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.