A possible mechanism of reversible photocyclization of [5]-helicene in Shpol'skii-type matrices at 4.2 K

Highly resolved fluorescence and phosphorescence excitation and emission spectra of [5]-helicene in the Shpol' skii-type matrix n-pentane were measured at 4.2 K. New additional narrow lines have been found in emission and excitation spectra upon irradiation of [5]-helicene solutions. These lines have been attributed to dihydrobenzoperylene (DHBP)-a product of the reversible photocyclization of [5]-helicene. Energies of the singlet and triplet states of [5]-helicene, DHBP and benzoperylene (BP) (a product of a subsequent dehydrogenation of DHBP) were compared with the results of quantum-chemical calculations. Fundamental frequencies of [5]-helicene and DHBP in the states S-0 and S-1 were determined from the vibrational analysis of the excitation and emission spectra. Vibrational frequencies detected in the emission and excitation spectra of [5]-helicene, DHBP and BP as well as data for [5]-helicene and BP obtained from IR and Raman spectra are in good agreement with the calculated vibrational frequencies. (C) 2002 Elsevier Science B.V. All rights reserved.