Intercorrelation between Structural Ordering and Emission Properties in Photoconducting Polymers

We investigated the structural properties of anthracene containing poly( p -phenylene−ethynylene)- alt -poly( p -phenylene−vinylene) polymers with general constitutional unit: (−Ph−C≡C−Anthr−C≡C−Ph−CH═CH−Ph−CH═CH−) n by means of wide-angle X-ray scattering and fluorescence spectroscopy. Interchain interactions were systematically modified by decorating the conjugated polymer backbone with linear or branched or combinations of linear and branched alkoxy side chains. Special emphasis is taken on an evaluation method for fiber scattering spectra that allows the deduction of important structural details of polymer materials with limited degree of order. These include positional correlations along the backbones, interlayer and π−π stacking distances between different chains as well as a quantification of the degree of order in terms of number of lattice planes per domain. The polymers with all-linear side chains attached close to the anthracenylene−ethynylene unit (AnE) show a layered structure. In contrast attachment of branched side chains at the AnE unit leads to amorphous polymers. These structural differences are reflected in their photophysical properties, whereby the first group of polymers exhibit red-shifted emission as compared to the second group. The larger the π−π stacking distance the more blue-shifted is the main emission band, but no clear correlation of the emission peak position is found with respect to the degree of order of the π−π stacking in terms of number of π−π lattice planes per domain. We also studied the effect of annealing on the as synthesized samples and observed structural transitions, which we attribute to side chain melting and backbone reorganization, respectively. Both transitions lead to enhanced structural order in the polymers which can be preserved during cooling the samples back to room temperatures. Once annealed, the polymers do not show significant structural changes over a wide temperature range, a property which we consider to be beneficial for photovoltaic applications.