J. Phys. Chem. C 2009, 113, 783–790
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A Study of the Interactions between I-/I3- Redox Mediators and Organometallic Sensitizing Dyes in Solar Cells Timofei Privalov,*,† Gerrit Boschloo,‡ Anders Hagfeldt,‡ Per H. Svensson,§ and Lars Kloo§ Department of Organic Chemistry, Arrhenius Laboratory, Stockholm UniVersity, SE-10691, Stockholm, Sweden, Physical Chemistry, Uppsala UniVersity, SE-75123 Uppsala, Sweden, and Inorganic Chemistry, Royal Institute of Technology (KTH), SE-10044 Stockholm, Sweden ReceiVed: NoVember 20, 2008
Specific interactions of the I-/I3- redox mediators with the reduced and oxidized dye, Ru(4,4′-dicarboxy2,2′-bipyridyl)2(NCS)2, referred to as N3 or Ru(dcbpy)2(NCS)2, have been studied by means of density functional theory (DFT) with the focus on the charge transfer process involving {dye+ I-} adducts; computations had been performed with a series of density functionals (gradient-corrected density functional BP86, and the hybrid density functionals B3LYP, MPW1K, B3PW1K, and MPW1PW91). Different pathways leading to {dye+ I-} adducts have been studied. First, mechanistic insights into the interaction of I- with RuIII(dcbpy)2(NCS)2 via an SCN- ligand directly giving rise to RuII(dcbpy)2(NCS)2I0 have been obtained with the distinctive S-I bonding. Second, the binding of I- to the N3 dye cation via I--dcbpy interactions has been analyzed. We also report experimental and computational evidence that sheds light on the interaction of the redox mediator with bipyridyl moieties. Evidence for a charge transfer process in the presence of only one I- anion in the outer coordination sphere of the ruthenium center has been identified. Finally, geometries and electronic structures of plausible intermediates have been computationally analyzed based on an innersphere interaction between the metal center and the redox mediator, including a two-step regeneration reaction: RuIII(dcbpy)2(NCS)2+ + I- f RuIII(dcbpy)2(NCS)I+ + SCN-, followed by the interaction of a second Iwith the intermediate RuIII(dcbpy)2(NCS)I+ complex. Conclusive evidence of a charge-transfer process that gives rise to the regenerated RuII complex, where I- interacts with the intermediate RuIII(dcbpy)2(NCS)I+ complex has been identified. Introduction The announcement of a surprisingly high efficiency of dyesensitized solar cells (DSSCs), made by O‘Regan and Gra¨tzel in the early 1990s,1 has attracted significant attention and initiated a very quick development of this new class of promising light-harvesting electrochemical devices. The best performing DSSCs reported to date are based on a working electrode of a nanostructured TiO2 film sensitized by ruthenium-based dyes, in particular ruthenium polypyridyl complexes.2 A detailed description of the operating principles of a DSSC may be found in ref 3 and references therein. New promising electrolytes and redox couples are continuously being developed in various laboratories,3 as well as quasi-solid or solid-state electrolytes.4 However, the traditional I-/I3- redox couple so far remains the primary choice in the DSSC design. The search for a more efficient and practical redox couple requires detailed understanding of the dye regeneration chemistry, which is essentially a multidisciplinary problem. The energy conversion in a DSSC is based on the ultrafast injection of an electron from the photoexcited dye into the conduction band of the nanocrystalline oxide semiconductor. This process transforms the sensitizing dye into an oxidized form. The light-induced oxidized state of the dye is subject to a relatively quick chemical degradation, which for stability reasons should be prevented by an efficient regeneration reaction * Corresponding author,
[email protected]. † Stockholm University. ‡ Uppsala University. § Royal Institute of Technology.
(a reduction to the reduced state) using an appropriate electrolyte containing a redox couple, such as I-/I3-, the most common mediator. While the dye itself is obviously the most central single component of a DSSC, determining the light-harvesting and light-conversion efficiency, the redox couple, playing a vital role in closing the light-conversion cycle via the dye regeneration chemistry, is vital for high photon conversion efficiency and chemical stability of the device as a whole. The dye regeneration process, though it is relevant for and related to a variety of charge recombination reactions in both organic and inorganic donor-acceptor complexes not being limited to technological applications of dye-sensitized solar cells as such, is still poorly understood. It was suggested that the reduction of the oxidized dye by iodide should lead to the formation of short-lived I2- radicals,5 I2- then dismutates to yield iodide and triiodide. The formation of I2- species was later confirmed by laser flash photolysis studies.6,7 Very recently, spectroscopic evidence for a further intermediate species, an oxidized dye-iodide complex, was found.5-8 On the basis of these data, the general mechanism of the regeneration reaction can be divided into two steps as follows
dye+ + I- f {dye+ I-} + -
] f dye + I2-
-
[{dye I } + I
(1) (2)
According to an experimental kinetics study,5-8 the first intermediate (1) is formed kinetically fast, and it is the reaction with the second iodide anion (2) that is the rate-limiting step of the regeneration reaction. The underlying chemistry of the regeneration, simplistically outlined by (1) and (2), and the
10.1021/jp810201c CCC: $40.75 2009 American Chemical Society Published on Web 12/22/2008
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Figure 1. The optimized structure of the RuII(dcbpy)2(NCS)2 dye in its ground singlet electronic spin state. All distances are in angstroms.
competing degradation of the photogenerated dye cations are quite complex. The latter effect has been studied in relation to the long-term stability of DSSCs.9 Our study, which focuses on the interaction of the I-/I3- redox couple with the N3 dye after photoexitation and ultrafast electron injection into the conduction band of the TiO2 semiconductor, is inspired by and based upon recent transient absorption studies5-8 of the dye-dependent regeneration dynamics in DSSCs. Though these suggested (1) the formation of a transient dye+-iodide intermediate complex and (2) the regeneration of the dye affording short-lived I2-, the mechanism is currently unknown. The rational search for a more efficient and practical redox couple requires detailed understanding of the dye regeneration chemistry. In order to gain insight into relevant reaction pathways from dye+-iodide complexes, including those which are highly relevant for the recombination reactions in donor-acceptor complexes without a metal center, fully organic dye-sensitizing compounds in particular, and subsequently to regenerate the dye giving rise to short-lived radical I2- species, we have carried out detailed structural and electronic analyses of dye+-iodide, {dye+ I-} and [{dye+ I-} + I-], complexes within several plausible reaction pathways. The main tool in this effort, which will allow us to draw conclusions about geometrical features, spin density distributions, and oxidation states of the Ru centers of key reaction intermediates, is quantum chemical calculations at the density functional theory (DFT) level. The computational model consists of the N3 dye, RuL2(NCS)2 (L ) dcbpy ) 4,4′-dicarboxy-2,2′-bipyridyl), and the oxidized RuIII center with components of the I-/I3- redox couple. The size of the molecular model precludes the use of direct wave-function-based computational methods, while the DFT method is well suited for studies of large molecular systems, such as N3. The detailed computational procedure is outlined below. We will also present experimental findings that support the hypothesis of an interaction by the reducing agent with the dcbpy ligand. Since the study of the thermodynamics of the inner- and outer-sphere interaction between charged species, the oxidized dye, N3, and the active agent of the redox couple (i.e., I-), requires careful consideration of solvent effects, we leave it out of the scope of the present investigation. 1. Mechanistic Considerations Since the exact chemical nature of the {dye+ I-} complex and its evolution is presently unknown, as its short lifetime
Figure 2. The optimized geometry of the Ru-I complex with S-I bonding; charge ) 0, electronic spin state ) doublet; dashed black line highlights the S-I bond. 2′: The spin density of the ground electronic state of complex 2 with one unpaired electron. 3: The {dye I2-} complex with an outer-sphere binding of I2-; dashed green lines highlight close contacts of I2- with C atoms of the dcbpy ligand; dashed yellow line highlights the S-I distance. All distances are in angstroms.
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Figure 3. The coordination of the polyiodide fragment to the bipyridyl cation according to the crystallographic data for 4,4′-bipyridinium-1,1diethyl di(triiodide), 4, and di(tri/pentaiodide), 5. All hydrogen atoms are removed for clarity. All distances are in angstroms.
complicates detailed characterization by means of spectroscopy, we consider plausible dye+-I- complexation as follows: (i) binding of I- to the S atom of the SCN- group; (ii) direct interaction of I- with the dcbpy ligand; (iii) an exchange with the chemically similar thiocyanate ligand; (iv) an associative seven-coordination to the ruthenium(III) center. The first hypothesis, the direct interaction between I- and a SCN- group, is based on the chemical similarity between these two ions and the analogy between I2- and (NCS)I-. Alternatively, the {dye+ I-} complex could be formed due to the interaction of I- with the dcbpy ligand via iodine-π/π* and iodine-hydrogen interactions. Such an interaction is highly relevant to the regeneration of purely organic sensitizing dyes, which have shown high DSSC efficiencies.10 We will present computational and experimental evidence supporting the plausibility of such interactions. It is also worth noting that I-H (aromatic and aliphatic) and I-π interactions in part are responsible for the aromatic iodine-assisted self-assemblies.11 The hypothesis of the ligand exchange of I- with SCN- at the Ru center is based on the chemical analogy between these two ions and on the reported ligand substitution of thiocyanate by solvent and water in the dye degradation products.9 However, the experimentally observed amount of the SCN--related degradation products9 seems to be less than one can expect from the formed {dye+ I-} complex via an SCN- T I- ligand exchange. Finally, the direct coordination of the I- ion to the RuIII center retaining both SCNgroups appears to be a plausible alternative. However, sevencoordination is well established only for higher oxidation states of the Ru center, such as the RuIV(Me2CNS2-)I complex.12 A further comment is that the latter two hypothesizes are not applicable to highly efficient DSSCs based on purely organic dyes. We begin with a brief analysis of the neutral RuII and monopositively charged RuIII complexes, which serve as models for the regenerated and the oxidized state of the dye, respectively. The actual study of the dye cation-I-/I3- interactions follows. First, we are going to study {dye+ I-} complexes found by the interaction of the I- anion with the SCN- ligand or with the bipyridyl ligand of the dye cation without a direct coordination of the reducing agent to the ruthenium center, a so-called outer-sphere interaction. Second, we will study the direct interaction of I- anions with the RuIII ion, also referred to as an inner-sphere interaction.
2. Reference Neutral RuII and Cationic RuIII Complexes The optimized geometry of the RuII(dcbpy)2(NCS)2 dye (1, Figure 1) in its ground singlet electronic state is the starting point of our study. All six coordination sites of the quasioctahedral coordination sphere of the ruthenium central ion in the structure of the RuII(dcbpy)2(NCS)2 dye are occupied (1, Figure 1). It is worth noting that S coordination of the one of the two SCN- ligands increases the total energy of the complex by 5.5 kcal/mol, which is a rather small change in energy. The S coordination of both SCN- ligands increases the total energy by 10.8 kcal/mol with respect to complex 1. Thus, from a theoretical point of view N/S-coordination dynamics cannot be ruled out. After the electron injection into the conducting band of TiO2, the dye cation becomes the analogue of complex 1, formally with monopositive charge, a 1+ complex with a ground doublet electronic spin state, which should formally correspond to a RuIII center. For such a complex, approximately 40% of the spin density is localized on the ruthenium center (the Mulliken atomic spin density of Ru is close to 0.4 in 1+), while approximately 60% of the spin density is equally divided between the two SCN- ligands. The major part of the ligand spin density is localized on the S atoms (the Mulliken atomic spin density of S is close to 0.2 in 1+). The difference in the electronic density of 1+ versus 1 results in a shortening of Ru-N bonds for the Ru-dcbpy binding and a shortening of N-C and C-S bonds of the SCN- ligands. Again, it is worth noting that S coordination of one of the two SCN- ligands in 1+ increases the total energy of the complex by 8.0 kcal/mol, while the S coordination of both NCS- ligands increases the total energy by almost 21 kcal/mol with respect to the (N-bonded) reference complex 1+. 3. Interaction of I- with the Dye Cation: Outer-Sphere Pathway 3.1. (I-)-(SCN-) Interaction. Interaction of I- with the dye cation via a Ru-coordinated SCN- ligand seems quite plausible, considering the chemical similarity between the I2- and (NCS)Ispecies. This makes us expect that, if in fact I- binds to the dye+ complex via the SCN- ligand, this will result in the RuIII f RuII recombination/reduction. Starting with the cationic complex 1+, the structure of an RuIII(NCS)2I complex was optimized with a total charge of 0
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Figure 4. The optimized geometry of the dcbpy-I3- complex 6; the closest I-C and I-N distances are 4.27 Å and 4.36 Å, respectively. 7: The {dye+ I3-} complex (see the Supporting Information for an alternative 2D projection of the 3D structure of 7); dashed green lines highlight close C-I and N-I contacts, the corresponding bond length labels are in green as well; closest S-I, C-I, and N-I distances are 4.437, 4.053, and 4.193 Å, respectively. All distances are in angstroms.
and a multiplicity of 2 (2, Figure 2). In the optimized doublet electronic spin state, which is the ground electronic state, the spin distribution is localized on the I-S fragment of the (NCS)Igroup (0.60 and 0.31 for I and S atoms, respectively, according to the Mulliken spin distribution). Only a very small fraction of spin density is localized on the central ruthenium ion (see 2′ in Figure 2).13 A comparison with the Mulliken spin distribution in the reference RuII dye clearly indicates that the ruthenium center is in the formal +II oxidation state in complex 2. This result is not surprising considering the analogy between the complex pseudohalide SCN- anion and the iodide I- anion. With respect to the RuIII f RuII recombination, (NCS)I- acts as a competent redox couple (vide infra comparison with I2- within an inner-sphere pathway), which requires only one I- anion for the regeneration of the ruthenium center. The dye regeneration, RuIII f RuII, appears to be a one-step process. Following the complete RuIII f RuII recombination reaction for the complex 2, and in order to gain insight into the role of a second I- in this mechanism, the inclusion of an additional I- in the model complex 2 was studied: the interaction of Iwith 2 seems to result in the formed {dye I2-} adduct, complex 3 in Figure 2. A comparison of 2 and 3 shows a substantial elongation of the S-I distance in the latter. The I-I distance
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Figure 5. The optimized geometry of the dcbpy-I- complex 8 and the {dye+ I-} adduct, 9 (total charge ) 0, the electronic doublet spin state), with the distinctive dcbpy-I- interaction. Dashed green lines highlight close C-I and N-I contacts. All distances are in angstroms.
in 3 is very close to the equilibrium bond distance in an isolated I2-. According to the Mulliken spin population analysis, the electronic doublet spin state of 3 is localized on the I-I couple (Mulliken spins are 0.43 and 0.54), with the ruthenium center firmly in the +II oxidation state. It seems that the role of the second I- in this mechanism is to generate the radical I2species, while the RuIII f RuII recombination is accomplished within the first step. 3.2. Triiodide/Iodide-dcbpy Interactions and {dye+ I-} Adduct Formation. It is known that the iodide ion interacts strongly with bypyridyls, particularly if the latter is positively charged through alkylation of the nitrogen atom.14 In order to determine if such an interaction is consistent with that observed for the Ru-based bipyridyl complexes, the compounds were isolated from a mixture of iodide, iodine, and biethylated bipyridyl. Both the tri-, 4, and tri/pentaiodide, 5, salts were isolated and structurally characterized (Figure 3). The coordination of the polyiodide anion to the bipyridinium cation is quite similar in both compounds. The triiodide fragment aligns with the extension of the bipyridiunium ion. The closest I-C and
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Figure 6. Ru-I complexes: 10, RuIII-I- complex with only one remaining SCN- ligand; 11, RuIII-I complex with two SCN- ligands and one cleaved Ru-N(dcbpy) bond. All distances are in angstroms.
I-N contacts are 3.641(11)/3.812(12) and 3.774(8)/3.709(9) Å for the triiodide and pentaiodide structures, respectively. The idea that dcbpy- I-/I3- and {dye+ I-/I3-} complexes might be formed in this fashion leads us to pursue an energy minimization of such plausible adducts. Though the search for the optimized geometry with a dcbpy-bound iodide proved to be complicated and time-consuming in all cases because of the necessity to search candidates for complete geometry optimization via potential energy scans in the dcbpy-I-/I3- and {dye+ I-/I3-} systems alike, we were finally able to determine reliable optimized structures. Since crystallographic data are available for the polyiodide fragments only, structures of dcbpy-I3- and {dye+ I3-} complexes have been computed. The comparison of crystal structures in Figure 3 and computed structure 6 in Figure 4 shows strong resemblance between iodide-fragment interactions in both systems. It is noteworthy that the orientation of the triiodide in 7 is strongly affected by the Ru-bound SCN- group; the close I-C contacts indicate nearly symmetrical positioning of the central I atom on top of the pyridine fragment of the dcbpy ligand. Encouraged by the qualitatively realistic description of the dcbpy-I3- interactions (specific anion-π/π* interactions) by the density functional method, we proceeded to search for optimum geometries of the dcbpy-I- and the more complex {dye+ I-} adducts; final results are shown in Figure 5. Formally, 9 should be an RuIII complex (total charge ) 0, doublet electronic spin state). Contrary to our expectations, the ground electronic spin state of 9, which is a doublet, is not localized on the ruthenium center. According to the Mulliken spin distribution, only ca. 20% of the spin density is localized on the Ru ion, while almost 50% of the spin density is localized on the iodide, with the remaining spin density distributed over the SCN- ligand in the closest proximity to the iodide. On the basis of our experimental and computational results, it seems quite plausible that the {dye+ I-} complex could indeed be formed due to the dcbpy--I- interactions, assisted by I--SCN- binding. It is notable that any form of the outer-sphere coordination via thiocyanate or dcbpy ligands (2 or 9, respectively) gives rise to changes in the spin density with respect to 1+ affording an oxidized form of a molecular halogen or
pseudohalogen anion. The important difference between 2 and 9 is in the remaining spin density on the Ru center: the spin density is almost completely transferred to the (NCS)I- in 2 with only a negligible contribution on the Ru center, while in 9 the Ru center retains a small but non-negligible amount of the spin density. In order to gain better insight into step 2 of the regeneration reaction the outer-sphere interactions between complex 9 and I- (2), a second I- was added to the model complex 9 and a series of plausible {dye I2-} adducts computed; all candidates for the complete geometry optimization were found via multidimensional potential energy scans as explained in the Computational Details. From all evaluated structures, the complex 3 (Figure 2) has emerged as the lowest energy {dye I2-} adduct of 9 and I-. The dcbpy-related pathways seems to strictly follow the proposed two-step reaction mechanism (1) and (2), 4. Inner-Sphere Pathway As previously pointed out, all six coordination sites of the quasi-octahedral coordination sphere of the ruthenium central ion in the structure of the RuII or RuIII dyes are occupied (1, Figure 1). For the inner-sphere Ru-I interaction to take place, either one of the SCN- ligands has to be replaced or one of the Ru-N bonds connecting Ru and the bipyridyl ligands must be cleaved. Alternatively, an associative seven-coordination of the ruthenium center has to be taken into consideration. The latter coordination mode appears not to exist in the literature for a RuIII center, and all our attempts to computationally optimize a seven-coordinated structure failed. 4.1. Inner-Sphere Pathway: SCN- T I- Ligand Exchange at the Ru Center. In order to model the {dye+ I-} complex resulting from the interaction of I- with the oxidized RuIII dye via the exchange of the SCN- ligand, the optimized geometry of the ground electronic spin state was found for the positively charged RuIII(NCS)I complexes (charge ) +1, multiplicity ) 2), complex 10 in Figure 6. A spin-density analysis of the electronic doublet state indicates that the ruthenium ion is indeed in the +III formal oxidation state (Mulliken spin density is 0.65). The remaining small part of spin density is distributed over the ligands, mainly on the SCN- ligand. As expected, the electronic
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Figure 7. RuIII-I2 complex with one SCN- ligand: 12′, spin density of complex 12; 13, RuII-I2 complex with two SCN- ligands and a cleaved dcbpy Ru-N bond (the dashed gray line). All distances are in angstroms.
ground state is still the RuIII-I- complex, and not the RuII-I* one, where asterisk indicates a radical character. This confirms the expected outcome of a typical ligand exchange reaction, RuIII(dcbpy)2(NCS)2 vs RuIII(dcbpy)2(NCS)I, in which the oxidation state of the metal center does not change. A similar Ru-I distance and the doublet spin distribution localized on the RuIII central ion is found in the neutral complex 11 (charge ) 0, multiplicity ) 2) with a cleaved Ru-N(dcbpy) bond. In summary, direct binding of I- to the RuIII center via the ligand exchange of one SCN- group or through cleavage of one dcbpy Ru-N bond does not result in any change of the oxidation state of the metal ion, following the formal breakdown of the regeneration reaction into two steps according to (1) and (2). This is in strong contrast to the aforementioned change of the oxidation state of the metal center caused by an outer-sphere binding of the single reducing agent, I-. 4.2. Inner-Sphere Pathway: Interaction of I- with {dye+ I }. The next plausible step of an inner-sphere regeneration reaction is the interaction of the RuIII-I- complexes 10 and 11 with a second I- anion, complexes 12 and 13 in Figure 7.
Complexes 12 and 13 were optimized in the electronic doublet state with a charge of 0 and of -1, respectively. The I-I distance was found to be close to the equilibrium bond length of noncoordinated I2-, the latter found to be 3.462 Å at unrestricted B3LYP/lacvp** level. The spin-density analysis of 12 and 13 conclusively shows that the ruthenium ion is in the formal oxidation state +II with a negligible spin density, while the spin density on the I2 ligand closely resembles the spin density of a free I2- molecule (0.36 and 0.62 on the two I atoms, respectively; see also 12′ in Figure 7). The spin density is lower on the iodine atom coordinated to the central ruthenium ion. In conclusion, the inner-sphere pathway [RuIII-I- + I- f RuII-I2-] provides a theoretically plausible pathway to the regeneration of the RuII dye by means of electron transfer from the reducing component, I-, of the redox couple. The electronic ground-state configuration of RuIII-I- and RuII-I2- is a doublet. In the former complex the major part of the spin density is localized on the ruthenium ion, while in the latter complex the spin density is localized on the I2- ligand. With respect to the outer-sphere pathway with a direct S-I interaction, a conclusive
Interactions of Redox Mediators with Dyes indication emerges that the (NCS)I- ion formed through the secondary coordination is completely analogous to I2- with respect to the RuIII f RuII recombination/reduction. Conclusions The regeneration process of a dye is vital for the efficiency and the stability of a photoelectrochemical solar cell. Therefore, an understanding of the process of dye regeneration/reduction is of central importance in the efforts to improve such a multicomponent device and in the rational design of the next generation of redox couples. The computational model used in this work is based on the N3 dye and the I-/I3- redox couple. The oxidation state of the metal center and the spin distribution was obtained via comparative Mulliken spin and orbital population analyses. All computations were performed with a series of density functionals. The present study has identified plausible pathways of a twostep regeneration mechanism that might take place in dyesensitized solar cells, namely, the interaction of a redox-active couple with the oxidized RuIII dye complex and the subsequent interaction of a second redox-active agent with the RuIII-Iintermediate complex giving rise to the regenerated RuII dye. We have demonstrated, that the generalized two-step regeneration, RuIII f RuII, may proceed via somewhat similar, yet significantly distinctive, pathways, one of which is highly relevant for the recombination reactions in donor-acceptor complexes without a metal center, fully organic dye sensitizing compounds in particular. First, considering the chemical similarity of the SCN- and I ions it is reasonable to expect that an outer-sphere interaction of I- with the oxidized dye via S-I binding will provide a pathway for the regeneration of the +II oxidation state of the metal center. Indeed, a study of Ru(dcbpy)2(NCS)2I0 showed that the uncoupled electron is localized on the (NCS)I- ligand (cf. I2- in an inner-sphere pathway) and the spin density on the ruthenium is in agreement with the expected formal +II oxidation state. In this pathway the dye regeneration reaction, when only focusing on the reactions at the dye complex, does not seem to follow the suggested two-step mechanism. The RuIII f RuII recombination is a one-step process. However, the release (dissociation) of I2- may constitute a subsequent reaction step. Second, we have computationally identified an alternative outer-sphere pathway that strictly follows the experimentally suggested two-step mechanism: (1) the combined dcbpy-I- and SCN--I- interactions result in the {dye+ I-} complex with distinctive charge transfer features; (2) the subsequent interaction with a second I- results in the regeneration of the RuII dye with formation of I2-. In addition, the interaction of I3- with the dye itself and with the dcbpy ligand has been analyzed. Experimental and theoretical evidence support the interaction of I-/I3- directly with the dcbpy ligand. Third, the ligand exchange of one SCN- ligand with I- is plausible from a mechanistic point of view, though it should probably result in the degradation of the dye or lowering of the photoefficiency of the dye due to the formation of the Ru-I bond. Such a ligand exchange does not affect the oxidation state of the coordination center. This, of course, is expected considering the chemical similarity of NCS- and I-. The second step in the process, the interaction of I- with RuIII(dcbpy)2(NCS)I+, proved to constitute a facile way to form a I2- radical ligand coordinated to the regenerated RuII dye. Finally, our computations demonstrated that the spin density distribution in {dye+ I-} intermediates and the oxidation state
J. Phys. Chem. C, Vol. 113, No. 2, 2009 789 of the Ru-center depends on the coordination mode of I- (the S, the dcbpy, or Ru bonding). We have also computed electronic spin distribution of all involved complexes, using the gradientcorrected density functional BP86 and the hybrid density functionals MPW1K, B3PW1K, and MPW1PW91. Very good internal consistency of all computed spin densities had been found with no discrepancy between the recomputed spin densities and B3LYP data. Experimental and Computational Details Computational Details. All calculations were performed with the Jaguar 6.015 program and the molecular model system formally in gas phase. Our approach is somewhat similar to the procedure described in ref 16. We initially performed a conformational search using potential energy scans at the B3LYP/lacvp level. The geometries of up to ten conformers lowestinenergywerethenreoptimizedusingtheB3LYP17/lacvp*18,19 level of theory. In some complicated cases, i.e., dcbpy-I-/I3interaction, where we could identify a large number of possible local minima within a flat potential energy surface, many more than ten lowest energy conformers were evaluated. The conformer with the lowest energy was optimized at the B3LYP/ lacvp** level and then further studied at the B3LYP/lacvp*+ level. All degrees of freedom were optimized. This procedure has been used in the determination of all reported structures for the {dye+ I-/I3-}, [{dye+ I-} + I-], and {dcbpy I-/ I3-} complexes. The existence of the so-called self-interaction error (SIE) of DFT warrants additional checks of delocalized electron spin distributions in all cases. The SIE appears because of the nonmatching formulations of the exchange and correlation functionals in DFT (unlike Hartree-Fock, where it cancels exactly). It is known that the SIE is difficult to correct for, since the correction needed typically becomes highly orbital (basis set) and functional dependent. However, it is also known that the SIE is less of a problem for weakly correlated systems, and it is less of a problem when using hybrid functionals containing a large weight on HF exchange (like B3LYP). Because of the causes of the SIE, it is expected to be revealed by inconsistencies and significant discrepancies in the electron spin distributions computed using different hybrid and gradient-corrected density functionals. We have computed electronic spin distribution of all involved complexes, using the gradient-corrected density functional BP86, and the hybrid density functionals MPW1K, B3PW1K, and MPW1PW91.20 We have found very good internal consistency of all computed spin densities with no discrepancy between the recomputed spin densities and our presented B3LYP data. Therefore we believe that SIE has no bearing on the results we report in our study. The assignment of the oxidation state of the metal center is based on the absolute and relative Mulliken spin populations21 combined with the orbital population analyses. Mulliken spin population analyses were made for all conformers within the 10-20 kcal/mol margin relative to the lowest energy conformer. In the vast majority of systems, the Mulliken spin population turned out to be essentially insensitive to the small changes in geometry of the model systems. Synthesis. The compounds 3 and 4 were prepared from the ethyl viologen diiodide salt (99% Sigma-Aldrich) and iodine (Merck, p.A.) by direct reaction of the stoichiometric amounts in acetone. More specifically, for compound 3 0.56 g (4.4 mmol) iodine was dissolved in 25 mL of acetone, after which 1.03 g (2.2 mmol) of viologen diiodide was added. Crystals of both compounds formed in large yield within a week during slow
790 J. Phys. Chem. C, Vol. 113, No. 2, 2009 solvent evaporation. The yields are estimated to at least 60%. Compound 4 was made in an analogous manner using 0.61 g (4.8 mmol) of iodine and 0.56 g (1.2 mmol) of viologen diiodide. X-ray Crystallography. Single crystals of 4 and 5 were obtained from the reaction mixture after evaporation of the solvent at room temperature. The data for 4 and 5 were collected at room temperature on a Bruker SMART2000 CCD diffractometer with Mo KR radiation. The data were corrected for Lorentz polarization and absorption effects (empirical absorption SADABS22 [C1]). The structures were solved by direct methods and refined with full-matrix least-squares (SHELX9723 [C2]). Crystal Data. C14H18I6N2 4, M ) 975.70, triclinic, a ) 7.6209(5), b ) 8.4510(5), c ) 9.8841(6) Å, R ) 91.797(2), β ) 94.043(2), γ )105.902(2)°, U ) 609.84(7) Å3, T ) 293 K, space group P1j (no. 2), Z ) 1, 5318 reflections measured, 2118 unique which were used in all calculations (Rint ) 0.032). The final wR(F2) was 0.1847 (all data). C14H18I8N2 5, M ) 1229.50, triclinic, a ) 12.1883(11), b ) 13.6143(12), c ) 17.4668(16) Å, U ) 2815.6(4) Å3, R )100.257(2), β ) 91.869(2), γ ) 98.491(2)°. T ) 293 K, space group P1j (no. 2), Z ) 4, 24901 reflections measured, 9878 unique which were used in all calculations (Rint ) 0.073). The final wR(F2) was 0.206 (all data). CCDC 682191 and CCDC 683192 contain the supplementary crystallographic data for the two compounds in this work. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Rd, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; e-mail:
[email protected]). Acknowledgment. We acknowledge the Swedish Research Council and K & A Wallenberg Foundation for financial support of this work. Supporting Information Available: Computational details and coordinates and structural parameters of all optimized complexes. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) O‘Regan, B; Gra¨tzel, M. Nature 1991, 353, 737. (b) See also: O’Regan, B.; L.-Duarte, I.; M.-Diaz, M. V.; Forneli, A.; Albero, J.; Morandeira, A.; Palomares, E.; Torres, T.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, 2906–2907. (2) (a) Vlachopoulos, N; Liska, L.; Augustynski, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1988, 110, 1216. (b) Nazeeruddin, M. K.; Kay, A.; Rodicio, I; Hamphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (c) Kohle, O.; Gra¨tzel, M.; Meyer, A. F.; Meyer, T. B. AdV. Mater. 1997, 9, 904. (3) (a) Nusbaumer, H.; Zakeeruddin, S. M.; Moser, J.-E.; Gra¨tzel, M. Chem. Eur. J. 2003, 9, 3756. (b) Sapp, S. A.; Elliott, C. M.; Contado, C.; caramori, S.; Bignozzi, C. A. J. Am. Chem. Soc. 2002, 124, 11215.
Privalov et al. (4) (a) Caramori, S.; Cazzanti, S.; Marchini, L.; Argazzi, R.; Bignozzi, C. A.; Martineau, D.; Gros, P. C.; Beley, M. Inorg. Chim. Acta 2008, 361, 627. (b) Fukuri, N.; Masaki, N.; Kitamura, T.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2006, 110, 25251. (c) Gorlov, M.; Kloo, L. Dalton Trans. 2008, 2655. (5) Pelet, S.; Moser, J.-E.; Gra¨tzel, M. J. Phys. Chem. B 2002, 104, 1791–1795. (6) Montanari, I.; Nelson, J.; Durrant, J. R. J. Phys. Chem. B 2002, 106, 12203–12210. (7) Bauer, C.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A. J. Phys. Chem. B 2002, 106, 12693–12704. (8) Clifford, J. N.; Palomares, E.; Nazeeruddin, Md. K.; Gra¨tzel, M.; Durrant, J. R. J. Phys. Chem. C 2007, 11, 6561–6567. (9) (a) See: Nour-Mohhamadi, F.; Nguyen, S. D.; Boschloo, G.; Hagfeldt, A.; Lund, T. J. Phys. Chem. B 2005, 109, 22413–22419. (b) Agrell, H. G.; Lindgren, J.; Hagfeldt, A. Sol. Energy 2003, 75, 169–180, and references therein concerning the chemistry of the photoanode. (10) Hara, K.; Kurashige, M.; ito, S.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. Chem. Commun. 2003, 252. (11) Barcelo-Oliver, M.; Terron, A.; Garcia-Raso, A.; Molins, E. Polyhedron 2007, 26, 1417–1426. (12) Matson, B. M.; Pignolet, L. H. Inorg. Chem. 1977, 16, 488. (13) See Computational Details for a general discussion of selfinteraction error (SIE) in current DFT implementations. All spin distributions were recomputed with various hybrid and gradient-corrected density functionals, such as MPW1K, B3PW1K, MPW1PW91 and BP86. Very good internal consistency of all computed spin densities had been found with no discrepancy between the recomputed spin densities and our presented B3LYP data, conclusively showing that the SIE has no bearing on the results we report in our study. (14) Svensson, P. H.; Kloo, L. Chem. ReV. 2003, 105, 1649. (15) Jaguar 6.0; Schro¨dinger, LLC., Portland, OR, 2005. (16) Staubiz, A.; Besora, M.; Harvey, J. N.; Manners, I. Inorg. Chem. 2008, 47, 5910–5918. (17) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (18) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (19) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (b) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; Defrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (c) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (20) Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. J. Phys. Chem. A 2000, 104, 4811–4815. (21) The assignment of the spin state of the ruthenium center on the basis of a Mulliken spin population analysis is the most commonly used procedure despite its well-known limitations. The well-known problem is that calculated Mulliken spin values come out somewhat lower than the ideal spins, i.e., 1.0 for an electronic doublet spin state. However, relative Mulliken spin values are more reliable and informative than absolute values. Therefore, we compare Mulliken populations for different dye complexes in order to reliably determine the change in oxidation state of the ruthenium center. The localization of the spin density is illustrated via spin density maps, and it has been verified by orbital population analyses. (22) Sheldrick,G. M. SHELXS-97: Program for the Solution of Crystal Structures,University of Go¨ttingen, Germany, 1997. (23) Sheldrick,G. M. SHELXS-97: Program for the Solution of Crystal Structures,University of Go¨ttingen, Germany, 1997.
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