Electronic spectra of porphycenes in rare gas and nitrogen matrices

Fine-line electronic absorption and fluorescence spectra of porphycene and 2,7,12,17-tetra-n-propylporphycene have been measured in low-temperature ni trogen, argon, krypton, and xenon matrices. A well-resolved emission spectr um of porphycene was also obtained in an isotropic glassy matrix using the fluorescence line-narrowing technique. Analysis of the observed matrix spec tral shifts enabled the assignment of the origin of the S-0-S-2 electronic transition. Vibrational frequencies have been obtained for the ground and t wo lowest excited singlet states. Vibronic and site structure in the region of S-0-S-1 and S-0-S-2 electronic transitions of porphycenes have been com pared with the corresponding data for porphyrin. Both similarities and diff erences were observed. In particular, contrary to the case of porphyrin and its derivatives, no site structure was observed in the spectra of porphyce nes in xenon and krypton matrices. This was explained by a model that takes into account different positions of the transition moment directions in th e two chromophores with respect to the matrix cage. No evidence was obtaine d for the presence of the cis tautomeric forms of porphycene.