Three series of diesters of 4-alkoxyphenols containing 12-vertex p-carborane (1A[n], n=1-22), 10-vertex p-carborane (1B[n], n=1-12) or bicyclo[2.2.2]octane (1C[n], n=1-12) as the central structural element were prepared and investigated by optical and calorimetric methods. All carborane diesters exhibited exclusively nematic behaviour, whereas the carbocyclic analogues 1C[n] and also cyclohexane (1D[n]) and benzene (1E[n]) derivatives, showed early onset of SmA phase and complete disappearance of nematic behaviour. The isotropic transition temperatures, T MI, for the five series of mesogens were analysed numerically using a three-parameter exponential function. The resulting limiting values, T MI(∞), provided a quantitative assessment of the central element ability to support the mesogenic state. They demonstrated that, whereas the T MI(∞) values for the carbocycles, C, D, and E, are around 125°C, for carboranes A and B this value is 70±2°C and 49±19°C, respectively. Two types of comparative analysis of trends in T MI relative to those of the terephthalate series 1E[n] demonstrated abnormal behaviour of both carborane series (1A[n] and 1B[n]) and also the cyclohexane series (1D[n]). The former showed progressive destabilisation of the mesophase, whereas the series 1D[n] exhibited increasing mesophase stability relative to 1E[n] with increasing chain length. Both of these effects were explained using conformational analysis of theoretical models and experimental molecular structures for 1A, 1B and 1C. The increasing relative destabilisation of the mesophase in the carborane derivatives was rationalised by the high order rotational axes in A and B and D 4d symmetry for B. The trend of the ΔT MI values for series 1D[n] was explained with the existence of the equatorial-axial conformational equilibrium for the cyclohexane derivatives. The clearing temperatures for the hypothetical pure diequatorial conformers 1D[n]-ee were estimated.
Bibliographical noteFunding Information:
This project was supported in part by the NSF grants (DMR-0111657 and DMR-0606317) and by Grant-in-Aid for Scientific Research (B) No. 13470468, the Ministry of Education, Culture, Sports, Science and Technology, Japan. We thank to Dr. Jadwiga Laska and Dr. Andrew G. Douglass for their help with obtaining the X-ray quality crystal of 1B. We also thank Prof. Roman Dabrowski for the gift of 4-decyloxyphenol (3).
- Homologous series
- Numerical analysis