Interplay between Fluorescence and Isomerism: The Excited-State Intramolecular Proton-Transfer Phenomenon in Chiral Liquid Crystalline Dimers

Abstract

One of the fascinating and unique behaviors observed in the tautomeric systems is the excited-state intramolecular proton-transfer (ESIPT) process. It occurs due to the interconversion between keto and enol tautomeric states during excitation–emission. Hitherto, many tautomeric fluorophores exhibiting the ESIPT phenomenon have been reported. However, chiral (cholesterol-based) liquid crystal (LC) dimers containing ESIPT active salicylaldimine core have been investigated for the first time to explore their four-level photochemical process. Herein, we report the synthesis and characterization of four novel chiral liquid-crystal (LC) dimers, comprising two epimeric (diastereomeric) pairs, in which natural cholesterol is covalently tethered via an oxyoctanoyloxy spacer to the two-ring chiral calamitic core, internally linked by a salicylaldimine(imine) linkage. The two-ring mesogenic core features two distinct chiral tails derived from (R)- and (S)-citronellyl bromide, as well as (R)- and (S)-2-octanol. These chiral mesogens commonly show enantiotropic chiral nematic (N*), twist grain boundary (TGBA*), and smectic A (SmA*) phases along with a monotropic chiral smectic C (SmC*) phase; notably, the technically significant SmC* phase exists well below room temperature. Most importantly, they exist in a photoinduced keto-enol tautomeric form and thus display ESIPT activity in all their condensed states (solid, liquid crystal, and liquid) and micromolar solutions. The CD studies revealed the reversal of the helical twist sense during the transition from the chiral nematic to the chiral smectic C phase.