A comparison of mesogenic properties ofp‐carborane‐1,12‐dicarbaldehyde schiff's bases with their terephthaldehyde analogues

A homologous series of carborane‐containing Schiff's bases 1A[n] (n = 1–10) was prepared and compared with the analogous series 1B[n] derived from terephthaldehyde. An exponential fit of the T NI values for both series yielded a quantitative assessment of the effect of ring structure on mesophase stability. This includes the T NI value for n→∞ (86°C for 1A[n] and 209°C for 1B[n]) and steepness of descent (0.135 for 1A[n] and 0.095 for 1B[n]). The difference in behaviour of the two series was attributed, largely, to conformational properties of the central rings A and B. Electronic interactions between the central rings and the π‐substituents were investigated by UV spectroscopy and by quantum‐mechanical calculations. The effect of replacement of O with CH2 in the terminal chain of 1[n] on the namatic phase stability was assessed for n = 5–7.