Spectroscopic studies of hydrogen bonded aromatic complexes at low temperatures: external deuterium isotope effects

Abstract

Association complexes of benzene and its alkyl homologues with weakly acidic halogeno-alkanes and -alkenes have been studied spectroscopically in frozen 3-methylpentane solutions at 77 K. Investigation of steric, and external deuterium isotope effects on their absorption and luminescence spectra and phosphorescence decay indicate that (i) photo-excitation into the lowest singlet or triplet state alters the equilibrium conformation of the complexes from one in which the C—H bond lies perpendicular to the plane of the benzene ring to one in which the bond is tilted with respect to the perpendicular axis, (ii) intermolecular vibronic coupling in the triplet state with the acidic proton only occurs when the proton lies off the central axis, and (iii) the coupling vanishes when bulky groups prevent the excited complex from relaxing into a tilted conformation.

While association with CHCl₃ or CHCl₂CCl₃ promotes the molecular distortion of the triplet state in benzene, the effect rapidly decreases when alkyl groups are substituted into the aromatic ring. In complexes with the di- or tri-chloroethylenes, the phosphorescence of the aromatic hydrocarbon is quenched by triplet–triplet energy transfer. Photosensitised cis-trans isomerisation has been demonstrated in dilute solution in frozen 3-methylpentane at 254 nm.