Singlet Fission in Quaterrylenediimide Thin Films

Singlet
fission (SF) creates two triplet excitons following absorption
of a photon by two electronically interacting chromophores. Quaterrylene-3,4:13,14-bis­(dicarboximide)
(QDI) is a strongly absorbing chromophore that readily fulfills the
energetic requirements for SF, E(S1) >
2E(T1), and thus should undergo rapid
and efficient SF. SF was studied in thin films of the QDI derivative N,N-bis­(2,6-diisopropylphenyl)-QDI (ArQDI), which undergoes SF in <300 fs to form the correlated
triplet pair state, 1(T1T1), which
dissociates with a (7.3 ± 1.2 ns)−1 rate constant.
The observed triplet yield for a thin film that has been solvent-vapor
annealed with CH2Cl2 is 135 ± 20% instead
of 200%, which is typically expected of chromophores that undergo
ultrafast formation of the 1(T1T1) state. The lower SF yield in ArQDI results from the
failure of the 1(T1T1) state to dissociate
before returning to the ground state. In contrast to other molecules,
like hexacene, which have low triplet energies, the SF rate in ArQDI is not limited by a multiphonon relaxation bottleneck,
largely due to the fact that the S–T energy gap in the film
is substantially smaller than that measured for monomeric ArQDI. The ability to maintain a favorable S–T energy gap in a
film is a design consideration when chromophores are considered for
use to enhance solar cell performance.