Disc-like solvents

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

As their name implies, liquid crystals are materials whose structures and properties are intermediate between those of isotropic liquids and crystalline solids (2). They can be of two primary types. Thermotropic liquid crystalline phases are formed at temperatures intermediate between those at which the crystalline and isotropic liquid phases of a mesogenic compound exist. Substances which exhibit thermotropic phases are generally rod- or disc-like in shape, and contain flexible substituents attached to a relatively rigid molecular core. Lyotropic liquid crystalline phases are formed by amphiphilic molecules (e.g. surfactants) in the presence of small amounts of water or other polar solvent. In general, the constituent molecules in a liquid crystal possess orientational order reminiscent of that found in the crystalline phase, yet retain some degree of the fluidity associated with the isotropic liquid phase. 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

Just as chiral induction can be realized in discotic liquid crystals, so it can in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical fm-amides (Fig. 6), which exhibit discotic liquid-crystalline phases, also form chiral columnar stacks through n-n interactions when dissolved in apolar solvents,which are depicted schematically in... 

A typical time evolution of fluorescence anisotropy is a monotonously decreasing function. However, the sum of several exponentials with both positive and negative prefactors derived on the basis of a rigid rotor model does not preclude increasing or even a non-monotonous time evolution. The non-monotonous time evolution has been observed for perylene excited to S2 quite far in the blue region with respect to the emission [11], It starts, as predicted for the perpendicular orientation of dipole moments, at ro = -0.2, but increases rapidly to a slightly positive transient value and then decreases more slowly to = 0. The non-monotonous r t) decay can be rationalized by the solvent effect on the rotation of the flat disc-like perylene around three different axes. 

In lyotropic liquid crystals theses micelles are the mesogens which built up the liquid crystalline phases. Depending on the solvent concentration, different types of micelles are possible. The most common micelles, i.e. rod-like micelles, disc-like micelles and spherical micelles, are depicted in the lower part of Fig. 3.1. Furthermore, the surfactant molecules may also aggregate into lamellas which represent full or partially interdigitated bilayers of the molecules. Those lamellas are, strictly speaking, no micelles as they extend infinitely into two dimensions, but yet the driving force for their formation is the same. 

For values of the packing parameter II smaller than 1/3, spherical micelles can be expected. For values up to 1/2, rod-like micelles are most likely, followed by disc-like micelles at increasing values of II. For values of approximately 1 the formation of lamellas dominates. At very low solvent concentrations or if using apolar solvents the packing parameter may take values larger than 1. Under these conditions inverse micelles are formed. They look similar to the micelles shown in Fig. 3.1, but instead of the alkyl chains, the hydrophilic head groups are located in the centers of the micelles. 

Under just the right conditions, a mixture of a highly polar liquid, a slightly polar liquid, and an amphiphilic molecule form micelles that are not spherical. They can be rodlike, disc-like, or biaxial (all three axes of the micelles are different). These anisotropic micelles sometimes order in the solvent just as liquid crystal molecules order in thermotropic phases. There is a nematic phase of rod-shaped micelles, another nematic phase of disc-shaped micelles, and even a biaxial nematic phase, in which the molecular axes transverse to the long molecular axis partially order. Chiral versions of these phases with the same structure as the chiral nematic phase also form. 

While normally amorphous, and generally featureless on a micron scale, crystallization of polycarbonate was solvent-induced with butyl acetate, generating a disc-like spherulitic structure of ca 10 fina in diameter surroimded by an amorphous matrix. Within the spherulite, the twisted fibrils emanating from the point of nucleation were observed in these afm images, and is consistent with known lamellar growth mechanisms (103). 

The terms liquid crystal and mesophase are interchangeable. Meso, in Greek, means between , so a fluid can be called a mesophase if it has some properties that are characteristic of crystals. The ability of the fluid to form a liquid crystal is due to the molecules ability to align with each other and create local ordering. So, liquid crystalline polymers are those polymers that form liquid crystalline phases either in solution or in the melt. Molecules that form a mesophase are usually rod-like or disc-like. In the case of rodlike polymers, such as poly(p-phenyleneterephthalamide) (PPTA), the rigidity of the backbone is primarily responsible for the formation of a mesophase. The rigidity is, of course, dependent on a variety of factors such as the nature of the solvent used, the temperature of the solution, and the chemical structure of the molecule. 

Some experimental results on the rates of photoassociation of various fluorophores are given in Table 6.1 [26]. The figures in the last column are typical in that most of the rate constants are within a factor of 2 of the diffusion-controlled value as calculated by the Smoluchowski theory for slip conditions (Section 3.4.3.1), regardless of solvent and temperature. From spectroscopic evidence, it appears that the dimer has a considerable dipole moment, suggestive of a charge-transfer complex. We may suppose that when these large disc-like molecules collide they usually form such a complex without stenc hindrance and with little or no activation-energy requirement. 

The linear growth of micelles is the dominant type, but disc-like or plate-like structures can also form, albeit over a narrow range of conditions. Linear growth can also lead to branched structures, which at a high enough concentration may lead to interconnected structures (Figure 3.6) that are referred to as bicontinuous , since the solutions are not only continuous in the solvent but also in the surfactant [ ] ... 

This particular type of mesophase can also occur on heating mixtures of certain organic solids of which at least one has a disc-like molecular structure, as we shall see. Furthermore, it should be mentioned here that suitable discotic compounds dissolved in various solvents, thus constituting binary lyotropic systems, may also give rise to a nematic phase. 

Lyotropic nematic phases (see Section A) can also be produced by preparing, for instance, binary or ternary mixtures of organic disc-like compounds in suitable solvents such as hydrocarbons [20]. In linear saturated [20,21] or, as found recently [21], even better in cyclic saturated hydrocarbons, preferably cyclohexane [21], almie or in such a solvent plus an achiral or a chiral electron acceptor compound, induction of lyotropic Ncd or N coi phases, respectively, can occur. Sometimes, an Ncd phase can be formed in addition to a columnar phase [21]. Furthermore, it has also been observed that even two different No>i phases can be induced in diat way in the same system [22,23] showing a nematic-nematic phase transition [22-24] due to a diffa ence in the construction of their columns. In one of these Ncoi phases the constituent discs of the columns spontaneously formed are tilted with respect to the column axis, but in the second, parallel Ncoi phase they are untilted [22,23]. However, reliable data about the length of the columns in Ncoi phases do not yet seem to exist... 

Figure 5a shows CD spectra of tartaric acid, which has an absorption i the short wavelength region and thus is prone to suffer from dispersion effect as compared with transition metal complexes. Two solution spectra in solvent of different polarity, water and dioxane, are similar to each other, but the CD C a nujol mull is quite different from that in solution. A KBr disc prepared t avoid dispersion effects gave a solid-state tartaric acid spectrum similar to thi in solution (Fig. 5b). Thus the difference between the nujol mull CD and solutid CD is not due to the different molecular conformation or intermolecular intera tion in the two phases. Most likely, it is due to the dispersion effect in the cas of the nujol mull form. Many nujol mull CD spectra of organic compound have been reported recently, but most of them appear to suffer from substanth dispersion effects. It is to be noted that the dispersion terms for molecules C...

---