Organic reactions in liquid crystalline solvents. 4. Nanosecond laser flash photolysis studies of intramolecular motions of rod-like solutes in liquid crystals

The rates of triplet decay of a series of β-aryl-(4-alkoxypropiophenone)s in the smectic, nematic, and isotropic phases of 4′-butyl- and 4′-ethylbicyclohexyl-4-carbonitrile (BCCN and ECCN, respectively) have been measured over the 30–95 °C temperature range by nanosecond laser flash photolysis. The rates of triplet decay for these probe molecules in fluid solution are governed by the rates of Cα—Cβ bond rotation, which allows intramolecular quenching of the carbonyl triplet state by the β-aryl ring. The ketones are substituted with alkyl groups of varying length, shape, and flexibility in the para positions of the β-phenyl (H, n-hexyl, cyclohexyl) and benzoyl (meth-, n-pent-, and n-octoxy) rings. With the exception of β-phenyl-(4-methoxypropiophenone), for each ketone the Arrhenius parameters for triplet decay in the smectic phase of BCCN are similar to those in the nematic phase of the same solvent, and in all cases, the Arrhenius plots exhibit perfect continuity at the S—N transition temperature. A solvation model is tentatively advanced to explain these results. In the nematic phase of BCCN, the Arrhenius activation energy and entropy are significantly more positive than those in isotropic ECCN for all the ketones studied, but variations in the energetics of triplet decay in the nematic phase as a function of solute structure are parallelled in the isotropic solvent. Thus, the inhibiting effect of the nematic solvent on the bond rotations leading to intramolecular triplet quenching in these probes is attributed to the predominant influence of microviscosity (viscous drag) effects; the presence of solvent orientational order appears to have little or no effect on the intramolecular mobility of these ketones. The results and conclusions of earlier studies of unimolecular reactions in nematic solvents are discussed in light of these results.