Unraveling the mechanisms of triplet state formation in a heavy-atom free photosensitizer

Kavli Affiliate: David T. Limmer

| First 5 Authors: Thomas P. Fay, David T. Limmer, , ,

| Summary:

Triplet excited state generation plays a pivotal role in photosensitizers,
however the reliance on transition metals and heavy atoms can limit the utility
of these systems. In this study, we demonstrate that an interplay of competing
quantum effects control the high triplet quantum yield in a prototypical boron
dipyrromethene-anthracene (BD-An) donor-acceptor dyad photosensitizer, which is
only captured by an accurate treatment of both inner and outer sphere
reorganization energies. Our fully textit{ab initio} derived model provides
excellent agreement with experimentally measured spectra, triplet yields and
excited state kinetic data, including the triplet lifetime. We find that both
spin-orbit coupled charge transfer and El-Sayed’s rule breaking intersystem
crossing facilitate rapid triplet state formation. Our calculations also reveal
that competing effects of nuclear tunneling, electronic state recrossing, and
solvent polarizability dictate the rate of non-productive ground state
recombination. This study sheds light on the quantum effects driving efficient
triplet formation in the BD-An system, and offers a promising simulation
methodology for diverse photochemical systems.

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