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Proceedings of the National Academy of Sciences of the United States of America logoLink to Proceedings of the National Academy of Sciences of the United States of America
. 2012 May 7;109(39):15628-15633. doi: 10.1073/pnas.1118340109

Visible light generation of I–I bonds by Ru-tris(diimine) excited states

Byron H Farnum 1, Jeffrey J Jou 1, Gerald J Meyer 1,1
PMCID: PMC3465397  PMID: 22566656

Abstract

Seven Ru-tris(diimine) compounds were prepared to study the photooxidation of iodide. Iodide oxidation results in the formation of I–I bonds, and it is therefore relevant to the conversion and storage of solar energy. Iodide oxidation is also a key step for electrical power generation in dye-sensitized solar cells. The mechanistic details of iodide oxidation and I–I bond formation were elucidated through time-resolved spectroscopic measurements. Bimolecular electron-transfer reactions between Ru-tris(diimine) excited states and iodide first yielded the iodine atom that subsequently reacted with excess I- to yield the I–I bond of diiodide (Inline graphic). An important finding was that excited-state iodide oxidation was rapid (k > 109 M-1 s-1) even for thermodynamically uphill reactions. These results indicated that iodide oxidation to the iodine atom may account for a significant fraction of sensitizer regeneration within dye-sensitized solar cells.

Keywords: chemical bond formation, solar fuel

The oxidation of iodide in fluid solution results in the formation of diiodide (Inline graphic) and triiodide (Inline graphic) (1–6). Photoinitiation of iodide oxidation with visible light thus provides a fundamental means by which solar photons can be converted to chemical energy in the form of I–I bonds. This bond formation chemistry is also of major importance in dye-sensitized solar cells where I-/Inline graphic is the most widely used redox mediator (7–11).

We have previously reported the photooxidation of iodide using Ru-tris(diimine) compounds in fluid solution (1, 2, 12–14). These compounds are known for their exceptional absorption properties in the visible region, and their long-lived metal-to-ligand charge transfer (MLCT) excited states (15, 16). Visible light generation of Ru-tris(diimine) excited states in the presence of I- resulted in electron-transfer reactions that generated I–I bonds. In acetonitrile solutions, iodide oxidation proceeded through an iodine atom intermediate, and Inline graphic appeared as a secondary reaction product (1, 2, 13). In dichloromethane solutions, iodide oxidation was greatly enhanced due to strong ion-pair interactions between the Ru-tris(diimine) compounds and iodide (1, 12, 14).

A series of Ru-tris(diimine) photooxidants have been prepared to characterize photo-initiated I–I bond formation in acetonitrile. The diimine ligands employed in this study are shown in Fig. 1, where a vertical arrow is drawn to indicate increased electron donation of the chelating nitrogens to the ruthenium metal center. The compounds prepared allowed the free-energy change for electron transfer (ΔG°) to be systematically varied over a 400-meV range. A strong dependence of the observed rate constants for iodide oxidation on ΔG° was apparent and rate constants on the order of 109 M-1 s-1 were determined for ΔG°≥0 eV.

Fig. 1.

Fig. 1.

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Collection of diimine ligands used in this study where dmb, 4,4′-dimethyl-2,2′-bipyridine; bpy, 2,2′-bipyridine; deeb, 4,4′-di-ethylester-2,2′-bipyridine; bpm, 2,2′-bipyrimidine; and bpz, 2,2′-bipyrazine.

Results

The series of Ru-tris(diimine) compounds was synthesized following literature procedures and are hereby referred to as Ru2+. To our knowledge, [Ru(dmb)2(bpz)](PF6)2 and [Ru(bpm)2(deeb)](PF6)2 are compounds that have not been previously reported. The photophysical properties for all other compounds have been well characterized in the literature, and they agree with the results presented in this study (12, 14, 17, 18).

The Ru2+ compounds ranged in color from bright orange for [Ru(bpz)2(deeb)](PF6)2 to a deep red for [Ru(dmb)2(bpz)](PF6)2, and they displayed absorption features in the 400- to 500-nm range, which is consistent with MLCT transitions. Extinction coefficient spectra of [Ru(bpy)2(deeb)]2+ and, its one electron reduced form [Ru(bpy)2(deeb-)]+ are shown in Fig. 2. Similar extinction coefficient spectra for all Ru2+ and Ru+ compounds can be found in the SI Text. Among the series of compounds, room temperature photoluminescence (PL) spectra exhibited a broad featureless band centered between 620 and 720 nm. PL spectra recorded at 77 K in 4∶1 ethanol∶methanol glass displayed a blue-shifted peak, with respect to those recorded at room temperature, and well-defined vibronic structure at lower energies. Time-resolved, excited-state decays measured after pulsed 532-nm laser excitation of Ru2+ were described by a first-order kinetic model, from which the excited-state lifetimes (τo) were abstracted. The PL quantum yields (ϕPL) were calculated by comparative actinometry using [Ru(bpy)3]Cl2 in H2O (ϕPL = 0.042) as a standard (19, 20). When combined with excited-state lifetimes, radiative (kr) and nonradiative (knr) rate constants for excited-state decay were calculated for each Ru2+∗ excited state. Table 1 displays a summary of these photophysical data.

Fig. 2.

Fig. 2.

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Extinction coefficient spectra of [Ru(bpy)2(deeb)]2+ and [Ru(bpy)2(deeb-)]+ in acetonitrile. [Ru(bpy)2(deeb-)]+ was generated by steady-state photolysis of [Ru(bpy)2(deeb)]2+ in the presence of triethylamine.

Table 1.

Photophysical properties of Ru-tris(diimine) compounds

Compound Ru2+ λmax* Ru+λmax* PLλmax*,† τo‡ ϕPL kr§ knr§
[Ru(dmb)2(bpz)](PF6)2 485 (14,000) 485 (17,200) 720 0.25 0.005 0.20 40
[Ru(bpy)2(deeb)](PF6)2 478 (16,000) 488 (17,800) 675 0.95 0.032 0.34 10
[Ru(deeb)2(dmb)](PF6)2 485 (20,000) 491 (17,600) 663 1.19 0.100 0.84 7.6
[Ru(deeb)3](PF6)2 467 (21,800) 527 (22,700) 626 2.10 0.130 0.62 4.1
[Ru(bpm)2(deeb)](PF6)2 457 (12,000) 496 (11,500) 625 0.45 0.028 0.62 22
[Ru(deeb)2(bpz)](PF6)2 456 (21,000) 502 (19,600) 647 1.06 0.015 0.14 9.3
[Ru(bpz)2(deeb)](PF6)2 ∥ 448 (17,500) 495 (14,000) 620 1.75 0.140 0.80 4.9
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*λmax given in nanometers (ε M-1 cm-1).

†PL max at room temperature.

‡10-6 s.

§105 s-1, kr = ϕPL/τ, knr = (1-ϕPL)/τ.

Ref. 1.

∥Ref. 2.

PL spectra measured at room temperature and 77 K were modeled by a single-mode Franck–Condon (FC) line-shape analysis to determine the E00 transition energy and full width at half maximum (Inline graphic) (19, 21, 22). A detailed description of the FC line-shape analysis, along with fitting parameters and representative room temperature PL spectra, are provided in the SI Text. The free energy stored in the excited state (ΔGes) was then calculated from Eq. S2 (22), as summarized in Table 2 for the series of photooxidants.

graphic file with name pnas.1118340109eq33.jpg [1]

Table 2.

Thermodynamics of Ru-tris(diimine) compounds

Compound
 E°*
ΔGes†
 3+/2+
 2+/+
 2+∗/+
[Ru(dmb)2(bpz)](PF6)2 1.87 1.39 −0.92 0.95
[Ru(bpy)2(deeb)](PF6)2 1.99 1.37 −1.00 0.99
[Ru(deeb)2(dmb)](PF6)2 2.02 1.39 −0.96 1.06
[Ru(deeb)3](PF6)2 2.13 1.56 −0.92 1.21
[Ru(bpm)2(deeb)](PF6)2 2.15 1.62 −0.91 1.24
[Ru(deeb)2(bpz)](PF6)2 2.07 1.66 −0.81 1.26
[Ru(bpz)2(deeb)](PF6)2 2.16 1.79 −0.77 1.39
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*Volt vs. SCE.

†Electronvolt, room temperature.

Cyclic voltammetry was used to determine electrochemical reduction potentials. At positive applied potentials, metal-based RuIII/II waves were observed, whereas at negative potentials, ligand localized redox waves were apparent. In all cases, the redox chemistry was classified as quasi-reversible as the peak anodic and peak cathodic currents were equal; however, the peak-to-peak splitting was greater than 59 mV. Typical cyclic voltammograms for [Ru(bpz)2(deeb)]2+, [Ru(bpm)2(deeb)]2+, [Ru(deeb)2(dmb)]2+, and [Ru(bpy)2(deeb)]2+ in the range of 0 to -2 V vs. SCE are shown in Fig. 3. Within this potential range, three successive redox waves were observed for each compound, and they represent reduction/oxidation of each of the diimine ligands. Comparisons of these data along with others in the literature indicated the order of ligand reduction within a given compound (18, 23). A summary of electrochemical results for the Ru2+ series is included in Table 2. Here, E°(3+/2+) is indicative of the metal-based reduction potential, and E°(2+/+) indicates the first ligand localized reduction potential. The excited-state reduction potential, E°(2+∗/+) relevant to the photooxidation of iodide was calculated using ΔGes and E°(2+/+) by Eq. 1.

Fig. 3.

Fig. 3.

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Cyclic voltammograms of [Ru(bpz)2(deeb)](PF6)2, [Ru(bpm)2(deeb)](PF6)2, [Ru(deeb)2(dmb)](PF6)2, and [Ru(bpy)2(deeb)](PF6)2 in argon purged 0.1 M TBAClO4/acetonitrile at a scan rate of 0.1 V/s.

Iodide oxidation by Ru2+∗ was quantified by time-resolved PL and transient absorption spectroscopies. For all photooxidants in the series, the excited-state lifetime decreased as a function of the tetrabutylammonium iodide (TBAI) concentration, whereas the initial concentration of excited states remained constant. Time-resolved PL decays of [Ru(bpy)2(deeb)]2+∗ as a function of [I-] in argon purged acetonitrile can be found in the SI Text. Excited-state quenching followed the Stern–Volmer law, as it was observed to be first-order in iodide concentration, and yielded a Stern–Volmer constant KSV = 7,530 M-1 for [Ru(bpy)2(deeb)]2+∗. The second-order rate constant for excited-state quenching (kq) was abstracted from the relation KSV = kqτo. In the case of [Ru(bpy)2(deeb)]2+∗, kq was calculated to be 7.9 × 109 M-1 s-1. Among the other Ru2+∗ compounds, kq ranged from 5.7 × 109 M-1 s-1 for [Ru(dmb)2(bpz)]2+∗ to 6.6 × 1010 M-1 s-1 for [Ru(bpz)2(deeb)]2+∗.

Transient absorption spectroscopy was used to identify products of the excited-state quenching reactions. Fig. 4 displays the transient absorption spectrum recorded 1 μs after pulsed 532-nm laser excitation of [Ru(bpy)2(deeb)]2+ in the presence of 7.8 mM TBAI. At this concentration of I-, 98% of the excited states were quenched, and the observed absorption features were consistent with an electron-transfer reaction that yielded the reduced Ru-tris(diimine) and oxidized iodide products. The feature at 520 nm was assigned to the reduced species, [Ru(bpy)2(deeb-)]+, whereas the broad transition at λ > 600 nm was attributed to the oxidized I–I bonded product (Inline graphic). Overlaid on the data is a simulation based on equal concentrations of [Ru(bpy)2(deeb-)]+ and Inline graphic calculated from their known extinction coefficient spectra (Fig. 4, Inset). Cage-escape yields for the electron-transfer quenching reactions were measured by comparative actinometry using the calculated [Ru+] produced at the condition of approximately 100% excited-state quenching. For all of the photooxidants employed, Ru+ and Inline graphic were the only products observed after excited-state quenching.

Fig. 4.

Fig. 4.

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Transient absorption spectrum (black points) observed 1 µs after pulsed 532-nm laser excitation of a 35 μM [Ru(bpy)2(deeb)]2+ acetonitrile solution containing 7.8 mM [I-]. The blue line is a simulation based on a 1∶1 mixture of [Ru(bpy)2(deeb-)]+ and Inline graphic. (Inset) Extinction coefficient spectra for ([Ru(bpy)2(deeb-)]+ - [Ru(bpy)2(deeb)]2+), and Inline graphic.

Single wavelength transient absorption data were used to quantify the reaction kinetics. Wavelengths were chosen based on the measured absorption spectra of Ru2+∗ and Ru+ for a given photooxidant. For example, Fig. 5 shows representative transient absorption data obtained for [Ru(bpy)2(deeb)]2+ recorded at 408 and 525 nm as a function of [I-]. Observations at 408 nm represented a Ru2+/Ru+ isosbestic point (see Fig. 4, Inset); therefore, Ru2+∗ was observed independently as a bleach that decayed to the ground state. At higher iodide concentrations, the appearance of Inline graphic was observed as a residual positive growth above the excited-state bleach. A similar scenario can be used to describe the data at 525 nm where features are mostly representative of Ru+. To ensure a complete description of the transient absorption features, up to three wavelengths were used to study each Ru2+∗ + I- reaction.

Fig. 5.

Fig. 5.

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Single wavelength transient absorption data recorded after pulsed 532-nm laser excitation of [Ru(bpy)2(deeb)]2+ as a function of [I-] at 408 nm. (Inset) Transient absorption recorded at 525 nm under the same conditions.

Single wavelength transient absorption data were well described by a pseudo-first-order kinetic model. In most cases, a single exponential function was used to model the data; however, when two species were observed at a given wavelength, a sum of two exponential functions was necessary. Kinetic derivations for this treatment are provided in the SI Text. Second-order rate constants for the iodide oxidation reactions were abstracted from pseudo-first-order kinetic plots like the one shown in Fig. 6 for [Ru(bpy)2(deeb)]2+. In this example, time-resolved PL recorded at 650 nm and ΔAbs at 408 nm were representative of Ru2+∗, whereas ΔAbs recorded at 525 and 398 nm were indicative of Ru+ and Inline graphic. Similar data were reported previously for [Ru(bpz)2(deeb)]2+ and [Ru(deeb)3]2+ (1, 2). In all cases, the rate constant associated with formation of the Ru+ product was found to be within error equal to that for the loss of Ru2+∗, indicating that Ru+ was a primary photoproduct of the quenching reaction. Therefore, these rate constants were averaged and reported as a single value, k1. The second-order rate constants extracted for the I–I bond formation step to yield Inline graphic were within error the same for the series of photooxidants except for [Ru(dmb)2(bpz)]2+∗ and [Ru(bpy)2(deeb)]2+∗. In these two cases, it was found that k1 < 2.4 × 1010 M-1 s-1 and the production of Inline graphic was rate limited by that of I•. Table 3 provides a summary of these measured rate constants along with cage-escape yields for the series of photooxidants.

Fig. 6.

Fig. 6.

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A plot of kobs vs. [I-] for the pseudo-first-order kinetics of single wavelength transient absorption measurements observed after pulsed 532-nm laser excitation of [Ru(bpy)2(deeb)]2+ in the presence of I-. The time-resolved PL decay rate constants recorded at 650 nm and the transient absorption (ΔAbs) rate constants recorded at 408 nm were representative of Ru2+∗, whereas ΔAbs recorded at 525 and 398 nm were indicative of Ru+ and Inline graphic, respectively. Second-order rate constants abstracted from the slopes were k1 = (7.9 ± 1.1) × 109 M-1 s-1 and k2 = (7.1 ± 2.0) × 109 M-1 s-1.

Table 3.

Rate constants and cage-escape yields for iodide oxidation and I–I bond formation

Excited-state ΔG°* k1† kact†,‡ ϕce k2†
[Ru(dmb)2(bpz)]2+∗ 0.04 (5.7 ± 0.6) × 109 (6.2 ± 0.6) × 109 0.025 (5.4 ± 2.0) × 109
[Ru(bpy)2(deeb)]2+∗ 0.00 (7.9 ± 1.1) × 109 (9.0 ± 1.3) × 109 0.050 (7.1 ± 2.0) × 109
[Ru(deeb)2(dmb)]2+∗ −0.07 (3.5 ± 0.7) × 1010 (7.9 ± 1.7) × 1010 0.044 (2.6 ± 0.1) × 1010
[Ru(deeb)3]2+∗§ −0.22 (4.9 ± 0.2) × 1010 (2.1 ± 0.3) × 1011 0.035 (2.2 ± 0.2) × 1010
[Ru(bpm)2(deeb)]2+∗ −0.25 (5.6 ± 0.4) × 1010 (4.5 ± 0.1) × 1011 0.043 (2.2 ± 0.3) × 1010
[Ru(deeb)2(bpz)]2+∗ −0.27 (5.4 ± 0.7) × 1010 (3.3 ± 0.4) × 1011 0.012 (2.3 ± 0.2) × 1010
[Ru(bpz)2(deeb)]2+∗ −0.40 (6.6 ± 0.2) × 1010 - 0.027 (2.4 ± 0.2) × 1010
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*Electronvolt, Eq. 3 where E°(I•/I-) = 0.99 V vs. SCE.

†M-1 s-1.

‡Eq. 5 where kact = Kdket and kd = 6.4 × 1010 M-1 s-1.

§Ref. 1.

Ref. 2; error given as SD.

Discussion

A family of seven Ru-tris(diimine) compounds was prepared to study the photooxidation of iodide whereby the driving force for excited-state electron transfer was systematically varied by more than 400 meV. The MLCT excited states were found to oxidize I- to I•, which subsequently reacted with excess I- to yield the I–I bond of Inline graphic. We highlight the key findings of the MLCT excited states and iodide oxidation mechanism below. We conclude with a discussion of the relevance of these finding to solar energy conversion.

MLCT Excited States.

The photophysical properties for the series of Ru-tris(diimine) compounds under study were typical of MLCT excited states (15, 16). Visible light absorption resulted in charge transfer from the RuII metal center to the diimine ligand shown in Eq. 2 for [Ru(bpy)2(deeb)]2+. The formality of the RuIII oxidation state is most meaningful for the initially formed Franck–Condon state, and it should be regarded with caution in the thermally equilibrated excited state because significant mixing of metal-t2g and ligand-π∗ orbitals occurs (15, 16, 24, 25). In fluid solution, the excited electron was best described as being localized on a single diimine ligand as opposed to being delocalized over all three (16, 26, 27). For heteroleptic Ru-tris(diimine) compounds, where the diimine ligands are inequivalent, it has been shown that the excited state is localized on the ligand that is most easily reduced. Among the set of diimine ligands investigated here, the order of reduction potentials was determined to be π∗(bpz) < π∗(bpm) < π∗(deeb) < π∗(bpy) < π∗(dmb) through a comparison of the electrochemical data along with careful inspection of the absorption spectra for the one electron reduced compounds.

graphic file with name pnas.1118340109eq34.jpg [2]

A Franck–Condon line-shape analysis of the photoluminescence spectra yielded confident estimates of the Gibbs free energy stored in the excited states. The values ranged from 1.87 eV for [Ru(dmb)2(bpz)]2+∗ to 2.16 eV for [Ru(bpz)2(deeb)]2+∗. These two extremes resulted from inductive effects of the electron donating methyl groups of dmb relative to the electron withdrawing bpz and ester containing deeb ligands (18, 23, 28). Similar trends were observed for electrochemical reduction potentials where the exchange of diimine ligands about the metal center resulted in a variance of over 400 mV for the excited-state reduction potentials.

Photooxidation to Yield I–I Bonds.

A proposed mechanism for the oxidation of iodide by Ru-tris(diimine) excited states consistent with the data presented here is shown in Fig. 7. The Ru2+∗ excited state underwent diffusional interactions with iodide that ultimately yielded an activated complex, [Ru2+∗,I-]+, with an association constant of Kd = kd/k-d. Electron transfer within the activated complex occurred based on the equilibrium constant Ket = ket/k-et established by the Gibbs free energy for electron transfer (ΔG°) using the exact relation Ket = exp[-ΔGo/RT]. The ΔG° for electron transfer was defined by Eq. 3 where E°(2+∗/+) is given in Table 2 [E°(I•/I-) = 0.99 V vs. SCE, and electrostatic work corrections have been omitted] (6, 29). Cage escape from the activated complex (kce) to yield solvated products occurred in competition with k-et to reform the excited state and kbt to yield the ground state. Further reaction of I• with I- to yield Inline graphic occurred with an average rate constant of k2,avg = (2.4 ± 0.2) × 1010 M-1 s-1, which is consistent with other studies (2, 30, 31).

graphic file with name pnas.1118340109eq35.jpg [3]

Fig. 7.

Fig. 7.

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Proposed mechanism for the oxidation of I- by Ru-tris(diimine) excited states.

A steady-state approximation applied to the activated complex yields the relationship (Eq. 4) between the observed second-order rate constant for excited-state iodide oxidation (k1), and the intimate rate constants defined in Fig. 7 (32). The observed rate constant will be diffusion limited when Kdket is much greater than the diffusion rate constant (kd). An estimate for kd = 6.4 × 1010 M-1 s-1 for the reaction of [Ru(bpz)2(deeb)]2+∗ and I- was previously reported in acetonitrile (2). The experimental values for k1 presented here approach the diffusion limited threshold; therefore, correction of k1 to yield Kdket was necessary to understand the reaction kinetics further. In the case of [Ru(bpz)2(deeb)]2+∗, this correction could not be applied because k1 was within experimental error equal to kd. For bimolecular rate constants, Kdket is often referred to concisely as the activation rate constant (kact), and it has second-order units of M-1 s-1 (33).

graphic file with name pnas.1118340109eq36.jpg [4]

The activation rate constants calculated for the series of photooxidants are given in Table 3, and they show a clear dependence on the ΔG° for electron transfer. The magnitude of these values exceeds 1011 M-1 s-1 for the strongest oxidizing excited states and 109 M-1 s-1 for reactions that are thermodynamically uphill in nature. This behavior was an indication of highly favorable electron transfer between Ru-tris(diimine) excited states and I-. Unfortunately, treatment of these data in terms of outer sphere electron-transfer theory was difficult, as there are many unknown parameters (i.e., reorganization energy, electronic coupling, interaction distance, etc.) that could not be independently calculated. Nevertheless, qualitative considerations are discussed below that are consistent with the observed data.

Previous studies of reductive quenching of Ru-tris(diimine) excited states in acetonitrile have reported large rate constants (i.e., kact > 108 M-1 s-1) over similar free-energy changes (34–38). This behavior was explained by the large self exchange rate constant (109 M-1 s-1) for the excited state Ru2+∗/Ru+ couple due to metal-based electron transfer (33, 39, 40). In the case of iodide oxidation, reaction within the activated complex can be viewed as electron transfer from an iodide p orbital to a ruthenium t2g orbital, [5]. Therefore, the favorable kinetics associated with RuIII/RuII self-exchange may in part account for the observed rate constants. The kact values determined here were much greater than those in previous reports; therefore, further reasoning is still required.

graphic file with name pnas.1118340109eq37.jpg [5]

The orientation of the iodide ion with respect to the excited state in the activated complex is also expected to be an important factor in the electron-transfer reaction. Iodide is known to form adducts with aromatic acceptors (41–44), and previously published crystal structures of [Ru(bpy)2(deeb)](I)2 and [Ru(deeb)3](I)2 revealed that iodide was within 4 Å of the nearest carbon atom on the deeb ligands, whereas the Ru-I distance was determined to be approximately 6 Å (1, 14). Direct coordination of iodide to ruthenium to form a seven-coordinate compound also cannot be ruled out; however, to our knowledge there is no precedence for such behavior with Ru-tris(diimine) compounds. The presence of any inner-sphere intermediates would have a significant impact on the electronic coupling, reorganization energy, and cage-escape yields for excited-state electron transfer (33, 37, 45, 46). In one limit, the π∗ orbitals of the diimine ligand could provide a pathway for electron transfer from iodide to the RuIII metal center (39). Although speculative, this hypothesis could provide an explanation for the rapid reaction observed at low driving force.

Although details of the electron-transfer reaction within the activated complex are unknown, the free-energy changes associated with the generation of I–I bonds can be accurately determined. An energetic scheme for the oxidation of iodide by [Ru(bpy)2(deeb)]2+∗ is shown in Fig. 8. The oxidizing power of the MLCT excited state was determined to be 0.99 V vs. SCE; therefore, it exhibited no change in free-energy for the oxidation of iodide to the iodine atom. The charge-separated state of Ru+/I• would, therefore, store the same free energy as that of the excited state—i.e., ΔGes = 1.99 eV. Rapid formation of Inline graphic via the reaction of I• with I- leads to a loss in free energy of 0.3 eV based on the upper limit of Inline graphic vs. SCE (6). For the series of photooxidation reactions, the free energy stored in the charge-separated state of Inline graphic was greater than 1.5 eV. This free energy is lost to recombination of Ru+ and Inline graphic to yield ground state products; however, generation of the charge-separated state via rapid electron-transfer reactivity with no significant loss in free energy is a valuable property of these reactions.

Fig. 8.

Fig. 8.

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Energetic scheme describing the oxidation of iodide by [Ru(bpy)2(deeb)]2+∗.

Summary and Relevance to Solar Fuels.

The results show that the Ru-tris(diimine) sensitized oxidation of iodide represents a general method for the conversion of light into potential energy in the form of redox equivalents and the transient storage in the form of I–I bonds. The mechanism identified for I–I bond formation involved two sequential steps: (i) the oxidation of I- to I•, and (ii) the reaction of I• with excess I- to form Inline graphic. A key result from this study was that the excited-state iodide oxidation reaction could be driven rapidly with no free-energy loss, a finding that has relevance to dye-sensitized solar cells.

In dye-sensitized solar cells, iodide oxidation is responsible for sensitizer regeneration after excited-state electron injection into the TiO2 semiconductor. Therefore, the oxidized form of the sensitizer initiates iodide oxidation as opposed to the excited-state redox chemistry reported here. The mechanism of iodide oxidation remains unknown, though it has tacitly been assumed to occur by a concerted pathway (Inline graphic) where the reaction is second-order in iodide concentration (7, 8). This assumption stems mainly from the unfavorable thermodynamics for iodide oxidation to the iodine atom relative to the concerted mechanism. For example, the popular Ru(dcb)2(NCS)2 sensitizer, where dcb is 4,4′-(CO2H)2-bpy, has an E°(RuIII/II) = 0.85 V vs. SCE, giving a ΔG° = 0.14 eV for the iodine atom pathway, where the concerted pathway is thermodynamically downhill, ΔG° ≤ -0.16 eV (8, 47, 48).

Based on the free-energy dependence of the activation rate constants, an estimate of kact = 1 × 109 M-1 s-1 for the oxidation of iodide to the iodine atom by [RuIII(dcb)2(NCS)2]+ would be reasonable. Assuming this estimate is relevant to the semiconductor interface and correction for operational dye-sensitized solar cell conditions (i.e., 0.5 M [I-]/ionic strength), a pseudo-first-order rate constant of k′ = 2 × 106 s-1 can be calculated for sensitizer regeneration. Although no definitive experimental rate constants exist for sensitizer regeneration, estimates based on half lives for the oxidized sensitizer are in the range of a few hundred nanoseconds to microseconds (8). The pseudo-first-order rate constant calculated here corresponds to a half-life of 300 ns, well within the range to satisfy the experimental results in functioning solar cells. Therefore, the kinetic data reported herein indicate that iodine atom formation in dye-sensitized solar cells may play a significant role in the sensitizer regeneration process, and it should not be ruled out based on thermodynamic arguments alone.

Materials and Methods

The synthesis and characterization of compounds, details of the spectroscopic and electrochemical methods, UV-visible absorption data, kinetic derivations, and Franck–Condon line-shape analyses of photoluminescence spectra can be found in the SI Text.

Supplementary Material

Supporting Information
supp_109_39_15628__index.html (986B, html)

Acknowledgments.

The authors want thank Patrik G. Johansson for help with the Franck–Condon line-shape analysis. The authors acknowledge support by a grant from the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, US Department of Energy (DE-FG02-96ER14662), and The Johns Hopkins University Chemistry Alumni Graduate Fellowship.

Footnotes

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1118340109/-/DCSupplemental.

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