A novel boron-stereogenic fluorophore with dual-state circular polarization luminescence via a self-dispersing strategy

Abstract

Molecular engineering is a reliable approach for the development of circularly polarized luminescence (CPL) materials for various applications. However, creating dual-state CPL platforms that possess chirality while achieving a delicate balance between molecular rigidity and flexibility remains a formidable challenge. In this study, a novel bisarylboron-anchored pyrrolylsalicylhydrazone (BOPSH) platform was synthesized via a facile "one-pot" condensation. These key aryl-boron substituents not only provide structural rigidity to the fluorophore, enhancing the bright emission and suppressing emission quenching from π-π stacking in solid states due to their twisting and bulky steric effects, but also generate a boron-stereogenic center and enable strong CPL by promoting intramolecular charge-transfer transitions. As a result, these BOPSHs show intense absorption and strong dual-state emissions in both solution and solid states (with Φ _PL value approaching unity), emitting across the visible region with excellent chemical, photostability, and thermal stability. Meanwhile, their enantiomers display dual-state CPL performance, with luminescence dissymmetry factors (g _lum) up to 9.40 × 10^-3, and CP electroluminescence (EL) with a dissymmetry factor (g _EL) of 3.07 × 10^-3, along with excellent maximum external quantum efficiencies (η _ext,max) of 5.0%, approaching the theoretical limit for fluorescent molecules. We expect our study to break new ground in the construction of chiral dual-state materials with diverse structures.

Fig. 1. (a) Previously reported chiral BODIPY and its analogues. (b) The chiral DSE BOPSH fluorophore designed in this work.

Scheme 1. Syntheses of BOPSHs 1a–k and 2a–e.

Fig. 2. (a)–(c) X-ray crystal, dimmer, and packing structures of 1a. (d) and (e) X-ray crystal and dimer structures of 1j. (f) NICS(1) values for 1a. Peripheral substituents and hydrogen atoms were removed for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths in 1a: B1–N1, 1.572 Å; B1–N3, 1.545 Å; B2–N2, 1.597 Å; B1–O1, 1.464 Å; B2–O2, 1.574 Å. C, gray; N, light blue; B, pink; O, red; S, yellow.

Fig. 3. Photophysical and chiroptical properties of 1a and 1h–j in dichloromethane and solid states. (a) Fluorescence photos of respective 1a and 1h–j taken under a 365 nm lamp. (b) Normalized UV-vis absorption and (c) emission spectra in dichloromethane. (d)–(f) CD (dash lines) and CPL spectra (solid lines) of (S/R)-1a (d) and (g), 1i (e) and (h), and 1j (f) and (i) in dichloromethane and solid states.

Fig. 4. Calculated magnetic (blue) and electronic (red) transition moment densities for BOPSHs 1a (a)–(c) and 1i (d)–(f) with an iso-value of 0.005.

Fig. 5. (a) Configuration diagram of the fabricated devices. (b) EL spectra measured at 1000 cd m⁻², (c) external quantum efficiency curves with the change of luminance, and (d) luminance and current density curves with the change of voltage of devices A1 and A2. (e) CPEL spectra were collected under 5 V and (f) plots of g_ELversus CP-EL wavelength of devices A1 and A2.