Spacer semi-flexible

Semi-flexible bipyridine derivatives incorporating alkyl spacer groups between the pyridyl rings have been demonstrated to yield dinuclear palladium(II) species related to those discussed in Section 7.2 of this chapter.Thus, insertion of -CHjCHj- or -CH2(CgF4)CH2- groups between the 4- and 4 -positions in the parent bipyridine ligand leads to the formation of 2 2 (metal ligand) complexes of type 32 on reaction of these extended bipyridine derivatives with dinitro(ethane-... 

Electron transfer was followed by the exciplex fluorescence method LE and exciplex-type emissions were recorded as a function of internal energy, as in the previously cited papers (Section 4.2). Here, however, the initial excitation was either of the electron donor or the acceptor. The electron-donating power was varied by simple substitution, with minimal interference with the nature of the spacer. Typical donors were para-substituted anilines or substituted naphthalenes. Typical acceptors were cyano-substituted naphthalenes (when anilines served as the donors) and dicyanoethylene or dicarbomethoxyethylene when naphthalenes served as donors. Spacers were of varying flexibility semi-flexible (a piperidine ring which may have a... 

A semi-flexible main chain liquid crystal polymer is composed of mesogenic units separated by flexible spacers, normally alkyl chains [ 7]. These polymers are not only of interest for their application potential [8] they are also of major fundamental interest because of their unusual liquid crystalline properties. It is well known, for example, that the transitional behaviour of a semi-flexible main chain liquid crystal polymer shows a dramatic dependence on the length and parity of the flexible spacer linking the mesogenic units [9]. Other fascinating behaviour includes the observation of a nematic-nematic transition [ 10] and the occurrence of alternating smectic phases [11-15]. 

Thus on varying the length and parity of the flexible spacer in dimers the nematic-isotropic transition temperature exhibits a dramatic odd-even effect which attenuates as the length of the spacer is increased while the nematic-isotropic entropy also exhibits the same pronounced alternation but which appears not to attenuate as the spacer length is increased. Such behaviour is also observed for semi-flexible main chain liquid crystal polymers for example, Figs. 5 and 6 show the dependence of and ASni/R, respectively, on the length of the flexible spacer for the poly a,co-[4,4 -(2,2 -dimethylazoxy-phenyl)]alkandioates [9],... 

The mesogenic groups can be connected to the main polymer backbone of LCPs with the help of some flexible spacers that can be introduced at the side of LCP backbones (Baron and Stepto 2002). This is termed as SGLCPs. Then also the LCPs become semi flexible (Fig. 5.6). 

Because of the structural similarity between semi-rigid thermotropic polymers and LCPs, i.e. the presence of rigid moieties separated by flexible spacers, all polymers of this kind are commonly referred to as the main chain liquid crystal polymers (mcLCP). 

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