Liquid-Crystalline Solvents as Mechanistic Probes. 8. Dynamic Quenching of Pyrene Fluorescence by Pyrene in the Liquid-Crystalline and Isotropic Phases of a Cholesteric Solvent

Rate constants and activation parameters for the quenching of pyrene fluorescence by pyrene have been determined in the cholesteric and isotropic phases of a 59.5/15.6/24.9 (w/w/w) mixture of cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride. From the small differences in the energies and entropies of activation (E₃(cholesteric) = 9.3 ± 0.3 kcal/mol and E₃(isotropic) = 7.0 ± 0.4 kcal/mol; ΔS^⧧(cholesteric) = 5 ± 2 eu and ΔS^⧧(isotropic) = −2 ± 3 eu), it is concluded that cholesteric solvent order has little effect on the quenching efficiency. The influence of solvent anisotropy on the efficiency of this process is compared to that observed with other reactions which are retarded or enhanced by solvent order.