Synthesis and Properties of Cationic and Neutral Radical Diaza[4]helicenes

Abstract

This PhD describes the synthesis and characterization of various functionalized cationic diaza[4]helicenes based on dimethoxyquinacridine (DMQA) scaffolds. Based on organometallic catalysis, late-stage functionalization strategies are developed and applied to introduce a diversity of functional groups. Chapter 1 provides a general introduction of chirality in organic molecules and their interaction with circularly polarized light; generalities on helical dyes are also presented. In Chapter 2, a pH-sensitive helicene is functionalized via C-H insertions of electrophilic metal carbenes (Ru or Rh). Up to four malonate moieties are introduced and, through the study of the electronic and chiroptical properties of these malonate-functionalized helicenes, the stereoelectronic influence of the added moieties is elucidated. In Chapter 3, the core transformation of this manuscript is described. Thanks to Ir-catalyzed direct C-H borylation of DMQA, regioselective functionalization of the three positions in para to the formal positive charge is obtained for the first time. In this chapter, Ir-catalysis is coupled, in tandem, with Suzuki-Miyaura cross coupling reactivity to give a series of tris(arene) derivatives. Furthermore, in Chapter 4, after Ir-catalysis, PdII-catalyzed alkoxycarbonylation and hydroxylation reactions are performed. Thanks to these two transformations, functional groups with strong ED or EW character are introduced in para positions. An extensive tuning of the optical and electronic properties of the helical core is observed with para-EDGs and EWGs, with absorption and emission spectra covering a range of ≈ 300 nm. Derivatives with EDGs have blue-shifted optical properties with high fluorescence quantum yields up to 70%. On the contrary, derivatives with EWGs have red-shifted absorption and emission and are poor fluorophores. However, they present interesting electronic properties, mainly due to their facile first one-electron reduction that makes them good candidates for the production of neutral radical helicenes. The formation of neutral persistent radicals, via electrochemical or chemical (CoCp2) mono electron reductions, is described in Chapter 5. The reversibility of this redox process, the chiroptical properties of the obtained radicals, their EPR signatures, spin density delocalization and half-life values under air are described. Finally, in Chapter 6, a family of strongly fluorescent para-OMe derivatives, based on helical scaffolds bearing primary or secondary N-alkyl chains, is presented. These compounds are employed in two-photon excitation studies, showing good values of cross section at 810 nm. Additionally, two-photon excitation circularly polarized emission spectra are reported. Further reactions are briefly described in the conclusions to give a prospective on future projects.</p