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Preparation and Characterization of Aqueous Silica-Based Colloidal Composites Containing Immobilized Organometallic Photosensitizers Petr Serguievski, William E. Ford, and Michael A. J. Rodgers* Center for Photochemical Sciences, Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403 Received July 27, 1995. In Final Form: September 25, 1995X Aqueous colloidal silica suspensions modified by incorporation of tris(2,2′-bipyridyl)ruthenium(II) and zinc(II) tetrakis(N-methyl-4-pyridyl)porphyrin into the particle matrix were prepared. The photophysical properties of the complexes in this environment were investigated using laser flash photolysis and steadystate luminescence techniques. The excited states exhibit nonexponential kinetics and their lifetimes are increased compared to homogeneous aqueous solutions. The metal-to-ligand charge transfer state of the Ru(II) complex is not affected by the presence of dissolved oxygen, whereas a fraction of the triplet states of the porphyrin is quenched by O2. Didodecyldimethylammonium ion was used to build surfactant bilayers supported on the silica surface. The possibility of using this system for transmembrane electron transfer is discussed.
Introduction Microheterogeneous systems capable of mediating photoinduced electron and/or energy transfer are of great interest for technological as well as for purely scientific reasons.1 Studying such systems is extremely beneficial to the understanding of the processes governing natural photosynthesis,2 solar energy conversion and storage, and photographic processes.3 Inorganic semiconductor colloidal suspensions have been commonly used for mediating photoinduced electron transfer.1,4 Stability, ease of preparation and photosensitization, and ability to provide a certain spatial arrangement of the reactants have contributed to the popularity of these systems. However, the magnitude of the band gap of most semiconductors forces one to use UV light to efficiently use these materials as photocatalysts.5 In order to take advantage of visible excitation, the wide band semiconductor suspensions can be modified by incorporating photosensitizers into the system. These photoactive species can inject electrons into the conduction band of the semiconductor when promoted into the excited state via visible light excitation.6 Potentially, inert (redox inactive) colloidal suspensions modified in a similar fashion can be used in initiating electron transfer.4 The role of the colloidal particles in such systems reduces to providing a template for arranging reactants and products in a desired configuration.5 Such modifications have been achieved via electrostatic adsorption of the chromophores to the surface of the particles,7-9 association of the * To whom correspondence may be addressed: e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1995. (1) Gratzel, M. Heterogeneous Photochemical Electron Transfer; CRC: Boca Raton, FL, 1989. (2) Willner, I.; Willner, B. In Photoinduced Electron Transfer III; Mattay, J., Ed.; Springer-Verlag: Berlin, 1991; Vol. 159; pp 153-218. (3) Lymar, S. V.; Parmon, V. N.; Zamaraev, K. I. In Photoinduced Electron Transfer III; Mattay, J., Ed.; Springer-Verlag: Berlin, 1991; Vol. 159; pp 1-65. (4) Kamat, P. V. Chem. Rev. 1993, 93, 267-300. (5) Fox, M. A. In Photoinduced Electron Transfer III; Mattay, J., Ed.; Springer-Verlag: Berlin, 1991; Vol. 159; pp 67-101. (6) Kalyanasundaram, K.; Vlachopoulos, N.; Krishnan, V.; Monnier, A.; Gratzel, M. J. Phys. Chem. 1987, 91, 2342-2347. (7) Willner, I.; Yang, J.-M.; Laane, C.; Otvos, J. W.; Calvin, M. J. Phys. Chem. 1981, 85, 3277-3282. (8) Willner, I.; Degani, Y. Isr. J. Chem. 1982, 22, 163-167.
0743-7463/96/2412-0348$12.00/0
photosensitizers with the colloid stabilizing polymers,10 covalent attachment,11 and coprecipitation of the photosensitizers and dyes in the colloidal media.12-18 Hydrophobic self-assembling membranes in aqueous systems comprise another popular environment for carrying out photoinduced charge transfer processes. Surfactant or lipid vesicles offer the biggest advantages by providing separated compartments for arranging participants of the electron transfer scheme.3 Photoinitiated electron transfer across vesicle membranes has been extensively studied and documented.1,3,19,20 Appropriately designed systems can provide efficient and long-lived charge separation across the bilayer. A surfactant or a lipid bilayer can be built on the surface of a charged colloidal particle suspended in an aqueous system yielding a structure similar to that of a vesicle with the particle replacing the water pool in the inner compartment.21-28 By modifying the colloidal particle with an appropriate photosensitizer, it is possible to take (9) Furlong, D. N.; Johansen, O.; Launikonis, A.; Loder, J. W.; Mau, A. W.-H.; Sasse, W. H. F. Aust. J. Chem. 1985, 38, 363-367. (10) Willner, I.; Eichen, Y.; Frank, A. J.; Fox, M. A. J. Phys. Chem. 1993, 97, 7264-7271. (11) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 38223831. (12) Castellano, F. N.; Heimer, T. A.; Tandhasetti, M. T.; Meyer, G. J. Chem. Mater. 1994, 6, 1041-1048. (13) Levy, D.; Ocana, M.; Serna, C. J. Langmuir 1994, 10, 26832687. (14) Lin, R.-J.; Onikubo, T.; Nagai, K.; Kaneko, M. J. Electroanal. Chem. 1993, 348, 189-199. (15) Narang, U.; Wang, R.; Prasad, P. N.; Bright, F. V. J. Phys. Chem. 1994, 98, 17-22. (16) Ocana, M.; Levy, D.; Serna, C. J. J. Non-Cryst. Solids 1992, 147, 621-626. (17) Wheeler, J.; Thomas, J. K. J. Phys. Chem. 1982, 86, 4540-4544. (18) Shibata, S.; Taniguchi, T.; Yano, T.; Yasumori, A.; Yamane, M. J. Sol-Gel Sci. Technol. 1994, 2, 755-759. (19) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic: Orlando, FL, 1987. (20) Takagi, K.; Sawaki, Y. Crit. Rev. Biochem. Mol. Biol. 1993, 28, 323-367. (21) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 74157418. (22) Esumi, K.; Watanabe, N.; Meguro, K. Langmuir 1989, 5, 14201422. (23) De Cuyper, M.; Joniau, M. Langmuir 1991, 7, 647-652. (24) Capovilla, L.; Labbe, P.; Reverdy, G. Langmuir 1991, 7, 20002003. (25) Brahimi, B.; Labbe, P.; Reverdy, G. New J. Chem. 1992, 16, 719-726. (26) Meguro, K.; Adachi, T.; Fukunishi, R.; Esumi, K. Langmuir 1988, 4, 1160-1162.
© 1996 American Chemical Society
Colloidal Composites Containing Photosensitizers
advantage of the capabilities of both microheterogeneous substructures of the supported bilayer system (the bilayer itself and the colloidal particle) in achieving an efficient photoinduced charge transfer process. The colloidal particle may serve as a template for catalytic reactions4 while at the same time making it straightforward to introduce an asymmetry3 between inner and outer surfaces of the bilayer, which is usually a requirement for vectorial electron or energy transfer. The hydrophobic bilayer serves as a medium for dissolving appropriate acceptors as well as acting as a barrier to the recombination of charges. This design has been tested using SnO2 colloidal particles and ruthenium(II) polypyridine derivative dissolved in didodecyldimethylammonium bilayer.21 The work reported here concentrated on the preparation of colloidal silica suspensions modified by incorporating tris(2,2′-bipyridyl)ruthenium(II) or zinc(II) tetrakis(Nmethyl-4-pyridyl)porphyrin complexes into the particle matrix. Ruthenium complexes were used in a similar fashion in preparing doped silica sols17 and gels.12,29 Free base tetrakis(4-N-trimethylaminophenyl)porphyrin has been incorporated into silica particles prepared by solgel hydrolysis of tetraethoxysilane.18 In that work micrometer size particles were analyzed for the efficiency of the dye uptake and the degree of porphyrin dimerization within the spheres. However, the issue of cationic porphyrin adsorption at the negatively charged silica surface was not addressed. Both kinds of metal complexes used in our study are useful photosensitizers of electron transfer reactions.30 Bilayers consisting of didodecyldimethylammonium ion were assembled on the surface of the colloidal particles yielding stable suspensions. The photophysical properties of the photosensitizers in this novel environment were examined and are reported here. Experimental Section Materials. Dimethylformamide (99+%, Aldrich) and ethanol (190 proof, McCormick Distilling Co.) were used as received. Tris(2,2′-bipyridyl)ruthenium(II) dichloride hexahydrate (Aldrich), zinc(II) tetrakis(N-methyl-4-pyridyl)porphyrin tetrachloride (Midcentury), didodecyldimethylammonium bromide (Eastman), Trizma base (Reagent grade, Sigma), and sodium metasilicate nonahydrate (Sigma) were also used as received. Dowex-50W (Sigma) was preswollen and repeatedly washed with water with the supernatant being discarded upon sedimentation prior to column packing. Sephadex G25 (Sigma) was similarly treated with 8.5 mM Tris buffer (pH 9). SP Sephadex C25 was preswollen in water and washed with 1 M NaCl. Distilled water was passed through Sybron/Barnstead NANOpure II system (resistivity greater than 17 MΩ). Suspension Preparation. The suspensions were prepared using a variation of a procedure for obtaining a stable silica sol described by Bechtold and Snyder.31 In a typical synthesis 1.4215 g of Na2SiO3‚9H2O was dissolved in 25 mL of deionized water. The resulting solution was passed through Dowex-50W cationexchanger that was previously regenerated with 0.3 M H2SO4. The necessary amount of 0.1 M NaOH (usually ∼0.65 mL) was added to the eluent to adjust the weight ratio of SiO2:Na2O to approximately 150:1. Five milliliters of the resulting suspension (the “heel”) was refluxed for 1.5 h. If a suspension was to be modified by incorporating a photosensitizer, an aliquot of a solution of the desired compound was added to the remaining volume of eluent. The final concentration of the photosensitizer was varied from 40 to 170 µM. The rest of the eluent (“feed”) was (27) Soderlind, E.; Bjorling, M.; Stillbs, P. Langmuir 1994, 10, 890898. (28) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1995, 99, 51395145. (29) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. J. Am. Chem. Soc. 1991, 113, 3984-3985. (30) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic: London, 1992. (31) Bechtold, M. F.; Snyder, O. E. U.S. Patent 2 574 902, 1951.
Langmuir, Vol. 12, No. 2, 1996 349 added to the refluxing “heel” at a constant rate of 7.5 mL/h using a syringe pump (Sage Instruments). The refluxing was stopped upon the addition of at least 25 mL of the “feed”. The resulting sol was filtered through Whatman paper filter to remove any precipitate. Suspensions prepared this way were stored at room temperature in subdued light and showed no signs of aggregation for weeks. Bilayer Building. In a typical preparation 6.5 mg of didodecyldimethylammonium bromide (DDAB) was dissolved in 0.05 mL of ethanol. A 0.45-mL portion of 8.5 mM Tris buffer was added and the system was heated to ca. 60 °C and sonicated for approximately 1 min in a sonication bath (Branson). This resulted in a clear suspension that became slightly turbid upon cooling. A 1.0-mL portion of a silica suspension was added dropwise to the warm surfactant system while the sample was repeatedly sonicated. The fluffy precipitate that formed initially was redispersed during heating and sonication. The resulting slightly turbid suspension was stable to aggregation for several days. Removal of Unassociated Photosensitizer. A fraction of the photosensitizers added to the system during the formation of the colloidal particles ended up electrostatically associated with the silica surface rather than embedded into the matrix. Removal of this fraction was accomplished with the following procedure. A 10 × 0.7 cm chromatography column (Sigma) packed with 0.4 mL of preswollen Sephadex G-25 was equilibrated with 8.5 mM Tris buffer. The bed was not equilibrated with a vesicle suspension of DDAB since it was found from the spectroscopic measurement of the concentration of the photosensitizer in the eluent that the amount of the colloid eluted was unchanged within the experimental error whether or not this procedure was carried out. The void volume and the sample dilution were determined with Blue Dextran (MW 2 000 000, Sigma). One milliliter of the supported bilayer suspension was chromatographed using 8.5 mM Tris as a mobile phase. The separation of two colored bands was clearly visible. The faster moving band eluted in the void volume, while the other remained stationary in the top layer of the bed. The bed was subsequently discarded due to virtually irreversible electrostatic binding of cationic photosensitizer complexes to the carboxylic groups of the gel.32 The ion-exchange removal of Ru(II) tris(bipyridyl) complex electrostatically attached to the surface of the particles was accomplished by passing an aliquot of the silica suspension through a column packed with Sephadex C25 that was previously regenerated with 1 M NaCl. Measurements. Spectroscopic measurements were performed at ambient temperature with 1 cm path length quartz cuvettes. The ground-state absorption spectra were recorded on a diode array spectrophotometer (Perkin-Elmer, Lambda Array 3840) in the high-resolution mode (0.25 nm). For quantitative measurements of extinction coefficients of the photosensitizers, the light scattering due to the colloidal aggregates was subtracted from the total optical density of the suspensions. Uncorrected steady-state emission spectra were measured with a PerkinElmer LS-5B spectrofluorimeter. Time-resolved experiments were done using a Q-switched Nd: YAG laser (Continuum, YG660 or Surelite I-10) with the pulse width of 6-7 ns. The output of the laser was frequency-converted to obtain a second-harmonic (532 nm) or third-harmonic (355 nm) radiation. When necessary a tunable laser source (MagicPRISM, OPOTEK), capable of producing pulsed output in the 410-650 nm spectral region when pumped with 355 nm light from the Nd:YAG, was employed. The laser beam incident upon the sample was attenuated with a diverging lens that provided an even irradiation of the front face of the cuvette. A computercontrolled spectrophotometer system (Kinetic Instruments) was used to collect emission and absorption data. The monitoring light in the absorption mode was supplied by a 150-W xenon arc lamp in a microsecond time scale regime and by a 100-W tungsten lamp in a millisecond regime. Bandpass (BG 37, Schott) and longpass filters (OG 570, Schott) were placed in front of the monochromator to remove scattered laser light. The signal was digitized on a LeCroy 9450 digitizer interfaced with a computer capable of storing as well as producing a hard copy of the data. (32) Fischer, L. Gel Filtration Chromatography, 2nd ed.; Elsevier: Amsterdam, 1980; Vol. 1.
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Serguievski et al. Table 1. Absorption and Emission Characteristics of the Two Photosensitizers in Homogeneous Aqueous Solutions and in Colloidal Composites absorption Soret/ vis
emission peak position (quantum yield)
452a 455b 436,562, 602d 11.3d 450, 575, 616b 7.1
613 (φe ) 0.046)a 596c (φe ) 0.138) 626, 666d 650, 700c
peak position 2+
Ru(bpy)3
ZnTMPyP4+
H2O coll comp H2O coll comp
a Reference 30. b Error (1 nm. c Error (2 nm. d Kalyanasundaram, K.; Neumann-Spallart, M. J. Phys. Chem. 1982, 86, 5163-5169.
Figure 1. Electron micrograph of the silica colloidal particles containing ZnTMPyP4+. Magnification 280 000× (reproduced at 70% of original size). Spectroscopic and kinetic measurements were performed using samples of colloidal silica suspensions modified with the photosensitizers and containing surfactant bilayers. These systems are called colloidal composites. Transmission electron micrography studies were carried out using an EM-10 Zeiss microscope. The samples were prepared by drying 10 times diluted suspensions on Formvar-coated copper grids. Photoimages and video images used in analysis had an overall magnification of 280 000.
Results Suspension Preparation and Purification. Silica sols were prepared by refluxing silicic acid solutions with the ratio of silica to alkali adjusted to a value of 150:1 (by weight) by cation-exchange. This ratio has been found to produce a stable suspension.33 The slightly turbid suspensions prepared this way were stable for months. Transmission electron microscopy images revealed that the systems contained nonagglomerated, almost spherical particles (Figure 1). The particles showed a relatively narrow distribution of sizes with the average diameter of 15 nm. The solid content of the suspension was 8 g/L. The final pH in the systems varied from 9 to 9.8. Tris(2,2′-bipyridyl)ruthenium complex (Ru(bpy)32+) and zinc tetrakis(methylpyridyl)porphyrin (ZnTMPyP4+) were incorporated into the suspensions resulting in the complexes being embedded in the lattice of the particles. However, not all the added photosensitizer molecules ended up trapped within the silica matrix. A fraction of the complexes remained electrostatically attached to the surface of the particles. The removal of the surface (33) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979.
attached Ru(bpy)32+ could be accomplished by cationexchanging it from the silica surface with sodium ions. The efficiency of the exchange of the complexes from the particle surface is close to 100%. This was confirmed by a control experiment in which the Ru(II) complex was added to preformed silica particles. Approximately 50% of the chromophores remained associated with the particles following ion exchange when the suspensions were prepared with the Ru(bpy)32+ embedded in the silica matrix. The amount of the complex present in the suspension was determined by absorption spectroscopy assuming that the extinction coefficient of the maximum of the metal-to-ligand charge transfer (MLCT) band is unchanged in going from aqueous solution to silica. The porphyrin cations electrostatically attached to the surface of silica particles could not be removed from the colloidal suspension using ion-exchange. It was found, however, that DDAB ions can displace porphyrin complexes from anionic sites on the silica surface. Such surfactants are expected to form stable bilayers surrounding each particle. The resulting composite suspensions were stable to aggregation and appeared only slightly more turbid than the original silica sols. The displaced cations of the photosensitizer were separated from the colloidal assemblies using gel-filtration chromatography. The removal was confirmed by a control experiment similar to the one used in the case of Ru(bpy)32+-doped silica sols. The concentration of ZnTMPyP4+ present in the suspensions was determined using absorption spectroscopy. It was assumed that the extinction coefficient of the Soret band maximum (λ ) 450 nm) of the porphyrin in the silica matrix is equal to that of the porphyrin adsorbed on the silica surface (λmax ) 439 nm, ) (1.30 ( 0.07) × 105 M-1 cm-1). It was estimated that in the range of concentrations used ca. 90% of Zn(II) porphyrin originally present was embedded in the silica and could not be removed from the suspension. Steady-State Measurements. The absorption spectra of Ru(bpy)32+ and ZnTMPyP4+ incorporated in the silica matrix were similar to their spectra in aqueous solutions. The spectra remained unchanged over a period of several months. The results of the steady-state measurements on the colloidal composites are summarized in Table 1 together with some literature values. The absorption spectrum of Ru(bpy)32+-doped silica suspension modified with the DDAB bilayer showed a maximum at 455 nm. The emission maximum in the same system was located at 596 nm. The emission quantum yield determined using an aqueous solution of Ru(bpy)32+ as a standard (φe ) 0.04630) was 0.138 ( 0.014. These observations confirm results reported by Wheeler and Thomas.17 The Soret band maximum of the porphyrin was located at 450 nm in the silica particles surrounded by the surfactant bilayer. Similar to the absorption spectrum of ZnTMPyP4+ in water, only two Q-bands were observed in
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Figure 2. Tris(2,2′-bipyridyl)ruthenium(II) MLCT-state decay monitored by following luminescence at 600 nm (trace a) and ground-state bleaching at 455 nm (trace b) in colloidal silica particles surrounded with didodecyldimethylammonium bilayer: excitation, 532 nm; inset, semilogarithmic plot of the emission signal vs time. The suspension was deoxygenated with a stream of argon for 10 min.
Figure 3. ZnTMPyP4+ triplet-state decay in silica particles surrounded with DDAB bilayer monitored by transient absorption at 500 nm following a 7-ns 450-nm laser pulse. Oxygen was removed by a successive application of vacuum and argon pressure to the cuvette. The solid line represents the best fit of the data to a three-exponential decay function (〈τ〉 ) 13.8 ms). Inset: The residuals of the fit.
the colloidal composites. The ratio of the band intensities was unchanged. Fluorescence of ZnTMPyP4+ excited state in colloidal composite system showed a maximum at 650 nm with a shoulder at 700 nm. The ratio of the intensity of the maximum peak to that of the shoulder was increased by a factor of almost 2 for the porphyrin incorporated into silica matrix compared to that for the aqueous homogeneous solution of ZnTMPyP4+. No room temperature phosphorescence or delayed fluorescence could be observed in deoxygenated colloidal composite samples contrary to what may be expected for a zinc porphyrin.34 Time-Resolved Experiments. The behavior of the excited states of the photosensitizers in the prepared colloidal assemblies was studied with time-resolved absorption and emission experiments. Upon photoexcitation the excited states of both complexes decayed nonexponentially. The nonexponentiality of the decay can be seen most readily from the semilogarithm plot of the signal versus time (Figure 2, inset). The traces were fitted with either two- or three-exponential decay models.4 The average lifetimes (〈τ〉) of the excited states were obtained from the fits using
Table 2. A Comparison of the Average Lifetimes of the Excited MLCT State of Ru(bpy)32+ and Excited Triplet State of ZnTMPyP4+ in the Colloidal Assemblies and Homogeneous Aqueous Solutions
N
〈τ〉 )
N
Riτi2/∑Riτi ∑ i)1 i)1
in colloidal composites deoxygenated 2+
Ru(bpy)3 ZnTMPyP4+
µsa
1.2 6.6-13.8 msa
air 1.2
µsa
in aqueous solutions deoxygenated µsb
0.6 1.2 msd
air 0.4 µsc
a Error in the lifetime values is (10%. b Lin, C.-T.; Sutin, N. J. Phys. Chem. 1976, 80, 97-105. c Boletta, F.; Maestri, M.; Moggi, L. J. Phys. Chem. 1973, 77, 861-862. d Harriman, A.; Porter, G.; Walters, P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 1335-1350.
decayed concomitantly to a zero baseline (Figure 2). It was found that the presence of oxygen did not affect the lifetime of the excited state. ZnTMPyP4+-Containing Composites. The kinetics of the triplet excited state of the porphyrin complex were followed by monitoring absorbance changes at 490-500 nm. The maximum of the triplet difference absorption spectrum was observed in this region, which constitutes a 30-nm red-shift from the homogeneous aqueous solution. Ground-state bleaching of the complex was monitored in the 440-450-nm region. The decay of both transients to a zero baseline was nonexponential. In the presence of oxygen the decay became strongly biphasic (Figure 4), showing that a portion of the porphyrin triplets was accessible to quenching, unlike the Ru(bpy)32+ complex. The variation in the average lifetimes determined for ZnTMPyP4+ triplet state in deoxygenated samples (Table 2) was probably due to variable amounts of residual O2.
where Ri is the contribution of an ith component, τi is its lifetime, and N is the number of components of the fit. An example of the fit is presented in Figure 3. Although the contributions of the individual components of the fit varied from sample to sample, the average lifetimes were reproducible and independent of the laser intensity. The excited states of both complexes incorporated into the colloidal composites were long-lived compared to their aqueous deoxygenated solutions. The results of the lifetime measurements are presented in Table 2 together with some literature values. Ru(bpy)32+-Containing Composites. The kinetic behavior of the MLCT state of Ru(bpy)32+ was investigated by following the decay of the emission at 600 nm. Groundstate recovery was monitored by observing the decay of the absorption bleaching at 455 nm. Both transients
A 0.2 M aqueous solution of sodium silicate, otherwise known as water-glass, has a pH of 10-11.33 Once passed through a bed of cation-exchange resin in H+ form, a solution of active silicic acid is obtained.35 It consists of SiO2 colloidal particles with a diameter of 2 nm and has a pH of 2-4. At a temperature of at least 60 °C and in the presence of alkali (a pH of 8-10.5 and molar ratio of SiO2:Na2O of 20-500), a dilute silica sol is formed. The
(34) Aota, H.; Morishima, Y.; Kamachi, M. Photochem. Photobiol. 1993, 57, 989-995.
(35) Yoshida, A. In The Colloid Chemistry of Silica; Bergna, H. E., Ed.; American Chemical Society: Washington, DC, 1994; pp 51-66.
Discussion
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Figure 4. ZnTMPyP4+ triplet-state decay monitored by observing absorption change at 500 nm following a 7-ns 355nm laser pulse. Sample: suspension containing silica particles modified with incorporation of ZnTMPyP4+ and surrounded with DDAB bilayers in argon (filled triangles), air (circles), and oxygen (open triangles) atmosphere. The signals were normalized to the same initial ∆OD for better comparison of the relative decay rates.
mechanism responsible for the particle growth is a dehydrating condensation, reaction 1.35
Initially, active silicic acid is polymerized into nuclei (∼4 nm diameter). More silicic acid polymerizes subsequently around the nuclei thus increasing the size of particles by a process often referred to as “buildup”.35 The spherical particles in the final suspension are stabilized from aggregating by a negative charge on the surface. This charge is produced in alkaline environments when a metal cation (e.g. Na+) substitutes the proton of the surface silanol group.33 The interior matrix of silica in the colloidal particles is not uniform. Sometimes internal silanol groups are present within the particles. That causes small cavities, which can contain impurities (e.g. cations), to be formed inside the silica lattice.36 Highly charged cations that are strongly associated with the anionic silanol group may end up trapped in these internal cavities during the polymerization of the silicic acid. The photophysical properties imply that the complexes are trapped within the silica particle’s interior away from the aqueous phase. A photoactive molecule in such an environment may exhibit changes in its photophysical and photochemical properties due to the strong influence of the adjacent silanol groups and restricted freedom of rotation or molecular motion. The effects of the medium surrounding the photosensitizer in the silica particles can be compared to the influence that low-temperature glasses project onto trapped molecules. The emission maximum of the Ru(bpy)32+ in ethanol-methanol glass at 77 K is located at 582 nm and the excited-state lifetime is 5.1 µs.30 A similar trend in the changes of the complex properties is observed in the silica particles (see Results). Similarly, the zinc porphyrin triplet-state lifetime increased more than 3-fold when it was immobilized in the silica particles. In comparison, the triplet-state lifetime of zinc(II) tetraphenylporphyrin increases from 1.2 to 26 ms on going from (36) Bergna, H. E. In The Colloid Chemistry of Silica; Bergna, H. C., Ed.; American Chemical Society: Washington, DC, 1994; pp 1-47.
Serguievski et al.
room temperature solutions to glass at 77 K.30 It is probable that in the silica the complexes are located in a semirigid environment: the molecular movements are constrained by the rigidity of the surrounding silica matrix, however enough solvent molecules are present in the cavities to provide some pathways for the excited state deactivation. It is interesting to note that for the Ru complex the increase in the emission yield is 1.5 times higher than the increase in excited-state lifetime, suggesting that the radiative rate constant may be affected as well. Changes in the steady-state fluorescence spectrum of ZnTMPyP4+ are also brought about by excited state interaction with the silica matrix (Table 1). The red-shift in the maximum suggests decreased energy gap between the first excited and the ground electronic states, which may occur as a result of a relative stabilization of the excited state by the medium. On the other hand, the emitting MLCT state of Ru(bpy)32+ is destabilized by the medium. The observed change in the ratio of the Soret peak intensity to that of the first Q-band for the silicabound ZnTMPyP4+ may indicate that the geometry of the second and/or first excited singlet state of the porphyrin is altered by the silica matrix. All these observations point to a very strong interaction between the excited states of the chromophores and silica. It may be speculated that such influences can be exerted only on the complexes located within the particle interior. Heterogeneous environments with relatively rigid components restrict motions of species incorporated into them. There exists a distribution of microenvironments within the system, each projecting a different influence upon the incorporated species. These microenvironments do not interconvert between each other during the lifetime of the excited state. This produces a distribution of excitedstate properties which manifests itself spectroscopically (band broadening) and kinetically (nonexponential decays) (see, for example, Figure 2). There exist a number of ways of treating excited-state kinetics in microheterogeneous systems.12,37-39 The best results, judging by the quality of the fit, were obtained with two- or three-exponential models. Even though this approach lacks physicochemical meaning, it provides a way of comparing excited-state lifetimes combined with ease and speed of calculations. Nonlinear least-squares optimization of the parameters was performed until the change in the sum of squares for the fit was lower than 0.01% of its value (Figure 3). The fits to curves containing about 200 points resulted in final lifetime values determined with approximately 10% error. Ionic amphipathic substances adsorb onto charged surfaces of solids from aqueous solutions.40 The head group of the surfactant is electrostatically attracted to the oppositely charged interface, while the hydrophobic chains are attracted to each other. An interplay of these forces favors arrangement of surfactant molecules in bilayers supported on the charged surface. Such bilayer structures, usually referred to as supported bilayers, form on charged colloidal particles as well as on the surface of the bulk solid. In the colloidal suspensions the aggregate is reminiscent of a vesicle with an inner aqueous core replaced with a solid particle. (37) Carraway, E. R.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1991, 63, 332-336. (38) Albery, W. J.; Barlett, P. N.; Wilde, C. P.; Darwent, J. R. J. Am. Chem. Soc. 1985, 107, 1854-1858. (39) Siemiarczuk, A.; Wagner, B. D.; Ware, W. R. J. Phys. Chem. 1990, 94, 1661-1666. (40) Hough, D. B.; Rendall, H. M. In Adsorption from Solution at Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic: London, 1983; pp 247-319.
Colloidal Composites Containing Photosensitizers
In the studied system didodecyldimethylammonium bilayers were built on colloidal silica particles. Didodecyldimethylammonium chloride (DDAC) forms complete bilayers on spherical silica particles.27 Thus it is reasonable to assume that the bromide salt will behave similarly. The critical micelle concentration (cmc) of DDAB in water has been reported to be 0.16 mM. It has also been reported that the surface area occupied by one molecule of that surfactant in liquid crystal phase is 30 or 34 Å2 per hydrocarbon chain depending on the particular phase.27 Taking the lower value and using 15 nm for the diameter of the silica particle leads to the conclusion that some 1200 surfactant molecules are needed for the complete monolayer. Consequently, approximately 2800 DDAB molecules are required for a complete bilayer, taking into account that the diameter of the sphere produced by a monolayer of adsorbed surfactant is 2.4 nm larger than that of the silica particle.41 A typical sample used in this work contained 1 µM particles and 7 mM DDAB, which provides 4.2 mM excess of surfactant over the amount needed for the bilayer. In the studies of DDAC adsorption on silica surface, it has been noted that some amount of the surfactant (concentration slightly higher than cmc) not associated with the surface was present in the system.27 Thus, apparently, it is essential for the bilayer stability that different surfactant aggregates are present with which the bilayer exists in a dynamic equilibrium. Some of the surfactant is expected to be removed from the system during the gel filtration in our studies, thus justifying using a high excess of it to construct the bilayer. Substitution of DDAB with a surfactant with a longer hydrocarbon chain may significantly reduce the excess needed for a stable bilayer. The thermodynamic force that induces surfactant molecules to organize in bilayers around silica particles is strong enough to overcome electrostatic attraction between the cationic chromophores and the negatively charged surface, thus displacing the complexes into the bulk of the solution (see Results). This ensures that all the photosensitizer molecules studied are located within the silica particle matrix. The proposed location of the metal complexes within the interior of the silica particles renders them virtually inaccessible for a direct contact with any species located outside of the colloids. However, it is reasonable to assume that a certain degree of porosity exists in the silica lattice.36 The pores would provide for small molecules to diffuse inside the particles and come into contact with the photosensitizer. Diffusion of such species will be retarded to a certain extent by the constraining environment of the (41) Thomas, J. K. The Chemistry of Excitation at Interfaces; American Chemical Society: Washington, DC, 1984.
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solid particle. This can explain the absence of excitedstate quenching by oxygen for Ru(bpy)32+-doped silica suspensions, the process that is reported to be near diffusion controlled in aqueous solutions.42 Oxygen quenching cannot effectively compete with the decay of the excited state in this system due to its restricted diffusion within the silica pores. On the other hand, with the much longer-lived porphyrin triplets the presence of oxygen strongly influences the triplet-state lifetime of a fraction of the zinc porphyrin complexes (approximately 75% of the total) located within the silica particles (Figure 4). This process in homogeneous aqueous solution should occur with a rate close to diffusion controlled. By assuming that the porphyrin molecules are distributed randomly within the silica sphere (excluding the inner core), O2 must penetrate into the particle as far as 2.5 nm from the surface to quench 75% of the excited states. Diffusion of oxygen deeper into the particle is most likely prevented by the decreased volume of pores at farther distances from the surface. Conclusion Stable colloidal assemblies of silica particles modified with photochemically active metal complexes and surface supported bilayers were prepared and characterized. The general techniques for obtaining such systems were developed. The photosensitizers incorporated into the particles retain their photophysical properties and may be used as photoinitiators of electron and energy transfer to species located in the bilayers. Some characteristics of the incorporated complexes are modified by the silica environment making them potentially more efficient photosensitizers. Small molecules such as O2 are able to penetrate into the silica particle interior and quench a fraction of the excited states of the porphyrin due to their long lifetime. This structure may become a basis for a system capable of storing solar energy by separating the products of the transfer step. The insulating nature of silica may retard the desired process by rendering the material unpenetrable to electrons. Semiconductor colloids will be used in the future to improve the efficiency of the system. Acknowledgment. This research was supported by Grant CHE-9208551 from the National Science Foundation. We are grateful to Drs. Daniel W. Schwab and James Olesen (Electron Microscopy Facility) for carrying out the transmission electron microscopy analysis. LA950624W (42) Kirk, A. D.; Porter, G. B. J. Phys. Chem. 1980, 84, 2998-2999.
