Structural Insight into Electrogenerated Chemiluminescence of Para-Substituted Aryl–Triazole–Thienyl Compounds

There has been a strong push to develop new thiophene-based monomers to tailor the electrical and optical properties of the resulting polymers. However, the synthesis of these elaborated thienyl compounds is difficult to realize. Here, we report the successful click coupling of thienyl azides and para-substituted aryl alkynes to synthesize eight thiophene-based luminophores intended for electrochemical and electrochemiluminescence (ECL) study. These thiophenes could be separated into two series: monothienyl and bithienyl analogues and further categorized based on the nature of the ligand attachment to their phenyl rings (electron-donating or -withdrawing characteristics: NMe2, OMe, H, or F) on the other side of the triazole bridge. The electrochemical experiments indicated these compounds lacked stability when they were oxidized or reduced, with the exception of those with a dimethylamine ligand attached (quasireversible oxidations). Cyclic and differential pulse voltammetries revealed the redox potentials of these compounds were affected by the extent of the conjugation and the nature of the ligands, while the electrochemical gaps correlated well with the energy differences between the excited and ground state species. ECL in the annihilation route confirmed the weak light-emitting nature of these thiophenes; however, great improvement was made with the use of a coreactant species (benzoyl peroxide or ammonium persulfate). ECL spectroscopy revealed that the excimer or polymeric excited states were more favorable in formation than their monomeric excited states, which was tunable based on the applied potentials. Structural insight into ECL will guide us to discover optimized thiophene-based luminophores.