Origin of Intersystem Crossing in Red-Light Absorbing Bodipy Derivatives: Time-Resolved Transient Optical and Electron Paramagnetic Resonance Spectral Studies with Twisted and Planar Compounds

We studied the intersystem crossing (ISC) property of red-light absorbing heavy atom-free dihydronaphtho[<i>b</i>]-fused Bodipy derivatives (with phenyl group attached at the lower rim via ethylene 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 π-conjugated framework. <b>BDP-1</b> and <b>BDP-3</b> show strong and sharp absorption bands (i.e., ε = 2.0 × 10⁵ M^-1 cm^-1 at 639 nm, fwhm ∼491 cm^-1 for <b>BDP-3</b>). <b>BDP-1</b> is significantly twisted (φ = 21.6°), while upon mono-oxidation, <b>BDP-2</b> becomes nearly planar on the oxidized side (φ = 3.5°). Interestingly, <b>BDP-2</b> showed efficient ISC (triplet state quantum yield, Φ_T = 40%) due to <i>S</i>₁/<i>T</i>₂ state energy matching. Long-lived triplet excited state was observed (τ_T = 212 μs in solution and 2.4 ms in 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 state is populated only with addition of ethyl iodide. Time-resolved electron paramagnetic resonance spectra of <b>BDP-1</b> revealed the coexistence of two triplet states, with drastically different zero-field splitting <i>D</i> parameters of -2047 MHz and -1370 MHz, respectively, along with varying sublevel population ratios. We demonstrate that the ISC is not necessarily enhanced by torsion of the π-conjugation framework; instead, <i>S</i>₁/<i>T</i>_<i>n</i> state energy matching is more efficient to induce ISC even in compounds that have planar molecular structures.