Conjugation lengths, distribution

When PVAPAC films, optimized with respect to conjugation length distribution and PAC concentration, are stretched in a controlled manner they turn into highly dichroic transparent neutral grey POLPAC filters (Fig. 1.10). The absence of coloration is indicative of an exceptionally highly ordered PAC state in these novel all polymer broadband polarizers based on polyacetylene (POLPAC ). 

Figure 22.17 shows the absorption and PL spectra of MEH-PPV in toluene solution and film cast fi om toluene solution [157]. It is seen that although the peak absorption is the same, occurring at 2.5 eV, there is enhanced absorption tail in the film. In addition, the PL spectrum in the film is redshifted and broadened compared to that in the solution. The absorption tail cannot be formed in the film due to a different conjugation length distribution in the chains, since the film was cast fi om the same solution and thus the conjugation length distribution is the same for the polymer chains in... 

A Ag state (see Table 11.4) and has an energy of around 4.4 eV. Although polydispersity would appear to rule out this scenario, this alternative interpretation is possible if we assume that the conjugation length distribution is sharply peaked around N = 8 rings in thin films (because, unlike the l R state, the state has strong A-dependence). Interestingly,... 

The authors postulated that since it was obvious from X-ray data that polypyrrole is a poorly crystalline material, for which there is no evidence for long conjugation lengths, it does not seem unreasonable to suppose that there is a wide distribution of conjugation lengths, a hypothesis supported by other workers. The cations of shorter conjugated units will be reduced... 

The electronic properties and thus the absorption bands of a polymer depend on the conjugation length Z. The absorption energy decreases as Z increases reaching a limiting value of 16 000 cm l as Z approaches infinity. If the polymer solution consists of a distribution of conjugation lengths (i.e. chromophores), photoselection of the different chromophores within the... 

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