Matrix isolation studies of vibrational structure of hemiporphycene

Parent, unsubstituted free base hemiporphycene, a constitutional isomer of porphyrin, was synthesized only a few years ago, and the detailed characterization of vibrational structure is lacking. We studied vibrations of hemiporphycene using site-selected fluorescence and IR measurements in low temperature argon and nitrogen matrices, combined with recording Raman spectra of polycrystalline samples. Temperature dependence of IR and Raman spectra was also investigated. The spectra were analysed with the help of DFT calculations of vibrational energies and transition intensities. Most of the bands observed in the 600-1600 cm−1 range could be reliably assigned. Moreover, it was possible to photoconvert the lowest energy trans1 tautomer, the only species present at low temperatures, into a higher energy trans2 form. However, this process is not efficient and strongly depends on the chromophore microenvironment.