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Photon Upconversion by Triplet−Triplet Annihilation in Ru(bpy)3and DPA-Functionalized Polymers Philip C. Boutin,† Kenneth P. Ghiggino,*,‡ Timothy L. Kelly,*,† and Ronald P. Steer*,† †

Department of Chemistry University of Saskatchewan, Saskatoon, Saskatchewan S7N 5C9, Canada School of Chemistry University of Melbourne, Parkville, Victoria 3010, Australia

‡

S Supporting Information *

ABSTRACT: Two photophysically active polymers, each containing one absorbing Ru(bpy)3 triplet sensitizing core and multiple 9,10diphenylanthracene (DPA) triplet energy acceptors, are shown to exhibit noncoherent photon upconversion by triplet−triplet annihilation (TTA) in both fluid solution and in thin solid films. An analysis of the kinetics and absorbed power dependencies of the upconverted fluorescence demonstrates that, in fluid solution, TTA is occurring in the low-power, pseudo-first order limit, and that the energy transfer mechanism involves fast, efficient intramolecular triplet energy transfer from the excited Ru(bpy)3 core to the DPA acceptors in the macromolecule’s pendant arms, followed by intermolecular TTA. Intermolecular triplet energy transfer, diffusion coefficient, and quantum yield measurements using Ru(bpy)3 and DPA controls suggest that, although a minority of the intermolecular encounters result in TTA in this one absorbing core system, efficient intramolecular TTA could be realized in a polymer with multiple absorbing centers. SECTION: Kinetics and Dynamics

R

some test OPV cells employing NCPU have been constructed and their efficiencies evaluated.20−22 In addition, there has been one report of upconversion in a sensitizing platinum porphyrin tethered to a light emitting conjugated oligomer.23 The most successful polymer matrices reported to date are those such as the thermoplastic polyurethanes with low glass transition temperatures that permit large diffusion rates. Short-range intermolecular (interexciton) energy transfer is thus facilitated and relatively efficient triplet sensitization and upconversion by TTA can be observed in these polymer matrices without strict exclusion of triplet-quenching oxygen. Recently, NCPU has also been observed in a photophysically active polymer containing both an absorber and upconverter.24 In this most recent report, Ghiggino and co-workers prepared two novel polymers containing a ruthenium tris-bipyridine (Ru(bpy)3) core and either two or six arms containing multiple, pendant 9,10-diphenylanthracene (DPA) moieties, and measured their intramolecular energy transfer kinetics in solution. Weak delayed fluorescence attributed to TTA in DPA was observed, but was otherwise not characterized. The use of Ru(bpy)3 as an absorber-sensitizer in a variety of donor− acceptor arrays is well-known,25−27 and upconverted fluorescence has been observed previously in a Ru(bpy)3-DPA dendrimer.28 The present report, however, is the first to

ecent interest in improving the efficiencies of organic photovoltaic (OPV) cells has prompted the exploration of methods to circumvent the Shockley−Queisser solar power conversion limit1 that applies to single threshold devices. Two approaches appear promising: singlet exciton fission2,3 and noncoherent photon upconversion4−11 (NCPU), both of which can, in principle, be applied to OPVs. Previous research on NCPU in organic materials has centered on triplet−triplet annihilation (TTA) as the basic upconversion process. Two schemes have been proposed. The more thoroughly investigated process4 involves photon absorption by a sensitizer with a large absorption cross-section in the visible and/or near-infrared and a high triplet quantum yield, followed by triplet electronic energy transfer to an acceptor with a large triplet-singlet energy gap (but 2E(T1) > E(S1)) and a large fluorescence quantum yield. TTA in this acceptor then produces its lowest excited singlet, S1, state, which radiates in a region to the blue of the absorption wavelength. A second scheme12,13 involves use of a combined absorber-upconverter such as a metalloporphyrin in which, following absorption to the lowest excited singlet state and intersystem crossing, homomolecular TTA occurs via the triplet state of the absorber, producing a higher singlet state (S2 in the porphyrins). Such states are normally relatively short-lived and exhibit small radiative quantum yields but can nonetheless act as energy or electron donors in a device. Both schemes have been shown to be operative in polymer matrices4,13−18 and on wide bandgap semiconductors,5,19 and © 2013 American Chemical Society

Received: October 25, 2013 Accepted: November 19, 2013 Published: November 19, 2013 4113

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determine the nature and efficiencies of the energy transfer processes leading to NCPU in polymers containing both a single absorber-sensitizer and multiple triplet acceptorupconverters. Information that can guide the design of new upconverting polymers for OPVs is obtained. The structures of the relevant molecules are shown in Chart 1. Chart 1. Structures of Polymers and Reference Materials Employed

Figure 1. Upconverted fluorescence and scaled phosphorescence for polymers 1 (black), 2 (red), 4 (orange), and control 3 (blue) with a 14 mW/cm2 532 nm excitation. Spectral bandwidths were 12.6 nm for upconverted fluorescence spectra and 1.8−3.6 nm for phosphorescence spectra. Phosphorescence spectra were divided by the following scaling constants: 2.5× for 1; 8× for 2; 75× for 3. The 705 nm peak is an artifact, the likely origin of which is described in the Supporting Information. It was modeled out prior to any quantitative calculations of quantum yields.

emission bands that exhibit only remnants of the vibronic structure characteristic of the monomer.24 Figure 2a and 2b show double logarithmic plots of the upconverted fluorescence intensity versus residual phosphorescence intensity for polymers 1 and 2 as a function of incident power density. In the low power region, the slopes of these plots are 1.88 ± 0.07 and 1.93 ± 0.04, respectively, values that are characteristic of TTA in which triplet decay occurs primarily by kinetically first or pseudo-first order processes.11,30 About 90% of the core Ru(bpy)3 triplet states of 1 and 2 transfer their energy to DPA moieties in the polymer solutions.24 Nevertheless, the relative yields of their residual phosphorescence, which strictly follow a linear power deopendence, can serve as an accurate metric for the relative absorbed intensities in the power dependence studies. These power dependence experiments therefore demonstrate conclusively that the upconverted emission observed in the blue region of the spectrum is a result of TTA between DPA moieties of the polymers. Quenching experiments were then conducted to determine whether the energy transfer processes involved are intramolecular or intermolecular. The results are shown in the Stern−Volmer plots: Figure 3a for quenching of the phosphorescence of the control Ru(dmb)3, 3, by monomeric DPA, and Figure 3b by DPA-polymer 4. Good linear correlations are found in both plots, and second order triplet energy transfer rate constants of kTET = (1.00 ± 0.01) × 109 M−1 s−1 and (2.2 ± 0.3) × 109 M−1 s−1, respectively, are calculated from the Stern−Volmer constants and the measured unquenched lifetime of triplet Ru(dmb)3 (as the perchlorate in chloroform), τ0. These calculations employ a value of τ0 = 871 ns measured by laser flash photolysis at the 74 μM Ru(dmb)3 concentration used in the quenching experiments. Data concerning the small variation in this lifetime with Ru(dmb)3 concentration are given in the Supporting Information. This lifetime of triplet Ru(dmb)3 compares with values of 704 ns,24 920 ns,31 and 840 ns32 previously reported for different solute concentrations and/or different solvents. The value of kTET = 1.00 × 109 M−1 s−1 obtained here for triplet energy transfer from Ru(dmb)3 to monomeric DPA should be compared with a diffusion-limited value of kTET = 1.1 × 1010 M−1 s−1 (not reduced by a spin-statistical factor) for chloroform solvent at

Continuous wave (cw) excitation at 532 nm on the red edge of the MLCT absorption band system of the Ru(bpy)3 chromophores of polymers 1 and 2 and control 3 (hereafter labeled Ru(dmb)3) in deoxygenated chloroform solution results in phosphorescence from the corresponding triplet state that is produced by intersystem crossing with near unit quantum yield.29 Upconverted delayed fluorescence to the blue is clearly seen when exciting 1 and 2 but not 4 at this same wavelength. Figure 1 shows the spectra. Both the Ru-core phosphorescence and the upconverted DPA fluorescence disappeared when oxygen was admitted to the solutions. The delayed upconverted fluorescence observed in the polymers exhibits substantial reabsorption in the region of the 0−0 band envelope of DPA at 400−420 nm, minor reabsorption in the blue end of the Ru(bpy)3 MLCT band, and does not exhibit the clear vibronic structure characteristic of prompt fluorescence generated by one photon excitation of monomeric DPA in its S1−S0 absorption band system. Some of this observed loss of vibronic structure can be attributed to the larger spectral bandwidth used to collect the upconverted emission in the spectra presented in Figure 1. However, intramolecular interactions between adjacent DPA moieties in the ground state and excitonic interactions associated with their orbital overlap required by TTA also contribute to the increased breadth of the observed upconversion emission bands of the polymers. One-photon excitations of polymers 1, 2 and 4 in their characteristic DPA absorptions in the violet region of the spectrum also produce broad fluorescence 4114

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Figure 2. Double logarithmic plot of upconverted fluorescence intensity versus phosphorescence intensity of 75 μM solutions of Ru-DPA polymer 1 (a) and 2 (b) in degassed chloroform at room temperature. The corresponding incident power density (PD) range is 1 to 50 mW/cm2 for the 532 nm cw laser excitation source. Details of the data analysis yielding these slopes are given in the Supporting Information.

Figure 3. Phosphorescence quenching of degassed Ru(dmb)3 in CHCl3 by the addition of DPA (a) and polymer 4 (b). A 532 nm Nd:YAG laser with a power density of 14 mW/cm2 was used as the excitation source. The slopes are the Stern−Volmer constants used to calculate the values of kTET.

room temperature calculated using the Stokes−Einstein model.33 These results beg analysis, since the slower-diffusing DPA polymer, 4, exhibits a triplet quenching rate constant that is a factor of 2.2 larger than that of the faster-diffusing DPA monomer. The diffusion coefficients measured by diffusion ordered spectroscopy (DOSY) for compounds 1 to 4 in CDCl3 are given in Table S2 in the Supporting Information, together with the ratios of these values normalized to that of monomeric DPA. The measured ratio of the diffusion coefficients of DPA to DPA-polymer 4 by DOSY is 3.3. It therefore follows that the encounter rate between 3Ru(dmb)3 and DPA-polymer 4, should be 3.3 times smaller than that for monomeric DPA in the same solvent at the same temperature. The fact that the actual rate of triplet energy transfer between 3Ru(dmb)3 and DPA-polymer 4 is a factor of 2.2 × 3.3 = 7.3 times faster than this encounter rate then suggests that only a fraction of the 30 DPA units of the polymer are available to accept triplet energy per encounter. Thus the probability of intermolecular triplet energy transfer from 3Ru(dmb)3 to polymer 4, per encounter per DPA moiety, appears to be 7.3/30 or about 1/4 of that for a single unaggregated DPA molecule. Given that polymer 4 will be folded in solution so that its interior DPA species are not accessible to the donor at the short interaction distances needed for triplet energy transfer, this seems reasonable. We now consider the probability of triplet energy transfer between macromolecules. Carrying forward the same argument used above reveals that intermolecular triplet energy transfer from the excited triplet Ru(bpy)3 core of polymer 1 or 2 to the DPA moieties of a second macromolecule can not contribute significantly to the energy transfer processes in these polymer solutions. The weighted averages of the four component

lifetimes of phosphorescence of the Ru(bpy)3 core in polymers 1 and 2 are 99 and 33 ns, respectively.24 At the low concentrations of polymer used in these experiments (100 μs in degassed chloroform.24 The data of Figure 2 reveal that the majority of triplet DPA species produced in the steady-state experiments reported here are also being quenched by processes that are kinetically first or pseudofirst order16 at the absorbed power densities employed. Secondorder rate constants for homomolecular TTA in monomeric polynuclear aromatic molecules such as DPA in fluid solution at room temperature typically range from ca. 1 to 5 × 109 M−1 s−1.4,11 Assuming that spin statistical limitations are the same for DPA in the two polymers as they are for monomeric DPA and calculating interaction cross sections and relative diffusion coefficients (Supporting Information) as described above, a simple kinetic analysis thus shows that the photon upconversion observed in polymers 1 and 2 must be occurring 4115

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photoexcited polymer are decaying by first-order (or pseudofirst-order) kinetic processes. The measured upconversion quantum yields are also expected to be substantially lower than the initial yields of upconverted singlet states owing to competing intramolecular singlet energy transfer following TTA. That is, a large fraction of the emitting upconverted singlet states of the DPA formed by TTA in the bifunctional polymers will transfer their energy to the lower energy singlet MLCT state of the Ru(bpy)3 core in the same polymer molecule by a FRET mechanism. Thus, Ghiggino and coworkers24 showed that the intensities of DPA fluorescence of polymers 1 and 2 are 83% and 69% smaller respectively compared with the polymeric DPA reference, 4, when each is excited directly into its DPA S1−S0 absorption band under otherwise identical conditions. The initial quantum yields of upconverted DPA excited singlet states will therefore be a factor of about 7 or 8 larger than the values of Φ1 and Φ2, above, which are derived from measurement of the residual upconverted S1 emission intensities. Thin films of 1 and 2 were prepared by drop casting chloroform solutions of the polymers onto glass slides, evaporating the solvent in air, and then mounting the slides, sample-side in, on an evacuable holder for cw 532 nm excitation in the spectrofluorometer. Weak, but nevertheless measurable, upconverted fluorescence to the blue of the excitation wavelength was observed (Figure S4 in the Supporting Information), and is readily attributable to upconversion via TTA in the sensitized DPA moieties. The relative intensities of upconverted fluorescence observed in the liquid solution compared with the solid film, normalized to the same absorbance and correcting for spectral bandwidth differences, are approximately 1000:1. A large reduction in the upconversion efficiency is anticipated because intermolecular TTA due to molecular diffusion will be greatly restricted in the solid films and any intermolecular triplet exciton migration could also be restricted via trapping in the solid. Finally, we examine the suitability of photophysically active upconverting polymers for possible use in augmenting the efficiencies of OPVs. On the basis of the results presented above, two problems must be addressed. First, in order to ensure that fast intramolecular processes rather than slow intermolecular ones dominate the TTA mechanism, polymers must be constructed that contain a large number of absorbing chromophores as well as nonabsorbing upconverting moieties. Such an improvement will not only increase the molar absorptivity in usable regions of the solar spectrum but would also markedly increase the probability of multiple onephoton absorptions in the same polymer molecule. The data presented above suggest that about thirty chromophores per molecule, each with an oscillator strength of ca. 1, would be sufficient to swing the preponderance of TTA events from intermolecular to intramolecular with the polymers and average incident powers used here (which, although monochromatic, are in the range of 1 kW/m2 or one Sun). Second, the absorbing and upconverting segments of the polymer must be capable of rapid intramolecular triplet−triplet energy transfer and rapid triplet−triplet annihilation, but must not exhibit competitive rates of back singlet−singlet energy transfer such as observed in the two polymers investigated here. This requirement will be difficult, but not impossible, to manage. Electron transfer to a suitable acceptor from the upconverted singlet state must be considerably faster than its intramolecular

by intermolecular TTA at the absorbed power densities used in these experiments. This conclusion can be confirmed by calculating the probability that two-photon 532 nm excitation of a single macromolecule contributes to the NCPU intensity via intramolecular TTA. Using a maximum incident power density of 1 W cm−2 in a 2 mm diameter beam at a typical maximum absorbance of 0.15 at 532 nm provided by a 100 μM Ru(bpy)3 core concentration in a 1 cm cell leads to an estimate that polymers 1 and 2 will absorb a maximum of ca. 10 photons per second per macromolecule (cf. Supporting Information). Because the triplet DPA lifetimes in these solution measurements are at most several hundreds of microseconds,24 the probability of sequential two-photon excitation in the same Ru(bpy)3-core leading to unimolecular TTA is exceedingly small. The quantum yields of both Ru(bpy)3 phosphorescence and upconverted fluorescence of the two polymer samples excited cw at 532 nm were measured under the same conditions as those used for the spectroscopic and photophysical experiments described above using rhodamine 6G as a reference (R) (ΦR = 0.75 for R6G in chloroform34). Corrections for emission selfabsorption, variations in detector sensitivity with emission wavelength, and missing segments of the emission spectrum eliminated due to scatter and filter absorption are described in detail in the Supporting Information. The upconverted, corrected integrated emission intensity from the DPA in polymers 1 and 2 varies quadratically with absorbed power density, whereas the standard’s integrated emission intensity varies linearly. Thus, the quantum yield data for polymers 1 and 2 are given in the form Φx = bP, which are the equations of the best linear correlations of the two data sets in Figure 4, with P

Figure 4. Upconverted fluorescence quantum yield of polymers 1 (black) and 2 (red) as a function of power density. The correlations are force-fit linear regressions from the origin.

in mW cm−2, for the low power density regime. For polymer 1, Φ1 = (3.1 ± 0.1) × 10−5 |P| and for polymer 2, Φ2 = (3.9 ± 0.1) × 10−6 |P|. Note that polymer 2 has a quantum yield of upconverted emission that is a factor of about 8 smaller than that of polymer 1 at the same excitation power. This is consistent with the above interpretation of the relative accessibilities of the triplet DPA moieties in the two macromolecules; intermolecular TTA is expected to be less efficient in the polymer with the larger number of DPAcontaining arms. The upconversion fluorescence quantum yields are small, as expected when the upconversion process requires intermolecular diffusional interaction of a pair of triplet excited polymer molecules and the vast majority of DPA triplets in the 4116

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electronic energy transfer via any of the spin-allowed channels and would provide a means of utilizing the excitation energy.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental methods, details of quantum yield calculations, upconversion and phosphorescence emission spectra as a function of incident laser power, diffusion coefficients, calculation of maximum photon absorption rates, Stern− Volmer plots, upconversion spectra from thin films and the Gaussian modeling protocol, details of data analysis. This information is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada for continuing support of this research. K.P.G. thanks the Australian Research Council (Grants DP 0986166 and LE 0989197) for financial support. T.L.K. is a Canada Research Chair in Photovoltaics. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. We are grateful to Dr. Ming Chen, CSIRO, and Dr. Evan Moore, University of Queensland, for the provision of compounds; Dr. Keith Brown, Department of Chemistry, University of Saskatchewan, for assistance with the DOSY measurements, and Mr. Brook Danger and Ms. Chelsea Greenwald for assistance with the triplet lifetime measurements.

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REFERENCES

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