Investigation of NH-tautomerism of alkyl derivatives of porphine by means of low-temperature selective excitation spectroscopy

In isotropic media at 4.2°K with selective laser excitation, fine-structure fluorescence spectra have been obtained for NH-tautomers of 2,3,12,13-tetramethylporphine (TMP) and their derivatives in which the central protons have been replaced by deuterons. For each of the phototautomers, polarization measurements have been made for individual background-free lines, and the symmetry of the normal vibrations has been determined. From an analysis of the experimental data and the results obtained in calculating the normal vibrations of the 2,3,12,13-TPM molecule with various possible positions of the imine protons, it has been shown that the phototautomer with fluorescence in the shorter-wave region corresponds to location of the central protons on the pyrrole rings with methyl substituents, and the tautomer with fluorescence in the longer-wave region corresponds to location of the protons on the porphine pyrrole rings. For porphine 2,3,12,13-TMP, and also for 2,3,7,8-TMP and 2,3,7,8-TEP, no phototautomers with adjacent positions of the NH-protons were detected.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 554–560, September–October, 1985.