Dynamic cooperativity of chromophores in promoting singlet fission in perylenediimide crystals

Abstract

Structure dynamics of crystals affects the initial configurations and electronic properties of chromophores upon photoexcitation for singlet fission (SF), but how such structure dynamics including monomer vibrations and surrounding structure fluctuations governs the SF energetics and possibility in the chromophore crystals remains elusive. Here, we explore the cooperativity of proximate monomers in promoting SF under the influence of structure dynamics in perylenediimide (PDI) crystals using combined ab initio molecular dynamics simulations and time-dependent density functional theory calculations. Although the PDI chromophore does not meet the SF energetic requirements in its static structure, our simulations reveal that structure dynamics fluctuations can markedly adjust the structures and energies of all monomers and make two arbitrary proximate chromophore monomers to cooperatively find opportunities to meet the SF energetic requirements transiently, exhibiting high cooperativity in fulfilling SF. The cooperativity of two arbitrary proximate monomers mainly originates from relative lowering of the triplet excited energy and HOMO–LUMO energy gap of a monomer under transient distortion compared with those of the other. Structurally, two chromophores with a cofacial slip-stacked arrangement have stronger dynamic electronic coupling than those with the head-to-head parallel contacted and tilted face-to-edge stacked structures, and thus exhibit more favorable SF conditions. Clearly, multiple chromophores in their local structure can cooperate with each other in a quasi-dimer mode to realize SF in the PDI crystal, exhibiting anisotropic SF channels featuring different probabilities and efficiencies. Our findings not only reveal the dynamic cooperative effect of the correlated chromophores on the SF energetics but also provide a strategy for constructing SF crystal materials with high SF efficiency by considering the structure fluctuation dynamics of crystals.