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Spectral properties of TMPyP intercalated in thin films of layered silicates
Journal of Colloid and Interface Science 324 (2008) 240–245
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Spectral properties of TMPyP intercalated in thin films of layered silicates ∗ , A. Czímerová, M. Pentrák, J. Bujdák ˇ A. Ceklovský Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovakia
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 17 March 2008 Accepted 29 April 2008 Available online 2 June 2008
The objective of this study was to investigate the spectral characteristics of tetracationic porphyrin dye (TMPyP), intercalated into films of three smectites. The smectites represented the specimens of high (Fluorohectorite; FHT), medium (Kunipia F montmorillonite; KF), and low layer charge (Laponite; LAP). Intercalation of TMPyP molecules was proven by XRD measurements. The molecular orientations of the dye cations were studied by means of linearly polarized ultraviolet–visible (UV–vis) and infrared (IR) spectroscopies. Both the UV–vis and the IR spectroscopy proved the anisotropic character of the films. The spectral analysis of the polarized UV–vis spectra and consequent calculations of tilting angles of the transition moments in the region of Soret band transitions were in the range of 25–35◦ . The determined angles indicated that the molecular orientation of the dye cations was almost parallel to the surface of the silicates. Slightly higher values, determined for a FHT film, indicated either a slightly more tilted orientation of the dye cations or the change of molecular conformation after the intercalation of the dye. Quenching of TMPyP fluorescence was observed, resulting from the formation of bimolecular layer arrangements with sandwich-type assemblies of the dye molecules. © 2008 Elsevier Inc. All rights reserved.
Keywords: Layer charge Clay minerals Porphyrin Smectites Fluorescence spectroscopy Visible spectroscopy Intercalation
1. Introduction Investigation of the interaction between host layered inorganic matrices and the organic guest chromophores has been the subject of numerous studies [1–8]. Intercalation of organic dyes into layered inorganic solids can lead to the formation of novel materials with superior properties. Specific interactions of organic dyes with inorganic surfaces often lead to chemical and thermal stabilization of the guest molecules, dye molecules’ self-assembly, and control of the molecular orientations of the chromophores. The distribution and orientation of adsorbed organic species can be controlled by the interactions between the surface of inorganic solids and the adsorbent. The presence of coadsorbed species or modifier as the third component may significantly influence the optical properties of adsorbed organic chromophores [9,10]. Photofunctional properties are one of the most promising properties of dye supramolecular systems, which can be used in various applications [11]. Therefore, study of the photophysical and photochemical processes of organic photoactive species can lead to a wide variety of applications in such areas as reaction media for controlled photochemical reactions or in construction of devises for optics, memory storage devices, etc. [12]. Generally, dye molecules tend to aggregate on the clay mineral surface or in the interlayer spaces. The layer charge of layered silicates controls the molecular aggregation of the dyes of various
*
Corresponding author. ˇ E-mail address: [email protected] (A. Ceklovský).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.04.073
© 2008
Elsevier Inc. All rights reserved.
structural types [13,14]. The occurrence of molecular aggregation is considered to be a serious problem during the preparation of luminescent hybrid materials, based on organic dyes embedded in inorganic solid hosts [15], because it leads to decreasing of the excited state lifetimes of photoactive species. In this study, we focused on investigation of the spectral behavior of tetracationic porphyrin 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP), intercalated in the layered silicate matrices with various layer charge. The effect of the layer charge of silicate hosts is expected to play an important role, both in modifying the optical properties of embedded TMPyP and also the molecular orientation of TMPyP species on a silicate surface. 2. Experimental 2.1. Materials In this study we used Fluorohectorite (FHT), Na+ -saturated montmorillonite Kunipia F (KF), and Na+ -saturated hectorite Laponite RD (LAP) as representative layered silicates of high, medium, and low layer charge densities, respectively. Synthetic trioctahedral silicates FHT and LAP were obtained from Corning Inc. (New York) and Laporte Industries Co. (UK), respectively. Dioctahedral KF was purchased from Kunimine Industries Co. (Japan) and is of natural origin. The CEC values represent the amount of cations that compensate the negative layer charge and are listed in Table 1. The tetra-p-tosylate salt of TMPyP (Scheme 1) was obtained from Frontier Scientific Europe, Ltd. (UK), and was used as received. Quartz slides (1 × 1 inch) were obtained from SPI Supplies (PA). The slides were transparent in a UV–vis region at wavelengths above 200 nm.
ˇ A. Ceklovský et al. / Journal of Colloid and Interface Science 324 (2008) 240–245
on the film tilt and the orientation of the transition moment with respect to the surface normal, according to [1,4,9,16]
Table 1 CEC values of clay mineral samples Sample
CEC (mmol g−1 )
σa
FHT KF LAP
1.48 1.24 0.78
0.03 0.09 0.03
a
σ denotes standard deviation.
2.2. Methods Quartz slides were treated using Piranha solution, 7:3 (v/v) mixture of concentrated H2 SO4 , and 35% H2 O2 for 30 min at 90 ◦ C in order to hydrophilize the surface and to remove the organic residues and impurities. Water suspensions of layered silicates were prepared by mixing 0.5 g sample/50 ml water with consequent ultrasonic disaggregation treatment for 30 min. Oriented thin films of layered silicates were prepared by covering the surface with suspensions and dried at room temperature. Dried films were immersed into the solution of TMPyP (c = 10−3 mol dm−3 ) and the system was equilibrated for 6 h. Prepared films were washed with water and dried at room temperature. X-ray diffraction patterns were recorded on diffractometer Bruker D8 Discover (CuK α radiation, 40 kV/30 mA). Polarized UV–vis absorption spectra of thin films were recorded in the region 350–700 nm by a Cary 100 Varian UV–vis spectrophotometer using a Glan-Taylor polarizer (PGT-S1G). A series of the spectra were recorded using both the x- and y-polarized light, varying the angle between the surface normal of the film specimen and the direction of the light propagation −70, 0, +70◦ . Absorption intensities were corrected for the substrate and the host material absorptions to obtain the absorption intensity of porphyrin chromophores only. The calculations of orientation angles were based on the dependence between light absorption and orientation of transition moments. Generally, the probability of the transition due to photon absorption is proportional to cos2 Θ , where Θ represents the angle between the electric vector of the radiation and the orientation of transitional moment. Maximum absorption probability occurs when the vectors are parallel, and the probability approaches zero when the transitional moment is perpendicular to the electric vector of radiation. Dichroic ratios (R) were calculated for each measured wavelength from 350 to 700 nm using the formula R (λi ) = A x (λi )/ A y (λi ),
241
(1)
where A x and A y denote the absorption values measured using x- and y-polarized light, respectively. The dichroic ratio depends
R=
2 sin2 α − (3 sin2 α − 1) sin2 γ sin2 γ
.
(2)
To calculate the angle γ , the data of R (λi , αi ) (where λi = 350–700 nm and αi = −70, 0, and +70◦ ) were applied. The calculation was performed on the basis of the functional relation among R, α , and γ in Eq. (2). Consequently, angles between the silicate surface and the transition moment of TMPyP molecule can be expressed by
Θ = 90◦ − γ .
(3)
Fluorescence measurements were performed on a Shimadzu RF5000 spectrofluorimeter. Spectra were recorded on excitation at 440, 460, and 520 nm at room temperature. Linearly polarized infrared spectra were recorded on a FTIR Nicolet Magna 750 spectrometer in the middle infrared (IR) region (4000–400 cm−1 ) using a ZnSe polarizer (Thermo Spectra-Tech). Principally the measurements were the same and the method is based on the same theory as in the case of linearly polarized UV– vis spectroscopy. Spectra were recorded for film tilt angles of 0, 40, and 50◦ . Due to a more complex character of the IR spectra, the dichroic measurements were only qualitatively evaluated to obtain information on the orientation of some functional groups of TMPyP, which could confirm the data of UV–vis-polarized spectroscopy. 3. Results and discussion 3.1. XRD analysis Fig. 1a shows the XRD patterns of sample FHT (dashed line) and diffraction patterns of FHT intercalated with TMPyP (solid line). Fluorohectorite has one of the largest values of particle diameter among expandable layered silicates, up to more than 10 μm [17], which results in sharp, well-resolved reflections in the XRD patterns. The position of d001 peak in the case of FHT is 1.24 nm, which is referred to the sample with one intercalated layer of water molecules including respective inorganic cations [18]. After intercalation of TMPyP, we observed the changes in the position of the 001 band. The position changed from 1.24 to 1.90 nm. In Fig. 1b are plotted the XRD patterns for the film of KF before (dashed line) and after intercalation with TMPyP (solid line). The XRD patterns are very similar to the patterns of FHT and FHT_TMP (Fig. 1a). The
Scheme 1. Structural formula of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP), tetra-p-tosylate salt.
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Fig. 2. Absorption spectra of TMPyP in solution (solid line) and intercalated in layered silicates. Layered silicates include Fluorohectorite (FHT), Kunipia F (KF), and Laponite (LAP).
Fig. 1. XRD patterns of the smectite films and materials obtained by porphyrin intercalation. Smectites: Fluorohectorite (a), Kunipia F (b), Laponite (c). Films with and without porphyrin are shown as solid and dashed lines, respectively.
values of d001 peaks are 1.26 and 1.73 nm for KF and KF_TMP, respectively. The much lower intensity of the reflection peaks of the KF-based films is because of the lower crystallinity and particle size of KF montmorillonite. The thickness of the smectite layer is approximately 0.96 nm [19]. Thus, the thickness of the interlayer space containing TMPyP intercalated molecules in FHT is 0.94 nm. The thickness of one TMPyP molecule is ca. 0.5 nm [20], considering rotation of pyridinium rings to a more flat position, parallel to the planes of porphyrin rings. Based on the XRD measurements, one would consider the arrangement of TMPyP cations mainly as bimolecular layers oriented to the silicate surface in a parallel fashion. However, TMPyP is built from large cations, which, under specific conditions, may not fit to form ordered mono- or bimolecular layers. Indeed, if the negative layer charge of layered compounds is too high,
intercalated porphyrin cations can take a more tilted or nearly perpendicular position [20]. The relationship between layer charge of clay mineral and arrangement of porphyrin cations is in accordance with the observations of Takagi and his co-workers and their theory of size-matching rules [3,6]. The highest layer charge density of FHT could contribute to the arrangement of a more tilted orientation of intercalated TMPyP cations. In case of a middle charge KF, the thickness of the interlayer space containing intercalated TMPyP molecules is only 0.77 nm. Taking into account lower charge density and lower interlayer space of KF layers, TMPyP cations are not supposed to be as densely packed as in the film of FHT. Consequently, the arrangement should be either a mixture of monomers and dimers parallel to the mineral surface plane or rather less densely packed bimolecular layers of TMPyP cations occurring in a disordered fashion in the interlayer spaces of KF. Fig. 1c shows the XRD patterns of LAP film before (dashed line) and after the intercalation with TMPyP (solid line). No significant changes in d001 spacing were observed for LAP treated with TMPyP solution. However, coloring of the LAP film indicates a sufficient penetration of TMPyP cations into the LAP film, in amounts which are comparable to those in FHT and KF. The changes in the interlayer spaces of LAP might be under a detection limit of XRD due to the lack of large reflection domains in the film which leads to lower XRD signal. This is because of a small diameter of LAP particles, which is approximately 20 nm [21], resulting in basal reflection bands of broad and low intensities. The occurrence of a relatively larger fraction of monomolecular layers of the TMPyP cations with flat orientation would further decrease sensibility of the method to distinguish phases of monolayers of water molecules in LAP film. The lowest charge density of LAP would contribute to the formation of the phases with less significant changes in the d001 spacing. 3.2. Polarized UV–vis spectra Fig. 2 shows visible absorption spectra of free-base TMPyP in solution and intercalated in thin films of layered silicates. Intercalation of the TMPyP cations on a silicate surface leads to significant changes in optical properties of the dye. The Soret band is redshifted (440–465 nm) with respect to the absorption maximum observed for the dye solution (420 nm). This trend has been re-
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peatedly observed for similar systems, including dye/clay aqueous dispersions [5,22,23]. There is still no agreement on the interpretations of the spectral changes. The red shift of the Soret band is mostly assigned to the changes of the molecular shape, namely to flattening of the porphyrin cations, on the dye adsorption or intercalation [2,24]. Alternative interpretations would be based on the formation of molecular assemblies, driven mainly by van der Waals intermolecular forces. The changes of the electronic properties of TMPyP are further supported by different spectral characteristics in the range of Q-bands, which represent forbidden transitions in the wavelength range of 500–700 nm. A series of spectra recorded using x- and y-polarized light and applying the variation of the film orientation were used for the calculation of the transition moment orientation. Relatively small total amounts of TMPyP were intercalated in the silicate films due to a relatively small film thickness. Therefore, only allowed transitions, namely at a Soret band in the 400–500 nm range, were used for the calculations. Due to the lower sensitivity of the method at the range of Q-bands, background, absorption, and scattering from the silicate would interfere with the dichroic absorptivity of the chromophores. According to the data of linearly polarized UV–vis spectra, the determined angles indicate that transition moments of chromophores exhibit slightly tilted orientation with respect to the surface of the silicates. The estimated angles were in the range of 25–35◦ in the region of a Soret band and did not significantly change in this spectral range. Since transition moments are parallel to the plane of the macrocycle rings, the orientation of the transition moment reflects a good approximation of molecular orientation of the dye planar cation on the silicate surface. Almost parallel orientation of the dye cation would be expected based on the molecular structure of TMPyP molecules and positions of charged groups. Four positively charged groups located in four corners of the TMPyP cation would dominantly contribute to the interaction with the silicate surface. The electrostatic attraction between the positively charged pyridinium groups and the negatively charged basal oxygen atoms at the silicate surface would preferentially lead to a flat orientation of the molecules. Determined angles 25–35◦ are in a good agreement with this model. Slight tilt of the transition moments can be due to a nonhomogeneous formation of bimolecular layers, or influenced by the changes of molecular conformation on the dye adsorption. Dye cations can be at a slightly tilted position also because of the steric adjustment of the cationic groups to optimize electrostatic interaction with the silicate surface. For the system containing FHT, the estimated angles were slightly higher. We assume that this deviation can be attributed to the high charge density of this silicate. In this case, fully packed interlayer spaces with two layers of the cations would have been insufficient to accommodate a larger number of TMPyP cations per surface unit to saturate all charge sites of FHT. If compared to the other films, the orientation angle was about 2–3◦ higher to give values ∼35◦ . 3.3. Polarized IR spectra One must consider intercalation compounds of clay minerals as nonperfect dichroic materials. More subtypes of similar molecular orientation of the dye cations in the interlayer spaces may coexist and their occurrence cannot be neglected. As a result of tilting the sample with respect to an incident light beam, the intensity of some vibrational modes increases or decreases, which depends on the vibrational transition moment orientation. The vibrational modes with the moment either randomly oriented or inclined at 35.26◦ would exhibit no change of the dichroic ratio with tilting [25]. The dichroic ratio would increase in the case of
243
Fig. 3. IR absorption spectra of TMPyP (a) and LAP (b) in KBr.
Fig. 4. Polarized difference IR spectra of TMPyP/LAP film (a, b, c) and spectrum of nontilted film (d). Shown are difference spectra, substracted at 0◦ inclination angle (a), 40◦ (b), and 50◦ (c).
more perpendicularly oriented vibrations on tilting and would be weakened for vibrations oriented in a parallel fashion. Fig. 3 shows spectra of pure single components measured using the KBr method. Fig. 4d shows a representative IR spectrum of TMPyP/LAP film perpendicularly oriented with respect to the light beam and measured using x-polarized light. The assignments of the bands (Fig. 4) are given in Table 2. Due to an orientation of the film, some vibrations, which were more perpendicular to the film surface, could be nonactive and invisible in the spectrum regardless of the light polarization. Calculated difference spectrum, obtained by the subtraction of x-polarized spectrum from y-polarized one, led to a linear, constant value line with no peak (Fig. 4a). That fact can be attributed to parallel orientation of the electric vectors of both the x- and y-polarized light to the film surface. It means no vibration was enhanced or suppressed as a consequence of light polarization change; i.e., there was no preferential orientation in either x- or y-directions on silicate surface. Tilting the
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Table 2 IR absorption band assignments of TMPyP/LAP system
3.4. Fluorescence spectroscopy
Wavenumber (cm−1 )
Band assignments
3690 3500–3200 3320 3020 1637 1600–1200
ν (OH) silicate ν (OH) water ν (NH) pyrrole ν (CH) pyridyl rings ν (C=N) pyridyl rings ν (C=C) porphyrin rings ν (C=N) porphyrin rings ν (C=pyrrole) ν (RSO− 3) ν (Si–O) silicate
Steady-state fluorescence emission spectra of TMPyP intercalates were recorded on excitation at 440, 460, and 520 nm (not shown). In all cases, relative fluorescence intensities of intercalated TMPyP cations were extremely low. The effective quenching of fluorescence cannot be assigned only to a scattering of emitted light by layered silicate particles, nor to quenching by the active moiety in the silicate structure. Only KF, clay mineral of natural origin, contains a small amount of Fe in its structure, which can partially quench fluorescence. However, this effect was found to be rather negligible for the case of clay mineral/TMPyP colloids (not shown). Quenching of TMPyP fluorescence can be assigned to the structure of the intercalated phase. Whereas the TMPyP cations are adsorbed on silicate surface in colloids in the way of a monolayer coverage to saturate efficiently the surface negative charge, bimolecular layers are formed in the films. Bilayer arrangements with sandwichtype assemblies can probably efficiently quench excited states of TMPyP cations. As noted above, polarized UV–vis spectral data support the assumption about the interaction between adsorbed TMPyP cations. Similar properties have been observed for solid silicate films with intercalated dyes of other types [15]. We assume that more promising materials based on dyes and silicates could be prepared from premodified materials with alkylammonium cations [30]. This type of the material is being investigated.
1100–1000 815 700 650
Out-of-plane (CH) porphyrin rings Out-of-plane δ (aromatic rings) δ (OH) silicate
film led to only slight changes in an x-polarized spectrum, which were due to the alterations of light beam trajectories crossing the film (spectrum not shown). However, substantial differences in the spectra obtained using y-polarized light were observed depending on film tilt, which are due to changes of the angle between the film surface and the electric vector of electromagnetic radiation. Difference spectra for the film inclinations at angles 40 and 50◦ are shown in Figs. 4b and 4c, respectively. They clearly indicated strengthening of some of the bands and the decrease of the others. Direct evidence of the preferential orientation of silicate layers is the appearance of –O–H stretching vibrations at 3690 cm−1 , which is otherwise invisible in the spectrum of a nontilted film (Fig. 4d). Perpendicular orientation of hydroxyl structural groups with respect to the layer plane is a typical feature of trioctahedral minerals [26]. Positive values of the difference spectra for 40 and 50◦ are also observed for Si–O stretching vibrations (two broad bands near 1005 and 1090 cm−1 ), especially for the higher energy one assigned to the perpendicular Si–O vibrations. Bands of lower positive absorbance include those at 650, 700, 815, and 1637 cm−1 and a broad absorption around 3200–3500 cm−1 . The latter broad band is assigned to the vibrations of adsorbed water molecules which overlap a weak band of an N–H stretching vibration at 3320 cm−1 . A band at 650 cm−1 is assigned to the –OH bending vibrations of structural OH groups, but together with those at 700 and 815 cm−1 can include transitions due to outof-plane deformations of aromatic rings and of C–H vibrations, respectively [27]. A strong and sharp band at 1637 cm−1 is assigned to a C=N vibration of pyridinium rings [28,29]. The contribution of the deformation vibrations assigned to water molecules at 1637 cm−1 is probably negligible considering adsorption of the porphyrin cations as is clear from the comparison of the spectra of pure components (Fig. 3). Positive absorption of the band at 1637 cm−1 probably relates to the tilting of the pyridinium group with respect to the porphyrin macrocycle plane. Vibrations within the region of 1600–1200 cm−1 (Table 2) exhibit negligible changes. Most negative values in difference spectra are shown for strong −1 [29]. There is no vibration assigned to SO− 3 vibrations at 1194 cm the porphyrin macrocycle, indicating perpendicular orientation of the porphyrin moiety. Such vibrations would have to be strengthened in y-polarized spectra on tilting of the film and absent in the standard mode of the measurement with the film perpendicular to the light incident beam. Vibrations perpendicular or strongly tilted with respect to the silicate plane have been assigned to only O–H and Si–O vibrations, occurring in the structure of inorganic components. Some partially tilted vibrations were assigned to originate at pyridinium rings. Most of the vibrations from the TMPyP cation were relatively weakly affected with variation of direction of the light polarity. Consequently, only slightly tilted orientation of the cations with respect to the substrate plane is expected.
4. Conclusions Intercalation of tetracationic porphyrin into the oriented layered silicate films was achieved via direct ion exchange from the aqueous phase. Intercalation of the dye was confirmed by XRD, indicating an increase of the interlamellar distance on the dye adsorption. Based on XRD measurements, the amount of adsorbed dye cations increased in the order of LAP, KF, FHT. Significant changes of optical properties of the intercalated porphyrin, namely changes of absorption spectra and quenching of the fluorescence, were proven by spectroscopy methods. Linearly polarized absorption spectroscopies in the UV–vis and IR region proved the anisotropic properties of the materials. Porphyrin cations probably are arranged in the interlayer spaces as bimolecular layers arranged in an almost parallel fashion with respect to the silicate surface. This arrangement can likely contribute to the occurrence of sandwich-type dimers, which can be the main cause of the fluorescence quenching. Acknowledgments This work was supported by the Slovak Research and Development Agency under Contract Nos. APVV-51-027405 and APVV-51050505 and grant agency VEGA (2/6180/27). References [1] R. Sasai, N. Iyi, T. Fujita, F. López Arbeloa, V. Martínez Martínez, K. Takagi, H. Itoh, Langmuir 20 (2004) 4715–4719. [2] Z. Chernia, D. Gill, Langmuir 15 (1999) 1625–1633. [3] M. Eguchi, S. Takagi, H. Tachibana, H. Inoue, J. Phys. Chem. Solids 65 (2004) 403–407. [4] J. Bujdák, N. Iyi, Y. Kaneko, A. Czímerová, R. Sasai, Phys. Chem. Chem. Phys. 5 (2003) 4680–4685. [5] P.M. Dias, D.L.A. de Faria, V.R.L. Constantino, J. Inclusion Phenom. Macrocycl. Chem. 38 (2000) 251–266. [6] S. Takagi, T. Shimada, M. Eguchi, T. Yui, H. Yoshida, D.A. Tryk, H. Inoue, Langmuir 18 (2002) 2265–2272. [7] C. Sanchez, B. Lebeau, F. Chaput, J.P. Boilot, Adv. Mater. 15 (2003) 1969–1994. [8] Ch.A. Schalley, A. Lutzen, M. Albrecht, Chem. Eur. J. 10 (2004) 1072–1080. [9] J. Bujdák, N. Iyi, J. Phys. Chem. B 109 (2005) 4608–4615.
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[10] M. Ogawa, K. Kuroda, Chem. Rev. 95 (1995) 399–438. [11] T. Shichi, K. Takagi, J. Photochem. Photobiol. C (2000) 113–130. [12] C. Sanchez, B. Julián, P. Belleville, M. Popall, J. Mater. Chem. 15 (2005) 3559– 3592. [13] J. Bujdák, P. Komadel, J. Phys. Chem. B 101 (1997). [14] J. Bujdák, N. Iyi, Clays Clay Miner. 50 (2002) 446–454. [15] A. Czímerová, N. Iyi, J. Bujdák, J. Colloid Interface Sci. 306 (2007) 316. [16] G. Chen, N. Iyi, R. Sasai, T. Fujita, K. Kitamura, J. Mater. Res. 17 (2002) 1035– 1040. [17] P.D. Karivatna, T.J. Pinnavaia, P.A. Schroeder, J. Phys. Chem. Solids 57 (1996) 1897. [18] G.J. da Silva, J.O. Fossum, E. DiMasi, K.J. Måloy, S.B. Lutnaes, Phys. Rev. E 66 (2002) 011303–011308. [19] D.M. Moore, R.C. Reynolds Jr., X-Ray Diffraction and the Identification and Analysis of Clay Minerals, Oxford Univ. Press, New York, 1989.
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[20] H. Wang, D. Han, N. Li, L. Ke-An, J. Inclusion Phenom. Macrocycl. Chem. 52 (2005) 247–252. [21] D. Gournis, M.A. Karakassides, D. Petridis, Phys. Chem. Miner. 29 (2002) 155– 158. [22] J. Dargiewicz, M. Makarska, S. Radzki, Colloids Surf. A 208 (2002) 159–165. [23] M. Makarska, S. Radzki, J. Legendziewicz, J. Alloys Compd. 341 (2002) 233–238. [24] S. Takagi, T. Shimada, T. Yui, H. Inoue, Chem. Lett. 30 (2001) 128–129. [25] J. Bujdák, N. Iyi, Y. Kaneko, R. Sasai, Clay Miner. 38 (2003) 561–572. [26] V.C. Farmer, in: Infrared Spectra of Minerals, in: Monograph, vol. 4, Mineralogical Society, New York, 1974. [27] M.M. El-Nahass, H.M. Zeyada, M.S. Aziz, M.M. Makhlouf, Spectrochim. Acta, Part A 61 (2005) 3026–3031. [28] J. Qu, P.M. Fredericks, Spectrochim. Acta, Part A 56 (2000) 1637–1644. [29] J. Nový, M. Urbanová, K. Volka, J. Mol. Struct. 748 (2005) 17–25. [30] R. Sasai, N. Iyi, T. Fujita, K. Takagi, H. Itoh, Chem. Lett. 32 (2003) 550–551.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Spectral properties of TMPyP intercalated in thin films of layered silicates ∗ , A. Czímerová, M. Pentrák, J. Bujdák ˇ A. Ceklovský Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, 845 36 Bratislava, Slovakia
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 17 March 2008 Accepted 29 April 2008 Available online 2 June 2008
The objective of this study was to investigate the spectral characteristics of tetracationic porphyrin dye (TMPyP), intercalated into films of three smectites. The smectites represented the specimens of high (Fluorohectorite; FHT), medium (Kunipia F montmorillonite; KF), and low layer charge (Laponite; LAP). Intercalation of TMPyP molecules was proven by XRD measurements. The molecular orientations of the dye cations were studied by means of linearly polarized ultraviolet–visible (UV–vis) and infrared (IR) spectroscopies. Both the UV–vis and the IR spectroscopy proved the anisotropic character of the films. The spectral analysis of the polarized UV–vis spectra and consequent calculations of tilting angles of the transition moments in the region of Soret band transitions were in the range of 25–35◦ . The determined angles indicated that the molecular orientation of the dye cations was almost parallel to the surface of the silicates. Slightly higher values, determined for a FHT film, indicated either a slightly more tilted orientation of the dye cations or the change of molecular conformation after the intercalation of the dye. Quenching of TMPyP fluorescence was observed, resulting from the formation of bimolecular layer arrangements with sandwich-type assemblies of the dye molecules. © 2008 Elsevier Inc. All rights reserved.
Keywords: Layer charge Clay minerals Porphyrin Smectites Fluorescence spectroscopy Visible spectroscopy Intercalation
1. Introduction Investigation of the interaction between host layered inorganic matrices and the organic guest chromophores has been the subject of numerous studies [1–8]. Intercalation of organic dyes into layered inorganic solids can lead to the formation of novel materials with superior properties. Specific interactions of organic dyes with inorganic surfaces often lead to chemical and thermal stabilization of the guest molecules, dye molecules’ self-assembly, and control of the molecular orientations of the chromophores. The distribution and orientation of adsorbed organic species can be controlled by the interactions between the surface of inorganic solids and the adsorbent. The presence of coadsorbed species or modifier as the third component may significantly influence the optical properties of adsorbed organic chromophores [9,10]. Photofunctional properties are one of the most promising properties of dye supramolecular systems, which can be used in various applications [11]. Therefore, study of the photophysical and photochemical processes of organic photoactive species can lead to a wide variety of applications in such areas as reaction media for controlled photochemical reactions or in construction of devises for optics, memory storage devices, etc. [12]. Generally, dye molecules tend to aggregate on the clay mineral surface or in the interlayer spaces. The layer charge of layered silicates controls the molecular aggregation of the dyes of various
*
Corresponding author. ˇ E-mail address: [email protected] (A. Ceklovský).
0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.04.073
© 2008
Elsevier Inc. All rights reserved.
structural types [13,14]. The occurrence of molecular aggregation is considered to be a serious problem during the preparation of luminescent hybrid materials, based on organic dyes embedded in inorganic solid hosts [15], because it leads to decreasing of the excited state lifetimes of photoactive species. In this study, we focused on investigation of the spectral behavior of tetracationic porphyrin 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP), intercalated in the layered silicate matrices with various layer charge. The effect of the layer charge of silicate hosts is expected to play an important role, both in modifying the optical properties of embedded TMPyP and also the molecular orientation of TMPyP species on a silicate surface. 2. Experimental 2.1. Materials In this study we used Fluorohectorite (FHT), Na+ -saturated montmorillonite Kunipia F (KF), and Na+ -saturated hectorite Laponite RD (LAP) as representative layered silicates of high, medium, and low layer charge densities, respectively. Synthetic trioctahedral silicates FHT and LAP were obtained from Corning Inc. (New York) and Laporte Industries Co. (UK), respectively. Dioctahedral KF was purchased from Kunimine Industries Co. (Japan) and is of natural origin. The CEC values represent the amount of cations that compensate the negative layer charge and are listed in Table 1. The tetra-p-tosylate salt of TMPyP (Scheme 1) was obtained from Frontier Scientific Europe, Ltd. (UK), and was used as received. Quartz slides (1 × 1 inch) were obtained from SPI Supplies (PA). The slides were transparent in a UV–vis region at wavelengths above 200 nm.
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on the film tilt and the orientation of the transition moment with respect to the surface normal, according to [1,4,9,16]
Table 1 CEC values of clay mineral samples Sample
CEC (mmol g−1 )
σa
FHT KF LAP
1.48 1.24 0.78
0.03 0.09 0.03
a
σ denotes standard deviation.
2.2. Methods Quartz slides were treated using Piranha solution, 7:3 (v/v) mixture of concentrated H2 SO4 , and 35% H2 O2 for 30 min at 90 ◦ C in order to hydrophilize the surface and to remove the organic residues and impurities. Water suspensions of layered silicates were prepared by mixing 0.5 g sample/50 ml water with consequent ultrasonic disaggregation treatment for 30 min. Oriented thin films of layered silicates were prepared by covering the surface with suspensions and dried at room temperature. Dried films were immersed into the solution of TMPyP (c = 10−3 mol dm−3 ) and the system was equilibrated for 6 h. Prepared films were washed with water and dried at room temperature. X-ray diffraction patterns were recorded on diffractometer Bruker D8 Discover (CuK α radiation, 40 kV/30 mA). Polarized UV–vis absorption spectra of thin films were recorded in the region 350–700 nm by a Cary 100 Varian UV–vis spectrophotometer using a Glan-Taylor polarizer (PGT-S1G). A series of the spectra were recorded using both the x- and y-polarized light, varying the angle between the surface normal of the film specimen and the direction of the light propagation −70, 0, +70◦ . Absorption intensities were corrected for the substrate and the host material absorptions to obtain the absorption intensity of porphyrin chromophores only. The calculations of orientation angles were based on the dependence between light absorption and orientation of transition moments. Generally, the probability of the transition due to photon absorption is proportional to cos2 Θ , where Θ represents the angle between the electric vector of the radiation and the orientation of transitional moment. Maximum absorption probability occurs when the vectors are parallel, and the probability approaches zero when the transitional moment is perpendicular to the electric vector of radiation. Dichroic ratios (R) were calculated for each measured wavelength from 350 to 700 nm using the formula R (λi ) = A x (λi )/ A y (λi ),
241
(1)
where A x and A y denote the absorption values measured using x- and y-polarized light, respectively. The dichroic ratio depends
R=
2 sin2 α − (3 sin2 α − 1) sin2 γ sin2 γ
.
(2)
To calculate the angle γ , the data of R (λi , αi ) (where λi = 350–700 nm and αi = −70, 0, and +70◦ ) were applied. The calculation was performed on the basis of the functional relation among R, α , and γ in Eq. (2). Consequently, angles between the silicate surface and the transition moment of TMPyP molecule can be expressed by
Θ = 90◦ − γ .
(3)
Fluorescence measurements were performed on a Shimadzu RF5000 spectrofluorimeter. Spectra were recorded on excitation at 440, 460, and 520 nm at room temperature. Linearly polarized infrared spectra were recorded on a FTIR Nicolet Magna 750 spectrometer in the middle infrared (IR) region (4000–400 cm−1 ) using a ZnSe polarizer (Thermo Spectra-Tech). Principally the measurements were the same and the method is based on the same theory as in the case of linearly polarized UV– vis spectroscopy. Spectra were recorded for film tilt angles of 0, 40, and 50◦ . Due to a more complex character of the IR spectra, the dichroic measurements were only qualitatively evaluated to obtain information on the orientation of some functional groups of TMPyP, which could confirm the data of UV–vis-polarized spectroscopy. 3. Results and discussion 3.1. XRD analysis Fig. 1a shows the XRD patterns of sample FHT (dashed line) and diffraction patterns of FHT intercalated with TMPyP (solid line). Fluorohectorite has one of the largest values of particle diameter among expandable layered silicates, up to more than 10 μm [17], which results in sharp, well-resolved reflections in the XRD patterns. The position of d001 peak in the case of FHT is 1.24 nm, which is referred to the sample with one intercalated layer of water molecules including respective inorganic cations [18]. After intercalation of TMPyP, we observed the changes in the position of the 001 band. The position changed from 1.24 to 1.90 nm. In Fig. 1b are plotted the XRD patterns for the film of KF before (dashed line) and after intercalation with TMPyP (solid line). The XRD patterns are very similar to the patterns of FHT and FHT_TMP (Fig. 1a). The
Scheme 1. Structural formula of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP), tetra-p-tosylate salt.
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Fig. 2. Absorption spectra of TMPyP in solution (solid line) and intercalated in layered silicates. Layered silicates include Fluorohectorite (FHT), Kunipia F (KF), and Laponite (LAP).
Fig. 1. XRD patterns of the smectite films and materials obtained by porphyrin intercalation. Smectites: Fluorohectorite (a), Kunipia F (b), Laponite (c). Films with and without porphyrin are shown as solid and dashed lines, respectively.
values of d001 peaks are 1.26 and 1.73 nm for KF and KF_TMP, respectively. The much lower intensity of the reflection peaks of the KF-based films is because of the lower crystallinity and particle size of KF montmorillonite. The thickness of the smectite layer is approximately 0.96 nm [19]. Thus, the thickness of the interlayer space containing TMPyP intercalated molecules in FHT is 0.94 nm. The thickness of one TMPyP molecule is ca. 0.5 nm [20], considering rotation of pyridinium rings to a more flat position, parallel to the planes of porphyrin rings. Based on the XRD measurements, one would consider the arrangement of TMPyP cations mainly as bimolecular layers oriented to the silicate surface in a parallel fashion. However, TMPyP is built from large cations, which, under specific conditions, may not fit to form ordered mono- or bimolecular layers. Indeed, if the negative layer charge of layered compounds is too high,
intercalated porphyrin cations can take a more tilted or nearly perpendicular position [20]. The relationship between layer charge of clay mineral and arrangement of porphyrin cations is in accordance with the observations of Takagi and his co-workers and their theory of size-matching rules [3,6]. The highest layer charge density of FHT could contribute to the arrangement of a more tilted orientation of intercalated TMPyP cations. In case of a middle charge KF, the thickness of the interlayer space containing intercalated TMPyP molecules is only 0.77 nm. Taking into account lower charge density and lower interlayer space of KF layers, TMPyP cations are not supposed to be as densely packed as in the film of FHT. Consequently, the arrangement should be either a mixture of monomers and dimers parallel to the mineral surface plane or rather less densely packed bimolecular layers of TMPyP cations occurring in a disordered fashion in the interlayer spaces of KF. Fig. 1c shows the XRD patterns of LAP film before (dashed line) and after the intercalation with TMPyP (solid line). No significant changes in d001 spacing were observed for LAP treated with TMPyP solution. However, coloring of the LAP film indicates a sufficient penetration of TMPyP cations into the LAP film, in amounts which are comparable to those in FHT and KF. The changes in the interlayer spaces of LAP might be under a detection limit of XRD due to the lack of large reflection domains in the film which leads to lower XRD signal. This is because of a small diameter of LAP particles, which is approximately 20 nm [21], resulting in basal reflection bands of broad and low intensities. The occurrence of a relatively larger fraction of monomolecular layers of the TMPyP cations with flat orientation would further decrease sensibility of the method to distinguish phases of monolayers of water molecules in LAP film. The lowest charge density of LAP would contribute to the formation of the phases with less significant changes in the d001 spacing. 3.2. Polarized UV–vis spectra Fig. 2 shows visible absorption spectra of free-base TMPyP in solution and intercalated in thin films of layered silicates. Intercalation of the TMPyP cations on a silicate surface leads to significant changes in optical properties of the dye. The Soret band is redshifted (440–465 nm) with respect to the absorption maximum observed for the dye solution (420 nm). This trend has been re-
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peatedly observed for similar systems, including dye/clay aqueous dispersions [5,22,23]. There is still no agreement on the interpretations of the spectral changes. The red shift of the Soret band is mostly assigned to the changes of the molecular shape, namely to flattening of the porphyrin cations, on the dye adsorption or intercalation [2,24]. Alternative interpretations would be based on the formation of molecular assemblies, driven mainly by van der Waals intermolecular forces. The changes of the electronic properties of TMPyP are further supported by different spectral characteristics in the range of Q-bands, which represent forbidden transitions in the wavelength range of 500–700 nm. A series of spectra recorded using x- and y-polarized light and applying the variation of the film orientation were used for the calculation of the transition moment orientation. Relatively small total amounts of TMPyP were intercalated in the silicate films due to a relatively small film thickness. Therefore, only allowed transitions, namely at a Soret band in the 400–500 nm range, were used for the calculations. Due to the lower sensitivity of the method at the range of Q-bands, background, absorption, and scattering from the silicate would interfere with the dichroic absorptivity of the chromophores. According to the data of linearly polarized UV–vis spectra, the determined angles indicate that transition moments of chromophores exhibit slightly tilted orientation with respect to the surface of the silicates. The estimated angles were in the range of 25–35◦ in the region of a Soret band and did not significantly change in this spectral range. Since transition moments are parallel to the plane of the macrocycle rings, the orientation of the transition moment reflects a good approximation of molecular orientation of the dye planar cation on the silicate surface. Almost parallel orientation of the dye cation would be expected based on the molecular structure of TMPyP molecules and positions of charged groups. Four positively charged groups located in four corners of the TMPyP cation would dominantly contribute to the interaction with the silicate surface. The electrostatic attraction between the positively charged pyridinium groups and the negatively charged basal oxygen atoms at the silicate surface would preferentially lead to a flat orientation of the molecules. Determined angles 25–35◦ are in a good agreement with this model. Slight tilt of the transition moments can be due to a nonhomogeneous formation of bimolecular layers, or influenced by the changes of molecular conformation on the dye adsorption. Dye cations can be at a slightly tilted position also because of the steric adjustment of the cationic groups to optimize electrostatic interaction with the silicate surface. For the system containing FHT, the estimated angles were slightly higher. We assume that this deviation can be attributed to the high charge density of this silicate. In this case, fully packed interlayer spaces with two layers of the cations would have been insufficient to accommodate a larger number of TMPyP cations per surface unit to saturate all charge sites of FHT. If compared to the other films, the orientation angle was about 2–3◦ higher to give values ∼35◦ . 3.3. Polarized IR spectra One must consider intercalation compounds of clay minerals as nonperfect dichroic materials. More subtypes of similar molecular orientation of the dye cations in the interlayer spaces may coexist and their occurrence cannot be neglected. As a result of tilting the sample with respect to an incident light beam, the intensity of some vibrational modes increases or decreases, which depends on the vibrational transition moment orientation. The vibrational modes with the moment either randomly oriented or inclined at 35.26◦ would exhibit no change of the dichroic ratio with tilting [25]. The dichroic ratio would increase in the case of
243
Fig. 3. IR absorption spectra of TMPyP (a) and LAP (b) in KBr.
Fig. 4. Polarized difference IR spectra of TMPyP/LAP film (a, b, c) and spectrum of nontilted film (d). Shown are difference spectra, substracted at 0◦ inclination angle (a), 40◦ (b), and 50◦ (c).
more perpendicularly oriented vibrations on tilting and would be weakened for vibrations oriented in a parallel fashion. Fig. 3 shows spectra of pure single components measured using the KBr method. Fig. 4d shows a representative IR spectrum of TMPyP/LAP film perpendicularly oriented with respect to the light beam and measured using x-polarized light. The assignments of the bands (Fig. 4) are given in Table 2. Due to an orientation of the film, some vibrations, which were more perpendicular to the film surface, could be nonactive and invisible in the spectrum regardless of the light polarization. Calculated difference spectrum, obtained by the subtraction of x-polarized spectrum from y-polarized one, led to a linear, constant value line with no peak (Fig. 4a). That fact can be attributed to parallel orientation of the electric vectors of both the x- and y-polarized light to the film surface. It means no vibration was enhanced or suppressed as a consequence of light polarization change; i.e., there was no preferential orientation in either x- or y-directions on silicate surface. Tilting the
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Table 2 IR absorption band assignments of TMPyP/LAP system
3.4. Fluorescence spectroscopy
Wavenumber (cm−1 )
Band assignments
3690 3500–3200 3320 3020 1637 1600–1200
ν (OH) silicate ν (OH) water ν (NH) pyrrole ν (CH) pyridyl rings ν (C=N) pyridyl rings ν (C=C) porphyrin rings ν (C=N) porphyrin rings ν (C=pyrrole) ν (RSO− 3) ν (Si–O) silicate
Steady-state fluorescence emission spectra of TMPyP intercalates were recorded on excitation at 440, 460, and 520 nm (not shown). In all cases, relative fluorescence intensities of intercalated TMPyP cations were extremely low. The effective quenching of fluorescence cannot be assigned only to a scattering of emitted light by layered silicate particles, nor to quenching by the active moiety in the silicate structure. Only KF, clay mineral of natural origin, contains a small amount of Fe in its structure, which can partially quench fluorescence. However, this effect was found to be rather negligible for the case of clay mineral/TMPyP colloids (not shown). Quenching of TMPyP fluorescence can be assigned to the structure of the intercalated phase. Whereas the TMPyP cations are adsorbed on silicate surface in colloids in the way of a monolayer coverage to saturate efficiently the surface negative charge, bimolecular layers are formed in the films. Bilayer arrangements with sandwichtype assemblies can probably efficiently quench excited states of TMPyP cations. As noted above, polarized UV–vis spectral data support the assumption about the interaction between adsorbed TMPyP cations. Similar properties have been observed for solid silicate films with intercalated dyes of other types [15]. We assume that more promising materials based on dyes and silicates could be prepared from premodified materials with alkylammonium cations [30]. This type of the material is being investigated.
1100–1000 815 700 650
Out-of-plane (CH) porphyrin rings Out-of-plane δ (aromatic rings) δ (OH) silicate
film led to only slight changes in an x-polarized spectrum, which were due to the alterations of light beam trajectories crossing the film (spectrum not shown). However, substantial differences in the spectra obtained using y-polarized light were observed depending on film tilt, which are due to changes of the angle between the film surface and the electric vector of electromagnetic radiation. Difference spectra for the film inclinations at angles 40 and 50◦ are shown in Figs. 4b and 4c, respectively. They clearly indicated strengthening of some of the bands and the decrease of the others. Direct evidence of the preferential orientation of silicate layers is the appearance of –O–H stretching vibrations at 3690 cm−1 , which is otherwise invisible in the spectrum of a nontilted film (Fig. 4d). Perpendicular orientation of hydroxyl structural groups with respect to the layer plane is a typical feature of trioctahedral minerals [26]. Positive values of the difference spectra for 40 and 50◦ are also observed for Si–O stretching vibrations (two broad bands near 1005 and 1090 cm−1 ), especially for the higher energy one assigned to the perpendicular Si–O vibrations. Bands of lower positive absorbance include those at 650, 700, 815, and 1637 cm−1 and a broad absorption around 3200–3500 cm−1 . The latter broad band is assigned to the vibrations of adsorbed water molecules which overlap a weak band of an N–H stretching vibration at 3320 cm−1 . A band at 650 cm−1 is assigned to the –OH bending vibrations of structural OH groups, but together with those at 700 and 815 cm−1 can include transitions due to outof-plane deformations of aromatic rings and of C–H vibrations, respectively [27]. A strong and sharp band at 1637 cm−1 is assigned to a C=N vibration of pyridinium rings [28,29]. The contribution of the deformation vibrations assigned to water molecules at 1637 cm−1 is probably negligible considering adsorption of the porphyrin cations as is clear from the comparison of the spectra of pure components (Fig. 3). Positive absorption of the band at 1637 cm−1 probably relates to the tilting of the pyridinium group with respect to the porphyrin macrocycle plane. Vibrations within the region of 1600–1200 cm−1 (Table 2) exhibit negligible changes. Most negative values in difference spectra are shown for strong −1 [29]. There is no vibration assigned to SO− 3 vibrations at 1194 cm the porphyrin macrocycle, indicating perpendicular orientation of the porphyrin moiety. Such vibrations would have to be strengthened in y-polarized spectra on tilting of the film and absent in the standard mode of the measurement with the film perpendicular to the light incident beam. Vibrations perpendicular or strongly tilted with respect to the silicate plane have been assigned to only O–H and Si–O vibrations, occurring in the structure of inorganic components. Some partially tilted vibrations were assigned to originate at pyridinium rings. Most of the vibrations from the TMPyP cation were relatively weakly affected with variation of direction of the light polarity. Consequently, only slightly tilted orientation of the cations with respect to the substrate plane is expected.
4. Conclusions Intercalation of tetracationic porphyrin into the oriented layered silicate films was achieved via direct ion exchange from the aqueous phase. Intercalation of the dye was confirmed by XRD, indicating an increase of the interlamellar distance on the dye adsorption. Based on XRD measurements, the amount of adsorbed dye cations increased in the order of LAP, KF, FHT. Significant changes of optical properties of the intercalated porphyrin, namely changes of absorption spectra and quenching of the fluorescence, were proven by spectroscopy methods. Linearly polarized absorption spectroscopies in the UV–vis and IR region proved the anisotropic properties of the materials. Porphyrin cations probably are arranged in the interlayer spaces as bimolecular layers arranged in an almost parallel fashion with respect to the silicate surface. This arrangement can likely contribute to the occurrence of sandwich-type dimers, which can be the main cause of the fluorescence quenching. Acknowledgments This work was supported by the Slovak Research and Development Agency under Contract Nos. APVV-51-027405 and APVV-51050505 and grant agency VEGA (2/6180/27). References [1] R. Sasai, N. Iyi, T. Fujita, F. López Arbeloa, V. Martínez Martínez, K. Takagi, H. Itoh, Langmuir 20 (2004) 4715–4719. [2] Z. Chernia, D. Gill, Langmuir 15 (1999) 1625–1633. [3] M. Eguchi, S. Takagi, H. Tachibana, H. Inoue, J. Phys. Chem. Solids 65 (2004) 403–407. [4] J. Bujdák, N. Iyi, Y. Kaneko, A. Czímerová, R. Sasai, Phys. Chem. Chem. Phys. 5 (2003) 4680–4685. [5] P.M. Dias, D.L.A. de Faria, V.R.L. Constantino, J. Inclusion Phenom. Macrocycl. Chem. 38 (2000) 251–266. [6] S. Takagi, T. Shimada, M. Eguchi, T. Yui, H. Yoshida, D.A. Tryk, H. Inoue, Langmuir 18 (2002) 2265–2272. [7] C. Sanchez, B. Lebeau, F. Chaput, J.P. Boilot, Adv. Mater. 15 (2003) 1969–1994. [8] Ch.A. Schalley, A. Lutzen, M. Albrecht, Chem. Eur. J. 10 (2004) 1072–1080. [9] J. Bujdák, N. Iyi, J. Phys. Chem. B 109 (2005) 4608–4615.
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