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
manifest themselves this region. The interpetation of the Soret band of
porphyrins [2] that attributes this band to one electron transition G®
Bx is there by 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.
Key words: porphyrin, low-temperature fluorescence polarization
spectrum, absorption and fluorescence spectra, fluorescence-excitation
spectrum, Soret band.