Nature of long-lived photoexcited states in polydiacetylenes: the photoinduced absorption spectra of PDA-4BCMU

We report results of steady-state photoinduced absorption experiments on PDA-4BCMU grown and oriented on silica substrate. A high degree of axial order, checked by linear and nonlinear optical dichroism and correlated with X-ray measurements was found. Polycrystalline films were obtained with a preferential orientation of crystallites. Unlike the solution cast films exhibiting the so called `red form' only, the deposited films are blue thus showing a better (pi) -electron conjugation than in the former case due to a better planarity of polymer chains. The films were used to perform steady-state photoinduced absorption experiments. Three photoinduced absorption bands at 0.82, 0.95, and 1.39 eV were observed. The 0.82 and 0.95 eV absorption features are assigned to charged states while the 1.39 eV peak is originated by triplet excitons. The photogeneration in PDA- 4BCMU of both charged and neutral states is discussed and compared with the results obtained in polydiacetylenes having carbazolyl groups as substituents. The relative weight of one excited species over the other one is dictated by the molecular and supramolecular organization of the polymer.