The isosteric replacement of C-C with B-N units is emerging as a powerful method to tune the optoelectronic properties of organic π-conjugated materials. In this work, copolymers based on ladder-type B-N Lewis pair-fused dipyridylfluorene monomers and a thienylated benzothiadiazole (BTDDT) are prepared by Stille-type polycondensation. The polymers containing exocyclic ethyl or pentafluorophenyl substituents on boron are characterized by multinuclear NMR spectroscopy and gel permeation chromatography, which provides estimates of the average degrees of polymerization (DPn) of 30 (R = Et) and 10 (R = C6F5). Electrochemical analyses on thin films reveal two distinct reversible reductions that are assigned to benzothiadiazole- and boron-fused dipyridylfluorene-centered processes. UV‒vis analyses show that the band gaps are significantly lowered relative to both BTDDT and the boron-containing monomers. As a result, the absorption and emission bands are shifted to much lower energies, leading to intense orange emission with maxima at 641 nm (R = Et) and 627 nm (R = C6F5), respectively, with quantum yields of over 50%. Computational studies on molecular model dyads offer insights into the effects of the isosteric replacement of C-C units in ladder-type fused bisfluorene with B-N Lewis pairs on the electronic structures of the polymers.
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FJ thanks the US National Science Foundation (NSF Grant CHE-1954122) and Rutgers University for support. The 500 MHz NMR spectrometers (NSF MRI 1229030, ELF III 047-04) were purchased with support from the NSF and the State of New Jersey. AFA thanks the Saudi Arabia Government for a graduate fellowship. We would like to thank Dr. Pavel Kucheryavy at Rutgers University-Newark for assistance with the acquisition of 2D NMR data, Dr. Roman Brukh for assistance with the acquisition of mass spectral data, and Dr. Monika Baraniak for assistance with GPC measurements.
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Alahmadi, A.F., Zuo, J. & Jäkle, F. B-N Lewis pair-fused dipyridylfluorene copolymers incorporating electron-deficient benzothiadiazole comonomers. Polym J (2022). https://doi.org/10.1038/s41428-022-00723-y