Polar terminal substituent on symmetrical liquid crystal dimers: Synthesis, photophysical studies, mesomorphism and DFT studies

Abstract

A novel symmetrical liquid crystal dimer has been reported featuring a three-ring rod-shaped monomer with methylene ether-linked flexible spacers and different terminal polar substituents. The presence of lateral bulky methoxy groups in dimer molecules enhances intermolecular interactions and improves mesophase stability. The photophysical, thermal stability, liquid crystalline behaviour, and chemical reactivity parameters of dimers have been highlighted. The compounds show good thermal stability, among them presence of trifluorocarbon in the dimer shows higher thermal stability. The polar dimers are fluorescent with a large Stokes shift (∼8353–11,912 nm) and interesting photophysical behaviour in solution and solid thin films. The optical band gap of the dimer molecule in the solution is in the range of a semiconductor (2.1–2.8 eV), whereas the dimer 4FV-6 is found to exhibit the lowest value in solid thin films (1.9 eV). Their fluorescence quantum yield is greatly affected by concentration and solvent polarity, signifying higher fluorescence efficiency. All dimers exhibited a broad range of enantiotropic nematic phases, except fluoro-substituted dimers exhibit additional enantiotropic N_x with the lowest clearing temperature. A DFT study explored their structural and electronic properties, including geometry optimization and TDDFT, CDFT, and ESP analyses. TDDFT revealed solvent-dependent redshifts in absorption spectra, with polar solvents stabilising excited states. Dipole moment and reactivity analyses showed 4CNV-6 had the highest polarity, while 4NO₂V-6 was most electrophilic.