Light-induced tautomerization in porphyrin isomers

Double proton tautomerization occurring in porphyrin and its structural iso mers represents a special case of a chemical transformation in which the su bstrate and the product are formally the same. The methods used for the inv estigation of this kind of processes are based on polarized spectroscopy an d high-resolution techniques, such as matrix isolation. Their combined use results in obtaining information pertinent to the mechanism of proton trans fer, regarding e.g., the values of proton transfer rates, structure of the tautomeric forms or the shape of the potential energy surfaces. In addition , these procedures provide a way of obtaining spectral, photophysical and s tructural data that would be otherwise difficult to gain. The examples incl ude determination of transition moment directions, assignment of electronic and vibrational states, elucidation of the character of the substitutional replacement of the rare gas matrix atoms by the chromophore, and the analy sis of the nature of the symmetry lowering due to the matrix cage.