Conjugation, interrupted

Cycloheptatriene lacks cyclic conjugation interrupted by CH2 group... 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

Other fluorene-oxadiazole copolymers, such as fully conjugated 268a,b [360] or 269a-c, with conjugation interrupted by o-links [361], have been synthesized. For both series of polymers, emission was in the blue region at very similar wavelengths, but no LED device was reported. 

A number of N-confused systems are known with macrocyclic conjugation interrupted by sp3 meso bridges. These systems, which include analogs of calixpyrroles [252] and calixphyrins [253], fall outside the scope of the present review. A number of fully conjugated N-confused expanded porphyrins have been obtained by use of predesigned N-confused substrates. The most representative examples of such structures are collected in Fig. 32. 

Iodine is commonly used to accelerate the equilibration of Wittig cisltrans-alkene products. Computational chemistry has been employed to study the mechanism in detail for seven different examples of 1,2-disubstituted alkenes.26 The iodo intermediates of the conventional three-step reaction path have been found to be weakly stable, bound by less than 7 kJ mol-1 in five cases and non-existent in the other two. These variations in relative stability have been interpreted as being closely related to the degree of conjugation interruption in the alkene upon attachment of iodine. 

As shown in Section IV,A,1, reduction of porphyrins by electron-transfer reactions may proceed via intermediates with interrupted conjugation, which subsequently rearrange to hydroporphyrins, thus reestablishing cyclic conjugation. Interruption of conjugation can also be achieved by catalytic hydrogenation of hydroporphyrins (Section IV,A,4). 

TAZ, c.f. Figure 9.5, is a low electron mobility material. It can be used as a hole blocking layer, which may limit electron injection and transfer in electroluminescent devices. The triazole unit was found to be an effective r-conjugation interrupter and can play the rigid spacer role in determining the emission color of the resulting copolymer. ... 

We have widened our scope in the current work to form a new series of platinum aryl-acetylide polymers and their dinuclear counterparts containing 0, S, SO, and SOj functional groups. These are remarkable model systems that can be used to evaluate how chalcogen-based conjugation-interrupters would limit the effective conjugation length of polymetallaynes. 

Main Chain Polymers with Non-conjugated Interrupters... 

More important from a synthetic point of view, however, is the concept of non-conjugative interrupters. In such a polymeric material the optoelectronic properties can be tailored by the proper selection of the chromophore unit whereas the physical properties can be adjusted by the non-chromophoric part. Bum et al. [112] introduced the notion of isolated chromophores in 1992 by selectively eliminating one of two leaving groups of the precursor polymer to give a conjugated-non-conjugated polymer. 

The design of such polymers with controlled chromophore length is quite a large area, as chromophores and spacers can be combined in an almost infinite way. Chromophores can be hole-transporting triarylamine or electron-transporting ones such as oxadiazoles, with ethers, esters [113], amines, amides, imides [114-116], silanes, fluorenylidene as possible conjugation interrupters [117,118],... 

Fluorenylidene linkages have been widely used as conjugation interrupters due to their stiff conformation and to the ability to increase solubility of the resulting polymer and were therefore incorporated in several PPP based polymers by the Suzuki polycondensation reaction [127]. Following this concept a blue emitting polymer based on... 

We and others have shown that the polymer chains in the template-synthesized materials are preferentially aligned (enhanced supermolecular order) and that these chains contain fewer conjugation-interrupting defects (improved molecular structure) [13,15,16,32,44]. These improvements in molecular and supermolecular structure are responsible for the enhancements in conductivity discussed above. 

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