Synthesis, characterization, mesogenic properties, and DFT studies of unsymmetrical liquid crystalline dimers of biphenyl

Abstract

ABSTRACT A series of asymmetrical liquid crystal dimers (E)-4((4(n([1,1’-biphenyl]-4-yloxy)alkoxy)phenyl) diazenyl)benzonitrile (BpnCN) were synthesized, consisting of two distinct rigid cores: 4-cyanoazobenzene and biphenyl units. Characterization was accomplished through various spectroscopic techniques. The mesomorphic properties were explored via polarizing optical microscopy and differential scanning calorimetry. UV–Visible spectroscopy assessed the photophysical properties. The presence of an even number of atoms in the flexible spacer resulted in linear molecular shapes for Bp4CN and Bp6CN, while Bp3CN and Bp5CN showed nonlinearity, explaining their non-mesomorphic properties. Computational studies showed a clear correlation between theoretical and experimental wave numbers. The enhanced stability of the mesophase in Bp4CN and Bp6CN was attributed to their higher aspect ratios, promoting ordered structures within the material. UV–visible spectroscopy of dimer Bp4CN revealed absorption bands at 445 nm (n–π* transitions) and 365 nm (π–π* transitions), with photoisomerization observed under UV exposure, reaching a photostationary state in 2.5 min and reversible isomerization back to the trans form in approximately 2.3 h. Notably, even in the absence of substitutions on the biphenyl core, the dimers in our present series demonstrated an enantiotropic nematic mesophase.