Three-Ring-Based Thermotropic Mesogens with a Dimethylamino Group: Structural Characterization, Photophysical Properties, and Molecular Order

Abstract

Thermotropic liquid crystals exhibiting light-emitting properties are gaining popularity as functional materials in view of their application in organic light-emitting diodes. Such mesogens essentially require active chromophoric moieties in the mesogenic core so that the mutual light-emitting and liquid crystalline properties can be realized. In this work, three-ring-core-based mesogens with a terminal dimethylamino unit are subjected to structural characterization by various techniques. These mesogens exhibit enantiotropic nematic as well as smectic A phase with interdigitated layer organization (SmAd). This is a surprising observation because the SmAd organization is commonly observed for calamitic mesogens with terminal polar groups. Interestingly, the single-crystal structure of the C6 homologue indicates antiparallel packing. Furthermore, the photophysical properties of a representative C12 mesogen in solution disclose yet another exciting feature. The steady-state and time-resolved fluorescence studies indicate negative solvotochromism in solvents with differing polarity. To obtain greater insight, density functional theory (DFT)-based highest occupied molecular orbital–lowest unoccupied molecular orbital studies are carried out which support intramolecular charge-transfer interactions in this class of mesogens. Additionally, the DFT calculations also provide the 13C chemical shifts which are compared with the solution NMR values for the structural assignment of all carbons in the core unit. Furthermore, the two-dimensional separated local field measurements for the C12 homologue in nematic and SmAd mesophases offer 13C–1H dipolar couplings from which the molecular order is determined to be 0.59 and 0.70, respectively.