In singlet fission (SF) the initially formed correlated triplet pair state, 1(TT), may evolve toward independent triplet excitons or higher spin states of the (TT) species. The latter result is often considered undesirable from a light harvesting perspective but may be attractive for quantum information sciences (QIS) applications, as the final exciton pair can be spin-entangled and magnetically active with relatively long room temperature decoherence times. In this study we use ultrafast transient absorption (TA) and time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to monitor SF and triplet pair evolution in a series of alkyl silyl-functionalized pentadithiophene (PDT) thin films designed with systematically varying pairwise and long-range molecular interactions between PDT chromophores. The lifetime of the (TT) species varies from 40 ns to 1.5 μs, the latter of which is associated with extremely weak intermolecular coupling, sharp optical spectroscopic features, and complex TR-EPR spectra that are composed of a mixture of triplet and quintet-like features. On the other hand, more tightly coupled films produce broader transient optical spectra but simpler TR-EPR spectra consistent with significant population in 5(TT)0. These distinctions are rationalized through the role of exciton diffusion and predictions of TT state mixing with low exchange coupling J versus pure spin substate population with larger J. The connection between population evolution using electronic and spin spectroscopies enables assignments that provide a more detailed picture of triplet pair evolution than previously presented and provides critical guidance for designing molecular QIS systems based on light-induced spin coherence.
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This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the U.S. Department of Energy, Office of Basic Energy Sciences (ERW7404) for spectroscopy, film fabrication and characterization at NREL and for pentadithiophene synthesis at the University of Kentucky. Single-crystal structure determination was supported by NSF-MRI (CHE-1625732). K. J. T., S. P, and J. E. A. thank the National Science Foundation (DMREF 1627428) for support of development of synthetic routes for pentadithiophene derivatives.
Electronic supplementary information (ESI) available: Crystal structure information, additional transient spectroscopic data, global analysis, film X-ray diffraction, temperature and power dependence, raw TR-EPR spectra, calculation details, synthesis details. CCDC 2000560 and 2000561.
Pace, Natalie A.; Rugg, Brandon K.; Chang, Christopher H.; Reid, Obadiah G.; Thorley, Karl J.; Parkin, Sean; Anthony, John E.; and Johnson, Justin C., "Conversion between Triplet Pair States Is Controlled by Molecular Coupling in Pentadithiophene Thin Films" (2020). Chemistry Faculty Publications. 182.