  Intramolecular singlet fission (iSF) materials provide remarkable advantages
in terms of tunable electronic structures, and quantum chemistry studies have
indicated strong electronic coupling modulation by high frequency phonon modes.
In this work, we formulate a microscopic model of iSF with simultaneous
diagonal and off-diagonal coupling to high-frequency modes. A non-perturbative
treatment, the Dirac-Frenkel time-dependent variational approach is adopted
using the multiple Davydov trial states. It is shown that both diagonal and
off-diagonal coupling can aid efficient singlet fission if excitonic coupling
is weak, and fission is only facilitated by diagonal coupling if excitonic
coupling is strong. In the presence of off-diagonal coupling, it is found that
high frequency modes create additional fission channels for rapid iSF. Results
presented here may help provide guiding principles for design of efficient
singlet fission materials by directly tuning singlet-triplet interstate
coupling.
