Naphthalimide-Annulated [<i>n</i>]Helicenes: Red Circularly Polarized Light Emitters

Abstract

Two [<i>n</i>]heliceno-bis(naphthalimides) <b>1</b> and <b>2</b> (<i>n</i> = 5 and 6, respectively) where two electron-accepting naphthalimide moieties are attached at both ends of helicene core were synthesized by effective two-step strategy, and their enantiomers could be resolved by chiral stationary-phase high-performance liquid chromatography (HPLC). The single-crystal X-ray diffraction analysis of enantiopure fractions of <b>1</b> and <b>2</b> confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both <b>1</b> and <b>2</b> exhibit high molar extinction coefficients for the S₀-S₁ transition and high fluorescence quantum yields (73% for <b>1</b> and 69% for <b>2</b>), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for <b>2</b> with CPL brightness (<i>B</i>_CPL) up to 66.5 M^-1 cm^-1 demonstrates its potential for applications in chiral optoelectronics. Time-dependent density functional theory (TD-DFT) calculations unambiguously showed that the large transition magnetic dipole moment |<i>m</i>| of <b>2</b> is responsible for its high absorbance dissymmetry (<i>g</i>_abs) and luminescence dissymmetry (<i>g</i>_lum) factor.