2018 Zhang, Zeng, ACS Appl. Mater. Interfaces 2018

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 23247−23253

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Reversibly Switching Molecular Spectra Yong Zhang,†,§ Hulie Zeng,*,†,§ Sifeng Mao,† Shun Kondo,† Hizuru Nakajima,† Shungo Kato,† Carolyn L. Ren,‡ and Katsumi Uchiyama*,† †

Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397, Japan ‡ Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada Downloaded via TSINGHUA UNIV on July 25, 2018 at 08:44:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Manipulation of light transmission/absorbance and reflection/emission has a great significance in smart windows and displaying media like liquid crystal. Here, we report the usage of an external electric field to reversibly switch the molecular spectra of a model molecule on the basis of its interaction with an electroresponsible polymer brush. Both the UV−vis absorbance spectrum and the fluorescence emission spectrum of the model molecule were confirmed to be electroswitchable. The electroswitchable spectra were experimentally demonstrated to be induced by the electroswitchable statuses of medium anionic poly-allyloxy hydroxypropyl sulfonate (poly-AHPS) brush. Insightfully, the molecular aggregated status of model proflavine molecules could be electrically controlled via the electroresponsible polyAHPS brushes and then the molecular spectra of the model proflavine molecule also could be electrically and controllably shifted. The success in the manipulation of molecular spectra opens up a wide range of applications not only for displaying but also for nonlinear optics, in vivo imaging, sensors, and environmental inspection. KEYWORDS: electroswitchable molecular spectrum, electrocontrollable spectrum, electroswitchable aggregate, electrocontrollable assembly, polymer brush, poly-AHPS

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alkenes.20,21 Furthermore, a switchable molecular structure has been utilized to switch the qubits, to switch the molecular electronics, to manipulate the molecular tweezers, to tune the frequency of microwave signals, to control the pore opening of molecular sieving membrane,22−26 and to sense the proteins in the living cells or the bacterial gene regulation processes.27,28 Generally, switching the chemical, biotechnical, and optical functions by smart stimulus responsive matrices and materials plays the main role in potential artificial intelligence, automatic control, and digital system. Usually, the manipulation of molecular emission is completed by tuning the aggregated states of molecules and it has been applied in biochemistry and environmental engineering. For example, aggregated induced emission (AIE), a photophysical phenomenon induced by chromophore aggregation, has been developed and applied in sensors, cellular imaging, organic light emitting diode devices, optical waveguides, and environmental inspections.29,30 In this case, the manipulation of molecular emission was completed by

INTRODUCTION There is an ever-increasing demand in developing nanoscale switching functions driven by their needs in high-performance computing, micromechanics, and biomimetic engineering.1,2 Specifically, utilizing molecular materials to complete the functional switching has been boosted up during the past few decades.3−5 The molecular bearable robotic arms have been usually adopted to manipulate the position of a chemical cargo to get the single-molecule switching.6 For instance, the position of the aromatic ring could be switched by a proton or by electrochemical means.7 The differential supramolecular structure could be constructed by adjusting the molecular conjugation.8−10 Moreover, the various functions and devices have been reversibly switched utilizing the stimulus responsive materials in recent decades. For example, the resistivity has been reversibly tuned by sliding the anchoring points of a rotatable group or by tuning the conformer of bridge molecule11,12 and the resistivity of the polymer has been manipulated by a redox13 or by doping it into acids.14 The molecular crystals and molecular assembly state also have been artificially manipulated.15−19 In addition, the molecular rotator has been obtained via the temperature responsible polymeric spin-crossover compound and the functionalized overcrowded © 2018 American Chemical Society

Received: March 20, 2018 Accepted: June 18, 2018 Published: June 20, 2018 23247

DOI: 10.1021/acsami.8b04530 ACS Appl. Mater. Interfaces 2018, 10, 23247−23253

Research Article

ACS Applied Materials & Interfaces

electrode was the reference) of voltage for 5 min. To perform continuous observation of the electroswitchable fluorescence spectra of the model proflavine molecules, the 300 nm depth of proflavine solution on the ITO glass slide that was modified with poly-AHPS was settled under the fluorescence microscope with a continuous voltage supply that was circulated between +400 and −400 mV.

aggregating the nonemissive liminogens to emit the aggregated fluorescence via restricting intramolecular rotations, vibrations, and motions.30 But the manipulation to molecular emission by AIE was limited at the internal stimuluslike solvent. In addition, the triplet−triplet annihilation photon upconversion (TTA-UC) emission also has been reported to be thermally switched by tuning the aggregated state of donor−acceptor pairs in terms of solid−liquid phase transition. And the TTAUC could be enhanced up by freezing the ambient solvent31 or aggregating the donor−acceptor pairs on the supramolecular gel nanofiber media.32 Excepting the selection of the appropriate donor−acceptor pair, the phase transition of the surroundings has been the critical factor to manipulate the emission via the TTA-UC emission. To our best knowledge, the direct and reversible manipulation of molecular spectra by an external stimulus in a homogeneous environment is still difficult. For the requirement of the instant switch to molecular spectrum in intelligence operations for the biosensor, optical devices, and imaging systems, we conceived an idea to directly switch the molecular absorbance and the molecular emission by an external electric field.

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RESULTS AND DISCUSSION On the basis of the foundation of reversible manipulation of biomatters33 and friction factor34 via anionic polymer brushes, we adopted the poly-AHPS brushes that were grafted on an electrically conductive substrate as the matrices to confine the proflavine model molecules to construct the switch to the aggregated states of model molecules, then switch the absorption spectrum and the emission spectrum of the model molecules (Figure 1). In addition, we selected

EXPERIMENTAL MATERIALS AND METHODS

Synthesis of the Polymer Brushes of Allyloxy Hydroxypropyl Sulfonate (AHPS). The poly-AHPS brushes were synthesized on the indium tin oxide (ITO) glass slide by the graftpolymerization, as before.33 Switching Molecular Assembly. To manipulate the molecular assembly of proflavine on the extended poly-AHPS, the poly-AHPSmodified glass slide was immersed into 5 μM aqueous proflavine solution with −400 mV applied voltage for 5 min. Then, the extra proflavine solution was gently removed using a filter paper from the glass slide. Thereafter, the glass slide was washed by pure water for three times and blown by an air duster. Finally, the aggregated proflavine on the extended poly-AHPS brushes was obtained, which could be characterized by atomic force microscopy (AFM), attenuated total reflection infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectra (EIS). For reference, the shrunk poly-AHPS brushes without the capability to conjugate the proflavine were prepared. The polyAHPS modified glass slide was immersed into 5 μM aqueous proflavine solution with +400 mV applied voltage for 5 min. Then, the extra proflavine solution was gently removed by a filter paper from the glass slide. Thereafter, the glass slide was washed by pure water for three times and blown by an air duster. Measurement of UV−Vis Spectra and Fluorescent Spectra. The poly-AHPS brush-modified ITO glass slide, rectangle of 1 × 4 cm2 area, was immersed into 5 μM aqueous proflavine solution with −400 mV or +400 mV of applied voltage for 5 min. Then, the extra proflavine solution was gently removed by a filter paper. Thereafter, the glass slide was washed by pure water for three times and blown dry by an air duster. Finally, it was settled on the sample stage at a 45° incident angle to observe the UV−vis spectra and fluorescence spectra of poly-AHPS brushes with or without proflavine assembly. To take the video of electroswitchable fluorescence of proflavine, the poly-AHPS-modified ITO glass slide was first doped with 100 μL of 5 μM aqueous proflavine solution, which is doped with 300 nm diameter polystyrene particles and then covered by a microcover glass slide with a small pressure to settle the depth of the solution as 300 nm with the support of polystyrene particles. The slide was supplied with −400 mV (Ag/AgCl electrode was the reference) of voltage for 5 min to assist the full formation of the extended poly-AHPS brushes. Consequently, the poly-AHPS-modified ITO glass slide adsorbed with proflavine was measured for its fluorescence intensity using an Olympus IX71 fluorescence microscope with a U-FBNA filter cube. The same procedure was repeated by applying +400 mV (Ag/AgCl

Figure 1. Molecular models of poly-allyloxy hydroxypropyl sulfonate (poly-AHPS) brush and proflavine. Carbon atoms, oxygen atoms, sulfur atoms, nitrogen atoms, and silicon atoms are, respectively, shown in cyan, red, yellow, blue, and gray in the molecular model of the poly-AHPS brush. To discriminate the molecular model of polyAHPS brush, carbon atoms, nitrogen atoms, and hydrogen atoms are, respectively, shown in purple, pink, and white in the molecular model of proflavine.

proflavine as the model molecule, which was confirmed of having reversibly aggregated statuses.35 We presumed the polarized nitrogen atom at the middle pyridine ring of the model proflavine molecule would be attracted by the anionic sulfonic groups at the ends of side chains of poly-AHPS brush, when the poly-AHPS brush was supplied with a negative bias to stretch the side chains and main chain (Figure 1). Then, both the absorption spectrum and the emission spectrum of model molecules would be electrically switched via directly electrocontrolling the aggregated state of the model molecule, which would be triggered by an external electric field. The proflavine molecule was approved to be electrochemically inactive during the electroswitching of the poly-AHPS brushes (Figure S1); therefore, the molecular structure of proflavine would not be changed by the supplied low external electrical voltage ranging from 400 to −400 mV. In this research, the matrices, the poly-AHPS brushes on an electrically conductive substrate, could be produced by grafting sodium AHPS on an indium tin oxide (ITO) glass slide.33 To act as a medium to support the electroswitchable molecular aggregated state and electroswitchable molecular spectra of the model molecule, the electroswitchable properties of polyAHPS brush on the support ITO glass substrate were primarily 23248


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Figure 2. Characterization of poly-AHPS brush media (a) The AFM imaging of the extended (left) and shrunk (right) poly-AHPS brushes after the negative bias and positive bias were charged, respectively. The supplied voltages for negative charging and positive charging were, respectively, −400 and +400 mV. The reference electrode was Ag/AgCl electrode in all of the experiments. (b) ATR-IR characterization of the extended (blue line) and the shrunk (red line) poly-AHPS brushes; the peaks of the sulfonic group were highlighted by a purple dashed ellipse. (c) The S 2p peaks of extended (blue line) and the shrunk (red line) poly-AHPS brushes in XPS spectrum. (d) The extended (blue square) and the shrunk (red cycle) poly-AHPS brushes in the EIS spectrum in the Nyquist plot.

exterminated ahead. The synthesized poly-AHPS brushes exhibited a uniform appearance when the substrate was negatively charged; in contrast, they presented grainy superficies with contractive knolls when the substrate was positively charged (Figure 2a). We assumed the uniform appearance of poly-AHPS brushes was due to the stretching of poly-AHPS brushes when a negative bias was supplied, whereas the grainy superficies with contractive knolls was due to the shrinking of poly-AHPS brushes when a positive bias was supplied (the upper inserts in Figure 2a). The supposition of the morphologies of poly-AHPS brushes were first supported by the quite different wettabilities of the extended poly-AHPS brushes and the shrunk poly-AHPS brushes (Figure S2). On the basis of the molecular structure of the poly-AHPS brush, we deduced the exposed strongly hydrophilic sulfonic groups due to the extended poly-AHPS brushes supplied high wettability. The exposed carbonic chains with relatively high hydrophobicity owing to the shrunk poly-AHPS brushes showed low wettability. The deduction was further confirmed by the characterizations of the ATR-IR for poly-AHPS brushes. The exposure of the sulfonic groups on the surface of the extended poly-AHPS brushes was evidenced by the presence of infrared absorbance at 1043 and 1143 cm−1, both belonging to sulfonic groups, when the poly-AHPS brushes were extended by negative voltage. In contrast, the above two absorbance peaks would vanish when a positive bias was supplied to the glass slide to reveal the carbonic chains at the surface of the rolled poly-AHPS brush (Figure 2b). Furthermore, the hypothesis of the shrinking of the poly-AHPS brushes after a positive bias was supplied to the slide also could be confirmed

by the increased exposure of ITO glass, which was evidenced by the high abundance of In 3d5/2 and In 3d3/2 in the X-ray photoelectron spectroscopy (XPS) analysis. But the abundance of In 3d5/2 and In 3d3/2 was almost negligible in the XPS spectrum of the extended poly-AHPS brushes (not shown). At the same time, the exposure of the sulfonic groups that were at the ends of the extended poly-AHPS brushes also addressed a low bonding energy and a high abundance of the S 2p peak in XPS spectra after they were negatively charged. Relatively, the concealing of the sulfonic groups of the shrunk poly-AHPS brushes permitted a high bonding energy and a low abundance of S 2p peak after a positive bias was supplied to the ITO glass slide (Figure 2c). Additionally, the extended poly-AHPS brushes, after a negative bias was supplied, also indicated an obvious resistance property compared with the shrunk polyAHPS brushes after a positive bias was supplied (Figure 2d). The motion between the extended poly-AHPS brushes and the shrunk poly-AHPS brushes under the bias of 400 and −400 mV, respectively, was demonstrated and simulated by the EIS analysis (Supporting Information). To verify the differential molecular absorbance and molecular emission of the model proflavine molecules, which were induced by the states of electroswitchable poly-AHPS brush, we immersed ITO glass slide modified with poly-AHPS brushes into 5 μM proflavine aqueous solution. Then, a negative or positive bias was supplied to the glass slide at −400 or 400 mV for 5 min to get the different aggregated states of the poly-AHPS brushes. Here, we assumed the ambient aqueous surrounding supplied a free environment to the motion of proflavine. The poly-AHPS brush-modified substrate 23249


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Figure 3. Electroswitchable molecular spectra of proflavine. The UV−vis spectrum (blue dash) and the fluorescence emission spectrum (blue line) of aggregated proflavine on the extended poly-AHPS brushes after negative charging and the UV−vis spectrum (red dash) and the fluorescence emission spectrum (red line) of 5 μM proflavine aqueous solution. The exciting wavelengths for the aggregate of proflavine and for the proflavine aqueous solution in fluorescence emission spectra were 444 and 422 nm, respectively.

Figure 4. Characterizations of electroswitchable aggregation and molecular spectra of proflavine. (a) AFM images of the congregations of the aggregates of proflavine on the extended poly-AHPS brushes (left) and the shrunk poly-AHPS brushes (right) without proflavine molecules. (b) XPS spectra of the extended (blue line) and the shrunk (red line) poly-AHPS brushes on the ITO glass after conjugating proflavine molecules. The typical peak of N 1s was highlighted by the purple triangle. (c) The amplified peaks of N 1s characterized the abundance of nitrogen atoms on the extended poly-AHPS brushes and the shrunk poly-AHPS brushes. (d) Fluorescence images of 5 μM proflavine solution in a depth of 300 nm when the support poly-AHPS brushes matrices were extended (left) or shrunk (right). The images are obtained by a fluorescence microscope. (e) The relative intensity of green fluorescence of 5 μM proflavine solution in a depth of 300 nm when the polarity of the glass slide modified with polyAHPS brushes was continuously and electrically switched.

brushes by using the blank ITO glass that was modified with poly-AHPS brushes as the reference (Figure S3a). Comparatively, no specific UV−vis spectrum of the shrunk poly-AHPS brushes could be obtained when a clear ITO glass modified with poly-AHPS brushes acted as the reference in the measurement. Relatively, the maximum absorbance of random free proflavine in water was at 422 nm. We found the maximum absorbance of proflavine could be red-shifted from

was consequently taken out from the proflavine aqueous solution and washed with pure water. Then, the UV−vis spectrum and the fluorescence spectrum of the substrate were measured, respectively. We found the UV−vis spectrum of adsorbed proflavine on the extended poly-AHPS brushes having an obvious peak of maximum absorbance at 444 nm. Here, the absorption spectrum of the aggregated proflavine was deducted from the background absorbance of the poly-AHPS 23250


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4c). The only source of nitrogen was proflavine in this experiment; therefore, we could further experimentally confirm the aggregation of proflavine on the extended poly-AHPS brushes. Comparatively, no obvious abundance of N 1s was found on the shrunk poly-AHPS brushes; which further evidenced the absence of proflavine molecules on the shrunk poly-AHPS brushes. (Figure 4c). To further clarify the electrically operative states of proflavine, the electrocontrollable aggregation of proflavine on the extended poly-AHPS brushes was also verified by continual fluorescence observation by a fluorescence microscopy. The distinct fluctuation of green fluorescence of free random proflavine molecules was captured in 5 μM proflavine solution at a depth of 300 nm with the support of electroswitchable poly-AHPS brushes because of the shifting between the aggregated proflavine and the free random proflavine in solution (Figure 4d). The reversibility of electroswitchable aggregation of proflavine, actually in the molecular emission spectrum, was further demonstrated by the reasonable repeatability of continual fluctuation of the fluorescence emission at the green ray range (Figure 4e). The real video of continually switching the green fluorescence emission of proflavine with the support of electrocontrollable poly-AHPS brushes is also supplied to further confirm the manipulability of electroswitchable spectral characteristics (Video S1). Since the aggregated proflavine was assumed to J-aggregate according to a previous report,35 the simulation of aggregated proflavine, which was carried out by time-dependent density functional theory calculation at the B3LYP/6-31+G(p) level plus Grimme’s D3 correction, confirmed the red-shifting of both the UV−vis absorbance and the fluorescence emission of the aggregated proflavine on the stretched poly-AHPS brushes.36,37 The distance between each of the two planar proflavine molecules was calculated to be 3.43 Ȧ , and the dimensional transplacement between each of the two proflavine was obtained to be 1.43 Ȧ in the aggregated proflavine.

422 nm to a longer wavelength of 444 nm with the induction of the aggregation of proflavine molecules on the extended poly-AHPS brushes (Figure 3). The absorption spectrum could exclude the influence of active oxygen in solution since the measurement of the UV−vis spectrum of the solid polyAHPS-modified ITO glass with aggregated proflavine molecules was carried out in the air. On the other hand, the peak of the maximum fluorescent emission of proflavine on the extended poly-AHPS brushes was found at 660 nm, which was also compared with the fluorescent emission of the shrunk poly-AHPS brushes (Figure S3b). Relatively, the maximum fluorescence emission of free proflavine in aqueous solution was at 534 nm (Figure 3). Conspicuously, the fluorescent emission of proflavine was also red-shifted from 534 to 660 nm with the aggregation of proflavine on the extended poly-AHPS brushes. Excluding the above red-shifting in fluorescence emission spectra due to protonation of proflavine in aqueous solution (Figure S4), we assumed the above electrocontrollable shiftings of the molecular absorption and the fluorescent emission of proflavine were induced by the previously electrocontrollable morphologies of poly-AHPS brushes because no other factors in the whole experiment were switchable or manipulatable except the evaluated electroswitchable morphologies of poly-AHPS brushes. Briefly, the aggregated states of proflavine were switchable between the aggregations and the free random molecules with the induction of electrocontrollable poly-AHPS brushes and then the molecular spectra of proflavine were demonstrated to be electroswitchable. Proflavine was actually reported of forming J-aggregate and having the red-shifting molecular spectra in aggregated state.35 Therefore, we deduced the above red-shifting of the maximum absorbance in the UV−vis spectrum and the red-shifting of the maximum emission in the fluorescence spectrum of proflavine were induced by the aggregation of proflavine molecules on the extended poly-AHPS brushes, which were triggered by a negative voltage. To validate the formation of the aggregate of proflavine on the extended poly-AHPS brushes, we observed the superficial appearance of the extended or the shrunk polyAHPS brushes with or without aggregated proflavine by AFM. The obvious clusters of the congregations of the aggregates of proflavine and the extended poly-AHPS brushes were observed on the substrate (Figure 4a), which were also discriminated against the uniform appearance of the blank extended polyAHPS brushes (Figure 2a), whereas the image of a grainy superficies with the knolls of the shrunk poly-AHPS brushes that lost the capability to aggregate proflavine molecules was also obtained by AFM (Figure 4a). Interestingly, the imaging of the shrunk poly-AHPS brushes obtained in 5 μM proflavine aqueous solution exhibited more obvious knolls than that of the shrunk poly-AHPS brushes developed in 100 mM KCl aqueous solution (Figure 2a). To further confirm the generation of aggregated proflavine on the extended poly-AHPS brushes under a negative voltage, we observed the surfaces of the extended and the shrunk polyAHPS brushes with and without aggregated proflavine by XPS. Certainly, the shrinking of the poly-AHPS brushes led to a extreme exposure of In on the ITO glass, which was evidenced by the high abundances of In 3d3/2 and In 3d5/2 in the XPS spectral analysis (Figure 4b). In addition, an obvious abundance of N 1s was also achieved in the amplified XPS spectrum of the extended poly-AHPS brushes when a negative bias was supplied to have the aggregated proflavine (Figure

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CONCLUSIONS In conclusion, the molecular spectra of the model molecule were turned out to be reversibly electroswitchable and the switch to the molecular spectra could be triggered by an external electric field. Intrinsically, the reversibly electroswitchable molecular spectra were induced by the electrocontrollable aggregated states of the model molecule, Herein, the two aggregated states of the model molecule were demonstrated to be dependent on the electrocontrollable morphologies of anionic poly-AHPS brushes matrices. Both the UV−vis absorption spectrum and the fluorescence emission spectrum of the model proflavine molecule were confirmed to be reversibly electroswitchable, and both the UV−vis absorption spectrum and the fluorescence emission spectrum could be directly and conveniently manipulated by an electric switch. On the basis of the above evidenced concept, the possibility to manipulate the molecular spectra of the polar molecules, which could be aggregated into an ordered assembly, was experimentally confirmed. The finding opens up a way to automatically and intelligently control manipulation in both the molecular assembly and nonlinear optics. 23251


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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/acsami.8b04530.

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Some more details about the characterization of proflavine and poly-AHPS brushes; the experimental procedures for ATR-IR, EIS, and CV; the method for theoretical simulation of proflavine; the discussion of the motion of poly-AHPS brushes (PDF) Green fluorescence emission of proflavine with the support of electrocontrollable poly-AHPS brushes (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Z.). *E-mail: [email protected]. Tel: +81 426771111 ext. 4883 (K.U.). ORCID

Hulie Zeng: 0000-0003-0484-836X Carolyn L. Ren: 0000-0002-9249-7397 Katsumi Uchiyama: 0000-0001-7179-9954 Author Contributions §

Y.Z. and H.Z. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS Scientific Research C (18K05178). We should thank Prof. Guozheng Zhang of the University of Science and Technology of China for his contribution on the calculations and simulations of J-aggregate of proflavine, Prof. K. Kajihara and Prof. H. Yoshida for their help in the measurement of ATR-IR, and Prof. H. Munakata for his contribution in the measurement of EIS.

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REFERENCES

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