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.