Heavy Atom at Bay of Perylene Significantly Improves\nIntersystem Crossing

We\nstudied the photophysical properties of substituted perylenes\nusing time-dependent density functional theory (TDDFT) with Tamm–Dancoff\nApproximation (TDA). The TDA-TDDFT method allowed us to examine how\nluminescence activity alters by substituting halogens at different\npositions (<i>bay</i>, <i>ortho</i>, and <i>peri</i>) of perylenes. Substituting larger halogens like chlorine\nand bromine at the <i>bay</i> position significantly affects\nthe planarity of the π-system in perylenes. Interestingly, <i>bay</i>-bromoperylene (P-<i>b</i>Br) showed pronounced\nspin–orbit coupling (SOC) between singlet and triplet excited\nstates. The heavy atom effect (HAE) functioned efficiently with a\ndistorted π-system and substantially enhanced the SOC in P-<i>b</i>Br. Therefore, a rapid intersystem crossing (ISC) is responsible\nfor turning off the fluorescence of P-<i>b</i>Br. In contrast,\nbromine substitution other than the <i>bay</i> position\n(i.e., <i>ortho</i>- and <i>peri</i>-bromoperylenes\n(P-<i>o</i>Br and P-<i>p</i>Br), which maintained\nplanarity), or substituting lighter elements like a methyl group (similar\nin size to Br) at the <i>bay</i> position of perylene did\nnot substantially improve the SOC. Thus, the ISC is insufficient to\nquench the fluorescence in these systems. Additionally, substituting\nmultiple bromines in perylene with at least one in the <i>bay</i> position (i.e., P-<i>bo</i>Br<sub>2</sub>, P-<i>bp</i>Br<sub>2</sub>, and P-<i>bop</i>Br<sub>3</sub>) further\nimproved the SOC, leading to much faster ISC (10<sup>11</sup> s<sup>–1</sup>) in P-<i>bop</i>Br<sub>3</sub>. While multiple\nbromine substitutions other than the <i>bay</i> position\n(i.e., P-<i>op</i>Br<sub>2</sub>) exhibited low ISC due\nto the planar π-system. So, the heavy bromine at the <i>bay</i> position of perylene causes significant enhancement\nof the ISC.