Singlet Fission via an Excimer-Like Intermediate in 3,6-Bis(thiophen-2-yl)diketopyrrolopyrrole Derivatives

Singlet fission (SF)
in polycrystalline thin films of four 3,6-bis­(thiophen-2-yl)­diketopyrrolopyrrole
(TDPP) chromophores with methyl (Me), n-hexyl (C6), triethylene glycol (TEG),
and 2-ethylhexyl (EH) substituents at the 2,5-positions
is found to involve an intermediate excimer-like state. The four different
substituents yield four distinct intermolecular packing geometries,
resulting in variable intermolecular charge transfer (CT) interactions
in the solid. SF from the excimer state of Me, C6, TEG, and EH takes place in τSF = 22, 336, 195, and 1200 ps, respectively, to give triplet
yields of 200%, 110%, 110%, and 70%, respectively. The transient spectra
of the excimer-like state and its energetic proximity to the lowest
excited singlet state in these derivatives suggests that this state
may be the multiexciton 1(T1T1) state
that precedes formation of the uncorrelated triplet excitons. The
excimer decay rates correlate well with the SF efficiencies and the
degree of intermolecular donor–acceptor interactions resulting
from π-stacking of the thiophene donor of one molecule with
the DPP core acceptor in another molecule as observed in the crystal
structures. Such interactions are found to also increase with the
SF coupling energies, as calculated for each derivative. These structural
and spectroscopic studies afford a better understanding of the electronic
interactions that enhance SF in chromophores having strong intra-
and intermolecular CT character.