Interest in materials
that undergo singlet fission (SF) has been
catalyzed by the potential to exceed the Shockley–Queisser
limit of solar power conversion efficiency. In conventional materials,
the mechanism of SF is an intermolecular process (xSF), which is mediated
by charge transfer (CT) states and depends sensitively on crystal
packing or molecular collisions. In contrast, recently reported covalently
coupled pentacenes yield ∼2 triplets per photon absorbed in
individual molecules: the hallmark of intramolecular singlet fission
(iSF). However, the mechanism of iSF is unclear. Here, using multireference
electronic structure calculations and transient absorption spectroscopy,
we establish that iSF can occur via a direct coupling mechanism that
is independent of CT states. We show that a near-degeneracy in electronic
state energies induced by vibronic coupling to intramolecular modes
of the covalent dimer allows for strong mixing between the correlated
triplet pair state and the local excitonic state, despite weak direct
coupling
