Liquid-Crystalline Solvents as Mechanistic Probes. 12. Solvent Anisotropy Effects on the Bending of Hydrocarbon Chains in α,ω-Bis(1-pyrenyl)alkanes

A comparison has been made of the effects of the cholesteric and isotropic phases of a 59.5/15.6/24.9 (w/w/w) mixture of cholesteryl oleate/cholesteryl nonanoate/cholesteryl chloride (CM) and of the nematic and smectic phases of trans,trans-4′-n-butylbicyclohexyl-4-carbonitrile (BCCN) on the dynamics of intramolecular fluorescence quenching in several α,⍵)-bis(1-pyrenyl)alkanes (PnP). It is demonstrated that whereas the quenching activation parameters are independent of chain length (n = 3, 10, 13, 22) in the isotropic phase of CM, they vary greatly with chain length in cholesteric CM. Very little quenching of P3P or P22P fluorescence occurs in smectic BCCN. In nematic BCCN, the activation energy for quenching increases with increasing chain length. No intramolecular quenching of P7P was detectable in any of our solvent systems. From analyses of these results, we conclude that the nature of the influence of mesophase order on hydrocarbon chain bending depends in a subtle way upon the length of the chains. In isotropic media, bending efficiency is solvent dependent but solute chain length independent.