Intramolecular singlet fission (iSF) materials provide remarkable advantagesin terms of tunable electronic structures, and quantum chemistry studies haveindicated strong electronic coupling modulation by high frequency phonon modes.In this work, we formulate a microscopic model of iSF with simultaneousdiagonal and off-diagonal coupling to high-frequency modes. A non-perturbativetreatment, the Dirac-Frenkel time-dependent variational approach is adoptedusing the multiple Davydov trial states. It is shown that both diagonal andoff-diagonal coupling can aid efficient singlet fission if excitonic couplingis weak, and fission is only facilitated by diagonal coupling if excitoniccoupling is strong. In the presence of off-diagonal coupling, it is found thathigh frequency modes create additional fission channels for rapid iSF. Resultspresented here may help provide guiding principles for design of efficientsinglet fission materials by directly tuning singlet-triplet interstatecoupling.
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