Tuning the fluorescence behavior of liquid crystal molecules containing Schiff-base: Effect of solvent polarity

Abstract We report the influence of intermolecular and intramolecular hydrogen bonding on the excited state proton transfer (ESPT) emission behavior of two bent core liquid crystal (BLC) molecules, C 54 H 63 NO 9 (BLC3) and C 60 H 75 NO 9 (BLC4), having a Schiff-base and two long alkyl chains at its two ends. Fluorescence spectra of these BLC molecules dispersed in different solvents show dual emission (at ∼ 365 nm and ∼425 nm) from the keto and enol tautomers. We observed that the population of these keto and enol tautomers and the corresponding intensities of fluorescence emission are strongly influenced by the solvent polarity. In protic solvents, formation of intermolecular hydrogen bond with the Schiff-base of the BLC molecules is highly favoured than the intramolecular hydrogen bonding. This intermolecular hydrogen bonding drastically reduces the population of the keto tautomers in the excited state, resulting in enhanced enol fluoroscence band along with a weak keto emission band. The observed intensity of the enol fluorescence band is the highest for the most studied polar solvent (methanol). On the other hand, in aprotic solvents, the intramolecular hydrogen bonding is highly favoured, which leads to the formation of keto tautomers in the excited state. Hence, an intense keto emission band is observed for the aprotic solvents along with a weak enol emission band. From the time resolved fluorescence studies we observed a longer life time for the keto band than that for the enol band. This is also related to the delayed emission associated with the vibrational bands resulting from the bulky alkyl chains attached to the ends of the BLC molecules.