Phenylene rings

This modular concept allows for the synthesis of monodisperse dendritic phen-ylene dendrimers of the first (43a, 22 benzene rings) and second (43b, 62 benzene rings) generation [65]. Due to the dense packing of the phenylene rings, shape-persistent nanostructures result. Several of these large phcnylene-type dendrimers (e.g. 43a) can be further cyclized to giant polycyclic PAHs. 

Similar behaviour has been observed in the photoreaction of methyl a-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (7 OMe) crystals in which the yield of [2.2] paracyclophane reached 65% on irradiation at — 78°C (see Scheme 10 p. 153) (Hasegawa et al., 1989b). From the crystal structure analysis of the same type of [2.2] paracyclophane, which is topochemically derived from alkyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamate crystals, a highly strained molecular shape is confirmed in which two phenylene rings are severely bent (Maekawa et al., 1991b). 

Fig. 11 (a) Schematic polymer structure of poly-7 OEt. Phenylene rings are omitted in order to simplify, (b) Molecular model of repeating structure of poly-7 OEt. Four chiral centres on each of two cyclobutane rings in both sides are enantiomeric to each other. 

The NMR chemical shifts of the polymers 10 and 11 are only slightly dependent on the molecular weight. The NMR spectrum of the polymer 10 shows three shrap resonances at 6 0.34, 0.94, and 7.28 ppm, due to MeSi, EtSi, and phenylene ring protons, while the polymer 11 reveals shrap resonances at 6 0.62, 7.28, and 7.35 ppm, attributed to MeSi and phenyl ring protons. The polymers 10 and 11 show characteristic strong absorption bands at 262 and 254 nm, respectively, significantly red-shifted relative to pentamethylphenyl-disilane which exhibits an absorption band at 231 nm. 

A novel cyclooctaphane (94) (Fig. 57) was assembled by the condensation reaction between the C,C -dicopper(I) derivative of m-carborane and l,2-bis(4-iodophenyl)-o-carborane.124 The macrocycle was found to adopt a butterfly (dihedral angle 143°) confirmation with the o-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes of the confirmation. 

Ar=para-phenylene ring Ar = mefa-phenylene ring... 

Figure 32 Structure of compound 194. The hydrogen atom at the 2-position of the mefa-phenylene ring is shown other hydrogen atoms are omitted for clarity. 

The band gap in substituted PPPs can be tuned to some extent by diluting the substituted phenylene rings with unsubstituted phenylenes. A bathochromic shift in absorption was observed for copolymer 473 (Eg = 2.95 eV) compared to the all-substituted homopolymer 472k (Eg = 3.1 eV) [581]. Sandwiching this polymer between ITO and Ca electrodes afforded a... 

Lindner (171) developed his own tt-SCF MO force field that is similar to MMPI in construction. This program was applied to simulate racemization of metacyclophane (48) and hexahelicene (50). In metacyclophane the m-phenylene ring flips readily at room temperature. Two mechanisms can be conceived one operates by way of a high steric energy conformation (48b) the other involves a biradical intermediate (49). The calculated activation energies are 17 and 32 kcal/mol, respectively. The experimental value is 17.7 kcal/mol, in accord with the first mechanism (172). The structures and energies of seven types of cyclophane have been calculated (172). 

The biphenyl core 9 promotes the growth of the dendrimer from the 3,3, 5,5 positions. The aforementioned tetraphenylmethane core 4 allows a tetrahedral growth of the dendrimer, whereas the 1,3,5-triethynylbenzene core 10 and the hexa-(4-ethynylphenyl)benzene core 11 promote an anisotropic extension along the plane of the central phenylene ring, so that propeller and pancake-shaped dendrimers should be obtained (Scheme 4). From all these cores it is possible to synthesize dendrimers up to the fourth generation, the properties of which will be presented later. 

Table 1. Overview of the number of phenylene rings, molecular masses, and calculated sizes of different generations of unfunctionalized polyphenylene dendrimers from us and those synthesized by Miller et al. [27]...

The structure of 51-(THF)4 consists of two ort/io-phenylene indium dichloride moieties linked through a central Hg atom, giving rise to a pseudo-centric core (Fig. 16). The mercury atom is, as expected, linearly coordinated. Each indium atom is penta-coordinated in a trigonal-bipyramidal fashion with two THF molecules at the axial positions and two chloride ligands as well as a phenylene ring at the equatorial sites. 

U, unknown anti or syn (with respect to the phenylene ring). 

Following the above conclusion it is clear that the rather bizarre spectra of these polymers derive from the special features of 1,2-enchainment. Examination of molecular models reveals that runs of 1,2-enchained segments are considerably more restricted in their degrees of motional freedom than are runs of 1,4- enchained segments. The restriction arises partly from the absence of a "crankshaft" mode with 1,2-enchainment and partly from the steric interference of substituents on adjacent phenylene rings. 

It was straightforward to apply the TRMC technique to study on-chain charge transport to ladder-type poly-phenylene (LPPP) systems because covalent bridging between the phenylene rings planarizes the chain skeleton, eliminates ring torsions, and reduces static disorder. One can conjecture that in these systems intra-chain motion should be mostly limited by static disorder and chain ends. To confirm this... 

Further intramolecular reaction of the poly(phenylene)-type polymer leads to more condensed polymers. Tour synthesized polymer 84 bearing a carbonyl moiety and a protected amino group in the phenylene rings by the reaction of boronate 83 and a dibromobenzene monomer. The polymerization takes place in the presence of a palladium catalyst in DME-H2O at 85 °C to give 84 that showed 3/n = 9850-28400 = 1.85-3.70) in 63-97% yields. The resulting polymer 84 is... 

In a series of diphenylpolyenes with a dimethylamino donor group attached to one phenylene ring and a penta-fluoro substitution on the other phenylene acting as an electron acceptor (d.6-8), the position of the 2PA peak shifts to longer wavelength and the cross section increases from 1.2 to 5.0 X 10 GM as the number of double bonds is increased from one to three (Table 4) [115]. This increase in cross section with the addition of double bonds is slightly larger for this series of dipolar diphenylpolyenes than for the centrosymmetric q.2, q.6, q.7 discussed earlier (from 2.0 to 3.2 X 10 GM). However, compounds d.6-8 have very low fluorescence quantum yields (2-3%), so their use in applications based on induced fluorescence could be limited. 

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