Intersystem crossing and spin dynamics of thionated perinones: combined steady state/transient optical and electron paramagnetic resonance spectral analysis

Abstract

We studied the intersystem crossing (ISC) and triplet states of thionated perinones to assess how the thionation of the carbonyl group in a chromophore affects these processes or properties. The thionated compounds show red-shifted absorption and strongly quenched fluorescence compared to the oxo-counterparts. Femtosecond transient absorption spectra show fast ISC kinetics (0.31−1.92 ps⁻¹) for the thionated chromophores. Nanosecond transient absorption spectra demonstrated short triplet excited state lifetimes (0.62−11.5 μs) for the thionated perinones, which is unusual for heavy atom-free small organic molecules. Time-resolved electron paramagnetic resonance spectroscopy following photoexcitation shows inversion of the electron spin polarization pattern of the triplet state, which is resulted from the overpopulation of one sublevel and its fast decaying with respect to the other two sublevels of the T₁ state. Spectral simulation shows that the Tz sublevel decays 25-time faster (1.0 × 10⁴ s⁻¹) than the Tx and Ty sublevels (ca. 4.0 × 10² s⁻¹). Interestingly, zero field splitting (ZFS) D parameters (ca. 0.268 cm⁻¹) was observed for S3, which is unusually large for an organic molecule. These results demonstrated the relationship between the thionation and the enhanced ISC, the triplet state lifetimes, as well as the triplet-state electron spin dynamics, it is useful for the development of heavy-atom-free triplet photosensitizers.