Liquid Crystalline Solvents as Mechanistic Probes. 6. Dynamic Quenching of Pyrene Fluorescence by 5α-Cholestan-3β-yldimethylamine in the Liquid Crystalline and Isotropic Phases of a Cholesteric Solvent

A method is described for determining the specificity of quenching of pyrene singlets by a tertiary alkylamine, 5a-cholestan-3β-yldimethylamine (CA), in a nonpolar cholesteric liquid crystalline solvent [59.5/15.6/24.9 (w/w/w) cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride]. It is found that the pyrene-amine orientation for quenching resembles closely the probable exciplex geometry. Activation parameters for the quenching process are phase dependent: from Stem-Volmer derived quenching rate constants at low CA concentrations, the Arrhenius activation energies and preexponential factors in the cholesteric phase are calculated to be E6 = 9.9 ± 0.2 kcal mol-1 and A6 = (2.1 ± 1.3) X 1014 M-1 s-1; the isotropic phase has E6 = 5.3 ± 0.1 kcal mol-1 and A6 = (1.8 ± 0.4) X 10u M-1 s-1. The value of E6(cholesteric) is found to change with CA concentration, approaching E6(isotropic) at high CA. A. cohesive explanation of these and other results, including the previously reported photodimerization of acenaphthylene in a different cholesteric phase, is advanced.