Abstract:
Aryl-aryl coupled polymers are a class of conjugated polymers that possess useful chemical, electrochemical and photochemical properties. Transition metal- catalysed C-C cross-coupling reactions employed in the synthesis of aryl-aryl coupled polymers require sophisticated reaction conditions and expensive catalysts. Therefore, this research project focused on the synthesis and characterisation of linear open-chain 4,4′-dibromo aromatic oligomers of 1,4- dibromobenzene using the Ullmann homocoupling reaction, which is feasible in the Sri Lankan context. Several synthetic attempts for the condensation of 1,4- dibromobenzene were carried out using Ullmann coupling in the presence of an activated Cu bronze catalyst, Cs2CO3 base in N-methyl-2-pyrrolidone (NMP) / dimethyl sulfoxide / dimethyl formamide solvent at refluxing conditions under normal atmospheric conditions / N2 over 3.5 h to 36 h of varying periods. The reaction progress of nine different reaction conditions investigated was monitored using thin-layer chromatography (TLC). Among them, the synthetic attempts that indicated the new product formation in TLC analysis were purified using column chromatography. The purified products were characterised using UV-visible spectroscopy and gas chromatography-mass spectrometry. Among the four out of nine reaction mixtures that demonstrated new product formation, the coupling of 1,4-dibromobenzene in NMP at 190 oC over 24 h in a normal atmosphere demonstrated the most intense peak in the TLC analysis. The column chromatography of the crude mixture yielded a clear, pale yellow colour liquid product with a relatively low yield, which was caused by the sublimation followed by the deposition of the starting material at temperatures above 130 oC.
The UV-visible spectra of the obtained liquid demonstrated the highest absorption at a wavelength of 287 nm in n-hexane. This absorption peak can be attributed to the π to π* transition of the conjugated aryls. Observation of a light blue fluorescence of the product under the UV light at 366 nm indicated the presence of an extended π system. In summary, condensation attempts of 1,4- dibromobenzene using the Ullmann coupling reaction yielded a fluorescence- active pale yellow colour liquid. Further characterisation of the product using 1H and 13C NMR spectroscopy is needed to confirm the product formation.
