Dissecting the Effect of Morphology on the Rates of Singlet Fission: Insights from Theory

The effect of morphology on singlet fission (SF) efficiency was investigated by using a combination of high-level electronic structure methods and a simple three-state kinetic model. The calculations reproduce the observed differences in SF efficiency in different polymorphs of 1,3-diphenylisobenzofuran (DPBF) and 1,6-diphenyl-1,3,5-hexatriene (DPH), as well as make predictions about 5,12-diphenyltetracene (DPT). The analysis of different factors contributing to the rates reveals that (i) there is more than one pair of adjacent chromophores that contribute to SF; (ii) not only slip-stacked configurations show efficient fission; and (iii) both electronic couplings and energy differences are responsible for different rates. The model predicts that the difference in SF efficiency in DPBF and DPH polymorphs increases at low temperature. In contrast, temperature dependence of the relative rates in the two DPT forms is predicted to be small. Our model predicts similar rates for the two polymorphs of DPT, although one form features much more favorable electronic couplings. This prediction depends strongly on the magnitude of Davydov's splitting; small changes in its value may change the ratio in favor of faster SF in xylene-grown crystals of DPT.