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Temperature-Dependent Electron Injection from Ru(II) Polypyridyl Compounds with Low Lying Ligand Field States to Titanium Dioxide Ping Qu, David W. Thompson, and Gerald J. Meyer* Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218 Received February 2, 2000 The Ru(II) compounds Ru(bpy)2(ina)2(PF6)2, Ru(deeb)2(py)2(PF6)2, Ru(deeb)(bpy)2(PF6)2, and Ru(dcb)(bpy)2(PF6)2, where py is pyridine, bpy is 2,2′-bipyridine, ina is isonicotinic acid, deeb is 4,4′-(CO2Et)22,2′-bipyridine, and dcb is 4,4′-(CO2H)2-2,2′-bipyridine, have been prepared, characterized, and anchored to colloidal ZrO2 and TiO2 thin films for excited state and interfacial electron-transfer studies. In neat acetonitrile at 22 ( 2 °C, Ru(bpy)2(ina)2(PF6)2 is photochemically unstable and nonemissive with a short excited-state lifetime, τ < 10 ns. When anchored to ZrO2, the lifetime of Ru(bpy)2(ina)2(PF6)2 increases to 60 ns at 22 ( 2 °C and is highly temperature dependent due to the population of a low-lying state(s) that are proposed to be ligand field (LF) state(s). The LF state(s) can be populated directly from hot vibrational excited states and from the thermally equilibrated metal-to-ligand charge-transfer excited state. On TiO2 the excited-state behavior of Ru(bpy)2(ina)2(PF6)2 is very similar to that on ZrO2, except that fast interfacial electron injection occurs, kinj > 108 s-1. Ru(bpy)2(ina)2/TiO2 displays temperature-dependent electron injection, intersystem crossing, and emission quantum yields. A model is proposed where the direct population of LF states competes kinetically with electron injection. This behavior is contrasted with other sensitizers that have excited states localized on a surface-bound ligand.
Introduction The luminescence from ruthenium trisbipyridine, Ru(bpy)32+, and its many derivatives continues to be a useful probe in molecular recognition,1 biology,2 and materials chemistry.3 The metal-to-ligand charge transfer, MLCT, excited states are also finding practical applications in sensors,4 displays,5 and photovoltaic devices.6 Real-world applications usually require that the coordination compounds be dispersed or organized in solid-state materials.4-6 From a fundamental point of view, key issues arise regarding how restricted translational motion and interfacial heterogeneity influence the molecular excited state. While it would be convenient if well-understood photophysics and photochemistry from fluid solution translated directly to the solid state, this is rarely the case.7 Ru(bpy)32+ dispersed in insulating materials is known to undergo less than unit intersystem crossing (1) (a) Breault, G. A.; Hunter, C. A.; Mayers, P. C. J. Am. Chem. Soc. 1998, 120, 3402. (b) Chin, T.; Lelouche, I.; Shin, Y. K.; Purandare, A.; Knapp, S.; Isied, S. S. J. Am. Chem. Soc. 1997, 119, 12849. (2) (a) Holmlin, R. E.; Tong, R. T.; Barton, J. K. J. Am. Chem. Soc. 1998, 120, 9724. (b) Hurley, D. J.; Tor, Y. J. Am. Chem. Soc. 1998, 120, 2194. (c) Szmacinski, H.; Castellano, F. N.; Terpetschnig, E.; Dattelbaum, J. D.; Lakowicz, J. R.; Meyer, G. J. Biochim. Biophys. Acta 1998, 1383, 151. (3) (a) Vo¨gtle, F.; Plevoets, M.; Nieger, M.; Azzellini, G. C.; Credi, A.; Cola, L. D.; Marchis, V. D.; Venturi, M.; Balzani, V. J. Am. Chem. Soc. 1999, 121, 6290. (b) Wang, Q.; Wang, L.; Yu, L. J. Am. Chem. Soc. 1998, 120, 12860. (4) Xu, W.; McDonough, R. C.; Langsdorf, B.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1994, 66, 4133. (5) (a) Handy, E. S.; Pal, A. J.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 3525. (b) Lyons, C. H.; Abbas, E. D.; Lee, J.-K.; Rubner, M. F. J. Am. Chem. Soc. 1998, 120, 12100. (c) Maness, K. M.; Masui, H.; Wightman, R. M.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 3987. (d) Clark, C. D.; Debad, J. D.; Yonemoto, E. H.; Mallouk, T. E.; Bard, A. J. J. Am. Chem. Soc. 1997, 119, 10525. (e) Maness, K. M.; Terrill, R. H.; Meyer, T. J.; Murray, R. W.; Wightman, R. M. J. Am. Chem. Soc. 1996, 118, 10609. (6) (a) Bonhoˆte, P.; Moser, J. E.; Humphry-Baker, R.; Vlachopoulos, N.; Zakeeruddin, S. M.; Walder, L.; Grta¨zel, M. J. Am. Chem. Soc. 1999, 121, 1324. (b) Bach, U.; Tachibana, Y.; Moser, J.-E.; Haque, S. A.; Durrant, J. R.; Gra¨tzel, M.; Klug, D. R. J. Am. Chem. Soc. 1999, 121, 7445.
yields,8 time-dependent emission spectral shifts,9 nonexponential relaxation,8-11 and excited state-excited-state annihilation reactions11 all of which have little or no precedence in fluid solution.7 Much less is known about MLCT excited states proximate to conductive interfaces, such as at semiconductor surfaces. This is unfortunate as semiconductors are the backbone of the electronics industry and many current and future applications will rely on mechanistic descriptions of molecular excited states organized at conductive interfaces.12,13 One reason little information exists for any molecular excited state on a semiconductor surface is that electron injection into the semiconductor generally results in shortlived excited states and weak photoluminescence, PL, that are experimentally difficult to rigorously quantify.14 In fact, most reported PL measurements of semiconductorbound MLCT excited states have been made as an indirect method for estimating electron injection rate constants.14 (7) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Horwood: Chichester, 1991. (b) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992. (c) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (d) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (e) Crosby, G. A. J. Chem. Educ. 1983, 60, 791. (f) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (8) Fan, J.; Tysoe, S.; Strekas, T. C.; Gafney, H. D.; Serpone, N.; Lawless, D. J. Am. Chem. Soc. 1998, 120, 12860. (9) Castellano, F. N.; Heimer, T. A.; Thandasetti, M.; Meyer, G. J. Chem. Mater. 1994, 6, 1041. (10) Avnir, D. Acc. Chem. Res. 1995, 28, 328 and references therein. (11) (a) Fan, J.; Shi, W.; Tysoe, S.; Strakas, T.; Gafney, H. D. J. Phys. Chem. 1989, 93, 373. (b) Kenelly, T.; Gafney, H. D.; Braun, M. J. Am. Chem. Soc. 1985, 107, 4431. (c) Turbeville, W.; Robins, D. S.; Dutta, P. K. J. Phys. Chem. 1992, 96, 5024. (d) Milosavijevic, B. H.; Thomas, J. K. J. Phys. Chem. 1983, 87, 616. (12) A theoretical framework for excited states organized at conductive interfaces does exist: Chance, R. R.; Prock, A.; Sibley, R. Adv. Chem. Phys. 1978, 37, 1 and references therein. (13) (a) Rossetti, R.; Brus, L. E. J. Chem. Phys. 1982, 76, 1146. (b) Rossetti, R.; Brus, L. E. J. Chem. Phys. 1980, 73, 572. (c) Campion, A.; Gallo, A. R.; Harris, C. B.; Robota, H. J.; Whitmore, P. M. Chem. Phys. Lett. 1980, 73, 447. (14) Heimer, T. A.; Meyer, G. J. J. Lumin. 1996, 70, 468 and references therein.
10.1021/la0001528 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/07/2000
Temperature-Dependent Electron Injection
We recently found that the quantum yield for electron injection from Ru(dcb)(bpy)22+*, where dcb is 4,4′-(COOH)22,2′-bipyridine, to titanium dioxide (anatase) could be reversibly tuned from below detection limits, ∼0, to near unity simply by altering the ionic strength of an external acetonitrile bath.15,16 Here we exploit this finding to quantify the excited state and interfacial electron-transfer properties of Ru(II) polypyridyl compounds anchored to colloidal TiO2 and ZrO2 thin films. An important and previously unexplored issue addressed herein concerns the role low-lying ligand field (LF) states play in deactivating semiconductor-bound excited states. In fluid solution, ruthenium polypyridyl coordination compounds are characterized by photochemistry from LF excited states and photophysics from MLCT excited states.7 For Ru(II) coordination compounds anchored to wide band gap semiconductors, MLCT f LF internal conversion may lower the efficiency of electron injection, the excited-state lifetime, and/or the photostability of the material. We have prepared Ru(II) polypyridyl compounds with low-lying LF states that can be bound to semiconductor surfaces through chromophoric or nonchromophoric ligands specifically to look for this behavior. An unprecedented observation is the appearance of temperature-dependent electron injection yields that appear to emanate from the competitive population of lowlying excited states. Experimental Section Materials. Reagents. HPLC grade acetonitrile and nitric acid, 70%, were obtained from Fisher Scientific. The LiClO4, 99.99%, titanium(IV) isopropoxide, zirconium(IV) propoxide (70%), and tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate were obtained from Aldrich Chemical Co. and used as received. Burdick and Jackson spectroscopic grade acetonitrile was used as received. All solvents for synthesis were of reagent grade or better. Preparations. Colloidal MO2 Films. TiO2 and ZrO2 films were prepared by a previously described sol-gel technique that produced mesoporous 10 µm thick film.17 For absorption and luminescence studies the films were coated onto glass slides rather than conductive glass. The glass slides were cut from plain microscope slides, VWR 25 × 75 × 1 mm, to ca. 12.5 × 50 mm, allowing the slides to be inserted diagonally into a 10 × 10 mm optical path length, quartz fluorescence cuvette. The thin films had dimensions of 12.5 mm × 15 mm × 10 µm. For infrared studies, the films were coated on the unpolished surface of CaF2 windows (25 × 12 × 3 mm) purchased from International Crystal Laboratories. Coordination Compounds. 4,4′-(COOH)2-2,2′-bipyridine (dcb) was prepared from 4,4′-(CH3)2-2,2′-bipyridine as previously described.18 In some cases dcb was converted to 4,4′-(COOEt)22,2′-bipyridine (deeb) by the method of Maerker and Case.19 Ru(bpy)2Cl2‚2H2O was prepared by a previously reported method.20 Ru(deeb)(bpy)2(PF6)2 and Ru(dcb)(bpy)2(PF6)2 were available from previous studies.16 cis-Ru(deeb)2Cl2 was prepared by a method analogous to one previously reported.20 A 20 mL argon saturated solution of 9:1 ethanol/water containing 200 mg (0.831 mmol) of RuCl3‚H2O (Johnson Matthey), 500 mg (1.667 mmol) of deeb, 230 mg (2.1 mmol) of hydroquinone, and 1.06 g (25 mmol) of LiCl were heated to reflux for 12 h under argon. After cooling, the volume was reduced under vacuum to a few milliliters. The solution was then added to ∼300 mL of CH2Cl2 and washed with five 80 mL (15) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999, 15, 731. (16) Kelly, C. A.; Thompson, D. W.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Langmuir 1999, 15, 7047. (17) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319. (18) Oki, A. R.; Morgen, R. J. Synth. Commun. 1995, 25 (24), 4093. (19) Maerker, G.; Case, F. H. J. Am. Chem. Soc. 1958, 80, 2745. (20) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17, 3334.
Langmuir, Vol. 16, No. 10, 2000 4663 aliquots of distilled water. The dichloromethane layer was dried over MgSO4, filtered, and rotary evaporated to dryness. A dark green powder was collected as product. 1NMR δ (ppm), CD2Cl2: 10.17 (2 H, d, J ) 5.7 Hz), 8.75 (2 H, d, J ) 1.8 Hz), 8.59 (2 H, d, J ) 1.8 Hz), 8.05 (2 H, d, J ) 5.7 Hz), 7.56 (2 H, d, J ) 5.7 Hz), 7.37 (2 H, d, J ) 5.7 Hz), 4.45 (8 H, m), 1.38 (12 H, m). cis-Ru(deeb)2(py)2(PF6)2. A 500 µL aliquot of pyridine (6.47 mmol) was added to 20 mL of an argon saturated solution of 1:1 ethanol/water containing 200 mg (0.25 mmol) of Ru(deeb)2Cl2. The solution was refluxed for 5 h in the dark under argon. The solution color changed form green to orange-red during this time. The solution was cooled and subsequently heated gently under an argon flow to reduce the volume to 10 mL. After cooling, the solution was added to 10 mL of water and was washed five times with 100 mL of CH2Cl2. Excess CH2Cl2 was removed from the water layer by rotary evaporation under vacuum. Approximately 2 mL of a saturated aqueous solution of NH4PF6 was then added to the water solution, and it was placed in the refrigerator overnight. An orange-red powder was collected on a medium frit that was subsequently washed with two 10 mL aliquots of distilled water and two 20 mL aliquots of 1:1 CH2Cl2/diethyl ether. The product was stored under vacuum in the dark until use. Yield was 30% based on Ru(deeb)2Cl2. Anal. Calcd for Ru(deeb)2(py)2(PF6)2: C, 43.87; H, 3.68; N, 7.31. Found: C, 43.24; H, 3.46; N, 7.43. 1H NMR δ (ppm), CD3CN: 9.07 (1 H, d, J ) 5.2 Hz), 8.03 (1 H, d, J ) 5.4 Hz), 7.76 (1 H, d, J ) 5.5 Hz), 8.89 (1 H, s), 8.79 (1 H, s), 8.22 (3 H, m), 7.32 (2 H, t, J ) 6.1 Hz), 7.88 (2 H, t, J ) 7.7 Hz), 4.45 (4 H, m), 1.39 (6H, m). cis-Ru(bpy)2(ina)2(PF6)2. One gram (8.06 mmol) of isonicotinic acid was added to 30 mL of an argon saturated 1:1 methanol/ water solution containing 250 mg (0.46 mmol) of Ru(bpy)2Cl2‚ 2H2O. The resulting suspension was heated at reflux for 5 h in the dark under argon, during which time the color of the suspension changed from pink to bright yellow. It was then heated gently under an argon flow to reduce the volume of methanol. After cooling, the solution was filtered on a fine frit. Water was then added to the solution to reach a final volume of ∼30 mL. Concentrated HCl was added until the pH reached 0.7, and then 2 mL of saturated aqueous solution of NH4PF6 was added. The solution was placed in the refrigerator overnight protected from light. A yellow powder was collected on a medium frit that was subsequently washed with 10 mL of distilled water, two 5 mL aliquots of ethanol, and two 20 mL aliquots of 1:1 CH2Cl2/diethyl ether. The product was stored under vacuum in the dark until use. The yield was 82% based on Ru(bpy)2Cl2‚2H2O. Anal. Calcd for Ru(bpy)2(ina)2(PF6)2: C, 40.3; H, 3.17; N, 8.81. Found: C, 40.89; H, 3.16; N, 8.72. 1H NMR δ (ppm), CD3CN: 8.86 (1 H, d, J ) 5.6 Hz), 7.89 (1 H, d, J ) 5.6 Hz), 8.36 (1H, d, J ) 8.1 Hz), 8.28 (1 H, d, J ) 8.2 Hz), 8.15 (1 H, t, J ) 7.8 Hz), 7.94 (1 H, t, J ) 7.9 Hz), 7.78 (1 H, t, J ) 6.8 Hz), 7.38 (1 H, t, J ) 6.8 Hz), 8.44 (2 H, d, J ) 6.7 Hz), 7.69 (2 H, d, J ) 6.7 Hz). Spectroscopy. All UV-vis optical measurements were acquired by placing the TiO2 on glass films diagonally in an acetonitrile filled 10 mm × 10 mm quartz cuvette, equipped with a 24/40 ground quartz joint. The cell was closed with a PTFE stopper and purged with argon through a glass capillary tube, when indicated. Ground-state absorption spectra were acquired at ambient temperature in air using a Hewlett-Packard 8453 diode array spectrometer. An unsensitized TiO2 film was used as the reference. Photoluminescence. Corrected photoluminescence (PL) spectra were obtained with a Spex Fluorolog that had been calibrated with a standard tungsten-halogen lamp using procedures given by the manufacturer. Quantum yields were measured as previously described with Ru(bpy)32+ as a standard.21 Sensitized films were placed diagonally in a 1.00 cm square fluorescence cell, immersed in CH3CN and Ar purged for at least 15 min. The excitation beam was directed 45° to the film surface, and the emitted light was monitored from the front face of the sample assembly. Electrochemistry. Cyclic voltammetry was performed in 0.1 M tetrabutylammonium (TBA+) perchlorate or 0.1 M LiClO4 acetonitrile electrolyte. A BAS model CV27 potentiostat was used in a standard three-electrode arrangement consisting of a Pt (21) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
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working electrode, a Pt gauze counter electrode, and a SCE or a Ag/AgCl reference electrode. Approximately millimolar concentrations of the compounds were dissolved in the electrolyte. The electrochemical measurements were performed in a Vaccum Atmospheres nitrogen-filled drybox. Cyclic voltammetry of the sensitizers bound to TiO2 was performed in a similar manner with a modified TiO2 electrode as the working electrode. Photoelectrochemistry. Photoelectrochemical and incidentphoton-to-current efficiency (IPCE) measurements were performed in a two-electrode sandwich cell arrangement as previously described.17 Briefly, ∼10 µL of electrolyte was sandwiched between a TiO2 electrode and a Pt-coated tin oxide electrode. The supporting electrolyte was 0.5 M LiI/0.05 M I2 in acetonitrile. TiO2 was illuminated with a 450 W Xe lamp coupled to a f/0.22 m monochromator. Photocurrents and voltages were measured with a Keithly model 617 digital electrometer. Incident irradiances were measured with a calibrated silicon photodiode from UDT Technologies. NMR. 1H NMR spectra were obtained on a Bruker 300AMX FT-NMR spectrometer. IR. Infrared measurements were made on a Perkin-Elmer Spectrum RX I Fourier transform IR spectrometer with 2 cm-1 resolution and 64 or 256 scans. IR spectra of the free sensitizers was performed in standard KBr pellets. For measurements on TiO2, CaF2/TiO2 substrates were placed in millimolar sensitizer/ acetonitrile solutions overnight. Measurements were made in transmission mode with unsensitized CaF2/TiO2 as reference. Fast Atom Bombardment Mass Spectroscopy (FAB-MS). FABMS was obtained on a VG70S mass spectrometer. FAB-MS samples were suspended in a p-nitrobenzyl alcohol matrix. Transient Absorption. Transient absorption data were acquired as previously described.16 Briefly, an ∼7 ns, 532 nm laser pulse from a Surelite II Nd:YAG, Q-switched laser was used as the excitation source. The 1 cm beam was expanded using a quartz concave lens as a means of both course attenuation of the excitation energy and ensuring homogeneous irradiation of the sample. The energy was often further attenuated with a polarizing prism of local design. Each kinetic trace was acquired by averaging 10-400 laser shots (typically 40) at a repetition rate of 1 Hz. Excitation was carried out such that the entire exposed TiO2 surface of the sample, positioned at a ca. 45° angle to the excitation beam, was irradiated. The Xe probe (150 W, Applied Photophysics, operating in pulsed mode) was positioned normal to the excitation beam and was focused on the exposed TiO2 surface. The transmitted light was collected and refocused on the entrance slit of an Applied Photophysics monochromator and detected using a Hamamatsu R928 photomultiplier. 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. Electron Injection Quantum Yield (φinj) Determinations. Laser intensities were determined by comparative actinometry,22 using tris(2,2′-bipyridyl)ruthenium(II) chloride in a poly(methyl methacrylate) (PMMA) thin film deposited on a microscope slide as a reference actinometer ∆450 ) (-1.0 ( 0.09) × 104 M-1 cm-1.23 The ∆ at the ground state-excited state isosbestic point was determined by spectroelectrochemistry of the TiO2-bound sensitizers. The ground-state absorbance of the actinometer and the sensitized colloidal films were approximately absorbance matched at the excitation wavelength. Quantum yield calculations were corrected for any differences in light absorbed by the actinometer and the thin film samples. Elemental Analysis. Elemental analyses were performed by Atlantic Microlab, Inc., Atlanta, GA.
Results Shown in Figure 1 are the visible absorption spectra of Ru(bpy)2(ina)2(PF6)2 and Ru(deeb)2(py)2(PF6)2 in acetoni(22) Bensasson, R.; Goldschmidt, C. R.; Land, E. J.; Truscott, T. G. Photochem. Photobiol. 1978, 28, 277. (23) Yoshimura, A.; Hoffman, M. Z.; Sun, H. J. Photochem. Photobiol., A 1993, 70, 29.
Qu et al.
Figure 1. Absorption spectra of Ru(bpy)2(ina)2(PF6)2 (dotted line) and Ru(deeb)2(py)2(PF6)2 (solid lines) in acetonitrile. The inset shows absorption spectra of Ru(bpy)2(ina)2(PF6)2 in aqueous solution as a function of pH. The pH 1.21 and 10.54 spectra are identified, and the intermediate spectra correspond to pHs of 2.03, 2.37, and 3.22. Chart 1
trile. For clarity the ligands and their abbreviations are given in Chart 1. The lower energy bands are assigned to Ru(II) f bpy or Ru(II) f deeb transitions, and the higher energy bands are assigned to Ru(II) f py or Ru(II) f ina. The assignments are made based on previous studies and the pH dependence in aqueous solution.7 For example, the higher energy charge-transfer band in the Ru(bpy)2(ina)2(PF6)2 absorption spectrum shifts from ∼390 nm at pH 1.2 to 360 nm at pH 10.5, Figure 1 inset. Roomtemperature photoluminescence, PL, was observed for Ru(deeb)(bpy)2(PF6)2 and Ru(deeb)2(py)2(PF6)2 in acetonitrile. Single-exponential kinetics were measured in argonsaturated acetonitrile, Table 1. No detectable PL was observed from Ru(bpy)2(ina)2(PF6)2 under these same conditions. The Ru(II) compounds bind to the semiconductor surface in acetonitrile solution. The concentration-dependent binding is well described by the Langmuir adsorption isotherm model from which surface adduct formation constants have been abstracted, Table 2. When anchored to TiO2, the higher energy MLCT band is obscured by the semiconductor fundamental absorption, valence band-toconduction band, that rises steeply below 400 nm, Figure 2. Typical surface coverages were (7 ( 2) × 10-8 mol/cm2. The infrared spectra of the unsensitized films displayed broad bands at ∼3500 and ∼1670 cm-1, characteristic of surface water.24 With an unsensitized TiO2 film as a (24) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1999, 15, 2402.
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Table 1. Electrochemical and Photophysical Properties of Sensitizers in Acetonitrile compounda Ru(deeb)2(py)2(PF6)2 Ru(bpy)2(ina)2(PF6)2 Ru(deeb)(bpy)2(PF6)2 Ru(dcb)(bpy)2(PF6)2
λAbs, nm λPL, E1/2 (, M-1 cm-1)b τ, nsc 102φPL d nme (V)f 485 (1.5 × 104) 429 (1.1 × 104) 475 (1.6 × 104) 478 (1.4 × 104)
625
