ORIGINS OF THE DIFFERENCES IN SOLVATION BY ALKANES AND PERFLUOROALKANES - EVIDENCE FROM THE S-2-S-0 SPECTRA OF JET-COOLED VAN-DER-WAALS COMPLEXES OF XANTHIONE AND AZULENE

The S-2-S-0 fluorescence excitation spectra of xanthione (XT) and azul ene (AZ) complexed with 1 or 2 molecules of the C-1 to C-10 n-alkanes and the C-1 to C-6 perfluoro-n-alkanes have been measured. The 1:1 com plexes exhibit microscopic solvent shifts, Delta<(nu)over bar>, which are larger for the alkanes than the corresponding perfluoroalkanes, de spite the larger molecular polarizability of the latter. The values of Delta<(mu)over bar> increase monotonically with carbon number of the adduct to C-10 in 1:1 n-alkane complexes with XT and to C-5 in 1:1 n-a lkane complexes with AZ. However, Delta<(mu)over bar> exhibits no furt her increase beyond C-2 in 1:1 perfluoro-n-alkane complexes with XT an d beyond C-3 in 1:1 perfluoro-n-alkane complexes with AZ. The results are interpreted in terms of a model in which the n-alkanes stretch out along the long axis of the chromophore and 'wet' its surface whereas the perfluoro-n-alkanes with carbon numbers greater than or equal to 3 away from the surface of the chromophore and are 'non-wetting'.