Time-Resolved Electron Spin Resonance Spectroscopy of Photoredox Reactions of Polyoxometalates: Formation of Paramagnetic-Species Pair by Triplet Mechanism

Abstract

Continuous-wave time-resolved electron spin resonance (TRESR) spectroscopy has been successfully employed to elucidate photochemical reaction mechanisms, since chemically induced dynamic electron polarization (CIDEP) spectra observed give information about the spin dynamics of short-lived intermediates such as radical pairs (RPs) and free radicals (FRs) [1, 2, 3]. CIDEP spectra were usually interpreted by two main mechanisms: triplet mechanism (TM) and radical pair mechanism (RPM). In the TM, electron spin polarization (ESP), which existed in the excited triplet state, is transferred to each of the radicals created on its reaction. The intersystem crossing (ISC) process is usually spin-selective and produces the excited triplet state with a non-equilibrated population in the spin sublevels, spin polarized triplet state. When a reaction occurs from such a polarized triplet state before the relaxation, spin-polarized radicals are produced, generating net polarized CIDEP spectra. In this paper we describe CIDEP phenomena for the photoredox reaction between the polyoxometalate (POM) and both electron- and proton-donor (DH), which allows us to detect large emissive ESP of deproto- nated one-electron oxidized species (D•) generated by the electron transfer (with an accompanying transfer of proton) from DH to the oxygen-to-metal charge transfer (O→M LMCT) excited triplet states of POM (3(O→M LMCT)), and to investigate the primary processes of the solution chemistry of POM [4]. If the photoredox reaction between POM and DH occurs rapidly via3(O→M LMCT), ESP in the 3(O→M LMCT) can be expected to be transferred to D• and /or POM- H (one-electron reduced protonated species of POM) to give rise to CIDEP To be observed, the production of D• must rapidly take place before thermal equilibrium of the triplet spin-lattice relaxation of3(O→M LMCT) has been established. The TM seems to be common in the photoredox reactions of POMs which produce emissive ESP in elementary steps.