Self-aggregation process

FIG. 42 Schematic of the self-aggregation process for semiconductor nanoparticles. 

The most elusive of self-aggregation processes is presented by the so-called hydrotropes amphiphilic compounds with the hydrophobic moiety being typically too small to induce micelle formation. Examples of hydro tropes are butylmonogly cosulfate (BMGS),/7-toluenesulfonate (PTS), and cumenesulfonate (CS) (Scheme 1). 

The assembling of molecules to form stable thiee-dimensional aggregates is the quintessence of the selfrecognition and self-aggregation processes that are behind the construction of any supramolecular system. The understanding of these processes requires a profound knowledge of the interactions among molecules and ions in the solid state. ... 

In relation to self-assembling, natural elastin suffers a self-aggregation process in its natural environment. Elastin is produced from a water-soluble precursor, tropoelastin, which spontaneously aggregates yielding fibrilar struc-... 

Poly(ethylene oxide)-Z)/oc -poly(organophosphazene) copolymers (274a-c) and (275a-b) have been prepared by controlled cationic polymerization of phosphoranimines. The self-aggregation process of (274a) in aqueous solution as a function of the temperature has been studied. ... 

NMR experiments are widely used to investigate the self-aggregation process of amphiphilic polymers. Most of these substances have been designed as drug carriers and present a multi-block architecture. Poly(ethylene glycol) blocks are often found in self-associating copolymers. 

One straightforward way to predict the occurrence of a self-aggregation process using volumetric data is to calculate the apparent molar volumes of the solute, Vi ), as a function of the concentration of the solutions... 

The presence of a critical St content in ASt-x can also be seen in fluorescence spectra [29], This copolymer in aqueous solution shows an excimer emission peaking at 325 nra. As shown in Fig. 8, the intensity of the excimer emission increases, while the monomer emission decreases, with increasing St content. Eventually the excimer dominates the monomer emission at an St content of 72 mol%. The excimer emission becomes apparent at an St content of about 50 mol%, which agrees with the critical St content estimated by viscometry and NMR spectroscopy. The existence of the critical St content suggests the hydro-phobic self-aggregation to be a cooperative process. 

Probably the best example of the manifestation of self-assembly processes in dendritic systems via H-bonds is seen in the recent work of Zimmerman et al. [156]. Dendritic wedges possessing tetraacid moieties (67) self-assembling into a hexameric, disc-like framework (68) was confirmed by SEC and -NMR studies. The tetraacid unit (69) is known to form cyclic as well as linear aggregates in solution via carboxylic acid dimerization (Fig. 30). However, with incorporation of larger dendritic wedges on 69, the hexamer form is preferred. 

Figure 6.9 Schematic representation of hierarchical self-assembly process for chiral phthalocya-nine 64. Phthalocyanine molecules self-assembly into helical columns with right-handed screw sense (left). These right-handed helices subsequently aggregate to give left-handed super-helices.

Figure 6.12 Self-assembly process of 24 from single molecules into helical columns via nonhelical intermediates. The average number of molecules, (IV), participating in one column in n-butanol at three different concentrations is given as function of temperature. Arrows mark transition from achiral to helical aggregates.

Size and shape Certain steric effects can be achieved using characteristically wedge-shaped dendrons. Thus, self-assembling dendrons have been connected to supramolecular aggregates with defined dimeric or hexameric structures. Such aggregates can form columnar superstructures which reveal liquid-crystalline properties. Spherical superstructures arise from the self-assembly process when conical dendrons are used14 14,161 Similar... 

The dependence of the fluorescence quantum yields and lifetimes of these stabilizers on the nature of the solvent suggests that the excited-state, non-radiative processes are affected by solvation. In polar, hydroxylic solvents, values of the fluorescence quantum yield for the non proton-transferred form are significantly lower, and the fluorescence lifetimes are shorter, than those calculated for aprotic solvents. This supports the proposal of the formation, in alcoholic solvents, of an excited-state encounter complex which facilitates ESIPT. The observed concentration dependence of the fluorescence lifetime and intensity of the blue emission from TIN in polymer films provides evidence for a non-radiative, self-quenching process, possibly due to aggregation of the stabilizer molecules. 

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