2020, Vol. 31 
b School of Chemistry and Chemical Engineering, Henan University of Technology, Zhengzhou 450001, China
Photochromic materials have attracted increasing interest due to their multiple applications in molecular switches, molecular logic gates, optical data storages, optoelectronic devices, photo-controllable biological processes and molecular machines [1-16]. In 2015, Hecht's group reported a potential photochromic system of acylhydrazones, which exhibited many outstanding advantages including low cost, simple synthesis and high modifiability compared with some classic photochromic systems such as azobenzene, spiropyran, dithienylethene [17]. Take benzoylhydra-zine-2-aldehyde-pyridine Schiff base (1) as an example, the photochromic mechanism is shown in Scheme 1A. An isomerism from (E)-isomers (1-E) to (Z)-isomers (1-Z) in acylhydrazones occurs upon UV light irradiation. When UV light is removed, the conformation of acylhydrazones recovered by heating. This process endows acylhydrazones with reversible color change, highly fatigue resistance and stable illumination products. Nevertheless, the absorption wavelength changes of acylhydrazones usually locate in ultraviolet region, which makes the band separation between the absorbance maxima of (E)-isomers and (Z)-isomers cannot be observed by naked eyes and greatly limits their practical applications [18-22]. Thus, it is necessary to find a way to endow the acylhydrazones photochromic systems with visible color changes. Introducing chromophores to the system is one of the feasible methods.
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| Scheme 1. The reported photochromic mechanism of (A) acylhydrazone system and (B) Rhodamine derivative metal complex system. (C) Proposed photo-response mechanism for 3 in methanol andTHF. | |
Rhodamine dyes (aka Fluoran) are one kind of classic organic chromophores, which have many advantages such as low cost, good oil-solubility, high absorption coefficient and high chromaticity [23-26]. More importantly, due to their intrinsic halochromic properties, Rhodamine dyes can be used as chromogenic reagents to build piezochromic, thermochromic and electrochromic materials [27-30]. In our previous work, a series of Rhodamine derivative metal complexes based photochromic systems were developed, which exhibited reversible photo-induced color/ emission changes both in solution and in a solid matrix [31-37]. Take Rhodamine 6G hydrazine salicylaldehyde Schiff base zinc complex (2) as an example, UV light irradiation induces a spirolactam ring-opening reaction in it (from 2-Close to 2-Open), along with color change and emission turn-on response (Scheme 1B). However, excess metal ions are indispensable to stabilize the complexes because the binding constants of these complexes are limited. The existence of excess metal ions somewhat limited their further applications in photochromic materials. Thus, developing a pure organic Rhodamine derivative based photochromic system is still a challenge.
Inspired by above-mentioned works, a novel acylhydrazone photochromic system of Rhodamine 6G hydrazine-2-aldehyde-pyridine Schiff base (3) is designed and synthesized by combining acylhydrazone with Rhodamine 6G structure. As shown in Scheme S1 (Supporting information), compound 3 can be facilely synthesized through a two-step reaction with a yield of 69% using commercially available reagents. As expected, compound 3 not only remains all the advantages of acylhydrazone photochromic system but also exhibits visible photo-induced color/emission changes both in solution and in a solid matrix. Upon UV light irradiation, the colorless methanol solution of 3 turned orange gradually, along with a significant increase of yellow fluorescence. The absorption spectra and fluorescence spectra of 3 before and after UV light irradiation were investigated. As shown in Fig. 1A, there was no absorption band over 450 nm for 3 before UV light irradiation. On the contrary, a strong absorbance band around 531 nm could be observed after UV light irradiation.The molar extinction coefficient of 3 at 531 nm was turned from 6.42 L mol-1 cm-1 to 623.02 L mol-1 cm-1 before and after UV light irradiation.These absorptionspectra were highly analogous to those of Rhodamine 6G in alkaline and acidic environments, respectively. According to the reports, the addition of acid could promote the ring-opening reaction of the spirolactone moiety in Rhodamine 6G, along with the transformation from the leuco form (Rh-Close) to the colored form (Rh-Open) (Scheme S2 in Supporting information) [38, 39]. Thus, the absorption spectra change of 3 suggested that its spirolactam group turned from close to openafter UV light irradiation. Similar change could be observed in the fluorescence spectra of 3. Before UV light irradiation, no fluorescence could be observed while intense orange fluorescence (quantum yield 8.42%) emerged once the UV light irradiation was applied. The fluorescence spectra of 3 after UV light irradiation were also analogues to those of Rhodamine 6G in acidic environment (Fig. 1B wine dash line). Combining above results with the reported photochromic mechanism of acylhydrazones, a mechanism for the photo-induced color/emission changes of 3 in methanol was proposed (Scheme 1C): Before UV light irradiation, 3 existed in an (E)-isomer and the Rhodamine 6G moiety of 3 was in a spirolactam ring-closed form (3-E-Close). After UV light irradiation, an isomerization reaction occurred in 3, the C=N double bond turned into Z form along with the ring-opening of spirolactam group (3-Z-Open).
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| Fig. 1. (A) Absorption and (B) fluorescence spectra of 3 in methanol before and after UV light irradiation (solid lines). Thedashline was the spectra of Rhodamine 6G in alkaline (gray lines) and acidic (wine lines) environments, respectively. Inset photo:3 in methanol before and after UV light irradiation. | |
To confirm this machanism, a series of experiments were carried out. Molecular configuration of 3 before UV light irradiation was characterized by X-ray crystallography. As shown in Fig. S1 (Supporting information), the C=N double bond in 3 before UV light irradiation was in an (E)-form, which was consistent with literature [17]. Then NMR spectra were used to characterize the configuration change of 3 upon UV light irradiation. Firstly, 1H-1H COSY spectra were recorded to assign the NMR signals of the hydrogen atoms in 3 (Figs. S2 and S3 in Supporting information). As shown in Fig. 2, after UV light irradiation, the signals of the hydrogen atoms in C=N double bond (proton a) and pyridyl (proton b, c, d, e) moved to low field significantly. On the contrary, the signals of the hydrogen atoms in Rhodamine 6G moiety (proton f, g, h, i) barely moved. These results demonstrated that the chemical surroundings of pyridyl were changed after UV light irradiation, which fitted well with the proposed mechanism. According to the reports, when carbon tetrachloride was irradiated by UV light, complicated free radicals such as Cl· and CCl3· could be produced. These free radicals were highly active, which could reactive with 3, resulting in complicated by-products [40, 41]. Thus, there were some small peaks showed in the 1H NMR of 3 after UV light irradiation, which might belong to the by-products. Meanwhile, it was noticed that proton n in 3 disappeared after UV light irradiation, which originated from the proton exchange between this active hydrogen and deuterium in the solvent. The stability of 3-Z-Open was further investigated. As shown in Fig. S4 (Supporting information), the absorbance and fluorescence spectra of 3-Z-Open in methanol were almost the same before and after being kept in dark for 1 h, indicating its good stability.
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| Fig. 2. 1H NMR spectra of 3 before and after UV light irradiation in DMSO-d6 and carbon tetrachloride (v:v = 1:1). | |
The photo-induced color/emission changes of 3 in different solvents were further investigated. As shown in Figs. 3A and B, and Fig. S5 (Supporting information), the absorption band of 3 around 310 nm decreased after UV light irradiation in all the solvents. In methanol and carbon tetrachloride, new absorption band could be observed for 3 after UV light irradiation while no absorption band in the visible region emerged after UV light irradiation in other solvents. These results suggested that the photo-response mechanism of 3 in methanol orcarbon tetrachloride was different from that in other solvents. According to the reports, the decrease of absorption band around 310 nm could be attributed to photo-induced Z/E isomerization in acylhydrazones moiety [17]. Thus, the solvent might be a key factor for the photo-response of 3 (Scheme 1C): The photo-induced Z/E isomerization of 3 occurred in all the solvents, but only methanol and carbon tetrachloride could induce the ring-opening reaction of spirolactam group to get 3-Z-Open. In other solvents, 3-Z-Close was produced. To confirm our hypothesis, 3 was dissolved in THF and irradiated by UV light for 30 s. As shown in Fig. 3C no color change could be observed, demonstrating that 3 was in spirolactam ring-close form after UV light irradiation (3-Z-Close). After removing the UV light, methanol was slowly added into the system and the methanol layer turned red immediately, i.e., 3-Z-Close turned to 3-Z-Open in methanol layer without UV light irradiaiton. These results fitted well with our hypothesis.
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| Fig. 3. Absorbance spectra of 3 inmethanol (A) and THF (B) before and after UV light irradiation for 1 min. (C) From left to right: 3 in THF without UV light irradiation; 3 in THF after UV light irradiation for 1 min; Methanol was added to the THF solution containing UV light irradiated 3. | |
One possible reason for the photochromism of 3 in methanol and carbon tetrachloride is the proper acidity of solvents. The acidity of methanol is stronger than other solvents showed in Fig. S5 (Supporting information), which is beneficial to get a spirolactam ring-openproduct. For carbon tetrachloride, HCl could be generated afterUV light irradiationinwet air, resulting in an increase of acidity (CCl4 + H2O = COCl2 + 2HCl, CCl4 + 2H2O = CO2+ 4HCl) [42, 43]. To confirm the hypothesis, the photochromic property of 3 in trifluoroethanol (an analogue of ethanol, which is more acidic) was investigated. As shown in Fig. S6 (Supporting information), a strong absorbance band around 531 nm of 3 could be observed in trifluoroethanol after UV light irradiation for 1 min, which was similar as that in methanol. This result suggested that a proper acidity of solvents is beneficial to the photochromic property of 3.
As mentioned above, the stability of 3-Z-Open in methanol was very good. To adjust the stability of 3-Z-Open, mixed solvent of THF/CCl4 (v:v = 500:1) was utilized [30, 34]. Because 3-E-Close had no absorptionband above 500 nm, the absorbance at531 nm should be proportional to the concentration of 3-Z-Open. Therefore, time-dependent absorption spectrum at 531 nmwas recorded tomonitor the recovery kinetics from 3-Z-Open to 3-E-Close in THF/CCl4 (v:v = 500:1). As shown in Fig. S7 (Supporting information), after removing UV light, the absorbance at531 nm of UV light irradiated 3 turned from 1.8 to 0 within 1 min, demonstrating that the recovery rate of 3-Z-Open could be adjusted effectively with different solvents. Moreover, the decay curve could fit well with first-order reaction kinetics (lnA = -kt, A was absorbance, k was rate constant and t was time). These results indicated that the reaction from 3-Z-Open to 3-E-Close might be a monomolecular reaction, which further supported the proposed mechanism [44].
The fatigue resistance of 3 was a key indicator to evaluate its performance as a photochromic system. As shown in Fig. S8 (Supporting information), 3 was toggled repeatedly between 3-E-Close and 3-Z-Open for 10 times. The absorbance at 531 nm stayed almost constant without any apparent degradation, indicating a good fatigue resistance of 3.
Beside in solutions, the photo-induced color/emission changes of 3 in a solid matrix were also investigated. The film of 3 was prepared by following steps: 3 and β-cyclodextrin (mass ratio was 1:100) were dissolved in THF/methanol (v:v = 1:1) mixed solution. Then the solvent was evaporated and white powders were obtained. After grinding and tabletting, a film of 3 was obtained. As shown in Fig. 4, 19-fold absorbance enhancement at 526 nm and 5-fold emission enhancement at 560 nm were observed for the film after UV light irradiation. These spectra changes were analogous to those of 3 in methanol. This feature suggested that 3 could serve as a promising material for photo-patterning. As shown in inset images in Figs. 4A and B, a pattern of Chinese knots was successfully visualized in the film of 3 upon UV light irradiation. The resolution was as high as 0.1 mm.The preparation procedure of patterns wasrather simple (Fig. S9 in Supporting information): Pre-organized pattern was printed in a transparency, then the UV light was pass through the transparency onto the film of 3 and the patterns were subsequently recorded.
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| Fig. 4. (A) UV diffuse reflectance spectra and (B) fluorescence spectra of 3 before and after UV light irradiation in β-cyclodextrin film. Inset photo: Patterns on the film generated with UV light irradiation. | |
In conclusion, a novel small molecule photochromic system of 3 was designed and synthesized by combining acylhydrazone with Rhodamine 6G structure. Compared with classic acylhydra-zone photochromic system, 3 exhibited visible photo-induced color/emission changes both in solution and in a solid matrix. Mechanism studies showed that UV light irradiation triggered an E/Z isomerization reaction and a spirolactam ring-opening process in 3, resulting in significant color change and emission enhancement. The photochromic process was reversible and the stability of illumination product of 3 was controllable by different solvents. The fatigue resistance of 3 was also very good. Moreover, 3 was successfully used as a material for photo-patterning. This work provided a new strategy for designing acylhydrazone photochromic systems with visible color/emission change. Currently, efforts toward developing more acylhydrazone photochromic molecules with multifunction are under investigation in our laboratories.
Declaration of competing interestThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
AcknowledgmentsThis research was supported by the National Natural Science Foundation of China (Nos. U1904172, 21501150, 51502079 and 21671174) and Key Scientific and Technological Project of Henan Province (No. 202102310006).
Appendix A. Supplementary dataSupplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.05.007.
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