Mean conjugation length

Temperature also has a considerable influence on the extent of conjugation and, hence, on the optical and electrical properties of the material PT films produced at 40°C have a shorter mean conjugation length than those prepared at 5°C [147,148,154]. 

XPS and IR data indicate that, although PT is more ordered than PP [168], it presents some a-jS couplings [141]. On the basis of IR spectra of polythiophenes selectively labeled with deuterium at the a and/or /3 position, a mean conjugation length of 18 units is found with an average degree of polymerization of 150 [152J. 

Increasing the length of the alkyl chain beyond 7-9 carbon atoms produces an increase in both the mean conjugation length of the polymer chain and the electrochemical reversibility as illustrated by the symmetry of the cyclic voltammograms [64,128]. The absence of hysteresis, i.e. of a high separation between forward and backward peak potentials, may be due to a better solvation, with electrolyte permeation, of the polymers. 

Because of possible ion-selective effects, pseudo-crown ethers were prepared by electropolymerization from suitably substituted thiophenes by Roncali et al. [1093]. The polymer formed from l,14-(3-thienyl)-3,6,9,12-tetraoxatetradecane showed an absorption maximum at A = 430 nm in the undoped state. Compared to the respective value for poly(3-(3,6,9-trioxade( l))thiophene, a blue shift of about SO nm, indicating a shorter mean conjugation length, was found. In the oxidized form, this band was considerably reduced in intensity, and a new band at A = 750 nm, attributed to a transition into the upper bipolaron band, was seen. The results were found to be inferior to those of polymers prepared from 3-polyether-substituted monomers. A general review of this field was provided by Fabre and Simonet [1094] for further details, see also [1095]. 

Since conjugated polymers, such as polypyrrole, usually yield stiff chains with interchain interactions and little flexibility (36), a long alkyl chain is added at the 3-position to change the characteristics of the polymer. This leads to more regular polymeric structures and higher electrical conductivity (37-39). The mean conjugation length of poly(3-alkylthiophenes) can be increased by... 

J Roncali, M Lemaire, R Garreau, F Gamier. Enhancement of mean conjugation length in conducting polythiophenes. Synth Met 1987 18 139-144. 

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