Photoexcited-state dynamics of meso-diarylamino subporphyrins 4-6 in cyclohexane, toluene, and acetonitrile have been studied by steady-state/time-resolved absorption and fluorescence experiments and quantum calculations. While 4 emits fluorescence from the locally excited state regardless of solvent polarity, the fluorescence of 5 and 6 are solvent-polarity dependent. The observed efficient fluorescent quenching of 5 and 6 has been ascribed to twisted intramolecular charge transfer (TICT), in which the two N-Cipso bonds in 5 and the Cmeso-N bond in 6 are twisted to facilitate the intramolecular electron transfer. In 2-methyltetrahydrofuran (2-MeTHF), the fluorescence of 5 and 6 are both almost completely quenched at 297 K, but restored at 77 K (below the melting point of 2-MeTHF) because of the frozen molecular twisting. Furthermore, electrochemical studies also revealed the charge separation processes of 4-6 are thermally unfavorable in nonpolar toluene and cyclohexane unless their structures change contrary to the observed efficient fluorescence quenching. These observations support the TICT mechanism. In addition, the formation of TICT state is affected by steric effect as the size of the meso-substituents increases.
Bibliographical noteFunding Information:
This research at Yonsei University supported by the Global Research Laboratory Program (2013K1A1A2A02050183) funded by the Ministry of Science, ICT & Future, Korea. The quantum mechanical calculations were supported by the National Institute of Supercomputing and Network (NISN)/Korea Institute of Science and Technology Information (KISTI) with supercomputing resources including technical support. The work at Kyoto was supported by JSPS KAKENHI (Grant numbers 25220802 and 25620031).
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