A method is described for determining the specificity of quenching of pyrene singlets by a tertiary alkylamine, 5α-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 Stern-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) × 1014 M-1 s-1; the isotropic phase has E6 = 5.3 ± 0.1 kcal mol-1 and A6 = (1.8 ± 0.4) × 1011 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.
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1981|
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