Thiophene-based donor–acceptor co-oligomers by copper-catalyzed 1,3-dipolar cycloaddition

Herein we present a three-component one-pot procedure to synthesize co-oligomers of a donor–acceptor–donor type, in which thiophene moieties work as donor and 1,2,3-triazoles as acceptor units. In this respect, terminally ethynylated (oligo)thiophenes were coupled to halogenated (oligo)thiophenes in the presence of sodium azide and a copper catalyst. Optoelectronic properties of various thiophene-1,2,3-triazole co-oligomers were investigated by UV–vis spectroscopy and cyclic voltammetry. Several co-oligomers were electropolymerized to the corresponding conjugated polymers.

In this study, we aimed at the combination of electron-rich (oligo)thiophenes as donors and electron-deficient 1,2,3-triazole rings as acceptors to conveniently build up novel donor-acceptor co-oligomeric and copolymeric materials by click chemistry. Thereby, the inherent instability of 2-azidothiophene was a problem.

Results and discussion
In 2005, Liang et al. [32] described a mild, copper-catalyzed method to synthesize aromatic azides from halogenated arenes and sodium azide. We have now transferred this method to the synthesis of 3-azidothiophene (2) from 3-iodothiophene (1) in excellent yield, which in the following was used for further click reactions to form novel thienyl-1,2,3-triazole co-oligomers (Scheme 1). In contrast, 2-azidothiophene could not be obtained by this protocol, because it is inherently instable. This finding corresponds to the observations of Zanirato et al. that, depending on the nature of the substituents, 5-substituted 2-azidothiophenes are more or less instable at elevated temperatures [33]. In order to overcome this inherent problem, we used a one-pot, two-step sequence, whereby an organic azide was generated in situ from a corresponding halide and immediately consumed in a reaction with copper acetylide [34][35][36]. Thus, as a model, we optimized the reaction of 2-halogenothiophene 3 and 2-ethynylthiophene (4) in the presence of sodium azide and a copper(I) catalyst to yield co-oligomer 1,4-di(thien-2-yl)-1,2,3triazole (5) (Scheme 2).
As expected, 3-halogenothiophenes gave higher yields than 2-halogenothiophenes (e.g., 8 and 9), because of the higher nucleophilicity in azide formation. Additionally, 3-azidothiophenes are much more stable than 2-azidothiophenes [37]. In the series of halides, benzenes gave higher yields than monoand bithiophenes; the lowest conversion was observed for the branched terthiophene to form co-oligomer 19, which is a result of the instability of the intermediate azide. The electrondonating methyl group in the 5-position of the thiophene ring destabilizes the corresponding azide, therefore low conversion to 15 was observed. The yield was increased with decreasing temperature. Electron-withdrawing ester groups showed an opposite tendency. Due to the lower nucleophilicity of ethyl 5-iodothiophene-2-carboxylate, the conversion to 17 rose with increasing temperature.
Substituents in the 3-position of 2-iodothiophenes gave low or no conversion to 16 and 18, respectively, because of steric hindrance in the copper-catalyzed azide formation. Twofold reactions were also investigated under the optimized reaction conditions. 1,4-Dihalogenobenzene was reacted with 2-ethynylthiophene in excellent yields (Scheme 3). 1,4-Diiodobenzene gave the desired product 21 in 99% yield at 50 °C, whereas 1,4-dibromobenzene was disubstituted at 95 °C in 98% yield.   [41]; c Stokes shift is given for the 0→0 * transition (Δν = ν abs max -ν em max ), d irreversible redox process, E 0 ox determined at I 0 = 0.855 × I p [42]. UV-vis absorption spectra of co-oligomers 5, 6 and 10-14 revealed absorption bands of the individual subunits ( Figure 1, Table 3). Thus, for triazole 5 an absorption band at 279 nm was observed, which is rather comparable to the absorption of 2-vinylthiophene (276 nm in ethanol) [38]. The second absorption band at 256 nm was assigned to the thiophene ring attached to the 1-position of the triazole. The evidence for intramolecular charge-transfer (ICT) [39] in 5 was investigated in several solvents with different dielectric constants: n-hexane (ε = 1.9), THF (ε = 7.6), methanol (ε = 32.6), and acetonitrile (ε = 37.5). Typically, no ICT was found in the described thiophene-triazole co-oligomers, but aggregate formation was observed in THF and n-hexane due to low solubility. For derivative 14 an absorption maximum at 353 nm was observed, which is red-shifted in comparison to that of 12 (λ max = 337 nm) and 13 (λ max = 339 nm). This band is not ascribed to individual bithienyl subunits, but to a weak conjugation through the triazole ring, caused by the donor-acceptor-donor system. Maarseveen et al. recently published synthesis and optical prop-erties of 1,2,3-triazole containing co-polymers, suggesting that triazole rings interrupt conjugation and therefore no interaction of the various moieties of the polymer was observable [21].
Fluorescence spectra (10 −6 M for 12 and 13, 5 × 10 −7 M for 14 in dichloromethane, Table 3) of investigated compounds 12-14 showed structured bands due to vibronic splitting. The red-shift of the emission maximum of 14 in comparison to 12 and 13 confirmed the facts assumed from UV-vis spectra that there should be weak electronic communication in the oligomers going through the 1,2,3-triazole ring. Fluorescence quantum yields of 6 to 14% were determined which are rather high for bithiophenes (1.8%) [40] and increased with increasing molecular size and conjugation. Obviously, the 1,2,3-triazole ring stabilizes the excited state by decreasing the probability of nonradiative deactivation.
From the optoelectronic data we deduced a HOMO-LUMO energy level diagram including band gaps for the novel donor-acceptor materials 5, 6 and 10-14 ( Figure 2). HOMO values were taken from the onset of oxidation and the internal reference Fc/Fc + was set to −5.1 eV versus vacuum. The LUMO values were calculated by taking the optical band gaps into account, which were taken from the absorption onset at the lowest energy band. As a trend it can be seen that 3-thienyl derivative 11 has the largest band gap (4.04 eV), because the thiophenes are linked to the triazole by unfavorable β-connections. The gap successively decreases the more extended the conjugated π-system is and approaches 3.11 eV for α-connected bithienyl derivative 14.
Oxidative oligo-and polymerization of derivatives 12-14 was carried out by potentiodynamic cycling in the appropriate potential range. The films, which were deposited on the platinum working electrode within 30 cycles, were investigated in monomer-free dichloromethane solution. The CVs indicated that only dimers of 12 and 13 were formed, which showed quasi-reversible oxidation waves at 0.28 V and 0.68 V versus Fc/Fc + , respectively, corresponding to a divinyl-quaterthio-

Conclusion
In summary, a series of novel thiophene-1,2,3-triazole co-oligomers was synthesized in good to excellent yields by a three-component two-step procedure using copper-catalyzed [3 + 2]-Huisgen cycloaddition reactions. Spectroscopic and redox properties of selected donor-acceptor-donor derivatives were investigated, in which thiophene units act as donors and triazoles as acceptors. As a general result we find that weak electronic conjugation through the 1,2,3-triazole ring is operative.

Experimental General information
All reactions were carried out under an inert atmosphere of argon. All chemicals were used as received without further purification unless otherwise specified. were determined with respect to 9,10-diphenylanthracene (DPA, Φ = 0.9 in dichloromethane) [43]. Cyclic voltammetry experiments were performed with a computer-controlled EG&G PAR 273 potentiostat in a three-electrode single-compartment cell (2 mL). The platinum working electrode consisted of a platinum wire sealed in a soft glass tube with a surface area of A = 0.785 mm 2 , which was polished down to 0.5 µm with Buehler polishing paste prior to use. The counter electrode consisted of a platinum wire and the reference electrode was an Ag/AgCl secondary electrode. All potentials were internally referenced to the ferrocene/ferricenium couple. For the measurements, the electroactive species were used in freshly distilled and degassed dichloromethane and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6  The aqueous phase was extracted with ethyl acetate (2 times, 10 mL). After the organic phases were washed with brine (15 mL) they were dried over sodium sulfate and evaporated to dryness in vacuum at room temperature. Because of the instability of the product on silica it was used without further purification. The NMR data was consistent with literature data [45]. 1  General procedure for the synthesis of 1,4disubstituted 1H-1,2,3-triazoles Halide (1 equiv) and terminal acetylene (1 equiv) were dissolved in an ethanol/water mixture (4 mL, 7:3). After the addition of sodium azide (2 equiv), sodium ascorbate (10 mol %), N,N'-dimethylethylenediamine (DMEDA, 20 mol %) and copper(I) iodide (10 mol %), the mixture was stirred in a closed Schlenk tube at 50 °C for about 15 hours. The cooled mixture was poured into 50 mL ice-water. If the product precipitated (method A) it was filtered off and washed with NH 4 OH (25 %) and water. The dried product was purified by column chromatography. The non-precipitating products (method B) were treated with 10 mL NH 4 OH (25 %). The aqueous solution was washed three times with 50 mL ethyl acetate. After the organic phase was dried over sodium sulfate, the crude product was concentrated at the rotary evaporator and purified on silica.