Aromaticity, molecular nonlinearities

However, dendrimeric and hyperbranched polyesters are more soluble than the linear ones (respectively 1.05, 0.70, and 0.02 g/mL in acetone). The solution behavior has been investigated, and in the case of aromatic hyperbranched polyesters,84 a very low a-value of the Mark-Houvink-Sakurada equation 0/ = KMa) and low intrinsic viscosity were observed. Frechet presented a description of the intrinsic viscosity as a function of the molar mass85 for different architectures The hyperbranched macromolecules show a nonlinear variation for low molecular weight and a bell-shaped curve is observed in the case of dendrimers (Fig. 5.18). 

Phthalocyanines 244 and hemiporphyrins 245 and 246 are aromatic systems. Extended conjugation confers special properties to these molecules that make them building blocks for new molecular organic materials with useful electric and nonlinear optical applications (Scheme 85).288... 

Self-poisoning is more prominent in solvents consisting of smaller nonlinear aromatic molecules as compared to the linear aliphatic solvent octacosane. The effect may be due to varying degrees of molecular preordering in solution. 

CpCo(CO)2 under UV irradiation and from the polymerizations of metallolyl (57 and 58) and silyldiynes (66-70) catalyzed by tantalum halides [86-91]. The homopolycyclotrimerizations of other aromatic diynes (38-56) all proceeded very rapidly, giving polymeric products that were only partially or totally insoluble in common organic solvents due to the involved cross-linking reactions. The large free volumes and irregular molecular structures generated by the nonlinear carbazolyl, diphenylamine, metallolyl and silyl groups may have helped endow the homopolymers (hb-P57 to hb-P70) with the excellent solubility. 

Work is currently in progress on the synthesis of these and similar polyhedral-based NLO compounds with charged aromatic donor and acceptor groups in order to experimentally determine their molecular structures and measure their nonlinear responses. 

Quantum mechanical analysis based on a simple two-level model [22] and bond-order alternation (BOA) principle exploiting aromaticity [23] have worked surprisingly well in providing useful structure/property relationships for the design of chromophores with ever improving molecular hyperpolarizability. Table 1 provides some representative examples with improved molecular optical nonlinearity developed over the past decade. It has been shown that very large nonlinearities... 

Some examples of intuition-based or modeling-based advances in nonlinear properties, specifically the first hyperpolarizability response p, have been developed in the past three decades. These include the bond-length alternation motif, the idea of octopolar molecular structures, the stronger responses of excited states and the use of purposely twisted -ir-electron molecules to modify the admixture of quinoid and aromatic structures. 

Recently, Jen and coworkers developed a new class of molecular glasses based on the reversible self-assembly of aromatic/perfluoroaromatic (Ar-ArF) dendron-sub-stituted nonlinear high-p/i chromophores [156]. In these molecules (Fig. 4.27), both phenyl and pentafluorophenyl rings are incorporated as peripheral dendrons on the re-bridge and the donor-end of the chromophores Cl, C2, and C3. 

The RTV silicones, in general, exhibit considerably greater outgassing of both condensable and noncondensable materials than the HV silicones. Their condensable-time curves were usually linear and only one condensable product was being evolved. The Infrared spectra of the condensable products from all of the silicones evaluated indicated these materials to be predominately low molecular weight alkyl linear and/or cyclic polysiloxanes (Table II). Silicone A gave a nonlinear condensable-time curve and evolved two types of condensable products an alkyl linear and/or cyclic polysiloxane, and an aromatic ester. 

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