First Observation of the Excited Doublet State of a Radical−Triplet Pair in Solution: W-Band High-Field Time-Resolved Electron Paramagnetic Resonance Spectroscopy

Using W-band time-resolved EPR (TREPR) (95 GHz), the lowest excited doublet (D1) state was observed in solution for the first time together with the excited quartet (Q1) and doublet ground (D0) states in a complex of MgTPP (TPP; tetraphenylporphyrin) and an axial-ligating p-pyridylnitronylnitroxide radical (nit-p-py). The g value obtained for D1 was in good agreement with the calculated value. The spin polarization of D1 varied with time from an emission to an absorption as well as that of Q1 and D0. The result shows that the spin polarizations are produced by a radical−triplet pair mechanism with a ferromagnetic coupling (J > 0). The sign of the exchange coupling is consistent with that obtained in the X-band TREPR experiment (Fujisawa, J.; Ishii, K.; Ohba, Y.; Yamauchi, S.; Fuhs, M.; Mobius K. J. Phys. Chem. A 1997, 101, 5869).