Heavy Atom at Bay of Perylene Significantly Improves Intersystem Crossing

We studied the photophysical properties of substituted perylenes using time-dependent density functional theory (TDDFT) with Tamm-Dancoff Approximation (TDA). The TDA-TDDFT method allowed us to examine how luminescence activity alters by substituting halogens at different positions (bay, ortho, and peri) of perylenes. Substituting larger halogens like chlorine and bromine at the bay position significantly affects the planarity of the π-system in perylenes. Interestingly, bay-bromoperylene (P-bBr) showed pronounced spin-orbit coupling (SOC) between singlet and triplet excited states. The heavy atom effect (HAE) functioned efficiently with a distorted π-system and substantially enhanced the SOC in P-bBr. Therefore, a rapid intersystem crossing (ISC) is responsible for turning off the fluorescence of P-bBr. In contrast, bromine substitution other than the bay position (i.e., ortho- and peri-bromoperylenes (P-oBr and P-pBr), which maintained planarity), or substituting lighter elements like a methyl group (similar in size to Br) at the bay position of perylene did not substantially improve the SOC. Thus, the ISC is insufficient to quench the fluorescence in these systems. Additionally, substituting multiple bromines in perylene with at least one in the bay position (i.e., P-boBr₂, P-bpBr₂, and P-bopBr₃) further improved the SOC, leading to much faster ISC (10¹¹ s^-1) in P-bopBr₃. While multiple bromine substitutions other than the bay position (i.e., P-opBr₂) exhibited low ISC due to the planar π-system. So, the heavy bromine at the bay position of perylene causes significant enhancement of the ISC.