Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (nu similar to 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T(1))(3) pi pi* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g (z) = 2.0048, g (x) = g (y) = 2.0035 for Pt(b-iq)(CN)(2) (b-iq = 3,3'bi-isoquinoline) and g (z) = 1.9968, g (x) = g (y) = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g (e) (=2.0023, g value of free electron) and that g (z) of ZnTPP is smaller than g (e). These results were interpreted in terms of the nature of the perturbed states: the higher triplet pi pi'* state mixes with T(1)(pi pi*) via spin-orbit coupling in ZnTPP. In contrast, the higher triplet d pi* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T(1)(pi pi*) state.