Thin-Film Morphology and Optical Properties of Photoisomerizable Donor-Acceptor Oligothiophenes.

It was recently reported that the most popular electron-accepting units introduced to π-conjugated oligomers studied for organic photovoltaic applications are susceptible to unwanted and even destructive photochemical reactions. The consequences of / photoisomerization of the popular 2-(1,1-dicyanomethylene)rhodanine (RCN) unit on the optical and morphological properties of a homologous series of RCN-functionalized oligothiophenes are studied here. Oligomers consisting of one, two, or three thiophene units were studied as pure isomers and with isomer compositions of 25, 53, and 45%, respectively, for / mixtures. Solutions of isomers and / mixtures were characterized by UV-vis and photoluminescence spectroscopy, wherein changes to optical properties were evaluated on the basis of isomer content. X-ray diffraction of thin-film / mixtures reveals crystalline domains of both and forms after thermal annealing for mono- and bithiophene oligomers, with greater interplanar spacing for crystalline domains than the counterparts along the substrate normal direction. The surface morphology viewed by atomic force microscopy also shows fiberlike structures for the form with a much larger aspect ratio than for the domains in the bithiophene oligomer. Optical characterization reveals drastic changes in the solid state upon introduction of the form for the mono- and bithiophene derivatives, whereas subtle consequences are noted for the terthiophene analogue. Most notably, a 132 nm redshift in maximum absorption occurs for the bithiophene oligomer films containing 53% isomer compared to the pure counterpart. Finally, although solid-state photoisomerization experiments find no evidence of → isomerization in polycrystalline films, → isomerization is observed and becomes more restrictive in films with higher crystallinity (i.e., after thermal annealing). This structure-property study, which elucidates the consequences of the RCN configuration on solid-state packing and optical properties, is expected to guide the development of more efficient and stable organic optoelectronic devices.