Unsymmetrical achiral four ring hockey stick shaped mesogens based on 1,3,4-oxadiazole: Photophysical, mesogenic and DFT studies

Abstract

New unsymmetrical achiral bent core molecules containing 1,3,4-oxadiazole motif have been synthesized. These new bent core molecules resemble hockey-stick shapes due to presence of two arms having different number of the phenyl rings and 4-n-alkyloxy chains. Of these, one arm of the molecule possesses two phenyl rings and 4-n-alkyloxy chain of different length (n=4–8, 10, 12, 14, 16 and 18) while other arm consists of one phenyl ring and fixed 4-n-octyloxy chain. UV–visible and steady state fluorescence in solution and solid thin film were applied in order to characterize the photophysical behaviour of the compounds. The bent core molecules exhibited absorption in the UV region with fluorescence emission in the green wavelength region in dichloromethane and blue fluorescence emission in DMF with a large Stokes shift. It is found that bent core molecules are self-assembled to both H- and J-type aggregates at concentration range (75–50μM) in dimethylformamide. The phase behaviour of the bent core molecules was characterized by polarized optical microscopy, differential scanning calorimetry and variable temperature powdered X-ray diffraction. The bent core molecule having lower 4-n-butyloxy chain exhibited Iso-N-SmA-DC phase sequence. The dark conglomerate (DC) phase is optically isotropic mesophase and consists of two optically active domains of opposite handedness. For carbon chains (n=5, 6), bent core molecule displayed Iso-N-SmA-SmC-DC phase whereas the higher homologues (n=16, 18) exhibited Iso-SmA-SmC-SmX-SmY-Cr phase sequence. The unidentified smectic phases appeared and DC phase disappeared on increasing carbon chains. The DC phase persists over wide temperature range and down to room temperature with crystallization in lower chain length. The density functional theory (DFT) study provides information related to molecular conformation, reactive site of the molecule and reactivity parameters which obey the maximum hardness (MHP) and minimum polarizability (MPP) principles. Our study confirms that inclusion of solvent makes the molecule more reactive but less stable under ambient condition.