Phenylene chromophores

A comparison of the fluorescence behavior PBA solutions in DMAC-LiCl and in H2S0 indicates that probably the broad emission band around X = 440 nm in isotropic dilute solutions in DMAC-LiCl or H2SO11 may be attributed to exciplex formation between Li+ or in the two cases. The exciplex formation between aromatic rings and was first elucidated by Chen et al. This interpretation, of course, needs further studies and confirmation. In DMAC-LiCl solution the presence of Li - apparently hinders the stacking of phenylene chromophores of PBA chain as no difference in the fluorescence spectra of a dilute and a nematic solution could be noted. 

The solid films precipitated from both DMAC-LiCl and H2SO4 solutions showed similar excitation spectrum and a well-defined excimer band in emission peaked at X = 400 nm for the film from DMAC-LiCl and at X = 413 nm from the film from H2SO11. The blue shift of the excimer band in the solid film is compared to that in a nematic solution in H2S0i indicates a loosening of the stacking distance or the degree of overlap of phenylene chromophores in the solid. 

An important challenge in the design of novel conjugated polymers is the synthesis of materials with tailor-made solid-state electronic properties. This section outlines the synthesis of the most significant classes of poly(para-phenylenevinylene)s (PPVs), poly(para-phenylene)s (PPPs), and related structures. Furthermore, this review demonstrates that the chromophoric and electronic properties of conjugated rr-systems are sensitive to their molecular and supra-molecular architecture. 

The logical continuation of the stepladder strategy outlined above for minimizing the mutual distorsion of adjacent main chain phenylene units was the incorporation of the complete PPP-parent chromophore into the network of a completely planar ladder polymer. The complete flattening of the conjugated 7T-system by bridging of all the subunits should then lead to maximum conjuga-tive interaction. As with the PTHP 11 systems, alkyl or alkoxy side chains should lead to solubilization of the polymers. 

Miller s group [415-417] at IBM reported two series of statistical PF copolymers using perylene and cyano-substituted phenylene vinylene chromophoric segments (Scheme 2.51). 

It is interesting that statistical copolymers 343, containing m-phenylene linkages that are supposed to interrupt conjugation, showed a PL maximum of 475 nm, similar to 342. Due to efficient energy transfer from the meta- to the para-linked chromophores, the emission maxima did not depend on the ratio of m- and p-divinylbenzenes, unless 100% loading of the meta units was used [420] (Scheme 2.54). 

Wu T-Y, Chen Y (2004) Poly(phenylene vinylene)-based copolymers containing 3,7-phenothiazylene and 2, 6-pyridylene chromophores fluorescence sensors for acids, metal ions, and oxidation. J Polym Sci A Polym Chem 42 1272-1284... 

Table 2 2PA data for class I linear chromophores with phenylene-vinylene backbones (solvent toluene)... 

Tazuke, Ikeda, and their co-workers [53,54] were the first to use phenylene-diacrylate (PDA) as a mesogen and chromophore to explore the possibility of forming two-dimensionally reinforced polymer materials by photochemical cross-linking of a thermotropic LC polymer. Polyesters, 23, of para-phenylene-diacrylic acid and several diols were synthesized and found to exhibit LC behavior upon annealing above Tg. Polymer 23a exhibited [53] the texture of a N meso-... 

In the chromophore-quencher system (1) a ferrocenyl unit is connected to a Ru(tpy)2 + unit via a p-phenylene-acetylene spacer [141]. The weak emission of the Ru(tpy)2 chromophore is completely quenched in the dyad [142]. For this system, an EnT mechanism could be inferred by analogy with the behavior of related bimolecular systems [140], but PET followed by fast back electron transfer cannot be ruled out. This mechanistic ambiguity is common to most studies on related chro-mophore-ferrocene dyads [143-146]. 

The peak shift due to aggregation is observed not only in LBK films containing azobenzene chromophores, but also for other chromophores with extended Ji-systems, such as viologen polymers. For monolayers of the poly(p-phenylene sulfonate) 9/ dioctadecyldimethylammonium bromide (DODA) complex, the peak shift due to aggregation results in a piezo-chromic effect—that is, upon compression of the monolayer, a significant shift of the poly(p-phenylene sulfate) A band is observed (see Figure 6.9). This photochromic effect has been shown to be based on the improved 7t-Jt interaction upon compression of the monolayer. ... 

Alternatively, the dendrimer backbone itself can concurrently be used as the energy donor or acceptor. Several types of chromophoric dendrimer backbones such as poly(phenylacetylene) [3], poly(phenylene) [4], and poly(benzylether) [5] have been used as light absorbers, and the energy was efficiently transferred to the core acceptor. While most of these systems have high energy transfer efficiencies, they still suffer from a weak fluorescence or a low fluorescence quantum yield. However, polyphenylene dendrimers composed of tens or hundreds of out-of-plane twisted phenyl units can be used as chromophoric backbones [6] carrying highly luminescent dyes at the periphery. 

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