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 (D 1 ) state was observed in solution for the first time together with the excited quartet (Q 1 ) and doublet ground (D 0 ) states in a complex of MgTPP (TPP; tetraphenylporphyrin) and an axial-ligating p -pyridylnitronylnitroxide radical (nit- p -py). The g value obtained for D 1 was in good agreement with the calculated value. The spin polarization of D 1 varied with time from an emission to an absorption as well as that of Q 1 and D 0 . 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.; Möbius K. J. Phys. Chem. A 1997, 101, 5869).