LOW-TEMPERATURE ABSORPTION AND FLUORESCENCE-SPECTRA OF TRANS-2 AND 3-STYRYLPHENANTHRENE ROTAMERS

We have studied the quasiline electronic spectra (Shpolskii spectral o f the single-bond rotamers of trans-3- and trans-2-styrylphenanthrenes embedded in n-hexane and n-decane matrices at 5 K. With 3-styrylphena nthrene, in both n-paraffin host matrices, well-resolved absorption sp ectra due to the two almost isoenergetic s-trans and s-cis rotamers we re obtained with comparable contribution in agreement with experiments conducted at room temperature by Bartocci et al. Vibronic analysis of the spectra indicates a localized phenanthrenic character for the S-1 <-- S-0 transition of the s-trans rotamer and a delocalized stilbenic character in the transition of the s-cis rotamer. In the case of 2-st yrylphenanthrene, in n-hexane and n-decane matrices, the two expected rotamers which were not detected in liquid-solution experiments could be clearly distinguished by their absorption and fluorescence spectra, thus giving conclusive proof for the separate existence of the s-cis and s-trans rotamers in this compound. Contrary to the case for 3-styr ylphenanthrene, the vibronic analysis of 2-styrylphenanthrene spectra of both rotamers did not provide evidence for st clear-cut localized o r-delocalized character but the long fluorescence lifetime is in favor of a localized phenanthrenic character. Moreover, the n-decane matrix was seen to shift the rotameric equilibrium toward the postulated s-t rans rotamer presumably as a result of specific Shpolskii matrix host- guest interactions.