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. The g values of Q1 in the ZnPc-3-NOPy and ZnPc-4-NOPy complexes were found to be g = 2.0025 and g = 2.0036, respectively. The obtained g 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. The g value (2.0025) is just an average of the Q1 and D1 (g = 2.0013) states for ZnPc-3-NOPy. The g 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 calculated g 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 the g 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 109 s-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.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics