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 (ϕ_f = 0.64) than the planar counterpart P-PDI (ϕ_f = 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.