Langmuir 2007, 23, 9443-9446
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Inclusion Complex of Spironaphthoxazine with γ-Cyclodextrin and Its Photochromism Study Shu-Xiao Zhang, Mei-Gong Fan, Yuan-Yuan Liu, Ying Ma, Guang-Jin Zhang, and Jian-Nian Yao* Beijing National Laboratory of Molecular Sciences (BNLMS), Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, PR China, and Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing, 100080 PR China ReceiVed January 30, 2007. In Final Form: June 12, 2007 An environmentally benign transparent photochromic film, 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3′-[3H]naphtho[2,1-b][1,4]oxazine] (SPO) /γ-cyclodextrin (γ-CD), was prepared using the cast-coating method, and the nanocavity effect of γ-CD on the photochromism of SPO was studied. The film mainly consists of the inclusion complex SPO@γ-CD, which has been verified by EA, TGA, XRD, MS, and ICD to comprise a 1:1 host-guest stoichiometry. The film shows normal photochromism. The decoloration of photomerocyanine (PMC) fits biexponential decay: PMCs located in the cavity of γ-CD decay with a rate constant of 6.0 × 10-2 s-1, which is nearly one order faster than those PMCs outside of the cavity.
Introduction Photochromic molecules undergo a color change on irradiation, and the photoproduct can be reversed to the initial species either thermally or by subsequent irradiation with suitable wavelength of light.1 This interesting effect can be used in commercial applications such as in ophthalmic lenses and nonlinear device components, the development of optical waveguides and shutters, light modulators, optical storage media and delay generators, as well as other optical devices depending on the response time and other properties of the photochromic molecules. There are mainly two kinds of methods known to modulate the response time of photochromic molecules: one is chemical structural modulation and the other involves doping the photochromic molecules into appropriate matrices.2-4 For example, Evans et al. attached various flexible oligomers having low glass transition temperature to the photochromic dyes using living radical polymerization, which greatly increase their switching speeds in a rigid polymer matrix, whereas Song et al. synthesized a SPO derivative by covalently bonding a ferrocene moiety to the parent spironaphthoxazine through acetyl, which enabled the open form to be stable for at least 1 month when stored at room temperature in the dark.5 This property allows it to have potential application in optical information storage. In contrast, the method of doping is rather simple, economical, and therefore much more practicable. When certain block copolymers and silica mesostructures are used as the host material, the response time for the back-fading rate of photomerocyanine (PMC) is extremely fast (rate constant of k ) 0.2 s-1), equaling that in ethanoll,6,7 which might be applicable when a fast response is required, like in optical shutter. As for the matrix composition, it ranges from the purely organic matrices, such as poly(methyl methacrylate) (PMMA), to hybrid inorganic* To whom correspondence should be addressed. E-mail: jnyao@ iccas.ac.cn. (1) Evans, R. A.; Hanley, T. L.; Skidmore, M. A.; Davis, T. P.; Such, G. K.; Yee, L. H.; Ball, G. E.; Lewis, D. A. Nat. Mater. 2005, 4, 249-253. (2) Biteau, J.; Chaput, F.; Boilot, J.-P. J. Phys. Chem. 1996, 100, 9024-9031. (3) Ribot, F.; Lafuma, A.; Eychenne-Baron, C.; Sanchez, C. AdV. Mater. 2002, 14, 1496-1499. (4) Such, G. K.; Evans, R. A.; Davis, T. P. Macromolecules 2006, 39, 13911396. (5) Yuan, W. F.; Sun, L.; Tang, H. H.; Wen, Y. Q.; Jiang, G. Y.; Huang, W. H.; Jiang, L.; Song, Y. L.; Tian, H.; Zhu, D. B. AdV. Mater. 2005, 17, 156-160.
organic composites (ormosils), and finally to purely inorganic matrices, such as sol-gel derived silicates and various types of modified zeolite.6,8,9 Among various photochromic molecules, spirooxazines have attracted a great deal of attention due to their unusual fatigue resistance.10 Generally, the thermal back-fading speed of the colored forms of this family is faster in organic matrices than in inorganic ones. The photochromism and thermochromism of spirooxazines have also been studied in normal and reversed micelles,11 in the voids of molecular sieves,12-14 and as metal chelates in solution. On the other hand, cyclodextrin wins the favor of scientists due to the hydrophobic interior and hydrophilic exterior of its pocket, which not only possesses inclusion and recognition capabilities but also offers a unique opportunity for studying size controlled nanoenvironment effect on spectroscopy, kinetics, etc.15 Combining these two points, we studied the photochromism of SPO with γ-CD as host. Herein, we report for the first time to the best of our knowledge, the photochromic behavior of a good transparent photochromic film of SPO/γ-CD, and the inclusion complex of SPO@γ-CD is examined in detail. Surprisingly, we found that PMCs incorporated in the cavities of γ-CD decay nearly 1 order of magnitude faster than those not incorporated due to the hydrophobic nanocage effect. Experimental Section Materials. 1,3-Dihydro-1,3,3-trimethylspiro[2H-indole-2,3′-[3H]naphtho[2,1-b][1,4]oxazine] (SPO) was purchased from Aldrich and (6) Wirnsberger, G.; Scott, B. J.; Chmelka, B. F.; Stucky, G. D. AdV. Mater. 2000, 12, 1450-1454. (7) Schaudel, B.; Guermeur, C.; Sanchez, C.; Nakatani, K.; Delaire, J. A. J. Mater. Chem. 1997, 7, 61-65. (8) Casades, I.; Constantine, S.; Cardin, D.; Garcı´a, H.; Gilbert, A.; Ma´rquez, F. Tetrahedron 2000, 56, 6951-6956. (9) Casades, I.; A Ä lvaro, M.; Garcı´a, H.; Pillai, M. N. Eur. J. Org. Chem. 2002, 2074-2079. (10) Chu, N. Y. C. In Photochromism: Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier Press: Amsterdam, 1990; pp 493-500. (11) Favaro, G.; Ortica, F. J. Chem. Soc., Faraday Trans. 1995, 91, 40994103. (12) Kopelman, R. A.; Snyder, S. M.; Frank, N. L. J. Am. Chem. Soc. 2003, 125, 13684-13685. (13) Wo¨hrle, D.; Schulz-Ekloff, G. AdV. Mater. 1994, 6, 875-880. (14) Chemyshev, A. V.; Voloshin, N. A.; Raskita, I. M.; Metelitsa, A. V.; Minkin, V. I. J. Photochem. Photobiol. A: Chem. 2006, 184, 289-297. (15) Douhal, A. Chem. ReV. 2004, 104, 1955-1976.
10.1021/la700252u CCC: $37.00 © 2007 American Chemical Society Published on Web 07/27/2007
9444 Langmuir, Vol. 23, No. 18, 2007 used as received. Polyvinyl alcohol (PVA; Mw ) 72 000) was obtained from Fluka and used without further purification. γ-Cyclodextrin (γ-CD) was purchased from Wacker Chemical Corp. (USA) and recrystallized in water before use. 1-Adamantanol (Ada) was obtained from F&F Chemical Industrial Corp. (China) and used as received. Preparation of Inclusion Complex.16 A clear solution of γ-CD (1.297 g, 1 mmol) in water (50 mL) and ethanol (5 mL) was combined with a solution of SPO (32.8 mg, 0.1 mmoL) in ethanol (10 mL) under vigorous stirring. Instantly, a precipitate was formed. After 12 h of stirring at room temperature, the mixture was kept at 4 °C for 20 h. The precipitate obtained was filtered out, washed three times with 5 mL of cold water-ethanol (40% v/v), and then dried at room temperature in vacuo in the dark overnight to afford a light blue complex. The sample, denoted as SPO@γ-CD, was used for thermogravimetric analysis (TGA), X -ray diffraction (XRD), mass spectrometry (MS), and elemental analysis (EA). Preparation of the Film by the Cast-Coating Method. In a typical preparation, a solution (250 µL, 1 × 10-2 M) of SPO in ethanol was injected into 5 mL of γ-CD (1 × 10-2 M) or PVA (5% w/w) aqueous solution at 30 °C under sonication for 10 min. After equilibrium at room temperature for about 6 h, the mixture was deposited onto a quartz plate and dried at room temperature in vacuo overnight. Finally, a colorless film of about 5 µm thickness was obtained when γ-CD was used as host (denoted as SPO/γ-CD), whereas a light blue film of about 10 µm thickness was obtained when PVA was employed as host (denoted as SPO/PVA). These samples were used for induced circular dichroism (ICD) and absorption spectra detection. Apparatus and Measurement Conditions of Absorption and ICD Spectra. Absorption and ICD spectra of the sample film were recorded on a Lambda 35 double-beam spectrophotometer and a JASCO J-810 CD spectrophotometer, respectively. The sample film was irradiated for 15 s with a 500 W high-pressure mercury lamp to produce the colored PMC form, and its optical density change with time at the absorption maximum wavelength was recorded. In the process of measuring the ICD spectra, the sample was placed perpendicular to the light path and rotated within the film plane.
Results and Discussion As is well-known, both SPO and spiropyran are photochromic compounds. However, there is controversy about whether spiropyran can form an inclusion complex with γ-CD17 or not,18 due largely to lack of proof. With this in mind, we performed a large volume of work on characterizing the inclusion complex of SPO with γ-CD. PVA was used as a reference control owing to its well-known qualities for films. Although glucose has similar chemical components to cyclodextrin, it is difficult to form a good film. Additionally, glucose is rather sensitive to moisture, which has a serious effect on the decay rate of PMC. Characterization of the SPO@γ-CD Complex. Generally, when guest molecules are incorporated in the CD cavity, their melting, boiling, and sublimation points will change. The results of thermogravimetric analysis of SPO, γ-CD, the physical mixture of SPO and γ-CD (1:1, mol/mol), and the inclusion complex are all shown in Figure 1. One can see that the curve of the inclusion complex is obviously different from that of the physical mixture which is almost simple addition of the host and guest materials. The step from room temperature up to about 130 °C is assigned to the removal of the water molecules located in the γ-CD cavities and in the interstices between the macrocycles. The corresponding weight loss is 4.13% for the inclusion complex, which is in good agreement with the elemental analysis result and indicates that the approximate number of water molecules per γ-CD molecule (16) Krois, D.; Brinker, U. H. J. Am. Chem. Soc. 1998, 120, 11627-11632. (17) Tamaki, T.; Sakuragi, M.; Ichimura, K.; Aoki, K.; Arima, I. Polym. Bull. 1990, 24, 559-564. (18) Zhou, J. W.; Sui, Q.; Huang, B. H. J. Photochem. Photobiol. A: Chem. 1998, 117, 129-136.
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Figure 1. TGA curves for SPO (dashed gray line), γ-CD (dashed black line), the (1:1) SPO-γ-CD mixture (solid gray line) and the SPO@γ-CD inclusion complex (solid black line).
is four. For comparison, a 6.77% weight loss occurred for γ-CD, which corresponds to about 5 water molecules per γ-CD molecule. The reduction in the number of water molecules per γ-CD molecule in the inclusion complex is consistent with a partial occupation of the γ-CD cavities by the hydrophobic SPO guest. Although pure SPO starts to melt and decompose at about 180 °C, the corresponding point of the complex is 230 °C accompanying the enhanced stability of γ-CD. The 50 °C high indicates that SPO is more stable after being incorporated in the cavity of γ-CD, which is further evidence for the existence of a significant host-guest interaction.19 Elemental analysis data combined with TGA give the formula of the complex as (C48H80O40)‚(C22H20N2O)‚4H2O (1697.5). Calcd: C 49.53, H 6.41, N 1.65. Found: C 48.34, H 6.38, N 1.71. Electrospray ionization mass spectrometry (ESI-MS) has become an increasingly interesting tool because of two aspects. On the one hand, MS is more sensitive compared with other analysis methods and relatively simple when preparing samples for analysis. On the other hand, ESI is the softest technique for ionization; hence, no fragments are produced, which is very important for the examination of noncovalent complexes. Therefore ESI-MS provides molecular weight data that allow one to determine stoichiometry.20 Since the SPO@γ-CD complex shows limited solubility in water, a methanol/water ) 1/2 (v/v) mixture was applied as solvent for the MS experiment, which resulted in part dissociation of the complex inevitably. From Figure 2, the peaks observed at m/z 1320 and 1647 correspond to [γ-CD+Na]+ and [γ-CD+SPO+Na]+, respectively. Although [γ-CD+SPO+Na]+ is rather low due to the factors mentioned above, the MS data revealed apparently that the ratio of host to gust is 1:1 for the inclusion complex. X-ray diffraction (XRD) studies allow the identification of the true inclusion complexes of cyclodextrins, mainly based on the empirical evidence that the powder XRD patterns of these complexes should be clearly distinct from those obtained by the superimposition of the diffractograms of each component.19 Figure 3 shows the XRD patterns for SPO, γ-CD, their physical mixture, and the inclusion complex. It is clear that the diffractogram of the inclusion complex is different from the pattern of the physical mixture, which is only the superposition of the individual patterns of SPO and γ-CD. A comparison of this pattern with those for SPO, the parent γ-CD, and the physical mixture of SPO and γ-CD indicates that a true inclusion complex has been obtained. (19) Braga, S. S.; F. Paz, A. A.; Pillinger, M.; Seixas, J. D.; Roma˜o, C. C.; Gonc¸ alves, I. S. Eur. J. Inorg. Chem. 2006, 1662-1669. (20) Yang, G.-F.; Wang, H.-B.; Yang, W.-C.; Gao, D. Q.; Zhan, C.-G. J. Phys. Chem. B 2006, 110, 7044-7048.
Spironaphthoxazine with γ-Cyclodextrin
Figure 2. ESI-MS of the SPO@γ-CD inclusion complex.
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Figure 4. ICD spectra of the SPO/γ-CD film and the (SPO + Ada)/ γ-CD film. Scheme 1
Figure 3. Powder XRD patterns of (a) SPO, (b) γ-CD, (c) the (1:1) SPO-γ-CD mixture, and (d) the SPO@γ-CD inclusion complex.
Induced Circular Dichroism (ICD) Studies of the SPO/ γ-CD Film. As is well-known, the chromophore of an achiral guest may exhibit an induced circular dichroism in the chiral environment of a cyclodextrin.21,22 However, the observed CD spectra of the solid-state samples detected using the conventional method are usually accompanied by artifacts. Therefore, in the process of measuring the ICD spectra, the film was placed perpendicular to the light path and rotated within the film plane to avoid the polarization-dependent reflections and eliminate the possible angle dependence of the CD signals.23-25 As depicted in Figure 4, there is a very strong ICD spectrum of SPO observed for the cast-coating SPO/γ-CD film, whereas no signal appears when SPO is incorporated in either PVA matrix or glucose (not shown). What’s more, when 50 equiv of 1-adamantanol, an alcohol known to partition into γ-CD,26 was added to the solution of SPO and γ-CD, a poorly transparent film was obtained and the ICD signal nearly disappeared. This is due to the stronger binding of 1-adamantanol with γ-CD, which results in SPO expulsion from the cavity. Once again it is proved that the SPO is incorporated in the cavity of γ-CD with its π-π* transition moment parallel to the axis of the host. However, the sign of the chirality was rather undetermined for films made in different (21) Yoshida, N.; Yamaguchi, H.; Iwao, T.; Higashi, M. J. Chem. Soc., Perkin Trans. 1999, 2, 379-386. (22) Woodberry, R.; Ransom, S.; Chen, F.-M. Anal. Chem. 1988, 60, 26212625. (23) Yuan, J.; Liu, M. H. J. Am. Chem. Soc. 2003, 125, 5051-5056. (24) Spitz, C.; Da¨hne, S.; Ouart, A.; Abraham, H. W. J. Phys. Chem. B 2000, 104, 8664-8669. (25) Szyrszyng, M.; Nowak, E.; Gdaniec, M.; Milewska, M. J.; Połon´ski, T. Tetrahedron: Asymmetry 2004, 15, 3257-3261. (26) Chamberlain, R. V.; Slowinska, K.; Majda, M. Langmuir 2000, 16, 13881396.
batches. It is assumed that there are two kinds of exciton splitting positive and negative and that the predominant one determines the ultimate sign of the Cotton effect. Considering that the cavity of γ-CD is not large enough to accommodate two SPO molecules at the same time,27 which was also demonstrated by EA and MS, the strong split Cotton effect may be the result of the exciton coupling of two neighboring guests, each in the form of a 1:1 complex with the host.28 According to the oscillator theory, the exciton couplet is either positive or negative depending on whether the relative configuration of the electric dipole moment of the transition of these chromophores forms part of a right-handed or left-handed spiral, respectively, in the Newman projection.21,29-31 In a word, all of these results indicate together that SPO forms an inclusion complex with γ-CD with a host:guest ratio of 1:1. As for the possible structure of the complex, see the Supporting Information. Photochromism. As is well-known, guest molecules incorporated in the hydrophobic cavities of suitable CD (R-, β-, or γ-CD) often exhibit remarkably different physicochemical properties compared with the bulk material. Generally, photochromism is not observed for SPO in the single-crystal state because of the large molecular rearrangement involved in the SPO to PMC.18 However, SPO recovers its photochromic property in the solid state after it is incorporated into γ-CD, because the large cavity of γ-CD makes it possible. The interconversion is shown in Scheme 1, where only one of the several colored forms is drawn. Upon irradiation with UV light, the colorless SPO undergoes a heterolytic cleavage of the spiro C-O bond in the oxazine ring, resulting in the colored form of photomerocyanine, which then reverts back to SPO either thermally or after irradiation with visible light. The open structure is best described in the (27) Iyengar, S.; Biewer, M. C. Cryst. Growth Des. 2005, 5, 2043-2045. (28) Suzuki, M. Carbohydr. Res. 1991, 214, 25-33. (29) Harada, N.; Nakanishi, K. Circular Dichroic Spectroscopy: exciton coupling in organic stereochemistry, 2nd ed.; Oxford University Press: New York, 1983. (30) Zhdanov, Y. A.; Alekseev, Y. E.; Kompantseva, E. V.; Vergeichik, E. N. Russ. Chem. ReV. 1992, 61, 563-575. (31) Buss, V. Angew. Chem., Int. Ed. Engl. 1991, 30, 869-870.
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Figure 5. UV-vis absorption spectra of the SPO/γ-CD film before (a) and during (b) UV irradiation.
quinoidal form for the PMC dye.6 The typical absorption spectra of the SPO/γ-CD film before and during UV exposure are depicted in Figure 5 as panels a and b, respectively. Absorption of PMC in this system is similar to that in ethanol or PVA matrix (not shown) and absorption maxima at 615 and 575 nm are due to isomers of PMCs.10,32 Because decoloration speeds are normally more strongly affected by surrounding microscopic environments than coloration speeds in a rigid matrix, only the fade speeds of the colored complex (PMC in γ-CD) are studied in this paper. The kinetics of the ring closure reaction is studied following the fading of color at the maximum absorption wavelength of PMC. It is 615 nm for SPO/γ-CD (see Figure 5b), which is similar to that for the SPO/PVA film (not shown). The decoloration curves are analyzed using the standard biexponential equation1,2,7,33,34
A(t) ) A1e-k1t + A2e-k2t + Ath where A(t) is the optical density at λmax, A1 and A2 are contributions to the initial optical density, k1 and k2 are rate constants of the fast and slow components, and Ath is coloration when time approaches infinity. A2 ) 0 for monoexponential decay. As for PMC in the PVA matrix, monoexponential decay (k ) 6.7 × 10-3 s-1, R2 > 0.99) is observed which is indicative of a homogeneous dye dispersal type. The fading of PMC in the SPO/γ-CD film fits biexponential decay very well (see Figure 6, R2 > 0.99) with k1 ) 6.0 × 10-2 s-1, k2 ) 7.5 × 10-3 s-1, and the ratio of A1 to A2 is 2.70. Two-component kinetics can be explained in terms of two kinds of species with different microscopic environments which may be due to different micropolarity and microviscosity, where the relaxing species encounter different energy barriers to closure. However, when aggregation of the photomerocyanines occurred, it can be explained by assuming that decoloration occurred by the successive detachment of PMC molecules from H or J aggregates which fits the biexponential model.35,36 Herein no shift of the λmax of PMC is observed during the thermal back process, so no aggregation takes place. In our study, the thermokinetic simulation indicates that one of the two rate constants of the SPO/γ-CD film is almost equal to that of the SPO/PVA film. What’s more, it is well-known that the polarity (32) Christopher, J. W.; Darius, K. J. Phys. Chem. B 2005, 109, 2218622191. (33) Krongauz, V. A. In Photochromism: Molecules and Systems; Du¨rr, H., Bous-Laurent, H., Eds.; Elsevier Press: Amsterdam, 1990; pp 793-820. (34) Hobley, J.; Pfeifer-Fukumura, U.; Bletz, M.; Asahi, T.; Masuhara, H.; Fukumura, H. J. Phys. Chem. A 2002, 106, 2265-2270. (35) Gentili, P. L.; Costantino, U.; Nocchetti, M.; Miliani, C.; Favaro, G. J. Mater. Chem. 2002, 12, 2872-2878. (36) Eckhardt, H.; Bose, A.; Krongauz, V. A. Polymer 1987, 28, 1959-1964.
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Figure 6. Time-dependent thermal bleaching of the PMC/γ-CD film.
of the γ-CD cavity is much lower than that outside of the cavity; therefore, the fast decay component is attributed to PMCs located in the cavities of γ-CD and the slow one to those outside of the cavities. From the ratio of A1 to A2, we know that in the SPO/ γ-CD film about 73% of the SPO molecules are incorporated in γ-CD, whereas a small number of the SPO molecules lie outside of the cavities, as necessary in the reaction equilibrium. Considering the similarity of the chemical compositions of γ-CD and PVA, the difference in decay kinetics of PMC inserted into these two matrices is ascribed to the hydrophobic nanocavity effect of γ-CD.
Conclusions Through the cast-coating method, we obtain a transparent photochromic SPO/γ-CD film. The kinetic simulation of its thermal fading suggests that PMCs locate in two different microscopic environments: about 73% of the SPOs are inside the cavity of γ-CD, and their PMCs decay with a rate constant of 6.0 × 10-2 s-1, whereas 27% of the SPOs lie outside and decolorate with a rate constant of 7.5 × 10-3 s-1 at room temperature. Compared with the kinetics of PMCs embedded within the PVA matrix, the observed results are due to the hydrophobic nanocavity effect of γ-CD. The samples have been examined in detail using various techniques, such as EA, TGA, XRD, MS, ICD, and absorption spectra.17,20,21,22,37 The results support the existence of an inclusion complex with a 1:1 hostto-guest stoichiometry even though direct evidence for the complex through NMR or other experiments is absent up to now. To the best of our knowledge, this is the first report of a good transparent photochromic film of SPO with γ-CD as host. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 50221201, 90301010, 50502033, and 20302008), the Chinese Academy of Sciences, the National Research Fund for Fundamental Key Projects (973 Program 2006CB806200), and the National High-Tech R&D Program (863 Program; 2006AA03Z314). Supporting Information Available: Additional structures and interaction energies calculated for the SPO@γ-CD inclusion complexes. This material is available free of charge via the Internet at http: //pubs.acs.org. LA700252U (37) Wen, X. H.; Liu, Z. Y.; Zhu, T. Q. Chem. Phys. Lett. 2005, 405, 114-117. Lemesle-Lamache, V.; Wouessidjewe, D.; Che´ron, M.; Ducheˆne, D. Int. J. Pharm. 1996, 141, 117-124. Figueiras, A.; Ribeiro, L.; Torres-Labandeira, J. J.; Veiga, F. J. B. J. Incl. Phenom. Macrocycl. Chem. 2007, 57, 531-535.
