Ionomeric control of interchain interactions, morphology, and the electronic properties of conjugated polymer solutions and films

It is becoming increasingly clear that the electronic properties of conjugated polymer films are strongly dependent on factors such as the conformation and the degree of aggregation of the polymer strands in the solution from which the film was cast. In this paper, we show how we can take advantage of conjugated ionomers (conjugated polymers that have been functionalized with side groups that can be electrically charged) to control the polymer morphology and degree of interchain interactions in both solutions and the films cast from them. The particular ionomer we study in this work, poly(2,5-bis[N-methyl-N-hexyl amino] phenylene vinylene) (BAMH-PPV), has dialkyl amino side groups that can be controllably charged by protonation with organic acids. In dilute BAMH-PPV solutions, protonation of just a few percent of the amino side groups leads to tight coiling of the polymer backbone, resulting in an enormous blueshift of the polymer's absorption and photoluminescence (PL) spectra. At higher BAMH-PPV solution concentrations, however, protonation of the side groups leads to redshifted emission, indicative of increased interactions between polymer chromophores that presumably result from counterion-mediated attractive interactions. The results suggest that conjugated polymer chromophores in solution interact by interpenetration of neighboring chains rather than by self-aggregation of the chromophores on a single chain. Scanning force microscopy experiments indicate that the surface topography of BAMH-PPV films varies directly with the degree of side-group protonation in the solution from which the film was cast. In addition, BAMH-PPV films cast from protonated solutions have a redder PL spectrum and a higher degree of exciton-exciton annihilation than films cast from neutral solutions, verifying that memory of the chain conformation and degree of chromophore interaction in solution carries through the spin-coating process. The charge-induced changes in the morphology of BAMH-PPV films also lead to dramatic differences in the performance characteristics of BAMH-PPV-based light-emitting diodes. Overall, we believe that the degree of control over the electronic properties of conjugated ionomers makes them attractive candidates for use in a wide variety of optoelectronic devices. (C) 2002 American Institute of Physics.