Origin of Intersystem\nCrossing in Red-Light Absorbing\nBodipy Derivatives: Time-Resolved Transient Optical and Electron Paramagnetic\nResonance Spectral Studies with Twisted and Planar Compounds

We studied the intersystem crossing (ISC) property of\nred-light\nabsorbing heavy atom-free dihydronaphtho[<i>b</i>]-fused\nBodipy derivatives (with phenyl group attached at the lower rim via\nethylene bridge, taking constrained geometry, i.e., <b>BDP-1</b> and the half-oxidized product <b>BDP-2</b>) and dispiroflourene[<i>b</i>]-fused Bodipy (<b>BDP-3</b>) that have a twisted\nπ-conjugated framework. <b>BDP-1</b> and <b>BDP-3</b> show strong and sharp absorption bands (i.e., ε = 2.0 ×\n10<sup>5</sup> M<sup>–1</sup> cm<sup>–1</sup> at 639\nnm, fwhm ∼491 cm<sup>–1</sup> for <b>BDP-3</b>). <b>BDP-1</b> is significantly twisted (φ = 21.6°),\nwhile upon mono-oxidation, <b>BDP-2</b> becomes nearly planar\non the oxidized side (φ = 3.5°). Interestingly, <b>BDP-2</b> showed efficient ISC (triplet state quantum yield, Φ<sub>T</sub> = 40%) due to <i>S</i><sub>1</sub>/<i>T</i><sub>2</sub> state energy matching. Long-lived triplet excited state was\nobserved (τ<sub>T</sub> = 212 μs in solution and 2.4 ms\nin polymer matrix), and ISC takes 4.0 ns. Differently, twisted <b>BDP-1</b> gives weak ISC only 5%, ISC takes 7.7 ns, and the triplet\nstate is populated only with addition of ethyl iodide. Time-resolved\nelectron paramagnetic resonance spectra of <b>BDP-1</b> revealed\nthe coexistence of two triplet states, with drastically different\nzero-field splitting <i>D</i> parameters of −2047\nMHz and −1370 MHz, respectively, along with varying sublevel\npopulation ratios. We demonstrate that the ISC is not necessarily\nenhanced by torsion of the π-conjugation framework; instead, <i>S</i><sub>1</sub>/<i>T</i><sub><i>n</i></sub> state energy matching is more efficient to induce ISC even in compounds\nthat have planar molecular structures.