Aggregation-induced emission mesogens formed by intermolecular hydrogen bonding of 4-alkyl-4′-cyanobiphenyl molecules

Abstract

The luminescent characteristics of liquid crystalline molecules based on 4-alkyl-4′-cyanobiphenyl, such as 4′-pentyl-4-cyanobiphenyl (5CB), have been less explored due to the fluorescence quenching arising from intermolecular dipole–dipole interactions and free intramolecular motion. Herein, we reported three 4-alkyl-4′-cyanobiphenyl-based molecules, i.e., 2CB-NC2O, 2CB-NC3O and 2CB-NC4O, with aggregation-induced emission (AIE) properties. Notably, 2CB-NC2O and 2CB-NC3O demonstrate a liquid crystalline smectic A phase at room temperature in addition to their AIE properties. The AIE properties of these molecules are caused by the intermolecular hydrogen bonding between –OH and –NH groups on the alkyl chain, which restricted the intermolecular motion to facilitate the radiative decay in their state of aggregation. Furthermore, in contrast to 5CB which exhibits an anti-parallel alignment, the 4-alkyl-4′-cyanobiphenyl-based molecules exhibited a parallel alignment induced by intermolecular hydrogen bonding, resulting in the inhibition of dipole–dipole quenching of their fluorescence. This study not only introduces a novel design concept for luminescent LCs, but also offers a strategic approach to enhance the fluorescence of dipolar molecules through intermolecular hydrogen bonding.