Excited state, formation spin restrictions

The role of the dopant potential on the stability and magnetic and optical properties of polarons and bipolarons in conducting polymers is shown with the aid of calculations of singlet and triplet states of a bipolaron [167] and by spectroelectrochemical and conductivity measurements [168-170]. The X-band optically detected magnetic resonance of PHT and PDDT shows that the distant intrachain polaron recombination is temperature-independent and identical in films and solutions. However, the triplet polaronic excitation decay is observable in films, but not in solutions [171], Electrochemical in situ conductivity and EPR measurements of PT films were performed in several solutions [172]. The results indicate that polarons merely seem to initiate the electrical conductivity. The electronic delocalization of polarons is restricted to a relatively short chain length at low potentials. As the polaron concentration increases (spin density maximum), bipolarons are generated immediately (probably too fast for the detection of polarons by EPR). Thus the bipolarons prevail in the fully conducting polymer films and as a consequence should be mainly responsible of the intrinsic conductivity [172]. Asymmetrically disub-stituted PBT display well-defined redox processes which are correlated to the consecutive formation of radical cations, dimerized radical cations, and dications [173]. 

---