Reversible Surface Dual-Pattern with Simultaneously Dynamic

Letter Cite This: ACS Macro Lett. 2018, 7, 540−545

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Reversible Surface Dual-Pattern with Simultaneously Dynamic Wrinkled Topography and Fluorescence Mingxuan Xie,† Fugui Xu,† Luzhi Zhang,† Jie Yin,†,‡ and Xuesong Jiang*,† †

State Key Laboratory for Metal Matrix Composite Materials, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ‡ School of Physical Science and Technology, Shanghai Tech, Shanghai 201210, People’s Republic of China S Supporting Information *

ABSTRACT: The reversible surface patterns with fluorescence and topography can possibly enable information recording and reading and provide an important alternative to realize the higher information security. We demonstrated a reversible dual-pattern with simultaneously responsive fluorescence and topography using an anthracene (AN) and naphthalene diimide (NDI) containing copolymer (PAN-NDI-BA) as the skin layer, in which the reversible photodimerization of AN can simultaneously control the cross-linking and CT interaction between AN and NDI. Upon irradiation with UV light and thermal treatment, the resulting pattern assumes a reversible change between smooth and wrinkled states, and its fluorescence changes reversibly from red to white to blue-green. The smart surfaces with dynamic hierarchical wrinkles and fluorescence were achieved by selective irradiation with photomasks and can be employed for potential applications in smart displays and anticounterfeiting.

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could be softened, and then the compressive stress could be released by the trans/cis photoisomerization of azobenzene moieties upon illumination.24 Crosby and Sun’s groups25,26 reported that wrinkle patterns could be reversibly tailored through externally imposed solvent and moisture stimuli, respectively. Recently, our group also developed a facile strategy for the fabrication of multiresponsive dynamic wrinkling patterns by regulating the modulus and cross-linking density of the skin layers via dynamic chemistry.27−29 In these reversible patterns, only one parameter for the recording and reading of information, fluorescence or topography, can be controlled dynamically by external stimuli, probably reducing the information capacity and security. A reversible pattern with both fluorescence and topography responses to a single stimulus will undoubtedly increase the information capacity and security but has rarely been reported so far. Here, we demonstrate an approach to fabricate a reversible dual-dynamic pattern based on a bilayer system whose fluorescence and wrinkled topography are controlled simultaneously by light (Scheme 1). Anthracene (AN) and naphthalene diimide (NDI) moieties containing a copolymer (PAN-NDI-BA) were used as the top layer, in which the πelectron-rich AN has a charge transfer (CT) interaction with the π-electron-poor NDI.30−34 Additionally, various elastomers, such as PDMS, can serve as a substrate. The CT interaction leads to the red fluorescence of PAN-NDI-BA, while both AN and NDI units emit a blue-green fluorescence. Upon irradiation

ynamic surface patterns with morphologies responsive to environmental stimuli can enable the on-demand control of surface properties and have found a wide range of potential applications in the tunable optical devices,1−4 switchable wettability surfaces,5,6 smart adhesion and friction properties,7−9 and especially information security endeavors.10−12 As two characteristic parameters of the morphology of surface patterns, surface color and topography are always used for information recording and reading. By taking advantage of photochromic materials or tunable supramolecular interactions, various dynamic patterns with reversible fluorescence have been fabricated and bear significant applications in information security and storage, sensors, and anticounterfeiting.13−17 Compared with these reversible fluorescence patterns, however, it is substantially more challenging to fabricate responsive patterns with dynamically controlled topographies. As one important alternative process for generating patterned surfaces with a dynamic topography, the surface wrinkles of a stiff skin bound to an elastomeric substrate have been harnessed to create reversible patterns that are responsive to external stimuli and provide unique dynamic characteristics of surface topographies and their properties.18−20 Wrinkles occur to minimize the total energy of a bilayer system when the compressive strain, caused by the modulus mismatch between the skin layer and the substrate, exceeds a critical threshold.21−23 By precisely regulating the modulus of the top layer or the applied strain, a series of dynamic wrinkles with a responsive topography have been developed based on bilayer systems. For example, light-erasable wrinkles on a poly(dimethylsiloxane) (PDMS) substrate were demonstrated using an azobenzene-containing polymer as the top layer, which © XXXX American Chemical Society

Received: March 22, 2018 Accepted: April 11, 2018

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DOI: 10.1021/acsmacrolett.8b00211 ACS Macro Lett. 2018, 7, 540−545

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ACS Macro Letters

with red fluorescence from the CT complex converted into a cross-linked and rigid layer with blue-green fluorescence from NDI because of the photodimerization of AN. Because of the mismatch in moduli between the rigid skin layer and soft PDMS, winkles occurred when the system cooled down to room temperature. After heating at 150 °C for 2 h or by irradiating with 254 nm UV light for 10 min with a thermal treatment at 100 °C, the wrinkles disappeared, and the red fluorescence was recovered. To gain detailed insight into the dependence of the wrinkled topography and fluorescence upon the photodimerization of AN, we traced the kinetics of the photodimerization of AN by UV−vis spectra (Figure S5), monitored the morphological evolution of the surface pattern by atomic force microscopy (AFM) and recorded the fluorescence variations by photography (Figures 1 and S6). Upon irradiation under 365 nm UV

Scheme 1. Strategy for the Reversible Dual-Pattern with Responsive Wrinkled Topography and Fluorescencea

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(a) Chemical structure of PAN-NDI-BA and its reversible photodimerization. (b) Schematic illustration for the fabrication process of the reversible dual-pattern based on the bilayer system with PANNDI-BA and PDMS as the top layer and substrate, respectively.

by 365 nm UV light, the fluorescence of the top layer gradually turns from red to blue-green because the anthracene photodimerization weakens the CT interaction between AN and NDI, and the anthracene dimer has no effect on the surface color because of the quenching of the fluorescence intensity. At the same time, photodimerization leads to a cross-linked top layer with a higher modulus, resulting in the generation of a wrinkled pattern that is caused by the mismatch in the moduli and thermal expansion ratios of the bilayers to minimize the total energy of the bilayers. As the anthracene photodimerization is reversible upon exposure to 254 nm UV light or 150 °C, the wrinkles can be erased to a smooth state because of the strain release, and the fluorescence returns red because of the regeneration of the CT interaction. This simple approach to fabricate a reversible dynamic dual-pattern with wrinkled topography and fluorescence in response to light stimuli has the advantages of controllable operability and region-selectivity and, more importantly, improvements in information security, and may find potential applications in information recording, smart displays, and anticounterfeiting. The entire strategy for production of the responsive dynamic dual-pattern with wrinkles and fluorescence is illustrated in Scheme 1. The main point in this strategy is that charge transfer interaction between AN and NDI and modulus of the copolymer in the skin layer can be controlled simultaneously by reversible photodimerization, which leads to the tunable fluorescence and wrinkled topography. Anthracene and naphthalene diimide containing a copolymer (PAN-NDI-BA, Mw = 9000, Mw/Mn = 1.47, the molar ratio of AN, NDI, and nbutyl acylate is approximately 4:1:10) was synthesized through free radical copolymerization, and the incorporation of n-butyl acrylate was to tune the mechanical properties. A detailed description of the synthesis and characterization of materials is in the Supporting Information (Schemes S1−S3, Figures S1− S3). The copolymer PAN-NDI-BA exhibited a unique red photoluminescence completely differed from those of the AN and NDI monomers under both natural and UV light (Figure S4), suggesting the formation of a CT complex between NDI and AN moieties. The toluene solution of PAN-NDI-BA was spin-coated on a PDMS substrate as the skin layer with a typical thickness of 100 nm. Upon irradiation with 365 nm UV light at 70 °C for several minutes, the soft top layer of PAN-NDI-BA

Figure 1. Evolution process of the generation/erasure of the dualpattern with spontaneously responsive wrinkled topography and fluorescence photography. (a−f) 3D AFM images of wrinkles when the sample was exposed to 365 nm UV light for 0, 2.5, 5, 7.5, 10, and 15 min. The samples were heated at 70 °C in an atmosphere of nitrogen gas when irradiated by UV light and cooled down to room temperature after irradiation (the 365 nm UV light intensity was approximately 15 mW·cm−2). (g, h) 3D AFM images of wrinkles erased by 254 nm UV light. The sample was exposed to 254 nm UV in an atmosphere of nitrogen gas and underwent a thermal treatment at 100 °C. The insets correspond to fluorescent photographs of PANNDI-BA film under UV light. (i) Fluorescence emission spectra of a PAN-NDI-BA film for different irradiation times of 365 nm UV light. The “recovery” line represents the cross-linked sample PAN-NDI-BA heated at 150 °C for 0.5 and 1 h. (j) The fluorescence cone of PANNDI-BA film at different irradiation (365 nm) time illustrated in the CIE color space.

light with an intensity of 15 mW·cm−2, the absorption peaks of PAN-NDI-BA decreased with the increasing irradiation, and approximately 80% of AN was photodimerized in 40 min (Figure S5). As shown in Figure 1, a sequence of 3D AFM images and corresponding fluorescence photographs under UV light provide visualization of the irradiation time-dependent growth of the dual-patterns of random wrinkles and 541

DOI: 10.1021/acsmacrolett.8b00211 ACS Macro Lett. 2018, 7, 540−545

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ACS Macro Letters fluorescence induced by the dimerization reaction. A rapid increase in the characteristic wavelength (λ) and amplitude (A) of the wrinkles could be observed from the flat state to the random wrinkles upon 365 nm UV irradiation because of the Young’s modulus of the top layer increasing from an initial 200 MPa to 4 GPa after 20 min. For example, after irradiation for 5 min, the wavelength and amplitude increased to 10 μm and 250 nm (Figure 1a−c), respectively, and further irradiation led to the continuous growth of wrinkles (λ ≈ 13 μm, A ≈ 650 nm; Figures 1d,e and S6a). When exposed to 254 nm UV light or 150 °C, the reversible photodimerization of the skin layer enabled the feasible erasure of the wrinkle morphology because of the decreasing modulus of top layer and the release of the compressive stress caused by the photocleavage of the AN dimer (Figure 1g,h). After irradiation under 254 nm UV light for 1 min, the amplitude of the wrinkles rapidly decreased to 300 nm and the wrinkled surface converted into a completely smooth state after irradiation for 10 min. The wrinkled pattern was subjected to multiple cycles of alternating UV-irradiation at different wavelengths and exhibited the reversible morphology (Figure S6b). Furthermore, the gradual process of the photodimerization reaction upon continuous irradiation of 365 nm UV light resulted in a noticeable but gradual fluorescence change from red to white to blue-green. The photodimerization of AN destroyed its conjugated and planar structure, weakening the red emission of the CT complex between AN and NDI,35−37 and releasing the noncomplexed NDI, which emits blue-green fluorescence. Therefore, the sample displayed a white color that combined the red fluorescence of the CT complex and NDI after a certain time of irradiation. To verify the variations in the fluorescence, emission spectra of the PAN-NDI-BA film were recorded by fluorescence spectroscopy, and the color space coordinates as defined by the Commission Internationale de L’Eclairage (CIE) were calculated from the fluorescence spectrum. As shown in Figure 1i,j, the emission at 650 nm assigned to the CT complex was clearly weakened with increasing 365 nm UV irradiation and nearly disappeared after approximately 20 min, while the emission near 495 nm related to NDI moiety increased significantly. Correspondingly, the color coordinates changed linearly from the red region (x = 0.5169, y = 0.4168) to the white (x = 0.3145, y = 0.3015) to the blue-green (x = 0.216, y = 0.236) according to the CIE, which were consistent with the photographs. The fluorescence could also return to the initial state displaying red after irradiation with 254 nm UV for 10 min with a thermal treatment at 100 °C. The high spatial resolution and noncontact characteristics of the light-induced dimerization reaction provide possibilities for controlling the spatial cross-linking and CT interaction of the top layer, resulting in the selective generation of the pattern with wrinkles and fluorescence. The dual-pattern with fluorescence and hierarchical wrinkles can be realized simultaneously by UV-irradiation with a photomask. After exposure with 365 nm UV light using different photomasks such as an annulus, stripes and the letters “SJTU,” the resulting micropatterns with fluorescence and hierarchical wrinkles were observed by super resolution multiphoton confocal microscopy (STED). As shown in Figure 2, positive images with a green fluorescence pattern of the annulus, stripes or “SJTU” within the red fluorescence background were obtained. Furthermore, arrays of hierarchical wrinkles arranged in the annulus and stripes corresponding to the green fluorescent areas were also

Figure 2. Reversible dual-pattern with fluorescence and hierarchical wrinkles obtained through photomasks. (a−d) STED images exhibiting the fluorescence of micropatterns. The scale bar represents 500 μm. (e, f) The corresponding optical microscope images exhibiting characteristic wrinkles. The samples were prepared using PAN-NDI-BA and by alternately overlaying/removing masks (“stripe”, “annulus”, and “SJTU”), followed by irradiation with 365 nm UV light for 15 min with a thermal treatment at 70 °C. The erasure procedure was achieved by heating the patterned film at 150 °C for 2 h. The excitation wavelength for STED was fixed at 405 nm.

observed. In the case of the exposure under 365 nm UV light, the uncovered areas undergo photodimerization, thus, the CT interaction between AN and NDI was weakened, and the areas emitted fluorescence only from NDI. Concurrently, the wrinkles were selectively generated in the exposed regions, while the unexposed area remained smooth. The wrinkles were oriented nearly perpendicular to the boundary of the exposed regions because of the boundary effect.38,39 The masked areas remained unchanged both in modulus and CT interaction, remaining in the initial state. After heating at 150 °C for 2 h, the surface with the dual-pattern of fluorescence and wrinkles converted into a smooth state of solely red color. Therefore, this reversible dual-pattern can realize double messages written and erased through a single input. The reversible dual-patterned surface with dynamic fluorescence and wrinkles can serve well for the development of a new type of smart material surface that might find applications in smart displays and message storage. As shown in Figure 3, the initial PAN-NDI-BA/PDMS bilayer was transparent under ambient conditions and emitted red fluorescence under UV illumination (Figure 3a,d). After 365 nm UV irradiation for 7.5 min through a macro-photomask of a “flower,” the specific wrinkling pattern of the flower appeared and could be identified by the naked eye because of light scattering by wrinkles, which provides an advantage in easily distinguishing patterns (Figure 3e,g). Under UV illumination, the white “flower” pattern in the exposed region was observed (Figure 3b), while the unexposed region emitted the red fluorescence. Further irradiation with 365 nm UV light resulted in the “flower” pattern emitting bluegreen (Figure 3c) and becoming more visible under natural light because of the increased amplitude of the wrinkles (Figure 3f,h). It should be noted that the resulting dual-pattern of the flower was very stable and remained nearly unchanged under ambient conditions for at least 4 months because of the excellent stability of the anthracene dimer (Figure S7), which is 542

DOI: 10.1021/acsmacrolett.8b00211 ACS Macro Lett. 2018, 7, 540−545

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letter changed to blue-green, and the letter “C” emerged under ambient conditions because of the light scattering caused by the wrinkled structure (Figure 4b). Next, another letter, “T,” was written in the same way; however, this time, the UV-light irradiation and heating steps were separated (Figure 4c). Because the wrinkles cannot generate after irradiation without thermal treatment, different wrinkle information can be obtained so that a difference in optical information can be observed under natural light even though the fluorescence information is the same (Figure 4d,e). The ridge ending and branching also arises in the wrinkle pattern, which offers potential security tags (Figure 4f). Thus, our responsive dualpattern capable of generating two sources of information is substantially more difficult to be cloned in counterfeiting compared with single patterns based only on fluorescence or wrinkles. Moreover, taking advantage of the reversibility of the dual-patterns, bar codes for information can be repeatedly written and erased (Figure S8). The selective exposure under 365 nm UV light can be regarded as the process of “writing” information, and heating at 150 °C or irradiation with 254 nm UV light represents the step of erasing the information. More importantly, the reversible dual-patterns can greatly increase both the capacity and safety of the information. In summary, we demonstrated a facile and robust strategy for the fabrication of a reversible dual-pattern with simultaneously responsive fluorescence and wrinkled topography using a copolymer containing AN and NDI (PAN-NDI-BA) as the skin layer. Both the fluorescence and wrinkled topography of a surface pattern were spatially and simultaneously tunable by simply controlling reversible photodimerization of AN. The generation and elimination of wrinkles were the consequence of the dynamic cross-linking of the copolymer controlled by the reversible photodimerization of AN, which also led to the reversible fluorescence from red to white to blue-green because of the tunable CT interaction between AN and NDI in the copolymer PAN-NDI-BA. By combining the remotely and spatially controlled characteristics of light, smart surfaces with dynamic hierarchical wrinkles and fluorescence were realized and can be used for smart displays and anticounterfeiting. Thanks to their dynamic characteristics and excellent stability, we believe that this dual-pattern has great potential in fields such as information security and anticounterfeiting.

Figure 3. Reversible dual-pattern with fluorescence and hierarchical wrinkles obtained through a macro-mask. Photographs of “flower” macro-patterns with reversible fluorescence and wrinkles: (a−c) under UV light and (d−f) under natural light. (g, h) The corresponding 3D AFM exhibited wrinkle characteristic. The sample were prepared by PAN-NDI-BA irradiated by 365 nm UV light through a photomask “flower” with a thermal treatment at 70 °C. Erasure procedure was achieved by heating the patterned film at 150 °C for 2 h. The scale bar represents 2 mm.

rarely found with other responsive fluorescent patterns based on photochromic molecules such as spiropyran.40−42 This “flower” pattern, with dynamic fluorescence and wrinkles, was reversible and could be erased by heating at 150 °C or by irradiation with 254 nm UV light. Smart fluorescent materials have been widely utilized in the field of information security, while wrinkled topography has also been used as a biomimetic microfingerprint in anticounterfeiting for two types of fields: ridge ending and ridge branching.43 Our PAN-NDI-BA/PDMS bilayer exhibiting dualdisplay properties with fluorescence and wrinkles combines the two kinds of anticounterfeiting technologies, undoubtedly further improving the security level. Here, we further demonstrate its application in anticounterfeiting. As shown in Figure 4, the letter “C” was written on PDMS by a writing brush with a PAN-NDI-BA solution as ink. After drying under ambient conditions, the letter “C” could be observed via red fluorescence emission under UV illumination but was not visible under natural light (Figure 4a). Upon irradiation with 365 nm UV light for 15 min at 70 °C, the fluorescence of the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00211.

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Synthesis and characterization, UV−vis and dynamic studies, property of long-term stability, and application for smart display (PDF).

AUTHOR INFORMATION

Corresponding Author

Figure 4. Photographs of letters on PDMS under UV light and natural light. (a) The letter “C” was brushed on PDMS. (b) The sample was exposed to UV light (365 nm) and underwent a thermal treatment at 70 °C. (c) The letter “T” was brushed on PDMS. (d) The sample was exposed to UV light and (e) followed by thermal treatment at 70 °C. The scale bar represents 5 mm. (f) An optical microscope image exhibiting the wrinkles of the letter. The scale bar represents 50 μm.

*E-mail: [email protected]. ORCID

Xuesong Jiang: 0000-0002-8976-8491 Notes

The authors declare no competing financial interest. 543

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ACKNOWLEDGMENTS The authors thank National Nature Science Foundation of China (51773114, 21522403) and Shanghai Municipal Government (17JC1400700) for their financial support.

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