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Thin films of poly(arylene ethynylene) conjugated polymers, including low-energy-gap donor–acceptor polymers, can be prepared via stepwise polymerization utilizing surface-confined Sonogashira cross-coupling. This robust and efficient polymerization protocol yields conjugated polymers with a precise molecular structure and with nanometer-level control of the organization and the uniform alignment of the macromolecular chains in the densely packed film. In addition to high stability and predictable and well-defined molecular organization and morphology, the surface-confined conjugated polymer chains experience significant interchain electronic interactions, resulting in dominating intermolecular π-electron delocalization which is primarily responsible for the electronic and spectroscopic properties of polymer films. The fluorescent films demonstrate remarkable performance in chemosensing applications, showing a turn-off fluorescent response on the sub-ppt (part per trillion) level of nitroaromatic explosives in water. This unique sensitivity is likely related to the enhanced exciton mobility in the uniformly aligned and structurally monodisperse polymer films. 

Controlled preparation of structurally precise complex conjugated polymer systems remains to be a major synthetic challenge still to be addressed, and this push is stimulated by the improved device performance as well as unique fundamental characteristics that the well-defined conjugated polymer materials possess. Catalyst-transfer polymerization (CTP) based on Pd-catalyzed Suzuki-Miyaura cross-coupling reaction is currently one of the most promising methods towards achieving such a goal, especially with the recent implementation of N-methyliminodiacetic acid (MIDA) boronates as monomers in CTP. Further expansion and development of practical applications of CTP methods will hinge on a clear mechanistic understanding of both the entire process and the particular steps involved in the catalytic cycle. In this work, we introduced Ag⁺-mediated Suzuki-Miyaura CTP and demonstrated that presence of Ag⁺ shifted a key transmetalation step toward the oxo-Pd pathway, leading to direct participation of MIDA-boronates in the transmetalation step and hence in the polymerization process, and resulting in the overall more efficient polymerization. In addition, we found that, under Ag⁺-mediated conditions, MIDA-boronates can also directly participate in small-molecule cross-coupling reactions. The direct participation of MIDA-boronates in Suzuki-Miyaura cross-coupling has not been envisaged previously and could enable new interesting possibilities to control this reaction both for small-molecule and macromolecular syntheses. In contrast to MIDA-boronates, boronic acid monomers likely undergo transmetalation through an alternative boronate pathway, although they may also be directed to react via the oxo-Pd transmetalation pathway in Ag⁺-mediated conditions. The interplay between the two transmetalation pathways which are both involved in the catalyst-transfer polymerization, and the opportunity to selectively enhance one of them not only improves mechanistic understanding of Suzuki-Miyaura CTP process but also provides a previously unexplored possibility to gain more effective control over the polymerization to obtain structurally better-defined conjugated polymers.