Communication Cite This: Inorg. Chem. 2018, 57, 3671−3674
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Negative Photochromism Based on Molecular Diffusion between Hydrophilic and Hydrophobic Particles in the Solid State Tetsuo Yamaguchi,† Ayan Maity,‡ Vivek Polshettiwar,‡ and Makoto Ogawa*,† †
School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology, 555 Moo 1 Payupnai, Wangchan, Rayong 21210, Thailand ‡ Division of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India S Supporting Information *
not efficient because of the low population of merocyanine (MC). The encapsulation of photochromic molecules in transparent solids has been reported so far to control the reaction as well as to construct novel photoresponsive supramolecular systems.21 During the study on the preparation and properties of hybrids composed of photochromic molecules and nanoporous silicas,21,22 it was found that the photochemically formed MC was adsorbed onto mesoporous silica by UV irradiation from the toluene solution of a SP containing mesoporous silica (“photoinduced adsorption”).23,24 In organic solvents, the photogenerated MC from SP by UV irradiation is not stable, giving a fast backward reaction, which makes it difficult to isolate the photogenerated MC. In the presence of hydrophilic particles (silica) in an organic solvent (toluene), the photogenerated MC was adsorbed on the silica particles prior to the backward reaction, thanks to the interactions between MC and the pore surface of the silica particles and the fast transfer from the solvent phase to the silica surface, resulting in “photoinduced adsorption”. “Photoinduced adsorption” is a unique phenomenon with possible controlled kinetics and bistability, so that the systematic study on the phenomenon using various nanoporous solids (hosts of photogenerated MC) is worth investigating. Herein, a photochemical reaction was used to prepare a hybrid composed of porous silica and photochemically formed MC, and the resulting hybrid exhibited unique negative photochromic behavior in the solid state. In addition, a new concept of solid-state materials design is where two materials (organophilic clay and nanoporous silica) are mixed in the sold state to construct a functional hybrid system, where mesoporous silica and organophilic clay accommodate colored (MC) and colorless (SP) isomers, respectively. The novel hybrid system was successfully constructed by the solid-state mixing of the two components together at room temperature. Excellent reversible negative photochromism was achieved by the novel materials design. MC was introduced into a dendritic fibrous nanosilica (DFNS)25,26 by the photoinduced adsorption from a toluene solution of SP. Transmission electron microscopy (TEM) images and a N2 adsorption study of DFNS with an average diameter of 600 nm are shown in Figures S3 and S4. The DFNS/MC hybrid was prepared from a toluene solution (20 mL) of 1,3,3-trimethylindolino-6′-nitro-
ABSTRACT: A colored hybrid based on a merocyanine adsorbed in a nanoporous-silica-composed dendritic fibrous silica was prepared by adsorption onto the nanoporous silica from a spiropyran solution during UV irradiation (photoinduced adsorption). The obtained red hybrid thus exhibited negative photochromism by visiblelight irradiation. The hybrid was further combined with an organophilic clay by a solid-state mixing without using solvent to achieve excellent reversibility of the color change, which was thought to be achieved by molecular diffusion through the two materials, where nanoporous silica and organophilic clay accommodated the colored (merocyanine) and colorless (photogenerated spiropyran) isomers, respectively.
P
hotochromic molecules have received attention for a wide range of future applications including optical recording, sensing and smart windows,1,2 and the trigger units for photoresponsive supramolecular systems. Improvement of the efficiency of the photochromic reaction and the accommodation of the photochromic reaction in the solid state are important for the practical devices. Negative photochromism, which is the photoinduced decoloration and thermal coloration, is a reverse phenomenon of normal photochromism. Negative photochromic materials are expected to show an effective conversion efficiency thanks to the absence of reabsorption of excitation visible light by the photogenerated colorless isomer,3−5 while that of the normal photochromic molecules is not high because of reabsorption of excitation light by the colored isomers. Negative photochromism of such molecules as dihydropyrenes, stenhouse salts, and spiropyrans in solution has been reported,6−12 while negative photochromism in the solid state has scarcely been reported.13−16 Negative photochromism of the crystals of N-salicylidenepyrenes and biindenylidenediones was reported from the colored isomer (the initial state of negative photochromism), which was stabilized by such intermolecular interactions as hydrogen bond and π−π stacking between adjacent molecules.17−19 Even at the photostationary state, the material was not colorless, which was explained by the dye aggregates, which absorb visible light. Negative photochromism has been observed for a spiropyran (SP) in solid doped in poly(2-ethylhexyl methacrylate-covinylpyridine)13 and a SP covalently immobilized to silica nanoparticles.20 In these cases, negative photochromism was © 2018 American Chemical Society
Received: December 13, 2017 Published: March 13, 2018 3671
DOI: 10.1021/acs.inorgchem.7b03132 Inorg. Chem. 2018, 57, 3671−3674
Communication
Inorganic Chemistry
The UV−vis absorption spectra of both DFNS/MC(High) and DFNS/MC(Low) had two absorption bands at 369 and 502 nm, which decreased by visible-light irradiation. The absorbance at 502 nm decreased from 0.83 to 0.25 for DFNS/ MC(High) and from 0.19 to 0.04 for DFNS/MC(Low) by visible-light irradiation (Figure 3a,c). Thermal coloration of
benzopyrylospiran (SP) containing DFNS (40 mg). The concentrations of SP in the initial suspension were 7.45 × 10−4 and 6.2 × 10−6 M. After the suspensions were mixed for 1 h at room temperature, UV light (365 nm; Ushio SPL-2 with 95 mW cm−2) was irradiated into the colorless suspensions for 2 min to see color development to red. After UV irradiation, the suspensions were filtered immediately in the dark to obtain red solids, which were subsequently dried under a reduced pressure overnight in the dark. From the changes in the concentration of SP remaining in the filtrate, the amount of adsorbed MC was derived as 14.4 and 0.68 mg of MC per g of DFNS for the samples prepared from 7.45 × 10−4 and 6.20 × 10−6 M SP suspensions, respectively (Figure S1). DFNS/MC hybrids obtained from 7.45 × 10−4 and 6.20 × 10−6 M SP suspensions are named DFNS/MC(High) and DFNS/MC(Low), respectively. The photographs of DFNS/MC(High) and DFNS/MC(Low) powders are shown in Figure 1. The red color was stable for more than 1 week when the samples were stored in the dark at room temperature (Figure 2).
Figure 3. (a) UV−vis absorption spectra and (b) time profiles of the absorbance of DFNS/MC(High) with 14.4 mg g−1 of MC and (c and d) those of DFNS/MC(Low) with 0.68 mg g−1 of MC at room temperature. Visible-light irradiation was done for 2 min.
both DFNS/MC(High) and DFNS/MC(Low) terminated in 10 h with the isosbestic points at 290 and 287 nm. The absorbances at 346, 369, and 502 nm were plotted to follow the thermal coloration process (Figure 3b,d). The time profile at 346 nm was well-fitted by the single-exponential equation (1)
Figure 1. Photographs of DFNS/MC(High) (a) before and (b) after visible-light irradiation for 2 min and DFNS/MC(Low) (c) before and (d) after visible-light irradiation for 2 min.
Abs(t ) = A exp( −kt )
(1)
Here, A and k are the amplitude and rate constant of thermal coloration and t is the reaction time. The rate constants were estimated to be 0.39 h−1 for DFNS/MS(High) and 0.35 h−1 for DFNS/MC(Low). The time profiles at 369 and 502 nm did not fit eq 1, while the double-exponential equation (2) explained them. Abs(t ) = A1 exp( −k1t ) + A 2 exp( −k 2t )
(2)
The rate constants for the two absorptions (369 and 502 nm) were similar, as summarized in Table 1. The doubleexponential fitting revealed that thermal coloration observed for DFNS/MCs included two components. The ratios of the
Figure 2. UV−vis absorption spectra of DFNS/MC(Low) after the sample preparation (blue line) and after 1 week (red line) in the dark at room temperature.
Table 1. Rate Constants of the Thermal Back-Reaction of DFNS/MCsa
Upon visible-light irradiation (100 W xenon lamp, ABET Technologies Sunlite solar simulator) to DFNS/MC(High) (Figure 1a), the red color faded, while the red color remained to some extent under the present irradiation conditions (Figure 1b). On the other hand, DFNS/MC(Low) (pink, as shown in Figure 1c) showed decoloration to give a white (colorless) product by 2 min visible-light irradiation (Figure 1d). The color of the DFNS/MCs returned to red by storing the samples in the dark overnight at room temperature. Thus, it was confirmed that zwitterionic MC was stabilized in a hydrophilic pore of DFNS compared to hydrophobic SP.
346 nm sample DFNS/ MC(High) DFNS/ MC(Low) a
3672
k/h
−1
0.39 0.35
369 nm −1
502 nm −1
k1/h
k2/h
0.41 (0.41) 0.37 (0.13)
2.0 (0.062) 1.8 (0.012)
−1
k2/h−1
0.39 (0.61) 0.37 (0.25)
2.1 (0.74) 2.0 (0.11)
k1/h
The numbers inside the brackets are amplitudes estimated from eq 2. DOI: 10.1021/acs.inorgchem.7b03132 Inorg. Chem. 2018, 57, 3671−3674
Communication
Inorganic Chemistry
on the hydrophilic surface of DFNS compared with that in organophilic clay, so that photochemically formed SP diffused into the organophilic clay domain through interparticle migration. The intercalation of hydrophobic molecules into organophilic clay have been reported by solid-state reaction with manual mixing.32,33 The interparticle migration observed in the present study will be controlled by constructing smooth interfaces between hydrophobic (organophilic clay) and hydrophilic (DFNS) domains and by mixing during irradiation. Thus, the efficiency of the decoloration/coloration cycle was significantly modified by combining hydrophilic DFNS for MC and hydrophobic organophilic clay for SP to achieve bistability in the solid state. This is a new phenomenon in which the diffusion of molecules between particles was induced by photoirradiation, which opened up a new opportunity for solid-state photochemistry. The photochromic reaction of SP in the presence of varied hydrophilic and hydrophobic materials under several different states including films is being investigated in our laboratory, and the results will be reported subsequently. In conclusion, a red hybrid based on MC adsorbed into a nanoporous-silica-composed dendritic fibrous silica was prepared by adsorption onto nanoporous silica from a SP solution during UV irradiation (photoinduced adsorption). The hybrid exhibited unique negative photochromic behavior in the solid state from red to colorless by visible-light irradiation. An organophilic clay was mixed with the silica/MC hybrids in order to modify the conversion efficiency. The mixed solid system was found to show more efficient photodecoloration/ thermal coloration cycles. The organophilic clay played a role in accommodating photogenerated SP. Diffusion of the photochromic molecules between particles was smooth, which enabled negative photochromic reaction within a few minutes in the solid state.
amplitude of the slow and fast components at 369 nm (A1/A2) were 6.6/1 and 10.8/1 for DFNS/MC(High) and DFNS/ MC(Low), respectively. Therefore, the contribution of the slow and/or fast components for thermal coloration depends on the initial concentration of MC. Taking the possible aggregation, which was reported for the high-concentration solution,27,28 of SP and MC into account, the organophilic SP was thought to be coadsorbed with MC in the hydrophilic mesopore by forming aggregate, which limited the conversion of MC to SP. Complete decoloration was possible for DFNS/MC(Low) because of the low concentration of MC corresponding to the initial pale color (Figure 1c,d); however, DFNS/MC(High) is still colored at the photostationary state, as shown in Figure 1b. It was thought that the lack of sites to accept photochemically formed SP limited the reaction. In order to accept (and stabilize) larger amounts of the photochemically formed SP for complete decoloration, an organophilic clay, dioctadecyldimethylammonium-exchanged montmorillonite,29−31 was mixed with DFNS/ MC(High). The solid-state mixing of organophilic clay (50 mg) with DFNS/MC(High) (50 mg) in an agate mortar and pestle manually for 10 min in the dark gave a red solid (no change in the color was seen during the solid-state reaction). The UV−vis absorption spectra and photographs of the mixture of organophilic clay and DFNS/MC(High) (organophilic clay− DFNS/MC) are shown in Figure 4. By visible-light irradiation
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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/acs.inorgchem.7b03132. Experimental details, UV−vis absorption spectra, plots of photoinduced decoloration/thermal coloration cycles, TEM images, N2 adsorption, lamp intensity, and photographs of the photoinduced decoloration/thermal coloration cycles (PDF)
Figure 4. (a) UV−vis absorption spectra, (b) time profiles, and photographs (c) before and (d) after visible-light irradiation of organophilic clay−DFNS/MC at room temperature. Visible-light irradiation was done for 2 min.
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for 2 min, the absorbance at 507 nm changed from 0.58 to 0.09 and the color of the mixture changed to colorless. The time profiles of the absorbance at 400 and 507 nm fitted a singleexponential equation (Figure 4b). The rate constants were determined to be 0.40 and 0.39 h−1 for absorption at 400 and 507 nm, respectively. These values are consistent with the rate constant of the slow component of the coloration of DFNS/ MC(High) without organophilic clay, as shown in Table 1. The fast reaction was not observed for organophilic clay−DFNS/ MC. It is considered that the photogenerated SP moved into organophilic clay to suppress the fast back-reaction from the aggregate of SP to MC. The photoinduced adsorption was not observed when SP was irradiated in the presence of organophilic clay. The adsorption of SP on organophilic clay in the dark was not efficient. The hydrophobic SP is not stable
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tetsuo Yamaguchi: 0000-0002-6697-2916 Vivek Polshettiwar: 0000-0003-1375-9668 Makoto Ogawa: 0000-0002-3781-2016 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a postdoctoral fellowship from Vidyasirimedhi Institute of Science and Technology. 3673
DOI: 10.1021/acs.inorgchem.7b03132 Inorg. Chem. 2018, 57, 3671−3674
Communication
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.7b03132 Inorg. Chem. 2018, 57, 3671−3674
