Boron difluoride formazanate copolymers with 9,9-di-n-hexylfluorene prepared by copper-catalyzed alkyne–azide cycloaddition chemistry

The synthesis and characterization of copolymers based on boron difluoride formazanate (BF₂L) and 9,9-di-n-hexylfluorene (hex₂Fl) units are described. A series of model compounds [(BF₂L)-(hex₂Fl), (hex₂Fl)-(BF₂L)-(hex₂Fl), and (BF₂L)-(hex₂Fl)-(BF₂L)] were also studied in order to fully understand the spectroscopic properties of the title copolymers [(BF₂L)-(hex₂Fl)]_n. The model compounds and copolymers, which were synthesized by copper catalyzed alkyne–azide cycloaddition chemistry, exhibited high molar absorptivities (25 700–54 900 M⁻¹ cm⁻¹), large Stokes shifts (123–143 nm, 3590–3880 cm⁻¹), and tunable electrochemical behaviour (ca. −0.75 V and ca. −1.86 V vs. ferrocene/ferrocenium). The low-energy wavelength of maximum absorption and emission of the model compounds red-shifted relative to the BF₂L repeating unit by ca. 30 nm per triazole ring formed, to maximum values of 557 nm and 700 nm in DMF, respectively. The low-energy absorption and emission properties of the copolymer were consistent with the model compound bearing two triazole rings [(hex₂Fl)-(BF₂L)-(hex₂Fl)] and were not dependent on copolymer molecular weight. However, the title copolymers may show promise as light-harvesting materials based on their thin-film optical band gap of 1.67 eV.