Photochemical Generation of Strong One-Electron Reductants via

Jun 1, 2014 - Tomoe ShimodaTakeshi MorishimaKoichi KodamaTakuji HiroseDmitry E. PolyanskyGerald F. ManbeckJames T. MuckermanEtsuko Fujita...
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Article pubs.acs.org/JPCA

Photochemical Generation of Strong One-Electron Reductants via Light-Induced Electron Transfer with Reversible Donors Followed by Cross Reaction with Sacrificial Donors Bing Shan and Russell Schmehl* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States S Supporting Information *

ABSTRACT: This work illustrates a modified approach for employing photoinduced electron transfer reactions coupled to secondary irreversible electron transfer processes for the generation of strongly reducing equivalents in solution. Through irradiation of [Ru(LL)3]2+ (LL= diimine ligands) with tritolylamine (TTA) as quencher and various alkyl amines as sacrificial electron donors, yields in excess of 50% can be achieved for generation of reductants with E0(2+/1+) values between −1.0 and −1.2 V vs NHE. The key to the system is the fact that the TTA cation radical, formed in high yield in reaction with the photoexcited [Ru(LL)3]2+ complex, reacts irreversibly with various sacrificial electron donating amines that are kinetically unable to directly react with the photoexcited complex. The electron transfer between the TTA+ and the sacrificial amine is an energetically uphill process. Kinetic analysis of these parallel competing reactions, consisting of bimolecular and pseudo first-order reactions, allows determination of electron transfer rate constants for the cross electron transfer reaction between the sacrificial donor and the TTA+. A variety of amines were examined as potential sacrificial electron donors, and it was found that tertiary 1,2-diamines are most efficient among these amines for trapping the intermediate TTA+. This electrondonating combination is capable of supplying a persistent reducing flux of electrons to catalysts used for hydrogen production.

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sacrificial electron donor such as triethanolamine (TEOA).17 Quantum yields for the formation of MV+ are typically only around 25% with Ru(II) tris-diimine chromophores because cage escape yields for the PS+, MV+ geminate pair are low.18−20 An additional disadvantage of generating reductants in this way is that the strength of the reductant is limited by the excited state potential of PS* (eq 1)

INTRODUCTION The development of catalysts for water reduction to hydrogen, either as electrocatalysts or as systems that function via reaction with chemical reductants, has received considerable attention in recent years.1−7 Often the process of investigation involves initial evaluation of homogeneous catalytic substances by electrochemical approaches, wherein the overpotential for hydrogen evolution, the faradaic efficiency, and the turnover number of the catalyst are the parameters of interest.1,8−10 This is followed by examination of the catalyst in a photochemical reactor that includes a chromophore, a redox partner, and a sacrificial electron donor.11 Such systems serve to illustrate the effectiveness of the catalyst under conditions where the rate of generation of reducing equivalents can vary significantly relative to electrochemical reactors.3,4 The photochemical method for generating a reducing agent for the catalyst typically involves one of two approaches. In the first, shown in Scheme 1, the photosensitizer, PS, reduces an electron mediator such as methyl viologen (MV2+), which subsequently reduces the catalyst.12−16 Regeneration of PS+ occurs via reaction with a

PS+ + e− ⇌ PS*

E 0(PS+ /PS*)

(1)

For transition metal complex sensitizers such as [Ru(bpy)3]2+, this potential is around 1.0 V vs NHE,21 thus the excited state is a modest reductant. An alternate approach is to reductively quench the excited state of the photosensitizer with a sacrificial electron donor, thereby making the reduced sensitizer the species to react with the catalyst (Scheme 2).22−26 This simplifies the overall photoreduction system and, Scheme 2. Three-Component System

Scheme 1. Four-Component System with an Oxidative Quencher

Special Issue: Current Topics in Photochemistry Received: April 21, 2014 Revised: May 29, 2014

© XXXX American Chemical Society

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Transient Absorption Measurements. Transient spectra for kinetic analysis of electron transfer were obtained from an Applied Photophysics LKS 60 optical system with an OPOTek pumped by a Quantel Brilliant Laser equipped with doubling and tripling crystals that supplies adjusted laser light. Excitation of the chromophores was typically at 450 nm using samples having an optical density of about 1.0. Samples (4 mL volume) containing the Ru complexes, TTA (0.04 M) and various concentrations of sacrificial amine in MeCN were degassed by nitrogen bubbling for 20 min immediately prior to data acquisition. Temperature Dependent Cross Electron Transfer Reactions. [Ru(bpy)3]Cl2 was chosen as the photosensitizer for the study of temperature dependence of electron transfer between TEA and TTA+•. Rate constants were determined by laser flash photolysis as described above. Temperature control was accomplished using a circulating water bath to provide heating/cooling to the sample cell; temperature measurements were made with a VWR type K Infrared Thermometer (at point blank range).

for Ru(II) diimine complex sensitizers, results in generation of stronger reductants. For instance, photoreduction of [Ru(decb)3]2+ (decb =4,4′-dicarboxyethyl-2,2′-bipyridine) by triethylamine (TEA) results in formation of the [Ru(decb)3]+ (E0(2+/+) = −0.8 V vs NHE).27 Once again, though, the reducing ability of the system is limited by the ability of the sacrificial donor to quench PS*. When sacrificial donors such as TEA are used to quench Ru(II) complex chromophores with more negative reduction potentials (i.e., E0(2+/+) < −1.0 V vs NHE), the rate of the excited state quenching process decreases to the point where reduction of the excited complex is ineffective even in solutions saturated with the sacrificial electron donor.27−29 Since the exact redox potentials (E0(+/0)) of most sacrificial electron donors are not known, the root cause of the ineffective quenching of the chromophores cannot be definitively ascribed. It is possible that the failure to quench the excited state is the result of kinetic rather than thermodynamic limitations. With this in mind, we examined the utility of a reductive quencher that has a reversible one-electron oxidation to serve as a quencher of the excited chromophore and as a mediator to oxidize the sacrificial electron donor. In this case, shown in Scheme 3, the sacrificial electron donor reacts with the oxidized

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RESULTS AND DISCUSSON

Forward and Back Electron Transfer of [Ru(LL)3]2+ with TTA and TEA. The generation of strongly reducing [Ru(LL)3]+ in solution by photoinduced electron transfer requires use of an electron donor capable of reducing the photoexcited complexes. For the bpy, 5-Clphen and dpp complexes, the excited state potentials are 0.84, 0.77, and 0.90 V vs NHE in water.32−34 Quenching of each of these complexes with TEA is not thermodynamically feasible based on literature estimates for E0(TEA+/TEA).35 Experimentally, rate constants for electron transfer quenching are immeasurably low, and, even with 0.55 M solutions of TEA, no quenching is observed for any of these [Ru(LL)3]2+ excited states.27,36 If an amine with a lower one-electron oxidation potential is used as quencher, for example tritolylamine (TTA, E0 = 0.99 V vs NHE), reaction with the excited complex occurs at rates that are competitive with excited state relaxation, despite the fact that the reaction is also endergonic. Electron transfer quenching of different [Ru(LL)3]2+ complexes by TTA was examined using luminescence intensity quenching and was found to follow Stern−Volmer kinetics. From each Stern−Volmer plot (Supporting Information), kq is calculated to be in the range of 3 × 107 ∼2 × 108 M−1 s−1 (Table 1). The transient absorption spectrum of [Ru(bpy) 3 ] 2+ quenched by TTA is shown in Figure 1a. The spectrum shows that absorption of [Ru(bpy)3]+ at both 380 and 510 nm37 decays simultaneously with that of TTA+• at 668 nm38 in about 70 μs after charge separation. Back electron transfer rates

Scheme 3. Four-Component System with a Reductive Quencher

mediating species (TTA+• in Scheme 3) on a time scale comparable to the back electron transfer reaction (PS− + TTA+•; ∼50−500 μs) rather than the much shorter time associated with excited state relaxation (∼1−5 μs time scale). This work reports kinetic studies of such a reaction sequence, using a variety of Ru(II) tris-diimine complexes, [Ru(LL)3]2+ as PS, tritolylamine (TTA) as an electron mediating quencher, and triethylamine and other alkyl amines as sacrificial electron donors. The results illustrate that TTA is much more effective at quenching the various PSs than the alkylamines. In addition, the work shows that the strategy of trapping TTA+• with alkylamines is effective and leads to the generation of strongly reducing [Ru(LL)3]+ complexes in high yield. Most of the complexes are commercially available and, thus, the reported system can be of immediate use to those investigating catalysts for hydrogen production under solar photochemical conditions.

Table 1. TTA in Acetonitrile in CH3CN with TTA and TEA As Well As Back Electron Transfer Rate Constants for TTA

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EXPERIMENTAL SECTION Reagents. Tri-p-tolylamine (TTA), triethylamine (TEA), triethanolamine (TEOA), triphenylamine (TPA), propylamine (PA), ethylene diamine (EDA) and N,N,N′N′-tetramethylethylenediamine (TMEDA) were purchased from Sigma-Aldrich and used without further purification. Solvent is distilled acetonitrile. [Ru(bipyridine) 3 ]Cl 2 , [Ru(5-Cl-1,10phenanthroline) 3 ]Cl 2 , and [Ru(4,7-diphenyl-1,10phenanthroline)3]Cl2 were prepared according to previously reported literature methods.30,31

[Ru(bpy)3]2+ [Ru(Clphen)3]2+ [Ru(dpp)3]2+

E(II*/ I)/ Va

E(II/ I)/ Va

τ0, ns CH3CN, RT

kq TTA/ 107 M−1 s−1

kb TTA/ 109 M−1 s−1

0.84 0.77

−1.28 −1.15

800 940

10.3 18.8

4.2 4.3

0.90

−1.31

4700

3.0

2.4

E*, E, and τ0 are the reduction potentials (vs NHE) of [Ru(LL)3]2+ excited states and ground states, respectively, and the excited state lifetimes in CH3CN, from the literature.31,33 a

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Figure 1. Transient absorbance changes vs wavelength following excitation at 450 nm of degassed acetonitrile solutions of (a) [Ru(bpy)3]Cl2 + 0.04 M TTA and (b) [Ru(bpy)3]Cl2 + 0.04 M TTA + 0.55 M [TEA]. T = 296.5 K. Figure 3. Transient absorbance changes at 668 nm following excitation at 450 nm of [Ru(LL)3]2+ (LL: (a) bpy; (b) 5-Cl-phen; (c) dpp.) + 0.04 M TTA + different concentrations of [TEA] (T = 296 K). (TEA concentrations: black: 0 M; dark blue: 0.16 M; light blue: 0.26 M; green: 0.37 M; orange: 0.45 M; purple: 0.55 M; fuchsia: 0.65 M; red: 0.73 M).

were measured by the decay of both the produced [Ru(LL)3]+ species at or near 510 nm (black curves in Figure 2) and the

of the initially formed [Ru(bpy)3]+ remains after reaction of TTA+• and TEA is complete. Evaluation of the TEA concentration dependence on the rate of disappearance of TTA+• allowed determination of the rate constant for the cross reaction (vide inf ra) to be 1.1 × 106 M−1 s−1. Considering literature values for the one electron amine cation radical/ amine reduction redox potentials, reduction of TTA+• by TEA should be endergonic by more than 0.2 V (>19 kJ/mol).35,39 The reaction sequence following formation of TEA+• is shown below and involves either deprotonation to yield an amine α carbon radical or H atom abstraction from solvent to yield a triethylammonium cation and an acetonitrile radical (which will likely dimerize). TTA+• + TEA ⇌ TTA + TEA+•

Figure 2. Transient absorbance changes at 510 nm following excitation at 450 nm of [Ru(LL)3]2+ (LL: (a) bpy; (b) 5-Cl-phen; (c) dpp.) + 0.04 M TTA + different concentrations of [TEA] (T = 296 K). (TEA concentrations: black: 0 M; dark blue: 0.16 M; light blue: 0.26 M; green: 0.37 M; orange: 0.45 M; purple: 0.55 M; red: 0.73 M.).

kcr , k −cr

(2)

TEA+• + TEA → TEAH+ + Et 2NC•HCH3

(3)

TEA+• + CH3CN → TEAH+ + •CH 2CN

(4)

+

+•

2+

[Ru(LL)3 ] + TTA → [Ru(LL)3 ]

+ TTA

kb

(5)

The key questions in determining the feasibility of using reactions 2−4 to remove the TTA+• from the system are (a) whether the reaction of TEA with TTA+• can compete with back electron transfer and (b) whether reactions 3 and 4 compete effectively with the energetically favorable reverse of reaction 2. To address the first point, it is useful to consider the half-life for the recombination reaction (eq 5). The reactants are formed in equal concentrations and the half-life will then depend on the initial concentration of ions formed in laser excitation. Typical concentrations in these experiments range from 20 to 60 μM, yielding half-lives of 4−12 μs (see Figure 1). With the cross reaction rate of 1.1 × 106 M−1 s−1, TEA concentrations of >0.1 M are necessary for reaction 2 to compete with the back reaction (eq 5) to yield significant concentrations of [Ru(LL)3]+.

TTA+• at 668 nm (black curves in Figure 3) in transient spectra. The back electron transfer follows equal concentration second-order kinetics and the rate constants, kb, are listed in Table 1. The charge recombination rates are nearly diffusion limited and there is little evidence of side reactions of either radical ion; thus the produced reducing equivalents are not persistent if the charge separated TTA+• cannot be excluded from the system. Addition of the sacrificial donor TEA at high concentration results in the rapid decay of TTA+• and leaves the majority of [Ru(bpy)3]+ as a strong reductant in solution. In the particular case shown in Figure 1b, with 0.55 M TEA, approximately 80% C

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To address the second question, the rate of the back reaction of TTA and TEA+• must be compared to the rates of proton loss and H atom abstraction from the solvent by TEA+•. If the cross electron transfer reaction is indeed endergonic by 0.2 V, the equilibrium constant will be approximately 5 × 10−4 for eq 2 and, given the measured rate for reaction of TTA+• with TEA at room temperature, the reverse reaction rate constant should be around 2 × 109 M−1 s−1. This also means that, with a typical TTA concentration of 0.04 M, the reverse reaction half-life will be less than 10 ns. Since Figure 1 clearly indicates that TEA is an effective sacrificial agent for trapping TTA+•, but the regeneration of TTA+• occurs on the nanosecond time scale, it must be the case that one or both of the reactions of TEA+• occur on a time scale that is fast relative to this regeneration process. While the processes of reactions 3 and 4 are discussed in the literature,27 they are linked to the detection of reaction products stemming from Et2NC•HCH3 (eq 3) or •CH2CN (eq 4). Rate constants for formation of these species are not readily available. In other systems, where TEA can be used directly to quench excited Ru(II) diimine complex chromophores, transient spectroscopic studies clearly show that essentially no back reaction occurs between the charge separated radical ions, indicating that decomposition of TEA+• is on the submicrosecond time scale.40 It is possible, however, that the free energy for the reaction of TTA+• and TEA estimated from reported electrochemical potentials is incorrect. Oxidation of TEA is electrochemically irreversible, and literature values of E0(TEA+•/TEA) are estimates based upon fitting of voltammograms.27,35,41,42 However, in essentially every literature example, the TEA+• reduction potential is more positive than the TTA+• potential. Rate Constants for Electron Transfer between TTA+• and TEA. As stated above, decays of TTA+• (λmax = 668 nm) become gradually faster with increasing concentrations of TEA, as shown in Figure 3. At the same time, the absorbance of [Ru(LL)3]+ at 510 nm (Figure 2) reflects less recombination with TTA+• as TEA concentrations increase. The photoproduced strongly reductive [Ru(LL)3]+ is stable for more than 100 μs in nitrogen degassed systems. In order to obtain the rate constants for electron transfer from TEA to TTA+•, in competition with the charge recombination reaction (eq 5), a kinetic analysis of the parallel reactions 2 and 5 was carried out. The decay of [Ru(LL)3]+ obeys equal concentration second order kinetics in the absence of TEA (kb, eq 5). The decay of TTA+• at 668 nm is the result of the parallel reactions with [Ru(LL)3]+ and TEA. A discussion of the kinetic analysis and fitting procedure is given in the Supporting Information, and the data fitting results are illustrated in Figure 4. The rate constant kcr for forward exchange electron transfer reaction shown in eq 2 at 296.5 K is 1.1 × 106 M−1 s−1. The value of kcr calculated from the three systems using different [Ru(LL)3]2+ chromophores is the same, within experimental error. Compared with the back electron transfer reaction between [Ru(LL)3]+ and TTA+•, reduction of TTA+• by TEA dominates with TEA concentrations above about 0.3 M. Since the bimolecular of the back electron transfer competes with the pseudo first order reaction of TTA+• with TEA, TEA trapping of TTA+• will be more efficient at lower irradiation intensities for any given fixed concentration of TEA. This is an important point, since steady state photolysis involves light sources with much lower intensities than those of pulsed laser experiments and the steady state concentrations of TTA+• will be lower.

Figure 4. Transient absorbance changes with fitted curves at 668 nm following excitation at 450 nm of [Ru(LL)3]2+ + 0.04 M TTA + different concentrations of [TEA] (T = 296.5 K). (TEA concentrations: blue: 0.16 M; blue green: 0.26 M; green: 0.37 M; orange: 0.45 M; purple: 0.73 M. Scattered: experimental data; red lines: fitted curves).

We also explored the temperature dependence of the cross electron transfer reaction (eq 2) to obtain activation parameters for the process. Figure 5 summarizes the temperature

Figure 5. kcr′ vs [TEA] under different temperatures (kcr′ = kcr × [TEA]). Inset is the Eyring−Polanyi plot for reaction 2.

dependent changes of kcr′ with different concentrations of TEA. The fitted plot of ln(k/T) vs 1/T, shown in the inset of Figure 5, gives a straight line with slope (−ΔH‡/R) and intercept [ln(κB/h) + ΔS‡/R] according to the linear form of the Eyring−Polanyi equation. ΔS‡ and ΔH‡ are estimated to be −35.2 J mol−1 K−1 and 27.7 kJ mol−1, respectively. The TTA+• and TEA form an encounter radical cation complex as an intermediate of reaction 2. The observed barrier to electron transfer corresponds to the energy required to distort the structures of the neutral and cation intermediate. The large negative entropy for the process indicates that the distortion associated with the process is significant. Quantum Efficiency for Production of Reducing Equivalents. The overall quantum efficiency (Φred) of the photolysis system containing the [Ru(LL)3]2+ chromophore, TTA and the sacrificial donor results from yields of light absorption, quenching, charge separation, and reduction of TTA+• by TEA, as shown in Table 2.43 The relatively high overall efficiencies are attributed to the combination of efficient quenching (ηq), charge separation (ηcs) and effective reduction of TTA+• by TEA (ηr), as interpreted in eq 6. Here F is simply the fraction of incident photons absorbed. The fraction of excited states quenched is obtained directly from the Stern− D

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factors in determining the yield of reducing equivalents. TPA, known to undergo dimerization following one electron oxidation, is ineffective as a trap of TTA+•. This is very likely due to the fact that the dimerization process requires reaction of two low concentrated radical cations, a process that has the same kinetic limitations as the back electron transfer between [Ru(LL)3]+ and TTA+•. This, coupled with the fact that the TTA+• /TPA thermodynamic barrier is also endergonic by nearly 0.2 V, severely limits the reducing ability of TPA.39 Tertiary alkyl amines are known to undergo hydrolysis after oxidation to yield aldehydes and secondary amines.41 This further decomposition also contributes to the decreased likelihood of reaction of [Ru(LL)3]+ with the accumulating amine oxidative decomposition products, since reduction potentials of aliphatic aldehydes are known to be quite negative.42 Another potential sacrificial amine class is 1,2diamines. The cation radicals are known to react by cleavage of the relatively weak C−C bond between the two nitrogen centers.44,45 TMEDA exhibits characteristic sacrificial electron donating ability in reaction with TTA+• and persistent production of [Ru(bpy)3]+ as illustrated in Figure 6. The fitted

Table 2. Yields of Each Step and the Overall Quantum Efficiencies

[Ru(bpy)3]2+ [Ru(Cl-phen)3]2+ [Ru(dpp)3]2+

ηqa (0.04 M TTA)

ηcsa

ηrb (0.55 M TEA)

Φred (F = 100%)

0.77 0.88 0.85

1.0 1.0 0.93

0.85 0.78 0.85

0.62 0.69 0.72

a

Details about determination of quenching and charge separation yields are in the Supporting Information. bηr was calculated from the ratio of [Ru(LL)3]+ that remains after full consumption of TTA+• by TEA to the initial amount of [Ru(LL)3]+ after charge separation.43

Volmer quenching ratio for any given TTA concentration (Supporting Information). Φred = F × ηq × ηcs × ηr × 100%

(6)

Charge separation yields were determined from the ratio of the concentration of radical ions formed to the concentration of excited states formed in pulsed laser excitation, corrected for the fraction of the excited states that are quenched. The concentration of excited states formed is determined from the absorbance of [Ru(LL3)]2+ excited state at 360 nm (the absorptivity of [Ru*(bpy)3]2+ is reported in the literature and the absorptivity of the other complexes was obtained relative to [Ru*(bpy)3]2+).17 The concentration of radical ions formed can be determined from the absorbance of either [Ru(LL)]+ at the absorbance maximum near 510 nm or the absorbance of the TTA+• at 670 nm (ε = 22 000 M−1cm−1).38 In CH3CN solutions, charge separation yields were found to be nearly 1.0 for the three sensitizers used with TTA as the reductive quencher. Very high charge separation yields are common for Ru(II) diimine complexes in photoinduced electron transfer reactions with aromatic amines and these are no exception. Having generated the reducing equivalents at the charge separation junction with an efficiency between 75% and 85%, the biggest loss at this particular quencher concentration being incomplete quenching of all the excited states with the reductive quencher, what is the efficiency of the cross reaction to yield the rapidly decomposing TEA+•? Cross reaction efficiencies are reported in Table 2 for the specific case of 0.55 M TEA. The value was determined from the concentrations of [Ru(LL)3]+ formed in the charge separation process (i.e., at t ∼ 500 ns) and the remaining [Ru(LL)3]+ following complete disappearance of TTA+• when the concentration of [Ru(LL)3]+ on the microsecond time scale is invariant (as in Figure 2). The values are around 80% for all three chromophores studied and increasing the TEA concentration results in only marginal increases in the resulting yields of [Ru(LL)3]+. Overall yields for generating [Ru(LL)3]+ range from 60 to 70%, assuming absorption of 100% of the incident photons and concentrations of TTA and TEA of 0.04 and 0.55 M, respectively. It is perhaps worth noting at this point that, with a fairly well optimized homogeneous charge separation system of this sort, operating with narrow bandwidth excitation under sacrificial conditions, the yield of ions approaches the yields of electrons per photon obtained in dye-sensitized solar cells operating under fully reversible conditions. In this case the stored energy is approximately 1.3−1.4 V, tied up in the powerful reducing agent produced. Other Amines as Sacrificial Electron Donors. In screening the sacrificial electron donors, their redox potentials and irreversible decompositions after oxidation are crucial

Figure 6. Transient absorbance changes at (a) 510 nm and (b) 668 nm (with fitted curves) following excitation at 450 nm of [Ru(bpy)3]2+ + 0.04 M TTA + different concentrations of [TMEDA] (T = 296.5 K). (For a, TMEDA concentrations: black: 0 M; blue: 0.27 M; blue green: 0.36 M; fuchsia: 0.63 M; gray: 0.92 M.) (For b, TMEDA concentrations: blue: 0.27 M; blue green: 0.36 M; orange: 0.52 M; purple: 0.71 M; gray: 0.92 M. Scattered: experimental data; red lines: fitted curves.) Inset shows k2 from fitted curves in (b) vs [TMEDA].

curves in Figure 6b give the pseudo first-order rate constant (kcr′) values that vary linearly with the concentrations of TMEDA (inset of Figure 6b), which yields a bimolecular rate constant for the cross reaction between TTA+• and TMEDA, kcr, of 2.4 × 106 M−1 s−1, roughly twice that of TEA. Interestingly, the yield of [Ru(LL)3]+ is actually slightly lower than that observed for TEA, possibly suggesting that the C−C fragmentation of the TMEDA+• is on the same time scale as reaction of the cation radical with TTA to regenerate TTA+•. Similar fragmentation has been reported for the cation radicals of aminoalcohols,46 and triethanolamine (TEOA) is frequently used as a sacrificial electron donor. All of these have the potential to replace TEA in this system. However, laser flash photolysis of TEOA in this system proved it to be a much less efficient reductant for TTA+• (Figure S6); the rate constant for the cross electron transfer in this system is calculated to be only E

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order and pseudo first-order reactions, allows determination of electron transfer rate constants for the cross electron transfer reactions between the sacrificial electron donor and the amine cation radical. Using this method, different amines were examined as potential sacrificial electron donors, and it turned out that tertiary 1,2-diamines are most efficient for trapping the intermediate amine cation radical. The overall effectiveness of any sacrificial donor is a combination of its ability to trap the intermediate cation radical (here TTA+•) and undergo decomposition on a time scale that is fast relative to reaction with the reduced chromophore. In the series of sacrificial amines examined in this work, TEA proved to have the optimal combination of rapid reaction with TTA and rapid decomposition of TEA+• subsequent to reaction with TTA.

one tenth that of TEA and likely reflects the inductive effect of the hydroxyl of TEOA on its oxidation potential. Primary amines like PA and EDA were also tested in this photolysis system. The rapid decay of TTA+• at 668 nm with either PA or EDA in transient absorption spectra in Figure 7b

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ASSOCIATED CONTENT

S Supporting Information *

Supporting Information contains luminescence quenching, charge separation yield determination, derivation of the expression of TTA+• decay, and transient absorption spectra with EDA or TEOA as sacrificial electron donors. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 7. Transient absorbance changes at (a) 510 nm and (b) 668 nm following excitation of [Ru(bpy)3]2+ + 0.04 M TTA + different concentrations of [PA] (T = 296 K) (PA concentrations: black: 0 M; blue: 0.46 M; green: 0.90 M; orange: 1.30 M).

AUTHOR INFORMATION

Corresponding Author

*Tel: +1(504) 862-3587. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

for PA (Figure S7b for EDA) clearly illustrates the feasibility of reduction of TTA+• by these two amines. However, both PA+• and EDA+• are rereduced by [Ru(bpy)3]+ before their degradation. This is observed directly from the decay of [Ru(bpy)3]+ at 510 nm, shown in Figure 7a (Figure S7a for EDA+•) for reaction with PA+•. A second implication from the combined decays of the TTA+• and the [Ru(bpy)3]+ is that the PA+• must back react with TTA at a rate that is slow relative to the reaction of PA+• with [Ru(bpy)3]+, otherwise the decays of TTA+• and [Ru(bpy)3]+ would not be affected by the presence of PA. This further suggests that PA+•/PA reduction potential is negative relative to the TTA+•/TTA potential so that the equilibrium position favors PA+•. The observations of [Ru(bpy)3]+ oxidation by PA+• and EDA+• illustrate that [Ru(bpy)3]+ generated in a photoredox reaction of this sort can serve as an indirect probe for the rates of decomposition of amine cation radicals generated in the cross reaction. A viable sacrificial electron donor for this photosystem needs to have a decomposition rate that is rapid relative to the reaction with [Ru(bpy)3]+.

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ACKNOWLEDGMENTS The authors wish to thank the U.S. Department of Energy, Office of Chemical Sciences (Grant DE-FG-02-96ER14617) for support of this research.

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REFERENCES

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SUMMARY This work illustrates a modified approach for employing photolysis of a system containing a reversible quencher and a sacrificial electron donor to generate strongly reducing equivalents in solution. The key to the system is the observation that trapping of an intermediate amine radical cation, formed efficiently from electron transfer quenching with appropriate chromophores, can be accomplished in high yield via an energetically uphill process involving oxidation of a sacrif icial electron donor, as long as the process can compete kinetically with back electron transfer between the reduced chromophore and the oxidized amine. Kinetic analysis of these parallel competing reactions (back electron transfer and trapping), consisting of initially equal concentration secondF

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The Journal of Physical Chemistry A

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