Push–pull bithiophene azo-chromophores bearing thiazole and benzothiazole acceptor moieties: Synthesis and evaluation of their redox and nonlinear optical properties

Three series of bithiophene azo dyes functionalized with thiazol-2-yl or benzothiazol-2-yl-diazene acceptor moieties were synthesized through azo coupling reaction using 2,2′-bithiophene, 5-alkoxy-2,2′-bithiophenes, 5-N,N-dialkylamino-2,2′-bithiophenes and thiazolyl- and benzothiazolyl diazonium salts as coupling components. The 5-alkoxy-2,2′-bithiophene precursors yielded the 5-thiazolylazo-5′-alkoxy-2,2′-bithiophenes, while the azo coupling reaction of 5-N,N-dialkylamino-2,2′-bithiophenes with the thiazolyl diazonium salt gave 4-thiazolyl-azo-5-N,N-dialkylamino-2,2′-bithiophenes. A different reactivity behavior was observed for 2,2-bithiophene with thiazolyl diazonium salts which gave rise to the expected azo dyes together with several arylation products. The redox behavior, thermal stability, and the first hyperpolarizability of the novel chromophores were evaluated through cyclic voltammetry, thermogravimetric analysis (TGA) and hyper-Rayleigh scattering (HRS) respectively. By varying the position of the thiazolyldiazene acceptor group on the bithiophene system, the electrochemical behavior as well as the optical (linear and nonlinear) properties of the donor–acceptor π-conjugated systems can readily be tuned. Push–pull 5-thiazolylazo-5′-alkoxy-2,2′-bithiophenes exhibit the most promising redox and the solvatochromic properties and second-order nonlinear optical response. The redox and the optical properties also show notable variations for the different heterocyclic spacers and were also sensitive to the electronic acceptor strength of the (benzo)thiazolyldiazene moieties.

Figure optionsDownload as PowerPoint slideHighlights
► Synthesis and characterization of novel bithiophene and thienylpyrrole azo dyes.
► An unexpected reactivity behavior was observed for 2,2-bithiophene with thiazolyl diazonium salts.
► Characterization of redox, thermal and optical properties of push–pull heterocyclic azo dyes.
► Comparison of the auxiliary donor effect of the pyrrole and thiophene heterocycles.
► This complete study shows that these azo dyes are good candidates as second-order NLO materials.