X-BAND 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 system s of metalloporphyrins, MgTPP, ZnTPP, and ZnOEP, and pyridine-substitu ted 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 re sonance (TREPR) in solution. Axial-ligations between the porphyrins an d the radicals were ascertained from visible absorption spectra except for the nit-o-py system. The TREPR spectra were composed of two signa ls, which were assigned to those of the ground (D-0) state of the radi cal and the excited quartet (Q(1)) state of the RTP. These components showed two kinds of CIDEPs in different time regions. The polarization s of the Q(1) state were attributed to radical-triplet pair mechanisms (RTPMs) with singlet and tripler 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 Q(1) state and the RTPM. The CIDEPs observed for the nit-o-py system were interpreted by ESPT and RTPM with the excited triplet (T-1) porphyrin. From the a nalysis of the RTPM polarizations, an exchange interaction between the T-1 porphyrin and the radical was found to be ferromagnetic for the n it-p-py system and antiferromagnetic for the nit-m-py and the nit-o-py systems.