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

The photophysical properties of acenaphthylene (ACN), fluoranthene, ru bicene and decacyclene were investigated in toluene by electronic spec troscopy and photoacoustic calorimetry. The photophysical parameters o f the related molecules pyracyclene (PYR), periflanthene (PF) and 1,16 -benzoperiflanthene, taken from the literature, were included in the d iscussion. A Siebrand plot was used to analyse the energy dependence o f the rate constant k(ic) of internal conversion. With the exception o f ACN and PF and possibly PYR, the molecules follow the energy gap law , demonstrating that internal conversion should be promoted by CH mode s. For ACN and PF, deviations to much larger values of k(ic) were obse rved. 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 differe nces in the C-C bond lengths in the two states.