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 (<i>bay</i>, <i>ortho</i>, and <i>peri</i>) of perylenes. Substituting larger halogens like chlorine and bromine at the <i>bay</i> position significantly affects the planarity of the π-system in perylenes. Interestingly, <i>bay</i>-bromoperylene (P-<i>b</i>Br) 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-<i>b</i>Br. Therefore, a rapid intersystem crossing (ISC) is responsible for turning off the fluorescence of P-<i>b</i>Br. In contrast, bromine substitution other than the <i>bay</i> position (i.e., <i>ortho</i>- and <i>peri</i>-bromoperylenes (P-<i>o</i>Br and P-<i>p</i>Br), which maintained planarity), or substituting lighter elements like a methyl group (similar in size to Br) at the <i>bay</i> 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 <i>bay</i> position (i.e., P-<i>bo</i>Br₂, P-<i>bp</i>Br₂, and P-<i>bop</i>Br₃) further improved the SOC, leading to much faster ISC (10¹¹ s^-1) in P-<i>bop</i>Br₃. While multiple bromine substitutions other than the <i>bay</i> position (i.e., P-<i>op</i>Br₂) exhibited low ISC due to the planar π-system. So, the heavy bromine at the <i>bay</i> position of perylene causes significant enhancement of the ISC.