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Photo-controlled Reversible Guest Uptake, Storage and Release by Azobenzene-modified Microporous Multi-layer Films of Pillar[5]arenes Tomoki Ogoshi, Shu Takashima, and Tada-aki Yamagishi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12893 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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Journal of the American Chemical Society

Photo-controlled Reversible Guest Uptake, Storage and Release by Azobenzene-modified Microporous Multi-layer Films of Pillar[5]arenes Tomoki Ogoshi,†,‡,§,* Shu Takashima† and Tada-aki Yamagishi† †

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan ‡JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan § WPI Nano Life Science Institute, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan ABSTRACT: Using the pillar-shaped architecture of pil-

lar[5]arenes, we constructed microporous multi-layer films with azobenzene groups on the top surface by layer-by-layer assembly of cationic and anionic pillar[5]arenes. Guest uptake, storage and release by the microporous films were regulated through photo-reversible isomerization of azobenzene groups attached to the pore outlets. Azobenzene was regarded as a “molecular valve” to control guest access: the trans form of the azobenzene acted as an open valve, allowing the guest free access from/to the micropores. Conversely, the cis form of the azobenzene behaved as a closed valve, completely blocking guest access from/to the micropores. Photo-responsive reversible uptake, storage and release of guest molecules were demonstrated through photo-isomerization of the azobenzene valves by irradiation with ultraviolet and visible light.

The switching of uptake/release of guest molecules is important for tuning molecular functions based on host– guest binding.1 To date, a number of the switching has been developed in solution state by taking advantage of various kinds of stimuli such as redox reactions,2 acid/base reactions3 and light irradiation.4 However, in the solid state, the switching is challenging topics,5 and few bulk porous materials showing photo-controlled reversible guest release systems have been reported to date using inorganic porous materials.6 In particular, in the thin film state, there are no examples displaying photo-responsive guest uptake and release. We produced a new generation of thin films with controlled micropores at the molecular level through layerby-layer (LbL) assembly of pillar-shaped macrocyclic Pillar[n]arenes were compounds, pillar[5]arenes.7 introduced by our group in 2008.8 Consecutive adsorption of cationic (P+) and anionic (P-) pillar[5]arenes (Figure 1a)30,31 on a solid substrate enabled us to construct thin films containing controlled pores at molecular level. The resulting thin films displayed excellent shape selectivity for guest molecules reflecting the size of the micropores: the microporous films took up para-dinitrobenzene (p-DNB) but not disubstituted isomers such as ortho-dinitrobenzene (o-DNB) and meta-dinitrobenzene (m-DNB). This is because the cavity size of the micropores constructed from pillar[5]arenes (ca. 4.7 Å) is larger than the length of pDNB (ca. 4.3 Å), but smaller than those of o-DNB (ca. 5.9

Å) and m-DNB (ca. 5.9 Å). The pore volume of the thin films can be also tuned by the number of deposited layers. As the number of deposited layers increased, the maximum number of guest p-DNB molecules adsorbed by the films increased. Such molecular-level guest recognition and tuning of pore volume are original features of the microporous thin films constructed from pillar[5]arenes, and have not been accomplished in porous thin films prepared using other materials. One important aspect of pillar[n]arene chemistry is versatile functionality: functional groups can be installed at desired positions of the pillar[n]arene substituents.8d, 8e Using this versatile functionality, in this study, we report that multi-layer films exhibit photo-responsive reversible uptake, storage and release of guest molecules by attaching azobenzene derivatives to the pore outlets. We regard the azobenzene substituents as “molecular valves” to control the access of the guest from/to the micropores. First, according to our previous report,7 pillar[5]arene multi-layer films were constructed by LbL assembly involving alternating adsorption of cationic pillar[5]arene P+ and anionic pillar[5]arene P- layers on quartz substrates four times (Figure 1b). Each step of multi-layer construction was monitored by UV-vis measurements (Figure 1c). The absorption intensity at 293 nm, corresponding to the absorption of phenyl moieties of pillar[5]arenes, increased linearly with the number of deposited layers. Such a linear absorption increase indicates that multi-layer films (nL, where n is the number of deposited layers) were obtained by repeated alternating immersion steps in P+ and P− solutions. The thin film with four layers (4L) had an anionic surface. Thus, to attach azobenzene valves onto the micropore outlets, we used an aqueous solution of cationic pillar[5]arene with one azobenzene moiety (Figure 1a, azoP+) instead of P+. A new absorption peak appeared at 350 nm after immersing 4L in the aqueous solution of azo-P+ (Figure 1c), confirming successful introduction of azo-P+ on the top surface of 4L. The resulting film had trans form azobenzene (denoted trans-azo-5L). The cis-azo-5L was produced with UV light irradiation (340 nm) of trans-azo-5L, which caused the intensity of the absorption band centered at 354 nm to decrease (Figure 2a). In contrast, irradiation with visible light (436 nm) or heating cis-azo-5L at 80 °C regenerated the absorption band of the trans azobenzene derivative, indicating reversible photo-isomerization of the azobenzene

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Figure 1. C Construction of microporou us multi-layerr films with azzobenzene valvves attached tto pore outletss. (a) Chemicaal structures o of cationic (P+ +) and anionicc (P−) pillar[5 5]arenes and a cationic pillaar[5]arene beaaring one azob benzene moietty (azo-P+). (b) ( LbL assem mbly by conseccutive adsorpttion of P+, P− − and azo-P+ +. (c) UV-vis absorption sp pectra of multiilayer films formed by Lb bL assembly w with azobenzen ne valves attacched to the po ore outlets usin ng ionic pillarr[5]arenes (P+ +, o-P+). P− and azo

Figure 2. Photo-respon nsive guest upttake. (a) UV-vvis absorption n spectra of tr rans-azo-5L upon alternatting irradiation n with UV or visible light. UV-vis U absorp ption spectra of o (b) trans-a azo-5L and (c)) cis-azo-5L before (dash llines) and afteer immersion in p-DNB forr 28 h (solid liines). (d) Abso orbance at 293 3 nm plotted aagainst immerrsion time for trans-azo-5L L (orange triaangles), cis-az zo-5L (purplee squares) and d 5L (blue circles). (e) Scheematic represeentation of ph hoto-responsivve guest uptak ke. derivatives aattached to tthe pore outtlets. This photoisomerization of trans/ciis-azo-5L co ould be reverrsibly

witched many times by alteernating irrad diation with U UV sw ligh ht and heating g (Figure S6). The conversio ons from transs to


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Figure 3. Photo-respon nsive guest storage and relea ase. (a) p-DN NB@trans-azo o-5L and (b) p-DNB@cis--azo-5L beforre mersion in a so olution containing excess co ompetitive gueest 1,4-dicyano obutane for 28 8 h (solid lines)). (dash lines)) and after imm (c) Absorbaance at 293 nm m plotted agaainst immersio on time for p-D DNB@trans-azo-5L (oran nge triangles), p-DNB@cis sazo-5L (pin nk squares) an nd p-DNB@5L (blue circless). (d) Schemaatic representaation of photo-responsive gu uest storage an nd release. (e) Photo-respon nsive guest uptake and releasse upon alternating irradiatiion with UV orr visible light. cis forms by U UV irradiation n and from ciss to trans form ms by heating at 80 oC, which werre determined using a mono olayer trans/cis-az zo-1L, were 9 98.1% and 9 97.9%, respecttively (Figure S7). Photo-isomeri P ization quantiitatively proceeeded even in the film m state. The photo-responsive hosst–guest adsorption behaviior of the thin film was investig gated using p-DNB as a g guest. Because p-DN NB has a UV V absorption band at 293 3 nm, encapsulation n of p-DMB can be mon nitored by U UV-vis measurementts. When tran ns-azo-5L w was immersed in a chloroform so olution contain ning p-DNB, tthe intensity o of the absorption att 293 nm inccreased as th he immersion time lengthened (F Figure 2b and d d, orange triiangles). The same change was also observed d during uptake of p-DN NB by pillar[5]arenee multilayer ffilms.7 These results indiicated encapsulation n of p-DNB into the micropo ores of trans--azo5L. A pillar[5]]arene multi-layer film with h five layers without the azobenzeene valves (5 5L), which was prepared d by consecutive adsorption a of cationic pilla ar[5]arene P+ + and anionic pillarr[5]arene P- layers five tiimes, also sh howed similar guest aadsorption beh haviour (Figurre 2d, blue cirrcles). Uptake amou unts of p-D DNB were saame with/witthout azobenzene vaalve.7 This sug ggests that thee trans form o of the azobenzene va alves on the po ore outlets did d not interferee with uptake of p-DNB. In th he trans form valves, att the equilibrium sstate, the max ximum adsorb bed amount of pDNB increaseed as the numb ber of depositted layers increeased (Figure S8). This indicattes that guesst molecules were included in n not only the to op surface layyer but also in n the pillar[5]arenee cavities of lo ower layers. In I contrast, th he cis form of the azobenzene valves indu uced by UV light irradiation diid not cause the absorptiion at 293 nm n to change when cis-azo-5L w was immersed in p-DNB sollution

forr a commensurate period off time (Figure 2c and d, purp ple squ uares), indicaating that cis-azo-5L did not take up pDN NB. The azobeenzene groupss attached on the pore outlets theerefore acted aas molecular vvalves to inhibiit guest accesss to thee micropores in the thin ffilm. To inveestigate how tthe azo obenzene moiieties work as a valves on the surface, we mo onitored fluoreescence chang ges of the 1,4-d dialkoxybenzeene un nits consisting of pillar[5]arrene because tthe fluorescen nce wa as enhanced b by complexation with guestts.9 Figure S110a sho owed the fluo orescence chan nge of azo-1L L film by phottoiso omerization frrom trans to cis form. T The fluorescen nce inttensity from the 1,4-dialko oxybenzene in n cis state w was larrger than 1.3 times larger than that in trans state. T The chaange was nott found in a film prepared d form the unit mo odel of azo-P P+ (Figure S S10b). These results suggest incclusion of th he cis from aazobenzene m moiety into tthe pilllar[5]arene caavity to inhib bit the compllexation with pDN NB. Pilllar[5]arenes fform highly stable host–g guest complexxes witth 1,4-dicyanobutane (asso ociation consstants K of the t com mplexes forrmed betweeen 1,4-dicya anobutane aand pilllar[5]arenes g generally exceeed 104 M−1).100 Thus, we ussed 1,4 4-dicyanobutan ne as a compeetitive guest to induce releaase of p p-DNB encap psulated in thee microporous thin film. Wh hen a p p-DNB@tran ns-azo-5L th hin film was immersed in n a chlloroform solu ution containiing excess co ompetitive guest 1,4 4-dicyanobutan ne (4.48 M), tthe absorption n intensity at 2 293 nm m originating from p-DNB decreased (F Figure 3a and d c, oraange triangless), indicating rrelease of p-D DNB. A decreaase of the absorptio on intensity aat 293 nm waas also observved wh hen p-DNB@ @5L, which d did not have the azobenzeene vallves, was im mmersed in a solution containing 1,4diccyanobutane (Figure 3c, blue circles).. These resu ults



suggest that the trans azobenzene valves did not interfere with the release of p-DNB (Figure 3d). In contrast, for the film with cis azobenzene valves, the absorption intensity at 293 nm did not change even after immersion in 1,4dicyanobutane solution (Figure 3b and c, purple squares). This indicates that p-DNB was not released because the cis form of the azobenzene valves blocked the guest-containing pores. Based on the guest uptake and release system using azobenzene moieties as molecular valves, we demonstrated photo-responsive reversible p-DNB uptake and release (Figure 3e). When trans-azo-5L was immersed in a solution containing p-DNB, trans-azo-5L took up p-DNB. Upon UV irradiation, p-DNB uptake did not occur because the cis form of the azobenzene valves induced by UV irradiation closed the entrances of the pores. Upon visiblelight irradiation, p-DNB uptake started again because the azobenzene valves changed from the “closed” cis form to the “open” trans form. Thus, the photo-responsive p-DNB uptake was reversibly switched on/off by alternating UV or visible light irradiation. When p-DNB@trans-azo-5L was immersed in a solution containing excess competitive guest 1,4-dicyanobutane, release of p-DNB occurred in the system with “open” trans azobenzene groups, but not in that with “closed” cis azobenzenes. The photo-responsive pDNB release was also reversibly switched on/off with alternating exposure to UV or visible light. In conclusion, attaching azobenzene valves onto the pore outlets of microporous films allowed effective direct photocontrol over p-DNB uptake and release. On/off switching of p-DNB uptake and release was controlled even during the uptake and release processes. Photo-patterning of the films with open and closed micropore sites will be possible using a photomask. Furthermore, the end-functionalization procedure used to attach functional groups to the pore outlets demonstrated in this study is novel method to obtain functionalized microporous multi-layer films, and very simple, involving adsorption of pillar[n]arenes bearing functional groups in the top layer formed by LbL assembly. Thus, the present study provides a new powerful method to construct new-generation microporous films with various functions at desired positions at the molecular level. Supporting Information. Experimental section, 1H NMR, reversible photo-isomerization, uptake of p-DMB by trans-azo-nL films and fluorescence spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *Department of Chemistry and Chemical Engineering, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan TEL: +81-76-234-4775; FAX: +81-76-234-4800 E-mail: [email protected]

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Note The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by Grant-in-Aid for Scientific Research on Innovative Areas (2601): π-System Figuration (15H00990 and 17H05148), Kiban B (16H04130), JST PRESTO (JPMJPR1313) and Kanazawa University Chozen Project.


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Photocontrolled Reversible Guest Uptake, Storage ... - ACS Publications

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