X- and W-band time-resolved electron paramagnetic resonance studies on radical-excited triplet pairs between metalloporphyrins and axial-ligating nitroxide radicals

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 pyridinesubstituted 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.