TY - JOUR

T1 - Conformational effects on mesophase stability

T2 - Numerical comparison of carborane diester homologous series with their bicyclo[2.2.2]octane, cyclohexane and benzene analogues

AU - Kaszynski, Piotr

AU - Januszko, Adam

AU - Ohta, Kiminori

AU - Nagamine, Takashi

AU - Potaczek, Piotr

AU - Young, Victor G.

AU - Endo, Yasuyuki

PY - 2008/10/1

Y1 - 2008/10/1

N2 - 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[3], 1B[4] and 1C[4]. 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.

AB - 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[3], 1B[4] and 1C[4]. 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.

KW - Carborane

KW - Conformation

KW - Homologous series

KW - Nematic

KW - Numerical analysis

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U2 - 10.1080/02678290802409775

DO - 10.1080/02678290802409775

M3 - Article

AN - SCOPUS:57049130483

VL - 35

SP - 1169

EP - 1190

JO - Liquid Crystals

JF - Liquid Crystals

SN - 0267-8292

IS - 10

ER -