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

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 (SmA(d)). This is a surprising observation because the SmA(d) organization is commonly observed for calamitic mesogens with terminal polar groups. Interestingly, the single-crystal structure of the C-6 homologue indicates antiparallel packing. Furthermore, the photophysical properties of a representative C-12 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 C-13 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 C-12 homologue in nematic and SmAd mesophases offer C-13-H-1 dipolar couplings from which the molecular order is determined to be 0.59 and 0.70, respectively.