Liquid crystalline solvents as mechanistic probes. 9. Dynamics of intramolecular fluorescence quenching processes of N,N-dimethyl-4-[3-(1-pyrenyl)propyl]aniline in the liquid crystalline and isotropic phases of a cholesteric solvent

The dynamics of the intramolecular quenching of pyrene singlet (¹P) fluorescence by an aromatic amino group in N,N-dimethyl-4-[3-(1-pyrenyl)propyl]aniline (A) have been investigated in a nonpolar, cholesteric liquid crystalline solvent [59.5/15.6/24.9 (w/w/w) cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride]. The disappearance of the ¹P fluorescence is shown to correlate kinetically with the formation of a fluorescent exciplex state. The monoexponential ¹P emission decay disfavors the existence of energetically distinct ground-state conformers of A. Employing 1-ethylpyrene as a model compound allows determination of rate constants and activation parameters for the intramolecular ¹P quenching process. The small differences in the activation parameters [E₆′(cholesteric) = 9.7 ± 1.2 kcal mol^-1 and E₆′(isotropic) = 7.8 ± 0.3 kcal mol^-1; ΔS^‡(cholesteric) = 3 ± 3 eu and ΔS^‡(isotropic) = -4 ± 1 eu] suggest that cholesteric order has little effect on the quenching efficiency. A mechanistic explanation is advanced.