Spectroscopy and photophysics of a highly nonplanar expanded porphyrin: 4,9,13,18,22,27-hexaethyl-5,8,14,17,23,26-hexamethyl-2,11,20-triphenylrosarin

The structure and spectra of neutral and protonated forms of rosarin, a non aromatic hexapyrrolic expanded porphyrin bearing three phenyl and twelve al kyl substituents, were studied by means of stationary and picosecond time-r esolved spectral techniques and by molecular mechanics and quantum-chemical calculations. The photophysics of the lowest excited singlet state is domi nated by rapid internal conversion to the ground state. No fluorescence cou ld be detected. This behavior was attributed to the nonplanarity and confor mational flexibility of the molecule. Triplet formation efficiency is negli gible as long as the solvent does not contain heavy atoms. In the presence of iodine, a significant fraction of the excited-singlet-state population w as found to cross over to the triplet state. The absorption spectral patter n consists of a weak transition in the red spectral region, well separated from the strong bands observed at higher energy. Analysis of the calculatio ns reveals why only one transition is predicted in the low-energy region. I n contrast to porphyrin, whose spectra can be understood in terms of a four orbital model, the interpretation of the spectral features of rosarin requ ires six orbitals: the three highest occupied and the three lowest unoccupi ed ones.