Synthesis and characterization

Bis-2,6-pyridinedicarboxylic acid ligand (L) was synthesized by a two-step procedure and comprehensively characterized (Supplementary Methods and Supplementary Figs. 1–7). Luminescence titration revealed that the coordination stoichiometry between 2,6-pyridinedicarboxylic acid (DPA) and Eu³⁺ is 3:1 (Supplementary Fig. 8), which is inconsistent with the previous report^51,52. Trimeric lanthanide coordination polymer (Eu³⁺-L) was prepared by mixing compound L and EuCl₃ in water with a molar ratio of 1.5:1 and characterized by FTIR spectra (Supplementary Fig. 9) and ¹H NMR spectra. Compared to individual L, the absorption band at 1724 cm^‒1 assigned to the C = O stretching vibration of DPA underwent a red shift to 1625 cm^‒1 in the FTIR spectrum of Eu³⁺-L, implying the successful coordination of DPA with Eu³⁺ ion⁵³. In the ¹H NMR spectra (Fig. 2a, b), the proton signals assigned to ligand L became highly broadening after the coordination with Eu³⁺, further confirming the formation of the coordination polymer. The high coordination number of Eu³⁺-L not only benefits to sufficient sensitization of the Eu³⁺ ion based on the antenna effect, but also prevents luminescent quenching caused by the infiltration of water molecule, thus endowing the lanthanide coordination polymer with characteristic emission color and brightness in both aqueous solution and the solid state under UV light excitation (Supplementary Figs. 10, 11)⁵⁴. The luminescence quantum yield of Eu³⁺-L aqueous solution was measured to be 23.31%. The imidazolium salt modified open-form diarylethene (OF-1) was synthesized through a robust two-step procedure in a yield of 72%, along with full characterizations (Supplementary Figs. 12–21). UV–Vis (Supplementary Figs. 22–26) and ¹H NMR spectra (Supplementary Figs. 27, 28) revealed that compound 1 had excellent reversible ring-open/ring-close photoisomerization behavior (Supplementary Notes 1 and 2).

Fig. 2

¹H NMR spectral studies.

Partial ¹H NMR spectra (DMSO:D₂O = 4:1, 400 MHz, 25 °C) of a compound L, b Eu³⁺-L, c-e Eu³⁺-L-OF-1: c before and d after the irradiation by UV light (300 nm, 60 min), and e subsequent irradiation with visible light (> 450 nm, 60 min). [Eu³⁺] = 1.4 × 10^-4 M, [L] = [OF-1] = 2.1 × 10⁻⁴ M.

The lanthanide coordination polymer carries three negative net charges (6COO⁻ + Eu³⁺) per coordination center, allowing it to further assemble with positively charged OF-1 based on electrostatic interaction^55–59. The SCP (Eu³⁺-L-OF-1) was then prepared by mixing Eu³⁺-L and OF-1 at charge stoichiometry (Eu³⁺:OF-1 = 1:1.5). Zeta potential experiments were carried out to verify the existence of electrostatic interaction between Eu³⁺-L and OF-1 (Supplementary Fig. 29). Individual Eu³⁺-L displayed a negative potential of −19.53 mV, while the ζ-potential value of individual OF-1 was measured to be 20.15 mV. The Eu³⁺-L-OF-1 solution was almost electrically neutral (1.54 mV). These results confirmed the presence of electrostatic interaction between Eu³⁺-L and OF-1, enabling suitable distance between the energy donor and acceptor. Dynamic light scattering (DLS) measurements confirmed the formation of supramolecular assembly between Eu³⁺-L and OF-1. The DLS experiment of Eu³⁺-L (Fig. 3a) shows a hydrodynamic radius of 220 nm, indirectly proving the formation of large-scaled coordination polymer in solution. The hydrodynamic radius of Eu³⁺-L-OF-1 increases to around 500 nm, much larger than that of Eu³⁺-L, revealing that Eu³⁺-L assembles with OF-1 to form the supramolecular polymer^60,61. Meanwhile, uniform spheres with an average diameter of 300 nm were observed by transmission electron microscopy (Supplementary Fig. 30), providing intuitive evidence for the formation of the self-assembly between Eu³⁺-L and OF-1.

Fig. 3

DLS size distribution and UV–Vis spectral studies.

a DLS size distribution of Eu³⁺-L (black curve) and Eu³⁺-L-OF-1 (red curve) ([Eu³⁺] = 1.4 × 10⁻⁴ M, [L] = [OF-1] = 2.1 × 10^-4 M). b UV–Vis spectral changes and corresponding photographic images of Eu³⁺-L-OF-1 and Eu³⁺-L-CF-1 with alternating 300 nm UV and >450 nm visible light irradiation in water for up to 60 s each time ([Eu³⁺] = 1.4 × 10^-5 M, [L] = [OF-1] = 2.1 × 10⁻⁵ M).

Photoresponsive property

We then investigated the photoresponsive property of Eu³⁺-L-OF-1 resulting from the isomerization of the diarylethene moiety. The UV–Vis spectra of Eu³⁺-L-OF-1 (Fig. 3b) showed an absorption band at 294 nm corresponding to OF-1 unit, and no absorption over 400 nm was observed. Upon the irradiation with UV light (300 nm), the absorption at 294 nm gradually decreased, two new absorption bands centered at 380 nm and 596 nm appeared. Meanwhile, the colorless aqueous solution changed to dark blue (insert of Fig. 3b). These phenomena jointly demonstrated the OF-1 transformed to its close form (CF-1) after the irradiation. All these changes levelled off in 60 s (Supplementary Fig. 31). Moreover, a well-defined isosbestic point was observed at 323 nm, indicating that ring-open isomer cleanly transformed into the photocyclized form in SCP^62,63. We further measured the photocyclization yield at the photostationary state by ¹H NMR spectra (Fig. 2c,d). Since the proton signals of OF-1 showed serious broadening in aqueous media (Supplementary Fig. 32), the ¹H NMR spectral study was carried out in mixed deuterated solvent (DMSO-d₆:D₂O = 4:1). After irradiated by UV light (300 nm, 60 min), the thiophene protons (H_b) underwent an obvious upfield shift from 7.30 to 6.81 ppm, mainly due to the electronic shielding effect in the large conjugated closed ring isomers⁶⁴. The methyl protons (H_a) of the diarylethene unit underwent an apparent downfield shift from 1.88 to 2.00 ppm. Meanwhile, the aromatic protons H_c and H_d showed downfield shifts from 7.52 ppm to 7.63 ppm and from 6.98 ppm to 7.04 ppm, respectively. All these shifts were thorough, and no apparent residual peaks retained in the original chemical shifts after the UV light irradiation (Supplementary Fig. 33). The molar ratio of CF-1: OF-1 was determined to be 0.94:0.06 according to the integrating resonance of protons H_a, indicating nearly quantitative (~94%) conversion from Eu³⁺-L-OF-1 to Eu³⁺-L-CF-1 upon exposure to UV light⁶⁵. Interestingly, a complete recovery in both UV–Vis (Supplementary Figs. 34, 35) and ¹H NMR spectra (Fig. 2e) was achieved upon subsequent irradiation of the resulting Eu³⁺-L-CF-1 solution with >450 nm visible light, accompanied by color change back to colorless, revealing that this photoisomerization behavior was fully reversible.

The photoresponsive luminescent behavior of SCP was then investigated. In the as-prepared Eu³⁺-L-OF-1, no FRET was observed, because there was no spectral overlapping between the UV–Vis absorption of OF-1 and the emission spectrum of Eu³⁺-L. Eu³⁺-L-OF-1 exhibited the characteristic spectral line of lanthanide. The excitation spectrum of Eu³⁺-L-OF-1 showed a broad band centered at 265 nm, attributed to the absorption of the DPA moiety (Supplementary Fig. 36). The corresponding emission spectrum was composed of five sharp peaks at 580, 594, 615, 649, and 692 nm, referred to the ⁵D₀ to ⁷F_J (J = 0-4) transitions of Eu³⁺ respectively, in which the ⁵D₀ → ⁷F₂ transition at 615 nm is dominant and responsible for the bright red emitting color (Supplementary Fig. 36)⁶⁶. On the other hand, the luminescence emission spectrum of lanthanide coordination polymer Eu³⁺-L completely overlapped with the absorption spectrum of CF-1 in the range of 500-700 nm (Fig. 4a), implying that efficient FRET process may occur from Eu³⁺ to CF-1 in Eu³⁺-L-CF-1. As expected, the luminescence of Eu³⁺ (Fig. 4b) was quenched gradually upon irradiating SCP with UV light. The luminescence quenching followed a biexponential attenuation law, containing a fast process, followed by a slow process to the photostationary state in 60 s (inset of Fig. 4b)⁴⁹. The luminescence intensity was quenched completely at the end, and the decay decreased from 1,289 to 12 μs (Supplementary Figs. 37–41), with concomitant decrease of the luminescence quantum yield from 15.84% to 0.85%. These phenomena confirmed the occurrence of the FRET process with an efficiency (E) of 98%, calculated according to the reported method⁶⁷. The quenched luminescence of Eu³⁺-L-CF-1 could completely recover to its original level upon subsequent visible light irradiation, ascribing to the photocycloreversion reaction (Fig. 4c). In particular, the photocontrolled luminescence on/off switch of SCP presented outstanding reversibility, and no apparent deterioration in the luminescence intensity (less than 4%) was observed after 20 consecutive cycles of alternating UV and visible light irradiations (Fig. 4d). Thus, SCP exhibited excellent fatigue resistance, which is of utmost importance for multiple anticounterfeiting applications.

Fig. 4

Photophysical studies.

a Partial emission spectrum (black curve) of Eu³⁺-L, and absorption spectra of OF-1 (red curve) before and (blue curve) after irradiation with 300 nm UV light for 60 s. b, c Luminescence emission spectral changes of Eu³⁺-L-OF-1 upon (b) UV light (300 nm) irradiation and c subsequent visible light (>450 nm) irradiation in water. Insets show corresponding emission intensity changes at 615 nm. d Luminescence emission changes of Eu³⁺-L-OF-1 upon consecutive alternating exposure to UV and visible light. Insets show corresponding intensity changes at 615 nm (upper) and the photographs of the SCP solution under 254 nm UV lamp (lower). [Eu³⁺] = 1.4 × 10⁻⁴ M, [L] = [OF-1] = 2.1 × 10⁻⁴ M.

It is worth noticing that the diarylethene derivative is bistable³⁷, which means that the spontaneous photocycloreversion reaction is extremely slow under natural conditions. The half-life (t_1/2) of Eu³⁺-L-CF-1 at 25 °C was estimated to be 376.7 min (Supplementary Figs. 42, 43, Supplementary Table 1, and Supplementary Note 3), ranking one of the longest t_1/2 values reported so far in diarylethene derivatives^68,69, which confirmed that the self-switching is negligible. Only slight self-switching of Eu³⁺-L-CF-1 was observed upon continuous exposure to sunlight for 90 min (Supplementary Fig. 44). When the Eu³⁺-L-CF-1 solution was kept at an elevated temperature (60 °C) in the dark, no sign of thermal ring opening was observed from the UV–Vis spectra, supporting the good thermal stability of Eu³⁺-L-CF-1 (Supplementary Fig. 45).

Pattern printing

The developed SCP with important features of rapid response, prominent anti-fatigue capability and thermally irreversible luminescence on/off photoswitch encouraged us to further explore its performance in smart anticounterfeiting. We directly filled the Eu³⁺-L-OF-1 aqueous solution in a commercial inkjet printer (canon PIXMA ip1180) cartridge with the concentration low to 2.1 × 10^-4 M (according to the concentration of OF-1), and printed various high-resolution quick response (QR) codes on commercial blue polyester terephthalate (PET) films (Fig. 5a, b). The obtained QR code was invisible under daylight due to the colorless nature of Eu³⁺-L-OF-1 aqueous solution (Fig. 5c and Supplementary Movie 1). However, bright red luminescent pattern was observed under 254 nm UV lamp, allowing to retrieve the encoded information quickly and accurately by scanning through a smartphone (Fig. 5d and Supplementary Movie 2). It should be noted that the UV absorbance intensity of OF-1 at 254 nm is low, and thus the conversion from OF-1 to CF-1 under 254 nm UV lamp is very slow, providing enough time for recognizing the authentic information recorded in the QR code. The luminescence was quenched upon 300 nm UV light irradiation, making the QR code invisible under UV light. Although the pattern turned to blue under daylight, it can be completely masked by the blue background of the PET film. Through which, the absolutely and really invisible security pattern was achieved under both daylight (Fig. 5e and Supplementary Movie 3) and UV light (Fig. 5f and Supplementary Movie 4), which was highly sufficient for confidential information encryption.

Fig. 5

Pattern printing using SCP as the ink.

a, b Schematic illustration of the pattern printing process. Light triggered QR code with visible/invisible transformation behavior was achieved by using supramolecular coordination polyelectrolyte (SCP) as the smart ink. c–f Digital photos of SCP-based luminescent QR code on commercial blue PET film (size: 5 × 5 cm) upon alternating UV (300 nm, 60 s) and visible light (>450 nm, 120 s) irradiation. c, d Photos under daylight. e, f Photos under 254 nm UV lamp. [Eu³⁺] = 1.4 × 10⁻⁴ M, [L] = [OF-1] = 2.1 × 10⁻⁴ M in the SCP ink.

As discussed above, CF-1 is the photostable state under daylight, especially in solid state. Consequently, the erased pattern remained unreadable even after placing under sunlight for one month (Supplementary Movies 5 and 6). The erased pattern could be completely recovered and recognized upon further irradiating with visible light (>450 nm). Moreover, even after 20 consecutive switching cycles, the quality of the remote light triggered information pattern with visible/invisible transformation process still remained unaffected (Supplementary Movies 7 and 8). Thus, the rapid response, noninvasive regulation, excellent fatigue resistance, and thermal irreversibility of SCP-based system made it a suitable anticounterfeiting ink for multiple authentic information encryption and decryption.

Synthesis of compound 5

A mixture of compound 6 (474 mg, 1.98 mmol), 1,4-dibromobutane (60 μL, 0.5 mmol) and K₂CO₃ (248 mg, 1.8 mmol) was stirred in N,N-dimethylformamide (10 mL) under N₂ at 80 °C for 48 h. Then, the reaction mixture was poured into water (100 mL). The resulting white precipitate was collected and washed three times with water. The precipitate was then dissolved in CH₂Cl₂ (100 mL) and washed with a solution of 5 % aqueous NaOH solution (2 × 50 mL). The organic phase was concentrated and dried under vacuum to give compound 5 as a white solid in 80% yield. ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.79 (s, 4H), 4.48 (q, J = 7.1 Hz, 8H), 4.27 (m, 4H), 2.08 (m, 4H), 1.46 (t, J = 7.1 Hz, 12H). ¹³C NMR (100 MHz, CDCl₃, ppm): δ 166.7, 164.7, 150.2, 114.1, 68.2, 62.4, 25.3, 14.2. HRMS [M + H]⁺ calcd. for C₂₆H₃₃N₂O₁₀⁺ 533.2135; found: 533.2126; Anal. Cald. for C₂₆H₃₂N₂O₁₀: C, 58.64; H, 6.06; N, 5.26; Found: C, 58.58; H, 6.10; N, 5.22.

Synthesis of compound L

A mixture of compound 5 (168 mg, 0.4 mmol), KOH (224 mg, 4 mmol), methanol (10 mL) and water (10 mL) was stirred at 60 °C for 12 h, and then acidified with HCl (3 M) to pH 4. The precipitate was collected by centrifugation, washed with H₂O, and dried under vacuum to give compound L as a white solid in 60% yield. ¹H NMR (400 MHz, D₂O, ppm): δ 7.59 (s, 4H), 4.34 (m, 4H), 2.08 (m, 4H). ¹³C NMR (100 Hz, H₂O, ppm): δ 172.7, 166.7, 154.9, 111.4, 68.3, 24.7. HRMS [M-H]^– calcd. for C₁₈H₁₅N₂O₁₀^– 419.0727; found: 419.0734; Anal. Cald. for C₁₈H₁₆N₂O₁₀: C, 51.44; H, 3.84; N, 6.66; Found: C, 51.29; H, 3.78; N, 6.60.

Preparation of the coordination polymer Eu³⁺-L

Compound L (42 mg, 0.1 mmol) and potassium hydroxide (22.4 mg, 0.4 mmol) were dissolved in water (10 mL). Then, europium chloride hexahydrate (24.5 mg, 0.067 mmol) was added with stirring for 30 min. The mixture was dried under vacuum to give Eu³⁺-L as a white solid.

Synthesis of compound 4

4-Hydroxyphenylboronicacidpinacolester (220 mg, 1 mmol), 1,2-dibromoethane (940 mg, 5 mmol), and K₂CO₃ (690 mg, 5 mmol) were added into acetonitrile (20 mL) with stirring. The mixture was heated at 70 °C under N₂ atmosphere for 24 h. After cooling down to room temperature, the reaction mixture was filtered and the residue was washed with CH₂Cl₂. Then, the filtrate was concentrated under reduced pressure. The residue was dissolved by CH₂Cl₂ (50 mL) and washed twice with saturated NaCl solution. The organic phase was concentrated. The crude product was purified by column chromatography over silica gel (eluent: petroleum ether/ethyl acetate = 20:1), and compound 4 was obtained as white powder in 80% yield. ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.75 (d, J = 8.5 Hz, 2H), 6.90 (d, J = 8.5 Hz, 2H), 4.32 (t, J = 6.4 Hz, 2H), 3.64 (t, J = 6.3 Hz, 2H), 1.33 (s, 12H). ¹³C NMR (100 MHz, CDCl₃, ppm): δ 160.6, 136.6, 114.0, 83.6, 67.6, 28.9, 24.9. HRMS [M + H]⁺ calcd. for C₁₄H₂₁BBrO₃⁺ 327.0767; found: 327.0754; Anal. Cald. for C₁₄H₂₀BBrO₃: C, 51.42; H, 6.16; Found: C, 51.38; H, 6.18.

Synthesis of compound 2

Compound 4 (327 mg, 1 mmol), compound 3 (210 mg, 0.4 mmol), Pd(PPh₃)₄ (70 mg, 0.06 mmol), and Na₂CO₃ (680 mg, 6.4 mmol) were added to a mixed solution of water (4 mL) and dimethoxyethane (30 mL). The mixture was refluxed under N₂ at 90 °C in dark for 24 h. After cooling down to room temperature, the solvent was removed under vacuum. The residue was extracted by dichloromethane, and purified on a silica gel column using petroleum ether/ethyl acetate (20:1) as the eluent. ¹H NMR (400 MHz, CDCl₃, ppm): δ 7.47 (d, J = 8.6 Hz, 4H), 7.17 (s, 2H), 6.93 (d, J = 8.6 Hz, 4H), 4.32 (t, J = 6.2 Hz, 4H), 3.66 (t, J = 6.2 Hz, 4H), 1.94 (s, 6H). ¹³C NMR (100 MHz, CDCl₃, ppm): δ 158.0, 141.9, 140.5, 127.0, 126.9, 125.8, 121.5, 115.2, 68.0, 28.9, 14.5. HRMS [M + H]⁺ calcd. for C₃₁H₂₅Br₂F₆O₂S₂⁺ 766.9546; found: 766.9537; Anal. Cald. for C₃₁H₂₄Br₂F₆O₂S₂: C, 48.58; H, 3.16; Found: C, 48.51; H, 3.19.

Synthesis of compound OF-1

Compound 2 (383 mg, 0.5 mmol) was dissolved in acetonitrile (10 mL), and then 1-methylimidazole (410 mg, 5 mmol) was added. The reaction mixture was stirred at 80 °C for 12 h. After cooling down to room temperature, the obtained precipitate was collected by centrifugation and washed with diethyl ether for three times to afford the desired product OF-1 in 90% yield. ¹H NMR (400 MHz, DMSO-d₆, ppm): δ 9.20 (s, 2H), 7.83 (s, 2H), 7.73 (s, 2H), 7.58 (d, J = 8.4 Hz, 4H), 7.39 (s, 2H), 7.02 (d, J = 8.6 Hz, 4H), 4.61 (t, J = 4.6 Hz, 4H), 4.39 (t, J = 4.7 Hz, 4H), 3.88 (s, 6H), 1.93 (s, 6H). ¹³C NMR (100 MHz, DMSO-d₆, ppm): δ 158.1, 142.0, 140.7, 137.5, 127.2, 126.4, 125.4, 124.0, 123.3, 121.8, 115.8, 66.4, 48.8, 36.3, 14.5. HRMS [M-2Br]²⁺ calcd. for C₃₉H₃₆F₆N₄O₂S₂²⁺ 385.1086; found: 385.1081. Anal. Cald. for C₃₉H₃₆Br₂F₆N₄O₂S₂: C, 50.33; H, 3.90; N, 6.02; Found: C, 50.39; H, 3.98; N, 5.94.

Preparation of the QR code

In a standard procedure, commercial PET film was first printed with blue background, and the QR-pattern was then directly printed on the blue PET film. The QR code was scanned by a commercially available smartphone APP.