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[]-fused Bodipy derivatives (with phenyl group attached at the lower rim via ethylene bridge, taking constrained geometry, i.e., and the half-oxidized product ) and dispiroflourene[]-fused Bodipy () that have a twisted π-conjugated framework. and show strong and sharp absorption bands (i.e., ε = 2.0 × 10 M cm at 639 nm, fwhm ∼491 cm for ). is significantly twisted (φ = 21.6°), while upon mono-oxidation, becomes nearly planar on the oxidized side (φ = 3.5°). Interestingly, showed efficient ISC (triplet state quantum yield, Φ = 40%) due to / state energy matching. Long-lived triplet excited state was observed (τ = 212 μs in solution and 2.4 ms in polymer matrix), and ISC takes 4.0 ns. Differently, twisted 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 revealed the coexistence of two triplet states, with drastically different zero-field splitting 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, / state energy matching is more efficient to induce ISC even in compounds that have planar molecular structures.