Effective Conjugation Length and Molecular

3) Effective conjugation length and molecular chain length from optical data. 

The conformation dependent delocalisation is proven by the systematic blue shift ( increase of the HOMO-LUMO energy gap) shown by the electronic spectrum of the oligomers in going from the solid to 

The chain length dependence of the delocalisation is proven by the systematic red-shift of the electronic spectra of the oligomers from 0 to O5 [168,175]. From the plot of HOMO-LUMO transitions vs. 1/N (N= the number of repeat units) and using an average value of the TT-TT transitions reported in the literature for PPV we conclude that the effective conjugation length for the solid polymer is not much larger than 7-10 units [175]. 

Information on chemical chain length can be derived from the intensity ratio of chain length independent end group modes with modes shows that the intensity depends on the number of monomer units. With some approximations (discussed in ref. 175) one finds good indications that the polymer chain length is considerably larger than 9-10 units. 

The experimental vibrational infrared and Raman data published or collected in our laboratory show the following characteristics. 

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