Layer-by-Layer Growth of Electrostatically Assembled Multilayer

Langmuir 1996, 12, 5393-5398

5393

Layer-by-Layer Growth of Electrostatically Assembled Multilayer Porphyrin Films Koiti Araki,† Michael J. Wagner,‡ and Mark S. Wrighton* Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received January 5, 1996. In Final Form: June 21, 1996X The feasibility of preparing multilayer porphyrin films with monolayer control of composition and thickness is demonstrated. Successive dip-coating of tetraruthenated zinc porphyrin, [ZnTPyPBpy]4+, and mesotetraphenylporphyrin sulfonate, [M-TPPS]4-, is shown to result in linear growth of the film thickness (12.7(6) Å/bilayer) and optical absorbance as a function of the number of bilayers. These results strongly suggest a layer-by-layer assembly of compositionally homogeneous films of up to 30 bilayers. The assembled multilayer composite films have been characterized by UV-vis spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and electrochemical methods. Electrodes modified with these films exhibit a reversible wave at 0.94 V and are photocatalytically active toward the reduction of O2. Our findings suggest that the layer-by-layer growth method may be a general route to compositionally modulated porphyrin films of arbitrary thickness and rationally tailored catalytic and photophysical properties.

Porphyrins, phthalocyanines, and related compounds have been extensively investigated due to their potential utility in optical,1 photoelectrochemical,2-5 and sensor6-11 applications. A great variety of reactions are catalyzed by porphyrins and metalloporphyrins,12-14 and they display strong photoelectrochemical activity.15-17 In addition, they tend to adsorb strongly on some surfaces, retaining many of their solution properties. However, the preparation of suitably tailored solid state assemblies with the desired physical, chemical, and photophysical properties for the envisioned device applications has proven difficult. A number of techniques have been developed to assemble thin homogeneous porphyrin films,18 but layerby-layer control of composition and thickness is challenging. Thermoevaporation19-21 is the most widely used technique to obtain homogeneous films. Indeed, very well defined layered materials can be obtained by molecular beam epitaxy and sublimation in high vacuum. However, * To whom correspondence should be addressed at the Department of Chemistry, Washington University, St. Louis, MO 63130. † Present address: Instituto de Quı´mica, Universidade de Sa ˜o Paulo, Caixa Postal 26.077, CEP 05599-970, Sa´o Paulo (SP), Brazil. ‡ Present address: Department of Chemistry, George Washington University, Washington, DC. X Abstract published in Advance ACS Abstracts, August 1, 1996. (1) Nalwa, H. S. Adv. Mater. 1993, 5, 341. (2) Giraudeau, A.; Fan, F. F.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 5138. (3) Kampas, F. J.; Yamashita, K.; Fajer, J. Nature 1980, 284, 40. (4) Chau, L.-K.; Arbour, C.; Collins, G. E.; Nebesny, K. W.; Lee, P. A.; England, C. D.; Armstong, N. R.; Barkinson, B. A. J. Phys. Chem. 1993, 97, 2690. (5) Schlettwein, D.; Kaneko, M.; Yamada, A.; Wohrle, D.; Jaeger, N. I. J. Phys. Chem. 1991, 95, 1748. (6) Shi, C.; Anson, F. C. Inorg. Chim. Acta 1994, 225, 215. (7) Araki, K.; Angnes, L.; Azevedo, C. M. N.; Toma, H. E. J. Electroanal. Chem. 1995, 397, 205. (8) Araki, K.; Angnes, L.; Toma, H. E. Adv. Mater .1995, 7, 554. (9) Malinski, T.; Taha, Z. Nature 1992, 358, 676. (10) Kolesar, E. S., Jr.; Wiseman, J. M. Anal. Chem. 1989, 61, 2355. (11) Lefevre, D.; Porteu, F.; Balog, P.; Roulliay, M.; Zalczer, G.; Palacin, S. Langmuir 1993, 9, 150. (12) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027. (13) Araki, K.; Toma, H. E. Inorg. Chim. Acta 1991, 179, 293. (14) Steiger, B.; Shi, C.; Anson, F. C. Inorg. Chem. 1993, 32, 2107. (15) Wasielewski, M. R. Chem. Rev. 1992, 92, 435. (16) Bedioui, F.; Devynck, J.; Bied-Charreton, C. Acc. Chem. Res. 1995, 28, 30. (17) Szulbinski, W. S. Inorg. Chim. Acta 1995, 228, 243. (18) Honeybourne, C. L. J. Phys. Chem. Solids 1987, 48, 109. (19) Yanagi, H.; Kanbayashi, Y.; Sclettwein, D.; Wohrle, D.; Armstrong, N. R. J. Appl. Phys. 1994, 98, 4760.

S0743-7463(96)00024-8 CCC: $12.00

Figure 1. Idealized structures of (A) µ-{meso-tetra(4-pyridyl)porphyrin}zinc-tetrakis{bis(bipyridine)(chloro)ruthenium} and (B) meso-tetra(4-phenyl)porphyrin sulfonate.

this technique is limited to volatile, thermally durable materials. The Langmuir-Blodgett (LB) method is another useful technique but is generally restricted to amphiphiles that are able to form stable monolayers on a liquid surface.22-25 However, (triazol)hemiporphyrazine multilayer films exhibiting a high degree of molecular organization by the LB method have been prepared recently,26 in spite of the fact that (triazol)hemiporphyrazine does not belong to the class of molecules for which the LB method is most suited. Films of porphyrins have also been prepared by dip-coating, spin-coating,3 and electro-polymerization16,27,28 methods. In this article, a new method of film preparation based on layer-by-layer formation of insoluble ion-paired porphyrins, Figure 1, is presented. Our method is similar to a recently reported strategy used to prepare polypeptide(20) Yanagi, H.; Kouzeki, T.; Ashida, M. J. Appl. Phys. 1993, 73, 3812. (21) Manivannan, A.; Nagahara, L. A.; Hashimoto, K.; Fujishima, A.; Yanagi, H.; Kouzeki, T.; Ashida, M. Langmuir 1993, 9, 771. (22) Bonnett, R.; Ioannou, S.; James, A. G.; Pitt, C. W.; Soe, M. M. Z. J. Mater. Chem. 1992, 2, 823. (23) Liu, J.; Chen, T.; Xu, L.; Shen, S.; Zhou, Q.; Liu, K.; Jiang, L.; Xu, H. J. Photochem. Photobiol., A 1993, 76, 91. (24) Honeybourne, C. L.; Barrell, K. J. J. Phys.: Condens. Matter 1991, 3, S35. (25) Bonosi, F.; Ricciardi, G.; Lelj, F. Thin Solid Films 1994, 243, 310. (26) Pfeiffer, S.; Mingotaud, C.; Garrigou-Lagrange, C.; Delhaes, P. Langmuir 1995, 11, 2705. (27) Curran, D.; Crimshaw, J.; Perera, S. D. Chem. Soc. Rev. 1991, 20, 391. (28) Malinski, T.; Ciszewski, A.; Bennet, J.; Fish, J. R.; Czuchajowski, L. J. Electrochem. Soc. 1991, 138, 2008.

© 1996 American Chemical Society

5394 Langmuir, Vol. 12, No. 22, 1996

Araki et al.

dye multilayers,29 except it does not require prior surface modification. Alternately dip-coating a surface into solutions of the cation and the anion can be used to prepare films of over 200 Å with monolayer compositional control. The resulting material is homogeneous, stable, and sparingly soluble in aqueous and nonaqueous solvents. The generality and simplicity of the method should allow its exploitation in the preparation of active materials for molecular devices. Experimental Section All reagents and solvents were of analytical grade and used without further purification. The synthesis and characterization of the tetraruthenated zinc porphyrin (ZnTPyPBpy, Figure 1A) is described elsewhere.30 The meso-tetraphenylporphyrin sulfonate (TPPS, Figure 1B) was purchased from Aldrich. The copper derivative was prepared by refluxing a stoichiometric ratio of the free-base with Cu(OAc)2‚H2O monohydrate in MeOH for 30 min. The solvent was removed by flash evaporation and the solid subsequently washed with dry Et2O to remove any excess Cu(OAc)2‚H2O. The yield was nearly quantitative. The indium-tin-oxide, ITO, slides (Delta Technologies, Limited, CG-80IN-CUV) were ultrasonically cleaned in acetone followed by MeOH (10 min each). The polished Si(100) wafers were cut into approximately 5 × 0.8 cm sections and cleaned with deionized H2O/NH3/H2O2 (5:1:1) solution for 2 h. Porphyrin films were prepared at room temperature by dipping the ITO or Si substrates into a 0.1 mM [ZnTPyPBpy](TFMS)4 MeOH solution for 1 min, air drying, and then dipping for 3 min in a 1 mM M-TPPS aqueous solution containing 1 mM LiTFMS (where TFMS ) trifluoromethanesulfonate and M ) 2H+ or Cu2+). Excess M-TPPS was removed by washing thoroughly with deionized H2O. The films were dried with N2 gas, and the procedure was repeated until the desired number of bilayers had been deposited. It should be noted that film formation was also observed on glass surfaces. The films on the glass side of all ITO samples were mechanically removed by wiping with a MeOHmoistened tissue. Voltammograms were recorded with a Pine Instruments Company RDE-4 bipotentiostat and a XYY′T (Kipp & Zonen) recorder. A conventional three-electrode system with an Ag/ AgCl (1.00 M KCl, 0.222 vs SHE) reference electrode, a coiled platinum wire counter electrode, and an ITO glass working electrode was employed. All potentials are referenced to SHE. The spectroelectrochemistry and the photoaction spectra were obtained using a three-electrode electrochemical system inside a 1.00 cm quartz cuvette. The conducting sides of the ITO plates were centered in the cuvette to assure equal light intensity for both front and backside illumination. The excitation system of a Perkin-Elmer MPF-44 spectrofluorometer, consisting of a 150 W Xe lamp and a Czerny-Turner monochromator (5 nm bandpass, 1200 lines/mm, ∼0.2 mW/cm2), was used as the light source for the photoaction spectra. The irradiated area of the surface was 0.6 cm2. The light intensity was measured with a calibrated Newport 1830-C photometer (818-SL detector, 400 to 1100 nm, (0.2%), and the photoaction spectra were corrected for the spectral distribution of the source. A Sylvania 500Q/CL tungsten/halogen lamp, filtered by 3.5 cm of H2O, was used to irradiate the sample during the photoelectrocatalysis experiments (∼20 mW/cm2). Film thickness was evaluated with a Nanoscope III (Digital Instruments, Inc.) atomic force microscope equipped with a Si tip, in the “tapping” mode. Well-defined edge steps were made by O2 or Ar plasma etching while protecting part of the film with a microscope cover slip (typical conditions: 100 Torr O2, 100 W, 10 min). Measurements of the thinner films on ITO surfaces were precluded by the surface roughness of the substrate. Therefore, the majority of the measurements were made on polished Si wafers. A Perkin-Elmer/Physical Electronics 5200C X-ray photoelectron spectrometer (XPS), with a Mg KR source operating at 15 kV and 300 W, was used to determine the relative Cu/Zn content of ZnTPyPBpy/Cu-TPPS films. Scanning electron micrographs (29) Cooper, T. M.; Campbell, A. L.; Crane, R. L. Langmuir 1995, 11, 2713. (30) Araki, K.; Toma, H. E. J. Photochem. Photobiol. 1994, 83, 245.

Figure 2. Electronic spectra of Na4[TPPS], [ZnTPyPBpy](TFMS)4, and the ion pair [ZnTPyPBpy]4+/[TPPS]4- in MeOH. Inset: titration curves of 3.00 mL of 1.84 µM Na4TPPS solution with 39.7 µM [ZnTPyPBpy](TFMS)4 solution at three wavelengths. The symbols n1 and n2 represent the number of moles of the respective species. were obtained with a JEOL S-800 microscope. The samples were coated with 2 nm of Au to dissipate surface charge. Optical spectra were recorded with a Hewlett-Packard HP8452A diode array UV-vis spectrophotometer.

Results and Discussion Study of the Ion Pair in Solution. The porphyrins [ZnTPyPBpy](TFMS)4 and Na4[M-TPPS], where M ) 2H+ or Cu2+, are ionic compounds soluble in a variety of polar solvents such as MeOH, N,N-dimethylformamide, and H2O. The molecular structures of both species are similar, Figure 1. The charges are localized on the pendant groups. Models show the center-to-center distance between charged groups to be about 19 Å.31-33 One might expect these compounds to readily associate in solution due to strong electrostatic interactions. Indeed, a dark brown voluminous precipitate is immediately formed when aqueous solutions of the anion and cation are mixed. However, dilute MeOH solutions (e10-6 M) of the ion-paired species are stable (no observable precipitate) for hours. The ion-paired species does have a unique spectrum, Figure 2. The optical spectral changes accompanying addition of the cation to a solution of the anion are consistent with strong interaction of the cation and anion. Titration curves show a characteristic profile of two strongly interacting species, Figure 2; the equivalence point is found at a 1:1 molar ratio of the cation to anion, consistent with the formation of the [ZnTPyPBpy]4+/ [TPPS]4- ion pair. Linschitz and co-workers34 observed similar behavior in a system containing M-TPPS and mesotetrakis-(p-trimethylphenylammonium)porphyrin. In that case, ion-pair formation constants larger than 107 M-1 were found in a mixture of acetone and H2O. However, prompt dissociation upon an increase in the ionic strength was observed, as expected for adducts formed mainly due to electrostatic interactions. We also find that the [ZnTPyPBpy]4+/[TPPS]4- ion pair dissociates with increasing ionic strength. The optical spectrum of ZnTPyPBpy in MeOH exhibits Soret (428 nm) and Q (558 and 602 nm) bands charac(31) Berreau, L. M.; Young, V. G., Jr.; Woo, K. Inorg. Chem. 1995, 34, 3485. (32) Li, N.; Su, Z.; Coppens, P.; Landrum, J. J. Am. Chem. Soc. 1990, 112, 7294. (33) Rillema, D. P.; Jones, D. S. J. Chem. Soc., Chem. Commun. 1979, 849. (34) Ojadi, E.; Selzer, R.; Linschitz, H. J. Am. Chem. Soc. 1985, 107, 7783.

Multilayer Porphyrin Film

teristic of the porphyrin as well as features attributable to the pendant Ru complexes: (293 nm {bpy (pπ f pπ*)}, 363 nm {RuII (dπ) f bpy (pπ*2) (MLCT2)}, ∼462 nm {RuII (dπ) f pyP (pπ*)} (pyP ) the pyridyl groups bonded to the porphyrin ring) and ∼486 nm {RuII (dπ) f bpy (pπ*1) (MLCT1)}).30 TPPS presents a narrow Soret band at 414 nm and Q bands at 512, 546, 586, and 647 nm. When [TPPS]4- is titrated with [ZnTPyPBpy]4+, the spectrum changes to that of the sum of the two species, except only a single Soret band, shifted to higher energy, is observed at 412 nm. The weaker transitions of the TPPS are hidden beneath the visible bands at 513, 558, 605, and 647 nm, Figure 2. The 16 nm blue shift of the ZnTPyPBpy Soret band allows the observation of a band at ∼450 nm due to a RuII (dπ) f pyP (pπ*) transition, previously inferred solely on the basis of the excitation profile in solid state resonance Raman spectra.35 The blue shift of the Soret band, while the remaining transitions appear to be insensitive to the formation of the ion-paired species, can be explained by the exciton theory. This theory has been used to understand the electronic transitions in layered materials of planar aromatic compounds as well as molecular systems.36,37 In the case of cofacial dimers, the coupling of the dipole moments of the fully allowed transitions of each molecule generates two distinct excited states. The respective transition moments are equal to the vector sum of those of the monomers. Therefore, the lower energy transition is prohibited while the higher energy transition is fully allowed, and a blue shift of the band should be observed. The coupling constant is proportional to the oscillator strengths and is inversely proportional to the cube of the interplanar distances. Chang confirmed this relationship by changing the distances in covalently linked dimeric cofacial porphyrins.38 Therefore, the observed blue shift of the [ZnTPyPBpy]4+/[TPPS]4- Soret band is consistent with a face-to-face orientation of the ion pairs, which maximizes the Coulombic and van der Waals interactions. However, the small size of the shift of only 2 nm relative to the Soret band of TPPS suggests that the electronic coupling of the rings is weak.34,36,37 Preparation and Characterization of the Multilayer Porphyrin Films. The tendency of the ZnTPyPBpy to interact with TPPS generating an ion-paired species can be exploited to prepare films. ZnTPyPBpy is deposited first on the substrate by dip-coating and airdrying. Formation of films by initial substrate coverage with TPPS is more difficult due to its relatively high solubility. Following the initial deposition of ZnTPyPBpy, the second layer of porphyrin is formed by dipping into a solution of TPPS containing 1 mM LiTFMS. The excess TFMS is needed to prevent dissolution of the initially deposited ZnTPyPBpy. Copious washing with deionized H2O removes any excess TPPS. The next layer of ZnTPyPBpy is formed by dipping the bilayer into a solution of ZnTPyPBpy. Alternate dipping into anion and cation solutions builds a well-defined multilayer assembly. Thin films on ITO exhibit yellow-orange color and are very adherent; no apparent damage is observed in the conventional “Scotch tape” peel test. Film growth can be easily monitored by UV-vis spectroscopy, as shown in Figure 3. The linear increase of the absorbance as a function of the number of bilayers suggests that the amount of material deposited each time is constant, as can be seen in the inset. A linear least-squares regression of the (35) Araki, K.; Santos, P. S.; de Oliveira, L. F. C.; Toma, H. E. Spectrosc. Lett. 1995, 28, 119. (36) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (37) Hochstrasser, F. M.; Kasha, M. Photochem. Photobiol. 1964, 3, 317. (38) Chang, C. K. J. Heterocycl. Chem. 1977, 14, 1285.

Langmuir, Vol. 12, No. 22, 1996 5395

Figure 3. UV-vis spectra of 4, 8, 16, 20, 25, and 30 bilayer films of ZnTPyPBpy/TPPS on ITO. Inset: growth of the absorbance as a function of the number of bilayers.

424 nm absorbance data for 10 different samples, ranging in thickness from 4 to 30 bilayers, passes, to within experimental error, through the origin (-0.03(3)) and has a slope of 0.039(2) absorbance unit per bilayer (the number in parentheses is the standard deviation in the final digit). Thus, it is possible to precisely control the amount of material deposited simply by controlling the number of dippings. Phthalocyanines are closely related to porphyrins and their films are expected to be similar. Comparison of the measured absorbance per bilayer of our ZnTPyPBpy/TPPS to that previously reported for phthalocynine films suggests that each bilayer consists of a single layer of ionpaired porphyrins. Assuming that there is no significant change of the transition moments upon film formation, the ratio of the oscillator strengths of the porphyrin bilayer Soret band and the phthalocynine monolayer Q band should be identical to that of porphyrin ion pairs and phthalocyanine molecules in solution. This relationship can be further simplified if Gaussian bandshapes are assumed in both solid and solution

( ∆νj1/2)bi ( ∆νj1/2)Pc

( ∆νj1/2)ip-s )

( ∆νj1/2)Pc-s

where  and ∆νj1/2 represent the absorptivities and full widths at half-maximum of the respective bands (bi ) porphyrin bilayers and ip-s ) ion-pairs in solution; Pc ) phthalocynine layers and Pc-s ) molecules in solution). The expected bilayer absorbance can be calculated using the above equation, our solution data (Figure 2), the measured solid state bandwidth (Figure 3), and published values of the respective parameters of chloroindium phthalocyanine films4 and disulfonated aluminum phthalocyanine MeOH solution.39 We find a value of 0.045 absorbance unit per bilayer, which compares well to our measured value of 0.039. The extent of the electronic coupling of the porphyrin rings in the solid state is difficult to judge from the optical spectra. The visible bands found at 518, 568, 614, and 648 nm, characteristic of porphyrins, and the bands at 360 and ∼490 nm, characteristic of relevant Ru species, appear to be a superposition of the bands found in the cation and in the anion comprising the ion pair. The Soret band is found at 424 nm, which is 12 nm red shifted from that found in solution; however it occurs in between those of ZnTPyPBpy (446 nm, red shifted 18 nm vs solution) (39) Dhami, S.; de Mello, A. J.; Rumbles, G.; Bishop, S. M.; Phillips, D.; Beeby, A. Photochem. Photobiol. 1995, 61, 341.

5396 Langmuir, Vol. 12, No. 22, 1996

Figure 4. Graph of the thickness of multilayered ZnTPyPBpy/ TPPS films, measured by AFM, as a function of the number of bilayers.

and TPPS (409 nm, blue shifted 5 nm vs solution) films. Solid state interactions complicate any assessment of the relative face-to-face vs the side-to-side electronic coupling between and among the respective porphyrins. However, it is clear that some electronic interaction must be present, because the solid state luminescence observed in pure ZnTPyPBpy films30 is totally quenched in these multilayered films at room temperature. In order to analyze the relative composition of the films by XPS, we deposited ZnTPyPBpy/CuTPPS multilayers on ITO. TPPS retains its structural and electrostatic binding characteristics upon metalation. However, the incorporation of Cu simplifies the direct determination of the relative proportion of positive and negative porphyrin species in the films, because Cu and Zn are easy to assay by XPS. By calculating the area under the Zn(2p) and Cu(2p) bands and applying the standard sensitivity factors of those elements (5.321 and 3.726, respectively), we found the ratio of negative to positive porphyrins to be 1:1 within experimental error (20 bilayers, 3 independently prepared samples, Zn/Cu ) 1.04(7)). Furthermore, no fluorine signals were detected as expected for a material consisting of ion pairs and containing an insignificant amount of the TFMS anion. The surfaces of samples with 8 to 20 bilayers were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The micrographs showed a microcrystalline material, with grain sizes in the 0.12 µm range. The surfaces appear smooth and homogeneous but are easily fouled by dust. The dust appears as intense peaks in the AFM, but a smoother background could be observed under this “noise” when small areas were scanned. If the amount deposited in each dipping and the relative orientation of the molecules remain constant throughout the films, the average thickness should be proportional to the number of layers. Sharp steps were made by etching part of the film with O2 or Ar plasma, and the heights were evaluated by AFM. The graph of the measured thickness as a function of the number of bilayers is shown in Figure 4. The data are well fit by a straight line, which to within experimental error, passes through the origin (-0.6(8)). The slope of this line, 12.7(6) Å, corresponds to the average thickness of a bilayer. The interplanar distances in epitaxyally grown phthalocyanine films4,20 and stacked unidimensional porphyrin crystals40,41 were estimated to be approximately 3.2 and 3.5 Å, respectively. In the case of a rodlike nematic mesogenic porphyrin phase, Bard and co-workers reported that the value increases to about 4 Å.42 Thus, our finding is

Araki et al.

about 1.7 times larger than that expected from the interplanar distances in face-to-face-oriented porphyrin crystals. This result is inconsistent with an ideal edge on molecular orientation (molecule plane orthogonal to substrate plane), because in this case, the bilayer thickness should be greater than 20 Å. If the molecules stack coplanar with the substrate, the bilayer thickness should be mainly defined by the bulky pendent [Ru(bpy)2Cl]+ groups (∼10 Å diameter)33 of the ZnTPyPBpy, which are symmetrically positioned around the porphyrin ring. Thus our results are consistent with bilayers composed of one ZnTPyPBpy (∼10 Å) and one TPPS (∼3 Å) molecules cofacially or slip-stacked43,44 oriented and coplanar with the substrate. This is an unusual and very important property of this method of assembling multilayered porphyrins; it allows the preparation of films with their structure and thickness defined with monolayer resolution. The large (∼6.5 Å) spacing between the porphyrins of our ion pairs is consistent with a small degree of electronic coupling, Figure 2. Spectroelectrochemical Properties. The electrodeconfined multilayer ZnTPyPBpy/CuTPPS films exhibit a quasi-reversible pair of waves at Epa ) 0.98 and Epc ) 0.90 V, similar to the electrochemical behavior of previously described M-TPyPBpy films.7,8,30 The waveshape and the linear increase of the current with the scan rate are consistent with a surface-bound, reversibly redox active species, Figure 5. The spectral changes observed in the ZnTPyPBpy/ CuTPPS films, while applying potentials in the 0.221.32 V range, are shown in Figures 6. The spectra taken at potentials lower than 0.87 V exhibit the porphyrin bands at 420 (Soret), 568, and 612 nm (Q), as well as bands of the pendent Ru species at 322 and ∼500 nm. As the potential is increased in the 0.87-1.02 V range, the band at 500 nm disappears and a new band arises at 316 nm. This change is analogous to the spectroelectrochemical behavior of tetraruthenated porphyrins in solution7,8,30,45,46 and can be attributed to the reversible oxidation of the pendant Ru complexes. In addition, a red shift and hyperchromism of the Soret band is observed, consistent with the appearance of a band at 440 nm and a small decrease in the band at 420 nm. The well-defined isosbestic points at 304, 338, 436, and 452 nm are consistent with clean electrochemical interconversion of durable RuII/III centers. Application of the Nernst equation yields E1/2 ) 0.94 V for the reduction potential, in good agreement with the voltammetric results. When potentials in the 1.02-1.32 V range are applied, the Soret and Q bands of both species vanish and a new broad band that extends above 820 nm appears, Figure 6B. This second set of spectral changes can be attributed to the simultaneous oxidation of the porphyrin rings of CuTPPS and ZnTPyPBpy. Thus, each bilayer yields six electrons upon scanning the potential from 0.22 to 1.32 V. Calculation of the surface coverage from the electrochemical data suggests, as was the case for the optical and AFM data, that each bilayer consists of a single layer of ion-paired porphyrins. In the potential range of the cyclic voltammetry shown in Figure 5 (0.2-1.1 V), only the RuII/III redox process is accessible. By integration of (40) Newcomb, T. P.; Godfrey, M. R.; Hoffman, B. M.; Ibers, J. A. J. Am. Chem. Soc. 1989, 111, 7078. (41) McGhee, E. M.; Godfrey, M. R.; Hoffman, B. M.; Ibers, J. A. Inorg. Chem. 1991, 30, 803. (42) Liu, C.-Y.; Pan, H.-L.; Tang, H.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1995, 99, 7633. (43) Palmer, S. M.; Stanton, J. L.; Jaggi, N. K.; Hoffman, B. M.; Ibers, J. A.; Schwartz, L. H. Inorg. Chem. 1985, 24, 2040. (44) Chau, L.-K.; England, C. D.; Chen, S.; Armstrong, N. R. J. Phys. Chem. 1993, 97, 2699. (45) Araki, K.; Toma, H. E. J. Coord. Chem. 1993, 30, 9. (46) Araki, K.; Toma, H. E. J. Chem. Res. (M) 1994.

Multilayer Porphyrin Film

Figure 5. Cyclic voltammograms of a ZnTPyPBpy/CuTPPS (four bilayers) modified ITO electrode (LiTFMS 0.1 M, 0.05 M acetate buffer, pH ) 4.7, area = 0.8 cm2).

the area under these four electrons per ZnTPyPBpy molecule waves, we find a surface coverage of ∼4.5 × 1013 molecules/cm2. This value compares favorably to that calculated from scanning tunneling microscopy (STM) images of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (∼3.7 × 1013 molecules/cm2) monolayers on a Au(111) surface.47 The higher surface coverage found in the present study is probably due to the surface roughness of our ITO electrodes, which more than compensates for the slightly larger size of ZnTPyPBpy relative to meso-tetrakis(N-methylpyridinium-4-yl)porphyrin. The cyclic voltammograms of the ZnTPyPBpy/TPPSmodified electrodes are similar to those shown in Figure 5. However, for the first two or three cycles, the cyclic voltammograms are quite different, exhibiting an anodic peak which is much more intense than the cathodic peak. This suggests an irreversible oxidation reaction accompaning the oxidation of the RuII centers. Under a 0.22 V applied bias, the optical spectrum is nearly identical to that of the ion-pair spectrum in MeOH solution, but when potentials above 0.87 V are applied, a decrease of the 490 nm band and a rise of the 316 nm band are observed, Figure 7. This change, as was the case above for the ZnTPyPBpy/CuTPPS films, is assigned to oxidation of RuII to RuIII. However, above 0.97 V, the absorbance intensities of the 420 and 525 nm TPPS bands decrease, indicating that it too is being oxidized. In the 1.07-1.22 V range, the bands of the ZnTPyPBpy (440, 565, and 610 nm) also decrease. Nevertheless, the oxidized ZnTPyPBpy species is much more stable than the oxidized TPPS species. Even after 5 min at 1.22 V, about 50% of the ZnTPyPBpy absorbance can be regenerated by applying a potential of 0.22 V, while regeneration of the TPPS bands is impossible. (47) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337.

Langmuir, Vol. 12, No. 22, 1996 5397

Figure 6. Spectroelectrochemistry of a ZnTPyPBpy/CuTPPS (12 bilayers) modified ITO electrode (LiTFMS ) 0.1 M, 0.05 M acetate buffer, pH ) 4.7).

Figure 7. Spectroelectrochemistry of a ZnTPyPBpy/TPPS (five bilayers) modified ITO electrode (LiTFMS ) 0.1 M, 0.05 M acetate buffer, pH ) 4.7).

Therefore, the initial voltammetric behavior of the films can be unambiguously assigned to the irreversible oxidation of the TPPS ring occurring simultaneously with the oxidation of the RuII centers of the cation. Photoelectrocatalytic Properties. The cyclic voltammograms of ZnTPyPBpy/TPPS films, in the dark and under illumination from a tungsten halogen lamp, are shown in Figure 8. It should be pointed out that the potential range shown is one for which no oxidation or reduction of the film should occur. Note that the difference between the forward and reverse scans corresponds to the capacitive current, present even when the scan rate

5398 Langmuir, Vol. 12, No. 22, 1996

Figure 8. Dark and backside illuminated (∼20 mW/cm2) voltammograms of a ZnTPyPBpy/TPPS (five bilayers) modified ITO electrode showing photocatalytic activity toward the reduction of O2 (LiTFMS ) 0.1 M, 0.05 M acetate buffer, pH ) 4.7, scan rate ) 5 mV/s). The voltammogram of the modified electrode in an Ar-saturated solution under irradiation is also shown.

Figure 9. Corrected photoaction spectra of ZnTPyPBpy/TPPS multilayer films showing the influence of the thickness and illumination mode (front or back) on the photoresponse (LiTFMS ) 0.1 M, acetate buffer 0.05, pH ) 4.7, saturated with O2).

is as slow as 5 mV/s. This is consistent with a high surface area electrode. A cathodic photocurrent, which reaches a maximum around 0.0 V, is observed in a O2-saturated electrolyte solution. In the absence of O2, only a very small photocurrent could be measured, confirming that the photoprocess is the reduction of O2. In addition, when the free base TPPS is replaced by its copper derivative, the photoelectrochemical behavior is similar, albeit lower photocurrents are observed for a given film thickness. Photoaction spectra obtained for backside illumination (through the ITO/film interface) reproduce the absorption spectrum of the films, Figure 9. The photocurrent intensities increase with thickness, in accordance with previously published results.5,48 Spectra obtained under frontside illumination of thin samples are also similar to

Araki et al.

the absorption spectra. However, as the thickness increases the contribution of the Soret band decreases, as can be seen in Figure 9, while the photoaction profile above 500 nm remains almost unchanged. These results are consistent with photoactivity being confined to a very thin layer located next to the ITO/film interface. The porphyrin layers far from the interface act as a barrier to the light, providing a “filtering effect”.49,50 The photoelectrochemical behavior of our multilayered porphyrin films, in the presence of O2, is similar to that previously reported for other porphyrin-based systems.51,52 That behavior was attributed to n-type semiconducting properties in these molecular materials and a Schottky barrier at the ITO/film interface. However, in both the present and previous investigations, the potentials applied to the electrode are thermodynamically able to reduce the O2. Thus, the energy of the photons is used solely to enhance the reduction kinetics by generating a reactive excited state species. Therefore, the porphyrin films are probably more precisely defined as photoelectrocatalysts.53 The photocurrent for O2 reduction is degraded by white light irradiation (∼20 mW/cm2) over time, decreasing to about 60% of the initial value within an hour for an electrode potential of 0.0 V. After 3 h of irradiation, cyclic voltammograms in the dark reveal a new quasi-reversible pair of waves at 0.7 V in addition to the wave for the RuII/III couple (E1/2 ) 0.94 V), indicating partial film photodecomposition. We suspect that photorelease of RuII centers in the form of [Ru(bpy)2Cl(H2O)]+ occurs; this species should have a wave at ∼0.7 V and may be loosely held in the multilayer film.54 Degradation via reaction of the film with products of O2 reduction is also likely. Conclusion In the present study, we have demonstrated a method to assemble porphyrin multilayers with rationally designed properties. ZnTPyPBpy/M-TPPS multilayer films can be electrostatically assembled with monolayer control on glass, ITO, and Si wafers. By choosing the number of dipping cycles, the number of bilayers and thus the overall thickness of the film can be regulated. Furthermore, the thickness of the individual bilayers is largely defined by the bulky [Ru(bpy)2Cl]+ groups; this fact leads us to believe that the bilayer thickness and consequently the interlayer interactions may be controlled by choice of substituents. In addition, the electrochemical, photoelectrochemical, and catalytic behaviors are dependent upon the metal ions coordinated to the porphyrin rings, suggesting the possibility of tailoring these properties. Thus, by using our method, one should be able to prepare a wide variety of layered porphyrin composites with rational control of their reactivity and photophysical properties. The application of these porphyrin-based multilayer films as active materials for sensors and other molecular devices is currently being investigated. Acknowledgment. This research was funded by the National Science Foundation. K.A. is additionally thankful to Fundac¸ a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) and to the University of Sa˜o Paulo for financial support . LA960024C (48) Yamashita, K.; Harima, Y.; Iwashima, H. J. Phys. Chem. 1987, 91, 3055. (49) Yamashita, K.; Matsumura, Y.; Harima, Y.; Miura, S.; Suzuki, H. Chem. Lett. 1984, 489. (50) Lewis, N. S. Acc. Chem. Res. 1990, 32, 176. (51) Klofta, T. J.; Sims, T. D.; Pankow, J. W.; Danziger, J.; Nebesny, K. W.; Armstrong, N. R. J. Phys. Chem. 1987, 91, 5651. (52) Ghosh, A. K.; Morel, D. L.; Feng, T.; Shaw, R. F.; Rowe, C. A., Jr. J. Appl. Phys. 1974, 45, 230. (53) Tan, M. X.; Laibinis, P. E.; Nguyen, S. T.; Kesselman, J. M.; Stanton, C. E.; Lewis, N. S. Progr. Inorg. Chem. 1994, 41, 21. (54) Lever, A. B. P. Inorg. Chem. 1990, 20, 1271.