Conjugated C3 symmetric aryl tripyrroles and aryl bipyrroles: synthesis, optical and electronic properties

Abstract

Conjugated C3 symmetric molecules have been pursued for their fluorescent and electrochemical properties. The synthesis of conjugated C3 symmetric aryl tripyrroles has been performed by a route featuring three steps from trimethyl 1,3,5-benzene tricarboxylate 6: copper-catalyzed cascade addition of vinylmagnesium bromide to convert the carboxylates into γ,δ-unsaturated ketones, Tsuji–Wacker olefin oxidations to form tris(1,4-dione) 9, and Paal–Knorr condensation with ammonia and different amines to furnish the final tripyrroles (30–60% yields). In addition, incomplete reaction of vinylmagnesium bromide to 6 provided benzoate 8 possessing two γ,δ-unsaturated ketones, which were similarly converted to 3,5-dipyrrolylbenzoates 11. The absolute fluorescence quantum yields (Φfl) and electrochemical properties of 5a–c and 11a–c were investigated. The Φfl are the first reported for such compounds and they ranged between 2 and 40%, contingent on structure and solvent polarity. Cyclic voltammetry revealed that the compounds could be both oxidized and reduced, albeit irreversibly. The oxidation potentials (Eox) varied between 0.73 V and 1.2 V and the reduction potentials (Ered) varied from −0.83 V to −1.36 V. The lowest redox processes were measured for the NH–pyrrole 5a. Moreover, tripyrrole 5a was air stable and on oxidative doping with ferric chloride exhibited a 50 nm bathochromic shift in its absorbance spectrum.