Donor positional inversion in carbazole–cyanostilbene conjugates: reorienting connectivity for solid-state color modulation

Abstract

A donor–acceptor–donor (D–A–D) carbazole–cyanostilbene luminophore incorporating anthracene donors has been developed to probe the role of donor orientation on structure–property relationships. Compared with previously reported donor–donor–acceptor (D–D–A) analogues, the D–A–D architecture preserves absorption features but markedly alters emission behavior. While absorption spectra remain nearly solvent-independent, the emission undergoes a solvatochromic red-shift from 560 nm in hexane to 617 nm in DMF. In dichloromethane, the D–A–D system emits weakly at 575 nm, contrasting with the blue-green emission (468 nm) of the D–D–A analogue, underscoring the impact of donor inversion on excited-state relaxation. Aggregation studies reveal a combination of aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ), with maximum fluorescence intensity observed at 50% water fraction, supported by SEM and DLS analysis of nano-assemblies. In the solid state, the luminophores display bright yellow emission (Φ ≈ 52%) with reversible mechanochromic switching between crystalline and amorphous forms, as confirmed by PXRD. Thermal analysis demonstrates high stability (T_d = 361 °C; T_m = 206 °C). These results establish donor inversion as a powerful design strategy to tune color, enhance efficiency, and achieve stimuli-responsive luminescence in carbazole–cyanostilbene systems, offering a versatile platform for advanced solid-state emitters.