Electroactive and luminescent polymers: new fluorene-heterocycle-based hybrids

Abstract

The synthesis, characterization, and electrochromic properties of copolymers derived from 9,9-dialkyl-2,7-dibromofluorene (18a, alkyl=C ₁₀ H ₂₁ ; 24, alkyl=Et) and pyrrole, thiophene, 3,4-ethylenedioxythiophene, and furan are described. Two synthetic routes to 9,9-diethyl-2,7-bis(pyrrol-2-yl)fluorene (30) afford product in 30% and 20% yields, respectively. Monomer 30 undergoes electropolymerizationto yield electroactive polymer films. The lowest monomer oxidation potential (E _p,m =0.4 V vs. Ag/Ag ⁺ ) is found in tetraethylammonium tosylate (TEATOS)-CH ₃ CN, but film formation is slow. Spectroelectrochemical analysis of poly(30) reveals a band gap at 2.4 eV and upon polymer oxidation, two low energy absorptions peaking at 1.2 and 2.2 eV appear. This phenomenon is attributed to formation of bipolaron bands between the valence and conduction bands. Soluble fluorene-heterocycle polymers 34a-d have been synthesized by the Stille coupling reaction of 18a and 2,5-bis(trimethylstannyl)thiophene (21a), 5,5′-bis(trimethylstannyl)-2,2′-bithiophene (21b), 2,5-bis(trimethylstannyl)-3,4-ethylenedioxythiophene (21c), and 2,5-bis(trimethylstannyl)furan (22), respectively, in high yields. The NMR spectra are consistent with the proposed structures of the polymers 34a-d, and no evidence of ring opening of the furyl unit in 34d is seen in the NMR and IR spectra. The molecular weights of 34a-d are in the range of 8000 g mol ^–1 with polydispersity indices (PDI) of 2. Polymers 34a-c have band gaps measured at 2.4 eV, while polymer 34d has its gap at 2.6 eV. Polymers 34a-c undergo solution doping with SbCl ₅ to form new low energy bipolaron bands at the expense of the absorption in the UV-VIS. However, polymer 34d does not oxidatively dope with SbCl ₅ .