The rate of singlet fission is usually assumed to be optimized for a slip-stack molecular geometry. The inverse of fission is triplet-triplet annihilation, whose rate presumably would be optimized for the same geometry. In pyrene, excimer formation after creation of the singlet provides a way to probe the interaction geometry of the two pyrene triplets. Steady-state spectroscopy elucidates the kinetics of pyrene excimer formation/fluorescence and establishes a basis for the mechanistic analysis. Using tris(2-phenylpyridine) Iridium (III) as a triplet- sensitizer, direct excitation of pyrene’s singlet can be avoided, allowing us to isolate singlet formation due to TTA. We find that the up-conversion luminescence observed in pyrene is of mixed excimer/monomer character despite the orientation and distance threshold for TTA favoring exclusive excimer formation. Time-resolved photoluminescence still detects an appreciable amount monomer fluorescence following annihilation. Further analysis using the sensitivity of pyrene’s monomer band to the local polar environment provides insight on the local environment experienced by the pyrene singlet state after the TTA event. Our results suggest that it is possible that a long-range mechanism or non-slip-stack geometry plays a role in the TTA event.<br/>

Using pyrene excimer dynamics to probe the geometry of triplet-triplet annihilation

2019 Fall National Meeting

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