Molecular design to enhance binaphthyl-based chiroptics using organoboron chemistry in isomeric chiral scaffolds

Abstract

Access to high-performance circularly polarized luminescence is highly desired in materials science but it hitherto remains a considerable challenge. We herein present a conceptually universal design strategy to improve the chiral luminescent properties of a series of binaphthyl-based compounds (MeBTT, MeBTB, p-BTT, p-BTB, m-BTT and m-BTB). The binaphthyl skeleton in all cases was π-functionalized either by an electron donor (Ar₃N) or acceptor (Ar₃B), and correlations of the molecular structures to their photophysical characters were systematically investigated. They all exhibited strong photoluminescence both in solution with quantum efficiency (Φ_PL,DCM) up to 100% and as solids (Φ_PL,solid = 21–59%). The optical resolution into enantiomers via chiral HPLC was achieved for p-BTT, p-BTB, m-BTT and m-BTB with helically ring-structured binaphthyls. We further unveiled that the sterically more constrained m-BTT and m-BTB exhibit superior chiroptical properties in circularly polarized luminescence (CPL) relative to the isomers p-BTT and p-BTB, evident from the order of magnitude of luminescence dissymmetry factor (|g_lum|) increasing from 10⁻⁴ to 10⁻³ in solution. Moreover, the double functionalization of the binaphthyl moiety with an electron donor–acceptor charge-transfer system resulted also in an experimentally improved CPL activity, as rationalized by TD-DFT calculations of a key angle (θ_μ,m) between vectors of the electric and magnetic transition dipole moments in the excited states. The three organoboranes (MeBTB, p-BTB and m-BTB) displayed dual emissions in polar solvents and the low-energy charge transfer bands were demonstrated to be air-sensitive and thermally responsive at elevated temperature.