No abstract

With the advent of supramolecular chemistry and later nanotechnology a great deal of research has been focused on new types of molecular structures, which are not held together by covalent bonds but by non-covalent mechanical interactions. Examples include the catenane, rotaxane, and knot interlocked structures. The design and synthesis of these architectures is an art by itself and as such is worth being reviewed. In this tutorial review we will focus, however, on the functional aspects of interlocked molecules and discuss how these can find applications, e.g. as artificial muscles, as molecular valves, as components of electronic devices, and as catalysts.

N,N',N″-trialkylbenzene-1,3,5-tricarboxamides (BTAs) self-assemble by means of strong, threefold α-helix-type intermolecular hydrogen bonding into well-defined, helical, one-dimensional columnar aggregates. When a stereogenic centre is introduced into the alkyl side chains of these BTAs, strong Cotton effects are observed in dilute apolar solutions, indicating the preference for one helical conformation over the other. Here, we report the creation of a helical sense preference in self-assembled BTAs by introducing deuterium/hydrogen isotope chirality into the alkyl side chains. We determine the relative stabilities of the left- and right-handed helical conformations of these deuterated supramolecular polymers by performing a conformational analysis. Our findings show that the results of deuterium/hydrogen substitution in BTA-based supramolecular polymers and helical polyisocyanates are very similar, although the formation mechanisms differ. The selectively deuterated BTAs discussed here represent the first example of supramolecular chirality resulting from isotope substitution.

The amide bond is a versatile functional group and its directional hydrogen-bonding capabilities are widely applied in, for example, supramolecular chemistry. The potential of the thioamide bond, in contrast, is virtually unexplored as a structuring moiety in hydrogen-bonding-based self-assembling systems. We report herein the synthesis and characterisation of a new self-assembling motif comprising thioamides to induce directional hydrogen bonding. N,N',N''-Trialkylbenzene-1,3,5-tris(carbothioamide)s (thioBTAs) with either achiral or chiral side-chains have been readily obtained by treating their amide-based precursors with P2S5. The thioBTAs showed thermotropic liquid crystalline behaviour and a columnar mesophase was assigned. IR spectroscopy revealed that strong, three-fold, intermolecular hydrogen-bonding interactions stabilise the columnar structures. In apolar alkane solutions, thioBTAs self-assemble into one-dimensional, helical supramolecular polymers stabilised by three-fold hydrogen bonding. Concentration- and temperature-dependent self-assembly studies performed by using a combination of UV and CD spectroscopy demonstrated a cooperative supramolecular polymerisation mechanism and a strong amplification of supramolecular chirality. The high dipole moment of the thioamide bond in combination with the anisotropic shape of the resulting cylindrical aggregate gives rise to sufficiently strong depolarised light scattering to enable depolarised dynamic light scattering (DDLS) experiments in dilute alkane solution. The rotational and translational diffusion coefficients, D(trans) and D(rot), were obtained from the DDLS measurements, and the average length, L, and diameter, d, of the thioBTA aggregates were derived (L = 490 nm and d = 3.6 nm). These measured values are in good agreement with the value L(w) = 755 nm obtained from fitting the temperature-dependent CD data by using a recently developed equilibrium model. This experimental verification validates our common practice for determining the length of BTA-based supramolecular polymers from model fits to experimental CD data. The ability of thioamides to induce cooperative supramolecular polymerisation makes them effective and broadly applicable in supramolecular chemistry.

Small changes in the alkane solvent structure in combination with temperature effects lead to four different conformations of stereoselectively deuterated benzene-1,3,5-tricarboxamides in the aggregated state, affecting the expression of the supramolecular chirality and highlighting the role of the solvent structure in self-assembly processes.

A detailed analysis of the conformational states of self-assembled, stereoselectively deuterated benzene-1,3,5-tricarboxamides ((S,S,S)-D-BTAs) reveals four different conformers for the supramolecular polymers. The relative amount of the conformers depends on the solvent structure and the temperature. With the help of a model, the thermodynamic parameters that characterize the different conformational states were quantified as well as the amount of the species that occur at different stages of the polymerization process. The results show that small changes in the stability between different types of conformers formed by (S,S,S)-D-BTAs—in the order of a few J mol(–1)—arise from the combination of interactions between the solvent/supramolecular aggregate, temperature, and solvent structure. While the introduction of a deuterium label allows to sensitively probe the solvophobic effects in the supramolecular aggregation, a rationalization of the observed effects on a molecular level is not yet straightforward but is proposed to result from subtle effects in the vibrational enthalpy and entropy terms of the isotope effect.

Saluting the sergeant: Phg-BTA (see scheme) cooperatively self-assembles into helical aggregates and shows unprecedented racemization behavior in the presence of base. In thermodynamically controlled conditions, the addition of a small amount of chiral auxiliary to this mixture results in a deracemization reaction and a final enantiomeric excess of 32 %. A theoretical model is presented to understand in detail the results obtained.

Racemization, an irreversible process arising from the reversible interconversion of enantiomers, [1] is governed by simple first-order kinetics. [2] Racemization can occur under a variety of conditions and is associated with the disappearance of optical activity. [1] On the other hand, the conversion of a racemic mixture into an excess of one enantiomer, deracemization, is useful to yield nonracemic products out of a racemic mixture without the intermediate separation of materials. [3] In the examples of deracemization, chiral auxiliaries are used to favor the nonracemic product: such as chiral catalysts in dynamic kinetic resolutions [4][5][6][7] or chiral hostguest interactions. [8,9] More recently, attrition-enhanced Ostwald or Viedma ripening was used to obtain enantiomerically pure crystals from racemic mixtures, in which the preference of the handedness is determined by chiral external forces, including chiral seeds. [4,[10][11][12][13][14] Herein we present a supramolecular system based on cooperative self-assembled helicity, which exhibits strong chiral amplification and in which racemization and deracemization processes are in thermodynamic equilibrium and take an unprecedented form.In earlier work, we showed that enantiomeric N,N',N''trialkylbenzene-1,3,5-tricarboxamide (BTA) molecules form helical supramolecular polymers in solution and show strong majority-rules behavior. [15][16][17][18] In a system under majority-rules control, the major enantiomer dictates the helical sense of the supramolecular polymer to which the minor enantiomer adjusts. This unique behavior results in a strong nonlinear relationship between the optical activity of the supramolecular polymer and the overall enantiomeric excess (ee), which were described and quantified by one-dimensional Ising models adapted from the theory of covalent helical polymers. [19] Recently, we introduced a novel method for quantifying chiral amplification in a two-component self-assembling BTA system by taking into account the dynamic equilibrium between free monomers and the supramolecular polymer and the cooperative growth of the corresponding polymer. [20,21] Our method is based on a common reaction scheme in which supramolecular polymerization is described as a sequence of stepwise monomer addition and dissociation events. In this model, the equilibrium concentrations of the BTA molecules in the free monomer state and in the aggregated state are predicted as a function of the overall ee value by solving the mass-balance equations for the enantiomeric monomers. [20] Calculations indicate that the majority-rules behavior in BTA-based supramolecular polymers leads to a nonlinear change in the concentration of enantiomeric free monomers as a function of the overall ee value under the conditions at which BTA monomers are mostly aggregated. In other words, the ee value of the enantiomeric units participating in the aggregate differs from the ee value of the free enantiomeric molecules in the solution except at an ee value of either 0 or 100 %. Similar ...