A chameleon AIEgen exhibiting six distinct yet tunable thermal and photoswitchable states

Abstract

Seeking methods to realize multiple fluorescence changes in a single luminogenic system is of great importance for both chemistry and bionics research. Due to the lack of effective strategies and functional motifs, luminogens with multiple switching and controllable models are still scarce. Herein, we report a chromone-based aggregation-induced emission luminogen called Z-CDPM, which exhibit six distinct, tunable thermal and photoswitchable states, offering controllable thermochromic or photochromic behavior under varying conditions. Specifically, five different reactions are involved: reversible Z/E isomerization, irreversible cyclization and elimination under thermal treatment, and photoarrangement of Z-CDPM and its thermal cyclization product under UV irradiation. The relative independence of the switching states is effectively maintained. Experimental and theoretical analyses validate our design strategies and provide valuable insights into the detailed mechanisms of these reactions, and single crystals further confirm their structures. Additionally, practical applications, including multiple-colored images, quick response codes, and an advanced information encryption system, are developed to demonstrate the utility. This work thus provides effective strategies and structural motifs for the design of multiresponsive luminogens and multifunctional systems.

Fig. 1. Design strategy of molecules with multiple responses and controls.

a Conventional catalyst-free ring-opening and closing reactions in monomolecular systems and their characteristics. EWG electron-withdrawing group, Cy cyclization. b Schematic illustration of the thermal and photo reactions of Z-CDPM under different conditions: (i) annealing at 145 °C in C₂D₂Cl₄ for 1 d; (ii) annealing at 200 °C in diphenyl ether for 2 h; (iii) annealing at 145 °C in silica gel for 1 d; (iv, v) UV irradiation under a 365 nm UV lamp (0.12 mW/cm) at room temperature in C₂D₂Cl₄ for 2 h.

Fig. 2. Photophysical properties and Z/E isomerization.

Single crystal structures of a Z-CDPM and b E-CDPM. Insets: chemical structures and dihedral angles in the crystal structures. c Photographs of crystals of Z-CDPM and E-CDPM. Scale bar: 2 mm. d Absorption (Abs) and normalized photoluminescence (PL) spectra of Z-CDPM (solid line, 10 μM) and E-CDPM (dash line, 10 μM) in THF/water mixtures with different water fractions (f_w). e Plots of I/I₀ values (defined as α_AIE) versus the f_w of Z-CDPM (black, 10 μM, λ_ex/em = 365/505 nm) and E-CDPM (red, 10 μM, λ_ex/em = 350/480 nm), where I₀ = PL intensity in pure THF solution at the corresponding emission wavelength. f ¹H NMR spectra of Z-CDPM and E-CDPM before and after heating at 145 °C in C₂D₂Cl₄ for 1 d (500 MHz). Z-CDPM was highlighted in purple and E-CDPM was highlighted in orange.

Fig. 3. Thermal cyclization of Z-CDPM.

a ¹H NMR spectra of as prepared Z-CDPM before and after heating at 145 °C in C₂D₂Cl₄ (500 MHz) for different times (1, 5, 12, 20, and 30 d). Z-CDPM was highlighted in purple, E-CDPM was highlighted in orange, and DPXDC was highlighted in green. b Chemical and single-crystal structures of DPXDC. c X-ray diffraction (XRD) diffractograms of the crystal powder of DPXDC. Inset: photograph of crystals of DPXDC taken under UV illumination. Scale bar: 2 mm. d Absorption spectra of DPXDC (10 μM) in THF. Inset: photograph of crystals of DPXDC taken under day light. Scale bar: 2 mm. e Photoluminescence (PL) spectra of DPXDC (10 μM) in THF/water mixtures with different water fractions (f_w, 0 and 99%). λ_ex = 365 nm.

Fig. 4. Thermal elimination of Z-CDPM and theoretical calculations.

a ¹H NMR spectra of Z-CDPM before and after heating at 145 °C under different conditions (500 MHz). Z-CDPM was highlighted in purple, DPXDC was highlighted in green, and DPXC was highlighted in blue. b Proposed reaction routes for the formation of DPXC from Z-CDPM and DPXDC. c Photoluminescence (PL) spectra of Z-DPXC in THF/water mixtures (10 μM). Inset: plots of I/I₀ value versus the water fractions (f_w) of THF/water mixtures of DPXC, where I₀ = PL intensity in pure THF solution. λ_ex/em = 355/440 nm. d X-ray diffraction diffractograms of the crystal powder of DPXC. Insets: photographs of crystals of DPXC. Scale bar: 2 mm. e Energy profile for the thermal reactions of Z-CDPM calculated at the M06-2X/6-31 G(d,p) level with solvent correction. TS transition state, INT intermediate.

Fig. 5. Photoarrangement of Z-CDPM.

a Photoarrangement of Z/E-CDPM and single crystal structures of HBNPMM. b Photoluminescence (PL) spectra of HBNPMM (10 μM) in THF/water mixtures. Inset: plots of I/I₀ value versus water fractions (f_w) of THF/water mixtures of HBNPMM, where I₀ = PL intensity in pure THF solution. λ_ex/em = 365/460 nm. c PL spectral change of Z-CDPM before and after UV irradiation at 365 nm for different times (1, 2, 3, 5, 7, 10, and 30 s). λ_ex = 365 nm d ¹H NMR spectra of the photoarrangement of Z/E-CDPM in C₂D₂Cl₄ (500 MHz). Z-CDPM (red line) and E-CDPM (orange line) were irradiated by 365 nm UV irradiation from a 365 hand-hold UV lamp for 30 min, and the black line was HBNPMM. e Energy profile for the photoarrangement of Z-CDPM calculated at the M06-2X/6-31 G(d,p) level with solvent correction. TS transition state, INT intermediate.

Fig. 6. Photoarrangement of DPXDC.

a Changes in ¹H NMR spectrum of DPXDC in C₂D₂Cl₄ during the photoarrangement process (500 MHz). b Photoarrangement of DPXDC and single crystal structure of HBNPMM. c Photoluminescence (PL) spectra of HBNPMM (10 μM) in THF/water mixtures with different water fractions (f_w). λ_ex = 440 nm. d PL spectral of DPXDC before and after UV irradiation at 365 nm for different times (1, 2, 3, 5, 7,10, and 30 s). λ_ex = 440 nm. e Energy profile for the photoarrangement of DPXDC calculated at the M06-2X/6-31 G(d,p) level with solvent correction. TS transition state, INT intermediate.

Fig. 7. Applications for information encryption.

a Fluorescent images of different AIEgens on a thin-layer chromatography (TLC) plate. Scale bar: 1 cm. b Schematic illustration of the dip-coating of Z-CDPM on a TLC plate and the following writing, fixing, and erasing process through UV irradiation (10 s) or heating (200 °C for 10 s), and the corresponding fluorescent images. Scale bar: 5 mm. c Photographs of the developed colored “quick response code” of DPXDC. Scale bar: 5 mm. d Demonstration of a multicolor encryption system dual encryption models. The light irradiation was conducted under a UV lamp (0.12 mW/cm) at room temperature to trigger the photoarrangement of Z-CDPM and/or DPXDC, and the annealing procedure was performed to generate DPXC from Z-CDPM and/or DPXDC. Scale bar: 5 mm.