Star-shaped molecule

Star-shaped molecules have been developed as an interesting class of semiconducting materials and used in organic solar cells because of a number of advantages (122). 

By tailoring the functional groups in the core and the arm, star molecules can be designed to realize a low band gap, strong and broad absorption, together with high mobility, resulting in an improved performance (121,123-126) 

The intermediate compound 4-bromo-7-[5 - -hexyl-(2,2, 5, 2 -terthiophene)-5-yl]-benzo[c][l,2,5]thiadiazole was s)mthesized by a Stnie coupling reaction of the compound 4,7-dibromobenzo[c]-[l,2,5]thiadiazole with 5-(n-hexyl)-5 -(tributylstannyl)-2,2, 5, 2 -terthiophene. The terthiophene unit was used to extend the conjugated length in order to broaden the absorption spectrum (121). 

The impact of alkyl side chain substituents on conjugated pol5uners on the photovoltaic properties of bulk heterojimction solar cells has been studied extensively (128). However, their impact on small molecules has not received adequate attention. In order to assess the effect of side chains, a series of star-shaped molecules based on a triphenylamine core, bithiophene, and dicyanovinyl units deriva-tized with various alkyl end-capping groups of methyl, ethyl, hexyl and dodecyl have been S5mthesized. These molecules were studied to get structure-property relationships. 

UV-vis absorption and CV showed that the variation of the alkyl chain length has only little influence on the absorption and the highest occupied molecular orbital and lowest imoccupied molecular orbital levels. 

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Star-shaped molecules with oligophenyl arms derived from 9,9-spirobifluorene as the central unit have been synthesized by Tour et al. [58] and Salbeck et al. [59] and have been suggested as potential emitter materials for blue LEDs. 

Star-shaped polymer molecules with long branches not only increase the viscosity in the molten state and the steady-state compliance, but the star polymers also decrease the rate of stress relaxation (and creep) compared to a linear polymer (169). The decrease in creep and relaxation rate of star-shaped molecules can be due to extra entanglements because of the many long branches, or the effect can be due to the suppression of reptation of the branches. Linear polymers can reptate, but the bulky center of the star and the different directions of the branch chains from the center make reptation difficult. 

The main topic of interest is the properties of molecules of finite size, having no large rings, and in general having trifunctional branch-points. These are typically produced by chain-transfer with polymer in free-radical polymerizations, though they can of course be made in other ways. Molecules with branch-points of higher functionality are also of interest, especially star-shaped molecules with several arms, as these are both easy to synthesize and relatively easy to discuss theoretically. 

Chompff,A.J. Normal modes of branched polymers. I. Simple ring and star-shaped molecules. J. Chem. Phys. 53,1566-1576 (1970). 

These sites can be used to initiate the polymerization of a small amount of di-vinylbenzene (DVB)8-10. The bifunctional monomer will polymerize to small tightly crosslinked nodules each of them is connected with the /chain ends which have participated in its initiation process. The average functionality /of the nodules is not directly accessible. However, it was shown in the case of star-shaped molecules that /increases with the overall concentration of the precursor, and, to a smaller extent, with the amount of DVB added per living end./is almost independent of the precursor chain length11. ... 

Lehmann et al. developed the concept of star-shaped molecules composed of three phenylbenzoate based rod-like cores directly connected by ester groups to a central... 

Fig. 2 Classification of star-shaped three arm mesogens. Flexible (subgroup (i)), semi-flexible (subgroup (ii)) and shape-persistent (subgroup (Hi)) star-shaped molecules. Folding of star-shaped semi-flexible molecules to X- and -shaped mesogens [2], Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

The most comprehensive studies on shape-persistent Hekates have been performed on stilbenoid star-shaped molecules. Structures and mesomorphic properties are collected in Table 1. Core building blocks with only one repeating unit per arm and one flexible chain 26a,b, 28a-d, 30a did not show any liquid crystal properties [56-58]. In the series of two chain derivatives 28e, 30b,c the formation of meso-phases depend on the core [57-59]. The electron deficient triazine and the dicyanopyridine building block induced obviously columnar mesophases. The pyridine derivative 30b showed only a crystalline phase [58]. In the series of nine chain stars 26c-g [60-62] and 28f-n [57], the compounds formed columnar phases depending on the chain length of the peripheral chains. Propyloxy chains are too short but hexyloxy and dodecyloxy chains are sufficient for the formation of liquid crystal phases by nanosegregation [60-62]. This can be rationalised by a dense... 

Gasteier P, Reska A, Schulte P et al (2007) Surface grafting of PEO-based star-shaped molecules for bioanalytical and biomedical applications. Macromol Biosci 7 1010-1023... 

One form of high-density proteoglycan was obtained from NaCl extracts of EHS tumor and appears as a star-shaped molecule (Fig. 11). The... 

Other various copolymer systems, such as AB2 graft copolymers [119-121], star-shaped molecules of the (AB)n type [122, 123], (AB)n multiblock copolymer systems [124-126], AB ring-shaped copolymers [127, 128], and ABC star-shaped terpolymers [129-131], were also widely studied. 

Burchard, W. Statistics of star-shaped molecules. II. Stars with homodisperse side chains. Macromolecules 1974, 7 (6), 841-846. 

Cationic star-shaped molecules containing cyclopentadienyliron complexes, where the organo-metallic cations are evenly distributed throughout the molecule branches, 250, have been reported (Scheme 2.67).295 296 291 Electrochemical studies of these materials indicated that the iron centers underwent reversible reductions. For the hexametallic star-shaped molecule (n = 1), two redox potentials were observed at KU2 = I -20 and —1.30 V. 

Various first-generation dendrimers and star-shaped molecules were generated from the nucleophilic aromatic substitution of chlorosubstituted arenes coordinated to cyclopentadienyliron complexes (Schemes 2.68 and 2.69). These compounds contained either ether or ester bridges. The star-shaped molecules prepared with ether bridges showed a decrease in solubility with an increase in star size, whereas the incorporation of ester bridges resulted in a decrease... 

Star-shaped molecules containing cationic arene complexes of iron and ruthenium have been reported by Astruc and co-workers.298 Utilizing the activating nature of the cyclopentadienyliron moiety on the complexed arene, 260 was converted into 262 via bromobenzylation. The photolysis of 262 gave 263, which was subsequently reacted with 264 to give the hexametallic complex 265. Further nucleophilic aromatic substitution reactions with phenol 266 gave the allyl-substituted complex 267 (Scheme 2.70). 

A number of research groups have reported the preparation of a large number of star-shaped molecules and dendrimers containing ferrocenyl or arene cyclopentadienyliron complexes at the core or the peripheries.300-320 A number of these dendrimers were prepared via cyclopentadienyliron-mediated per-alkylation, -benzylation and -allylation reactions of cationic tri-, tetra- and hexamethylbenzene complexes. These dendrimers were multifunctional materials and have been used in the synthesis of branched organic and organometallic polymers. 

Scheme 2.71 shows the preparation of an octametallic star-shaped molecule containing cyclopentadienyliron arene complexes at the peripheries. The reaction of the organic core, 269, with the organometallic complex, 270, gave the organometallic star-shaped molecule, 271. A previously reported synthesis of 271 occurred via deprotonation of a permethylated iron complex. Astruc et al. have also reported silicon-containing star-shaped molecules, 272.312... 

Functional groups can be attached to the living polymer chains by choosing suitable termination reagents, resulting in linear or star-shaped molecules. With bi- or multifunctional organolithium initiators , a greater variety of polymer products can be synthesized,... 

Aromatic amines are frequently used as xerographic materials and incorporated in OLEDs,[139] while reports of their use as the semiconductor material in OFETs are few. Only very recently triarylamine polymers have been used in OFETs.[140,141] Star-shaped molecules with triphenylamine core and carbazole (20a, 20c-20d) or fluorene (20b) side groups have all been characterized in OFETs (Figure 3.1.7).[142] Devices containing these materials were fabricated using a drop casting technique and showed mobilities on the order of 10 cm V s with good stability under ambient conditions. 

Zimm and Kilb have investigated theoretical formulas for the limiting viscosity number of various model, branched molecules in dilute solution. For star-shaped molecules, it was found that the ratio Mbr/Mnn varies approximately as the square root of the ratio of the mean square radii of gyration. Zimm and Kilb then postulated that branched macromolecules, of any shape, will obey this relation to a degree of approximation sufficiently good for practical purposes thus... 

These types of macromolecules are shown in Fig. 8.1. Many other variants of macromolecules have been synthesized however, their overaU shape represents combinations of the above four basic types, for example, a globule with one or several long-coded chains grafted onto its surface. Simdarly, ad brush-type, worm-hke, star-shaped molecules, dumbbeUs, etc., can be classified according to combinations of the above basic structures. 

Use of a multifimctional core, e.g., (9) has been used to prepare star-shaped molecules by applying a RAFT process with styrene and acrylate monomers. 

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