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J. Phys. Chem. B 1998, 102, 8954-8957
Spectral Characterization of the One-Electron Oxidation Product of cis-Bis(isothiocyanato)bis(4,4′-dicarboxylato-2,2′-bipyridyl) Ruthenium(II) Complex Using Pulse Radiolysis Suresh Das† and Prashant V. Kamat*,‡ Notre Dame Radiation Laboratory, Notre Dame, Indiana 46556-0579 ReceiVed: July 29, 1998; In Final Form: September 20, 1998
The spectral characterization of the oxidized form of (ruthenium(II) cis-di(isothiocyanato)bis(4,4′-dicarboxy2,2′-bipyridyl), (commonly known as N3-dye) in aqueous solution is made by reacting the title compound with pulse radiolytically generated radicals N3• and Br2-•. The one-electron oxidized form of Ru(II)-dye initially formed in both these oxidation reactions shows absorption maxima at 320 and 740 nm. Following radiolysis, the one-electron oxidized form of Ru(II)-dye undergoes further transformations to yield a stable product that exhibits a characteristic absorption maximum at 440 nm. Our preliminary results provide the spectral evidence for the formation of two different species that follow the oxidation of Ru(II)-dye.
Introduction
Experimental Section
A recent debate1,2 on the charge-transfer dynamics between photoexcited Ru(II) dye (also known as N3-dye and referred to in the present paper as Ru(II)-dye) and nanostructured TiO2 film has raised a question on the spectral features of the oxidized form of the sensitizer. The Ru(II)-dye is considered to be one of the most promising sensitizers for light energy harvesting applications since it is capable of converting light energy into electricity with a power conversion efficiency of around 10%.3 In view of the practical interest of dye-sensitization phenomena, a number of research groups are now involved in elucidating the dynamics of charge injection from the photoexcited Ru(II)dye into TiO2 semiconductor nanoparticles (see, for example, refs 4-11). A proper identification of the one-electron oxidized species of the Ru(II)-dye using absorption spectroscopy has important implications in the investigation of charge-transfer dynamics at the semiconductor interface. Major questions that arise from the recent debate1,2 are whether the oxidized form exhibits an absorption band in the near-IR (740 nm) and whether the steadystate chemical oxidation methods generate oxidized species similar to those formed during photochemical oxidation. We have employed an independent, well-established pulse radiolysis method to investigate the oxidation of Ru(II)-dye. We were able to characterize the one-electron oxidation of Ru(II) dye using oxidizing radicals in aqueous solution using two different oxidizing radicals, N3• and Br2-•. In the past, similar methodology has been employed to characterize oxidized and reduced forms of various ruthenium polypyridyl complexes (see, for example, refs 12-15). The aim of the present study is to clarify the questions/concerns raised in the earlier debate1,2 and provide information regarding the stability of the oxidized form of the Ru(II)-dye.
cis-Bis(isothiocyanato)bis(4,4′-dicarboxylato-2,2′-bipyridyl) ruthenium(II) was obtained from Solaronix, Switzerland (www.solaronix.com) and used as supplied. All the chemicals were analytical reagents. Solutions were prepared using deionized water, and the pH was adjusted to 7 using phosphate buffer (10-3 M). Freshly prepared solutions were used for all the experiments. The absorption spectrum of the dye solution indicated that the dye was stable for more than 24 h under our experimental conditions. Pulse radiolysis experiments were performed utilizing 50 ns pulses of 8 MeV electrons from a model Titan Beta-8/16-1S Electron Linear Accelerator.16 Dosimetry was based on the oxidation of SCN- to (SCN)2-• (G value ) 6 in N2O-saturated aqueous solutions. The G value denotes the number of species generated per 100 eV, or the approximate micromolar concentration per 10 J of absorbed energy). The radical concentration generated per pulse amounts to (1-3) × 10-6 M for all the systems investigated in this study. The solutions were saturated with nitrogen or N2O and flowed continuously through the sample cell during radiolysis. γ-Radiolysis of N2O-saturated aqueous samples were performed in a Shepard-109, 60Co source. This is a concentric well type source delivering a dose of 69 Gy/min.
† Permanent address: Regional Research Laboratory, Trivandrum 695019, India. Email:
[email protected]. ‡ Email:
[email protected]; http://www.nd.edu/∼pkamat.
Results and Discussion One-Electron Oxidation. Radiolysis of dilute aqueous solutions by high-energy electrons and γ-rays leads to the generation of reactive species such as aqueous electrons, •OH radicals and H• atoms. In neutral and alkaline solutions, the H• atom formation is negligible. Also, by saturating the solutions with N2O most of the electrons can be scavenged, thereby creating an experimental condition that selectively favors oxidation of the added substrate with hydroxyl radicals. In the present experiments we have further converted hydroxyl radicals into secondary oxidants, viz., Br2-•(E° )1.63 V vs NHE17) and N3• (E° ) 1.33 V vs NHE18,19) via reactions with Br- and N3-.
10.1021/jp9832241 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/16/1998
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Compared to •OH radicals, both these radicals are milder oxidants and are capable of inducing one-electron oxidation of Ru(II)-dye (E° )1.1 V vs NHE) (eqs 1-3). •
OH + N3 f OH- + N3•
(1)
N3• + Ru(II) f N3- + Ru(III)
(2)
or Br2-• + Ru(II) f 2Br- + Ru(III)
(3)
The transient absorption spectra recorded following the reaction of Ru(II)-dye with Br2-• and N3• radicals are shown in parts A and B of Figure 1, respectively. The difference absorption spectrum recorded 20 µs after the pulse radiolysis exhibits characteristic absorption bands at 320 and 740 nm and a shoulder around 580 nm. The transient bleaching observed in the 400-550 nm region corresponds to the depletion of the ground-state Ru(II)-dye during the oxidation process. Similar pulse radiolysis studies have been carried out earlier to probe one-electron oxidation of ruthenium(II) polypyridyl and other complexes.13,14 The selectivity of the oxidizing radicals N3• and Br2-• in inducing one-electron oxidation of Ru(II) dye was evident from the kinetic traces of reactant decay and product formation. A representative example of absorption-time profiles recorded in the Br2-• reaction is shown in Figure 2. The decay of Br2-• (as monitored from its absorption at 340 nm) parallels the growth of the transient absorption at 740 nm, thereby confirming the formation of oxidized species during the radiolytic reaction over a period of 20 µs. The pseudo-first-order growth rate constant at 740 nm increased with increasing Ru(II) concentration. The bimolecular rate constants as determined from this concentration dependence were 5.4 × 109 M-1 s-1 for the reaction of the Ru(II)-dye with Br2-• and 1.8 × 109 M-1 s-1 for the reaction with N3• radicals. The radiolytically generated Ru(III) form is likely to undergo transformations since the SCN ligand of the Ru(II) complex is expected to be labile and is also capable of undergoing facile oxidation. Moreover, the irreversible oxidation of Ru(II)-dye has also been noted in cyclic voltammetric studies of this compound.20 Upon examination of the long-term transient decay and bleaching recovery over a period of 4 ms, it is evident that the fraction of 740 nm transient decayed (∼40%) more than the bleaching recovery at 490 nm (∼15%). This suggested that the one-electron oxidation product of Ru(II)-dye must have been undergone further transformations in aqueous solutions at room temperature. Steady-State γ-Radiolysis. To study the spectral changes that occur at longer time periods, the aqueous solutions of the Ru(II)-dye and KBr (or NaN3) were irradiated using a 60Co source. The spectral changes observed following γ-irradiation of N2O-saturated aqueous solutions of the Ru(II) dye containing 0.1 M KBr are shown in Figure 3. The difference spectrum obtained from the two spectra (before and after radiolysis) is shown in the inset. The long-term irradiation facilitates accumulation of stable products that follows initial oxidation. Interestingly, the difference absorption spectra exhibit spectral features (abs. max. 440 nm) that are significantly different than that observed in the primary oxidation step (Figure 1). The yellow-colored solution remained unchanged upon storage. Similar observations were also made following the γ-radiolysis of N2O-saturated solution containing Ru(II)-dye and NaN3.
Figure 1. (A) Transient absorption spectra recorded following the reaction between Ru(II) complex and radiolytically generated N3• in N2O-saturated aqueous solution. The concentration of Ru(II) dye was 10-4 M and 0.1 M NaN3. The difference absorption spectra were recorded (a) 20 µs and (b) 4 ms after pulse radiolysis. Insets show the absorption-time profiles at 740 and 500 nm. (B) Transient absorption spectra recorded following the reaction between Ru(II) complex and radiolytically generated Br2-• radicals in N2O-saturated aqueous solution. The concentration of Ru(II) dye was 10-4 M and 0.1 M KBr. The difference absorption spectra were recorded (a) 20 µs and (b) 1 ms after pulse radiolysis.
Figure 2. Absorption time profiles showing the decay of Br2-• (340 nm) and formation of Ru(III) (740 nm). Experimental conditions were same as in Figure 1B.
It may be noted that earlier radiolysis and photolysis studies have shown that the oxidized form ruthenium polypyridyl complexes undergo chemical changes.12,21-24 The formation of oxidation product such as hydroxypyridine has also been observed in these studies. Eventual cleavage of SCN bond and/ or oxidation is also another possibility. Although identification
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Figure 3. The absorption spectra of Ru(II) dye (a) before and (b) after 20 min γ-radiolysis of the aqueous solution (N2O-saturated) containing 10 mM Ru(II) and 0.1 M KBr. Inset shows the difference absorption spectrum of the oxidation product formed following the oxidation of Ru(II) with Br2-•.
of the final product is yet to be made, the radiolysis results of the present study highlight the spectral difference of the two species formed following the oxidation of Ru(II)-dye. Relevance to Dye-Sensitization Studies. The primary photochemical event in a photoelectrochemical cell is the charge injection process, which occurs with a quantum efficiency of >80%. Following charge injection from the excited sensitizer into semiconductor nanocrystallites, one observes formation of oxidized sensitizer at the semiconductor surface.
Ru(II)* + TiO2 f Ru(III) + TiO2(e)
(4)
For stable operation of a photoelectrochemical cell, it is necessary to regenerate the sensitizing dye by quickly reacting the oxidized sensitizer with a redox couple such as I3-/I-. The formation of an oxidized sensitizer in nanocrystalline semiconductor colloids and films has often been monitored in the investigation of the photoinduced charge-transfer dynamics at the semiconductor interface.9,10 Thus, a proper identification and characterization of the oxidized sensitizer becomes crucial in elucidating the mechanistic and kinetic details of the dyesensitization process. The recent debate related to the oxidized form of Ru(II)-dye by Moser et al.1 and Hannappel et al.2 raises serious questions concerning the nature of the one-electron oxidation product, (Ru(III)), and the influence of the surrounding medium on spectral features. The spectra a and b in Figure 4 show the difference absorption spectra of the oxidized Ru(II)-dye as reported in the literature by the two research groups. (The difference spectra shown in Figure 4 were obtained by subtracting the absorption spectrum of the Ru(II) complex from that of the oxidized form following the chemical oxidation.) The researchers who employed Ce(IV) as a chemical oxidant have found that the oxidized form has an absorption maximum at 740 nm,9 while those who employed Br2 as a chemical oxidant have found that the oxidized species absorbs only at 440 nm and there is no observable absorption band in the IR region.10 In accordance with these observations, the two research groups involved in this debate have separately probed the femtosecond charge-transfer dynamics of the excited Ru(II) dye-TiO2 system by monitoring the transient absorption at 750 nm9 and the transient bleaching at 550 nm,10 respectively. To explain the differences in spectral features, both the research groups have presented arguments to justify their spectral identification of the one-electron oxidation product. While Moser et al.1 expressed their concern regarding the stability of oxidized sensitizer, Hannappel et al.2 suggested that the differ-
Figure 4. Difference absorption spectra as extracted from the spectral data presented in refs 10 and 1. These spectra were obtained from the chemical oxidation of Ru(II) with (a) Ce(IV) and (b) Br2. Note that the difference absorption spectrum was obtained by subtracting the ground-state spectrum from the Ru(III) spectrum. (It is assumed that the spectra presented in figure 1 of ref 1 are normalized to reflect the corresponding extinction coefficient.)
ences could arise from the nature of surrounding environment and experimental conditions. (Their femtosecond measurements were carried out in an UHV chamber.) Hannappel et al. further justified their measurements by comparing the transient bleaching kinetics at 550 nm with the absorption growth at 1100 nm that arises from the electron trapping in TiO2 colloids. Hannappel also pointed out that the near-IR absorption band (740 nm) is mainly due to triplet excited state of the Ru(II)-dye. It is clearly evident from the present pulse radiolysis investigation that the one-electron oxidation product, Ru(III), indeed has an absorption in the near-IR region with a maximum at 740 nm (as shown in Figure 1) and agrees with the results of Moser et al.1 As inferred from the long-term oxidation experiment (Figure 3) using γ-irradiation, the one-electron oxidation product further undergoes chemical changes to yield a more stable product with an absorption band centered at 440 nm. The spectral features of this chemically transformed product (Figure 3) closely match the spectrum obtained by Hannapel et al. using Br2 as chemical oxidant10 (spectra b in Figure 4). The transient absorption spectra recorded using TiO2 films under UHV conditions show only transient bleaching in the 500-700 nm region.10 It would therefore be interesting to probe whether surface effects are capable of inducing dramatic differences in the absorption spectrum of the oxidized Ru(II)-dye. Such surface effects and the precise nature of the stable oxidation product will be the focus of our future studies. Acknowledgment. We thank Dr. Q. G. Mulazzani for helpful discussions. The work described herein was supported by the Office of the Basic Energy Sciences of the U.S. Department of Energy. This is contribution No. NDRL 4082 from the Notre Dame Radiation Laboratory. References and Notes (1) Moser, J. E.; Noukakis, D.; U., B.; Tachibana, Y.; Klug, D. R.; Durrant, J. R.; Humphry-Baker, R.; Gra¨tzel, M. J. Phys. Chem. B 1998, 102, 3649. (2) Hannappel, T.; Zimmermann, C.; Meisner, B.; Burfeindt, B.; Storck, W.; Willig, F. J. Phys. Chem. B 1998, 102, 3651. (3) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry, B. R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (4) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano, F. N.; Meyer, G. J. J. Am. Chem. Soc. 1995, 117, 11815.
Letters (5) Kamat, P. V.; Bedja, I.; Hotchandani, S.; Patterson, L. K. J. Phys. Chem. 1996, 100, 4900. (6) Rehm, J. M.; McLendon, G. L.; Nagasawa, Y.; Yoshihara, K.; Moser, J.; Gra¨tzel, M. J. Phys. Chem. 1996, 100, 9577. (7) Fessenden, R. W.; Kamat, P. V. J. Phys. Chem. 1995, 99, 12902. (8) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98, 3822. (9) Tachibana, Y.; Moser, J. E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. R. J. Phys. Chem. 1996, 100, 20056. (10) Hannappel, T.; Burfeindet, B.; Storck, W.; Willig, F. J. Phys. Chem. B 1997, 101, 6799. (11) Ellingson, R. J.; Asbury, J. B.; Ferrere, S.; Ghosh, H. N.; Sprague, J. R.; Lian, T.; Nozik, A. J. J. Phys. Chem. B 1998, 102, 6455. (12) Mulazzani, Q. G.; Venturi, M.; Bolletta, F.; Balzani, V. Inorg. Chim. Acta 1986, 113, L1. (13) Mulazzani, Q. G.; Venturi, M.; D’Angelantonio, M.; Bignozzi, C. A.; Scandola, F. J. Phys. Chem. 1989, 93, 736. (14) Maliyackel, A. C.; Waltz, W. L.; Lilie, J.; Woods, R. J. Inorg. Chem. 1990, 29, 340. (15) Parsons, B. J.; Beaumont, P. C.; Navaratnam, S.; Harrison, W. D.; Akasheh, T. S.; Othman, M. Inorg. Chem. 1994, 33, 157.
J. Phys. Chem. B, Vol. 102, No. 45, 1998 8957 (16) Whitham, K.; Lyons, S.; Miller, R.; Nett, D.; Treas, P.; Zante, A.; Fessenden, R. W.; Thomas, M. D.; Wang, Y. Linear Accelerator for Radiation Chemistry Research at Notre Dame. ’95 Particle accelerator conference and international conference on high energy accelerators, 1995, Dallas, TX. (17) Schwarz, H. A.; Dodson, R. W. J. Phys. Chem. 1984, 88, 3643. (18) Alfassi, Z.; Harriman, A.; Huie, R. E.; Mosseri, S.; Neta, P. J. Phys. Chem. 1987, 91, 2120. (19) DeFelippis, M. R.; Faraggi, M.; Klapper, M. H. J. Phys. Chem. 1990, 94, 4, 2420. (20) Gra¨tzel, M. Nanocrystalline electronic junctions. In Semiconductor NanoclusterssPhysical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier Science: Amsterdam, 1997; p 353. (21) Gillard, R. D. Coord. Chem. ReV. 1975, 16, 67. (22) Creutz, C.; Sutin, N. Proc. Nat. Acad. Sci. U.S.A. 1975, 72, 2885. (23) Ghosh, P. K.; Brunschwig, B. S.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1984, 106, 4772. (24) Vinodgopal, K.; Hua, X.; Dahlgren, R. L.; Lappin, A. G.; Patterson, L. K.; Kamat, P. V. J. Phys. Chem. 1995, 99, 10883.
