Switching between Coherent and Incoherent Singlet Fission via Solvent-Induced Symmetry Breaking

Singlet fission in
organic semiconductors causes a singlet exciton
to decay into a pair of triplet excitons and holds potential for increasing
the efficiency of photovoltaic devices. In this combined experimental
and theoretical study, we reveal that a covalent dimer of the organic
semiconductor tetracene undergoes activated singlet fission by qualitatively
different mechanisms depending on the solvent environment. We show
that intramolecular vibrations are an integral part of this mechanism,
giving rise to mixing between charge transfer and triplet pair excitations.
Either coherent or incoherent singlet fission can occur, depending
on the transient solvent-induced energetic proximity between the states,
giving rise to complex variation of the singlet fission mechanism
and time scale in the different environments. Our results suggest
a more general principle for controlling the efficiency of photochemical
reactions by utilizing transient interactions to tune the energetics
of reactant and product states and switch between incoherent and coherent
dynamics.