Near-infrared II photochromic behavior triggered by green light in an in situ protonated dithienylethene functionalized by quinoxalinone moieties

Abstract

Exploiting the near-infrared (NIR) photochromic dithienylethenes (DTEs) triggered by visible light is urgently needed for various biological scenarios. However, all the NIR photochromic DTEs reported so far are located in the first NIR window (NIR-I, 700–900 nm), which usually shows shallower penetration in biological tissues due to autofluorescence and photon scattering compared to NIR light in the second window (NIR-II, 1000–1700 nm). Herein, we present a novel quinoxalinone-functionalized DTE derivative (QDTE) with acceptor (A)–DTE (D)–acceptor (A) structural features, in which electron-withdrawing quinoxalinone groups ensure visible light-driven NIR I photochromism. Besides, the facile protonation of the quinoxalinone moieties favors the formation of the more electron-deficient A′–D–A′-type DTE (QDTE-2H, where A′ is a stronger electron-withdrawing unit) for a unique NIR II photochromism by reducing the HOMO–LUMO energy gap of a closed isomer after protonation. As expected, the resulting QDTE displays a blue light-controlled NIR I photochromic performance in various solvents. Furthermore, an unprecedented green light-triggered NIR II photochromism for the in situ protonated QDTE-2H is successfully implemented in CHCl₃ and toluene in the presence of trifluoroacetic acid (TFA), representing the first case of NIR II photochromic DTE. By virtue of these properties, QDTE has been successfully applied in dual information encryption, demonstrating its versatility in functional materials.

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Introduction

Photochromic dithienylethene (DTE) featured by ring-open/-closed isomerization has recently drawn much attention because of their potential as photoswitches in optical information storage, super-resolution imaging, photopharmacology, and anti-counterfeiting.^1–12 Their promising thermally irreversible properties, fatigue resistance and photoswitching efficiency have further inspired scientists from the fields of chemistry and materials to explore them extensively and develop a large number of DTE derivatives since it was first discovered by Kellogg et al.¹³ Although great advances have recently been made in photochromic DTEs, their further application is still limited by some disadvantages. For example, the ring-closed operation of DTE commonly depends on UV light with high energy, and visible light (400–700 nm) is usually used to excite the cycloreversion reaction of DTE (Fig. 1a), which undeniably shows a shallower penetration in biological tissues than near-infrared (NIR) light.^14–17 Therefore, exploiting visible light-controlled NIR photochromic DTE derivatives is highly desirable for further applications.

Fig. 1 Classification of light at different wavelengths ranging from 400–1700 nm (a). Schematic of protonation of QDTE and deprotonation of QDTE-2H upon addition of TFA and TEA, and photochromic reaction of QDTE and QDTE-2H upon alternating irradiation with blue light at 460 nm (or green light at 520 nm) and NIR I light at 710 nm (or NIR II light at 1050 nm) (b). Absorption spectra of QDTE(c) and QDTE-2H(c) at photostationary state in CHCl₃ (2.0 × 10⁻⁵ mol L⁻¹) (c).

In this context, substantial attempts have been made to design visible light-driven DTE switches with absorption and reactivity in the NIR window through a wide variety of strategies in recent years.^18–22 Among these strategies, extending the π-conjugation system is commonly employed to achieve NIR photochromic DTEs triggered by visible light by reducing the HOMO–LUMO energy gap for both open and closed isomers of DTEs to shift the corresponding absorption maxima to a longer wavelength.^23–25 However, elaborate molecular design and tedious synthesis are in demand accompanied by the depletion of the photochromic performance in most cases (e.g., low cyclization or cycloreversion quantum yields), and its convergence limit of the wavelength shift will also be eventually reached. Alternatively, the strategy of triplet sensitization has recently been employed to fabricate visible-light DTEs by intramolecular or intermolecular energy transfer between the DTE core and an energy-matched photosensitizer.^26–31 For example, Zhang and Tian et al. rationally designed and successfully prepared several visible light-driven DTEs by applying a facile triplet sensitization strategy based on a triplet–triplet energy transfer process.^29,30 However, the environment-sensitivity and distance-dependence features of triplet-sensitization limit their practical applications in biological scenarios. To address these issues, a nanoconfinement strategy has recently been presented by the same group to achieve enhanced visible light-triggered photochromism in water by self-assembling the sensitizer and DTE derivative into nanoconfined micelles,³¹ thus protecting the triplet sensitizer from environmental quenchers (e.g., O₂ and water). Still, most of visible light-DTE derivatives based on triplet sensitization strategy or other strategies (such as intramolecular proton transfer,³² multiphoton absorption,³³ and upconversion of nanoparticles^34,35) fail to achieve NIR-excited cycloreversion reaction. In stark contrast to the above strategies (vide supra), the acceptor (A)–DTE (D)–acceptor (A) strategy by incorporating two electron-withdrawing groups into both ends of the DTE center provides a straightforward approach to develop visible light-triggered NIR photochromic DTEs,^36–38 which is attributed to the reduction of the HOMO–LUMO energy gap of the open isomer. However, fewer cases of A–D–A type DTEs have been reported thus far. To sum up, some visible light-triggered DTEs with NIR photochromic performance have been developed by extending π-conjugation or the A–DTE–A strategy, but all these NIR photochromism are located in the first near-infrared (NIR-I, 700–900 nm) region (Fig. 1a). In contrast with NIR-I (∼1–3 mm), NIR light in the second near-infrared (NIR-II, 1000–1700 nm) window shows deeper penetration (∼5–20 mm) in biological tissues owing to diminished autofluorescence and reduced photon scattering.^39–42 Consequently, it is of great essentiality to develop novel visible light-triggered DTE derivatives with efficient NIR II photochromism for biological applications in the future.

In this contribution, a novel quinoxalinone-functionalized A–D–A type DTE derivative (QDTE) has been rationally designed and successfully synthesized (Fig. 1b), in which electron-withdrawing quinoxalinone groups endow QDTE with the A–DTE–A feature for visible light-driven NIR I photochromic properties. However, quinoxalinone moieties are easily protonated to form the more electron-deficient A′–D–A′ type DTE (QDTE-2H), which may be beneficial for achieving a unique NIR II photochromism triggered by longer wavelength visible light due to a reduction in the HOMO–LUMO energy gap of open and closed isomers after protonation. As expected, the as-prepared QDTE displays a blue light-controlled NIR I photochromic performance with negative solvent dependence. More importantly, the in situ protonated QDTE-2H presents an unprecedented green light-triggered NIR II photochromism in CHCl₃ and toluene in the presence of trifluoroacetic acid (TFA) (Fig. 1c). To the best of our knowledge, it represents the first example of NIR II photochromic DTE. Ultimately, QDTE has been preliminarily applied to double information encryption, further implying its versatility in functional materials.

Results and discussion

As depicted in Scheme S1 (ESI†), the symmetrical quinoxalinone-functionalized DTE derivative (QDTE) was prepared via the classical aldol condensation reaction between 4,4′-(cyclopent-1-ene-1,2-diyl)bis(5-methylthiophene-2-carbaldehyde) (1) and 1-butyl-3-methylquinoxalin-2(1H)-one (2) under a catalytic amount of concentrated sulfuric acid in a yield of 57% in acetic acid. Its chemical structure was confirmed by applying the standard characterization methods, i.e., ¹H NMR, ¹³C NMR and HRMS (Fig. S26–S28, ESI†). According to the ¹H NMR spectrum of QDTE, the coupling constants of double bonds between the quinoxalinone group and DTE core were ca. 15.8 Hz, thereby implying a stable trans configuration for QDTE.

Soon afterwards, the photochromic properties of QDTE were systematically explored in different media. Fig. 2a illustrates a strong absorption band centered at 425 nm (ε = 7.7 × 10⁴ M⁻¹ cm⁻¹) in the visible region in DMSO before irradiation, which is attributed to intramolecular charge transfer (ICT) transition from one thiophene group of the ring-open isomer to the quinoxalinone moiety.^43–45 When irradiated with blue light (λ = 460–470 nm) (8.9 mW cm⁻²), a new NIR I absorption band with 692 nm (ε = 2.3 × 10⁴ M⁻¹ cm⁻¹) as the maximum peak gradually emerged accompanied by the color from yellow to green (Fig. 2a, inset), thereby indicating the formation of the ring-closed isomer QDTE(c) (Fig. 1b). Evidently, an obvious isosbestic point at 466 nm was observed, implying a single photoconversion mechanism for QDTE.⁴⁶ Unexpectedly, it only took 30 seconds to reach the photostationary state (PSS), representing a relatively fast photochromic DTE case triggered by visible light in a large polar solvent (DMSO) to date. Subsequent irradiation with NIR I light (λ = 730–740 nm) (8.9 mW cm⁻²) allowed for the cycloreversion reaction to occur (Fig. 2b), causing a regeneration of the open form. In particular, no significant degradation was detected after 10 cycles of alternating blue and NIR I light irradiation (Fig. 2c and Fig. S1, ESI†), which means that QDTE has a decent fatigue resistance in DMSO. This is mainly attributed to the fact that such mild irradiation conditions inhibit the generation pathways of photolysis byproducts. Furthermore, the cyclization and cycloreversion quantum yields of QDTE were recorded as φ_o–c = 32% and φ_c–o = 0.8% in DMSO, respectively (Table 1). Then, when the less polar solutions of QDTE in CHCl₃ and toluene were exposed to the same light conditions, some similar photochromic performance was obtained, as depicted in Fig. S2–S11 (ESI†) and Table 1. In stark contrast to that in DMSO, the closed QDTE(c) displayed a negligible bathochromic shift (Δλ = 4–8 nm) in CHCl₃ and toluene, while the absorption maximum of the open isomer was largely unchanged (Fig. 2d and Table 1). Surprisingly, QDTE showed the fastest optical response rate to blue light in DMSO compared with those in CHCl₃ and toluene (Fig. 2e). As expected, relatively smaller φ_o–c and φ_c–o were obtained in CHCl₃ (φ_o–c = 11% and φ_c–o = 0.5%) and toluene (φ_o–c = 29% and φ_c–o = 0.9%) (Table 1). In brief, QDTE presented an extremely negative solvent-dependent photochromism, inconsistent with some previous cases of solvent-dependent DTE derivatives.^47–52 As far as we know, most DTE derivatives presented poor or even no photochromism in large polar solvents (such as MeOH, DMSO, and water). Therefore, this study will be of great interest for the design of novel DTE derivatives with outstanding photochromism under physiological conditions. Additionally, its photoisomerization was further investigated via¹H NMR spectral variations in CDCl₃ to determine the photocyclization conversion ratio at PSS. As depicted in Fig. 3, some protons belonging to closed isomer QDTE(c) (e.g., , , , , and ) underwent a palpable upfield shift compared to those on the open form QDTE(o), which is attributed to the electron shielding effect from the conjugated closed isomer. According to ¹H NMR analysis results at PSS, the photocyclization conversion ratio was determined to be 58.6%. Ultimately, the thermal stability of the closed isomer QDTE(c) was evaluated under dark conditions considering that it is a crucial criterion for evaluating the photoswitchable performance of DTE switches for applications in biological systems.^53–55 As depicted in Fig. 2f, the NIR I absorption intensity at 692 nm for QDTE(c) remained largely unchanged even after the DMSO solution of QDTE(c) was maintained at 40 °C for 600 min, thus suggesting that an excellent thermal stability and a larger ground state activation energy difference that is unfavorable to thermal open-ring reaction. Consequently, this blue-/NIR I light-driven DTE derivative with negative solvent-dependent photochromism, decent fatigue resistance and excellent thermal stability is promising to fulfill versatile applications in the future.

Fig. 2 Absorption spectra changes in QDTE in DMSO (2.0 × 10⁻⁵ mol L⁻¹) upon alternating irradiation with blue light at 460–470 nm (8.9 mW cm⁻²) (a) and NIR I light at 730–740 nm (8.9 mW cm⁻²) (b). Insets show the corresponding color changes upon photoirradiation. The fatigue resistance of QDTE in DMSO for ten cycles (c). Absorption spectra changes in QDTE in three different solvents before and after irradiation with blue light (d). Optical response rate of QDTE upon irradiation with blue light in three different solvents by monitoring changes in the maximum absorption wavelength (λ_max) of the closed isomer QDTE(c) (e). Thermal stability of the closed isomer QDTE(c) in DMSO at 40 °C by monitoring changes in the λ_max of QDTE(c) at PSS (f). Frontier molecular orbital profiles of QDTE(o), QDTE(c), QDTE-2H(o), QDTE-2H(c) (g), and their calculated electrostatic potential surfaces (blue region represents the positive work and the red region represents the negative work) (h) based on DFT calculations at the B3LYP/6-31G* level using the Gaussian 09 program.

Table 1 Photochromic parameters and fluorescence data of QDTE and QDTE-2H in various solvents (2.0 × 10⁻⁵ M)

Compounds	Solvents	λ max (nm) (ε × 10^−4, M^−1 cm^−1)	λ max (nm) (ε × 10^−4, M^−1 cm^−1)	φ o–c (%)	φ c–o (%)	λ em (nm)	Φ f (%)
a Absorption maxima of ring-open isomers. b Absorption maxima of ring-closed isomers. c Cyclization quantum yields. d Cycloreversion quantum yields. e Fluorescence emission maxima. f Fluorescence quantum yield determined by a standard method with rhodamine 6G in water (Φf = 0.75, λex = 488 nm) as a reference before irradiation with 570 nm light.
QDTE	DMSO	425 (7.7)	692 (2.3)	32	0.8	530	8.2
	CHCl3	427 (9.0)	700 (3.4)	11	0.5	513	11
	Toluene	425 (8.3)	696 (3.3)	29	0.9	530	14
QDTE-2H	DMSO	NA	NA	NA	NA	NA	NA
	CHCl3	548 (6.3)	1007 (1.2)	8.0	1.2	628	3.4
	Toluene	540 (5.4)	1001 (0.4)	5.0	2.3	616	2.5

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Fig. 3 Partial ¹H NMR spectral changes in QDTE upon irradiation with blue light in CDCl₃ at room temperature.

Given the strong fluorescence performance of quinoxalinone, the subsequent exploration focused on the fluorescence switching behavior in different solvents. As illustrated in Fig. 4a, QDTE exhibited a bright yellow fluorescence with the maximum emission wavelength at 530 nm in DMSO prior to irradiation, whose relative fluorescence quantum yield was determined to be Φ_f = 8.2%. Upon blue light irradiation for ca. 30 s, the yellow fluorescence gradually faded along with the emission intensity at 530 nm attenuating as a result of efficient energy transfer (ET) from the excited quinoxalinone to the closed DTE core (Fig. 4e).⁵⁶ Only ca. 81% of the emission intensity for QDTE(o) was quenched because of the partial overlap of the absorption spectrum of QDTE(c) with the emission spectrum of QDTE(o), as illustrated in Fig. 4b. Conversely, the original fluorescence intensity was turned on again upon NIR I light irradiation owing to the reformation of QDTE(o). Remarkably, such a fluorescent switching process can be reversibly operated for ten cycles with some perceptible attenuation, implying general fluorescence fatigue resistance for QDTE in DMSO (Fig. 4c). As shown in Fig. S12–S15 (ESI†), QDTE displayed an analogous fluorescent switching in CHCl₃ and toluene under the same irradiation conditions. Compared to those in DMSO and toluene (λ_em = 530 nm), a significant hypochromatic shift for its emission peak was detected in CHCl₃ (λ_em = 513 nm) (Fig. 4d and Table 1), which is exactly the opposite of the change in the absorption spectra. In brief, QDTE also presented a solvent-dependent fluorescent switching behavior triggered by blue-/NIR I light.

Fig. 4 Fluorescence spectra changes of QDTE in DMSO (2.0 × 10⁻⁵ mol L⁻¹) upon alternating irradiation with blue light at 460–470 nm (8.9 mW cm⁻²) and NIR I light at 730–740 nm (8.9 mW cm⁻²) (a). (inset) Corresponding fluorescent colour changes upon photoirradiation. Overlap between the absorption spectrum of QDTE(c) and the emission spectrum of QDTE(o) (b). Fluorescence fatigue resistance of QDTE in DMSO (2.0 × 10⁻⁵ mol L⁻¹) for ten cycles (c). Fluorescence spectra of QDTE in three different solvents (2.0 × 10⁻⁵ mol L⁻¹) before irradiation (d). Photochromic reaction of QDTE along with fluorescence changes, which is attributed to the ET process (e). ET: energy transfer.

Given the structural features of QDTE in which the nitrogen atoms in the quinoxalinone groups may respond to the various acids, it would be of great interest to explore its acid response properties, and even the photochromic performance regulated by the acid.⁵⁷ As illustrated in Fig. 5a, when 0–5.0 eq. trifluoroacetic acid (TFA) was added, the decreasing intensity of absorption at 427 nm for QDTE(o) was accompanied by the appearance of a low-energy absorption peak at 548 (ε = 6.3 × 10⁴ M⁻¹ cm⁻¹) in CHCl₃, during which the solution color changed from yellow to purple. In addition, an isosbestic point was observed at 458 nm, thereby implying the emergence of a new protonated product (i.e., QDTE-2H) from QDTE induced by TFA (Fig. 1b). Fig. 5b and Fig. S16 (ESI†) show a diminishing fluorescence intensity at 513 nm with increasing TFA concentration when excited with 440 nm light, while a significantly red-shifted emission peak at 628 nm is observed when more than 2.0 eq. TFA was added (λ_ex = 500 nm). In short, these experimental results indicated that the protonation of QDTE can apparently extend its π-conjugated system, which facilitates the regulation of the photochromic performance.

Fig. 5 Absorption (a) and fluorescence (b) spectra changes of the open isomer QDTE(o) in the presence of TFA (0–5.0 eq.) in CHCl₃ (2.0 × 10⁻⁵ mol L⁻¹) (λ_ex = 440 nm, 500 nm, respectively). Insets show the corresponding color and fluorescence changes before and after the addition of 5.0 eq. TFA. The absorption spectra changes in in situ protonated QDTE-2H in CHCl₃ (2.0 × 10⁻⁵ mol L⁻¹) upon irradiation with green light at 520–530 nm (8.9 mW cm⁻²) (c) and NIR II light at 1000–1050 nm (8.9 mW cm⁻²) (d). Insets show the corresponding color changes upon photoirradiation. Fatigue resistance of QDTE-2H in CHCl₃ for ten cycles (e). Absorption spectra changes in the closed isomer QDTE(c) in the presence of TFA (0–5.0 eq.) in CHCl₃ (2.0 × 10⁻⁵ mol L⁻¹) (f). Partial ¹H NMR spectral changes of the open isomer QDTE(o) in the presence of TFA-D1 (0–5.0 eq.) in CDCl₃ at room temperature (g).

Subsequently, the photochromism of in situ protonated QDTE was investigated thoroughly in the presence of 5.0 eq. TFA. Green light at 520–530 nm (8.9 mW cm⁻²) was selected to trigger the close ring reaction of the ring-open isomer QDTE-2H(o). Amazingly, an ultra-low-energy NIR II absorption band located at 1007 nm (ε = 1.2 × 10⁴ M⁻¹ cm⁻¹) emerged with inconspicuous color changes in CHCl₃ under the persistent irradiation of green light (Fig. 5c), resulting from the generation of the ring-closed isomer QDTE-2H(c) (Fig. 1b). Irradiation of 100–1050 nm NIR II light (8.9 mW cm⁻²) triggered the cycloreversion reaction to regenerate the initial open form (Fig. 5d). Compared with the unprotonated QDTE, QDTE-2H presented an inferior fatigue resistance in CHCl₃ after 10 cycles of alternating irradiation with green- and NIR II-light (Fig. 5e). Besides, the cyclization and cycloreversion quantum yields of QDTE-2H were measured as φ_o–c = 8.0% and φ_c–o = 1.2% in CHCl₃, respectively (Table 1). As depicted in Fig. 5f, when the green solution of the closed QDTE(c) in CHCl₃ was gradually added with 0–5.0 eq. TFA, two absorption peaks near 548 nm and 1007 nm appeared along with the gentle disappearance of the absorption peak at 700 nm for QDTE(c), which is largely consistent with the absorption peak of the open QDTE(o) treated with TFA, followed by green light in CHCl₃. As shown in Fig. S17–S21 (ESI†) and Table 1, QDTE-2H exhibited an analogous TFA-regulated NIR II photochromic behavior in toluene under the same treatment. Compared to the NIR II photochromism in CHCl₃, both open and closed isomers of QDTE-2H showed a blue-shifted absorption peak in toluene (i.e., 540 nm for QDTE-2H(o) and 1001 nm for QDTE-2H(c)), which agrees with the solvent-dependent feature of the NIR I photochromism for QDTE. However, negligible changes in absorption spectra were detected in DMSO probably because protonated QDTE-2H was not easily generated or was unstable in DMSO (Fig. S22, ESI†). Therefore, this in situ protonated QDTE-2H presented an unprecedented green light-triggered NIR II photochromic performance in CHCl₃ and toluene under the treatment with excess TFA.

For preliminary verification of the NIR II photochromic mechanism, subsequent ¹H NMR titration and density functional theory (DFT) calculations for QDTE-2H were performed. First, the protonated process of QDTE was explored via¹H NMR spectral variations in CDCl₃ upon the gradual addition of TFA (0–5.0 eq.). As depicted in Fig. 5g and Fig. S29 (ESI†), a broad singlet at 11.80 ppm emerged when 1.0 eq. TFA was added to the solution of QDTE, which was assigned as the proton signal of the ammonium NH⁺ on the quinoxalinone group. With the increase in TFA (1.0–5.0 eq.), the NH⁺ proton signal underwent a significant upfield shift (δ = 11.10 ppm upon the addition of 5.0 eq.). Compared to those on the TFA-untreated QDTE, all aryl protons except for H_f showed palpable low-field shifts when treated with excess TFA (e.g., vs. H_d: 7.04 ppm; vs. H_e: 8.18 ppm; and vs. H_i: 7.50 ppm). In addition, the saturated hydrogen protons on QDTE had different degrees of low-field shifts under the same conditions (Fig. S23 and S29, ESI†). This is mainly attributed to the electron de-shielding effect from the protonated quinoxalinone skeletons, thus confirming the formation of the protonated product (QDTE-2H). Then, DFT calculations were performed to explore the electronic features of QDTE and QDTE-2H at the B3LYP/6-31G* level using the Gaussian 09 program.⁵⁸ As depicted in Fig. 2g, the HOMO orbital energy of QDTE(o) was distributed in almost the entire molecule centered on the DTE core, whereas its LUMO was mainly localized around the quinoxalinone moieties at both ends. Such partially separated HOMO and LUMO orbitals indicate an unobvious A–DTE–A structural feature. For QDTE(c), the HOMO was distributed on the central DTE core, and the LUMO was nearly on the whole molecular skeleton. As expected, a narrower energy band gap (E_g = 1.74 eV) for QDTE(c) was detected compared to QDTE(o) (E_g = 2.95 eV) owing to the extended π conjugation system, which endowed it with blue light-excited NIR I photochromic performance. In stark contrast to QDTE(o), the HOMO electron cloud of QDTE-2H(o) moved closer to the DTE center due to the electron-withdrawing effect of the protonated quinoxalinone moieties, allowing for an efficient separation of the HOMO and LUMO orbitals for the formation of the more electron-deficient A′–D–A′ type DTE derivative. More importantly, both QDTE-2H(o) (E_g = 2.32 eV) and QDTE-2H(c) (E_g = 1.12 eV) presented a much narrower energy gap compared to the respective unprotonated QDTE(o) (E_g = 2.95 eV) and QDTE(c) (E_g = 1.74 eV), which resulted in the absorption redshift and then the achievement of NIR II photochromism triggered by green light. As shown in Fig. S24 and S25 (ESI†), the optimized structures of QDTE-2H(o) and QDTE-2H(c) were barely unaffected by protonation. As illustrated in Fig. 2h, the enhanced dipole moment for QDTE(c) was calculated to be 12.89 D compared to that of QDTE(o) (μ = 4.31 D), which suggests a palpable polarity change during photoisomerization. Evidently, the protonated QDTE-2H(o) (μ = 4.69 D) and QDTE-2H(c) (μ = 15.97 D) exhibited much larger dipole moment changes compared to both isomers of QDTE, in which the positive π-charge was mainly distributed in the NH⁺ groups (blue region). Accordingly, the ¹H NMR titration and DFT calculations confirmed that the green light-controlled NIR II photochromism benefited from the formation of the protonated DTE derivative, providing a promising guide for designing NIR II photochromic DTE derivatives triggered by long wavelength visible light.

Given the excellent photochromic properties of QDTE and QDTE-2H under irradiation with blue-/NIR I light and green-/NIR II light in organic solutions, respectively, we preliminarily evaluated their potential applications for information encryption in the polymer medium. First, QDTE was introduced into the polymer medium by immersing the filter paper (d = 7.0 cm) in a mixed solution containing QDTE and poly (methyl methacrylate) (PMMA) (m/m = 1 : 20) (Fig. 6a) and then drying naturally in the dark. The as-prepared PMMA film exhibited a yellow colour attributed to the doped QDTE in the film. When exposed to the blue light (460–470 nm) through a quick response (QR) code mask, a green QR code image could be successfully inscribed on filter paper, which could be handily decoded by a smartphone (Fig. 6b). Moreover, the yellow filter paper instantly turned red when exposed to TFA vapor, indicating the formation of a QDTE-2H-loaded PMMA film. Subsequent irradiation of green light at 520–530 nm through the same mask patterned a yellow QR code on this red filter paper. As expected, this yellow QR code could also be decoded into the same display information with a smartphone (Fig. 6b). These two different QR codes could be erased upon irradiation with NIR I light (730–740 nm) and NIR II light (1000–1050 nm), respectively, thus enabling erasable and recyclable information storage. Therefore, this DTE derivative with double scan and display features presented great potential for double information encryption and accurate anti-counterfeiting.

Fig. 6 Schematic of preparation and instant photo-patterning of QDTE and QDTE-2H-loaded PMMA film for double information encryption (a). Dual-colour readout QR code was inscribed onto and erased from QDTE and QDTE-2H-loaded PMMA film on the filter paper under irradiation with blue-/NIR I light and green-/NIR II light, respectively, which could be dual-decoded using a smartphone (b).

Conclusions

In summary, we successfully developed a novel A–D–A type DTE derivative (QDTE) by incorporating the electron-withdrawing quinoxalinone groups in both ends. Photoswitching experiments showed the as-prepared QDTE presented a blue-/NIR I light-driven photochromism with negative solvent dependence, decent fatigue resistance and excellent thermal stability, as well as excellent fluorescence switching performance in various solvents. Because of its outstanding photochromic performance in DMSO, it is of great significance to design novel DTE derivatives with excellent photochromism under physiological conditions. More importantly, under the treatment with excess TFA, the in situ protonated QDTE-2H presented an unprecedented NIR II photochromism that was triggered by green light in CHCl₃ and toluene. The NIR II photochromic mechanism was preliminarily verified via the ¹H NMR titration and DFT calculations. As far as we know, this was the first example of NIR II photochromic DTE. Ultimately, the proof-of-concept application of such a DTE derivative with both NIR I and NIR II photochromic performance in dual information encryption was successfully implemented, demonstrating its versatility in functional materials. We expect that this study will pave the way for the further development of NIR II photochromic DTE derivatives triggered by red or even NIR I light for various true biological applications in the future.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of Henan Province (No. 222300420501), the Science and Technology Project of Henan Province (No. 242102230119), and the Key Scientific Research Project of Higher Education of Henan Province (No. 22A430007), and Innovation and Entrepreneurship Training Program for College students in China (No. 202410482001, 202410482052).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm00719k

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