Triple <i>para</i>-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes
Bibiana Fabri, Tiziana Funaioli, Lucas Frédéric, Christina Elsner, Enrica Bordignon, Francesco Zinna, Lorenzo Di Bari, Gennaro Pescitelli, Jérôme Lacour
Abstract
Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E 1/2 red −1.50 V → −0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λ abs ∼ 700–900 nm) with g abs values above 10 –3 . The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.