Organic π-conjugated compounds have replaced inorganic materials in optoelectronic device applications such as light-emitting diodes (LED), field-effect transistors (FET), and photovoltaic cells (OPV), owing to their many benefits: low production cost, fast/easy processing, and potential application to flexible displays. π-conjugated polymers in particular are among the most promising candidates for optoelectronic device materials. Some polymers are now commercially available, but their fluorescence emission quantum efficiencies are normally too low for solid-state practical use because a strong cofacial π-π interaction, which leads to the formation of intermolecular excimers.
Many efforts have been made to date to improve the solid-state fluorescence emission quantum efficiency of conjugated polymers. Although complicated and time-consuming, monomer design was considered a routine method. While the introduction of longer and bulkier alkyl groups into the side chain could enhance the polymer solubility, it was not sufficient to collapse the excimer structure in the solid state.
In previous studies on synthesis and photophysical properties of polydiphenylacetylene (PDPA) derivatives, it was found that the polymer emission originates from an excimer due to the intramolecular stack structure in the side chain. The relationship between the photophysical properties and chain conformation in the solid states showed that the longer alkyl side chain behaves like a solvent or plasticizer, which relaxes the intramolecular stack structure to degenerate the excimer, leading to significant emission enhancement.
Giseop Kwak at Kyungpook National University together with co-workers and colleagues in Gumi, Fukui, and Gwangju in their new work demonstrate that the fluorescence emission of PDPA in solid film can be finely tuned simply by copolymerization of monomers with different alkyl side chain lengths. A higher content of longer alkyl chain unit in the polymers relaxed the intramolecular stack structure to significantly enhance the fluorescence emission in solid film.
This approach was very useful for fine-tuning the fluorescence emission because it was very difficult to change the properties of conventional conjugated polymers in bulk solid due to the highly cofacial chain packing structure and high crystallinity. The authors believe that their result will be helpful in the molecular design of light-emitting organic materials with high quantum efficiency in finely tuned fluorescence emission.