Conjugated polymers composed of tricoordinate boron and π-conjugated units possess extended conjugation with relatively low-lying LUMOs arising from p B –π interactions. However, donor–acceptor (D–A) polymers that feature triorganoboranes alternating with highly electron-rich donors remain scarce. We present here a new class of hybrid D–A polymers that combine electron-rich dithienosiloles or dithienogermoles with highly robust tricoordinate borane acceptors. Polymers of modest to high molecular weight are readily prepared by Pd-catalyzed Stille coupling reaction of bis(halothienyl)boranes and distannyldithienosiloles or -germoles. The polymers are obtained as dark red solids that are stable in air and soluble in common organic solvents. Long wavelength UV-vis absorptions at ca. 500–550 nm indicate effective π-conjugation and pronounced D–A interactions along the backbone. The emission maxima occur at wavelengths longer than 600 nm in solution and experience further shifts to lower energy with increasing solvent polarity, indicative of strong intramolecular charge transfer (ICT) character of the excited state. The powerful acceptor character of the borane comonomer units in the polymer structures is also evident from cyclic voltammetry (CV) analyses that reveal relatively low-lying LUMO levels of the polymers, enhancing the D–A interaction. Density functional theory (DFT) calculations on model oligomers further support these experimental observations.
Exploration and development of gold- and silver-catalyzed cross dehydrogenative coupling toward donor–acceptor π-conjugated polymer synthesis
π-Conjugated polymers are materials of interest for use in organic electronics. Within these polymers, donor–acceptor polymers are favorable for solar cell applications due to improved charge mobility, better absorption in the low energy region of the solar spectrum, and tunable band gaps. One of the barriers to commercializing these donor–acceptor materials is that their synthetic pathways are complex because of the alternating repeat units in the polymer. To address this, the application of cross dehydrogenative coupling (also called oxidative CH/CH cross-coupling) toward the synthesis of donor–acceptor polymers was explored. In this work, the roles of specific reagents in a one-pot gold- and silver-catalyzed cross dehydrogenative coupling and the factors that contribute to selectivity for cross-coupling rather than homo-coupling are analyzed. Based on our results, we postulate that the percentage of alternating repeat units in the final polymer is affected by the increased reactivity of the dimer that forms in the initial stages of the polymerization compared to the monomer, which ultimately may be exploited to control the ratio of electron-rich to electron-poor monomers.
- Award ID(s):
- 1700982
- Publication Date:
- NSF-PAR ID:
- 10093211
- Journal Name:
- Polymer Chemistry
- Volume:
- 10
- Issue:
- 4
- Page Range or eLocation-ID:
- 486 to 493
- ISSN:
- 1759-9954
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Publisher's Version of Record
- https://doi.org/10.1039/C8PY01162A
