Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

Abstract

A series of phenylguanidinium salts 3 . X, which are linked via an alkoxy spacer either to a 4-decyloxy-or 4-cyanosubstituted biphenyl mesogen, was prepared and the mesomorphism studied. A decyloxybiphenyl core and a spacer of at least C6 chain length were required for mesophase formation. Replacement of the chloride counterion by other anions like bromide or tetrafluoroborate improved the thermal stability of the mesophase. A comparison of substitution pattern (meta v. para) on the phenyl ring revealed decreased melting and clearing points for the bent cationic head group. All guanidinium ionic liquid crystals 3 displayed only smectic A (SmA) phases. A packing model is assumed where the molecules in a bilayer stack over each other in opposite direction with interdigitated terminal decyloxy groups and spacers.bromides 9 giving the tethered compounds p-10a,b and m-10b-e in 78-88 % yield. N-Boc-deprotection to derivatives p-11a,b and m-11b-e was achieved very cleanly with methane sulfonic acid in CHCl 3 . Derivative m-11a was obtained in 94 % yield by acidic hydrolysis [62] of acetamide m-7a. The aniline derivatives 11 were finally treated with chloro-N,N,N 0 ,N 0tetramethylformamidinium chloride [63] and either NEt 3 or NaHCO 3 . Workup strictly required an inert gas atmosphere to provide the neat target guanidinium salts 3 . Cl in 70-98 % yield. To further study the anion effect, decyloxybiphenyl guanidinium chloride m-3a . Cl was submitted to salt metathesis with NaBr, KI, NaOTf, NaBF 4 , KPF 6 , KOAc, and KSCN yielding the corresponding guanidinium ILCs m-3a . X (Scheme 2). Anion Effect in Guanidinium ILCsComparison of the 1 H NMR spectra of phenylguanidinium salts m-3a . X revealed a significant downfield shift of the N-H signal from ,7.5 to 12.0 ppm in the following order: PF 6 , BF 4 , OTf , I , SCN , Br ECl (Fig. 1).This correlation between the anion and the N-H proton shift indicates the presence of contact between the ion pairs that also led to small chemical shifts of the aromatic protons of the phenylguanidinium moiety (Fig. 1, grey). To estimate the effect of concentration on the N-H signal, 1 H NMR measurements of m-3b . Cl with concentrations varying from 0.83 to 2.50 mg mL À1 were carried out. The N-H signal shift of 0.1 ppm turned out to be negligible with respect to the strong dependence of the NH signal on the anion (see Supplementary Material for details). This effect rather might be due to either the interaction of the anion with the anisotropic cone of the aryl ring than due to the conjugation between the guanidinium cation and the aryl ring. Previous DFT calculations of several guanidinium salts revealed no or only very little conjugation, i.e. the guanidinium moiety behaves like an isolated cation. [39] In agreement with these studies, the N-H signal shifted upfield with increasing anion radii [64,65] with an almost linear correlation (Fig. 2).

Tyrosine‐Based Ionic Liquid Crystals: Switching from a Smectic A to a Columnar Mesophase by Exchange of the Spherical Counterion

“…The experimentally observed lattice parameters a and b are in good agreement with a calculated molecular length of around 28 Å for 3(14 , 14)Cl (Supporting Information, Table S1) and an estimated diameter of the aggregate of approximately 56 Å. This behaviour is in contrast to other calamitic guanidinium ILCs for which anion exchange to hexafluorophosphate resulted in a considerable reduction or complete loss of mesomorphism …”

“…The experimentally observedl attice parameters a and b are in good agreement with ac alculated molecular length of around2 8 for 3 (14,14)Cl (Supporting Information, Table S1) and an estimated diameter of the aggregate of approximately 56 .T his behaviour is in contrast to other calamitic guanidinium ILCs for which anion exchange to hexafluorophosphate resulted in ac onsiderable reduction [70] or complete loss of mesomorphism. [71] The change of mesophase typefrom SmA to Col r in the case of 3(14,14)PF 6 might be due to the larger ion radius of the spherical PF 6 as compared to the smaller tetrafluoroborate or halides, which cannot be accommodated in al amellar organization of the SmA mesophase but rather prefers the formation of ad isc-like aggregate, leading to ac olumnar packing. To explain the mesophase type change of ILCs 3(14,14)X,w ep ropose as imilarp acking mechanism as comparedt om icellef ormationi nl yotropic liquid crystals.F or this purpose, we applied the packing model by Israelachvili [72] for hydrocarbona mphiphiles and by Tschierske [73] for lyotropic liquid crystals to ILCs 3 (Figure 11).…”

Ionic Liquid Crystals: Versatile Materials

“…In recent years, many other LC guanidinium salts in which the cationic core is covalently connected to a substituted aromatic ring or ring system have been reported by Laschat and co-workers (Scheme ; see the Supporting Information for an explanation of structure codes). ,,,− They prepared both “acyclic” guanidinium compounds (such as 320-X- n / m ) and “cyclic” guanidinium salts (such as 329-R 1 /R 2 ). In the first instance, guanidinium salts with one or two phenyl rings directly attached to the acyclic ionic moiety ( 319-X-(a-k)- m and 320-X- n / m ) were studied extensively. , Structure–property relationships were investigated both experimentally and theoretically by varying a number of factors, including the length of the aromatic moiety attached to the ionic core, the counterion, the terminal alkyl chain length, the type of terminal alkyl chain, and the possibility of hydrogen bonding, which can be modulated via N -alkylation.…”

Hunting for smectic C in calamitic azobenzene ionic liquid crystals with different cationic head groups

“…For example, the lamellar nontilted SmA phase is the most common mesophase found in ILCs, while tilted SmC phases are much rarer as compared with the corresponding SmA/SmC ratio in nonionic thermotropic liquid crystals . There are relatively few examples of ILCs with SmC phases described in the literature, most notably calamitic biphenyls, phenylpyrimidines, and azobenzenes with various cationic head groups. However, general design principles for SmC promoting mesogens are unknown for ILCs in contrast to the guidelines successfully developed by Lemieux for noncharged liquid crystals .…”