they are known to undergo rapid intersystem crossing with unit quantum efficiency following excitation, and further the triplet energy can be tuned by varying the structure of the C^N ligand.23-26 We then turned to nanosecond transient absorption spectroscopy to probe the triplet state absorptivity (yield) of PPESO3-, allowing us quantify the relative triplet yield and how it varies as a function of the concentration and structure of the Ir(C^N)2(bpy)+ sensitizer. 3 ACS Paragon Plus Environment


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Chart 1 The spectral properties of PPESO3- and complexes 1 - 3 are shown in Figure 1 (complete photophysical data for 1 - 3 is provided Table S1 in the supporting information). The absorption spectra of the three iridium complexes in methanol feature high-energy π-π* transitions between 200 – 350 nm and weak, lower energy metal-to-ligand charge transfer (MLCT) transitions between 350-500 nm. The absorption spectrum of PPESO3in methanol exhibits a strong π-π* transition centered near 426 nm. The emission spectrum of PPESO3- in methanol consists of a structured band with origin at λ ~ 450 nm, indicating a singlet energy of 2.81 eV. Based on the small Stokes shift and the lack of a distinct long-wavelength emission band it is believed that PPESO3- is molecularly dissolved in methanol, with little contribution of aggregate states to its photophysics.22,27 The photoluminescence of the 1 – 3 appears as a single broad emission band that arises from a state of largely triplet spin character (phosphorescence).23-26,28 The triplet energies of the iridium complexes can be estimated from the phosphorescence spectra and are listed in Table S1. With a triplet energy of 2.36 eV, 1 has the highest triplet energy, followed by 2 (2.25 eV) and 3 (1.96 eV) .


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500 600 700 500 Wavelength, nm Figure 1. The absorption and emission spectra in argon out-gassed methanol for PPESO3- (―), 1 (•••), 2 (– –), and 3 (–••–). The emission spectra are normalized to the maximum intensity. See Table in supporting information for quantum yields and lifetimes for 1 – 3. 300

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Stern-Volmer (SV) fluorescence quenching studies of PPESO3- by 1 - 3 were carried out in order to determine their ability quench the singlet state of PPESO3-. Given the good spectral overlap between the PPESO3- fluorescence and the absorption of 1 – 3, quenching is anticipated to occur by energy transfer. Each iridium complex quenches PPESO3- fluorescence efficiently, with the fluorescence nearly quantitatively quenched upon addition of ∼ 60 µM of Ir(III) (where [PPESO3-] = 4.6 µM). Quenching constants (KSV) obtained from the initial slopes of the SV plots (see supporting information) are high, KSV ∼ 105 M-1 (Table S2), consistent with the amplified quenching effect.17 Quenching constants (KSV) obtained from the initial slopes of the SV plots (see supporting information) are high, KSV ∼ 105 M-1 (Table S2), consistent with the amplified quenching effect.17 Given our expectation that quenching occurs by energy transfer from PPESO3- to the iridium complexes, it was surprising to observe little phosphorescence from the polymer-iridium complex solutions. For example, as shown in Figure S1 (supporting information), there is only weak emission observed from 1 and 2 as the fluorescence of PPESO3- is quenched. Control experiments were carried out by titrating the Ir(III) complexes alone at the same excitation wavelength in the absence of PPESO3- with the results showing that the emission of 1 and 2 can be clearly observed in the concentration range used for the SV quenching experiments (Figures S5 and S6). 5 ACS Paragon Plus Environment


A possible explanation for the quenching without the observation of emission from the Ir(III) complexes is that back energy transfer is occurring from the photoexcited complexes to sensitize the PPESO3- triplet state. Thus, in order to gain more insight in the mechanism of the quenching, the transient absorption of PPESO3- was compared in the presence and absence of 1. Figure 2 compares the transient absorption spectra observed 50 ns following 355 nm pulsed excitation of solutions of PPESO3- alone and at the same concentration in the presence of 1 (c = 4.0 µM ), with the same laser excitation energy. In both cases the transient absorption that is observed is consistent with that of the PPESO3- triplet state (note that the transient absorption of the triplet state of 1 is distinctly different, Figure S2)21; however, the amplitude of the triplet-triplet absorption is approximately doubled in the presence of 1. This suggests that the triplet yield of PPESO3- is increased substantially in the presence of 1, consistent with the hypothesis that 1 is sensitizing the PPESO3- triplet state.

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Wavelength / nm Figure 2. Transient absorption difference spectra of PPESO3- (17.4 µM in methanol) 50 ns following 355 nm pulsed excitation (∆A, t=0): (a) PPESO3- alone, and (b) upon addition of 4 µM of 1 to PPESO3-. Note that the signal intensity of PPESO3- is enhanced in the presence 1 indicating a higher triplet yield. Having established that 1 enhances the PPESO3- triplet yield, we carried out titration experiments to monitor the relative triplet yield as a function of added Ir(III) complex concentration for all three complexes. As shown in Figure 3, the PPESO3- transient absorption at ~730 nm increases with increasing concentration of 1 and 2, consistent with triplet sensitization. However, by contrast, addition of 3 leads to quenching of the triplet, with quenching nearly complete at [3] ~ 10 µM. This indicates that 3 is does not transfer triplet energy back to 6 ACS Paragon Plus Environment

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PPESO3-, consistent with the fact that this complex has the lowest energy triplet (1.96 eV) among the series. The phosphorescence quantum yield of 3 is comparable to that of PPESO3- (0.77 vs. 0.78, Table S1). In view of this, we anticipated that since the complex does not transfer triplet energy back to the polymer, the phosphorescence intensity from the complex in the PPESO3-/3 mixture at the end of the Stern-Volmer titration should be comparable to that of the unquenched polymer.

However, it is evident from Figure S7 that the

phosphorescence intensity of 3 in the mixture is somewhat less than expected if polymer to complex energy transfer is efficient. We conclude from this result that there is a competing non-radiative decay channel operating in the excited state of 3 in the polymer/complex ion-pair.29 Time, sec 2.0e-6

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Figure 3. Triplet-triplet transient absorption titrations in argon out-gassed methanol PPESO3- by iridium dyes (a) 1, (b) 2, and (c) 3. The insets of each plot show the change in optical density as a function of the corresponding iridium dye concentration. The decays were recorded at 734 nm and the optical power of the pump was kept constant at 2.5 mJ. The concentration of PPESO3- was 17.4 µM for (a), (b) and 19.1 µM for (c). 7 ACS Paragon Plus Environment


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Importantly, the fact that 3 is unable to sensitize the formation of the triplet state of PPESO3- clearly supports the notion that the sensitization mechanism involves “back and forth” energy transfer, and not just due to an external heavy atom effect. Specifically, the triplet states of 1 and 2 are sufficiently energetic to transfer triplet energy to the polymer, whereas the triplet energy of 3 must be below that of the polymer. Thus, the results allow us to bracket the triplet energy of PPESO3-, and it is within the range 1.96 – 2.25 eV. This result is in good accord with previous investigations of poly(phenylene ethynylene)s which have indicated similar values for the triplet energy.30,31 Conclusion This study clearly demonstrates that heavy metal complexes with appropriate triplet energy can serve to sensitize the triplet state in a conjugated polymer system. The necessary condition is that the triplet energy of the metal complex sensitizer must be in between that of the polymer singlet and triplet states. The conjugated polyelectrolyte/Ir(III) complex system studied here provides a convenient means to investigate the mechanism and relative efficiency of the triplet sensitization as a function of the energy of the triplet state of the metal complex.

However, we believe that the results are more broadly applicable to conjugated polymer/metal

complex systems, and in particular for blends or covalently linked systems in the solid state. The results could point the way for the rational design and development of hybrid conjugated polymer/metal complex based systems where triplet exciton states are needed to enhance the efficiency of opto-electronic devices. Experimental Optical titrations. Ultraviolet-visible (UV-vis) absorption measurements were performed on a Varian-Cary 50 Bio spectrophotometer and photoluminescence (PL) measurements were obtained using a Jobin-Yvon Fluorolog-3 spectrophotometer. A 1 cm pathlength cuvette was used. Transient absorption (TA) measurements were acquired on previously described instrumentation.32,33 Methanol was used as the solvent for the donor and acceptor solutions. In a typical experiment, the concentrated acceptor solution was out-gassed with argon for twenty minutes. A volume of neat methanol was separately out-gassed in a cuvette for twenty minutes, after 8 ACS Paragon Plus Environment

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which time stock PPESO3- was added to give the appropriate concentration (4.61 µM for PL titrations and 17.419.1 µM for TA titrations). In all cases, no further methanol out-gassing was done after the addition of PPESO3- to prevent polymer aggregation. Transient absorption titrations were monitored at 734 nm, the wavelength corresponding to the PPESO3- T1-Tn maximum absorption. The pump power at 355 nm was held constant at 2.5 mJ during each titration. Photophysical characterization. The triplet lifetimes of each Ir(III) complex were estimated from the full transient absorption spectrum using SpecFit software by fitting the transient decay at each wavelength to a global first-order rate of decay. Phosphorescence quantum yields for the Ir(III) complexes were obtained using relative actinometry. Rose Bengal (Φf = 0.11 in absolute ethanol + 0.01 M KOH) was used as the quantum yield standard for Ir(bt)2(bpy)Cl (1) and Ir(ppy)2(bpy)Cl (2), while Rhodamine B (Φf = 0.69 in methanol) was the standard for Ir(hqx)2(bpy)Cl (3). Acknowledgment We thank the United States Department of Energy (Grant DE-FG02-03ER15484) for support of this work. Supporting information Experimental details, synthesis and characterization of Ir (III) complexes and additional photophysical data. This material is available free of charge via the internet at http://pubs.acs.org.

References (1)

Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices Nature 1998, 395, 151-154.

(2)

Shao, Y.; Yang, Y. Efficient Organic Heterojunction Photovoltaic Cells Based on Triplet Materials Adv. Mater. 2005, 17, 2841-2844.

(3)

Guo, F. Q.; Kim, Y. G.; Reynolds, J. R.; Schanze, K. S. Platinum-Acetylide Polymer Based Solar Cells: Involvement of the Triplet State for Energy Conversion Chem. Commun. 2006, 1887-1889.

(4)

Rand, B. P.; Schols, S.; Cheyns, D.; Gommans, H.; Girotto, C.; Genoe, J.; Heremans, P.; Poortmans, J. Organic Solar Cells with Sensitized Phosphorescent Absorbing Layers Org. Electron. 2009, 10, 10151019. 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 12

(5)

Laquai, F.; Im, C.; Kadashchuk, A.; Bassler, H. Sensitized Intrinsic Phosphorescence from a Poly(Phenylene-Vinylene) Derivative Chem. Phys. Lett. 2003, 375, 286-291.

(6)

Liu, Y.; Jiang, S. J.; Schanze, K. S. Amplified Quenching in Metal-Organic Conjugated Polymers Chem. Commun. 2003, 650-651.

(7)

Evans, N. R.; Devi, L. S.; Mak, C. S. K.; Watkins, S. E.; Pascu, S. I.; Koehler, A.; Friend, R. H.; Williams, C. K.; Holmes, A. B. Triplet Energy Back Transfer in Conjugated Polymers with Pendant Phosphorescent Iridium Complexes J. Am. Chem. Soc. 2006, 128, 6647-6656.

(8)

Burrows, H. D.; Fonseca, S. M.; Dias, F. B.; de Melo, J. S.; Monkman, A. P.; Scherf, U.; Pradhan, S. Singlet Excitation Energy Harvesting and Triplet Emission in the Self-Assembled System Poly{1,4Phenylene-[9,9-Bis(4-Phenoxy-Butylsulfonate)]Fluorene-2,7-Diyl} Copolymer/Tris(Bipyridyl)Ruthenium(II) in Aqueous Solution Adv. Mater. 2009, 21, 1155-1159.

(9)

Monkman, A. P.; Burrows, H. D.; Miguel, M. D.; Hamblett, I.; Navaratnam, S. Measurement of the S-0T-1 Energy Gap in Poly(2-Methoxy,5-(2 '-Ethyl-Hexoxy)-P-Phenylenevinylene) by Triplet-Triplet Energy Transfer Chem. Phys. Lett. 1999, 307, 303-309.

(10) Postacchini, B. B.; Zucolotto, V.; Dias, F. B.; Monkman, A.; Oliveira, O. N. Energy Transfer in Nanostructured Films Containing Poly(P-Phenylene Vinylene) and Acceptor Species J. Phys. Chem. C 2009, 113, 10303-10306. (11) Adachi, C.; Kwong, R. C.; Djurovich, P.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Endothermic Energy Transfer: A Mechanism for Generating Very Efficient High-Energy Phosphorescent Emission in Organic Materials Appl. Phys. Lett. 2001, 79, 2082-2084. (12) Luhman, W. A.; Holmes, R. J. Enhanced Exciton Diffusion in an Organic Photovoltaic Cell by Energy Transfer Using a Phosphorescent Sensitizer Appl. Phys. Lett. 2009, 94, 153304-1 - 153304-3. (13) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Sausalito, California, USA, 1991. (14) Haneline, M. R.; Tsunoda, M.; Gabbai, F. P. Π-Complexation of Biphenyl, Naphthalene, and Triphenylene to Trimeric Perfluoro-Ortho-Phenylene Mercury. Formation of Extended Binary Stacks with Unusual Luminescent Properties J. Am. Chem. Soc. 2002, 124, 3737-3742. (15) Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Photochemical and Photophysical Studies of OrganicMolecules Included within Zeolites Acc. Chem. Res. 1992, 25, 299-307. (16) Dalvi-Malhotra, J.; Chen, L. H. Enhanced Conjugated Polymer Fluorescence Quenching by DipyridiniumBased Quenchers in the Presence of Surfactant J. Phys. Chem. B 2005, 109, 3873-3878. (17) Jiang, H.; Taranekar, P.; Reynolds, J. R.; Schanze, K. S. Conjugated Polyelectrolytes: Synthesis, Photophysics, and Applications Angew. Chem. Int. Ed. 2009, 48, 4300-4316. (18) Zhou, Q.; Swager, T. M. Fluorescent Chemosensors Based on Energy Migration in Conjugated Polymers: The Molecular Wire Approach to Increased Sensitivity J. Am. Chem. Soc. 1995, 117, 12593-12602. (19) Chen, L. H.; McBranch, D. W.; Wang, H. L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Highly Sensitive Biological and Chemical Sensors Based on Reversible Fluorescence Quenching in a Conjugated Polymer Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 12287-12292. (20) Chemburu, S.; Corbitt, T. S.; Ista, L. K.; Ji, E.; Fulghum, J.; Lopez, G. P.; Ogawa, K.; Schanze, K. S.; Whitten, D. G. Light-Induced Biocidal Action of Conjugated Polyelectrolytes Supported on Colloids Langmuir 2008, 24, 11053-11062.


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(21) Corbitt, T. S.; Ding, L.; Ji, E.; Ista, L. K.; Ogawa, K.; Lopez, G. P.; Schanze, K. S.; Whitten, D. G. Light and Dark Biocidal Activity of Cationic Poly(Arylene Ethynylene) Conjugated Polyelectrolytes Photochem. Photobiol. Sci. 2009, 8, 998-1005. (22) Tan, C. Y.; Pinto, M. R.; Schanze, K. S. Photophysics, Aggregation and Amplified Quenching of a WaterSoluble Poly( Phenylene Ethynylene) Chem. Commun. 2002, 446-447. (23) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Highly Phosphorescent Bis-Cyclometalated Iridium Complexes: Synthesis, Photophysical Characterization, and Use in Organic Light Emitting Diodes J. Am. Chem. Soc. 2001, 123, 4304-4312. (24) Tani, K.; Fujii, H.; Mao, L.; Sakurai, H.; Hirao, T. Iridium(III) Complexes Bearing Quinoxaline Ligands with Efficient Red Luminescence Properties Bull. Chem. Soc. Jpn. 2007, 80, 783-788. (25) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Synthesis and Characterization of Phosphorescent Cyclometalated Iridium Complexes Inorg. Chem. 2001, 40, 1704-1711. (26) Chen, H. Y.; Yang, C. H.; Chi, Y.; Cheng, Y. M.; Yeh, Y. S.; Chou, P. T.; Hsieh, H. Y.; Liu, C. S.; Peng, S. M.; Lee, G. H. Room-Temperature Nir Phosphorescence of New Iridium (III) Complexes with Ligands Derived from Benzoquinoxaline Can. J. Chem. 2006, 84, 309-318. (27) Zhao, X. Y.; Pinto, M. R.; Hardison, L. M.; Mwaura, J.; Muller, J.; Jiang, H.; Witker, D.; Kleiman, V. D.; Reynolds, J. R.; Schanze, K. S. Variable Band Gap Poly(Arylene Ethynylene) Conjugated Polyelectrolytes Macromolecules 2006, 39, 6355-6366. (28) Ohsawa, Y.; Sprouse, S.; King, K. A.; Dearmond, M. K.; Hanck, K. W.; Watts, R. J. Electrochemistry and Spectroscopy of Ortho-Metalated Complexes of Ir(III) and Rh(III) J. Phys. Chem. 1987, 91, 1047-1054. (29) A possible mechanism for the non-radiative decay channel is excited state electron transfer (ET) from PPESO3- to *Ir(hqx)2(bpy)+. However, we estimate that the free energy change for this reaction is moderately endothermic, ∆GET = E(PPE/PPE+) - E(Ir+/Ir0) - E(*Ir) ≈ +0.4 eV, where E(PPE/PPE+) ≈ 0.9 V, E(Ir+/Ir0) ≈ -1.50 V and E(*Ir) = 1.97 eV. On the basis of the estimated ∆GET it is not likely that ET is the active non-radiative decay pathway. (30) Walters, K. A.; Ley, K. D.; Schanze, K. S. Triplet State Photophysics in an Aryleneethynylene πConjugated Polymer Chem. Commun. 1998, 1115-1116. (31) Kohler, A.; Wilson, J. S.; Friend, R. H.; Al-Suti, M. K.; Khan, M. S.; Gerhard, A.; Bassler, H. The Singlet-Triplet Energy Gap in Organic and Pt-Containing Phenylene Ethynylene Polymers and Monomers J. Chem. Phys. 2002, 116, 9457-9463. (32) Farley, R. Photophysics of Platinum and Iridium Organometallic Materials from Molecular Wires to Nonlinear Optics, Ph.D. Dissertation, University of Florida, 2007. (33) Wang, Y. S.; Schanze, K. S. Photochemical Probes of Intramolecular Energy and Electron Transfer Chem. Phys. 1993, 176, 305-319.



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Triplet Sensitization in an Anionic Poly(phenyleneethynylene

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