An ultrawide-range photochromic molecular fluorescence emitter

Abstract

Photocontrollable luminescent molecular switches capable of changing emitting color have been regarded as the ideal integration between intelligent and luminescent materials. A remaining challenge is to combine good luminescence properties with wide range of wavelength transformation, especially when confined in a single molecular system that forms well-defined nanostructures. Here, we report a π-expanded photochromic molecular photoswitch, which allows for the comprehensive achievements including wide emission wavelength variation (240 nm wide, 400-640 nm), high photoisomerization extent (95%), and pure emission color (<100 nm of full width at half maximum). We take the advantageous mechanism of modulating self-assembly and intramolecular charge transfer in the synthesis and construction, and further realize the full color emission by simple photocontrol. Based on this, both photoactivated anti-counterfeiting function and self-erasing photowriting films are achieved of fluorescence. This work will provide insight into the design of intelligent optical materials.

Fig. 1. The design strategy of ultrawide-range photochromic molecular light emitter.

a The reported strategies for photocontrollable luminescence, and pyrene-modified merocyanine-based photoswitch with wide range of photochromic fluorescence by controlling ICT and self-assembly effect in this work. (ICT: intramolecular charge transfer, PET: photoinduced electron transfer, FRET: Förster resonance energy transfer) b, c UV-Vis, and fluorescence spectra of photoswitch (0.1 mM) in strong-ICT-Self-assembly (PMC) and weak-ICT-monomer (PSP) state in CHCl₃. Inserts are image of PMC (I) and PSP (II) in natural light and PMC (III) and PSP (IV) in UV light.

Fig. 2. Chemical structures and ¹H-NMR spectra (in DMSO-d₆) of PMC and PSP.

a Partial ¹H-NMR spectrum of PMC. b Partial ¹H-NMR spectrum of PSP. The variations of chemical shift of protons are marked with different color dashed lines.

Fig. 3. Proposed mechanism of photoisomerization/thermal relaxation process and HOMO/LUMO simulation of PMC and PSP.

a, b Gibbs free energy profile (kcal/mol) and schematic representation of the photoswitch isomerization mechanism (obtained by M06-2X/6-311 + G(d,p)//M06-2X/6-31 + G(d,p)). c The frontier molecular orbitals of PMC and PSP (optimized by B3LYP/6-31 + G). (TS: transition state, G: Gibbs free energy).

Fig. 4. Investigation for the ICT-Self-assembly mechanism enabling photochromic fluorescence of PMC in CHCl₃ solution.

a Absorption at 500 nm of PMC from 0.0001 mM to 0.1 mM. b Normalized fluorescence spectra of PMC with concentration from 0.1 mM, 0.07 mM, 0.04 mM, 0.03 mM, 0.02 mM, 0.01 mM, 0.001 mM, and 0.0001 mM in chloroform (from top to bottom, λ_ex = 365 nm). c Optical transmittance at 625 nm of PMC from 0.0001 mM to 0.1 mM. d SEM image of PMC (0.1 mM). e The 640 nm emission, 400 nm emission, and the ratio of the two emission intensities (I₄₀₀/I₆₄₀) of PMC in different concentration. f, g The CIE 1931 chromaticity diagram, and fluorescence images of PMC in different concentration. (λ_ex = 365 nm).

Fig. 5. Characterization of the dynamics of photoisomerization and thermal relaxation, and the time-dependent fluorescence chromism of PMC photoswitch in CHCl₃ (25 mW cm^–2, 25 °C unless mentioned).

a, b Absorbance at 500 nm and emission at 400 nm/640 nm for 5 cycles of time-dependent photoisomerization and thermal relaxation process. The yellow areas represent photoisomerization process, and the blue areas represents thermal relaxation process. The rate constant of photoisomerization process k₂ is (0.29 ± 0.03) s^–1, and rate constant of thermal relaxation process k₃ is (223.10 ± 5.59) s^–1 (see Supplementary Tables 3 and 4). c Half-life of thermal relaxation process and photoisomerization extent of light irradiation and thermal relaxation process through time-dependent dynamic UV-Vis absorption and fluorescence emission spectra. n = 5 independent experiments, with the bar data indicating mean ± SD. d Time-dependent 3D-fluorescence spectra of thermal relaxation process. e, f The CIE 1931 chromaticity diagram, and fluorescence images of thermal relaxation process at certain time including 0 min (PSP state), 1 min, 5 min, 15 min, 30 min, 60 min, 90 min, 120 min, and 300 min.

Fig. 6. Solvent effect of photochromic fluorescence of PMC.

a, b Fluorescence images and CIE 1931 chromaticity diagram of PMC and PSP in CHCl₃, o-dichlorobenzene (DCB), 1,4-dioxane (Diox), toluene (Tol), DMSO, acetone (DMK), ethanol (EtOH) and water. The purple mark represents PMC state, and orange marks represents PSP.

Fig. 7. The control of photochromic fluorescence ranges using a series of mixed solvents (DMSO/DCB) and photowriting and self-erasing behavior of PMC-PMMA film.

a The CIE 1931 chromaticity diagram of PMC and PSP in the DMSO/DCB solvent with different DMSO fractions. b Fluorescence image of the photoswitch in the DMSO/DCB solvent with different DMSO fractions. The top ones are PMC and bottom ones are PSP. c, d The illustration, and digital images of anti-counterfeiting patterns of PMC or PSP in mixed solvents. e, f Fluorescence and UV-Vis spectra for photoisomerization and thermal relaxation process of PMC-PMMA film. The insert spectra are thermal relaxation dynamics of absorption at 479 nm of the film and CIE 1931 diagram, respectively. g, h The illustration, and digital images for photowriting, full light-exposure, and self-erasing process of the film.