Design, synthesis, and uniquely electron-spin-polarized quartet photo-excited state of a π-conjugated spin system generated via the ion-pair state

Abstract

A functionalized verdazyl radical, 1, was synthesized, which consists of a bodipy acceptor (A), a phenyl-anthracene donor (D), and a stable verdazyl radical (R). Optical measurements, electron spin resonance (ESR), time-resolved ESR (TRESR), and laser-excitation pulsed ESR experiments were carried out. The efficient intra-molecular energy transfer (EnT) from the anthracene moiety to the bodipy functional component was observed by time-resolved fluorescence spectroscopy and TRESR measurements. A unique dynamic electron-spin polarization (DEP) was detected for the quartet photo-excited state of 1 by TRESR and pulsed ESR. Such a unique DEP was not detected for either the parent verdazyl radical2 without the bodipy acceptor or new compound 3 with an anthraquinone moiety instead of the bodipy component. The spectral simulation of the quartet spectrum of 1 revealed that the unique DEP was generated by competition between a mechanism involving the intra-molecular ion pair *A⁻–D⁺–R and spin–orbit intersystem crossing. The dynamic electron-spin polarization via the ion-pair state and the mechanism were discussed based on the experimental data and theoretical calculations. The electronic structure and the spin delocalization in the photo-excited quartet state of 1 were discussed by comparison with the results of 3.