Supermolecular units

Notable in the tricolorin A (106) solid state is the presence of 18 water molecules in the unit cell, and an anisotropic repartitioning of the hydrophobic and hydrophilic sections. The water molecules form a dense network that creates a dividing layer between the hydrophilic faces (see Fig. 6). The high water content indicates that the conformation in the solid state is not dominated by intermolecular forces and could be indicative of a similar conformation in both solution and supermolecular... 

Mooney,M. Theory of the non-newtonian rheology of raw rubbers consisting of supermolecular rheological units. J. Appl. Phys. 27,691-696 (1956). 

Supermolecular metallocatalysts, by combining a substrate recognition unit with a catalytic metallic site, offer powerful entries to catalysts presenting shape, regio-and stereoselectivity. 

When the cracks first appear, the propagation rate is small, and the stressed state is preserved until the fracture occurs. Mechanical force orients both the structural units and the macro- and supermolecular formations into directions parallel to the direction of the force. 

In such a way, an ordered supermolecular structure could be spontaneously formed from polymers exhibiting a random coil structure. The fundamental cause of this transformation is naturally the first-order structure of the polymer, i.e. its chemical structure. If a polymer with the proper structural units was prepared, then fiber formation would be accelerated and would especially account for the formation of a highly ordered structure. 

Complementary dendritic hexamers based on a central scaffold made up of linked pentaerythritol and tri(hydroxymethyl)amino methane units have been found to also exhibit liquid-crystalline properties. This star-shaped scaffold was used to create supermolecules containing two different hemispheres, referred to thereafter as Janus supermolecular Hquid crystals (Fig. 76) [323,324]. One of the hemisphere contains three cyanobiphenyl end-groups, whereas the other lobe consists of three chiral phenyl benzoate mesogenic moieties laterally attached. The type of mesophase observed (N ... 

Supramolecular chemistry - broadly the chemistry of multicomponent molecular assemblies in which the component structural units are typically held together by a variety of weaker (non-covalent) interactions - has developed rapidly over recent years. Typically is used since, in a considerable number of systems, metal-donor bonds - often essentially covalent in nature - have also been employed to stitch together organic components into larger assemblies. Such metal-linked assemblies will be treated as part of the supramolecular realm in the present work (although not employed here, perhaps supermolecular is a better term for this category). 

In the first two parts of this chapter, electron transfer (ET) from atomic donors, e.g., alkali metals or the iodine anion, to an accepting unit composed of simple molecular or atomic solvents was discussed. It was demonstrated that even for a molecule without a stable anionic state or large dipole moment, e.g., water and ammonia, an ensemble of a relatively small number of the molecules can act as an electron acceptor. In the case of the solvated alkali metal atom clusters, ET takes place spontaneously as the number of solvent molecules increases, while the ET in the solvated 1 clusters is induced by photoexcitation into the diffuse electronic excited states just below the vertical detachment thresholds. These ET processes in isolated supermolecular systems resemble the charge delocalization phenomena in condensed phases, e.g., excess-electron ejection from alkali metals into polar solvents and the charge transfer to solvent in a solution of stable anions. 

Here, (> and /> are initial and final states of the supermolecular complex, 11 is the induced incremental polarizability (a second-rank tensor), fio and fij are unit vectors in the direction of the electric polarization of incident and scattered waves, which are often specified in the form of subscripts V and H, for vertical and horizontal, (aif = (Ej- — Ei)lh is the energy difference of initial and final state in units of angular frequency, Pi T) is the population of the initial state (a function of temperature), <5 (to) is Dirac s delta function, and the summation is over all initial and final states of the collisional complex. 

There is no fundamental qualitative difference in mechanisms of low molecular weight (MW) penetrant diffusion in polymers above and below glass transition temperature, Tg, of the polymers [5,6]. The difference lies only in the fact that the movement of structural units of the macromolecule that are responsible for the transfer of penetrant molecules takes place at different supermolecular levels of the polymer matrix. At T > Tg the process of diffusion takes place in a medium with equilibrium or near-equUibrium packing of chains, and the fractional free volume, P(, in the polymer is equal to the fractional free volume in the polymer determined by thermal mobUity of strucmral units of macromolecules V((T), i e., V(= vut). At r< Tg the process of diffusion comes about under nonequihbrium packing conditions, although there exists a quasi-equilibrium structural organization of the matrix, where Vf> It is assumed that in this case Vf= where is the fractional free volume... 

Physical methods include tuning of chain conformation [39-42] and manipulation of supermolecular structure [43]. Because of its highly coplanar backbone, PFO can be physically transformed into a variety of supermolecular structures [39-42], such as crystalline phases (i.e., a and a phases) and non-crystalline phases (i.e., amorphous, nematic, and /3-phase for /3-phases it has an extended conjugation length of about 30 repeat units as calculated from wide-angle X-ray diffraction measurements) [44]. However, studies on the effects of tuning chain conformation on EL are scarce, but reports on effects of manipulating supermolecular structures on PL behaviors of PFO are extensive [40-42,44]. 

Fig. 1 Templates for the design of liquid-crystalline supermolecular materials. The meso-genic units are shown as various shapes, however, the materials may be constructed with mesogenic units of the same type. In this case, many of the constructions shown are essentially dendritic...

Further studies by Nishiyama et al. [34-45] showed that when taken in isolation, only one of the aromatic units within a supermolecular system has a propensity to exhibit liquid crystal phases, then the supermolecular material itself could be mesomorphic, see Fig. 5. For example, for the top molecular structure, 5 [45], in Fig. 5, only the biphenyl unit at the center of the structure supports mesophase formation, whereas the benzoate units are too isolated from the biphenyl moiety in order to affect mesomorphic behavior. The second material, 6 [45] has terminal phenyl units, which are only connected by aliphatic chains to the benzoate units. Thus in this case, the material has four aromatic units out of six which are not in positions that can enhance mesophase formation. However, the second material has similar transition temperatures and phase sequences to the first, i.e., both materials exhibit an unidentified smectic phase and a synclinic ferroelectric smectic C phase. If the third material, 7 [38], is examined, it can be seen that the mesogenic unit at the center of the supermolecule is an azobenzene unit which is more strongly supportive of mesophase behavior than the simple biphenyl moiety. Thus the clearing point is higher for this material in comparison to the other two. The attachment of the terminal phenyl unit is by a methylene spacer of odd parity, and as a consequence the smectic C phase has an anticlinic structure rather than synclinic. 

These studies demonstrate that for supermolecular materials it is not necessary to have all of the aromatic/rigid units being supportive of mesophase formation for a supermolecular material to be mesomorphic. Odd par-... 

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