B←N Lewis pair fusion of [6]helicene: one way to integrate circularly polarized luminescence with two-photon absorption

Abstract

It is very challenging to integrate circularly polarized luminescence (CPL) and two-photon absorption (TPA) in a single organic molecule, which is of great interest for the advanced optoelectronic and bioimaging applications. We here disclose synthesis and properties of a new family of helicenes, in which [6]helicene is fused with six-membered B←N heterocycles. Benefiting from the strong electron-affinity of B←N Lewis pair, the introduction of electron-donating (para-diphenylamino)phenyl (p-Ph₂NC₆H₄ ) group induces the intramolecular charge transfer (CT) characteristic. For the mono-fused derivatives, both molar absorption coefficients (ε) and fluorescence quantum yields (Φ_F) progressively increase with enhancement of intramolecular CT feature through introduction of p-Ph₂NC₆H₄ and replacement of boron-bound phenyl group with more electron-withdrawing perfluorophenyl. Moreover, the structural evolution from mono-fused dipolar to bis-fused quadrupolar architecture results in more than double of ε and simultaneous significant enhancement of TPA cross section (δ_TPA) and luminescence dissymmetry factor (g_lum). It thus became possible to achieve integration of excellent CPL and TPA properties in a single molecule for the bis-fused quadrupolar [6]helicene BiNFBPy-HC, with δ_TPA up to 1361 GM and CPL brightness (B_CPL) reaching 13.2 M ^-1 cm ^-1 , owing to its large ε (8.70 × 10 ⁴ M ^-1 cm ^-1 ), good Φ_F (0.41) and moderate |g_lum| (7.37 × 10 ^-4 ).