Quantum
Chain Reaction of Tethered Diarylcyclopropenones
in the Solid State and Their Distance-Dependence in Solution Reveal
a Dexter S<sub>2</sub>–S<sub>2</sub> Energy-Transfer Mechanism
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Abstract
When
promoted to their second singlet excited state (S<sub>2</sub>) in
benzene, alkyl-linked dimers of diarylcyclopropenone undergo
a photodecarbonylation reaction with quantum yields varying from Φ
= 0.7 to 1.14. Quantum yields greater than 1.0 in solution rely on
an adiabatic reaction along the S<sub>2</sub> energy surface where
the immediately formed excited-state product transfers energy to the
unreacted molecule in the dimer to generate a second excited state.
By determination of the quantum yields of decarbonylation for the
linked diarylcyclopropenones with linkers of various lengths it was
shown that S<sub>2</sub> → S<sub>2</sub> energy transfer is
limited to distances shorter than ca. 6 Å. Notably, the quantum
chain reaction occurs with similar efficiency for all the linked diarylcyclopropenones
dimers in the solid state