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Thin Film Actinometers for Transient Absorption Spectroscopy: Applications to Dye-Sensitized Solar Cells Bryan V. Bergeron,† Craig A. Kelly,‡ and Gerald J. Meyer*,† Department of Chemistry and Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21210, and The Applied Physics Laboratories, Laurel, Maryland 20723 Received June 15, 2003. In Final Form: July 22, 2003 The chromophores Ru(bpy)3(PF6)2 and Os(bpy)3(PF6)2 were immobilized within poly(methyl methacrylate) (PMMA) thin films on glass substrates for applications as actinometers for nanosecond flash photolysis. Transient absorption difference spectra of M(bpy)3(PF6)2 (M ) Ru, Os), at ambient temperature and in an argon atmosphere, were the same when imbedded in PMMA films as in solution, within experimental error. Linear ranges of ∆A versus 532 nm pulsed laser energy where these actinometers were applicable were identified, up to 25 mJ/(cm2 pulse) for Ru(bpy)32+/PMMA and up to 5 mJ/(cm2 pulse) for Os(bpy)32+/ PMMA. Laser energy measurements were used to estimate the difference between the excited- and groundstate extinction coefficients at 450 nm for Os(bpy)32+, ∆450nm, which is -7300 M-1 cm-1. The Ru(bpy)32+/ PMMA actinometer was useful from 300 to ∼550 nm, while the Os(bpy)32+/PMMA actinometer extends the sensitivity to ∼700 nm. An application of these actinometers for dye-sensitized solar cells is described, wherein the quantum yield for electron injection from Ru(dcbH2)(bpy)22+*, where dcbH2 is 4,4′-(CO2H)22,2′-bipyridine, into mesoporous nanocrystalline (anatase) TiO2 thin films was quantified as a function of ionic strength.
Introduction The best method to determine quantum yields of intermediates formed after pulsed laser excitation relies upon the technique of comparative actinometry.1 Important insights into the mechanistic details of photodriven electron transfer, energy transfer, and excited-state reactivity have been gained through this approach.2-4 Organic and inorganic chromophores have been commonly used as actinometers, with applications historically limited to measurements in fluid solution.5-7 More recently, however, there exists growing interest in excitedstate processes in thin film solid-state materials.8,9 This desire is motivated by applications in photocatalysis, solar energy conversion, light-emitting diodes, and other photonic devices.10-12 The geometry and dimensions of these materials are generally not compatible with the existing fluid solution actinometers. As a result, the need for thin film actinometers is apparent and is addressed as the subject of this paper. A stringent definition states that an actinometer is a chemical system or physical device that determines the † ‡
The Johns Hopkins University. The Applied Physics Laboratories.
(1) McNaught, A. D.; Wilkenson, A. IUPAC Compendium of Chemical Terminology, 2nd ed.; Blackwell Science: Cambridge, MA, 1997. (2) Mallouk, T. E.; Krueger, J. S.; Mayer, J. E.; Dymond, M. G. Inorg. Chem. 1989, 28, 3507-3510. (3) Ruthkosky, M.; Castellano, F. N.; Meyer, G. J. Inorg. Chem. 1996, 35, 6406-6412. (4) Olmsted, J.; Meyer, T. J. J. Phys. Chem. 1987, 91, 1649-1655. (5) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York, 1993. (6) Hachard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235A, 518-536. (7) (a) Masson, C. R.; Boekeldeide, V.; Noyes, W. A., Jr. In Technique of Organic Chemistry; Weissberger, A., Ed.; Interscience Publishers: New York, 1956; Vol. II, pp 257-384. (b) Evans, T. R. Tech. Org. Chem. 1969, 14, 297-348. (c) Wagner, P. J. Tetrahedron Lett. 1968, 53855388. (d) Wagner, P. J.; Capen, G. Mol. Photochem. 1969, 1, 173-188. (8) Meyer, G. J.; Karlin, K. D. Molecular Level Artificial Photosynthetic Materials; Progress in Inorganic Chemistry, Vol. 44; John Wiley & Sons: New York, 1997.
number of photons in a beam integrally or per unit time.1 Chemical actinometers reported in the literature for nanosecond transient absorption spectroscopy have documented intersystem crossing yields and triplet-to-triplet extinction coefficients.13,14 With this knowledge, a single wavelength transient absorption measurement of the actinometer (A) can be used to quantify the moles of photons (i.e., einsteins) absorbed. More often, one measures the absorption change after excitation of a sample (S) and the actinometer under similar experimental conditions. The number of photons absorbed by the sample is comparatively estimated, with corrections made for differences in ground-state absorbance. If the extinction coefficient of the photoproduct(s) of S is known, one can then quantify the concentration of photoproducts, S′, and the quantum yield for the photodriven process S f S′. Experimentally, one commonly probes intermediates in a direction 90° relative to the excitation laser pulse (Scheme 1). According to Beer’s law, the excitation irradiance I (given in units of photons/cm2) decreases logarithmically with path length x, concentration [c] (given in units of mol/L), and extinction coefficient (given in units of M-1 cm-1), eq 1.15 Quantum yields deduced by this method necessitate optically dilute matrixes to ensure near homogeneous volume irradiation, whether solution or thin film based. Nevertheless, the concentration of intermediates sampled may rely upon the positioning of (9) Chopra, K. L.; Das, S. R. Thin Film Solar Cells; Plenum Press: New York, 1983. (10) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537-2574. (11) (a) Qu, P.; Meyer, G. J. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley & Sons: New York, 2001; Chapter 2, Part 2, Vol. IV, pp 355-411. (b) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: New York, 1991. (12) Nakumura, S. Jpn. J. Appl. Phys. 1996, 35, L74-L76. (13) Yoshimura, A.; Hoffman, M. Z.; Sun, H. J. Photochem. Photobiol., A 1993, 70, 29-33. (14) Bensasson, R.; Goldschmidt, C. R.; Land, E. J.; Truscott, T. G. J. Photochem. Photobiol. 1978, 28, 277-281. (15) Lachish, U.; Shafferman, A.; Stein, G. J. Chem. Phys. 1976, 64 (10), 4205-4211.
10.1021/la035053j CCC: $25.00 © 2003 American Chemical Society Published on Web 08/28/2003
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Scheme 1
the probe beam and also the width of the probe beam relative to the path length along the excitation direction. Therefore, solution actinometers present in a 1 cm path length cuvette are not ideally suited for applications of the previously mentioned thin film solid-state materials. To minimize errors, thin film actinometers are needed whose geometric and optical properties most closely duplicate those of the sample.
I(x) ) I010-[c]x
(1)
Here we describe the preparation of thin film actinometers for transient absorption spectroscopy. The actinometers are comprised of poly(methyl methacrylate) (PMMA) doped with M(bpy)3(PF6)2, where M ) Ru or Os, cast upon microscope slides. The excited-state extinction coefficient differences of M(bpy)32+* within PMMA films are reported and contrasted to previous solution values. Finally, an application of the actinometers for calculating the excitedstate interfacial electron injection yields at dye-sensitized semiconductor interfaces is reported. Experimental Section Materials. PMMA, Mw ∼ 15 000, was obtained from Aldrich. Plain microscope slides 25 mm by 75 mm by 1 mm were obtained from Fisher Scientific. Ru(bpy)3(PF6)2 and Os(bpy)3(PF6)2 were available from previous experiments. Reagent grade CHCl3 and CH3CN were obtained from EM Science. TiO2 thin films were produced as previously reported.16 PMMA Thin Films. Incorporation of the chromophores within the solid amorphous polymer was established by utilizing a chloroform/acetonitrile solvent mixture with proportions and concentrations as follows. A 0.2 g/mL mixture of PMMA/CH3Cl was vigorously stirred and stored in a sealed amber jar. M(bpy)3(PF6)2 (1-2 mg), where M ) Ru or Os, was dissolved in 0.1 mL of an acetonitrile solution. Chloroform and acetonitrile in a 6:1 (v/v) ratio were combined and thoroughly mixed within a test tube. A microscope slide was pre-scored 6.25 mm on each side and ∼15 mm at the ends as shown in the diagram (dotted lines).
mixture was deposited on the top of the slide and spread evenly using a Pasteur pipet. The material was covered for ∼1 h and was cleaved along the scored lines. Appropriate preparation using this procedure resulted in stable films that have not cracked for periods of years. Profilometry. Measurements were obtained using a Wyko NT1100 optical profiler. Data were acquired using a 20× objective and a 0.5× field of view. The instrument was operated in vertical scanning interferometry mode and in full resolution. Undoped films were 6 ( 1 µm thick, and M(bpy)3(PF6)2-doped films were 3 ( 1 µm thick. Absorbance. Steady-state absorption measurements were made on a Hewlett-Packard 8453 diode array spectrophotometer. A PMMA/microscope slide was used as a reference for the polymer films containing either Ru(bpy)3(PF6)2 or Os(bpy)3(PF6)2, and measurements were made in the absence of solvent. Neat acetonitrile was used as a reference for solution measurements. A TiO2/microscope slide was used as a reference for the TiO2 material sensitized with Ru(bpy)3(PF6)2 or Os(bpy)3(PF6)2, with a surrounding acetonitrile solvent. All measurements were obtained in an argon-purged, sealed, quartz cuvette. Transient spectra were acquired as previously described.17 Briefly, a ca. 7 ns, typically 6 mJ/(cm2 pulse), 532 nm laser pulse from a Surelite II Nd:YAG, Q-switched laser was used as the excitation source. A white light source was the probe beam (150 W Xe, Applied Photophysics, operating in pulsed mode), which was positioned normal to the excitation beam and was focused on the exposed surface. The excitation/probe orientation was chosen to minimize scattered light reaching the detector. The sample was protected from UV and IR light using suitable glass and water filters positioned between the lamp and the sample, and scattered laser light was attenuated using appropriate glass filters between the sample and monochromator. The transmitted light was collected and refocused on the entrance slit of an Applied Photophysics monochromator and detected using a Hamamatsu R928 photomultiplier. The data were digitized and stored on a Lecroy 9450 digital oscilloscope. Each kinetic trace was acquired by averaging 40-200 laser shots (typically 80) at a repetition rate of 1 Hz. Labview (National Instruments) was utilized with a GPIB programmable Berkeley Nucleonics 555 pulse/delay sequence generator for sequence timing and data acquisition. Photoluminescence. Steady-state photoluminescence (SSPL) spectra were obtained with a Spex Fluorolog that had been calibrated with a standard NIST tungsten-halogen lamp. PMMA thin film measurements were made in the absence of solvent and solution measurements were performed in an argon-purged acetonitrile solvent, both obtained in a sealed quartz cuvette.18 Time-gated photoluminescence spectra were acquired following pulsed 532 nm excitation by a Continuum Surelite II Nd:YAG, Q-switched laser (second harmonic, 7 ns fwhm, 8 mJ/(cm2 pulse)). Emitted light was collected at a right angle to the excitation path with a Princeton Scientific 576 G/RB intensified charge coupled device (ICCD) camera, controlled by a ST130 controller/ PG200 pulse generator.
Results Thin film actinometers consisting of either Ru(bpy)3(PF6)2 or Os(bpy)3(PF6)2 homogeneously dispersed within poly(methyl methacrylate) upon glass substrates were prepared; these samples are abbreviated throughout as Ru2+/PMMA and Os2+/PMMA. For brevity, the data for Ru(bpy)3(PF6)2 are included and those for Os(bpy)3(PF6)2 are available in the Supporting Information. Profilometry measurements displayed a typical film thickness of ∼5 µm ( 2.5 µm. The undoped thin films show high transparency from 300 to >1100 nm. A comparison of the ground-state absorbance of Ru2+ in neat acetonitrile and in PMMA films is shown in Figure 1. The spectral distribution and maximum absorbance wavelengths of The sides of the slide were taped to a flat surface with the perforated side down. Approximately 0.5-1.0 mL of the test tube (16) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737.
(17) Bergeron, B. V.; Meyer, G. J. J. Phys. Chem. B 2003, 107, 245254. (18) Castellano, F. N.; Heimer, T. A.; Thandasetti, M.; Meyer, G. J. Chem. Mater. 1994, 6, 1041-1048.
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Figure 1. Normalized ground-state absorbance and photoluminescence spectra for Ru2+ in an acetonitrile solution (s) and in a PMMA thin film (- - -).
Figure 3. The time-zero amplitude, ∆At)0, measured at 450 nm after pulsed 532 nm photoexcitation as a function of irradiance. (A) The ground-state absorbance at 532 nm was 0.0721 (b) and 0.024 (1) within PMMA and 0.0725 (O) and 0.024 (3) within acetonitrile solution Ru2+. (B) The groundstate absorbance at 532 nm was 0.0565 (b) and 0.0265 (1) within PMMA and 0.0575 (O) and 0.0265 (3) within acetonitrile solution for Os2+. Figure 2. Transient absorbance vs wavelength following 532 nm excitation with 5.6 mJ/(cm2 pulse) for Ru2+ in PMMA thin films. Time delays are at 0 ns (9), 200 ns (O), 600 ns (1), 1400 ns (]), and 3000 ns (b). A superposition of the normalized transient absorbance trace of Ru2+ in CH3CN is also shown (dotted gray line). The inset represents the transient absorbance signal observed at 380 nm with an overlaid fit to a biexponential kinetic model.
Ru2+ and Os2+ are identical in CH3CN solution and PMMA thin films, within experimental error. Corrected steady-state photoluminescence spectra were obtained following 480 nm excitation of Ru2+ and Os2+ in CH3CN solutions and in PMMA films. The PL maximum from Ru2+* blue shifts from 625 to 605 nm (∆E ) 40 meV) and from 745 to 715 nm (∆E ) 20 meV) for Os2+* following incorporation of the chromophore into PMMA from CH3CN solution. An energy decrease of 38 cm-1 of the full width at half-maximum (fwhm) emission from Ru2+* was also observed in PMMA relative to CH3CN. The bandwidth of the Os2+* PL extends beyond the long-wavelength limit of the detector sensitivity. Uncorrected time-gated photoluminescence spectra measured at t > 10 ns after pulsed 532 nm excitation of Ru2+/PMMA or Os2+/PMMA showed no evidence for spectral shifts. Transient absorbance difference spectra of Ru2+*/ PMMA are displayed in Figure 2. A superposition of a normalized ∆A spectrum of Ru2+ in CH3CN solution is shown for comparison. Excited/ground-state isosbestic
points were observed at 400 ( 2 nm and 407 ( 2 nm for Ru2+ and Os2+, respectively. Single wavelength kinetics were exponential in CH3CN with lifetimes of 800 ns for Ru2+*, 59 ns for Os2+*, and 95 ns for Os2+*/PMMA, eq 2. Excited-state decay kinetics of Ru2+*/PMMA were nonexponential and were well fit to the sum of two exponentials, eq 3. The insets display selected kinetic decay traces with a fit superimposed, demonstrating the goodness of the fit.
∆A(t) ) ∆At)0 exp(-kt)
(2)
∆A(t) ) (∆A1) exp(-k1t) + (∆A2) exp(-k2t)
(3)
In fluid solution and in PMMA, the maximum absorption bleach for the ruthenium and osmium compounds is observed at 450 nm. The amplitude of the transient absorbance signal measured at time zero, ∆At)0, was plotted versus 532 nm pulsed excitation energy for two representative PMMA and solution samples with their indicated ground-state absorbance (Figure 3A,B). The ∆At)0 was estimated from the maximum data point collected on the oscilloscope or from fits to the kinetic data where ∆At)0 ) ∆A1 + ∆A2. Amplitudes resolved from kinetic fits were found to be most accurate and less sensitive to light scatter. For Ru2+, the ∆At)0 was found to vary nearly linearly over the excitation energy range employed. However, this was not the case for Os2+, and
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Figure 4. Transient absorbance at 403 nm following excitation of TiO2/Ru(dcbH2)(bpy)22+ with increasing concentrations of lithium perchlorate (0, 10-5, 10-4, 10-3, 10-2, 10-1, and 1 M) in the external argon-purged acetonitrile solution. The horizontal lines indicate the estimated value of ∆At)0. The ground-state absorbance was 0.103 at 532 nm, and an irradiance of 3.1 mJ/pulse was used for the transient absorption experiment.
Figure 5. Interfacial electron-transfer yields, φinj, from Ru(dcbH2)(bpy)22+* to TiO2 as a function of the lithium perchlorate concentration in an external acetonitrile solution. Absolute quantum yields were measured with both Ru2+/PMMA (9) and Os2+/PMMA (O) actinometers.
saturation was clearly observed at high irradiance in CH3CN solution and PMMA thin films. The thin film actinometers were used to quantify interfacial electron injection yields following pulsed excitation of Ru(dcbH2)(bpy)22+ anchored to nanocrystalline TiO2 thin films. An extinction coefficient difference of -9000 M-1 cm-1 at 403 nm between the Ru(III) and Ru(II) forms was previously measured by spectroelectrochemistry.36 Electron-transfer efficiencies were quantified as a function of lithium perchlorate electrolyte concentration in an external acetonitrile solution (Figures 4 and 5). Discussion The powerful analytical technique of nanosecond transient absorption spectroscopy has been widely used to characterize the yields and dynamics of photoinitiated reactions.19-21 A common optical arrangement is shown in Scheme 1 where the excitation beam is a collimated (19) Das, S.; Thomas, K. G.; Ramanathan, R.; George, M. V.; Kamat, P. V. J. Phys. Chem. 1993, 97, 13625-13628. (20) Tyson, D. S.; Castellano, F. N. J. Phys. Chem. A 1999, 103, 10955-10960.
laser source positioned normal to an uncollimated white light probe beam. This geometry minimizes collection of the exciting light in the detection optics. For solution experiments, the sample is homogeneously dispersed through the cuvette. Following pulsed light absorption, the concentration of photoproducts decreases logarithmically with distance through the sample along the excitation beam axis. Therefore, the transient absorption signals are largest when the probe beam is closest to the incident laser source and decrease in magnitude when the probe beam is moved along the excitation axis away from the laser source. A low optical absorbance at the excitation wavelength helps minimize such concentration gradients; however, so-called inner-filter effects are well documented in the literature and are particularly problematic when high concentrations are needed to measure low quantum yield processes.22,23 For the thin film measurements reported here, the film is placed diagonally in the cuvette and the probe beam exceeds the film thickness (Scheme 2). Therefore, the probe beam quantifies all the photogenerated intermediates in the film. If a solution actinometer were used as a thin film reference, the quantum yields calculated should be dependent on the position of the probe beam, and this was found to be the case. In principle, identical amplitudes for solution and thin films could be obtained by expanding the probe beam such that it samples the entire excitation path length. However, in practice this is difficult as the probe beam is rarely collimated and losses are associated with beam expansion, overfilling, and light collection. Therefore, a thin film actinometer is far better suited to estimate excited-state concentrations of samples deposited as a thin film. Below we describe the absorption and emission properties of actinometers based on Ru(bpy)3(PF6)2 and Os(bpy)3(PF6)2 doped in PMMA thin films, abbreviated Ru2+/PMMA and Os2+/PMMA. Absorbance and Photoluminescence. Ru(bpy)3(PF6)2 and Os(bpy)3(PF6)2 were selected as suitable chromophores due to their high photostabilities, long lifetimes, and absorption characteristics.24,25 The metal-to-ligand charge transfer (MLCT) absorption bands are observed in the visible region with ligand localized bands in the ultraviolet.26,27 The absorption properties of the complexes in PMMA are within experimental error the same as those in fluid acetonitrile solution. (21) Galappini, E.; Guo, W.; Zhang, W.; Hoertz, P.; Qu, P.; Meyer, G. J. J. Am. Chem. Soc. 2002, 124 (26), 7801-7811. (22) Lees, A. J. Anal. Chem. 1996, 68, 226-229. (23) (a) He, H.; Li, H.; Mohr, G.; Kovacs, B.; Werner, T.; Wolfbeis, O. S. Anal. Chem. 1993, 65, 123-127. (b) Tohda, K.; Lu, H.; Umezawa, Y.; Gratzl, M. Anal. Chem. 2001, 73, 2070-2077. (24) (a) Casper, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 24442453. (b) Casper, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 55835590. (25) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967-3977. (26) Chen, P.; Meyer, T. J. Chem. Rev. 1998, 98 (4), 1439-1478. (27) (a) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. Rev. 2001, 101 (9), 2655-2686. (b) Dattelbaum, D. M.; Meyer, T. J. J. Phys. Chem. A 2002, 106, 4519-4524.
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The photoluminescence of the complexes was slightly blue shifted in the PMMA matrixes relative to fluid solution. This behavior has previously been observed and is consistent with destabilization of the thermally equilibrated excited state in the PMMA material.28 The excitation spectra of the films and those from acetonitrile solutions were within experimental error the same, indicating that the intersystem crossing yields were wavelength independent in both media. Transient absorption difference spectra, at ambient temperature in an argon atmosphere, of M(bpy)32+ (M ) Ru, Os) imbedded within PMMA films were the same as those in solution, within experimental error. This suggests that the excited- and ground-state extinction coefficients are within experimental error the same in these two media. Steady-state absorption measurements made before and after pulsed laser excitation revealed no evidence for permanent photochemistry. Transient photoluminescence and absorption data for Ru2+/PMMA could not be adequately fit to a first-order kinetic model and required a sum of two. Nonexponential kinetics has previously been observed for MLCT excited states in PMMA thin films and other heterogeneous materials. Since the objective of this paper was to develop thin film actinometers, we were less concerned with the kinetics and most concerned with the amplitude of the signal at time zero, ∆At)0. We therefore modeled the data as a sum of exponentials to ensure that the fits accurately represented ∆At)0. Excitation Irradiance Measurements. Transient absorption spectroscopy was employed as an optical tool for estimation of excited-state concentrations of Ru(bpy)32+* or Os(bpy)32+* in solution and PMMA thin films as a function of irradiance. To a first approximation, the transient absorbance difference spectra, ∆A, can be attributed to the individual absorptions of the reduced bipyridine ligand and oxidized metal center and to loss of ground state.29 More specifically, the amplitude of the transient absorbance is a function of the excited-state and ground-state extinction coefficients and the excited-state concentration, as expressed by eqs 4 and 5. Here, ∆λ refers to the difference in extinction coefficients between the excited state and ground state at the probe wavelength λ given in units of M-1 cm-1, and Γes refers to the number of excited states within a cross-sectional area, given in units of mol/cm2. Equation 4 was cast in this form since it applies to solution and thin films, without the need to know exact optical path lengths.
∆Aλ ) ∆λ1000Γes
(4)
∆λ ) es,λ - gs,λ
(5)
The observed transient absorbance magnitude following excitation is directly proportional to the concentration of excited states. Within optically dilute samples, the excitedstate concentration is nearly homogeneously dispersed within the matrix along the direction of incident irradiance, I. The number of excited states probed within a crosssectional area responds to the incident irradiance according to eq 6.15 Here, Γgs refers to the number of groundstate molecules probed, and gs refers to the ground-state extinction coefficient at the excitation wavelength. However, I was not directly measured, but a proportional value, termed the pulse energy E, was obtained instead.13 In (28) Jones, W. E.; Chen, P.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 387-388. (29) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992.
this case, Γes responds to the pulse energy according to eq 7. The constant b is proportional to the ground-state extinction coefficient at the excitation wavelength. Hoffmann demonstrated the validity of this exponential function through excitation of known concentrations of Ru(bpy)32+ with application of eq 8 to transient absorption measurements.13 The difference between the excited-state and ground-state extinction coefficients of Ru(bpy)32+ was determined to be -10 000 M-1 cm-1 at 450 nm in acetonitrile solution.13
Γes ) Γgs(1 - exp(-gsI))
(6)
Γes ) Γgs(1 - exp(-bE))
(7)
∆A ) ∆1000Γgs(1 - exp(-bE))
(8)
The concentration of excited state produced following pulsed excitation of optically dilute samples varies linearly with low excitation energies and can be expressed by eq 9.15 The transient absorption is related to the excitation energy according to eq 10. The proportionality constant is comprised of the difference between the excited-state and ground-state extinction coefficients and the groundstate absorbance.
Γes = ΓgsbE
(9)
∆A = ∆1000ΓgsbE
(10)
The ∆At)0 values were linear up to 25 and 5 mJ/(cm2 pulse) for Ru2+ and Os2+, respectively. Deviations from linearity are attributed to ground-state depletion and were most significant for Os2+. The ∆At)0 values for Os2+* in solution and PMMA converge at high irradiance and are essentially equal near 25 mJ/(cm2 pulse). In the linear region, the proportionality constant for the osmium relative to ruthenium is related to the ratio of the differences between excited- and ground-state extinction coefficients (∆λ). With this in mind and assuming ∆450nm for Ru2+ is -10 000 M-1 cm-1, ∆450nm for Os2+ was estimated to be ca. -7300 M-1 cm-1 from averages of multiple measurements. We note that the sequential twophoton absorption, often observed with 355 nm excitation, is unlikely here as the excited state absorbs very weakly at 532 nm. Applications. We anticipate that these thin film actinometers will provide insights into mechanisms of photoinduced processes occurring in a variety of photonic materials with real-world applications. An application that we have been specifically interested in is dye-sensitized solar cells based on mesoporous nanocrystalline TiO2 thin films. We and others have utilized time-resolved spectroscopy to gain insights into molecular level interfacial redox chemistry that promotes and inhibits the conversion of light into electrical power.30-32 To date, most of these studies have focused on the dynamics of charge transfer and relatively few have attempted to measure quantum yields.33,34 Here we report the quantum yields for excited(30) (a) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999, 15 (3), 731-737. (b) Kelly, C. A.; Thompson, D. W.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Langmuir 1999, 15, 70477054. (31) Gratzel, M. Nature 2001, 414, 338-342. (32) Dang, X.; Hupp, J. T. J. Am. Chem. Soc. 1999, 121, 8399-8400. (33) Tachibana, Y.; Haque, S. A.; Mercer, I. P.; Durrant, J. R.; Klug, D. R. J. Phys. Chem. B 2000, 104, 1198-1205. (34) Moser, J. E.; Noukakis, D.; Bach, U.; Tachibana, Y.; Klug, D. R.; Durrant, J. R.; Humphry-Baker, R.; Gratzel, M. J. Phys. Chem. B 1998, 102, 3649-3650.
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state electron injection from Ru(dcbH2)(bpy)22+*, where dcbH2 is 4,4′-(CO2H)2-2,2′-bipyridine, into TiO2 as a function of the ionic strength in an external acetonitrile solution as an example of how these actinometers can be applied. The TiO2 films were treated with an aqueous basic solution prior to attachment of the dye as has been previously described.35 This procedure is believed to shift the position of the conduction band edge negatively on an electrochemical scale (toward the vacuum level) resulting in energetically unfavorable electron injection from Ru(dcbH2)(bpy)22+*. The transient absorption spectrum of Ru(dcbH2)(bpy)22+* anchored to TiO2, abbreviated Ru2+/ TiO2, and immersed in an acetonitrile solution at room temperature is within experimental error the same as that of the deprotonated complex in acetonitrile.35 A ground/excited-state isosbestic point was observed at 403 nm. The oxidized ruthenium compound absorbs less light than the ground state at this wavelength, and the difference in extinction coefficients has been quantified by separate spectroelectrochemical measurements, ∆ ) -9000 M-1 cm-1.36 Therefore, the amplitude of the transient absorption bleach at 403 nm was a direct measure of the concentration of oxidized sensitizer. The concentration of excited state produced and hence the number of photons absorbed were estimated using the Ru2+/PMMA and Os2+/PMMA actinometers. A correction was made for differences between the ground-state absorbances between the Ru2+/TiO2 sample and the actinometer at the 532 nm excitation wavelength. Electron injection quantum yields measured with the two actinometers were within experimental error the same. For a large number of samples, we have found that quantum yields can be determined with an accuracy of (0.05. For the data shown in Figure 5, the quantum yields increased from
