AIE-Driven Chiral Covalent Organic Frameworks for Solid-State Circularly Polarized Luminescence, Hydrochromism, and Water-Induced Chiroptical Enhancement

Abstract

Chiral covalent organic frameworks (CCOFs) are promising candidates for chiral optoelectronics and sensing, but their weak solid-state fluorescence and chiroptical responses often limit practical utility. Here, we introduce a novel CCOF synthesized from achiral monomers, 2-hydroxy-1,3,5-benzenetricarbaldehyde and hydrazine, via imine condensation with a chiral induction strategy, yielding salicylaldehyde azine units with aggregation-induced emission. Optimized catalyst and chiral inducer stoichiometry endow the framework with exceptional chiroptical properties (|g_abs| = 2.2 × 10⁻², ellipticity ≈ 1000 mdeg). In the solid state, the CCOF exhibits intense red fluorescence (λ_em ≈ 645 nm) with a large stoke shift and favorable circularly polarized luminescence (CPL, |g_lum| = 5.2 × 10⁻²), marking the first CCOF derived solely from achiral building blocks with robust solid-state CPL. When integrated into polydimethylsiloxane, it forms flexible and semitransparent composite films suitable for CPL-based applications. The CCOF also functions as a highly enantioselective fluorescent sensor for chiral analytes, including 2-aminocyclohenanol and dimethyl-1,2-cyclohexanediamine. Furthermore, it demonstrates reversible hydrochromism, transitioning from yellow to orange (ΔE ≈ 42.7), and water-induced chiroptical enhancement (ellipticity up to 2100 medg, |g_abs| = 5.5 × 10⁻²), achieving the highest ground-state chirality reported for CCOFs through enol-to-keto tautomerism upon water adsorption. This stimuli-responsive CCOF overcomes persistent limitations in solid-state CPL and paves the way for chiral sensing, optical displays, and responsive materials.