Triplet Excited State Mechanistic Study of meso‐Substituted Methylthio Bodipy Derivative: Time‐Resolved Optical and Electron Paramagnetic Resonance Spectral Studies

Understanding the intersystem crossing (ISC) mechanism of organic compounds is essential for designing new triplet photosensitizers. Herein, we investigated the ISC mechanism of a heavy atom-free Bodipy derivative with thiomethyl substitution (S−BDP). A long-lived triplet state was observed with nanosecond transient absorption spectroscopy with lifetime of 7.5 ms in a polymer film and 178 μs intrinsic lifetime in fluid solution, much longer as compared with what was previously reported (apparent triplet lifetime=15.5 μs). Femtosecond transient absorption studies retrieved an ISC time constant of ∼3 ns. Time-resolved electron paramagnetic resonance (TREPR) indicated a special triplet electron spin polarization phase (ESP) pattern (a, e, a, e, a, e) for S−BDP, different from the ESP (e, e, e, a, a, a) typical for the spin-orbital coupling (SOC) mechanism. This indicates that the electron spin selectivity of the ISC of S−BDP is different from that of the normal SOC effect in iodo-Bodipy. Simulations of the TREPR spectra give a zero-field-splitting D parameter of −2257 MHz, much smaller as compared to the reference 2,6-diiodo-Bodipy (D=−4380 MHz). The computed SOC matrix elements (0.28–1.59 cm−1) and energy gaps for the S1/Tn states suggest that the energy matching between the S1 and T2/T3 states (supported by the largest kISC ∼109 s−1) enhances the ISC for this compound.