A. M. Zanin

Phenomenological description of the spontaneous formation of macroscopic strings in low-concentration chiral solutions and the formation of anisometric gels

36 Gel formation has recently been described in chiral solutions of low concentrations (10–3 to 10–2 M) [1], which are two orders of magnitude lower than the per colation threshold for the formation of an isotropic gel [2]. The forming gel has a well discernible microstruc ture [1]. Examination of the xerogel produced after solvent evaporation from the samples detected aniso metric (with a length to diameter ratio of 103–105) structural elements with observable rigidity—strings [1, 3, 4]. In this work, we identified the phenomena that lead to the solidification of low concentration chiral solu tions and studied the solidified phase by the example of solutions of trifluoroacetylated amino alcohols (TFAAA) (table). TFAAA molecules are chiral (except compound 4 in the table) and approximately isometric, which allows one to exclude the effect of nonchiral steric factors on solidification. The molecu lar design of TFAAA ensures the manifestation of the full range of weak intermolecular interactions in the formation of supramolecular structures. The solutions we used were cyclohexane, chloroform, carbon tetra chloride, benzene, ethanol, methanol, and acetone. The solution microstructure was investigated by optical microscopy. Solvent evaporation was pre vented because the samples of the studied solutions were placed in closed vessels. At a concentration on the order of 10–3 M and higher and a temperature of 300–340 K, a condensed phase separates out from homochiral TFAAA solu tions. On cooling a solution within a capillary 300 μm i.d., the phase separates out as an individual discrete anisometric structural element or elements—strings (Fig. 1). The string diameter (1–3 μm) is constant, and the string length reaches several millimeters. The strings form approximately along the capillary axis, definitely being repelled from the capillary walls and showing no signs of interacting with each other. Thus, the string formation in these experiments is a separate physical phenomenon unrelated to gel formation.

Macroscopic chirality of strings

The macroscopic chirality of strings in xerogel obtained from a solution of N-trifluoroacetyl-R(+)-valinol in cyclohexane was revealed by atomic force microscopy and circular dichroism. The characteristic plots on different scales are reported, namely, large-scale “chaos” (∼100 μm), string commutation (∼10 μm), and “fine” structure (∼1 μm).

Experimental Observation of Anisometric Structures in Solutions with a Low Gelator Content

Gelators with a molecular mass of 200 Da are synthesized, and the mechanisms of the formation of anisotropic supramolecular structures during the gelation of N-trifluoroacetyl derivatives of amino alcohols are discussed. The conditions of their stability are identified and the thresholds of gelation of the alcohols in various solvents are determined. It is shown that the gels are formed at concentrations of 10−2–10−3 M, values substantially lower the percolation threshold for isotropic molecules. An analysis of possible topological structures formed in the gel phase is performed.

Compaction of intermolecular bonds in the macroscopic chiral phase of strings

The microstructure of a cured state characteristic of a wide variety of low-concentration homochiral solutions in comparison with that of the condensed phase formed in achiral solutions is studied using optical and atomic force microscopy and X-ray diffraction analysis.

Chirooptical effects in dilute solutions of gelators

The chirooptical properties of a number of compounds in a variety of achiral solvents are studied. The results are interpreted within the framework of the standard model of chiral molecules. It is demonstrated that, with increasing concentration, chiral aggregates emerge, up to the formation of a macrophase (strings), which radically changes the chirooptical characteristics of the solution.

Commensurability effects in chiral strings

The previously constructed continuum model of aninfinite straight string inevitably assumes the presenceof a quasionedimensional supramolecular lattice,which intuitively leads to physical and geometricalnotions of stereospecific fit, or complementarity, ofhomochiral molecules and their stacking [1, 2].At the same time, it has been experimentally (byoptical microscopy) demonstrated that, even in nonpolar solvents (heptane, cyclohexane, benzene), thestrings of the same trifluoroacetylated amino alcohol(TFAAA) have essentially different morphology [1, 2].To explain this difference in string structure, the crucial question is whether the molecules constituting thestrings are commensurate, namely, whether the compact complementarybound supramolecular lattice ofstrings involves achiral solvent molecules. The notionof commensurability implies a continuous change inphysical properties and, thus, invariance of the majorgeometric pattern of the molecular lattice or its continuum model when an extra molecule is incorporatedinto its finite volume [3]. In particular, for an infinitestring, the pattern is undoubtedly determined by itsgeometry.In more general form, the problem of commensurability in formation of a supramolecular lattice shouldalso be considered for the case of simultaneous incorporation of chiral TFAAA molecules of different typesinto the lattice.To solve this problem, in this work, we studied byXray diffraction xerogels of homochiral TFAAAs,synthesized as described in [1], in different solventsand xerogels of their equimolar homo and heterochiral mixtures, also prepared according to [1].

Structural dynamics of chiral strings

It is demonstrated that a microscopic model of an antiferroelectric elementary string makes it possible to explain the specifics of the structural macrokinetics and macroscopic dynamics of chiral strings. It is established that the formation of strings is controlled by diffusion, whereas the supercoiling of strings, by Van der Waals interactions between them. Three modes of strings formation were identified: the uniform growth of cylindrical strings, π-assembly of an inverted cone of thin strings, and π-decay. The π-assembly is accompanied by the rotation of the string about its axis, which can cause instability, leading first to a bending of the string and then to the formation of loops.

Experimental investigation of anisometric chiral phase xerogel

The microstructure of a cured state (xerogel) of homochiral and achiral compounds belonging to a series of trifluoro acetylated amino alcohols has been investigated. It is shown that the substance of the cured homochiral compound consists of anisometric structures with an observed rigidity (strings) which are crystalline monophase. The xerogel of an achiral compound is represented by isometric microstructures. It is suggested that intermolecular hydrogen bonds comprising the molecular strings are apparently packed within the phase.

Initiation of the formation of chiral strings: Dimension of formation domain, microstructure, and nucleation mechanism

The initiation of the formation of strings in low-concentration chiral solutions was experimentally studied. The domains of initiation were classified according to dimensions: (0–3)D. The dipole-dipole mechanism of the formation of anisotropic nucleation in a mesophase was considered. The role of dispersion interactions in the assembly of supramolecular stacks was estimated at ∼0.3–0.4 eV. The threshold values of string formation in chiral mixtures were determined. A phenomenon of the formation of paired strings was established and characterized.

Self assembly of supramolecular homochiral structures in solutions of chiral biomimetics

The phenomenon of the formation of homochiral sets of supramolecular vortices in xerogels with a low concentration of chiral biomimetics of trifluoroacetylated amino alcohols that are metrically similar to certain macromolecular and macroscopic helical biological structures was discovered and described. Each vortex, with a characteristic scale of ∼ 1 mm, corresponds approximately to the rotational symmetry Sn groups depending on the number of strings that have a common anchorage or are formed from a single nucleation and experience a mutual agreed deviation. It is essential that the direction of deviation is changed to the opposite one with the change of the chirality of the enantiomer and does not depend on the solvent type. Both weakening and strengthening of this trend were observed in the mixtures of various chiral compounds. The result is an important confirmation of the synergetic rule that was proposed by authors. According to this rule, the sign of the chirality of a supramolecular helix is changed to the opposite with a change in the chirality sign of the basic monomers from which it is formed. This regularity has a fundamental character and is executed as a trend in the hierarchies of DNA, protein, and polysaccharide macromolecular structures.