X- and W-Band Time-Resolved Electron Paramagnetic Resonance Studies on Radical-Excited Triplet Pairs between Metalloporphyrins and Axial-Ligating Nitroxide Radicals

Abstract

A chemically induced dynamic electron spin polarization (CIDEP) study has been accomplished on radical-excited triplet pairs (RTP) in systems of metalloporphyrins, MgTPP, ZnTPP, and ZnOEP, and pyridine-substituted nitronyl nitroxide radicals, nit-R (R = o-py, m-py, and p-py), by X- (9.5 GHz) and W-band (95 GHz) time-resolved electron paramagnetic resonance (TREPR) in solution. Axial-ligations between the porphyrins and the radicals were ascertained from visible absorption spectra except for the nit-o-py system. The TREPR spectra were composed of two signals, which were assigned to those of the ground (D0) state of the radical and the excited quartet (Q1) state of the RTP. These components showed two kinds of CIDEPs in different time regions. The polarizations of the Q1 state were attributed to radical−triplet pair mechanisms (RTPMs) with singlet and triplet precursors. In the nit-p-py and the nit-m-py systems, the polarizations of the radical were generated via an electron spin polarization transfer (ESPT) from the Q1 state and the RTPM. The CIDEPs observed for the nit-o-py system were interpreted by ESPT and RTPM with the excited triplet (T1) porphyrin. From the analysis of the RTPM polarizations, an exchange interaction between the T1 porphyrin and the radical was found to be ferromagnetic for the nit-p-py system and antiferromagnetic for the nit-m-py and the nit-o-py systems.