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

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Abstract

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.

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Authors and Affiliations

  1. Graduate School of Pharmaceutical Science, Kyushyu University, Fukuoka, Japan

    S. Karawasa & N. Koga

  2. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 980-8577, Sendai, Japan

    Y. Iwasaki, K. Katano, Y. Ohba & S. Yamauchi

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  1. Y. Iwasaki
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  2. K. Katano
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  3. Y. Ohba
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  4. S. Karawasa
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  5. N. Koga
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  6. S. Yamauchi
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Iwasaki, Y., Katano, K., Ohba, Y. et al. On theg-value shift and intersystem crossing in photo-excited radical-triplet systems. Appl. Magn. Reson. 23, 377–391 (2003). https://doi.org/10.1007/BF03166628

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  • DOI: https://doi.org/10.1007/BF03166628

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