Naphthalimide-Annulated [<i>n</i>]Helicenes: Red Circularly Polarized Light Emitters
Xiaoqi Tian, Kazutaka Shoyama, Bernhard Mahlmeister, Felix Brust, Matthias Stolte, Frank Würthner
Abstract
Two [ n ]heliceno-bis(naphthalimides) 1 and 2 ( n = 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 1 and 2 confirmed their helical structure, and together with experimental and calculated circular dichroism (CD) spectra, the absolute configuration was unambiguously assigned. Both 1 and 2 exhibit high molar extinction coefficients for the S 0 –S 1 transition and high fluorescence quantum yields (73% for 1 and 69% for 2 ), both being outstanding for helicene derivatives. The red circularly polarized luminescence (CPL) emission up to 615 nm for 2 with CPL brightness ( B 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 | m | of 2 is responsible for its high absorbance dissymmetry ( g abs ) and luminescence dissymmetry ( g lum ) factor.