Two‐photon Excitation of Bright Diaza[4]Helicenes for Isotropic and Circularly Polarized Emission

Helicenes are chiral organic dyes that are attracting growing attention due to the high tunability of their (chir)optical and electronic properties. In this work, a series of functionalized cationic diaza[4]helicenes based on dimethoxyquinacridinium (DMQA) scaffolds are presented. By merging branched N-alkyl side chains and triple para-functionalization with OMe groups, structures combine improved chiroptical responses (5x increase) and strong fluorescence quantum yields (Φ_f ≈ 70% in acetonitrile). An overall improved efficiency of the emission of circularly polarized light with B_CPL values reaching 3.4 M^-1 cm^-1 is obtained. Additionally, two-photon excitation (2PE) studies were performed, showing good values of cross section (CS) at 810 nm. Interestingly, 2PE circularly polarized luminescence (CPL) spectra were acquired for the most performant derivatives (N-isopropyl and N-cyclohexyl); this type of measurement being usually challenging for organic molecular species. Finally, the viability of these compounds in single-photon (1PE) and 2PE microscopy is also shown.