Core-Twist Reduces the Triplet Formation Efficiency in Brominated Perylene Diimides.

Bromination is a recent approach to achieve intersystem crossing (ISC) in perylene diimides (PDIs). Herein, we explore the triplet formation dynamics in two tetrabrominated PDI (PDI-Br4) positional isomers with planar (P-PDI) and twisted (T-PDI) π-conjugated frameworks. In contrast to the known effect where the planar geometry favors fluorescence, T-PDI shows higher fluorescence (ϕf = 0.64) than the planar counterpart P-PDI (ϕf = 0.42). P-PDI possesses near-degenerate S1 and T3/T4 states and a larger spin-orbit coupling (SOC). Core-twisting has a pronounced effect on the absorption spectra due to symmetry breaking and would open up multiple ISC pathways, albeit with a lower SOC. Low-energy singlet-triplet state crossings within the Franck-Condon region would facilitate ultrafast triplet generation via the S1-T3/T4 ISC pathway in P-PDI. In comparison, such crossings occur at relatively higher energy, reducing the triplet formation efficiency in T-PDI.