Conformational effects on mesophase stability: numerical comparison of carborane diester homologous series with their bicyclo[2.2.2]octane,cyclohexane and benzene analogues |
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| Authors: | Piotr Kaszynski Adam Januszko Kiminori Ohta Takashi Nagamine Piotr Potaczek Victor G Young |
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| Institution: | 1. Organic Materials Research Group , Department of Chemistry , Vanderbilt University , Nashville, TN 37235piotr.kaszynski@vanderbilt.edu;3. Organic Materials Research Group , Department of Chemistry , Vanderbilt University , Nashville, TN 37235;4. Tohoku Pharmaceutical University , 4‐4‐1, Komatsushima, Aoba‐ku, Sendai 981‐8558, Japan;5. X‐ray Crystallographic Laboratory , Department of Chemistry , University of Minnesota , Twin Cities, MN 55455 |
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| Abstract: | Three series of diesters of 4‐alkoxyphenols containing 12‐vertex p‐carborane (1An], n = 1–22), 10‐vertex p‐carborane (1Bn], n = 1–12) or bicyclo2.2.2]octane (1Cn], 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 1Cn] and also cyclohexane (1Dn]) and benzene (1En]) 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 1En] demonstrated abnormal behaviour of both carborane series (1An] and 1Bn]) and also the cyclohexane series (1Dn]). The former showed progressive destabilisation of the mesophase, whereas the series 1Dn] exhibited increasing mesophase stability relative to 1En] with increasing chain length. Both of these effects were explained using conformational analysis of theoretical models and experimental molecular structures for 1A3], 1B4] and 1C4]. 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 1Dn] was explained with the existence of the equatorial‐axial conformational equilibrium for the cyclohexane derivatives. The clearing temperatures for the hypothetical pure diequatorial conformers 1Dn]‐ee were estimated. |
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| Keywords: | carborane homologous series nematic conformation numerical analysis |
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