Structure of PPV from Spectroscopy

Because of the shape of the /F plots in the upper range of values for the pristine materials the increase of effective conjugation is undoubtely revealed by the electronic spectra in a more relevant way than by the Raman spectra. Evidence has been found of an effective conjugation length 9-10 units and of a chain length 8 units. 

In order to reconcile the effective delocalisation measured from UV-Vis spectra of the oligomers and that measured for the polymer the experimental and theoretical problem of the coplanarity of the systems has to be condidered in more detail. 

The fitting between the doping induced of photoexcited spectra with the ones calculated by ECC theory is an evidence that a para-quinoid configuration is formed at the site of doping and in the photogenerated species. The decision between bipolaron and polaron cannot be taken on the basis of ECC theory the infrared spectra calculated by MNDO calculations seems to support the bipolaronic structure [176]. 

Since there are theoretical reasons to believe that the Raman spectra of doped polyconjugated materials should be extremely weak, if observable (3,17) the published Raman spectrum of doped PPV should find an explanation different from that proposed in the literature (174). Application of ECC theory helps in this case. Comparison of the reported Raman spectrum with the and I/F/j plots of figs. 

25 and 26 shows that the weak-strong-weak triplet nd  

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