Optical Humidity Sensing Using Transparent Hybrid Film Composed of

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Optical Humidity Sensing Using Transparent Hybrid Film Composed of Cationic Magnesium Porphyrin and Clay Mineral Takuya Fujimura, Tetsuya Shimada, Ryo Sasai, and Shinsuke Takagi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04006 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Optical Humidity Sensing Using Transparent Hybrid Film Composed of Cationic Magnesium Porphyrin and Clay Mineral Takuya Fujimura,1* Tetsuya Shimada,2,3 Ryo Sasai,1 Shinsuke Takagi2,3* 1

Department of Physics and Materials Science, Interdisciplinary Graduate School of Science and

Engineering, Shimane University, 1060, Nishikawatsu-cho, Matsue, 690-8504 Japan 2

Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo

Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan 3

Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1 Minami-osawa,

Hachioji, Tokyo 192-0397, Japan

ABSTRACT

A transparent hybrid film composed of cationic magnesium porphyrin and clay mineral was developed, and its chromic behavior depending on relative humidity (RH) was investigated. The hybrid film was obtained via intercalation of magnesium porphyrin into clay film; magnesium porphyrin was intercalated into the interlayer spaces of the clay mineral without aggregation. The

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absorption spectra of the hybrid film showed red shift compared to the aqueous solution of magnesium porphyrin because of the π-conjugated system extension with co-planarization of the meso-substituted pyridinium group and porphyrin ring. The absorption maximum of the hybrid film was gradually shifted to a shorter wavelength and the color of the hybrid film was changed with increasing RH. The XRD measurement suggested that the basal space of clay was expanded with increasing RH, indicating that the interlayer space of clay was expanded by water adsorption, and the spectral shift was induced by the change in co-planarization degree between the porphyrin ring and meso-substituted pyridinium groups.

INTRODUCTION The photochemical and photophysical behaviors of dyes in various host materials, such as micelle, clathrate, mesoporous materials, zeolites, and nano-sheet materials, have been investigated over several decades. The reason is that the guest dye molecules in the nano-spaces provided by these host materials show interesting photophysical and photochemical behaviors, which are different from their photochemical/photophysical behaviors in a solution.1-6 The layered materials have attracted great interest as host materials over the past decade because of their unique interlayer spaces.7-14 Especially, the clay minerals, which are typical layered materials, provide the characteristics of the interlayer spaces, such as very flat surface, negatively charged layer and ion exchange properties of the incorporated cation, exfoliation/stack ability of the nano-sheet of clay, and modifiability of their surface with cationic organic molecules.15-21 In particular, their flexible interlayer distance is reversibly changed by a change in relative humidity (RH), and it is extended with increasing RH.17-21 This characteristic property of the clay minerals


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suggests that they can provide reversible and controlled nano-space for incorporated cationic guests, such as functional dyes. Various researchers have investigated the photochemical properties of guest dyes on the surface of exfoliated clay nano-sheets or in interlayer spaces of clay minerals, and these hybrid materials show unique properties, which can be applicable to luminous devices, light harvesting systems, and sensors.22-30 However, intercalated/adsorbed dyes tend to form the aggregates showing complicated photochemical behavior which differ from the behavior of monomer in the gallery/surfaces of layered materials, thus it is difficult to design and to expect the functionality of the dye-layered material hybrid systems. In our previous work, we have reported on an interesting porphyrin−clay mineral hybrid in which the porphyrin molecules densely adsorb the exfoliated clay nano-sheets or clay film without aggregation. The crucial factor for the highdensity adsorption of porphyrins has been found to be the distance matching between the cationic sites in the porphyrin and the average distance between the anionic sites on the clay nano-sheet surfaces (intercharge distance matching effect or size-matching effect).31-33 This adsorption and intercalation property will help us to design the photo-functionalities of intercalated dyes because aggregates which shows the complicated photochemical behavior was suppressed. In these previous works, we reported that the absorption and fluorescence spectra of the porphyrin molecules show the red shift by adsorption on/intercalation into clay, and then, this spectral shift can be ascribed to the extension of the π-conjugated system with co-planarization between the peripheral meso-substituted pyridinium groups and the porphyrin ring.7, 31, 34-35 Considering this investigation and the dependency of the interlayer spaces of clay minerals on relative humidity, the absorption maximum of the intercalated porphyrins in clay minerals should depend on the


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relative humidity, as shown in Figure 1. Thus, chromism of hybrid materials would be expected, and it would be useful for the sensing devices for the detection of relative humidity. 36-38 In this report, we focused on this spectral shift of the porphyrin molecules intercalated into the interlayer space of clay and its RH dependency. Saponite, which is one of the synthetic clay minerals, and magnesium tetrakis(1-methyl-4-pyridiniumyl)porphyrin (MgTMPyP4+) were used as host and guest materials, respectively. A transparent hybrid film composed of magnesium porphyrin and saponite was prepared via an intercalation reaction. Judging from the absorption/fluorescence study, the intercalated MgTMPyP4+ did not aggregate, in spite of the high amount of dye loaded. As a result, a reversible and clearly visible color change was observed by the change of RH condition. It should be noted that the color change is not induced by bond cleavage, but by molecular structure change. Thus, high reversibility will be expected in the present system.

High RH condition clay nano-sheet

Porphyrin ring

+

Low RH condition

+

+

+

+

+

meso-substituted pyridinium group

Figure 1. Ideal image of the structural change of incorporated porphyrin in clay induced by change in the basal space of clay minerals (side view).


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EXPERIMENTAL Materials Magnesium

tetrakis(1-methyl-4-pyridiniumyl)porphyrin

tetrachloride

(abbreviated

as

MgTMPyPCl4) was purchased from Mid-century Chemicals, and its purity was checked by 1HNMR and thin-layer chromatography. Saponite (abbreviated as SAP) clay was purchased from Kunimine industry. The stoichiometric formula of SAP is [(Si7.20Al0.80)(Mg5.97Al0.03)O20(OH)4]0.77

(Na0.49 Mg0.14)+0.77, its theoretical surface area is 750 m2 g-1, and its cation exchangeable

capacity (CEC) is 1.00 meq g-1.38 The average area per anionic site is calculated at 1.25 nm2, and the average distance between anionic sites on the SAP sheets is estimated to be 1.2 nm on the basis of a hexagonal array. The glass substrate (cover glass, 24 mm × 24 mm, thickness of 0.145±0.025 mm) was purchased from Matsunami Glass IND., LTD. The structural images of MgTMPyP4+ and SAP are shown in Figure 2.

(a)

(b)

Mg

Figure

2. Structural

image of magnesium

tetrakis(1-methyl-4-pyridiniumyl)porphyrin

(MgTMPyP4+(a)), Saponite (SAP, (b)).


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Preparation of Hybrid Film Composed of SAP and MgTMPyP4+ We previously reported the method for preparation of a transparent hybrid film composed of cationic porphyrin and SAP, and MgTMPyP4+/SAP hybrid film was prepared according to the literature.39 The glass substrate was sonicated with water and ethanol for 30 min. The glass substrate was treated in sulfuric acid (Kanto Chemical, 96%) overnight at room temperature and then it was washed with sufficient water. A total of 2 mL of SAP dispersion (100 mg L−1 in water:dioxane = 4:1 (v:v)) was filtered through a PTFE membrane filter (pore size = 0.1 µm). Because not much SAP was observed in the filtrate, SAP was almost completely filtered (ca. 99%). The transparent SAP thin film was obtained by transferring the residue upon the glass substrate. The thin film was dried up under vacuum overnight. The SAP thin films were immersed in a water:ethanol = 1:2 (v:v) solution containing MgTMPyP4+ for 48–72 h at room temperature with constant stirring.

Relative Humidity Control and Sample Preparation for UV-Vis. Absorption Spectra, Fluorescence Spectra, and XRD Pattern at Each Relative Humidity Condition Dried N2 gas and moist N2 gas were mixed and flowed in the globe bag. The flow ratio of dried and moist N2 gas was changed to control the relative humidity, and total flow ratio was set at 200 mL min-1.40 The relative humidity in the globe bag was monitored by hydrometer. The hybrid film was treated under N2, having constant relative humidity in the above globe bag, and it was closed in the cell to measure the absorption spectra, fluorescence spectra, and XRD


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pattern. To control the loading level of the MgTMPyP4+, its concentration was changed. The actual loaded amount of MgTMPyP4+ was calculated from absorbance of the solution after immersion.

Analysis The UV-Vis. absorption spectra were obtained on Jasco V670. The fluorescence spectra were measured with a Jasco FP-6600 spectrofluorometer, and the wavelength of excitation light was set at the absorption maximum wavelength of each sample. The glass substrate was set at 45° against the direction of the excitation light source and the detector of emission. The X-ray diffraction (XRD) pattern was measured with Mini Flex II (Rigaku). Time resolved absorption spectra were measured by USB2000+ sopectrometer (Ocean Optics).

RESULTS AND DISCUSSION XRD pattern, Absorption spectra and Fluorescence spectra of the Hybrid Film A photograph of the obtained hybrid film is shown in Figure 3. The obtained film is highly transparent, enough to measure the transmission spectra. Although MgTMPyP4+ in water is unstable because of an occurrence of demetallation, the obtained hybrid film was stable even in water.

UV-Vis.

absorption

spectral

change

of MgTMPyP4+

aqueous

solution

and

MgTMPyP4+/SAP hybrid film in water were shown in Figure S1-(a) and (b), respectively. The


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spectral shape of the MgTMPyP4+ solution was changed even 1 day later. It suggested that some of the MgTMPyP4+ was decomposed and/or demetalated in water. On the other hand, spectral shape of the hybrid film was not changed even 5 day later. This result indicated that intercalated MgTMPyP4+ in SAP is stable compared to the MgTMPyP4+ in aqueous medium, and it will be because that MgTMPyP4+ was sandwiched in SAP sheets as described below, and then demetallation will be suppressed by the steric effect. The XRD pattern of the MgTMPyP4+/SAP hybrid film is shown in Figure 4. We previously reported that the basal space of the nano-sheet was 0.31 nm in the case of only a SAP film. The diffraction pattern of the MgTMPyP4+/SAP hybrid film was observed around 6.1° to 6.3°, and the basal space was estimated as 0.47 to 0.44 nm, considering the thickness of the saponite nano-sheet. It indicates that MgTMPyP4+ was intercalated into SAP, and the basal space of the nano-sheet was expanded. Considering the thickness of the porphyrin ring (approx. 0.3–0.35 nm), MgTMPyP4+ did not form the face-toface type dimer (H-aggregate). Thus, MgTMPyP4+ was intercalated between the SAP nanosheets as a monolayer. In addition, orientation of the MgTMPyP4+ intercalated in the SAP nano-sheets would be almost in parallel to these nano-sheets. It is known that the basal space of clay minerals depends on the incorporated cation spaces under the same RH. Thus, the basal spaces of SAP depended on the loaded amount of MgTMPyP4+.20


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Figure 3. Photograph of MgTMPyP4+/SSA hybrid film.


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Figure 4. XRD pattern of the SAP film and MgTMPyP4+/SAP hybrid films at various MgTMPyP4+ loadings.

The UV-Vis. absorption spectra for the MgTMPyP4+ solution, MgTMPyP4+ adsorbed on the exfoliated SAP surface (dispersion), and MgTMPyP4+/SAP hybrid film are shown in Figure 5. The absorption maximum of the MgTMPyP4+ adsorbed on the exfoliated SAP surface shows a red shift compared to that of an aqueous solution (450.0 nm to 472 nm). Several researchers revealed that adsorption spectra of the porphyrin adsorbed on clay surfaces could be shifted to longer wavelength because of theπ-conjugated system extension with co-planarization of the


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meso-substituted pyridinium group and porphyrin ring7,31,34-35,41 Judging from these investigations, this spectral shift has been ascribed to the coplanarization of the peripheral mesosubstituted pyridinium groups and the porphyrin ring on the SAP surface. Judging from these investigations, this spectral shift has been ascribed to the coplanarization of the peripheral mesosubstituted pyridinium groups and the porphyrin ring on the SAP surface. The absorption maximum for the MgTMPyP4+/SAP hybrid film showed further red shift compared to the MgTMPyP4+ on SAP surface. This indicates that co-planarization between the meso-substituted pyridinium groups and porphyrin ring is enhanced because porphyrin was sandwiched between clay nano-sheets.35 The absorption spectra of the MgTMPyP4+/SAP at various loading levels and the absorbance−adsorption density plot are shown in Figures S2 and S3, respectively. The shape of the absorption spectra did not depend on the loading level of the dyes, and the linearity of the absorbance–adsorption density plots was retained up to 32% versus CEC. It indicated that MgTMPyP4+ was intercalated as a monomer without J-type aggregate (head to tail type aggregate) formation because if MgTMPyP4+ aggregated, the absorption spectra of films would show drastic changes by change of concentration of dyes.


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2

Extinction coefficient / 108 cm2・mol-1

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449 nm 472 nm

1.8 1.6

499 nm

1.4 1.2 1 0.8 0.6 0.4 0.2 0 350

400

450

500

550

600

650

700

750

Wavelength / nm Figure 5. UV-Vis. absorption spectra of MgTMPyP4+ solution (without SAP, solid line), MgTMPyP4+ on exfoliated SAP surfaces (broken line), and MgTMPyP4+/SAP hybrid film (dotted line)

The Fluorescence spectra of MgTMPyP4+ in solution, adsorbed on SAP surfaces, and MgTMPyP4+/SAP hybrid film are shown in Figure 6. The fluorescence spectra of MgTMPyP4+ adsorbed on SAP and MgTMPyP4+/SAP film also showed the red shift, which is similar to the absorption spectral shift. It would also be ascribed to the coplanarization of the porphyrin ring and meso-substituted pyridinium groups. In addition, the fluorescence spectral shapes of MgTMPyP4+ in each matrix are almost the same. This suggested that new luminescence spaces, such as excimer and J-dimer (head-to-tail type dimer), were not formed in the interlayer space of SAP sheets.


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Normalized fluorescence intensity / a. u.

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1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 600

650

700

750

800

850

Wavelength / nm Figure 6. Normalized fluorescence spectra of MgTMPyP4+ solution (without SAP, solid line), MgTMPyP4+ on exfoliated SAP surfaces (broken line), and MgTMPyP4+/SAP hybrid film (dotted line)

The typical AFM image of MgTMPyP4+/SAP hybrid film was shown in Figure S4. Similar structure was observed in several areas in the hybrid film. Judging from this result, surface structure of the hybrid film must be uniform. The AFM image of SAP only film was previously reported, and then obtained images are almost same as SAP only film.39 It indicated that the structure of the hybrid film was not changed by intercalation of MgTMPyP4+.

Chromism of the MgTMPyP4+/SAP Hybrid Film Depending on Relative Humidity


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As described above, the basal space of SAP depended on the loaded amount of MgTMPyP4+. Thus, the loaded amount of MgTMPyP4+ was unified in this experiment (18% vs. CEC of SAP). The photographs of the MgTMPyP4+/SAP hybrid film under various relative humidity conditions are shown in Figure 7. The color of the hybrid film was pink under RH5%. Then, the color was continuously changed to orange and green with increasing RH. The adsorption spectra of the hybrid film under various RH conditions are shown in Figure 8. It should be noted that the expected mechanism for color change, which is a conformational change of the molecule induced by the expansion/contraction of the interlayer space of nano-sheets, is unique, because the typical chromism of the dyes occurred by cleavage of the bond in the chromophore, redox of the dye, and change of the structural assemblies (e.g., aggregate). 42-44

Figure 7. Photograph of MgTMPyP4+/SAP hybrid film under various RH conditions.

The absorption spectra of the MgTMPyP4+/SAP hybrid film was gradually shifted to a shorter wavelength with increasing RH, and then, the maximum absorption wavelength of the MgTMPyP4+/SAP hybrid film was changed from 512 nm (RH5%) to 500 nm (RH90%), as shown in Figure 8. The fluorescence spectra of the hybrid film under various relative humidity


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values are shown in Figure S5. The fluorescence spectra also shifted to a shorter wavelength with increasing RH. This chromic behavior is ascribed to a change of the co-planarization degree of the porphyrin ring and meso-substituted pyridinium groups. As described above, the interlayer space of the clay minerals depends on the relative humidity value, and the interlayer space under high relative humidity condition is expanded compared to the interlayer space under low relative humidity. Thus, coplanarization of MgTMPyP4+ under low relative humidity conditions will be stronger than that under a high relative humidity, and then, the absorption spectra of the MgTMPyP4+/SAP film under a low relative humidity condition would be shifted to a longer wavelength. To confirm the reversibility of the hybrid film, medium of MgTMPyP4+/SAP hybrid film was repeatedly changed from vacuum to wet air (RH30%) and absorption spectra of MgTMPyP4+/SAP hybrid film were measured. The λmax of the film against repeated time were shown in Figure S6-(a). The λmax of hybrid film were reversibly changed, and λmax of hybrid film under vacuum and under wet air were almost kept, respectively. The absorption spectra of hybrid film when medium of the film were repeatedly changed from vacuum and wet air (RH30%) were shown in Figure S6-(b). The spectral shape of the film under vacuum and wet air were almost same, respectively, and then this reversible spectral change was almost kept till 100 times. These results suggested that absorption spectra of hybrid film could be repeatedly changed, and then this reversibility was kept till 100 times. The responsiveness of the film was investigated by time-resolved absorption spectral change induced by the change of the medium from vacuum to wet air (RH30%). The absorption spectral changes by introducing the wet air between 0 msec. - 300 msec. and 300 msec. - 5000 msec. were shown in Figure S7-(a) and (b), respectively. λmax of MgTMPyP4+/SAP hybrid film was shifted to longer wavelength under vacuum condition, and λmax was gradually shifted to shorter wavelength by introducing the wet


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air. Spectral shift was observed in 30 msec. from introducing the wet air as shown in Figure S7(a), and then spectral shift was finished till 5000 msec. after introducing the air as shown in Figure S7-(b). This result indicated that spectral shift was quickly occurred and will be completed till ca. 5 sec.

1

1

0.8

Normalized Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.8 0.4 0.2

0.6 0 460

480

500

520

540

0.4

0.2

0 400

450

500

550

600

650

700

Wavelength / nm

Figure 8. Normalized absorption spectra of MgTMPyP4+/SAP hybrid film at RH5% (black solid line), RH15.0% (black dotted line), RH36% (black broken line), RH68% (gray solid line), and RH90% (gray dotted line). Inset: detail of the Soret band (460 nm–540 nm).


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To confirm the above-assumed mechanism of chromic behavior of this hybrid film, XRD measurements of this hybrid film under various RH conditions were carried out. The XRD pattern of the MgTMPyP4+ hybrid film under various RH values is shown in Figure S8. The broaden peak of 001 plane was observed between 6.0°–6.2°. The top peak of the pattern was slightly shifted to a high diffraction angle with increasing RH. The interlayer distance and λmax of the hybrid film under various RH values are summarized in Figure 9.

Figure 9. Interlayer distance (open circle) and λmax (closed circle) of the hybrid film under various RH.

The λmax of the hybrid film was gradually shifted to a shorter wavelength with increased RH, as described above. In addition, the basal space of the clay nano-sheet was extended with


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increasing RH. This result supported that the change of the interlayer spaces induced the color change and spectral shift of the MgTMPyP4+/SAP hybrid film.

CONCLUSION A transparent hybrid film composed of cationic magnesium porphyrin and clay mineral was developed, and its chromic behavior depending on relative humidity was demonstrated. The color of this hybrid film was changed from pink to green with increasing RH, and the absorption maximum owing to the Soret band of intercalated MgTMPyP4+ was shifted from 511 nm to 500 nm with increasing RH. The interlayer space of the MgTMPyP4+/SSA hybrid film was increased with increasing RH, and then, the chromic behavior of the hybrid film occurred because of the planation of MgTMPyP4+ in the SAP layer.

ASSOCIATED CONTENT UV-Vis absorption spectra of MgTMPyP4+ aqueous solution and MgTMPyP4+/SAP hybrid film after 0 day, 1 day and 5 days, UV-Vis. absorption spectra of the MgTMPyP4+/SAP hybrid film at various loading levels (in water), absorbance−adsorption density plot of MgTMPyP4+/SAP film at various dye loadings, AFM image of MgTMPyP4+/SAP film surface, fluorescence spectra of MgTMPyP4+/SAP hybrid film under various relative humidity conditions, λ max of MgTMPyP4+/SAP hybrid film under vacuum or wet air (RH30%) and


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absorption spectra of MgTMPyP4+/SAP hybrid film under vacuum condition, Time resolved absorption spectral change of MgTMPyP4+/SAP hybrid films by introducing wet air to under vacuum condition, XRD pattern of MgTMyP4+/SAP hybrid film under various RH conditions. These materials are available free of charge via Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail address: [email protected]; Tel: +81 852 329 843

Notes The authors declare no competing financial interest.

ABBREVIATIONS MgTMPyP4+, magnesium tetrakis(1-methyl-4-pyridiniumyl)porphyrin; SAP, saponite.

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Optical Humidity Sensing Using Transparent Hybrid Film Composed of

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