Electron-Transfer Reactions of Electronically Excited Zinc

Dec 14, 2014 - Transient absorption decay rate constants (kobs) for reactions of electronically excited zinc tetraphenylporphyrin (3ZnTPP*) with triru...
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Electron-Transfer Reactions of Electronically Excited Zinc Tetraphenylporphyrin with Multinuclear Ruthenium Complexes Jane Henderson,† Starla D. Glover,†,§ Benjamin J. Lear,†,∥ Don Walker,‡ Jay R. Winkler,‡ Harry B. Gray,‡ and Clifford P. Kubiak*,† †

Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman MC 0358, La Jolla, California 92093, United States ‡ Beckman Institute, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: Transient absorption decay rate constants (kobs) for reactions of electronically excited zinc tetraphenylporphyrin (3ZnTPP*) with triruthenium oxocentered acetate-bridged clusters [Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)]2(μ-pz), where pz = pyrazine and L = 4-cyanopyridine (cpy) (1), pyridine (py) (2), or 4-dimethylaminopyridine (dmap) (3), were obtained from nanosecond flash-quench spectroscopic data (quenching constants, kq, for 3ZnTPP*/1−3 are 3.0 × 109, 1.5 × 10 9, and 1.1 × 109 M−1 s−1, respectively). Values of kq for reactions of 3ZnTPP* with 1−3 and Ru3(μ3-O)(μCH3CO2)6(CO)(L)2 [L = cpy (4), py (5), dmap (6)] monomeric analogues suggest that photoinduced electron transfer is the main pathway of excited-state decay; this mechanistic proposal is consistent with results from a photolysis control experiment, where growth of characteristic near-IR absorption bands attributable to reduced (mixedvalence) Ru3O-cluster products were observed.

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over.4−9 The tunability of ΔG0, in particular, has found widespread use in the study ET rates.10−14 In inorganic mixedvalence chemistry, the energy levels of the metal donor and acceptor can be adjusted through synthetic modification and inductive effects. In this work, we have modulated the electrochemical reduction potentials of multinuclear ruthenium clusters by choice of ancillary ligands. This has the effect of controlling the driving force of their ET reactions with electronically excited zinc tetraphenylporphyrin. The strong electronic coupling observed in pyrazine (pz) bridged dimers of triruthenium clusters 1−3 (Figure 1) has been characterized within the Marcus−Hush theory of mixed valency.15−19 Singly reduced systems, 1−−3−, form stable [Ru3III,III,II-pz-Ru3III,II,II]− mixed valence ions, and ancillary ligand substitution has been shown to affect the ground-state coupling across the Ru3O-pz-Ru3O scaffold. The relative degree of electronic coupling can be estimated from electrochemical measurements, where the voltage separation between subsequent reductions in cyclic voltammograms yields a comproportionation constant, Kc, with values that range from ca. 104 for L = 4-cyanopyridine (cpy) (1) to 106 for pyridine (py) (2) to 107 for 4-dimethylaminopyridine (dmap) (3). This variation in stability of the mixed-valence state has been correlated with the electron-donating/withdrawing nature of the ligand and the

INTRODUCTION Excited-state electron transfer (ET) is the foundation of both natural and artificial solar energy conversion. In all cases, the rate of ET from the excited-state molecule to an electron acceptor molecule is a crucial parameter that must be carefully tuned for optimal efficiency. For instance, if the rate of ET is too fast, this can result in damage to the photoconversion system, while ET that is too slow places limits on the overall power of the energy conversion system. Thus, it is clear that control over excited-state ET is an important consideration in the design of efficient optical to chemical energy conversion systems. Here we consider a new way to control intermolecular excited-state ET: through the use of an electron acceptor capable of forming a strongly coupled mixed-valence state. The rate constant for ET (kET) is generally considered within the semiclassical theory of outer-sphere ET, eq 1, as1−3 kET =

π |V |2 exp[−(ΔG° + λ)2 /4λkBT ] ℏ λkBT 2

(1)

Here the pre-exponential factor includes the electronic matrix element, V, a function of the electronic overlap of the electron donor and acceptor orbitals. In most intermolecular ET reactions, |V|2 values are large enough that ET reactions occur but small enough that the energy of interaction is still negligible compared with the energy barrier of ET. In the limit of such small coupling, the rate of outer-sphere ET is determined by the driving force (ΔG0) and the reorganization of the system (λ). The relationship between kET, λ, and ΔG0 shown in eq 1 has been experimentally verified many times © XXXX American Chemical Society

Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: November 9, 2014 Revised: December 11, 2014

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Figure 1. Dimer of triruthenium clusters of the form [Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)]2(μ-pz), where pz = pyrazine and L = 4-cyanopyridine (cpy) (1), pyridine (py) (2), and 4-dimethylaminopyridine (dmap) (3), and monomer triruthenium clusters of the form Ru3(μ3-O)(μCH3CO2)6(CO)(L)2, where L = cpy (4), py (5), and dmap (6) used to study excited-state electron transfer from 3ZnTPP* to redox-active Ru3O complexes. Single-electron reduction of 1−3 forms the mixed-valence [Ru3III,III,II-pz-Ru3III,II,II]− species of interest.

electronic delocalization of the mixed-valence ion.20 The stabilization of the mixed-valence state also shifts the reduction potential to values more positive than would be expected in the absence of coupling. Thus, the stability that results from electronic coupling provides another way to fine-tune the redox chemistry of these systems. Trends in ligand-dependent mixed-valence properties have been further studied by analysis of the intervalence charge transfer (IVCT) bands in the near-IR of the singly reduced dimers, allowing these systems to be characterized as Robin Day Class II (1−) to Class II/III borderline (3−) (Figure 2).18 Further, ν(CO) band coalescence in the infrared (IR) spectrum of the mixed-valence state depends on the ancillary ligand and allows dynamics of nearly barrierless intramolecular ET on the picosecond time scale to be observed.15−18,21−25 Although the respective energetics of the mixed-valence ions have been well-

characterized, the chemistry of the ET reactions that produce these strongly coupled systems has not yet been studied. Complexes 4−6 represent the corresponding monomeric units, comprising the redox centers involved in the mixed valence dimers 1−3, allowing the two types of systems to be compared.26 Because the energetics of the singly reduced mixed-valence states are strongly dependent on the nature of the ancillary ligands, we sought to explore the ability to mediate excited-state ET via ligand substitution and study the kinetics and thermodynamics of the ET reactions. In the present study, we examined intermolecular photoinduced ET from the triplet excited state of zinc tetraphenylporphyrin (3ZnTPP*) to [Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)]2-pz, where L = 4-cyanopyridine (cpy) (1), pyridine (py) (2), and 4-dimethylaminopyridine (dmap) (3), and Ru3(μ3-O)(μ-CH3CO2)6(CO)(L)2, where L = cpy (4), py (5), and dmap (6). Upon photoexcitation, the singlet excited state (1ZnTPP*) was generated and undergoes rapid intersystem crossing to give the excited-state triplet (3ZnTPP*).28 In the presence of 1−6, the 3ZnTPP* decay rate constants increased linearly with concentration of ruthenium complexes, and the bimolecular quenching rate constant (kq) of 3ZnTPP* with each species was determined. The correlation of kq with ancillary ligand substitution effects on Ru3O acceptor energy levels and electronic coupling between donor and acceptor suggests that photoinduced ET is the main pathway of excited-state decay in dimers 1−3. Evidence of ET was confirmed under continuous photolysis of the donor/acceptor system in the presence of a sacrificial reductant, by observation of the singly reduced mixed-valence species 1− and 2− in the near-IR region.

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EXPERIMENTAL SECTION Materials. Complexes 1−6 were synthesized according to the literature.21,29 Acetonitrile (CH3CN) was used as received from Fisher Scientific. Zinc tetraphenylporphyrin (ZnTPP) was purchased from Sigma-Aldrich and used without further purification. Tetra-n-butylammonium hexafluorophosphate (TBAPF6) was received from Sigma-Aldrich, recrystallized

Figure 2. Electronic absorption spectra of dimer 2n in acetonitrile for n = 0 (black) and n = −1 (blue). The singly reduced mixed valence dimer 2− shows characteristic IVCT bands in the near-IR. Spectra were collected in acetonitrile at room temperature, and the mixedvalence species was generated by the stoichiometric addition of potassium graphite (KC8) as chemical-reducing agent.18,27 Reproduced from earlier work. B

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Figure 3. (A) 3ZnTPP* decay (λpump = 555 nm, λprobe = 470 nm) in the presence of increasing concentrations of 2. (B) Plot of the lifetime τ (right vertical axis) versus the concentration of 2. Stern−Volmer plot shows linear dependence of the single exponential decay rate constant, kobs (left vertical axis), versus concentration, and the bimolecular collisional quenching rate constant of 3ZnTPP* in the presence of 2, kq, was derived from the slope of the linear fit.

of 50 μM complex, which was then filtered directly into the cuvette. 250 μL of a filtered solution of 5 mM BNAH and 1 M TBAPF6 in CH3CN was added and the cuvette was sealed. An initial (pre-exposed) UV−vis/NIR spectrum was collected prior to photolysis performed using a 532 nm (40 mW) temperatureregulated laser diode (ThorLabs LDC200CV/TED200C), and samples were irradiated for 120 min with constant stirring.

from absolute ethanol, and dried under vacuum at 100 °C for 24 h. Air-sensitive reactions were carried out under a N2 inert atmosphere in CH3CN that had been passed through two alumina drying columns. 1-Benzyl-1,4-dihydronicotinamide (BNAH) was purchased from TCI and used as received. Flash-Quench Photolysis. UV−vis/NIR absorption spectra were collected with a Shimadzu UV 3600. The laser system employed for transient absorption measurements has been described elsewhere.8 All kinetics measurements were collected on solutions of 1.0 × 10−5 M ZnTPP in CH3CN that had been degassed by a minimum of three freeze−pump−thaw cycles. Excited-state 1ZnTPP* was generated by excitation at 555 nm. Decay curves of 3ZnTPP* were measured at 470 nm. A total of 105 data points were collected using an oscilloscope, which were then averaged over 103 scans. Decay curves in the absence of ruthenium complex were obtained for each system in this study. UV−vis and decay measurements were repeated for increasing concentrations of 1−6, typically between 0 and 6 × 10−5 M. Triplet excited-state decay fits were generated by IgorPRO Version 6.34A. Electrochemical Measurements. Cyclic voltammetry was performed using a BAS CV-50 computer-controlled potentiostat with a scan rate of 100 mV/s. A 3 mm platinum electrode was used as the working electrode. The counter electrode was a platinum wire, and the reference electrode was a Ag/AgCl wire. Electrochemical solutions were prepared as 3 mM solution of complex in 0.1 M TBAPF6 CH3CN electrolyte solution degassed with N2. All potentials are reported versus the ferrocene/ferrocenium (Fc/Fc+) couple, used as an internal reference for each sample (FeCp2+/0 = 0.380 V vs SCE in acetonitrile). Continuous Photolysis. Samples were prepared under a N2 atmosphere. A 1 cm × 1 cm quartz cuvette (Spectrocell, P/ N RF-4010-T) containing a mico-stirbar was prepared to prevent light exposure prior to photolysis. A solution of 90 μM ZnTPP in 50 mL of CH3CN was used to dilute a 2 mL solution

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RESULTS AND DISCUSSION ZnTPP* Decay Kinetics. Excited-state 1ZnTPP* was generated by excitation at 555 nm in CH3CN and was rapidly converted to the 3ZnTTP*. Transient absorption at 470 nm, diagnostic for the 3ZnTPP* excited state, was monitored as a function of time following laser excitation.28,30 Figure 3 shows a set of difference absorption decay curves of 3ZnTPP* in the presence of 2 from 0 to 4.17 × 10−5 M. Decay lineshapes shown are typical of those obtained for 3ZnTPP* in the presence of the ruthenium complexes shown in Figure 1 and correlate to published results.9,30 Under the condition that [3ZnTPP*] ≪ [Ru3O], the decay curves of 3ZnTPP* obey pseudo-first-order kinetics and were fit to a single exponential decay function. In the presence of complexes 1−6, the 3 ZnTPP* decay rate constant, kobs, continually increased as the concentration of ruthenium complex increased. The time constant for 3ZnTPP* decay in the absence of Ru3O (τ0 = 1/ kobs) was < 100 μs, which includes the effects of bimolecular ZnTPP triplet−triplet annihilation.28 The excited-state decay rate constants increase linearly with the concentration of 1−6, and the slopes of the pseudo-firstorder plots yielded second-order rate constants (kq) for bimolecular quenching of 3ZnTPP* by the triruthenium complexes. The linear response of kq to quencher concentration is observed in all data sets and allows us to determine that this quenching is the result of a dynamic interaction between 3 ZnTPP* and the ruthenium clusters, defined by the Stern− C

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The Journal of Physical Chemistry B Volmer relation.14 The kq values for 1−6 were evaluated to be on the order of 109 M−1s−1 in all but one case, as listed in Table 1.

thermodynamically favorable ET from 3ZnTPP* to the Ru3Opz-Ru3O dimers. The trends observed in ancillary ligand substitution effects on kq and ΔG0ET across the dimer series (1− 3) are consistent with the general physical characteristics of these complexes previously reported.21,30,33 As the donor strength of the ancillary pyridine ligands increases from cpy (pKa = 2) to py (pKa = 5.1) to dmap (pKa = 9.2), the Ru3O core experiences an inductive effect, and the dπ-based energy levels of the core are raised. This is observed electrochemically, where shifts to more negative reduction potentials values are measured with more electron-donating ligands, E0red of dmap (3) > py (2) > cpy (1) (Figure 4, left). The data for 1−3 in 0 Table 1 show that as Ered becomes more negative, ΔG0ET becomes less favorable and kq decreases. The experimentally observed trend in the kq thus follows the order predicted by ET theory, that is, 1 > 2 > 3 (Figure 4, right). The rate of energy transfer is a function of the integral of the spectral overlap between the ZnTPP donor emission and the Ru3O acceptor absorption. The spectral overlap is significant for these complexes (Supporting Information), and thus Förster energy transfer could be a viable mechanism of 3ZnTPP* decay. Förster analysis of the spectroscopic data for 1−3 would lead to the expectation that the rate of energy transfer (as a function of the molar extinction coefficient of the acceptor) should increase through the series 3 > 1 > 2. Because this trend was not observed in experimentally measured kq values, we conclude that while energy transfer may occur, it is not the primary mechanism of 3ZnTPP* decay in the presence of 1−3. In nonadiabatic ET reactions, kET depends on the coupling strengths between reactants and products at the transition state (eq 1). We have previously shown that intermolecular selfexchange reactions in complexes 4−6 were facilitated by electronic coupling resulting from orbital overlap of the ancillary pyridyl ligands.22,26 The more electron-withdrawing ancillary ligands delocalize a greater portion of the LUMO onto the pyridine ring, which increases orbital overlap between donor and acceptor, while electron-donating ligands deplete LUMO density on the pyridyl rings. Rates of self-exchange increased through the series 6 < 5 < 4, demonstrating the importance of ancillary ligand-dependent electronic coupling via orbital overlap in intermolecular electron-transfer when ΔG0ET = 0. In the present study, we observed the same trends for rate constants of 3ZnTPP* quenching with increased donor strength of the pyridine ligands for both the dimers 1−3 and

Table 1. Collisional Quenching Constants (kq), Electrochemical Reduction Potentials (E0red), and Thermodynamic Results of Free Energy for Electron Transfer from 3ZnTPP* (ΔG0ET) in Acetonitrile at 25 °C kq (× 109 M−1s−1)a

E0red (V)b

ΔG0ET (eV)c

3.0 1.5 1.1 1.5 1.2 0.08

−0.90 −0.98 −1.02 −1.06 −1.28 −1.49

−0.31 −0.23 −0.19 −0.15 0.07 0.28

1 2 3 4 5 6 a

Stern−Volmer derived bimolecular quenching constants for the observed excited state quenching of 3ZnTPP* with 1−6. bElectron reduction potentials versus Fc/Fc+. These potentials correspond to E1/2(0/−1) Ru3III,III,II-pz-Ru3III,III,II/Ru3III,III,II-pz-Ru3III,II,II for 1−3 and Ru3III,III,II/Ru3III,II,II for 4−6. cFree energy change for photoinduced + electron transfer from ZnTPP (E0/+ ox = 0.38 V vs Fc/Fc in MeCN). 3 Excited-state energy (EZnTPP*) of ZnTPP* is 1.53 eV.

Quenching of 3ZnTPP* Lifetime with 1−6. In principle, the quenching of 3ZnTPP* by 1−6 could occur by energy transfer, ET, or both. In the case of photoinduced ET from 3 ZnTPP*, the reaction would proceed by oxidative quenching to produce ZnTPP+ and a reduced Ru3O− complex. The standard free-energy change for ET from the excited state of Zn porphyrin is determined using eq 231 0 0 0 ΔG ET = −e(Ered − Eox ) − EZnTPP * − C

E0ox

(2)

E0red

Here and are the half-wave potentials for the donor oxidation and acceptor reduction, respectively, and EZnTPP* is the excited-state energy. When the reactants are neutral, the Coulombic stabilization (C) in the ET pair (A•−/D+) is estimated as (e2/εa) ≈ 0.06 eV in CH3CN.32 Using the + ZnTPP0/+ potential (E0/+ ox = 0.38 V vs Fc/Fc in CH3CN) and the reduction potential of the ruthenium metal cluster complex (Table 1), the free-energy changes for photoinduced ET (ΔG0ET) from 3ZnTPP* to 1−6 were estimated and are presented in Table 1. For complexes 1−3, excitation of ZnTPP provides sufficient energy to drive charge separation, allowing

Figure 4. Ancillary ligand effect on electrochemical reduction potential (left) and quenching rate constant (right) for 1− 3. Correlation with pyridyl ligand electronics is described by respective pKa values. D

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Figure 5. Difference near-IR absorbance of 50 μM 1 and 2 in CH3CN solvent with 0.1 M TBAPF6 and 0.50 mM sacrificial donor (BNAH) following photolysis with 532 nm light.

energy for the more electronically coupled ion 2−. The observation of a strongly coupled mixed-valence state following photoexcitation allows us to confirm photoinduced intermolecular ET from 3ZnTPP*. The lack of significant changes in absorbance for 3 is not surprising given the predicted freeenergy change for the ET reaction is smaller than that for the other dimers.

the monomers 4−6. This is consistent with the expected trend in electronic coupling based on the degree of LUMO character on the peripheral pyridine ligands of the Ru3O electron acceptors and the trend in ΔG0ET. However, in complexes 5 and 6, the standard free-energy changes for ET (ΔG0ET) calculated from eq 2 are uphill by as much as 0.28 eV. As previously discussed, energy transfer may provide another mechanism for the decay of 3ZnTPP* in the presence of 1−6. The triplet energy level of analogous [Ru3(μ3-O)(μ-CH3CO2)6(L)3]+ complexes has been measured to be 1.48 eV, suggesting that ET via 3ZnTPP* (E0 = 1.53 eV) may take place competitively with the more thermodynamically favorable energy-transfer process in systems 5 and 6.30 In complexes 1−3, there is sufficient driving force to allow thermodynamically favorable ET in each case. Photoinduced ET via Continuous Excitation. Following an intermolecular ET from photoexcited 3ZnTPP* to 1−3, + BNAH (E0/+ ox = 220 mV vs Fc/Fc ) was used as a sacrificial donor to reduce the resulting porphyrin cation.34 This reaction is predicted to inhibit back-ET (e.g., from a reduced ruthenium complex to ZnTPP+) following photoinduced ET to generate the stable mixed-valence dimer of interest through regeneration of neutral ZnTPP. The cell was exposed to 532 nm light (40 mW), with stirring, and spectra were collected over the course of 2 h. Figure 5 shows the changes in absorbance in the near-IR of 1 and 2 following photolysis (3 can be found in Figure S4 in the Supporting Information). After 10 min of continuous photolysis, absorbance at λ > 670 nm becomes nonzero. During the course of the experiment, the near-IR spectrum of 1 evolves into two well-defined peaks (λmax/nm = 923 and 1305). Similar results were observed for 2 with small variation in peak position (λmax/nm = 909 and 1383). No significant change in absorption in this region was observed for 3. The singly reduced (mixed-valence) dimer is generated following photoexcitation of ZnTPP. As previously described, the bands observed in the near-IR can be clearly identified as the IVCT transitions of the mixed-valence [Ru3III,III,II-pzRu3III,II,II]− dimers. These IVCT transitions are characterized as metal-to-bridge charge transfer (MBCT, high energy) and metal-to-metal charge transfer (MMCT, low energy), where peak energy and intensities indicate the strong electronic coupling resulting from electron delocalization across the dimer. Significantly, the intrinsic and respective peak energies show the previously described trends in the three-state model description of two IVCT transitions, where the MBCT band shifts to higher energy and the MMCT band shifts to lower

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CONCLUSIONS

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ASSOCIATED CONTENT

We have measured the bimolecular quenching rate constants (kq) of the photosensitizing electron donor 3ZnTPP* in the presence of triruthenium cluster monomer and dimer electron acceptors. The correlation of kq with the thermodynamic driving force for ET from the excited-state porphyrin to the dimers (1−3) suggests that photoinduced intermolecular ET is the main pathway of 3ZnTPP* decay. kq values reveal the effects of ancillary ligands in adjusting the electron acceptor energy level and electronic coupling facilitating ET in the presence of 1−3. Intermolecular photoinduced ET 3ZnTPP* to Ru3O-pz-Ru3O was confirmed by the observation of IVCT transitions of the singly reduced mixed-valence species 1− and 2− in the near-IR. Extensive electrochemical and spectroscopic investigations into the electronic structures of 1−−3− indicate that these complexes are strongly coupled mixed-valence complexes.15−18,21,22,26,35,36 In this study we have demonstrated the ability to generate a mixed-valence state with significant electronic delocalization from a photoinduced ET reaction. The evolution of delocalization following initial photochemical charge injection is predicted to occur significantly faster than the diffusion limit and could not be detected in this work. We have recently reported the synthesis of photoexcitable dimer dyads where the Zn porphyrin is coordinated to the Ru3O-pzRu3O scaffold, and future studies will aim to explore the time scale of delocalization observed in these strongly coupled mixed-valence systems.37

S Supporting Information *

Linear agreements of kobs and concentration for 1−6, ZnTPP fluorescence and 1−3 absorbance overlap, pKa dependence of Ered and kq of 4−6, and photolysis of 3. This material is available free of charge via the Internet at http://pubs.acs.org. E

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses §

S.D.G.: Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, SE75120 Uppsala, Sweden. ∥ B.J.L.: Department of Chemistry, Pennsylvania State University, 104 Chemistry Building, University Park, Pennsylvania 16802, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Jonas Petersson for his valuable collaboration. We also thank Professor Judy Kim for use of the Nd:YAG laser, Ignacio López-Peña for his spectroscopic support, and Dr. Brian Leigh for his continual enthusiasm. This work was supported by the National Science Foundation under Grants CHE-1145893 (C.P.K.) and CHE-1305124 (J.R.W., H.B.G.).

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