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

Abstract

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.; Möbius K. J. Phys. Chem. A 1997, 101, 5869).

Excited quartet and doublet states in the complex of tetraphenylporphine zinc (II) and a nitroxide radical in solution: X- and W-band time-resolved EPR studies

“…1. This complex is expected to have a larger exchange coupling J than the previously reported MgTPP-4-NITPy complex [8], because direct overlap between the electronic orbitals of the triplet porphyrin and the radical is more effective.…”

“…This might be explained by virtue of broadening effects, but also effects of spin polarization should be taken into account for a quantitative understanding. At any rate, the transition probability for the Q 1 transition is higher by a factor of 4 than that of the D i transition [3,4,8] (see below). Furthermore, the observed g-value of the D 1 state (2.0005) appears to be somewhat larger than that calculated from Eq.…”

“…These Q, and D, states stem from an intramolecular coupling of a stable radical such as nitroxide spin labels and an excited triplet state which is generated by optical excitation of a diamagnetic chromophore such as C60 [1][2][3][4], porphyrin [5][6][7][8], or phthalocyanine [9,10]. There are two interesting situations in these systems; one is an excited spin-multiplet state and the other a radical-triplet pair in photochemical reactions.…”

“…The experiments were carried out in the liquid and solid phases providing important information on the electronic interactions such as the exchange (J) and the magnetic dipolar couplings, and on the spin dynamics and kinetics. Recently, we have applied high-field W-band time-resolved EPR (TREPR) spectroscopy to study the Q, and D, states in liquid solution of meso-tetraphenylporphine magnesium (II) (MgTPP) and 4-(N- nitronyl-nitroxide) pyridine (4-NITPy) radical [6,8]. In the latest work [8], we reported the first observation of the D, signal in the radical-triplet pair system.…”

“…Recently, we have applied high-field W-band time-resolved EPR (TREPR) spectroscopy to study the Q, and D, states in liquid solution of meso-tetraphenylporphine magnesium (II) (MgTPP) and 4-(N- nitronyl-nitroxide) pyridine (4-NITPy) radical [6,8]. In the latest work [8], we reported the first observation of the D, signal in the radical-triplet pair system. Three kinds of EPR signals of the excited Q, and D 1 states and the doublet ground (Do) state were resolved, and approximate values for the g-factor and the signal linewidth were obtained for the D, state.…”