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 advanced optoelectronic and bioimaging applications. We here disclose the 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 the B ← N Lewis pair, the introduction of an electron-donating (para-diphenylamino)phenyl (p-Ph₂NC₆H₄) group induces intramolecular charge transfer (CT) characteristics. For mono-fused derivatives, both molar absorption coefficients (ε) and fluorescence quantum yields (Φ_F) progressively increase with enhancement of the intramolecular CT feature through introduction of p-Ph₂NC₆H₄ and replacement of the boron-bound phenyl group with more electron-withdrawing perfluorophenyl. Moreover, the structural evolution from a mono-fused dipolar to a bis-fused quadrupolar architecture results in more than a twofold increase in ε and simultaneous significant enhancement of the 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 (8.84 × 10⁴ M⁻¹ cm⁻¹), good Φ_F (0.41) and moderate |g_lum| (7.37 × 10⁻⁴).