Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet–triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields.
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G. D. S., Y.-L. L., R. D. P. and G. E. P acknowledge funding from the Princeton Center for Complex Materials, a MRSEC supported by NSF Grant DMR 1420541. G. D. S. acknowledges partial support for this work from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant No. DE-SC0015429. Y.-L. L. and G. E. P. acknowledge additional support through NSF Grant DMR 1627453. Portions of this work were conducted at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the NSF and NIH/NIGMS via NSF award DMR-1332208. J. E. A. and D. B. G. acknowledge funding from the National Science Foundation (CHE-1609974). D. S. S. acknowledges financial support from the National Sciences and Engineering Council of Canada (NSERC) through Discovery Grant RGPAS 477794-2015; D. G. O. acknowledges support from the Postgraduate Scholarships Doctoral Program of NSERC. C. G., G. S. D., and J. B. A. are grateful for support from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-SC0008120.
Electronic supplementary information (ESI) available: Additional steady-state absorption spectra, sample structural characterization, and nanosecond and femtosecond transient absorption spectra and associated modelling details. See DOI: 10.1039/c8sc00293b
Pensack, Ryan D.; Tilley, Andrew J.; Grieco, Christopher; Purdum, Geoffrey E.; Ostroumov, Evgeny E.; Granger, Devin B.; Oblinsky, Daniel G.; Dean, Jacob C.; Doucette, Grayson S.; Asbury, John B.; Loo, Yueh-Lin; Seferos, Dwight S.; Anthony, John E.; and Scholes, Gregory D., "Striking the Right Balance of Intermolecular Coupling for High-Efficiency Singlet Fission" (2018). Chemistry Faculty Publications. 121.