Electrochemical bandgaps of a series of poly-3-p-phenylthiophenes

The change of the UV-Vis optical absorption of electropolymerized substituted poly-3-p-X-phenylthiophenes (X = -H, -CH3, -OCH3, -COCH3, -COOC2H5, -NO2) has been followed in situ as a function of applied electrode potential in an electrolyte solution of tetraethylammoniumtetrafluoroborate Et4NBF4 in acetonitrile. The UV-Vis spectra show features between 300 and 900 nm similar to those observed with many other polythiophenes having a high degree of conjugation. During stepwise oxidation of the poly-3-p-X-phenylthiophene films, the intensity of the absorption due to the pi ->pi*-transition around 450-566 nm decreases, and a new broad absorption band associated with (bi)polaron states appears around 730-890 nm. On the other hand, during the oxidation (p-doping) of the poly-3-p-X-phenylthiophene films, a blue (hypsochromic) shift is observed for both absorption bands. This is explained by taking into account that a polymer contains a distribution of chain lengths, and the longest polymer chains (the absorption of which occurs at lower energies) start to oxidize at the relatively lowest potentials. The electrochemical bandgaps of poly-3-p-X-phenylthiophenes have been estimated based on results of cyclic voltammetry. Bandgaps obtained this way have been found to be generally higher than optical bandgaps; the actual discrepancy was found to depend on the mode of evaluation.