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The crystal structure of 2,4-bis(1,3,3-trimethyl-2-indolinylidenemethyl)cyclobutenediylium-1,3-diolate has been determined. The space group is P21⁄c, with a=9.5115(4), b=15.712(1), c=15.174(1) Å, β=96.138(5)°, and Z=4. The molecule adopts the trans conformation.

Thermodynamic characteristics of a dimer liquid crystal comprising a relatively long spacer -O(CH 2 -CH 2 O) 6were investigated. The contour length of the flexible spacer (∼25 Å) exceeds that of the hard segments (∼18 Å). The compound exhibits a nematic mesophase (N) between the crystal (C) and the isotropic melt (I) over a sizably wide temperature range (∼80 °C). Thermodynamic quantities such as thermal expansion coefficient R, isothermal compressibility β, thermal pressure coefficient γ ()R/β), and the volume change ∆V at the CN and NI interphase were carefully determined by the PVT measurements. The transition entropies due to volume expansion were estimated by the well-known relation ∆S V ) γ∆V. The latent entropies (∆S tr ) P for both CN and NI transitions were estimated according to the Clapeyron relation from the PVT and DTA data. The magnitude of the constant-volume entropy changes (∆S tr ) V calculated was found to be about 50-60% of the corresponding values of (∆S tr ) P . These results were consistent with the previous 2 H NMR observations that the chain segments adopt a nematic conformation in the LC state. The thermodynamic role of the flexible segment in determining the phase transitions has been critically discussed.

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