Design, synthesis, and characterization of the electrochemical, nonlinear optical properties, and theoretical studies of novel thienylpyrrole azo dyes bearing benzothiazole acceptor groups

Two series of related donor–acceptor conjugated heterocyclic azo dyes based on the thienylpyrrole system, functionalized with benzothiazol-2-yl (5–6) or benzothiazol-6-yl acceptor groups (7) through an Ndouble bond; length as m-dashN bridge, have been synthesized by azo coupling using 1-alkyl(aryl)thienylpyrroles (1) and benzothiazolyl diazonium salts (2–4) as coupling components. Their optical (linear and first hyperpolarizability), electrochemical, and thermal properties have been examined. Optimized ground-state molecular geometries and estimates of the lowest energy single electron vertical excitation energies in dioxane solutions were obtained using density functional theory (DFT) at the B3LYP/6-31+G(d,p) level. Hyper-Rayleigh scattering (HRS) in dioxane solutions using a fundamental wavelength of 1064 nm was employed to evaluate their second-order nonlinear optical properties. Of these systems, the benzothiazol-2-yl-diazenes 5–6 exhibit the largest first hyperpolarizabilities (β=460–660×10−30 esu, T convention) compared to benzothiazol-6-yl-diazenes 7 (β=360–485×10−30 esu, T convention). Good to excellent thermal stabilities were also obtained for all azo dyes (235–317 °C). This multidisciplinary study showed that modulation of the optical and electronic properties can be achieved by introduction of the benzothiazole acceptor group in the thienylpyrrole system through position 2 or 6 of the benzothiazole heterocycle.