EEC Theory

While no other theoretical simplifications can be predicted for the infrared spectra of the polymers the Raman spectrum is expected to be drastically simplified according to the theory of resonance Raman and of ECC. 

As usually, we focus at the 44 q=0 phonons of Ag species, all in principle Raman active, for the cs model of PHTh when recorded in resonance or preresonance conditions. 

In conclusion for the Raman spectrum we predict i) Since the spectrum is in resonance conditions Bg modes will not be observed 31 of the Ag modes will not be observed since they are not involved in the vibrational transitions resonantly enhanced. Of the remaining 13 modes those preferentially enhanced are the ones which contain a sizeable contribution by the Hcordinate, their intensity being proportional to iLflP. 

ECC predictions are valid both for the perfect 1-d crystal or for the disordered system. However, if the effective delocalisation of the n electrons changes, according to ECC, we expect both frequency and intensity dispersions. 

The last statement is proven (using the same theoretical concepts) when the Raman spectra of PHTh (or POTh) in the solid and solution phases are compared (fig.17). In agreement with the electronic spectra of the same samples, in solution the energy gap is larger because the molecule is distorted in a structure which we label as 20 (with the torsional angle 0 yet unknown) while in the solid the HOMO-LUMO transition is red shifted since the systems approaches coplanarity in a 2 structure with smaller. The observed Raman frequency and intensity dispersion is the following for the solid 1515, 1448 and 1380 cm , for the solution 1517, 1475 cm and (too weak ), in agreement with the predictions by ECC theory. 

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As a first application of EEC theory we discuss the case of polypyrrole (PPy) which has been one of the first polyaromatic systems studied with the hope to find a material with relevant technological interest. 

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