A comparison of mesogenic properties of p ‐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 CH 2 in the terminal chain of 1[ n ] on the namatic phase stability was assessed for n  = 5–7.