Thermotropic liquid–crystalline properties of 4,4′-dialkoxy-3,3′-diaminobiphenyl compounds and their precursors

<p>A series of 4,4ʹ-dialkoxy-3,3ʹ-diaminobiphenyl compounds were synthesised by three-step procedure that involves alkylation, nitration and reduction reactions. Their chemical structures were characterised by FTIR, <sup>1</sup>H and <sup>13</sup>C spectroscopy and elemental analysis. Their thermotropic liquid–crystalline (LC) properties were examined by a number of experimental techniques including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarising optical microscopy (POM) and variable temperature X-ray diffraction (VT-XRD). The 4,4ʹ-dialkoxy-3,3ʹ-dinitrobipheyl compounds, precursors to the diamine compounds, were also examined for their thermotropic LC properties. POM studies of focal conic textures and VT-XRD of the 3,3ʹ-diaminobiphenyl derivatives having flexible alkyl chains (C<sub>6</sub>–C<sub>12</sub>) exhibited the smectic A (SmA) phase independent of the length of alkyl chains. Similarly, the 3,3ʹ-dinitrobiphenyl derivatives containing alkyl chains C<sub>7</sub>, and C<sub>9</sub>–C<sub>11</sub> exhibit the SmA phase, those containing C<sub>8</sub> formed the smectic C (SmC) phase and C<sub>12</sub> formed both the SmA and smectic B (SmB) phases, respectively. The 3,3ʹ-diaminobiphenyl derivatives had excellent thermal stability in the temperature range of 237–329°C, while those of 3,3ʹ-dinitrobiphenyl derivatives were in the temperature range of 270–321°C. The 3,3ʹ-diaminobiphenyl derivatives emitted UV light both in chloroform and acetonitrile.</p>