Ultrafast photodriven intramolecular electron transferfrom an iridium-based water-oxidation catalystto perylene diimide derivativesMichael T. Vagninia, Amanda L. Smeigha, James D. Blakemoreb, Samuel W. Eatona, Nathan D. Schleyb, Francis D’Souzac,Robert H. Crabtreeb, Gary W. Brudvigb, Dick T. Coa, and Michael R. Wasielewskia,1

aDepartment of Chemistry and Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, IL 60208-3113; bDepartment ofChemistry, Yale University, P.O. Box 208107, New Haven, CT 06520-8107; and cDepartment of Chemistry, University of North Texas, Denton, TX 76203

Edited by Thomas J. Meyer, University of North Carolina, Chapel Hill, NC, and approved April 4, 2012 (received for review February 3, 2012)

Photodriving the activity of water-oxidation catalysts is a criticalstep toward generating fuel from sunlight. The design of a systemwith optimal energetics and kinetics requires a mechanistic under-standing of the single-electron transfer events in catalyst activa-tion. To this end, we report here the synthesis and photophysicalcharacterization of two covalently bound chromophore-catalystelectron transfer dyads, in which the dyes are derivatives ofthe strong photooxidant perylene-3,4:9,10-bis(dicarboximide) (PDI)and the molecular catalyst is the Cp�IrðppyÞCl metal complex,where ppy ¼ 2-phenylpyridine. Photoexcitation of the PDI in eachdyad results in reduction of the chromophore to PDI•− in lessthan 10 ps, a process that outcompetes any generation of 3�PDI byspin-orbit-induced intersystem crossing. Biexponential charge re-combination largely to the PDI-Ir(III) ground state is suggestiveof multiple populations of the PDI•−-IrðIVÞ ion-pair, whose relativeabundance varies with solvent polarity. Electrochemical studies ofthe dyads show strong irreversible oxidation current similar to thatseen for model catalysts, indicating that the catalytic integrity ofthe metal complex is maintained upon attachment to the highmolecular weight photosensitizer.

photoinduced electron transfer ∣ solar fuels ∣ ultrafast optical spectroscopy ∣water oxidation

Artificial photosynthetic systems for solar fuels generationmust integrate the functions of light harvesting, charge se-

paration, and catalysis, with water as the source of electrons forreductive fuel-forming chemistry (1–5). The design of a light-driven water-splitting system based on molecular catalysts re-quires a fundamental understanding of the individual electrontransfer steps involved in multielectron catalyst activation. To in-vestigate the energetic and kinetic demands of coupling photo-driven charge separation and catalysis, many research groupshave studied the photophysical properties of covalently linked re-dox-active organic dyes and transition metal complexes (6–16). Inseveral cases, electron transfer to or from the metal has been ob-served and characterized, though energy transfer and intersystemcrossing to the chromophore triplet state can be significant com-peting processes. In this work, we use derivatives of perylene-3,4∶9,10-bis(dicarboximide) (PDI) linked to an iridium complexof the type Cp�IrðN-CÞX to demonstrate light-driven single-elec-tron oxidation of a highly active molecular catalyst precursor forwater oxidation.

Derivatives of PDI are useful organic chromophores for solardevice applications (5, 17, 18). Their high molar absorptivity,stability, low cost, ease of synthetic manipulation, and often ad-vantageous self-assembly properties have led to their use in mo-lecular electronics and a variety of solar energy conversionsystems (19). For solar fuels applications, the mild reduction po-tentials of PDI derivatives make their excited states powerfulphotooxidants, though there are only a few literature examplesin which 1�PDI is used to produce higher-valence states of a metalcomplex (8, 9, 16). Rybtchinski, Wasielewski, and coworkers have

shown that upon selective photoexcitation of PDI in a PDI-(bpy)Ru(II)-PDI complex, in which the bpy ligand is bound to two PDIunits through acetylide linkages at their aromatic cores, chargetransfer generates PDI-Ru(III)-PDI•− in <150 fs, and the vibra-tionally hot ion-pair exhibits fast relaxation (τ ¼ 3.9 ps) andcharge recombination (τ ¼ 63 ps) (8). In a side-to-face ruthe-nium porphyrin/PDI assembly studied by Iengo, Scandola,Würthner, and coworkers, in which two Ru porphyrins are coor-dinated to pyridyl groups at the PDI imide positions, chargetransfer (τ ¼ 5.6 ps) and recombination (τ ¼ 270 ps) are ob-served following PDI photoexcitation (16). Related work fromthe groups of Guldi, Torres, and Wasielewski has demonstratedelectron transfer with a ruthenium phthalocyanine/PDI assemblyin which the metal complexes are coordinated to the organic dyethrough pyridyloxy groups at its 1, 6, 7, and 12 positions (9).

PDI has a fluorescence quantum yield that is close to unity andthus a very low intrinsic triplet yield due to spin-orbit-inducedintersystem crossing (SO-ISC) (20). Nevertheless, the competi-tiveness of heavy atom-induced SO-ISC is a concern for PDI/me-tal complex assemblies, and previous studies by the Castellanogroup of PDI linked to platinum(II) metal centers through acet-ylide groups at its 1-position have reported SO-ISC from 1�PDI to3�PDI in 2–4 ps (7, 12). However, when Pd(II) is directly attachedto the PDI aromatic core through metal-carbon σ-bonds, strongPDI fluorescence is observed and the yield of 3�PDI is estimatedto be only 6%, which is explained by the relatively weak couplingbetween the metal-based orbitals and the frontier π-orbitals ofPDI (14). Also, Pd(II)-PDI and Pt(II)-PDI complexes in whichthe metal centers are bound through pyridyl groups at the PDIimide positions—through which the PDI frontier orbitals eachhave a nodal plane—have fluorescence quantum yields of 86%and 88%, respectively (15). Once again, the heavy-atom effect isdiminished when the electronic coupling of the metal to the chro-mophore is weak.

The Cp�IrðN-CÞX complex used in our studies, where ðN-CÞ ¼2-phenylpyridine and X ¼ chloride, is a highly active catalyst pre-cursor for water oxidation, with turnover frequencies on the orderof 10 turnovers per minute and activity that continues over hours(21, 22). It catalyzes water oxidation both electrochemically andwith cerium(IV) as a 1-electron oxidant, which suggests that cat-alyst activity could be driven with photoinduced electron transfer(23). In addition, the activity of the catalyst is homogeneous and

Author contributions: M.T.V., A.L.S., J.D.B., S.W.E., and D.T.C. designed research; M.T.V.,A.L.S., J.D.B., S.W.E., N.D.S., and D.T.C. performed research; M.T.V., A.L.S., J.D.B.,S.W.E., F.D., R.H.C., G.W.B., D.T.C., and M.R.W. analyzed data; and M.T.V. and J.D.B. wrotethe paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence should be addressed. E-mail: m-wasielewski@northwestern.edu.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1202075109/-/DCSupplemental.

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not simply a precursor to catalytically active iridium oxides, asdetermined by piezoelectric gravimetry during water oxidation(24). The parent Cp�IrðppyÞCl complex, which has an Ir(III) va-lence state, displays three irreversible waves in electrochemicalstudies at about 0.85, 1.4, and 1.8 V versus SCE. The 0.85 V oxi-dation of Cp�IrðppyÞCl has been attributed to oxidation of thechloride-bound complex, which is followed by oxidation at higherpotentials of a cationic solvent-bound species (23). In this work,we are using the 1�PDI excited state to oxidize the chloride-bound complex, seeking to generate higher-valence states of thecatalyst using light.

There has been one PDI/iridium complex dyad reported in theliterature by Ortí and Gierschner, in which the metal is com-plexed by one PDI-linked 1,10-phenanthroline and two 2-phenyl-pyridine ligands (6, 25). Upon selective excitation of the PDImoiety, the dyad emits with a quantum yield of ΦF ¼ 0.55, re-duced from ΦF ¼ 0.86 of the PDI model compound, a decreasewhich the authors attribute to SO-ISC induced by the iridiumatom (6). The fluorescence lifetime of the 1�PDI excited stateis still long at 3.0 ns, which suggests that SO-ISC should not becompetitive with fast electron transfer when an iridium complexis bound to PDI through an imide position.

We have prepared PDI/metal catalyst dyads 1 and 2 fromligands 3 and 4, in which PDI is covalently appended to the phe-nylpyridine ligand (Fig. 1). Two different PDI structures are con-sidered: The first includes 3,5-di-t-butylphenoxy groups at the 1,7“bay region” positions, while the second generation of this com-pound uses 3,5-bis(trifluoromethyl)phenyl groups to make thePDI a more powerful photooxidant, as well as a 3-pentyl tail atone imide position to enhance solubility in highly polar organicsolvents relevant to solar fuels applications, like acetonitrile orpropylene carbonate (26). While both PDI variants undergo elec-tron transfer with the iridium complex, the more highly oxidizing1�PDI excited state in 2 will likely be necessary to access the pu-tative higher-valent iridium(V) state implicated in the proposedcatalytic cycle (21).

Results and DiscussionSynthesis and Characterization. The syntheses of ligands 3 and 4and dyads 1 and 2 are described in the SI Text. Briefly, eachPDI-based ligand was prepared by condensing the correspondingimide anhydride with an amino-functionalized 2-phenylpyridineunit. Metalation was accomplished by stirring each ligand withthe iridium pentamethylcyclopentadienyl dichloride dimer inCH2Cl2 in the presence of sodium acetate trihydrate (27). Thesteady-state absorption and emission spectra of each ligand(Fig. S1) resemble those of most PDI compounds (18). Upon me-

talation, the absorption spectrum of the chromophore is onlyslightly perturbed; λmax of 3 shifts from 551 to 553 nm, whileλmax of 4 shifts from 541 to 544 nm (Fig. S1). Absorption tothe blue of 450 nm is enhanced in both 1 and 2 due to absorptionby the metal complex (Fig. S1). The spectrum of each dyad is asum of their individual components, indicating that the Ir(III)complex and the PDI core are not strongly coupled. Importantly,the PDI unit can be selectively excited with visible wavelengthsgreater than 450 nm. The solubility of the ligands and their re-spective metal complexes are comparable: 1 and 3 are very solu-ble in toluene, CH2Cl2, and benzonitrile, but they are less solublein butyronitrile and only 3 is soluble in acetonitrile, and in thesetwo latter solvents the UV-Vis spectra show evidence of PDI ag-gregation (28, 29). Dyad 2 and ligand 4 are soluble in the entirerange of solvents, as a result of the 3-pentyl tail at the imide thathinders π-π stacking of PDI aromatic cores (30) and the directattachment of the phenyl side-groups, which may cause evengreater distortion of the PDI core from planarity relative tothe phenoxy-substituted dyes (31, 32).

Using cyclic voltammetry, the two reductions of the PDI chro-mophores are reversible, whereas the oxidation waves of the Ircomplex are irreversible, as has been previously reported (22).The redox potentials of each dyad were thus determined with dif-ferential pulse voltammetry (Table 1), and the potentials of thePDI and the metal complex components remain unchanged re-lative to their potentials as individual molecules. The bis(CF3)phenyl-substituted PDI in 2 is 220 mV easier to reduce thanthe phenoxy-substituted PDI in 1, without a significant differencein the energy of 1�PDI, enhancing the driving force for electrontransfer from the iridium complex to the chromophore.

Electron Transfer Energetics. The ion-pair energies within 1 and 2,shown schematically in Fig. 2, were determined using the expres-sion developed by Weller (33), as described in the SI Text. To de-termine the free energy for charge separation (ΔGCS) in eachdyad, the lowest-lying singlet excited state energy of each PDIligand (ES)—estimated by averaging the energies of the lowestenergy transition in the absorption spectrum and highest energytransition in the emission spectrum—was subtracted from ΔGIP.Table 2 summarizes this data for dyads 1 and 2. In each solventconsidered, electron transfer from the iridium complex to PDIshould occur upon photoexcitation of the chromophore.

Time-Resolved Spectroscopy. Femtosecond transient absorptionmeasurements were performed on 1–4 in a range of solventsof varying polarity using 150 fs, 550 nm pulses to selectively excitethe PDI. Ligand 3 demonstrates 1�PDI excited state relaxationto the ground state with τ ¼ 3.3 ns and 3.8 ns in toluene andCH2Cl2, respectively, as determined by the decay kinetics ofthe 1�PDI signal at 700 nm (Fig. 3 and Fig. S2). There is a shorterdecay component in each solvent (6.7 ps in toluene and 4.3 ps inCH2Cl2) that accounts for 15% of the signal amplitude at 700 nm,and we attribute this to vibrational relaxation of hot 1�PDI, whichis supported by the red-shift of the stimulated emission feature atabout 620 nm. For dyad 1 in both solvents, the initial 1�PDI signalred-shifts into a peak at 725 nm (Fig. 3 and Fig. S2), which is theabsorption of PDI•− as has been observed in numerous donor-acceptor systems containing 1,7-diphenoxy-PDI derivatives (34,35). Charge separation, as also evidenced by the fast disappear-ance of stimulated emission at 620 nm, occurs with τ ¼ 1.8 ps and3.6 ps in toluene and CH2Cl2, respectively. These time scales of

Fig. 1. PDI/Ir complex dyads 1 and 2 and their respective ligands 3 and 4.

Table 1. Redox potentials (V vs. SCE) of the PDI/Ir complex dyads

Dyad Eox (IrIII∕IV) Ered ðPDI0∕−Þ Ered ðPDI−∕2−Þ1 0.80 −0.57 −0.792 0.80 −0.35 −0.61

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charge separation match those found by monitoring the fluores-cence of 1 at 585 nm (Table 3 and Fig. S3).

The decay of PDI•− is biexponential in both solvents, as notedin Table 3. To help distinguish the spectral features of the speciesassociated with each charge recombination time scale, the tran-sient spectra were subjected to singular value decomposition(SVD) and global fitting using a sum of exponentials to obtainthe principal kinetic components and their associated spectra.The two decay-associated spectra in CH2Cl2 (Fig. 4) have timescales of 14 ps (81%) and 75 ps (19%), which correlate withthe two time scales of charge recombination from the transientabsorption kinetics. Fig. 4 shows that the loss of stimulated emis-sion and the rise of PDI•−, features that match previously re-ported spectra for PDI derivatives (29, 34, 35), are associatedwith τCS ¼ 3.6 ps, while the recovery of the ground state bleachand decay of PDI•− are associated with both of the longer timescales (τCR ¼ 14 ps and 75 ps), implying that two different ion-pair populations are generated upon charge separation and re-combine with different rates. Similar results are obtained fromSVD and global fitting of the data in toluene (Fig. S3). The pos-sibility of two structural conformers of each of the starting com-plexes is unlikely, since the 1H and 13CNMR spectra of the dyadsare each consistent with a singular structure, and the observedcharge separation kinetics are monoexponential. Structuralchanges resulting in two ion-pair populations may follow the fastelectron transfer and are more likely to occur at the Ir complex,since most donor-acceptor studies with 1,7-diphenoxy-PDI in-volve a single time constant for charge recombination (35–37).Yet, because the ground state of this Ir(III) complex and theIr(IV) state generated by electron transfer are spectroscopicallysilent in the 430–820 nm visible probe range of the transient ab-sorption experiment, it is difficult to draw definitive conclusionsabout geometry changes at the metal center. One possibility toconsider is dissociation of the chloride ligand in some fractionof the molecules upon oxidation to Ir(IV), as changes in the elec-tronic structure of the metal complex would likely affect ligandbinding. The Jahn-Teller d5 configuration of the iridium(IV)

complex in the ion-pair state would have unequal filling of degen-erate orbitals, which could lead to enhanced ligand exchangerates. Sykes and coworkers have reported that ½IrðH2OÞ6�3þ is aninert ion, but oxidation to Ir(IV) accelerates ligand exchange andthe formation of new complexes (38, 39). Furthermore, rates ofwater substitution are enhanced with Cp� as a ligand, as theligand exchange rate for ½Cp�IrðH2OÞ3�2þ is enhanced by up to14 orders of magnitude relative to ½IrðH2OÞ6�3þ (40). There areno changes to the UV-Vis spectrum of the dyad following eachlaser experiment, strongly suggesting there is no photodecomposi-

Fig. 2. Energy level diagrams for dyads 1 (A) and 2 (B), featuring the calculated ion-pair energies in different solvents.

Table 2. Singlet excited state energies, ion-pair energies andelectron transfer driving forces for the PDI/Ir complex dyads

DyadLigandλabs * (nm)

Ligandλems * (nm)

ES *(eV) Solvent

ΔGIP(eV)

ΔGCS(eV)

1 551 576 2.20 toluene 1.75 −0.45CH2Cl2 1.40 −0.80

2 541 579 2.22 toluene 1.53 −0.69CH2Cl2 1.18 −1.04PhCN 1.10 −1.12CH3CN 1.09 −1.13

*Measured in CH2Cl2.

Fig. 3. Femtosecond transient absorption spectra of 1,7-diphenoxy-PDI com-pounds in CH2Cl2 with 550 nm laser excitation. (A) Spectra of ligand 3 (inset:kinetic trace at 700 nm, with a nonlinear least-squares fit). (B) Spectra of dyad1 (inset: kinetic trace at 725 nm, with a nonlinear least-squares fit).

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tion. This observation is consistent with previous electrochemicalstudies with the Cp�IrðppyÞCl model catalyst, which indicate thatequilibration of chloride- and solvent-bound Ir(III) is relativelyfast. Thus, following any ligand changes upon oxidation to theIr(IV) transient, the initial chloride ligation can be restored (23).

The transient spectra of 1 in toluene (Fig. S2) show a populationof PDI triplet at long times, with a yield of approximately 8% (14,20). The yield is smaller in CH2Cl2—a modest 2%—which is rea-sonable in the context of electron transfer theory given that theion-pair energy lies higher than the PDI triplet energy in toluenethan it does in CH2Cl2. Nanosecond transient absorption, per-formed by using 7 ns, 570 nm pulses to selectively excite thePDI, confirms the presence of a long-lived PDI triplet (>45 μs)in toluene (Fig. S4).

Focusing on the bis(CF3)phenyl compounds, PDI ligand 4demonstrates 1 �PDI relaxation to the ground state in toluene,CH2Cl2, and benzonitrile with τ ∼ 4 ns (Fig. 5 and Fig. S5).The 1 �PDI signal is red-shifted relative to the excited state of phe-noxy PDI derivatives, appearing as a peak at 745 nm in CH2Cl2and as a broad shoulder in that region in toluene and benzoni-trile. As with the 1,7-diphenoxy-PDI ligand, there is a fast decaycomponent that ranges from 4.5–13 ps (16–19%) in these sol-vents, attributed to vibrational relaxation. For dyad 2 in toluene,CH2Cl2, benzonitrile, or acetonitrile, the initial 1�PDI signal red-shifts into a peak at 720–730 nm, consistent with the results fromdyad 1 and indicating electron transfer to generate PDI•− (Fig. 5and Fig. S5). The UV-Vis absorption spectra of an electrochemi-cally reduced bis(CF3)phenyl PDI model compound (Fig. S6)confirm the assignment of this signal. Charge separation, whichagain is evidenced by the fast disappearance of stimulated emis-sion, occurs with τ ¼ 0.46 ps in toluene, 1.4 ps in CH2Cl2, and

1.5 ps in the nitrile solvents, values determined by fitting thekinetic data at 725 nm. These time scales are faster than thecorresponding values for dyad 1, which is expected given the ad-ditional 220 mVof driving force conferred by the 3,5-bis(trifluor-omethyl)phenyl PDI side-groups. The charge separation timescales correlate well with those found by monitoring the fluores-cence of 2 at 595 nm (Table 3 and Fig. S7), with the exception of a200 fs fluorescence decay component in toluene, most likely dueto S2 → S1 internal conversion in PDI given that the fluorescencedata is obtained using 415 nm excitation.

The PDI•− decay is biexponential in all four solvents as notedin Table 3, mirroring the behavior of dyad 1. These transientspectra were also analyzed with SVD and global fitting, and theprincipal kinetic components correlate with the values in Table 3(Fig. S8). The spectra associated with PDI•− decay, with featurescorresponding to radical anion decay and recovery of groundstate bleach, again suggest the presence of two different ion-pairpopulations, each with its own recombination rate. The same pro-cess that results in two ion-pairs in 1 appears to be operative in 2,since the amplitudes of the two decay components in both to-luene and CH2Cl2 are similar for 1 and 2. If chloride loss is creat-ing the additional ion-pair population, then the stability andpotential abundance of the effectively dicationic Ir(IV) metalcenter would be more significantly influenced by solvent polarity,relative to the Ir(IV) center with bound chloride. This differenceis reflected in the relative contributions of the two recombinationtime scales, which vary strongly with solvent: the shorter timescale becomes more prominent as solvent polarity increases from

Table 3. Photophysical parameters for electron transfer in PDI/Ircomplex dyads

Dyad Solvent τCS* (ps) τCS† (ps) τCR * (ps)

1 toluene 1.8 ± 0.2 1.6 ± 0.1 4.1 ± 0.9 (54%)21 ± 1 (46%)

CH2Cl2 3.6 ± 0.2 3.9 ± 0.2 13 ± 1 (78%)65 ± 5 (22%)

2toluene 0.46 ± 0.01 0.20 ± 0.05 (75%)

1.3 ± 0.3 (25%)15 ± 1 (46%)56 ± 2 (54%)

CH2Cl2 1.4 ± 0.1 1.5 ± 0.1 6.2 ± 0.1 (80%)83 ± 2 (20%)

PhCN 1.5 ± 0.1 1.7 ± 0.2 11 ± 1 (88%)125 ± 10 (12%)

CH3CN 1.5 ± 0.1 2.2 ± 0.2 5.3 ± 0.2 (83%)72 ± 3 (17%)

*Determined by transient absorption.†Determined by time-resolved fluorescence.

Fig. 4. Spectra associated with the kinetic components obtained by globalanalysis of the transient absorption spectra of 1 in CH2Cl2.

Fig. 5. Femtosecond transient absorption spectra of bis(CF3)phenyl-PDIcompounds in CH2Cl2 with 550 nm laser excitation. (A) Spectra of ligand 4(inset: kinetic trace at 745 nm, with a nonlinear least-squares fit). (B) Spectraof dyad 2 (inset: kinetic trace at 725 nm, with a nonlinear least-squares fit).

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toluene to CH2Cl2 to nitriles. This variation in time scale contri-butions also argues against multiple ground state conformers asthe source of different ion-pair populations. Triplet PDI is againgenerated to a larger extent in toluene than in the more polarsolvents, consistent with the greater driving force in toluenefor recombination to 3�PDI.

Catalytic Activity of PDI/Ir Complex Dyads. Having thus demon-strated that 1�PDI can oxidize the Ir(III) complex in a covalentsystem, we were next interested in confirming that the catalyst moi-ety of these dyads is still competent for water-oxidizing chemistry.Because oxidation of the chloride ligand to chlorine or hypochlor-ite is possible under oxidizing conditions, which can be an interfer-ing signal in oxygen-evolution studies (21), dyad 1 was studied as asolvent-bound derivative, with acetonitrile displacing the chloride.Ligand substitution was accomplished by stirring dyad 1 in aceto-nitrile with AgPF6 (41), generating the now positively chargeddyad as a PF6 salt (1-CH3CN). Cyclic voltammetry was used toinvestigate the oxidation chemistry of the dyad in both the absenceand presence of water. As a comparison, we investigated the prop-erties of the analogous control catalyst Cp�IrðppyÞOTf (22), whichdoes not bear the light-absorbing PDI moiety.

As recently reported by the Meyer group, we used propylenecarbonate as the oxidation-resistant solvent for the electrochemi-cal investigation (26). The solvent-bound dyad and model catalystare both soluble in propylene carbonate, and the solution canaccommodate several mass percent of added water. The cyclicvoltammograms of both compounds in propylene carbonateare unremarkable, showing low currents up to 2 V versus NHEin the absence of water (Fig. 6 and Fig. S9). In the case ofCp�IrðppyÞOTf, there is little above-background current in theabsence of water. However, upon addition of water, a strong ir-reversible oxidation current is observed for both 1-CH3CN andCp�IrðppyÞOTf, consistent with evolution of oxygen* (Fig. 6and Fig. S9). Analogous experiments with the chloro-bound com-pound show a similar above-background oxidative current.

Recently, we reported a method for probing the ambiguity be-tween homogeneous and heterogeneous water-oxidation catalysisby use of an electrochemical quartz crystal nanobalance (EQCN)(24). In this method, piezoelectric gravimetry is used to monitorthe electrode mass in real time during catalyst oxidation. If a layerof catalytically active iridium oxide is deposited, the EQCN willregister this as an increase in electrode mass. If the catalyst re-mains in solution, there will be no such mass deposition. In orderto study the properties of Cp�IrðppyÞOTf by EQCN, we have now

extended our work to the propylene carbonate/water mixtureused for the electrochemistry as described above. Although thereis significant above-background current ascribable to catalysis(Fig. S9), there is no corresponding increase in electrode mass.Thus, we feel that we can confidently ascribe the oxidation ofCp�IrðppyÞOTf—and by extension, the solvent-bound PDI/Ircomplex dyad—to be a solution-based process.

ConclusionIn covalently bound chromophore-catalyst dyads 1 and 2, electrontransfer to 1�PDI occurs in less than 10 ps upon excitation ofthe dye, with unity quantum yields. The rates of electron transferare increased when bis(CF3)phenyl groups are appended to thebay positions of PDI, which make the chromophore a strongerphotooxidant and more soluble in polar, water-miscible organicsolvents relevant to the field of solar fuels. Charge recombinationis also relatively rapid, though biexponential decay of PDI•− sug-gests two different ion-pair populations, perhaps a result of struc-tural changes at the iridium center upon oxidation. Electrochemi-cal experiments with dyad 1 and its solvent-bound derivative showoxidative current consistent with catalytic water oxidation by themetal complex, suggesting that the attachment of the chromophoredoes not preclude catalytic activity. Current research directions in-clude lengthening the charge-separated state lifetime to be able tocharacterize high-valent states of iridium using time-resolved elec-tron paramagnetic resonance spectroscopy, as well as studying theelectron transfer behavior of dye-catalyst dyads bound to semicon-ductors, a prerequisite to building a photoelectrochemical water-splitting cell.

Materials and MethodsDetailed synthetic procedures and characterization of all compounds, includ-ing dyads 1 and 2 and ligands 3 and 4, are described in the SI Text. All solventswere spectroscopic grade and dried using a Glass Contour solvent system ordistilled prior to use.

Electrochemical measurements for basic characterization were performedusing a CH Instruments Model 622 electrochemical workstation. All measure-ments were performed under an argon atmosphere in benzonitrile contain-ing 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6). TBAPF6was recrystallized twice from ethanol prior to use. A 1.0 mm diameter pla-tinum disk, platinum wire, and silver wire were employed as working, aux-iliary, and pseudoreference electrodes, respectively. The ferrocene/ferroce-nium redox couple (Fc∕Fcþ, 0.47 V versus SCE) was used as an internalreference for all measurements. Spectroelectrochemistry was performedusing a platinummeshworking electrode, a platinumwire counter electrode,and silver wire pseudoelectrode in a 2-mm cell.

Cyclic voltammetry (CV) measurements to assess catalytic activity weremade on a Princeton Applied Research Versastat 4–400 or model 2273 poten-tiostat/galvanostat using a standard three-electrode configuration. A basal-plane graphite (surface area: 0.09 cm2) or gold (surface area: 0.017 cm2) elec-trode was used as the working electrode to minimize background oxidation

Fig. 6. Cyclic voltammograms of solvent-bound dyad 1-CH3CN in (A) propylene carbonate and (B) propylene carbonate with 6% water by volume.

*Although we were interested in observing the evolution of oxygen with a Clark-typeelectrode as we have done previously, the organic solvent used in the electrochemicalinvestigation is not compatible with the membrane on the Clark electrode.

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current. The preparation and treatment of the basal-plane graphite elec-trode have been described previously (21). A platinum wire was used as thecounter electrode, and a Ag∕AgCl electrode (Bioanalytical Systems, Inc.) wasused as the reference (Ag∕AgCl versus NHE: þ197 mV). Experiments werecarried out in the mixed solvent solution containing 0.1 M TBAPF6 as the sup-porting electrolyte. The instrumentation and methodology for simultaneousCV and piezoelectric gravimetry measurements have been described pre-viously (24) and are included in the SI Text.

Steady-state absorption spectroscopy was performed using a Shimadzu(UV-1801) spectrophotometer with a 1 cm quartz cuvette, and fluorescencemeasurements were performed using a Photon Technology Internationalphoton-counting spectrofluorimeter in a right angle configuration with a1 cm quartz cuvette.

Femtosecond transient absorption measurements were made using the550 nm, 110 fs output of an optical parametric amplifier using techniquesdescribed earlier (32). Samples were prepared with an optical density be-tween 0.65 and 0.75 at 550 nm in a 2 mm cuvette, and they were irradiatedwith 1.0 μJ∕pulse focused to a 200-μm spot. The total instrument responsefunction for the pump-probe experiments was 150 fs. Transient absorptionkinetics were fit to a sum of exponentials convolved with a Gaussian instru-ment function by using Levenberg-Marquardt least-squares fitting. Thethree-dimensional datasets of ΔA versus time and wavelength were sub-jected to singular value decomposition and global fitting to obtain the ki-netic time constants and their associated spectra using Surface Xplorersoftware (Ultrafast Systems LLC, Sarasota, FL).

The sample for nanosecond transient absorption spectroscopy was pre-pared in a 10 mm path length quartz cuvette and degassed with five freeze-pump-thaw cycles. The sample was excited with 7 ns, 2.0 mJ, 570 nm laser

pulses using the frequency-tripled output of a Continuum Precision II 8000Nd:yttrium-aluminum-garnet laser pumping a Continuum Panther opticalparametric oscillator, a setup described earlier (10). The excitation pulsewas focused to an 8 mm diameter spot and matched to the diameter ofthe probe pulse. The total instrument response function is 7 ns and is deter-mined primarily by the laser pulse duration.

Pump pulses at 415 nm for femtosecond time-resolved fluorescenceexperiments were generated using a home-built cavity-dumped Ti:sapphirelaser system (center wavelength, 830 nm; spectral width, 55 nm; pulse dura-tion, 25 fs; repetition rate, 820 KHz) followed by frequency doubling in a200 μm thick lithium triborate crystal. The remaining fundamental servedas the gate pulses for the fluorescence upconversion experiment, which de-livered sub-50 fs time resolution by utilizing a noncollinear sum frequencygeneration scheme, which has been described in detail elsewhere (42, 43).

ACKNOWLEDGMENTS. This material is based upon work supported as partof the ANSER Center, an Energy Frontier Research Center funded by the U.S.Department of Energy, Office of Science, Office of Basic Energy Sciences un-der Award Number DE-SC0001059. Further funding from the Division of Che-mical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciencesof the U.S. Department of Energy through grant DE-FG02-84ER13297 (R.H.C.and N.D.S.; synthesis and characterization of Cp*Ir(ppy)OTf) and the U.S.National Science Foundation through grant CHE-1110942 (F.D.; electrochemi-cal quartz crystal nanobalance studies) is gratefully acknowledged. S.W.E.was supported by the IMI program of the National Science Foundation underaward no. DMR-08-43962. M.T.V. acknowledges support from an NSF Grad-uate Research Fellowship and a National Defense Science and EngineeringGraduate Fellowship.

1. Alstrum-Acevedo JH, Brennaman MK, Meyer TJ (2005) Chemical approaches to artifi-cial photosynthesis 2. Inorg Chem 44:6802–6827.

2. Gust D, Moore TA, Moore AL (2009) Solar fuels via artificial photosynthesis. Acc ChemRes 42:1890–1898.

3. Meyer TJ (1989) Chemical approaches to artificial photosynthesis. Acc Chem Res22:163–170.

4. Song WJ, et al. (2011) Making solar fuels by artificial photosynthesis. Pure Appl Chem83:749–768.

5. Wasielewski MR (2006) Energy, charge, and spin transport in molecules and self-assembled nanostructures inspired by photosynthesis. J Org Chem 71:5051–5066.

6. Costa RD, et al. (2009) A deep-red-emitting perylenediimide-iridium-complex dyad:Following the photophysical deactivation pathways. J Phys Chem C Nanomater Inter-faces 113:19292–19297.

7. Danilov EO, Rachford AA, Goeb S, Castellano FN (2009) Evolution of the triplet excitedstate in Pt(II) perylenediimides. J Phys Chem A 113:5763–5768.

8. Gunderson VL, et al. (2011) Photoinduced singlet charge transfer in a ruthenium(II)perylene-3,4∶9,10-bis(dicarboximide) complex. J Phys Chem B 115:7533–7540.

9. Jimenez AJ, et al. (2011) Synthesis, characterization, and photoinduced energy andelectron transfer in a supramolecular tetrakis (ruthenium(II) phthalocyanine) peryle-nediimide pentad. Chem Eur J 17:5024–5032.

10. Poddutoori P, et al. (2011) Photoinitiated multistep charge separation in ferrocene-zincporphyrin-diiron hydrogenase model complex triads. Energy Environ Sci 4:2441–2450.

11. Qvortrup K, et al. (2008) Perylenediimide-metal ion dyads for photo-induced electrontransfer. Chem Commun 1986–1988.

12. Rachford AA, Goeb S, Castellano FN (2008) Accessing the triplet excited state in per-ylenediimides. J Am Chem Soc 130:2766–2767.

13. Samuel APS, Co DT, Stern CL, Wasielewski MR (2010) Ultrafast photodriven intramo-lecular electron transfer from a zinc porphyrin to a readily reduced diiron hydroge-nase model complex. J Am Chem Soc 132:8813–8815.

14. Weissman H, et al. (2007) Palladium complexes of perylene diimides: Strong fluores-cence despite direct attachment of late transition metals to organic dyes. Inorg Chem46:4790–4792.

15. Wurthner F, You CC, Saha-Moller CR (2004) Metallosupramolecular squares: Fromstructure to function. Chem Soc Rev 33:133–146.

16. Prodi A, et al. (2005)Wavelength-dependent electron and energy transfer pathways ina side-to-face ruthenium porphyrin/perylene bisimide assembly. J Am Chem Soc127:1454–1462.

17. Jones BA, et al. (2004) High-mobility air-stable n-type semiconductors with processing ver-satility: Dicyanoperylene-3,4 : 9,10-bis(dicarboximides).AngewChem Int Ed 43:6363–6366.

18. Wurthner F (2004) Perylene bisimide dyes as versatile building blocks for functionalsupramolecular architectures. Chem Commun 1564–1579.

19. Huang C, Barlow S, Marder SR (2011) Perylene-3,4,9,10-tetracarboxylic acid diimides:Synthesis, physical properties, and use in organic electronics. J Org Chem 76:2386–2407.

20. FordWE, Kamat PV (1987) Photochemistry of 3,4,9,10-perylenetetracarboxylic dianhy-dride dyes 3. Singlet and triplet excited-state properties of the bis(2,5-di-tert-butyl-phenyl)imide derivative. J Phys Chem 91:6373–6380.

21. Blakemore JD, et al. (2010) Half-sandwich iridium complexes for homogeneous water-oxidation catalysis. J Am Chem Soc 132:16017–16029.

22. Hull JF, et al. (2009) Highly active and robust Cp� iridium complexes for catalytic wateroxidation. J Am Chem Soc 131:8730–8731.

23. Brewster TP, et al. (2011) An iridium(IV) species, ½Cp�IrðNHCÞCl�þ, related to a water-oxidation catalyst. Organometallics 30:965–973.

24. Schley ND, et al. (2011) Distinguishing homogeneous from heterogeneous catalysis inelectrode-driven water oxidation with molecular iridium complexes. J Am Chem Soc133:10473–10481.

25. Costa RD, et al. (2009) Efficient deep-red light-emitting electrochemical cells based ona perylenediimide-iridium-complex dyad. Chem Commun 3886–3888.

26. Chen ZF, et al. (2010) Nonaqueous catalytic water oxidation. J Am Chem Soc132:17670–17673.

27. Boutadla Y, Al-Duaij O, Davies DL, Griffith GA, Singh K (2009) Mechanistic study ofacetate-assisted C-H activation of 2-substituted pyridines with ½MCl2Cp��2 (M ¼ Rh,Ir) and ½RuCl2ðp-cymeneÞ�2 . Organometallics 28:433–440.

28. Ahrens MJ, et al. (2004) Self-assembly of supramolecular light-harvesting arrays fromcovalent multi-chromophore perylene-3,4∶9,10-bis(dicarboximide) building blocks.J Am Chem Soc 126:8284–8294.

29. Giaimo JM, et al. (2008) Excited singlet states of covalently bound, cofacial dimers andtrimers of perylene-3,4 : 9,10-bis(dicarboximide)s. J Phys Chem A 112:2322–2330.

30. Rajasingh P, Cohen R, Shirman E, Shimon LJW, Rybtchinski B (2007) Selective bromina-tion of perylene diimides under mild conditions. J Org Chem 72:5973–5979.

31. Sivalmurugan V, et al. (2010) Synthesis and Photophysical Properties of Glass-FormingBay-Substituted Perylenediimide Derivatives. J Phys Chem B 114:1782–1789.

32. Ramanan C, Smeigh AL, Anthony JE, Marks TJ, Wasielewski MR (2012) Competitionbetween singlet fission and charge separation in solution-processed blend films of6,13-bis(triisopropylsilylethynyl)-pentacene with sterically-encumbered perylene-3,4∶9,10-bis(dicarboximide)s. J Am Chem Soc 134:386–397.

33. Weller A (1982) Photoinduced electron-transfer in solution—Exciplex and radical ion-pair formation free enthalpies and their solvent dependence. Z Phys Chem 133:93–98.

34. Colvin MT, et al. (2010) Competitive electron transfer and enhanced intersystem cross-ing in photoexcited covalent TEMPO-perylene-3,4∶9,10-bis(dicarboximide) dyads:Unusual spin polarization resulting from the radical-triplet interaction. J Phys ChemA 114:1741–1748.

35. Goldsmith RH, et al. (2005) Wire-like charge transport at near constant bridge energythrough fluorene oligomers. Proc Natl Acad Sci USA 102:3540–3545.

36. Rybtchinski B, Sinks LE, Wasielewski MR (2004) Photoinduced electron transfer in self-assembled dimers of 3-fold symmetric donor-acceptor molecules based on perylene-3,4 : 9,10-bis(dicarboximide). J Phys Chem A 108:7497–7505.

37. van der Boom T, et al. (2002) Charge transport in photofunctional nanoparticlesself-assembled from zinc 5,10,15,20-tetrakis(perylenediimide)porphyrin buildingblocks. J Am Chem Soc 124:9582–9590.

38. Castillo-Blum SE, Richens DT, Sykes AG (1989) Oxidation of hexaaquairidium(III) andrelated studies—preparation and properties of iridium(III), iridium(IV), and iridium(V) dimers as aqua ions. Inorg Chem 28:954–960.

39. Castillo-Blum SE, Sykes AG, Gamsjager H (1987) Substitution inertness of ½IrðH2OÞ6 �3þ.Polyhedron 6:101–103.

40. Cayemittes S, et al. (1999) Mechanistic investigation on the water substitution inthe η5-organometallic complexes Cp� IrðH2OÞ3 2þ and Cp�RhðH2OÞ3 2þ . Inorg Chem38:4309–4316.

41. Park-Gehrke LS, Freudenthal J, KaminskyW, DiPasquale AG,Mayer JM (2009) Synthesisand oxidation of Cp� IrðIIIÞ compounds: Functionalization of a Cp� methyl group. Dal-ton Trans 1972–1983.

42. Kim CH, Joo T (2008) Ultrafast time-resolved fluorescence by two photon absorptionexcitation. Opt Express 16:20742–20747.

43. Rhee H, Joo T (2005) Noncollinear phase matching in fluorescence upconversion. OptLett 30:96–98.

15656 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1202075109 Vagnini et al.

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