posted on 2021-03-26, 18:34authored byMichelle Chen, Adam F. Coleman, Ryan M. Young, Michael R. Wasielewski
Covalent chromophore dimers having
the required energetics can
undergo intramolecular singlet fission (SF) in solution; however,
in the solid state, intra- and intermolecular SF can compete. Here,
the structure and excited-state dynamics of a linear terrylene-3,4:11,12-bis(dicarboximide)
(TDI) dimer, TDI-Ph-TDI, in which the two TDI molecules
are linked via one of their imide nitrogen atoms to a 2,5-di-t-butylphenyl spacer at its 1,4-positions are studied in
solution and in thin films to understand the interplay between these
two SF mechanisms. TDI-Ph-TDI undergoes slow intramolecular
SF in toluene due to weak through-bond interactions and a lack of
through-space electronic coupling. The TDI-Ph-TDI dimers
in the films are π-stacked, allowing for through-space interactions
between neighboring TDI moieties. As a result, TDI-Ph-TDI undergoes intermolecular SF two orders of magnitude faster than
intramolecular SF. Using film-processing techniques, the SF dynamics
can be modified. The (T1T1) states in the unannealed
and thermally annealed films dissociate to form free triplet excitons,
whereas a long-lived (T1T1) state with mixed
charge-transfer character is observed in a chlorobenzene solvent vapor
annealed film. Although intramolecular SF in TDI-Ph-TDI cannot compete with intermolecular SF, the structure of TDI-Ph-TDI has a strong influence on the possible film morphologies and the
role of the charge-transfer state in SF.