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Core-Twist Reduces the Triplet Formation Efficiency in Brominated Perylene Diimides
Sai Vamsi Krishna Isukapalli1, Sivaranjana Reddy Vennapusa1
1School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Maruthamala PO, Vithura, Thiruvananthapuram695551, India.
Abstract:
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.
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