Chain conjugated polymers

An increase in polarizability of the w-electron system in a PCS is expected to be followed by an easy transfer of the chemical behavior along the conjugation chain and by a decrease in the internal energy of the PCS, thus enhancing the thermal stability of the polymers and the rigidity of the main chain. 

We have already pointed out that the reduction in conjugation efficiency in PCSs is followed by a short-wave shift of the CTC transfer band. This accounts for the fact that poly(schiff base)s and polyazines having conjugated sections separated by oxygen and sulfur atoms are characterized by a short-wave shift of the transfer band of CTC with all acceptors compared to the respective polymers having no interruption of the conjugated chain. This shift may reach 20-50 nm. 

Figure 13 shows the irreversible conversion of a nonconjugated poly (p-phenylene pentadienylene) to a lithiun-doped conjugated derivative which has a semiconducting level of conductivity (0.1 to 1.0 S/cm) (29). Obviously, the neutral conjugated derivative of poly (p-phenylene pentadienylene) can then be reversibly generated from the n-type doped material by electrochemical undoping or by p-type compensation. A very similar synthetic method for the conversion of poly(acetylene-co-1,3-butadiene) to polyacetylene has been reported (30), Figure 14. This synthesis of polyacetylene from a nonconjugated precursor polymer containing isolated CH2 units in an otherwise conjugated chain is to be contrasted with the early approach of Marvel et al (6) in which an all-sp3 carbon chain was employed.

In the via precursor method, however, it is difficult to prepare the ji-conjugated polymers with ideally developed -conjugation system the -conjugated polymer chains contain many conformational defects because the jc-conjugated chains are caused to develop from disordered precursor polymer, which form random coil conformation, in solid state. For the preparation of polymers with well-developed jc-conjugation system by the via precursor method, accordingly, it is necessary to introduce orientational and conformational orderliness of the precursor polymers in the films. 

Jin et al. [487] synthesized and studied the PL and EL properties of polymers 403 and 404 that differ by the position of the alkoxy substituent in the phenyl ring, expecting different distortion of the polymer main chain (and consequently conjugation length) due to different steric factors for para- and ort/zo-substitution (Chart 2.98). The absorption spectrum of the ortho-polymer 403 showed a substantial blue shift of 40 nm compared to para 404 and a decrease in EL turn-on voltage (4.5 and 6.5 V, respectively). Both polymers demonstrated nearly the same PL and EL maxima (Table 2.1). 

Copolymer 459, prepared by Stille coupling of dibromophenylene with 2,5-Mv(tributyl-stannyl)thiophene, represents another example of a phenylene-u/t-thiophene backbone, where the substituted phenylene unit forms an oligophenylene vinylene fragment that is not in the main conjugation chain [561]. A PLED fabricated with this polymer (ITO/459/A1) emitted green light (520 nm) with a turn-on voltage of ca. 9.5 V, but no other data on luminance or efficiency of the device were reported (Chart 2.111). 

L. Romaner, A. Pogantsch, P.S. de Freitas, U. Scherf, M. Gaal, E. Zojer, and E.J.W. List, The origin of green emission in polyfluorene-based conjugated polymers on-chain defect fluorescence,... 

Fig. 6.1 Chain polymer conjugated technology (EnVision System) developed by DakoCytomation...

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