Reversibly Switching Molecular Spectra - ACS Applied Materials

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04530 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 23, 2018

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ACS Applied Materials & Interfaces

Reversibly Switching Molecular Spectra Yong Zhang,†,º Hulie Zeng,†,º,* 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 Minamiohsawa, Hachioji, Tokyo 192-0397, Japan ‡ Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Ave W., Waterloo, ON N2L 3G1, Canada 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 based on its interaction with an electro-responsible polymer brush. Both the UV-Vis absorbance spectrum and the fluorescence emission spectrum of the model molecule were confirmed to be electro-switchable. The electro-switchable spectra were experimentally demonstrated to be induced by the electro-switchable statuses of medium anionic poly-allyloxy hydroxypropyl sulfonate (polyAHPS) brush. Insightfully, the molecular aggregated status of model proflavine molecules could be electrically controlled via the electro-responsible poly-AHPS 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 to displaying but also to nonlinear optics, in-vivo imaging, sensors and environmental inspection.

KEYWORDS: electro-switchable molecular spectrum, electro-controllable spectrum, electro-switchable aggregate, electro-controllable assembly, polymer brush, poly-AHPS

INTRODUCTION There is an ever-increasing demand in developing nanoscale switching functions driven by their needs in high-performance computing, micro mechanics 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 singlemolecule switching. 6 Take an instance, the position of the aromatic ring could be switched by a proton or by an 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

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 polyAHPS brush. To discriminate the molecular model of poly-AHPS brush, carbon atoms, nitrogen atoms and hydrogen atoms are respectively shown in purple, pink and white in the molecular model of proflavine.

examples, the resistivity has been reversibly tuned by sliding the anchoring points of a rotatable group or by tuning the conformer of bridge molecule. 11-12 And, the resistivity of the polymer has been manipulated by a redox 13 or by doping it into acids. 14 The molecular crystals and molecular assembly alsostate 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 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, the biotechnical and the optical functions by

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smart stimulus responsive matrices and materials are playing 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 (OLED) devices, optical waveguides, and environmental inspections. 29-30 In this case, the manipulation of molecular emission was completed by aggregating the non-emissive 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 stimulus like 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 TTA-UC could be enhanced up by freezing the ambient solvent 31 or aggregating the donor-acceptor pairs on the supramolecular gel nanofiber media. 32 Excepting the selection of the appropriate donoracceptor 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 in difficulty. With the requirement of the instant switch to molecular spectrum in intelligence operations to the bio-sensor, the optical devices, and the imaging systems, we conceived an idea to directly switch the molecular absorbance and the molecular emission by an external electric field.

EXPERIMENTAL MATERIALS AND METHODS Synthesis of the Polymer Brushes of AHPS. The poly-AHPS brushes were synthesized on the indium tin oxide (ITO) glass slide by the graft-polymerization as before. 33

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Switching Molecular Assembly. To manipulate the molecular assemble of proflavine on the stretched poly-AHPS, the polyAHPS modified glass slide was immersed into 5 µM aqueous proflavine solution with -400mV of applied voltage for 5 min. Then the extra proflavine solution was gently removed by filter paper from the glass slide. After that, the glass slide was washed by the pure water for three times and blown by an air duster. Finially, the aggregated proflavine on the stretched poly-AHPS brushes was obtained, which could be characterized by the atomic force microscopy (AFM), the attenuated total reflection infrared spectroscopy (ATR-IR), the X-ray photoelectron spectroscopy (XPS) and the electrochemical impedance spectra (EIS). For reference, the shrunk poly-AHPS brushes without the capability to conjugate the proflavine was prepared. The poly-AHPS modified glass slide was immersed into 5 µM aqueous proflavine solution with +400mV of applied voltage for 5 min. Then the extra proflavine solution was gently removed by filter paper from the glass slide. After that, the glass slide was washed by the pure water for three times and blown by an air duster. Measurement of UV-Vis spectra and fluorescent spectra. The poly-AHPS brushes modified ITO glass slide at 1×4 cm2 rectangle was respectively immersed into 5 µM aqueous proflavine solution with -400mV or +400mV of applied voltage for 5 min. Then the extra proflavine solution was gently removed by filter paper, After that, the glass slide was washed by the pure water for three times and blown dry by an air duster. Finally, it was settled on the sample stage at a 45ºincident angle to observe the UV-Vis spectra and the fluorescence spectra of poly-AHPS brushes with or without proflavine assembly. To take the video of electro-switchable fluorescence of proflavine, the poly-AHPS modified ITO glass slide was first dropped with 100 µL of 5 µM aqueous proflavine solution which is doped with 300 nm diameter of polystyrene particles, and then covered by a micro cover 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

Figure 2. Characterization of poly-AHPS brush media (a) The AFM imaging of the extended (left) and the shrunk (right) poly-AHPS brushes after 2 the negative bias and positive bias were charged, respectively. The supplied voltages for negatively charging or positively charging were respectively -400 mV and +400 mV. The reference electrod 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 dash ellipse. (c) The S2p peaks of extended (blue line) and the shrunk (red line) poly-AHPS brushes in XPS spectrum. (d) The stretched (blue square) and the shrunk (red cycle) poly-AHPS brushes in EIS specACS Paragon Plus Environment trum in Nyquist plot.

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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 electrode was the reference.) of voltage for 5 min. In order to perform continuous observation of the electro-switchble fluorescence spectra of the modle proflavine molecules, the 300 nm depth of proflavine solution on the ITO glass slide which was modified with poly-AHPS was settled under the fluorescence microscope with the continuous voltage supplying that was circulated between +400 mV and -400 mV.

RESULTS AND DISCUSSION Based on the foundation of reversible manipulation of bio-matters and friction factor 34 via anionic polymer brushes, we adopted the poly-AHPS brushes that grafted on an electrically conductive substrate as matrices to confine the proflavine model molecules to construct the switch to the aggregated states of model molecule, then switch the absorption spectrum and the emission spectrum of a model molecule (Figure 1). In addition, we selected proflavine as the model molecule which was confirmed 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, the absorption spectrum and the emission spectrum of model molecules both would be electrically switched via directly electro-controlling the aggregated state of the model 33

medium to support the electro-switchable molecular aggregated state and electro-switchable molecular spectra of the model molecule, the electro-switchable properties of poly-AHPS brush on the support ITO glass substrate were primarily exterminated ahead. The synthesized poly-AHPS brushes exhibited a uniform appearance when the substrate was negatively charged; oppositely they presented grainy superficies with contractive knolls when the substrate was positively charged (Figure 2a). We assigned the uniform appearance of poly-AHPS brushes were supplied by the stretching of poly-AHPS brushes when a negative bias was supplied, while the grainy superficies with contractive knolls were 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 firstly supported by the quite different wettabilities of the extended poly-AHPS brushes and the shrunk poly-AHPS brushes (Figure S2). Based on 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 owe 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 presenting of the infrared absorbance at 1043 cm-1 and 1143 cm-1 those both belong to sulfonic groups when the poly-AHPS brushes were extended by negative voltage. Oppositely, the above two absorbance peaks would be vanished when the 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

Figure 3. Electro-switchable 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 negatively charged and the UV-Vis spectrum (red dash) and the fluorescence emission spectrum (red line) of 5 µM proflavine aqueous solution. The exciting wavelengths for aggreaget of proflavine and for the proflavine aqueous solution in fluorescence emission spectra were 444 nm and 422 nm, respectively.

molecule, that would be triggered by an external electric field. The proflavine molecule was approved to be electrochemcially inactive during the electro-switching 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 400mV to -400mV. 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 the

positive bias was supplied to the slide also could be confirmed by the increased exposure of ITO glass, that was evidenced by the high abundances of In3d5/2 and In 3d3/2 in the X-ray photoelectron spectroscopy (XPS) analysis. But, the abundances of In3d5/2 and In 3d3/2 were almost negligible in the XPS spectrum of the extended poly-AHPS brushes (not shown). At the same time, the exposure of the sulfonic groups those were at the ends of the extended poly-AHPS brushes also addressed a low bonding energy and a high abundance of the S2p peak in XPS spectra after they were negatively charged. Relatively, the concealing of the sul-

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fonic groups of the shrunk poly-AHPS brushes permitted a high bonding energy and a low abundance of S2p 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 with the comparsion of the shrunk poly-AHPS brushes after a positive bias was supplied (Figure 2d). The motion between the extended polyAHPS brushes and the shrunk poly-AHPS brushes under the biased of 400 mV 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 induced by the states of electro-switchable poly-AHPS brush, we immersed ITO glass slide that modified with poly-AHPS brushes into 5 µM proflavine aqueous solution. Then, a negative or a positive bias was supplied to the glass slide at -400 mV 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 brushes modified substrate was consequently taken out from the proflavine aqueous solution and washed out 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

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water was at 422 nm. We found the maximum absorbance of proflavine could be red-shifted from 422 nm to 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 UV-Vis spectrum of the solid poly-AHPS 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 nm to 660 nm with the aggregation of proflavine on 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 electro-controllable shiftings of the molecular absorption and the fluorescent emission of proflavine were induced by the previously electro-controllable morphologies of poly-AHPS brushes. Because no other factors in the whole experiment were switchable or manipulatable expected the valuated 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 electro-controllable poly-AHPS brushes, and then the molecular spec-

Figure 4. Characterizations of electro-switchable 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 N1s was highlighted by the purple triangle. (c) The amplified peaks of N1s 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 300nm when the support poly-AHPS brushes matrices were stretched (left) or shrunk (right). The images are obtained by fluorescence microscope. (e) The relative intensity of green fluorescence of 5 µM proflavine solution in a depth of 300nm when the polarity of the glass slide that modified with poly-AHPS brushes was continuously and electrically switched.

was deducted the background absorbance of the poly-AHPS brushes by using the blank ITO glass that modified with polyAHPS 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 that modified with polyAHPS brushes acted as the reference in the measurement. Relatively, the maximum absorbance of random free proflavine in

tra of proflavine were demonstrated to be electro-switchable. Proflavine was actually reported forming J aggregate and having the red-shifting molecular spectra in aggregated state35. Therefore, we deduced the above red-shifting of the maximum absorbance at UV-Vis spectrum and the red-shifting of the maximum emission at the fluorescent spectrum of proflavine were induced by the


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aggregation of proflavine molecules on the extended poly-AHPS brushes, those were triggered by a negative voltage. To validate the formation of the aggregate of proflavine on the extended polyAHPS brushes, we observed the surficial appearance of the extended or the shrunk poly-AHPS 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 poly-AHPS brushes (Figure 2a). While 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 the 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 the negative voltage, we observed the surfaces of the extended and the shrunk poly-AHPS brushes with and without aggregated proflavine by XPS. Certainly, the shrinking of poly-AHPS brushes led to the extreme exposure of In on the ITO glass that was evidenced by the high abundances of In3d 3/2 and In 3d 5/2 in the XPS spectral analysis (Figure 4b). In addition, an obvious abundance of N1s was also achieved in the amplified XPS spectrum of the extended polyAHPS brushes when a negative bias was supplied to have the aggregated proflavine (Figure 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 N1s was found on the shrunk poly-AHPS brushes, it 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 electro-controllable aggregation of proflavine on extended poly-AHPS brushes was also verified by the continual fluorescence observation by fluorescence microscopy.The distinct fluctuation of green fluorescence of free random proflavine molecules was caught in 5µM proflavine solution in a depth of 300 nm with the supporting of electro-switchable poly-AHPS brushes because of the shifting between the aggregated proflavine and the free random proflavine in solution (Figure 4d). The reversibility of electro-switchable aggregation of proflavine, actually the molecular emission spectrum, was further demonstrated by the reasonable repeatability of continual fluctuation of the fluorescence emission at green rays range (Figure 4e). The real video of continually switching the green fluorescence emission of proflavine with the supporting of electro-controllable poly-AHPS brushes was also supplied to further confirm the manipuility of electro-switchable spectral characteristics (Video S1). Since the aggregated proflavine was assumed to J aggregate according to previous report 35, the simulation of aggregated proflavine that was carried out by a time-dependent density functional theory (TD-DFT) 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 two planer proflavine molecules was calculated to be 3.43 Ȧ, and the dimensional transplacement between

each two proflavine was obtained to be 1.43 Ȧ in the aggregated proflavine.

CONCLUSION In conclusion, the molecular spectra of the model molecule were turned out to be reversibly electro-switchable, and the switch to the molecular spectra could be triggered by an external field. Intrinsically, the reversibly electro-switchable molecular spectra were induced by the electro-controllable aggregated states of model molecule, Hereinto, 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 electro-switchable, and both the UV-Vis absorption spectrum and the fluorescence emission spectrum could be directly and conveniently manipulated by an electric switch. Based on the above proofed concept, the possibility to manipulate the molecular spectra of the polar molecules, those could be aggregated into an ordered assembly, was experimentally confirmed. The finding opens up a way to automatically control and intelligent manipulation to both the molecular assembly and the nonlinear optics.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Some more details about the characterization of proflavine and poly-AHPS brushes; the experimental procedures for ATR-IR, EIS and CV; the mothod to the theoretical simulation of proflavine; the discussion to the motion of poly-AHPS brushes; and related figures and video.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]. Tel: +81 426771111 ext. 4883 ºThe two authors have the same contribution to the paper. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by JSPS Scientific Research C (18K05178). We should thank Prof. Guozheng Zhang of 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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Reversibly Switching Molecular Spectra - ACS Applied Materials

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