Synthesis of tetrazine-tetracyanobutadienes and their transformation into pyridazines via inverse-electron demand Diels–Alder cycloaddition (IEDDA)

Abstract

We report the synthesis of novel triphenylamine–tetrazine–tetracyanobutadiene based asymmetric and symmetric push–pull chromophores (1T and 1DT) by [2 + 2] cycloaddition–retroelectrocyclization of tetrazine connected electron-rich ethynyl triphenylamine with tetracyanoethene (TCNE). The electron-deficient tetrazine and tetracyanobutadiene (TCBD) moieties in 1T and 1DT create strong intramolecular charge transfer (ICT) interactions with TPA units, resulting in strong visible absorption with red edges reaching up to 700 nm (bandgaps of 1.79–1.89 eV). Furthermore, the structural, optical, and electronic properties of 1T and 1DT have been further tuned by transforming tetrazine units into pyridazines (1T-P and 1DT-P) via inverse-electron demand Diels–Alder cycloaddition (IEDDA). The relatively electron-donating character of pyridazine raised the HOMO and LUMO energies and widened the bandgap by ∼0.2 eV. This is the first synthetic strategy which allows two levels of tuning of the properties. 1DT acts as a selective colorimetric sensor for CN⁻via a nucleophilic attack on the dicyanovinyl unit of TCBD. The transformation results in a distinct colour change from orange to brown, while no change was observed for a range of anions tested (F⁻, Br⁻, HSO₄⁻, NO₃⁻, BF₄⁻, and ClO₄⁻).