Tuning Singlet Fission in π‑Bridge‑π Chromophores

We
have designed a series of pentacene dimers separated by homoconjugated
or nonconjugated bridges that exhibit fast and efficient intramolecular
singlet exciton fission (iSF). These materials are distinctive among
reported iSF compounds because they exist in the unexplored regime
of close spatial proximity but weak electronic coupling between the
singlet exciton and triplet pair states. Using transient absorption
spectroscopy to investigate photophysics in these molecules, we find
that homoconjugated dimers display desirable excited-state dynamics,
with significantly reduced recombination rates as compared to conjugated
dimers with similar singlet fission rates. In addition, unlike conjugated
dimers, the time constants for singlet fission are relatively insensitive
to the interplanar angle between chromophores, since rotation about
σ
bonds negligibly affects the orbital overlap within the π-bonding
network. In the nonconjugated dimer, where the iSF occurs with a time
constant >10 ns, comparable to the fluorescence lifetime, we used
electron spin resonance spectroscopy to unequivocally establish the
formation of triplet–triplet multiexcitons and uncoupled triplet
excitons through singlet fission. Together, these studies enable us
to articulate the role of the conjugation motif in iSF.