Functional Covalent Layer-by-Layer Thin Films by [2 + 2

Jun 6, 2018 - Covalent layer-by-layer (LbL) thin films with the well-defined charge-transfer (CT) chromophores were fabricated by the highly efficient...
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Letter Cite This: ACS Macro Lett. 2018, 7, 716−719

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Functional Covalent Layer-by-Layer Thin Films by [2 + 2] Cycloaddition−Retroelectrocyclization Hiroyuki Fujita and Tsuyoshi Michinobu* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: Covalent layer-by-layer (LbL) thin films with the well-defined charge-transfer (CT) chromophores were fabricated by the highly efficient [2 + 2] cycloaddition−retroelectrocyclization (CA−RE) reaction between the dialkylaniline-substituted alkynes and 7,7,8,8-tetracyanoquinodimethane (TCNQ) moieties. The resulting thin films showed potent redox activities and Ag+ ion sensing ability due to the formed CT chromophores.

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occur at room temperature, and a homogeneous mixed solution of P1 and P2 in THF could be prepared. A thin mixed film of P1 and P2 was prepared by spray-coating on an indium−tinoxide (ITO) plate. In order to investigate the reaction progress, the spray-coated film was heated to 100 °C (Scheme 1). The

he layer-by-layer (LbL) technique is a powerful tool for the fabrication of functional multilayered nanofilms on a wide range of substrates or templates.1−5 Noncovalent weak physical interactions, such as electrostatic interactions, hydrogen bonding, and metal ion coordination, have been the main forces to construct the LbL films. However, recent advancements in click chemistry or other high-yielding chemical bond formation reactions have opened the door to a covalent LbL technique due to the high efficiency, rapid progress, and the absence of any side reactions.6 Successful LbL film formation has indeed been reported by using the Cu(I)-catalyzed azide− alkyne cycloaddition (CuAAC),7−18 thiol−ene reaction,19−22 and other new click reactions.23−25 However, despite the useful reactions in the LbL film production, these reactions simply formed new covalent bonds and no additional optical and electrical functionalities were directly endowed to the produced films except when functional groups were designed and introduced in advance.26,27 We previously developed a new high-yielding reaction based on the [2 + 2] cycloaddition−retroelectrocyclization (CA−RE) between electron-rich alkynes and electron-deficient olefins, which produces intramolecular charge-transfer (CT) chromophores.28−31 It was revealed that the formed CT products are useful for applications in ion sensors,32,33 nonlinear optics,34,35 photovoltaics,36−38 and electrical memory devices.39,40 We have now employed the [2 + 2] CA−RE reaction to fabricate LbL films and report for the first time the successful alternating polymer deposition and well-defined visual Ag+ ion sensing behavior. A polystyrene derivative P1 with N,N-dihexylanilinesubstituted alkyne side chains was prepared by free radical polymerization, and the 7,7,8,8-tetracyanoquinodimethane (TCNQ) polyester P2 was prepared by polycondensation. Although the unsubstituted TCNQ undergoes the [2 + 2] CA− RE with aniline-substituted alkynes at room temperature,41 the reactivity of the 2,5-dialkoxy-substituted TCNQ apparently decreased.42 Thus, the reaction between P1 and P2 did not © XXXX American Chemical Society

Scheme 1. Polymer Reaction Based on the [2 + 2] CA−RE Reaction between the Dialkylaniline-Substituted Alkyne and TCNQ Unit to Produce the Crosslinked Polymer with CT Chromophores

film gradually changed its color from yellow to green, being insoluble at this temperature after 10 min (for the reaction mechanism, see Scheme S1). Furthermore, in the IR spectra, the peak intensity at 2220 cm−1 ascribed to the cyano groups of P2 decreased upon heating, whereas a new peak at 2207 cm−1 appeared (Figure S1). This spectral change is consistent with the previous report about the reaction of P2 with dialkylanilinesubstituted alkyne molecules.43 The conversion of the TCNQ moiety, estimated from the peak intensities, was about 45% based on the assumption that the absorbance values at 2220 and 2207 cm−1 represent the TCNQ moiety and the formed donor−acceptor structure, respectively. All these results support the reaction progress between P1 and P2 without any noticeable side reactions. Received: May 11, 2018 Accepted: June 4, 2018

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DOI: 10.1021/acsmacrolett.8b00365 ACS Macro Lett. 2018, 7, 716−719

Letter

ACS Macro Letters Based on the test experiment of the spray-coated bulk film, the reaction conditions of the LbL assembly was determined to be 100 °C for 10 min so that a part of the reactive sites remains at the surface. The LbL deposition started with the spin-coating of P1 (10 mM repeat unit−1 in toluene) on an ITO plate at 1000 rpm for 60 s. This was followed by the spin-coating of P2 (10 mM repeat unit−1 in THF) on the P1 film at 1000 rpm for 60 s. Due to the lower solubility of P1 in THF, a type of bilayer was successfully formed. Heating the bilayer film to 100 °C for 10 min followed by washing the unreacted P2 with THF produced a chemically cross-linked film with the surface layer based on P2. This process was then repeated by the alternating deposition of P1 (from toluene) and P2 (from THF), thus, furnishing the covalent LbL films. The LbL film growth was monitored by its UV−vis-near IR absorption spectra (Figure 1a). The absorption peaks at 320,

Figure 2. (a) Plots of water contact angles vs the layer numbers and the water droplet images on (b) 7 layer and (c) 8 layer films.

conventional click reactions. Cyclic voltammograms (CVs) were employed to reveal the redox activities of the LbL films of P1 and P2. The LbL films with different layer numbers were prepared on an ITO electrode and subjected to CV measurements in CH3CN containing 0.1 M (nC4H9)4NClO4. All the films exhibited well-defined oxidation and reduction peaks ascribed to the CT chromophore (Figure S3a). The reversibility of the reduction peaks was better than that of the oxidation peaks. This was consistent with previous reports using this reaction in organic synthesis to construct n-type semiconducting materials.45 In addition, it is noted that the oxidation peak top (Eox,top) cathodically shifted and the reduction peak top (Ered,top) anodically shifted as the layer number increased (Figures S3b,c). This was probably caused by the dense multilayer films. The dense packing of each polymer hindered the diffusion of the supporting electrolytes and medium. Thus, both the oxidation and reduction became difficult when the film thickness increased. The dense multilayer films were also supported by atomic force microscopy (AFM) images. For example, the AFM image of a 6 layer film displayed a smooth surface with the roughness parameter (RMS) value of 3.18 nm (Figure S4). One of the promising application possibilities for the [2 + 2] CA−RE reaction is an ion sensor.46 Previously, linear polystyrenes bearing the CT chromophoric side chains showed colorimetric sensing of some metal ions, such as Fe3+ and Ag+. Among the recognized metal ions, Ag+ ions were a good target due to the well-defined color change caused by the multivalent coordination of the cyano groups.47 Thus, the Ag+ ion sensing ability of the LbL film was investigated. When the 6 layer film was soaked in a 0.2 mM CF3SO3Ag solution in 1,2dichloroethane, the film color visually changed from green to dark orange, resulting in a significant bathochromic shift in the CT band (ca. 100 nm) as shown in Figure 3a. This color change, caused by the Ag+ ion coordination to the multiple cyano groups of the CT chromophores within the films, was

Figure 1. (a) UV−vis-near IR spectra of multilayer films (1−10 layers) on the ITO substrate and (b) absorbance changes at 320, 415, and 700 nm during the LbL assembling process of P1 and P2.

415, and 700 nm are derived from P1, P2, and the formed CT chromophore, respectively. The absorbance of the CT band at 700 nm linearly increased with the layer number, whereas those of P1 (320 nm) and P2 (700 nm) showed a zigzag pattern (Figure 1b). This result indicated the successful alternating LbL assembly. In addition, the continuous film growth was supported by the film thicknesses determined by a surface profilometer (Figure S2). Similar to the linear increase in the CT band intensity, the film thickness also linearly increased with the increasing layer number. The thickness of a 10 layer film reached 63 nm. This result implied that the formed LbL film can be controlled with a thickness of several tens of nanometers. Furthermore, the LbL assembly was also investigated in terms of its surface property. It is well-known that the hydrophilicity (or hydrophobicity) of polymer films depends on the chemical components and nanostructures at the surface.44 In this study, as can be anticipated from the solubility in different solvents, the hydrophilicity (or hydrophobicity) of P1 is thought be different from P2. Thus, the surface property of the multilayer thin films was analyzed by water contact angle measurements. When the top layer was P1 (odd number), the films showed the water contact angle of about 90° (Figure 2a,b). In contrast, when the top layer was P2 (even number), the contact angles decreased to ca. 60° (Figure 2a,c). This result clearly reflects the chemical structures of P1 and P2 and again suggests the successful formation of the LbL films. Note that controlling the surface properties is a significant advantage of using the LbL technique. One of the most noticeable features of the [2 + 2] CA−RE reaction is the significant optical and electrical properties of the formed CT chromophores. This feature has, to the best of our knowledge, not been realized for the LbL films formed by other

Figure 3. (a) UV−vis-near IR spectra and images and (b) λmax values of the cross-linked films during capture/release cycles of Ag+ ions by repeated soaking in a 0.2 mM CF3SO3Ag solution in 1,2-dichloroethane and in trimethylamine (cycle 1: as-prepared six-layer film; cycles 2, 4, 6, 8, and 10: Ag+ complex state; cycles 3, 5, 7, 9, and 11: Ag+ ion release state). 717

DOI: 10.1021/acsmacrolett.8b00365 ACS Macro Lett. 2018, 7, 716−719

Letter

ACS Macro Letters

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rapidly completed within 1 min. More importantly, the original color of the LbL film could be regenerated by soaking the Ag+ ion loaded films into triethylamine (Figure 3a). This process could be repeated many times without any decomposition (Figure 3b). Note that the Ag+ ion recognition did not occur without the CT chromophore constructed by the [2 + 2] CA− RE reaction.48 In conclusion, covalent LbL films were fabricated for the first time by the [2 + 2] CA−RE reaction. The successful LbL assembly was revealed by its UV−vis-near IR spectra, film thicknesses, and surface properties. The LbL films possessed potent redox activities and visible absorption due to the formation of CT chromophores. It was demonstrated that the LbL film becomes a simple and reusable Ag+ ion sensor.

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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.8b00365. Synthesis, reaction mechanism, experimental details, IR spectra, film thicknesses, CVs, and AFM images of LbL films (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tsuyoshi Michinobu: 0000-0001-6948-1189 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the JSPS KAKENHI Grants 15H03863 and 15KT0140, the Izumi Science and Technology Foundation, and the Support for Tokyotech Advanced Researchers.

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DOI: 10.1021/acsmacrolett.8b00365 ACS Macro Lett. 2018, 7, 716−719