Influence of N‐Alkyl Substituents and Counterions on the Structural and Mesomorphic Properties of Guanidinium Salts: Experiment and Quantum Chemical Calculations

Abstract

A series of N-4-(4'-alkoxybiphenyl)-N',N',N",N"-tetramethylguanidinium salts was synthesized with varying alkoxy chain lengths and additional N-alkyl substituents, each with a number of different counterions. X-ray crystal-structure analyses of 1b I, 1b PF(6), 2a I, and 4a I reveal bilayer structures in the solid state and, for the 1b and 1b PF(6) salts, a hydrogen-bond-type connectivity between the guanidinium N-H group and the anion is found. For the N-alkyl homologues 2a I and 4a I the anion is still oriented close to the head group, although at a larger distance. Ion pairs are present also in solution, as demonstrated by (1)H NMR: the N-H chemical shift shows a good linear correlation with the radius, and hence the hardness, of the anion. The intramolecular conformational flexibility of 1b I, 2b I, 3b I, and 4b I was studied by temperature-dependent (1)H NMR spectroscopy and discrete activation barriers were determined for rotations about each of the three C-N partial double bonds of the guanidinium core. The relative heights of the individual barriers change between the N-H and the N-alkylguanidinium salts. A fourth barrier is observed for the rotation about the N-biphenyl bond. DFT calculations of charge densities show that the positive charge resides primarily on the central carbon atom. Rotational barriers were calculated for N'-substituted 2-amino-1,3-dimethylimidazolidinium cations as models, and are in qualitatively good agreement with the NMR data. Mesomorphic properties were studied by differential-scanning calorimetry, polarizing optical microscopy, and X-ray diffraction (WAXS/SAXS). All liquid-crystalline guanidinium salts exhibit smectic A mesophases. Clearing temperatures show a linear correlation with the anionic radius. Substitution of the N-H group with methyl, ethyl, or propyl results in decreasing mesophase widths and a concomitant shrinkage of the layer spacings.

Ionic liquid crystals derived from guanidinium salts: induction of columnar mesophases by bending of the cationic core

“…[18,[26][27][28] In all cases, a SmA mesophase was confirmed by the combination of POM studies and temperature-dependent XRD investigations. Although 3d · SCN also gave a broad mesophase, its comparison with respect to the anionic radius has been omitted from because it has an elliptical shape which cannot be directly compared to the other spherical anions.…”

“…The substitution pattern also influenced the phase behaviour, as demonstrated upon comparison with the corresponding para-substituted acyclic derivatives 10a, 10b and their cyclic counterparts 11a and 11b ( Figure 3). [18] In contrast to the linear-core compounds 10 and 11, which have much higher phase transition temperatures with almost constant melting points regardless of head group or chain length, meta-substitution clearly decreases the melting points but gives comparatively similar mesophase widths.…”

“…The experimentally calculated layer spacing for the cyclic derivatives 4 are very similar to those calculated for the acyclic derivatives 3 and show the expected temperature dependencies (Table 3). [18] This again can be rationalised by a high degree of interdigitation.…”

“…These observations are in contrast to the mesomorphism of congruent ion pairs with both para-substituted cations and anions which formed exclusively SmA phases. [18] Interestingly in the series of ion pairs 7, derivative 7e with the most differing chain lengths between cation and anion takes a prominent position. The melting point was lowered to room temperature (22°C) and gave a stable SmA phase until 113°C (Table 4, Figure 10).…”

“…Hence, the influence of chain lengths, type of head group and counterion on the mesomorphic properties of ILCs has been investigated by several groups. [2][3][4]17,18] Furthermore, recent work by Tschierske, Cheng [19,20] and Laschat [21] has shown that by increasing the number of alkyloxy side chains on the same mesogenic unit the whole range of mesophase types from smectic A (SmA) to columnar (Col) and even cubic (Cub) could be obtained from structurally similar compounds. The effect of bending of the mesogenic core, however, has been explored much less, although it is well known from typical organic crystals (e.g.…”