Excited-state properties of thiaporphyrins and their dications

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Abstract
Singlet excited-state properties of mono- and di-thiaporphyrins and their dications are reported. A comparison of vibrational frequencies of the ring stretching modes of neutral and protonated species in resonance Raman spectra show shifts to both higher and lower frequencies. An analysis of the shifts of various Raman modes reveals that the dications have predominantly an a2u ground state. The direction of the shifts of structurally sensitive modes indicates core expansion upon protonation. Protonation results in the red shifts of emission bands with considerable quenching of fluorescence, the magnitude of which depends on the number of sulfur atoms in the porphyrin core. The quantum yield and lifetime of various thiaporphyrins are found to be decreased relative to structurally similar 5, 10, 15,20-tetraphenylporphyrin (H2TPP). The estimated excited-state potentials suggest that thiaporphyrins are better reductants in the first singlet excited state. The structural change accompanying the protonation process accounts for most of these observations.

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