J. Phys. Chem. B 2002, 106, 5455-5460
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Photochemical Energy Transfer of Cationic Porphyrin Complexes on Clay Surface Shinsuke Takagi,* Donald A. Tryk,† and Haruo Inoue*,† Department of Applied Chemistry, Graduate Course of Engineering, Tokyo Metropolitan UniVersity, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397 Japan ReceiVed: January 14, 2002; In Final Form: March 7, 2002
Photochemical energy transfer of cationic porphyrins on an “anionic-type” clay (Sumecton SA) surface was investigated. Two types of complexes were formed between the cationic porphyrin and the “anionic-type” clay depending on the conditions. With the first type, the porphyrin molecules are adsorbed on both surfaces of the individual exfoliated clay sheets or layers (type b complexes). With the second type, the porphyrin molecules are intercalated between the stacked clay sheets (type c complexes). The aqueous solutions of both types of complexes do not scatter light in the UV-vis wavelength region. For the type b complexes, we have found recently that the porphyrin molecules adsorb in a flat orientation on the clay sheets as monolayers, without discernible aggregation. The high packing density is determined by the fact that the positive charges on the porphyrin precisely neutralize the negative charges of the clay surface. Efficient photochemical energy transfer was found between porphyrins in type b complexes. Two modes of energy transfer are proposed, depending on the conditions. The first is assigned to energy transfer between porphyrins adsorbed on the same clay sheet (intrasheet energy transfer). The second is assigned to energy transfer between porphyrins adsorbed on adjacent clay sheets (intersheet energy transfer).
Introduction Clay minerals are multilayered inorganic materials that provide quasi-two-dimensional spaces that are interesting as microenvironments for chemical reactions.1-3 Synthetic clay minerals have attracted recent interest regarding application to photochemical reactions that use many different types of dyes.1-10 However, these dye molecules tend to aggregate on the clay surface or in the interlayer space.3-5 Aggregation tends to significantly decrease the excited-state lifetimes of dyes, and thus the control of aggregation is very important to make photochemical processes possible. A fair amount of research has been carried out on porphyrinclay complexes; for example, the orientations of the porphyrins with respect to the clay sheets, as well as bathochromic shifts of the porphyrin Soret band, have been reported.11-28 However, little is known about photochemical reactions involving such complexes. Recently, we have developed a new technique for the conformational and orientational control of dye adsorption on clay surfaces.29,30 We have been able to prepare unique complexes in which the porphyrin molecule adsorbs on clay surfaces without aggregation. The crucial factor for high-density adsorption of porphyrin with controlled intermolecular gap distance is the matching of inter-charge distances on clay and porphyrin, i.e., the distance between negatively charged sites on the clay sheet and that between positively charged sites in the porphyrin molecule (“size-matching effect”).29,30 In these complexes, the excited-state lifetimes for the porphyrins adsorbed on clay were sufficient to participate in photochemical reactions. In the present work, Sumecton SA (SSA) was used; it is a synthetic cation-exchange-type clay (synthetic saponite). SSA is colorless, highly pure, and disperses well in water. Since * To whom correspondence should be addressed. E-mail:
[email protected];
[email protected]. † CREST, JST(Japan Science and Technology).
aqueous solutions of SSA are transparent in the UV-vis range, they are well suited as microenvironments for photochemical reactions. Zinc and free-base porphyrin derivatives were used as dyes because of their well-known properties and usefulness as sensitizers.31,32 In the present paper, photochemical energy transfer between adsorbed porphyrins on clay surfaces are examined. Experimental Section Chemicals. Sumecton SA was received from Kunimine Industries Co., Ltd. and used without further purification. Water was deionized just before use with an ORGANO BB-5A system (PF filter × 2 + G-10 column). The conductivity of the water used was below 0.02 µS cm-1. Tetrakis(1-methyl-pyridinium4-yl) porphyrin (H2TMPyP) and tetrakis(N,N,N-trimethyl-anilinium-4-yl) porphyrin (H2TMAP) were used as the +4-charged porphyrins and were purchased from Aldrich; the counterion (tosylate anion) was exchanged for chloride by use of an ion exchange column (ORGANO AMBERLITE IRA400JCL). The corresponding Zn complexes (ZnTMPyP and ZnTMAP) were synthesized according to a method reported in the literature.33 Analysis. Absorption spectra were measured with Shimadzu UV-2400 and UV-3150 spectrophotometers. For samples which had absorbances larger than 2.0 for 1 cm optical-length, a cell with a short optical length (5 or 3 mm) was used. Fluorescence spectra were measured with a Hitachi F-4010 and a JASCO FP-6500 spectrofluorometers. In steady-state absorption and fluorescence measurements, a poly(methyl methacrylate) cell was used for aqueous solutions of porphyrin in order to avoid adsorption of porphyrin onto the cell wall. The fluorescence decay was measured using a picosecond fluorescence lifetime measurement system under photon-counting conditions (a Hamamatsu, C4334 streak scope, connected with a CHROMEX 250IS polychromator) with an EKSPLA PV-401 optical parametric generator (424-480 nm, 25 ps fwhm, >100 µJ, 5 Hz)
10.1021/jp0200977 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002
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Figure 2. Schematic diagram of a possible structure of SSA-cationic porphyrin complexes; type b (exfoliated) and type c (intercalated).
CHART 1: Structures of Sumecton SA and Cationic Porphyrins
Figure 1. Absorption spectra of cationic porphyrins with clay and without clay in the Soret band region; (top, H2TMPyP-a, -b, -c, ZnTMPyP-a, -b, and -c; bottom, H2TMAP-a, -b, -c, ZnTMAP-a, -b, -c). [SSA] ) 500 mg L-1, [porphyrin] ) 1.0 × 10-6M (0.8% vs CEC).
SCHEME 1: Sample Preparation Methods for Fluorescence Measurements of SSA-(H2TMPyP-b)-(ZnTMPyP-b) Complexes (SSA + (H2TMPyP + ZnTMAP))], and (ii) each complex (H2TMPyP-b and ZnTMAP-b) was prepared and then the complexes were mixed [independent adsorption (IA) method ((SSA + H2TMPyP) + (SSA + ZnTMAP))] (Scheme 1). Results and Discussion
pumped by the third harmonic radiation of a Nd3+:YAG laser, EKSPLA PL2143b (355 nm, 25 ps fwhm, 15 mJ). The laser flux was reduced with neutral density filters to avoid multiphoton absorption processes and nonlinear effects. Sample Preparation. Clay-porphyrin complexes were prepared as follows. Type a Samples. Aqueous porphyrin solutions were prepared at appropriate concentrations as reference samples for the clayporphyrin complex. Type b Complexes (Exfoliated). These were prepared by mixing aqueous SSA solution and the respective aqueous porphyrin solutions. The SSA concentration was 500 mg L-1 unless otherwise noted. The concentration of anionic sites in solution was calculated to be 5.0 × 10-4 mol L-1 from the cation exchange capacity. Type c Complexes (Intercalated). These were prepared by repeating three times a freeze (liquid N2)-thaw cycle with type b complexes. Complexes for Energy Transfer Experiments. Two types of complexes were prepared: (i) aqueous solutions of H2TMPyP and ZnTMAP were mixed and the solution obtained was mixed with aqueous clay solution [coadsorption (CA) method
Estimation of Intercharge Distances for Clay and Porphyrins. The stoichiometric formula for Sumecton SA is [(Si7.20Al0.80)(Mg5.97Al0.03)O20(OH)4]-0.77‚(Na0.49Mg0.14)+0.77. From the surface area of 750 m2 g-1 (theoretical),34 and the cationic exchange capacity (CEC) of 99.7 mequiv/100 g, the average area per anionic site is calculated to be 1.25 nm2.30 Thus the average distance between the neighboring anionic sites is estimated to be 1.20 nm on the basis of hexagonal array. The free-base porphyrins H2TMPyP and H2TMAP and their corresponding zinc complexes ZnTMPyP and ZnTMAP (Chart) were estimated to have the following distances between adjacent cationic sites: 1.09 nm for H2TMPyP and ZnTMPyP and 1.35 nm for H2TMAP and ZnTMAP. Absorption Spectra. The UV-vis absorption spectra of the essentially nonlight-scattering porphyrin samples of types a, b, and c (Figure 1) show that, for each of the porphyrins, the spectrum for each type (a, b, or c) was somewhat different. As reported before,29,30 type b complexes are identified as those in which SSA exists as either exfoliated single, completely nonassociated sheets or very loosely associated sheets, and the porphyrin adsorbs on the clay surfaces. It was found that aggregation is completely inhibited, and the porphyrin exists as a single molecular unit for every loading level (0-100% vs CEC).29,30 Type c complexes were identified as those in which the SSA sheets are stacked, and the porphyrin molecules are intercalated in the interlayer space (Figure 2).
Cationic Porphyrin Complexes on Clay Surface
Figure 3. Absorption spectra of cationic porphyrins (H2TMPyP-b, ZnTMAP-b) with clay ([H2TMPyP-b] ) [ZnTMAP-b] ) 3.2 × 10-7 M, [SSA] ) 500 mg L-1).
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Figure 5. Absorption spectra of SSA-(H2TMPyP-b)-(ZnTMAP-b) complexes prepared by coadsorption (CA) and independent-adsorption (IA) methods ([H2TMPyP-b] ) [ZnTMAP-b] ) 3.2 × 10-7 M, [SSA] ) 500 mg L-1).
Figure 4. Fluorescence spectra of cationic porphyrins (H2TMPyP-b, ZnTMAP-b) with clay excited at 428 nm ([H2TMPyP-b] ) [H2TMPyPb] ) 3.2 × 10-7 M, [SSA] ) 500 mg L-1).
Figure 6. Fluorescence spectra of SSA-(H2TMPyP-b)-(ZnTMAP-b) complexes prepared by coadsorption (CA) and independent-adsorption (IA) methods with excitation at 428 nm ([H2TMPyP-b] ) [ZnTMAPb] ) 3.2 × 10-7 M, [SSA] ) 500 mg L-1). Dotted curves; a linear combination of H2TMPyP-b and ZnTMAP-b.
The spectral differences between the type b and c samples were ascribed to a flattening of the porphyrin molecule on the clay sheet, i.e., the four cationic methylpyridiniumyl moieties become parallel to the porphyrin ring, as reported earlier.13,14,21,22,29,30 Photochemical Energy Transfer between Adsorbed Porphyrins on Clay Surface. In general, the excited singlet lifetimes of dyes adsorbed on clay surfaces are much shorter than those of the corresponding free molecules in solution because of their aggregation. We have already reported the excited singlet lifetimes of porphyrins in SSA [(500 mg L-1)porphyrin (8.4 × 10-6 M (6.7% vs CEC)) complexes.29,30 Although the decay behavior for some of the complexes included a short-lifetime component, most of the curves could be analyzed with a single-exponential fitting and the lifetimes for the type b and c complexes turned out to be comparable to those of monomeric porphyrins in aqueous solution. In the present study, photochemical energy transfer from the excited singlet state of ZnTMAP-b to H2TMPyP-b was examined as a representative photochemical reaction. Since the absorption and fluorescence spectra (Figures 3 and 4) are relatively well separated from each other, quantitative estimation of the energy transfer reaction is facile. The SSA-(H2TMPyPb)-(ZnTMAP-b) complexes were prepared at various loading levels of the porphyrins (0.003-90% vs CEC),35 where the concentrations of the two porphyrins were equal ([H2TMPyPb] ) [ZnTMAP-b], [H2TMPyP-b] + [ZnTMAP-b] ) loading level). The excitation wavelength was set at 428 nm (bandwidth ) 3 nm), which is the λmax for ZnTMAP-b. The λmax of H2-
TMPyP-b is 450 nm, and 79% of the excitation light was absorbed by ZnTMAP-b. The absorption spectra of the samples (SSA-(H2TMPyP-b)-(ZnTMAP-b), [H2TMPyP-b] + [ZnTMAPb] ) 3.8% vs CEC) prepared by the coadsorption (CA) and independent adsorption (IA) methods (see Experimental Section) are shown with the absorption spectra of H2TMPyP-b and ZnTMAP-b in Figure 5. The absorption spectra of both samples (CA and IA) exhibited good coincidence with each other and also coincided well with the sum of the spectra for H2TMPyP-b and ZnTMAP-b. The agreement in the absorption spectra was retained at every porphyrin loading level (0.003-90% vs CEC). These observations indicate that porphyrin aggregation was completely suppressed even when different porphyrins adsorb on the clay sheet simultaneously. These complexes without discernible aggregation should be suitable for photochemical reactions such as energy transfer. The excited singlet-state lifetimes of porphyrins in the complex was estimated to be 3.8 ns for H2TMPyP-b and 0.7 ns for ZnTMAP-b.29,30 The fluorescence spectra of the samples ((SSA-(H2TMPyPb)-(ZnTMAP-b), [H2TMPyP-b] + [ZnTMAP-b] ) 3.8% vs CEC) prepared by CA and IA methods are shown in Figure 6. In contrast to the absorption spectra, the fluorescence spectra for the two types of samples were clearly different each other. Each fluorescence spectrum could be fitted with a linear combination of H2TMPyP-b and ZnTMAP-b. (Figure 6, dotted curves) These results indicate that processes other than energy transfer, such as exciplex formation, were negligible under the present conditions. From the fitting, the contribution ratio of each porphyrin was estimated. Although ZnTMAP-b absorbs
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Figure 7. Fluorescence spectra of SSA-(H2TMPyP-b)-(ZnTMAP-b) complexes prepared by the coadsorption (CA) method (left) and the independentadsorption (IA) method (right) with excitation at 428 nm at various porphyrin loading levels (1.6-90.0% vs CEC).
TABLE 1: Summary for the Properties of SSA-Porphyrin Complexes
complex b complex b′
interaction between porphyrins in the ground state
energy transfer between porphyrins on the same clay sheet
energy transfer between porphyrins on the different clay sheet
heterogeneity on visual observation
transparency on spectrometric observation
no no
yes yes
no yes
no yes
yes yes
79% of excitation light under the present conditions, the contribution ratio of ZnTMAP-b to the fluorescence was estimated to be 53% for IA-type complex and 13% for the CAtype complex. The estimations clearly indicate that efficient energy transfer from excited ZnTMAP-b to H2TMPyP-b proceeded, especially for the CA-type complex. This difference of energy transfer behavior between CA-type and IA-type complexes indicates that their microscopic structures are different from each other, even though their absorption spectra were quite similar. The influence of the porphyrin concentration on energy transfer behavior for both types of complexes was examined. Fluorescence spectra normalized at 625 nm for both types at various porphyrin loading levels are shown in Figure 7. As the porphyrin concentration increased, the contribution of H2TMPyP-b to the fluorescence spectra (λmax ) 691 and 754 nm) increased for both types of complexes. However, the magnitude of the concentration effect on the fluorescence spectra for each type was different. The values estimated for ZnTMAP-b at various porphyrin loading levels are plotted for the IA and CA complexes in Figure 8. A small contribution by ZnTMAP-b means that there is efficient energy transfer from excited ZnTMAP-b to H2TMPyP-b. In the case of the IA-type complexes, the relatively large contribution by ZnTMAP-b was not significantly affected by the loading level of porphyrin below 20%, but decreased as the loading level increased above 20%. For the CA-type complexes, the contribution ratio for ZnTMAP-b decreased drastically in the very low porphyrin concentration region (
