Interpretation of the soret band of porphyrins based on the polarization spectrum of N-methyltetraphenylporphin fluorescence
Tóm tắt
Low-temperature (77 K) absorption, fluorescence, and fluorescence-excitation spectra and the fluorescence polarization spectrum for N-methyl-meso-tetraphenylporphin (N-methyl-TPP) are measured. Based on the polarization spectrum the absorption spectrum in the visible region (a “generic” porphyrin spectrum) is interpreted. In particular, it is shown that the fifth absorption band (the “Longo band”) that manifests itself in some porphyrins is a Qy(0–2) band and does not belong to the individual electron transition. Emphasis is placed on the region of the Soret band. It is inferred that at least two allowed electron transitions (G →Bx and G→By) polarized mutually perpendicularly manifect themselves in this region. The interpretation of the Soret band of porphyrins [2] that attributes this band to one electron transition G→Bx is thereby rejected. This interpretation is confirmed by computer modeling of the polarization spectrum. Special features of the experimental polarization spectrum are explained by a more developed vibrational structure and, possibly, a larger half-width of the Bx band than the half-width of the By band. The contribution of the states of intramolecular charge transfer to the formation of the Soret band is discussed.