PHOTOPHYSICAL PROPERTIES OF SOME POLYCYCLIC CONJUGATED HYDROCARBONS CONTAINING 5-MEMBERED RINGS

The photophysical properties of acenaphthylene (ACN), fluoranthene, rubicene and decacyclene were investigated in toluene by electronic spectroscopy and photoacoustic calorimetry. The photophysical parameters of the related molecules pyracyclene (PYR), periflanthene (PF) and 1,16-benzoperiflanthene, taken from the literature, were included in the discussion. A Siebrand plot was used to analyse the energy dependence of the rate constant k(ic) of internal conversion. With the exception of ACN and PF and possibly PYR, the molecules follow the energy gap law, demonstrating that internal conversion should be promoted by CH modes. For ACN and PF, deviations to much larger values of k(ic) were observed. This effect could be caused by a strong displacement of the S-0 and S-1 potential energy curves, which is related to the large differences in the C-C bond lengths in the two states.