Illumination of Conjugated Polymer in Solution Alters Its Conformation and Thermodynamics

The importance of chain structure in conjugated polymer-based material active layers and its relation to device efficiencies in OPVs, organic field transistors, OLEDs, and other devices has been well established. However, the influence that the absorbance of the light inherent to these devices might have on the conjugated polymer structure is not well understood. Herein, we employ small-angle neutron scattering to investigate structural changes occurring in solutions of poly­(3-hexyl­thiophene-2,5-diyl) with exposure to white light. Results indicate significant decrease in both Kuhn length (<i>b</i>) and radius of gyration (<i>R</i>_g) of the polymer upon illumination, coupled with a drop in the second virial coefficient (<i>A</i>₂). We explain this phenomenon through a chain collapse model, proposing that the interaction of light with the polymer backbone alters its thermodynamic interactions with and solubility in the surrounding solvent. The presence of such an effect, which we observe in several conjugated polymers, introduces the possibility of a powerful, nondestructive, and tunable method for controlling polymer conformation in solution. This in turn opens a path to develop a broad range of new light-responsive materials, in that a variety of conjugated polymers could be used as the stimuli-responsive material. Additional implications include the identification of the importance of illumination in the reproducible fabrication of organic electronic active layers from conjugated polymer inks