On theg-value shift and intersystem crossing in photo-excited radical-triplet systems

The electron paramagnetic resonance (EPR) signals of photoexcited quartet (Q₁) states for zinc(II) tetra-tert-butyl-phthalocyanine (ZnPc) ligated by 3- and 4-(N-nitronyl-nitroxide) pyridine radicals (3-NOPy, 4-NOPy) were observed in toluene solution at room temperature by means of X-band (9.4 GHz) time-resolved EPR (TREPR) spectroscopy. Theg values of Q₁ in the ZnPc-3-NOPy and ZnPc-4-NOPy complexes were found to beg=2.0025 andg=2.0036, respectively. The obtainedg value (2.0036) for ZnPc-4-NOPy is in good agreement with the value (g=2.0037) of the Q₁ state calculated under the strong-exchange limit. Theg value (2.0025) is just an average of the Q₁ and D₁ (g=2.0013) states for ZnPc-3-NOPy. Theg value of Q₁ for zinc(II) meso-tetraphenylporphine (ZnTPP) ligated by 3-NOPy showed a slight shift (g=2.0027) at X-band and no shift (g=2.0031) at W-band from the calculatedg value (g=2.0031) (J. Fujisawa, Y. Iwasaki, Y. Ohba, S. Yamauchi, K. Koga, S. Karasawa, M. Fuhs, K. Möbius, S. Weber, Appl. Magn. Reson. 21, 483–493, 2001). These changes in theg value were found to originate from an averaging of the TREPR spectra over the Q₁ and photoexcited doublet (D₁) states via a fast intersystem crossing (ISC) process. The ISC rates between these two states were estimated by means of numerical calculations with the modified Bloch equations as 1.2·10⁸ and 6·10⁷ s^?1 for the ZnTPP-3-NOPy complex at the X- and W-bands, respectively. The lower limit of the ISC rate was obtained as 10⁹s^?1 for the ZnPc-3-NOPy complex and the higher limit was found to be 3.1·10⁸ s^?1 for the ZnPc-4-NOPy complex.