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.
Triplet Excited State Mechanistic Study of meso-Substituted Methylthio Bodipy Derivative: Time-Resolved Optical and Electron Paramagnetic Resonance Spectral Studies
Taddei M.;Mazzone G.
2024-01-01
Abstract
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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
