Inorganic−Organic Hybrid Materials: Layered Zinc Hydroxide Salts

J. Phys. Chem. C 2010, 114, 16321–16328

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Inorganic-Organic Hybrid Materials: Layered Zinc Hydroxide Salts with Intercalated Porphyrin Sensitizers Jan Demel,† Pavel Kuba´t,‡ Ivan Jirka,‡ Petr Kova´rˇ,§ Miroslav Pospı´sˇil,§ and Kamil Lang*,† Institute of Inorganic Chemistry of the AS CR, V.V.i., Husinec-Rˇezˇ 1001, 250 68, Rˇezˇ, Czech Republic, J. HeyroVsky´ Institute of Physical Chemistry of the ASCR, V.V.i., DolejsˇkoVa 3, 182 23 Praha 8, Czech Republic, and Charles UniVersity in Prague, Faculty of Mathematics and Physics, Ke KarloVu 3, 121 16, Praha 2, Czech Republic ReceiVed: July 2, 2010; ReVised Manuscript ReceiVed: August 13, 2010

Layered materials provide a two-dimensional interlayer space suitable for accommodating molecules with a designed functionality. In this study, inorganic-organic hybrids were prepared by intercalation of anionic porphyrin sensitizers into the host structure of layered zinc hydroxide salts. The inorganic host offers stabilization and protection, whereas the guest species provide the photofunction. The properties and arrangement of porphyrin molecules in the interlayer space were studied by a combination of experimental techniques and molecular simulations. Intercalation of porphyrins led to a gallery height that is comparable with the size of porphyrin molecules. Molecular simulations showed that the interlayer space is filled with disordered porphyrin units. The porphyrin sulfonate groups interact with the brucite-like layers via dominant electrostatic interactions similarly to layered double hydroxides. The photophysical experiments proved that intercalated anionic Pd porphyrins produce singlet oxygen, O2(1∆g), with long effective lifetimes, suggesting that layered zinc hydroxide salts are good carriers of porphyrin sensitizers. Introduction Anion-exchangeable layered hydroxides provide a twodimensional interlayer space of a flexible height for accommodating molecules imposing the designed functionality.1-3 The final properties of inorganic-organic functional hybrids are controlled by interactions between a host (inorganic layered matrix) and guest (functional organic molecule) because the strength and directionality of interactions affect the distribution, orientation, and electronic properties of guest molecules. A lot of effort has been devoted to studying spectral, photophysical, and photochemical properties of these hybrids4,5 because the inorganic host structure offers stabilization and protection, whereas the chromophore species provide the optical function such as color,6,7 thermochromicity,8 luminescence,6,9,10 formation of singlet oxygen,11,12 nonlinear optical properties, photooxidation,13 or UV absorption.14 These studies are crucial for the development of materials for energy storage and conversion, sunscreens, photocatalysis, and sensing or photochemical devices. Layered double hydroxides (LDH) have a privileged position in the field of layered hydroxides. The structural network consists of brucite-like layers with the general formula [M2+1-xM3+x(OH)2An-x/n] · mH2O, where M2+ and M3+ are divalent and trivalent cations forming the hydroxide layers, An- is interlayer anion, and R ) (1 - x)/x is the M2+/M3+ molar ratio. A positive charge is located on the hydroxide layers, and it is counterbalanced by anionic species aligned in the interlayer space. The lamellar structure, the wide range of cationic compositions of the hydroxide layers, and the ease with which various inorganic and organic anions can be accommodated in the interlayer space of LDH bring these materials to a high level of interest. * Author to whom correspondence should be addressed. E-mail: lang@ iic.cas.cz (Kamil Lang). † Institute of Inorganic Chemistry of the AS CR. ‡ J. Heyrovský Institute of Physical Chemistry of the ASCR. § Charles University in Prague.

Photoactive LDH materials have been obtained, among others, by the insertion of porphyrins,11,12 methyl orange,6 derivatives of naphthalene,9 benzophenone,10 or perylene.7 The functionalization of layered hydroxide salts15 by anions is comparably rare and was inspired by searching for materials with novel magnetic properties. This is the case of layered transition-metal hydroxide salts of the general formula [M2(OH)3A-] (M ) Co, Ni, Cu).16-18 In contrast to LDH, the brucite-like layers of [Cu2(OH)3NO3] contain one-fourth of hydroxide anions replaced by nitrate anions. The advantage is the presence of a paramagnetic single metal ion in octahedral positions and the fact that anions are directly coordinated to the metal sites. Similarly, layered rare-earth hydroxide structures [Ln2(OH)5A-] are very promising because they allow the targeting combination of the inherent luminescence properties of the hydroxide layers with luminescence of incorporated anions.19 To date, only few groups have studied layered zinc hydroxide (LZH) salts of the general formula [Zn5(OH)8(An-)2/n] · 2H2O. These materials derive the structure from LZH nitrate [Zn5(OH)8(NO3)2] · 2H2O having the brucite-like layers with onequarter of octahedrally coordinated zinc atoms replaced by tetrahedrally coordinated zinc atoms located below and above the plane.20 The layers are charged because of valence vacancies at tetrahedral zinc atoms. The layer charge is compensated by anions that are located in the interlayer space. The attractive aspect of LZH is their relatively simple synthesis, high anionexchange capacity (3.2 meq/g for LZH nitrate) comparable to LDH, the possibility to mix Zn atoms with other metals to form double hydroxide salts [(M32+Zn2)(OH)8(An-)2/n] · mH2O, where M atoms (Ni or Co) are placed in octahedral sites and Zn atoms in tetrahedral sites,21,22 and the use as a precursor for the preparation of a wide-band-gap semiconductor ZnO.15,23,24 ZnO in a combination with suitable dyes such as porphyrins is an interesting multifunctional material for development of dye-

10.1021/jp106116n  2010 American Chemical Society Published on Web 09/01/2010

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sensitized solar cells.25 Three strategies have been developed for the preparation of LZH hybrids. (i) The first one is ionexchange reactions of LZH nitrate. This frequently used strategy was applied for intercalation of dicarboxylic and benzoic acids,26,27 indole-3-acetic acids,28 or azo dyes.29 In contrast, larger diazo anions such as Evans Blue were only adsorbed on the outer surface.29 To overcome these limitations, the pre-expanded LZH structure with aliphatic diacid30 or acetate anions22 was used. (ii) The second strategy is the reaction of benzoic acid with zinc hydroxide in organic solvents.31 The drawback of this method is the contamination of final products by starting precursors. (iii) The third method is the direct coprecipitation of LZH in the presence of desired organic anion and has rarely been used.23,27,28 Porphyrins and related macrocycles are photosensitizers with rich fluorescence properties and ability to produce singlet oxygen, O2(1∆g).32 The photosensitized production of O2(1∆g) is based on excitation of a photosensitizer to the triplet states followed by energy transfer to the ground electronic state of oxygen that is excited to the lowest singlet state, O2(1∆g). Intercalation of porphyrins into LDH has proved to retain their photosensitizing properties.11,12 LDH-porphyrin hybrids are particularly interesting as handy sources of singlet oxygen, for construction of bactericidal coatings, and also offer potential applications in catalysis.33-35 In contrast, the LZH structure has not been considered as a host for photosensitizing compounds yet. In the present work, we have investigated for the first time the intercalation of anionic porphyrins into the LZH structure by the coprecipitation method. Molecular-dynamics simulations indicate an inclined orientation of the porphyrin planes with respect to the hydroxide layers. The structural and spectral properties and the formation of O2(1∆g) by LZH-porphyrin hybrids are also described. Experimental Section Materials. The tetrasodium salt of 5,10,15,20-tetrakis(4sulfonatophenyl)porphyrin, TPPS (Aldrich), Pd(II)-5,10,15,20tetrakis(4-sulfonatophenyl)porphyrin, PdTPPS, Pd(II)-5,10,15,20tetrakis-(4-carboxyphenyl)porphyrin, PdTPPC (both Frontier Scientific Europe, Ltd., UK), Zn(NO3)2 · 6H2O, NaOH, and Na2CO3 (all Penta, Czech Republic) were used as purchased. Synthesis. LZH nitrate Zn5(OH)8(NO3)2 · 2H2O (LZH-NO3) was prepared by the coprecipitation method described earlier.36 A total of 50 mL 0.75 Mol L-1 NaOH (37.5 mmol) was added dropwise to a vigorously stirred solution of 20 mL 3.5 Mol L-1 Zn(NO3)2 (70 mmol) during 1 h at room temperature. The white product was filtered immediately after the addition of NaOH, washed thoroughly with water, and dried at room temperature. Intercalation with porphyrins was performed analogously to produce solids, hereafter abbreviated as LZH-ZnTPPS, LZHPdTPPS, and LZH-PdTPPC. Solutions of tetrasodium salts of TPPS, PdTPPS, or PdTPPC (0.225 mmol; 2-fold molar excess) and Zn(NO3)2 (3.5 mmol) were mixed together, followed by the dropwise addition of 5 mL of 0.375 Mol L-1 NaOH (1.9 mmol) during 30 min. The colored precipitate was filtered, thoroughly washed with water and acetone, and dried. The slurry of LZH-PdTPPS was stirred overnight before the filtration in order to increase the sample crystallinity. Instrumental Methods. Powder X-ray diffraction (XRD) was performed on a PANalytical X’Pert PRO diffractometer in the conventional Bragg-Brentano geometry. Incident X-ray beam produced from a Cu X-ray tube (40 kV, 30 mA) passed through a 0.02 rad Soller slit, 1/8° divergence slit, 15 mm fixed mask, and a 1/4° antiscatter slit. The diffracted beam was detected by

Demel et al. a PIXcel linear position sensitive detector equipped with a β filter, 0.02 rad Soller slit, and 1/8° antiscatter slit. The XRD patterns were recorded in the range of 2-90° (2θ) with a step of 0.013° and an acquisition time of 80 s per step. Qualitative analysis was performed with the HighScorePlus software package (PANalytical, The Netherlands, version 2.2.5) and the JCPDS PDF-2 database.37 In-situ high-temperature XRD were collected by using a high-temperature Anton PAAR HTK-16 chamber installed on a PANalytical X’Pert PRO X-ray diffractometer (Co KR radiation, 40 kV, 30 mA, multichannel detector X’Celerator with an antiscatter shield) in the Bragg-Brentano geometry. The temperature was increased from 25 to 300 °C with a step of 5 °C (16 min acquisition at each temperature, heating rate 60 °C/min). Thermal analyses (TGA/DTA/MS) were carried out by using a Setaram SETSYS Evolution instrument. The gas-emission analysis was performed by using a Setaram SETSYS Evolution16-MS coupled with a mass-spectroscopy system. The measurements were performed in synthetic air atmosphere (flow rate 30 mL/min) from 30 to 1050 °C with a heating rate of 5 °C/ min. The infrared spectra (FTIR) were collected on a Nicolet NEXUS 670-FT spectrometer in KBr pellets. The diffuse reflectance absorption spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer equipped with a Labsphere RSA-PE20 integration sphere. The content of zinc was determined on an ICP-OES spectrometer (IRIS Intrepid II; Thermo Electron Corp.) equipped with an axial plasma and ultrasonic CETAC nebulizer, model U-5000AT+. The samples were mineralized by combustion in oxygen flame, and the residue was dissolved in aqua regia (HCl-HNO3, 3:1 v/v). The amount of intercalated/ adsorbed porphyrin was obtained spectrophotometrically upon the dissolution of porphyrins in 0.01 Mol L-1 NaOH (LZHPdTPPS) or 0.01 Mol L-1 HCl (LZH-ZnTPPS). The elemental analysis C, H, N was determined by a standard combustion technique. The photoelectron spectra were measured by using a spectrometer ESCA 3 Mk 2 (VG) with a hemispherical analyzer operated with the constant pass energy of 20 and 50 eV in the low-resolution (1000 eV scan, step 1 eV) as well as in the highresolution (30 eV scan, step 0.1 eV) regimes. The nonmonochromatized Al K R1,2 line was used to excite photoelectrons. The X-ray source was operated at 110 W, and vacuum was typically ∼1.10-9 Torr at the beginning of the experiment. The samples were introduced into the spectrometer on a doublesided Scotch tape (see Supporting Information for more details). The time-resolved near-infrared luminescence of O2(1∆g) at 1270 nm was monitored by using a Ge detector upon laser excitation by a Lambda Physik FL 3002 dye laser (λexc ) 425 nm, incident energy 0.2-0.3 mJ/pulse). The short-lived signal produced by the scattering of an excitation laser pulse and/or by porphyrin fluorescence was eliminated by exciting the sample in argon atmosphere and subtracting the obtained signal from the signal recorded in oxygen atmosphere. The powders were equilibrated in selected atmosphere by evacuating a cell and filling it with Ar or O2. The treatment was repeated three times to ensure the desired atmosphere. The signal-to-noise ratio of the signals was improved by averaging 200-500 individual traces. Molecular Modeling. Molecular mechanics and classical molecular dynamics38 were carried out in the Cerius2 and Materials Studio modeling environment.39 The structure of ZnTPPS was optimized by the quantum-chemistry computational program Turbomole v5.9. The LZH matrix was con-

LZH Salts with Intercalated Porphyrin Sensitizers

Figure 1. Powder XRD patterns of LZH-NO3 (a) compared with LZHZnTPPS (b), LZH-PdTPPS (c), and LZH-PdTPPC (d).

structed as a bilayered structure in the C2/m space group by using the cell parameters of LZH-NO3 (JCPDS No. 01-0720627);37 that is, b ) 6.238 Å, c ) 5.517 Å, and β ) 93.28°.20 The basal spacing in initial models was set to an experimental value of 22.8 Å (i.e., a ) 2 × d200 ) 45.6 Å) obtained from the XRD data. A layer of [Zn80(OH)128]32+ · 32 H2O bearing a charge of +32 with lattice parameters B ) 24.952 Å and C ) 22.068 Å was created by the linking of 16 individual cells. The complete saturation of the layer charge by ZnTPPS anions in the interlayer space leads to the two-layered supercell [Zn160(OH)256]64+[(ZnTPPS)16]64- · 64 H2O. A set of models with various orientations of ZnTPPS anions was created. The space group was set to P1 for all calculations. Charges were calculated by using the Qeq method (charge equilibrium approach).40 The models were optimized in the Dreiding and Universal force field,41,42 the electrostatic energy was calculated by the Ewald summation method,43 and the van der Waals energy was calculated by using the Lennard-Jones potential.44 The porphyrin pyrroles were kept as rigid units during the minimization, and the layers were kept frozen except for hydrogen atoms of the OH groups and tetrahedrally coordinated water molecules. The dynamics simulations were carried out in an NVT statistical ensemble (N, constant number of atoms; V, constant volume; T, constant temperature) at 300 K. One dynamic step was 0.001 ps, and dynamics of 200 ps was carried out. Porphyrin pyrrole atoms were kept fixed during quench dynamics, whereas other atomic positions in the interlayer space together with hydrogen atoms of the layers were variable. After quench dynamics, the interlayer space of selected minimized structures was minimized in the Dreiding force field to obtain the final structure models. Results and Discussion Preparation and Characterization of LZH Intercalated with Porphyrins. The observed XRD pattern of LZH nitrate (LZH-NO3) corresponds to the previously published data (JCPDS No. 01-072-0627).20,36,37 The position of the first diffraction peak (Figure 1a) corresponds to the basal spacing of 9.73 Å allowing nitrate anions to adopt an approximately perpendicular orientation with respect to the hydroxide layers. The prepared samples are well crystallized with no impurities such as ZnO, a product of thermal decomposition. Comparison of the data with results on LDH (i.e., Zn2Al(OH)6(NO3) abbreviated hereafter as Zn2Al LDH) having nitrate anions, similar composition, and layer charge shows that LZH-NO3 has a 1∼ Å larger basal spacing due to tetrahedral Zn atoms protruded into the interlayer.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16323 The most often used synthetic strategy, based on the exchange of nitrates for other anions, was not successful with porphyrin anions at room temperature. Higher temperatures were not applied because LZH-NO3 decomposes (see below). The exchange reaction did not proceed even when the LZH layers were pre-expanded by dodecylsulfate anions. The result is in agreement with findings that large multivalent anions do not intercalate but adsorb on the outer surface of the platelets.29 It is obvious that rigid two-dimensional large molecules do not have enough driving force to penetrate between stacked hydroxide layers because of the tetrahedral Zn(OH)3(H2O) units exposed to the interlayer space. In contrast, the strategy based on the coprecipitation in the presence of porphyrin salts proved to be a successful method for the synthesis of LZH-porphyrin hybrids. Tetraanionic porphyrins, TPPS or PdTPPS, were coprecipitated from solutions containing 100% excess of porphyrin over the anion-exchange capacity (AEC, 3.2 meq/g for LZH-NO3), yielding hybrids LZH-ZnTPPS and LZH-PdTPPS, respectively. Under the synthesis conditions, originally used TPPS is completely metalated to ZnTPPS. The same metalation occurs during the synthesis of ZnRAl LDH hybrids (R ) 2-4).12 Elemental analysis, XPS (see below), and the amount of ZnTPPS or PdTPPS in the solids show that both samples can be characterized by the formula [Zn5(OH)8(porphyrin)0.5] · mH2O (m ≈ 2) (Table S1 in the Supporting Information). The synthesis of LZH-PdTPPC was complicated by strong sorption of PdTPPC on the crystal surface probably due to the relatively low solubility of PdTPPC during the synthesis. Therefore, LZHPdTPPC is not further studied in detail. The prepared hybrids have a layered structure with typical h00 diffraction lines between 4 and 20° (Figure 1). The high affinity of anionic porphyrins toward the hydroxide layers is evidenced by the fact that all diffractions can be assigned to pure phases of respective hybrids with no impurities such as ZnO or LZH-NO3. LZH-ZnTPPS has the best crystallinity, whereas the low crystallinity of LZH-PdTPPC can be explained by the nonstoichiometric amount of PdTPPC and nonuniformed structure. The basal spacing expanded from 9.73 Å in pure LZHNO3 to 22.8 and 22.2 Å for LZH-ZnTPPS and LZH-PdTPPS, respectively. The interlayer space available for intercalated porphyrin anions can be estimated similarly to hybrid LDH by subtracting a thickness of the hydroxide layer (i.e., 4.80 Å) from the basal spacing supposing that anionic substituents are arranged between the zinc tetrahedra. The obtained gallery height of about 18.0 Å is comparable with the size of a porphyrin molecule and indicates a tilted orientation of the porphyrin planes to the hydroxide layers similarly to the alignment of the porphyrin units in LDH.11,12 Nonbasal diffractions located around 33, 60, and 70° confirm the identity of the hydroxide layers (Figures 1 and 4 inset). The LZH-ZnTPPS and LZH-PdTPPS hybrids were characterized by FTIR, TGA/DTA/MS, and XPS. The FTIR spectrum of LZH-NO3 is consistent with previously published data (Figure S1 in the Supporting Information).45,46 The broad absorption around 3490 cm-1 corresponds to the O-H stretching vibrations of the hydroxyl groups from the layers and water molecules. Molecularly adsorbed H2O is further identified by the characteristic bending mode at 1639 cm-1. The strong absorption around 1380 cm-1 belonging to interlayer nitrates disappears in LZH-ZnTPPS and LZH-PdTPPS because of the complete saturation of the hydroxide layer charge by porphyrins (Figure S1 in the Supporting Information). In addition, both hybrids display strong absorption bands belonging to sulfonated por-

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Figure 2. Normalized diffuse reflectance spectra of LZH-PdTPPC (a), LZH-ZnTPPS (b), Zn2Al LDH-PdTPPC (c), and Zn2Al LDH-ZnTPPS (d).

phyrins. Virtually, no shift of the phenyl related bands indicates that the structure of the porphyrin moiety in the interlayer space is preserved without a significant ring distortion. The observed bands at 1176 and 1037 cm-1 in LZH-ZnTPPS correspond to the fundamental stretching vibrations νas(SO3-) and νs(SO3-), respectively, whereas pure TPPS and ZnTPPS salts have these bands at 1188-1190 and 1038 cm-1.47 The LZH-PdTPPS sample has the bands at 1171 and 1038 cm-1. The positions and intensities of these two bands are practically the same in LZH and in structurally similar LDH materials (Figure S2 in the Supporting Information). For example, Zn2Al LDH-ZnTPPS and Mg2Al LDH-TPPS have these bands at 1171, 1036 and 1176, 1038 cm-1, respectively. This suggests that the SO3groups of intercalated porphyrins between the LZH layers are not directly coordinated to zinc atoms of the layers, but they rather interact with the hydroxide layers via noncovalent interactions such as in LDH.12,16,17 The nature of an intercalated molecule affects the thermal behavior (Figure S3 in the Supporting Information). The results on LZH-NO3 follow these described earlier.46,48 The TGA/DTA/ MS (Figure S4 in the Supporting Information) and XRD (Figure S5 in the Supporting Information) analyses document the loss of coordinated water leading to the structurally equivalent Zn5(OH)8(NO3)2. The process is completed at 110 °C, followed by the dehydroxylation of the hydroxide layers accompanied with the formation of ZnO. In contrast, LZH-ZnTPPS undergoes a mass loss of 18.1% between 30-300 °C (Figure S7 in the Supporting Information). It is ascribed to bound/interlayer water (30-119 °C) and hydroxide groups (120-300 °C). The decomposition of ZnTPPS starts at about 350 °C with the evolution of S, N, and C oxides. Detailed structural information can be obtained from the XRD patterns recorded at different temperatures (Figure S6 in the Supporting Information). The decomposition of the brucite-like layers is indicated by the decreasing intensities of the basal diffractions and by the disappearance of the nonbasal diffractions at 135 °C (Figure S6, inset, in the Supporting Information). In contrast to LZH-NO3, there is no formation of ZnO below 300 °C because the complete decomposition of amorphous mixture of salts is at about 820 °C. The surface composition of hybrids (∼5 nm in depth) was analyzed by X-ray photoelectron spectroscopy (XPS) (Table S2 in the Supporting Information). The Zn/N ratio of LZH-NO3 is higher than the stoichiometry (Zn/N ) 3.8 instead of 2.5). It may arise from the partial replacement of NO3- anions by OHduring the LZH formation, the amorphous character of the interface region addressed by XPS, and experimental uncertain-

Demel et al.

Figure 3. Time dependence of the O2(1∆g) luminescence signal of LZH-PdTPPS (λexc ) 425 nm, ∼0.2 mJ, oxygen atmosphere, recorded at 1270 nm). The solid line is a least-squares monoexponential fit.

Figure 4. Experimental (a) and calculated (b) XRD basal diffraction lines of LZH-ZnTPPS hybrid. Inset: corresponding experimental (a) and calculated (b) XRD nonbasal diffraction lines.

ties. The ratio S/N related to the intercalated porphyrin molecule fits the theoretical value well, whereas the Pd/N ratio shows the depletion of Pd atoms in the surface layer. The binding energies are collected in Table S3 in the Supporting Information. In all studied samples, the C 1s photoelectron spectra show only one type of C-containing species with a binding energy of Eb ) 284.8 eV (aromatic and contamination carbon). No carbonates are present (Eb ≈ 289 eV). The only one S 2p doublet in the spectra indicates one type of S-containing species that is in accordance with four equivalent sulfonate groups of intercalated porphyrins. The analysis of the Zn 3s, Zn 3p, and Zn 2p photoelectron spectra and Zn LM4,5M4,5 Auger spectra of Zn atoms in the layers and in ZnTPPS shows that the spectra are not sensitive to the environment of Zn atoms. The N 1s spectrum of LZH-NO3 was fitted by four lines assigned to NO3(Eb ) 406.9 eV), NO2- (Eb ) 404.2 eV), N-containing contamination (Eb ) 398.5 eV), and R3,4 satellite of the NO3line (Eb ) 396.9 eV), Figure S8 in the Supporting Information. It is interesting to compare the N 1s spectra of intercalated ZnTPPS and PdTPPS with pure TPPS. The N 1s spectrum of TPPS is composed of the lines at 397.6 eV (iminic N atoms: -C-Nd), 399.9 eV (pyrrolic N atoms: -NH-),49 and the highenergy line at Eb ) 401.1 eV assigned to a satellite (π f π* transition).50 In contrast, LZH-PdTPPS and LZH-ZnTPPS have the lines at 399.0 eV (satellite Eb ) 401.4 eV) and 399.5 eV (satellite Eb ) 401.3 eV), respectively, signifying that N atoms become chemically identical upon binding of porphyrin central metal. The high intensities of the N 1s satellites (Table S3 in the Supporting Information) are qualitatively explainable by relatively high crystallinity of the samples because an amorphous

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Figure 5. Top view of the interlayer space of the linked supercells.

Figure 6. Side view of the interlayer space: porphyrin units located in the front (red), in the middle (blue), and at the back (green). Small red and white balls represent oxygen and hydrogen atoms, respectively.

porphyrin phase does not have these satellites.51 In general, the N 1s spectra document the complete saturation of the samples with porphyrins because there are no signals of NO3- (Figure S8 in the Supporting Information). Spectral Properties. The results presented above show that LZH are suitable hosts for accommodating porphyrin photosensitizers of cytotoxic O2(1∆g).32 The absorption spectrum of pure solid TPPS displays the Soret band at 412 nm and four Q bands at 516, 550, 590, and 647 nm.12 In contrast, the Soret band of LZH-ZnTPPS is shifted to ∼427 nm with two Q bands at 561 and 602 nm (Figure 2), and the spectrum corresponds to that of ZnTPPS with the bands at 424, 561, and 603 nm.12 The results confirm that TPPS is metalated to ZnTPPS during the precipitation procedure and justify the use of the formula LZHZnTPPS. It is worth noting that TPPS has a high affinity toward

free Zn2+ in solution and that metalation also occurs during intercalation of TPPS into Zn-containing LDH.12 The spectra of PdTPPC and PdTPPS have a broad Soret band at about 416 nm and Q bands at ∼530 and ∼570 nm (Figure 2). The absorption spectra of porphyrins provide important information for studying their photosensitizing activity because the spectral features allow us to specify the extent of aggregation.52 Aggregation of porphyrins usually leads to a fast competitive relaxation of the excited states associated with the decrease of the quantum yields of the triplet states and O2(1∆g) formation. The mutual arrangement of porphyrin units in aggregates generally falls into two types: (i) J-aggregates (edgeto-edge) characterized by a red shift of the Soret band, whereas (ii) the formation of H-aggregates (face-to-face) is accompanied by a blue shift. As shown in Figure 2, the intercalation of

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TABLE 1: Comparison of Structural Parameters of LZH-ZnTPPS and Zn2Al LDH-ZnTPPS Hybrids

system Zn2Al LDH-ZnTPPSa LZH-ZnTPPS

basal tilted angle between distance between spacing angleb planesc planesd (Å) (deg) (deg) (Å) 23.0 22.8

14 22

0-10 0-20

5.5 4

a

Reference 12. b Average angle between the layer normal and porphyrin plane. c The angle between neighboring porphyrin planes varies between 0 (parallel arrangement) and the given maximal angle. d Average distance between neighboring porphyrin planes.

TABLE 2: Energy Characteristics Related to a ZnTPPS Molecule Intercalated in the Zn2Al LDH and LZH Hostsa system

Etotal/kcal mol-1

Eelst/kcal mol-1

EvdW/kcal mol-1

EHB/kcal mol-1

Zn2Al LDH-ZnTPPSb LZH-ZnTPPS

-1100 -1088

-1010 -1037

-20 -14

-70 -37

a Total sublimation energy Etotal and corresponding van der Waals (vdW), electrostatic (elst), and hydrogen-bonding (HB) contributions. b Structural model in ref 12.

porphyrins into the LZH host causes the significant broadening of the absorption bands when compared with the spectra of intercalated ZnTPPS12 and PdTPPC34 in Zn2Al LDH. It might indicate extensive electronic interactions of porphyrin moieties because of their closer packing in LZH (see below). Production of Singlet Oxygen. As shown earlier, TPPS and PdTPPC intercalated in LDH produce O2(1∆g) upon laser flash irradiation.11,12,33,34 These results were based on the measurements of the porphyrin triplet state interaction with molecular oxygen and directly evidenced by time-resolved luminescence of O2(1∆g) at 1270 nm arising from relaxation of O2(1∆g) to the ground triplet state. The effects of LZH on the photoactivity of porphyrins have not been addressed yet. Therefore, we performed detailed measurements. LZH-ZnTPPS hybrid behaves similarly to LDH with intercalated ZnTPPS,12 indicating that ZnTPPS between the hydroxide layers completely loses the photosensitizing activity. On the contrary, both LZH-PdTPPS and LZH-PdTPPC show intense signals of photoproduced O2(1∆g) (Figure 3). It is evident that O2(1∆g) is generated by the photosensitized reaction

of intercalated porphyrins because pure LZH-NO3 does not have any O2(1∆g) luminescence. The O2(1∆g) luminescence intensity decays monoexponentially, giving effective O2(1∆g) lifetimes of 30 and 41 µs for LZH-PdTPPS and LZH-PdTPPC in oxygen atmosphere, respectively. Both these hybrids appear to be good O2(1∆g) producers, and the long lifetime of O2(1∆g) suggests that the LZH host is a good carrier of Pd porphyrins. Molecular Modeling. Molecular modeling combined with the XRD results can give information on the potential arrangement of guest molecules in the interlayer space not available by direct measurements. Analyzed structural models were constructed by using the van der Waals radii of the porphyrin guest to avoid the overlap, when assuming that intercalation does not affect the structure of the hydroxide layers and that sulfonated porphyrins interact with the brucite-like layers of LZH similarly to LDH.12 The experimental basal lines of LZH-ZnTPPS are in good agreement with the calculated results (Figure 4). Low-intensity peaks between 5 and 7° and at about 16.5° in the calculated pattern are imposed by the forced periodicity of porphyrin Zn atoms and diminish after their removal from the models. It indicates a disorder of guest porphyrins in the interlayer space of the real sample (see below). The positions of the experimental and calculated nonbasal diffractions are also in good agreement (Figure 4, inset); however, the experimental diffractions are broadened because of low crystallinity and/or small particles dimensions. The optimized model presented in Figure 5 with a disordered filling of the interlayer space is based on the agreement between calculated and experimental XRD patterns and the arguments concerning the forced periodicity of ZnTPPS central zinc atoms due to a limited volume of the cell used for the calculations. The angle between neighboring porphyrin planes varies between 0 (parallel arrangement) and 20°; thus, the disorder is greater than that in structurally similar Zn2Al LDH-ZnTPPS.12 In addition, the ratio of free/occupied cell volumes calculated by using the van der Waals radii decreases from 0.72 for Zn2Al LDH-ZnTPPS to 0.44 for LZH-ZnTPPS. The low free volume in the latter case signifies a close arrangement of porphyrin planes with a distance of 4 Å in average (for comparison, this distance in LDH is between 5 and 6 Å). Repulsive interactions especially between neighboring sulfonato phenyl rings and pyrroles cause some deviations of

Figure 7. Detailed view of the hydrogen-bonding pattern between the porphyrin sulfonate groups and hydrogen atoms (light gray balls) of the hydroxide layer.

LZH Salts with Intercalated Porphyrin Sensitizers porphyrin unit-phenyl dihedral angles. The mutual horizontal shift of neighboring guests equals to about one half of a porphyrin unit (∼7 Å). Figure 6 represents a side view of the interlayer space of the optimized model. As in the LDH matrix,12 the porphyrin guests adopt an inclined orientation with respect to the layers. The deviation from perpendicular arrangement is characterized by the angle between the layer normal and the porphyrin plane and ranges in the interval of 22 ( 3°. The higher value of the tilted angle in comparison to LDH (∼14°) can be caused by (i) a slightly lower basal spacing and (ii) steric hindrance by water molecules coordinated to the apexes of zinc tetrahedrons. The arrangement of ZnTPPS in the LZH and Zn2Al LDH hosts is summarized in Table 1. Anions of ZnTPPS interact with the host layers via dominant electrostatic interactions with the contribution of van der Waals and hydrogen bonding interactions. The respective contributions are given in Table 2 for both LZH and Zn2Al LDH host structures. The total sublimation energies are similar, indicating very similar stability of both systems. A hydrogen-bonding pattern between the sulfonate groups of ZnTPPS and a LZH layer is shown in Figure 7. The porphyrin arrangement in the interlayer space can be further specified by an atom-concentration profile constructed on the assumption of equivalence of all atoms (Figure S9 in the Supporting Information). The peaks at 0 and ∼23 Å represent zinc atoms located in the octahedral positions of the layers. Because the layers contain vacant positions, the peaks have lower intensities than the peaks at ∼1 and ∼22 Å attributed to hydroxide oxygen atoms. The maxima at 4.5 and 18.5 Å represent tetrahedrally coordinated water molecules, and the sulfonate groups of the porphyrin units and porphyrin pyrroles show broad maxima at ∼7 and ∼15 Å. The presented model has porphyrin Zn atoms at ∼11 Å, which is ∼0.2 Å above the midplane. A maximal vertical deviation of 0.5 Å above and below the midplane in other minimized models suggests that the average position of central zinc atoms is close to the interlayer midplane. Conclusions A new approach to the development of solid state O2(1∆g) sensitizers is based on a series of LZH hybrids containing intercalated anionic porphyrins. The hybrids were prepared by the coprecipitation method because the ion exchange of nitrate anions in [Zn5(OH)8(NO3)2] · 2H2O for porphyrins does not occur. The XRD patterns confirm the identity of the hydroxide layers and the layered structure of the hybrids with a gallery height comparable with the size of porphyrin molecules. Thermal behavior is affected by intercalated anions. In the porphyrin hybrid, the hydroxide layers disappear at 135 °C, but the products of decomposition of the sulfonate groups can be detected even at 820 °C. The charge of the hydroxide layers is completely counterbalanced by porphyrins because possible contaminant anions NO3- or CO32- are not present. Molecular simulations of LZH-ZnTPPS give the gallery filled with the porphyrin guests: (i) the porphyrin planes are inclined with respect to the hydroxide layer normal by about 22°; (ii) the angle between the porphyrin planes varies between 0 (parallel arrangement) and 20°; (iii) the porphyrin units are closely packed; and (iv) the sulfonated porphyrins interact with the brucite-like layers of LZH similarly to LDH as also follows from FTIR results. The measured absorption spectra of the hybrids confirm the identity of intercalated porphyrins. The hybrids appear to be good O2(1∆g) producers with the large effective lifetimes of O2(1∆g) suggesting that the LZH host is a good carrier of Pd porphyrins.

J. Phys. Chem. C, Vol. 114, No. 39, 2010 16327 Acknowledgment. This work was supported by the Czech Science Foundation (P207/10/1447), the Research plan of the Academy of Sciences of the Czech Republic (AV0Z40320502, AV0Z40400503), and the Ministry of Education, Youth and Sports of the Czech Republic (MSM 0021620835). We are grateful to Petr Bezdicˇka (IIC, ASCR, Rˇezˇ) for measurement of XRD data. Supporting Information Available: Details of photoelectron spectra analysis, tables of elemental and XPS results, FTIR spectra, TGA curves, TGA/DTA curves and the evolution of gases, temperature dependence of the powder XRD patterns, N 1s photoelectron lines, and concentration profile in the [100] direction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Newman, S. P.; Jones, W. New. J. Chem. 1998, 105. (2) Khan, A. I.; O’Hare, D. J. Mater. Chem. 2002, 12, 3191. (3) Leroux, F.; Taviot-Gue´ho, C. J. Mater. Chem. 2005, 15, 3628. (4) Ogawa, M.; Kuroda, K. Chem. ReV. 1995, 95, 399. (5) Takagi, S.; Eguchi, M.; Tryk, D. A.; Inoue, H. J. Photochem. Photobiol. C 2006, 7, 104. (6) Costantino, U.; Coletti, N.; Nocchetti, M.; Aloisi, G. G.; Elisei, F. Langmuir 1999, 15, 4454. (7) Bauer, J.; Behrens, P.; Speckbacher, M.; Langhals, H. AdV. Funct. Mater. 2003, 13, 241. (8) Wang, X.; Lu, J.; Shi, W.; Li, F.; Wei, M.; Evans, D. G.; Duan, X. Langmuir 2010, 26, 1247. (9) Shi, W.; Wei, M.; Lu, J.; Li, F.; He, J.; Evans, D. G.; Duan, X. J. Phys. Chem. C 2008, 112, 19886. (10) Wang, Z.-L.; Kang, Z.-H.; Wang, E.-B.; Su, Z.-M.; Xu, L. Inorg. Chem. 2006, 45, 4364. (11) Lang, K.; Bezdicˇka, P.; Bourdelande, J. L.; Hernando, J.; Jirka, I.; Ka´funˇkova´, E.; Kovanda, F.; Kuba´t, P.; Mosinger, J.; Wagnerova´, D. M. Chem. Mater. 2007, 19, 3822. (12) Ka´funˇkova´, E.; Taviot-Gue´ho, C.; Bezdicˇka, P.; Klementova´, M.; Kova´rˇ, P.; Kuba´t, P.; Mosinger, J.; Pospı´sˇil, M.; Lang, K. Chem. Mater. 2010, 22, 2481. (13) Pigot, T.; Dupin, J. C.; Martinez, H.; Cantau, C.; Simon, M.; Lacombe, S. Microporous Mesoporous Mater. 2005, 84, 343. (14) Perioli, L.; Ambrogi, V.; Bertini, B.; Ricci, M.; Nocchetti, M.; Latterini, L.; Rossi, C. European J. Pharm. Biopharm. 2006, 62, 185. (15) Arizaga, G. G. C.; Satyanarayana, K. G.; Wypych, F. Solid State Ionics 2007, 178, 1143. (16) Delahaye, E´.; Eyele-Mezui, S.; Bardeau, J. F.; Leuvrey, C.; Mager, L.; Rabu, P.; Rogez, G. J. Mater. Chem. 2009, 19, 6106. (17) Taibi, M.; Ammar, S.; Jouini, N.; Fie´vet, F.; Molinie´, P.; Drillon, M. J. Mater. Chem. 2002, 12, 3238. (18) Kurmoo, M.; Day, P.; Derory, A.; Estourne`s, C.; Poinsot, R.; Stead, M. J.; Kepert, C. J. J. Solid State Chem. 1999, 145, 452. (19) Hu, L.; Ma, R.; Ozawa, T. C.; Geng, F.; Iyi, N.; Sasaki, T. Chem. Commun. 2008, 4897. (20) Sta¨hlin, W.; Oswald, H. R. Acta Crystallogr. 1970, B26, 860. (21) Tronto, J.; Leroux, F.; Dubois, M.; Taviot-Gueho, C.; Valim, J. B. J. Phys. Chem. Solids 2006, 67, 978. (22) Rajamathi, J. T.; Raviraj, N. H.; Ahmed, M. F.; Rajamathi, M. Solid State Sci. 2009, 11, 2080. (23) Morioka, H.; Tagaya, H.; Kadokawa, J.-I.; Chiba, K. J. Mater. Sci. Lett. 1999, 18, 995. (24) Hu, X.; Masuda, Y.; Ohji, T.; Kato, K. Thin Solid Films 2009, 518, 621. (25) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129, 4655. (26) Wypych, F.; Arı´zaga, G. G. C.; Gardolinski, J. E. F. C. J. Colloid Interface Sci. 2005, 283, 130. (27) Cursino, A. C. T.; Gardolinski, J. E. F. C.; Wypych, F. J. Colloid Interface Sci. 2010, 347, 49. (28) Yang, J.-H.; Han, Y.-S.; Park, M.; Park, T.; Hwang, S.-J.; Choy, J.-H. Chem. Mater. 2007, 19, 2679. (29) Marangoni, R.; Ramos, L. P.; Wypych, F. J. Colloid Interface Sci. 2009, 330, 303. (30) Arizaga, G. G. C.; Gardolinski, J. E. F. C.; Schreiner, W. H.; Wypych, F. J. Colloid Interface Sci. 2009, 330, 352. (31) Ogata, S.; Tagaya, H.; Karasu, M.; Kadokawa, J. J. Mater. Chem. 2000, 10, 321. (32) Lang, K.; Mosinger, J.; Wagnerova´, D. M. Coord. Chem. ReV. 2004, 248, 321.

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