Bright Electrochemiluminescence Tunable in the Near-Infrared of Chiral Cationic Helicene Chromophores

Cationic helicenes are ortho-fused polyaromatics which exhibit well-defined and stable helical conformations with original absorption and emission properties in the red to near-infrared spectral ranges. Herein, a selection of cationic [4] and [6]helicenes are studied for their electrochemical, fluorescence, and electrochemiluminescence (ECL) properties in acetonitrile solutions. Their photophysical and redox responses are drastically tuned by the introduction of auxochrome substituents at their periphery or the interconversion of oxygen and nitrogen atoms within the helical core. All diaza helicenes exhibit a reversible reduction process, whereas in the presence of oxygen instead of nitrogen atoms in the helical core, irreversible oxidations and a decrease of ECL intensity are observed. ECL emission was successfully produced with two sacrificial coreactants (benzoyl peroxide and tri-n-propylamine, TPrA). [4]Helicene DMQA+, [6]helicene DIAZA(Pr/Br)+, and DIAZA(Hex/Br)+ exhibit similar ECL emission wavelength in the near-infrared region and generate very intense ECL signals. Their ECL efficiencies are up to 2.6 times higher than that of the reference compound [Ru(bpy)3]2+ when using TPrA as coreactant. A thermodynamic map gathering both oxidation and reduction potentials and fluorescence data is proposed for the prediction of energy sufficiency needed in both coreactant ECL systems. Such a systematic overview based on the photophysical and electrochemical properties may guide the conception and synthesis of new chromophores with a strong ECL proficiency