Structure and molecular packing in smectic B_Cr and A_d phases of Schiff base liquid crystal compounds through the analyses of layer spacing, entropy and crystal structure

Abstract

Single-crystal structural analyses and heat capacity measurements were performed on two Schiff base liquid crystal compounds, 5CBAA (4-chlorobenzylidene-4'-pentyloxyaniline) and 5ABCA (4-pentyloxybenzylidene-4'-chloroaniline). The alkyloxy-chain of a 5CBAA molecule was conformationally ordered in the crystal at room temperature. While that of 5ABCA was partially disordered in the room temperature phase but ordered in a low-temperature phase at 100 K. The structural phase transition involving the disordering of the conformation was observed at 107 K in the heat capacity of 5ABCA. Both compounds showed two liquid crystalline phases, SmB and SmA. The net entropy change associated with the chain disordering was essentially the same in them despite the difference in the orientation of their central -CH[double bond, length as m-dash]N- moiety. The layer-spacings of SmB and SmA phases were analyzed for their chain-length dependence in both series of mesogens (nCBAA and nABCA), as well as in the case of nBBAA (4-bromobenzylidene-4'-alkyloxyaniline). The results reveal that these smectic structures are composed of alternately stacked core- and chain-layers with an antiparallel arrangement of cores and a bent-form of molecules.

Synthetic, Mesomorphic, and DFT Investigations of New Nematogenic Polar Naphthyl Benzoate Ester Derivatives

“…The stereo configurations of the central naphthalene moiety and terminal polar groups played an important role in the predicted thermal parameters, which will be discussed in the computational section. Moreover, the thermal cis/trans isomerization of the azo group was an essential factor in the lower entropy changes observed, as documented in previous studies [ 65 , 69 , 70 , 71 ]. Moreover, due to their nematic nature, of the mesophase, they exhibited the lowest order mesophase.…”

“…The data revealed that the N–I transition entropy changes showed relatively lower values. The estimated entropy changes values are independent on alkoxy chain length [ 64 , 65 ]. Additionally, these results were agreement with previous reports [ 66 , 67 , 68 ].…”

“…Thus, the e terminal aggregation strength increased along with the length of the alkoxy chain, but with a decrease in total thermodynamic energy. It was reported that [ 64 , 65 ], the terminals of the chains always change their conformation dynamically and are randomly pointed out within the chain-layer. The random packing of the chains results in the loss of the two-dimensional symmetry within the chain-layer.…”

Aggregation structure of chiral cubic liquid crystals revealed by X-ray diffraction utilizing a new algorithm

“…Based on this parallel arrangement, we revealed the details of molecular aggregation in the Gyroid phase. 15 Considering the possible variety of alkyl-chain orientations and interdigitations, 15,45–47 we can imagine the molecular arrangement reasonably compatible with the reported chain-length dependence of the cell constant 18 (see ESI† for details), as shown in Fig. 5: the parallel alkyl chains with little interdigitation between the two junctions on a body diagonal (3 B and 3 Y ), and the bent chains directed to the six-membered circuit (S B ).…”

“…The orientations of alkyl chains proposed above are literally on average relative to the body diagonal that coincides with the averaged molecular orientation there. 15,[45][46][47] The straight chains do not mean an all-trans state at all. Similarly, we do not claim anything concerning the tilt direction (around the body diagonal) of disordered alkyl chains, for which we need further studies having higher sensitivity even for dynamical correlation.…”