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Unique Solvatochromism of a Membrane Composed of a Cationic Porphyrin-Clay Complex Shinsuke Takagi,*,†,‡ Tetsuya Shimada,† Dai Masui,† Hiroshi Tachibana,† Yohei Ishida,† Donald A. Tryk,† and Haruo Inoue*,† † Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397, Japan, and ‡PRESTO (Precursory Research for Embryonic Science and Technology), Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan
Received February 24, 2010. Revised Manuscript Received March 9, 2010 A novel optically transparent membrane composed of porphyrin-clay mineral complexes was developed. Reversible solvatochromism behavior of the membrane was successfully observed, due to an orientation change of porphyrin in the clay interlayer space. The λmax value of porphyrin was 423 nm in acetone, while it was 464 nm in hexane. The color of the membrane changed from pink to green through to brown, when Sn porphyrin was used. The mechanism for solvatochromism in the present system is very unique compared to those for conventionally reported materials.
The reversible color change of materials depending on the surrounding circumstances is called chromism.1-4 In most cases, chromism is based on a change in the electronic states of molecules. In the present paper, the solvatochromism5,6 behavior in transparent membranes composed of clay mineral-porphyrin complexes is described. Dicationic porphyrin (cis-bis(N-methylpyridinium-4-yl)diphenylporphyrin) (cis-DPyP) and synthetic saponite (Sumecton SA (SSA)) were used as guest dye and host material, respectively (Figure 1). The structural formula for SSA is [(Si7.20Al0.80)(Mg5.97Al0.03)O20(OH)4]-0.77(Na0.49Mg0.14)þ0.77. Since the synthetic clay mineral is pure, colorless, and well characterized, its application to the inorganic/organic hybrid materials attracts much attention, especially from the viewpoint of photochemistry.7-15 The typical transparent clay mineral-porphyrin membrane was prepared as follows. The aqueous colloidal solution of SSA (1.0 g L-1, 150 μL) and cis-DPyP (1.0 10-4 M in water, 37.5 μL (5.0% vs CEC (cation exchange capacity) of the clay)) was added to water (3 mL) with stirring. Then dioxane (68 μL) was added to the clay-porphyrin solution. The obtained solution containing the *To whom correspondence should be addressed. E-mail: takagi-shinsuke@ tmu.ac.jp. (1) Crano, J. C.; Guglielmetti, R., Eds. Organic Photochromic and Thermochromic Compounds; Plenum: New York, 1999; Vols. 1 and 2, and references therein. (2) Bouas-Laurent, H.; Durr, H. Pure Appl. Chem. 2001, 73, 639. (3) Matsuda, K.; Irie, M. J. Photochem. Photobiol. C 2004, 5, 169. (4) Irie, M. Chem. Rev. 2000, 100, 1685. (5) Wetzler, D. E.; Chesta, C.; Fernandez-Prini, R.; Aramendia, P. F. Pure Appl. Chem. 2001, 73, 405. (6) Reichardt, C. Chem. Rev. 1994, 94, 2319. (7) Lucia, L. A.; Yui, T.; Sasai, R.; Takagi, S.; Takagi, K.; Yoshida, H.; Whitten, D. G.; Inoue, H. J. Phys. Chem. B. 2003, 107, 3789. (8) Takagi, S.; Shimada, T.; Eguchi, M.; Yui, T.; Yoshida, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 2265. (9) Takagi, S.; Tryk, D. A.; Inoue, H. J. Phys. Chem. B 2002, 106, 5455. (10) Eguchi, M.; Takagi, S.; Tachibana, H.; Inoue, H. J. Phys. Chem. Solids 2004, 65, 403. (11) Bujdak, J.; Iyi, N.; Kaneko, Y.; Sasai, R. Clay Miner. 2003, 38, 561. (12) Takagi, S.; Shimada, T.; Yui, T.; Inoue, H. Chem. Lett. 2001, 30, 128. (13) Kuykendall, V. G.; Thomas, J. K. Langmuir 1990, 6, 1350. (14) Yui, T.; Hirano, T.; Okazaki, K.; Inoue, H.; Torimoto, T.; Takagi, K. J. Photochem. Photobiol., A 2009, 207, 135. (15) Sato, H.; Tamura, K.; Taniguchi, M.; Yamagishi, A. Chem. Lett. 2009, 38, 14.
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clay-mineral-porphyrin complex was filtrated with a poly(tetrafluoroethylene) (PTFE) membrane filter with 0.1 μm pore size. The porphyrin or clay mineral-porphyrin complex was not detected in the filtrate judging from the absorption measurement. The residual membrane on the PTFE filter could be transferred to the cover glass. The membrane exhibits high transparency in the visible and ultraviolet regions. The uniformity of the membrane and small particle size of SSA (20-50 nm) could realize such a high transparency. The absorption spectra of the membrane and the solution were measured in the cell from the x and y directions as shown in Figure 2b. The solvatochromism behavior was analyzed by x direction absorbance. The dissolution of the membrane was estimated by y direction absorbance. The absorption spectra of the membrane were measured in the following conditions: (i) from the x direction under air (line a in Figure 2a), (ii) from the x and y directions in N,N-dimethylformamide (DMF) (lines b and c), and (iii) from the x direction after removal of DMF and drying (line d). The absorption spectrum of porphyrin in clay membrane under air (line a) is shifted to longer wavelength compared to that in solution without clay. The X-ray diffraction (XRD) pattern of the clay mineral-porphyrin membrane covered with DMF and under air is shown in Figure 2c. As can be seen, the reversible absorption spectral change accompanied with a XRD pattern change was observed with the change of surrounding solvent. Such a bathochromic shift of porphyrin on the clay has been reported in the case of H2TMPyP4þ (tetrakis(1-methyl-pyridinium-4-yl) porphyrin) with clays.8-10,12,13 These reports suggested that enhanced π-conjugation and the electron withdrawing effect of the pyridinium group, due to flattening of the TMPyP4þ on the clay, induce the spectral change. By flattening, one means that the four cationic tetramethylpyridiniumyl moieties become parallel to the porphyrin ring. While λmax of cis-DPyP in aqueous solution and that on exfoliated clay are 421 and 455 nm, respectively, that in the clay membrane is 464 nm. Since the porphyrin molecule is intercalated and sandwiched between clay sheets, the porphyrin molecule should suffer higher coplanarity in the membrane. The molecular orientation of adsorbed cis-DPyP on the clay surface has been confirmed to be parallel with respect to the clay surface by
Published on Web 03/15/2010
DOI: 10.1021/la1007928
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Figure 1. Structures of Sumecton SA and cis-DPyP.
Figure 2. (a) Absorption spectra of membrane from the x direction under air (line a), from the x and y directions in DMF (lines b and c), and from the x direction after removal of DMF and drying (line d). (b) Setup for absorption measurement. (c) XRD pattern of clay-porphyrin membrane covered with DMF and under air. Table 1. λmax and Clearance Space of Clay-Porphyrin Membranesa λmax of porphyrin in the membrane (nm)
solvent
Figure 3. Orientation change of the porphyrin molecule on the clay surface.
dichroic measurement using a waveguide system.16,17 The λmax value of cis-DPyP in the membrane was 424 nm in DMF, which is a 40 nm shorter wavelength compared to that under air. After removal of DMF and drying, the absorption spectra recovered reversibly (lines a and d). Judging from the absorption spectra from the y direction, desorption of porphyrin did not take place. Thus, the reason for the spectral change upon addition of DMF is supposed to be due to an orientation angle change of cis-DPyP on the clay surface (Figure 3). The change of the parallel orientation to the nonparallel one with respect to the clay surface would allow the porphyrin molecule to relax to the state with a less flattened conformation. To confirm this hypothesis, various organic solvents were adopted to the cis-DPyP-clay mineral membrane. The λmax values of porphyrin in the membrane, in the solvent, and after removal of solvent are summarized in Table 1. Very interestingly, a large shift to shorter wavelength was observed in acetone, DMF, acetonitrile (MeCN), and dimethyl sulfoxide (DMSO), in which the clearance space of the interlayer was expanded by swelling of the solvent. In a previous paper, we reported that cis-DPyP changes its orientation on the exfoliated (16) Eguchi, M.; Takagi, S.; Inoue, H. Chem. Lett. 2006, 35, 14. (17) Eguchi, M.; Tachibana, H.; Takagi, S.; Tryk, D. A.; Inoue, H. Bull. Chem. Soc. Jpn. 2007, 80, 1350.
4640 DOI: 10.1021/la1007928
clearance space of the in the after removal interlayer on exposure solvent of solvent to solvent (nm)
(i)
acetone DMF MeCN DMSO pyridine
423 424 425 427 436
463 464 464 463 463
1.77 0.92 1.07 0.88 0.56
(ii)
dioxane THF
452 456
462 464
0.50 0.48
(iii) formamide 452 463 0.81 MeOH 454 464 1.07 EtOH 458 464 0.73 2-PrOH 458 464 0.53 water 461 464 0.93 dichloromethane 462 464 0.88 cyclohexane 462 464 0.42 hexane 464 464 0.46 under air 464 0.42 a The membrane is composed of Sumecton SA and cis-DPyP (5.0% vs CEC of the clay). DMF, N,N-dimethylformamide; MeCN, acetonitrile; DMSO, dimethyl sulfoxide; THF, tetrahydrofuran; MeOH, methanol; EtOH, ethanol; 2-PrOH, 2-propanol. These solvents were classified into three groups (i), (ii), and (iii), depending on the solvatochromism mechanism. Classification details are described in the text.
clay surface in DMF, DMSO, acetone, dioxane, THF, pyridine, and MeCN.16 We concluded that the solvents with low hydrogen bonding character tend to induce the orientation change of porphyrin on the clay surface. Other than dioxane and tetrahydrofuran (THF), the same solvents induced the absorption spectral change of porphyrin both on the clay surface and in Langmuir 2010, 26(7), 4639–4641
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Figure 4. Color change of Sn(IV)TMPyP4þ-clay-mineral membrane when covered with various solvents, and porphyrin orientations in the clay interlayer space.
the membrane. In the case of dioxane and THF, the swelling of the clay membrane was not observed in the XRD measurements of the membrane when covered with dioxane and THF. Although clay sheets are known to swell well in water, water induced a small absorption spectral shift (∼3 nm) in the clay mineral-porphyrin membrane. Thus, it is apparent that two factors are controlling at once to induce the orientation and spectral change of porphyrin in
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the membrane system. One is the swelling ability of the clay membrane, and the other is the orientation change ability of porphyrin on the exfoliated clay. In the Table 1, group (i) includes solvents where both factors are fulfilled; group (ii) includes solvents where only orientation change ability is fulfilled; group (iii) includes solvents where orientation change ability is not fulfilled. It is noteworthy that the clay-porphyrin membrane was very stable in all solvents used here, despite their swelling ability. The membrane is completely stable in water for over 300 days, while the membrane prepared by the same procedure without the cationic porphyrins was rather unstable. Obviously, the cationic porphyrin acts as an adhesive agent for the clay sheet membrane. In the case of cis-DPyP as a guest porphyrin, the color change of the membrane in various organic solvents was not visually so clear, while Sn(IV)TMPyP4þ afforded a more distinct color change of the membrane from green (in cyclohexane) to wine-red (in DMF) color, as shown in Figure 4. In conclusion, the very stable and optically transparent clay-porphyrin membrane was successfully prepared on the glass substrate for the first time. We observed solvatochromism behavior in a clay-porphyrin membrane based on the structure change of the complex. Acknowledgment. This work has been partly supported by a Grant-in-Aid for Precursory Research for Embryonic Science and Technology (PRESTO), from Japan Science and Technology Agency (JST).
DOI: 10.1021/la1007928
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