diimine ChromophoreâAcceptor Dyads

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Spectroscopy and Photochemistry; General Theory

Ligand Mediation of Vectorial Charge Transfer in Cu(I)diimine Chromophore—Acceptor Dyads Dugan Hayes, Lars Kohler, Lin X. Chen, and Karen L Mulfort J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00468 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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The Journal of Physical Chemistry Letters

Ligand Mediation of Vectorial Charge Transfer in Cu(I)diimine Chromophore—Acceptor Dyads Dugan Hayes,a§+ Lars Kohler,a§ Lin X. Chen,ab Karen L. Mulforta* a

Division of Chemical Sciences and Engineering, Argonne National Laboratory, Argonne IL 60439; bDepartment of Chemistry, Northwestern University, Evanston IL 60208

§

These authors contributed equally; +Current affiliation: Department of Chemistry, University of Rhode Island, Kingston RI 02881

Corresponding Author *[email protected]

ACS Paragon Plus Environment

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ABSTRACT

In this work we present the photoinduced charge separation dynamics of four molecular dyads composed of heteroleptic Cu(I)bis(phenanthroline) chromophores linked directly to the common electron acceptor naphthalene diimide. The dyads were designed to allow us to 1) detect any kinetic preference for directionality during photoinduced electron transfer across the heteroleptic complex, and 2) probe the influence of excited state flattening on intramolecular charge separation. Singular value decomposition of ultrafast optical transient absorption spectra demonstrates that charge transfer occurs with strong directional preference, and charge separation occurs up to 35 times faster when the acceptor is linked to the sterically blocking ligand. Further, the charge-separated state in these dyads is stabilized by polar solvents, resulting in dramatically longer lifetimes for dyads with minimal substitution about the Cu(I) center. This unexpected but exciting observation suggests a new approach to the design of Cu(I)bis(phenanthroline) chromophores that can support long-lived vectorial charge separation.

TOC GRAPHICS (2”x2”)


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The successful development of molecular complexes as functional modules for artificial photosynthesis requires physical insight into how key structural features support efficient light absorption, stabilize electronic excited states, promote directional charge transfer and long-lived charge separation, and ultimately, facilitate the accumulation of multiple charges for multielectron redox chemistry.1-3 Considerable efforts have been focused on developing molecular modules based on earth-abundant elements, and a specific example of this is seen in work aimed at substituting Cu(I)(diimine) complexes for the benchmark molecular photosensitizer Ru(bpy)32+ (bpy = 2,2’-bipyridine) and its derivatives.4,5 Cu(phen)2+ (phen = 1,10phenanthroline) complexes in particular have strong absorbance in the visible region resulting from a metal-to-ligand charge transfer (MLCT) transition and are potent excited state reductants, with excited state reduction potentials at least 200 mV more negative than Ru(bpy)33+/2+*.6 Furthermore, the bis-coordination environment around the Cu(I) center represents a straightforward route to well-defined electron donor-acceptor geometries that can be approximated as simple two-dimensional molecular “wires”, an approach that is not possible using the tris-functionalized octahedral geometry of Ru(bpy)32+ and analogs.7 Another salient feature of Cu(I)diimine complexes is their unusual excited state structural dynamics, which could potentially be harnessed to stabilize charge separation.8-10 Through numerous studies employing transient optical and X-ray spectroscopies, these structural dynamics are well understood from the sub-picosecond to the microsecond time scale.11-20 Following visible excitation of the MLCT band, a Cu(I)diimine complex, such as [Cu(I)(phen)2]+, which has a pseudo-tetrahedral geometry in the ground state with 3d10 electronic configuration, is transformed to nominally Cu(II) (3d9) and then undergoes a Jahn-Teller flattening distortion in the MLCT state. The kinetics and extent of this flattening process is


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strongly influenced by the size and steric effects of the diimine substitution nearest the copper center.21-23 The flattening of the 1MLCT state changes the molecular symmetry and the extent of the Jahn-Teller distortion influences the kinetics of intersystem crossing (ISC) to form the 3

MLCT state. Subsequently, the 3MLCT state decays through a combination of radiative and

non-radiative pathways that is largely determined by the ability of the ligand substituents to shield the Cu(II) center from interaction with solvent molecules. This detailed knowledge of how the ground state structure influences the excited state dynamics has been applied to the design of Cu(I)diimine complexes capable of participating in intramolecular,24,25 intermolecular,26 and interfacial27 photoinduced electron transfer processes. Given the exciting potential for Cu(I)diimine complexes as modules for artificial photosynthesis, we are interested in understanding how to best design these complexes to drive and support directional charge transfer. The heteroleptic phenanthroline (HETPHEN) synthesis strategy developed by Schmittel et al. provides a route to asymmetric coordination of two different phenanthroline ligands around a Cu(I) center.28-31 Our group has used this strategy to synthesize several mononuclear32,33 and dinuclear34 CuHETPHEN model complexes and fully characterize their ground and excited states. Importantly, Odobel and co-workers have used this approach to demonstrate the prospects for CuHETPHEN complexes in solar energy conversion schemes.35-37 In the absence of strong electron donating or withdrawing groups on the phenanthroline periphery, however, a crucial question remains: how do we most effectively design the attachment of complementary functionality (i.e. electron relays, catalysts, surface binding) to light-harvesting Cu(I)diimine complexes? To specifically address this design challenge, we synthesized four heteroleptic Cu(I)diimine (CuHETPHEN) complexes with the well-known molecular electron acceptor 1,4,5,8-


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naphthalene-diimide (NDI) covalently bound to either the blocking ligand (2,9-dimesityl-1,10phenanthroline, bL) or the secondary ligand (1,10-phenanthroline, phen, or 2,9-dimethyl-1,10phenanthroline, dmp) (Figure 1). The model complexes (bL)Cu(phen) and (bL)Cu(dmp) were available from previous studies32 and (bL)Cu(dmp-NDI) was previously known from work by Sandroni et al.37 Since we know that the excited state dynamics are strongly influenced by 2,9phenanthroline substitution, we looked at complexes without and with 2,9-dimethyl substitution on the secondary ligand. We also varied the location of NDI to probe the rate of charge separation across bL compared to the secondary ligand (either phen or dmp). The NDIfunctionalized phenanthroline ligands phen-NDI38 and dmp-NDI37 were previously described in the literature. The more challenging bL-NDI was prepared starting from 2,9-dichloro-1,10phenanthroline in four steps with an overall yield of 19% via a nitration, Suzuki cross coupling, reduction and imidization sequence (see Supporting Information for complete synthetic details). Following synthesis of the phenanthroline ligands, CuHETPHEN complexes were readily obtained following the one-pot, two-step HETPHEN synthesis strategy.28-31,39 The structure and purity of all six complexes were verified by NMR, ESI-MS, and elemental analysis to explicitly confirm that there was no contamination by the homoleptic complexes that might complicate the analysis of the ground or excited state properties.


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Figure 1. Chemical structures of model CuHETPHEN complexes and CuHETPHEN-NDI dyads studied here. bL = blocking ligand, 2,9-dimesityl-1,10-phenanthroline; phen = 1,10phenanthroline; dmp = 2,9-dimethyl-1,10-phenanthroline. The UV-Vis absorbance spectra and cyclic voltammograms of the model CuHETPHEN complexes and dyads are shown in Figures 2, S22-24, and summarized in Tables 1 and S1. The visible region of the absorbance spectra is dominated by a broad absorbance peak centered at 460 nm, characteristic of the MLCT band of homoleptic and heteroleptic Cu(I)diimine complexes. The prominent vibronic progression of the π-π* transitions of the NDI group is clearly observed between 320 and 380 nm in the linked dyads. The Cu(II/I) potential of the phen-bearing complexes is approximately +800 mV vs. SCE, and it is shifted positive by almost 200 mV when 2,9-dimethyl substitution is introduced in complexes containing dmp. We observe two reversible, one-electron reduction peaks at approximately -500 mV and -1000 mV vs. SCE, which we assign to the first and second reductions of the NDI electron acceptor.40,41 Importantly, other than the differences attributed to the 2,9-phenanthroline substitution adjacent to Cu(I), we observe only very minor differences in both the optical spectra and electrochemical response,


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indicating that the relative thermodynamics between the Cu(I) center (electron donor) and NDI (electron acceptor) are not affected by linking location.

Figure 2. (A) Optical and (B) electrochemical characterization of (bL)Cu(dmp), (bL)Cu(dmpNDI), and (bL-NDI)Cu(dmp) in CH2Cl2. Table 1. Summary of optical and electrochemical characterization of CuHETPHEN-NDI dyads in CH2Cl2. λmax

ε (λmax)

Cu(II/I)

NDI0/-

NDI-/2-

(MLCT, nm)

(MLCT, M-1cm-1)

(V vs. SCE)

(V vs. SCE)

(V vs. SCE)

(bL)Cu(phen)a

467

7326

0.77

n/a

n/a

(bL)Cu(phen-NDI)

465

7609

0.81

-0.51

-0.99


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(bL-NDI)Cu(phen)

471

7930

0.83

-0.52

-0.98

(bL)Cu(dmp)a

460

5631

0.96

n/a

n/a

(bL)Cu(dmp-NDI)

464

6222

0.98

-0.51

-0.98

(bL-NDI)Cu(dmp)

461

5931

0.98

-0.50

-0.94

a

Optical and electrochemical data for model complexes from reference 33.

Ultrafast and nanosecond optical transient absorption spectroscopy (TA) was used to probe the photoinduced charge separation kinetics in the excited state for the linked CuHETPHEN-NDI dyads in both a coordinating and a non-coordinating solvent. NDI was in part selected as the electron acceptor for the strong signature spectrum of its radical anion, which has peaks at 470 nm and 602 nm. Also, the ground state NDI has virtually no absorption above 400 nm which enables selective excitation of the Cu(I)diimine MLCT band by the 415 nm laser pulse. A direct comparison of the transient spectra of the model complex (bL)Cu(dmp) and dyad (bL)Cu(dmpNDI) is shown in Figure 3. As expected, the spectra of NDI·- is clearly observed following excitation of all four dyads (Figures S28-S35), and matches well with the transient spectra reported for other linked complexes,42,43 confirming electron transfer from Cu(I)diimine to NDI.


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Figure 3. Transient spectra of (A) model complex (bL)Cu(dmp) and (B) CuHETPHEN-NDI dyad (bL)Cu(dmp-NDI) following 415 nm excitation in CH3CN. Time delay of spectra (in picoseconds) noted in legend. Singular value decomposition (SVD) of each TA dataset reveals that only two components contribute to the signal outside the temporal overlap region of the pump-probe pulses (Figure S48 and Table S2). These two components, shown in red and orange for (bL)Cu(dmp-NDI) in CH3CN in Figure 4A, may immediately be assigned to the Cu(II)*diimine (black)32,33 or NDI·(green)41-43 transient species, respectively, by comparison with published spectra. Here we only consider the NDI·- component, which greatly simplifies the analysis of the charge transfer dynamics by excluding contributions from copper-centered processes that occur within the first few picoseconds.19,20 Additionally, by focusing our analysis on the NDI·- kinetics, we wish to highlight the relevant timescale for formation and decay of the charge-separated state, which in this model system acts as a proxy for the catalytically poised redox species in a functional assembly. For each dyad/solvent combination, the time trace corresponding to the NDI·component was fit to the sum of four or five exponential decay terms convolved with a Gaussian instrument response function, and the results are summarized in Table 2.


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Figure 4. (A) SVD components of (bL)Cu(dmp-NDI) in CH3CN compared to literature spectra. Orange: NDI·- component; red: Cu(II) component; black: (bL)Cu(dmp) TA spectrum in CH3CN at a delay time of 25 ps from reference 32; green: NDI·- absorption spectrum in DMF obtained by UV-Vis spectroelectrochemistry from reference 41. (B) Comparison of kinetics of (bL)Cu(dmpNDI) (blue) and (bL-NDI)Cu(dmp) (gray) at 602 nm in CH3CN illustrating difference in rate of NDI·- formation. Inset shows a zoom of early time region (-20 to 300 ps) indicated by gray box. Table 2. Summary of excited state kinetics of model complexes and dyads in CH3CN and CH2Cl2. Amplitudes and time constants listed below; full details of fitting routine in Supporting Information. τ1 A (%) (bL)Cu(phen)a

(bL)Cu(phen-NDI)

(bL-NDI)Cu(phen)

(bL)Cu(dmp)a

(bL)Cu(dmp-NDI)

(bL-NDI)Cu(dmp)

τ2 ps

A (%)

τ3 ps

A (%)

τ4 ps

A (%)

τ5 ps

A (%)

nsd

CH3CNb

0.73

CH2Cl2c

3.4

CH3CN

-17

1.4

20

2.2

-26

16

11

380

26

22

CH2Cl2

-28

1.9

22

8.9

-12

79

12

95

26

3.3

CH3CN

-20

0.83

-19

6.6

15

360

46

14

CH2Cl2

-42

2.1

2

160

20

3.5

36

6.6

CH3CNb

21

CH2Cl2c

47

CH3CN

-18

0.83

16

3.4

-29

190

37

2.0

CH2Cl2

-12

1.7

26

30

-32

52

30

3.7

CH3CN

-30

1.4

30

3.4

-18

6.4

CH2Cl2

-44

2.2

31

16

4

210

18

1.7

4

350

21

4.2


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a

Only the ground state recovery (τ5) is reported here for model complexes to compare to charge recombination of CuHETPHEN-NDI dyads. bExcited state lifetime for model complexes in CH3CN from reference 32. cExcited state lifetime for model complexes in CH2Cl2 from reference 33. dLifetimes beyond 5 ns were measured separately by nanosecond TA shown in Figures S3639. The fit for each dyad/solvent combination includes a 1-2 ps rise component (τ1), corresponding at least in part to non-impulsive formation of the initial charge separated state. In all but one case, the fit also contains a decay component (τ2) ranging from 2.2 to 30 ps. This likely corresponds at least in part to geometry relaxation of NDI·-, although we cannot exclude a possible assignment to other nuclear and/or electronic motion. We note that in all cases the value of this time component is more than four times shorter in CH3CN than in CH2Cl2, consistent with the order-of-magnitude shorter dielectric relaxation time of CH3CN.44,45 The early-time signals are also likely influenced by intersystem crossing, solvation dynamics, and coherent artifacts, and thus the above assignments are not comprehensive. An accurate modeling of these processes would require improved temporal resolution and additional fit components; we do not attempt such analysis here and instead focus on dynamics at times beyond 10 ps. In this region, we assign τ3 and τ5 to interligand charge transfer (ILCT) and charge recombination, respectively, and these components will be discussed in detail below. The time traces of all dyads other than (bL)Cu(dmp-NDI) also contain a decay component (τ4) with a lifetime ranging from 100 to 400 ps. The origin of this component is unclear, but it is a relatively minor contribution to the excited state dynamics, accounting for at most 15% of the total signal. Detailed analysis of τ3 and τ5 for each dyad/solvent pair provides unprecedented insight into the directionality of charge separation in CuHETPHEN complexes. The charge recombination time, given by τ5, ranges from 1.7 to 22 ns across all dyad/solvent combinations. We emphasize that the state from which recombination occurs is a thermodynamic excited state rather than a


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purely electronic excited state in the traditional sense; although both the Cu(II) center and NDI·in this state have adopted relaxed nuclear geometries, the transient intramolecular charge separated state is thermodynamically unstable and relaxes to the ground state via charge recombination with a lifetime given by τ5. In some cases, the amplitude of this decay component is correlated with the amplitude of a second rise component (τ3) that corresponds to formation of the charge-separated state. For example, for (bL)Cu(phen-NDI) in CH3CN, the kinetic components with decay time constants τ3 and τ5 are equal in magnitude but opposite in sign. This correlation demonstrates that formation of the long-lived charge-separated state occurs following ILCT from bL to phen-NDI, and the overall process (ILCT and charge separation) has a time constant of 16 ps. Importantly, this interpretation suggests that the initial MLCT process occurs with nearly complete directionality, preferentially toward bL, as depicted in Scheme 1. The same behavior is observed for (bL)Cu(dmp-NDI) in CH2Cl2, with an ILCT time constant of 52 ps. In CH3CN, however, the amplitude of τ3 is ~20% lower than that of τ5, suggesting that MLCT only occurs with ~80% net directionality toward bL. The 190 ps ILCT time constant of this dyad/solvent combination is by far the slowest, resulting in the dramatic rise in the TA signal highlighted in Figure 4B.

Scheme 1. Model for photoinduced charge transfer events in (bL)Cu(phen-NDI) in CH3CN with corresponding time components indicated. In this particular dyad and solvent, MLCT occurs almost entirely toward bL, from where it must first migrate to phen (ILCT, τ3) before


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migrating to NDI to form the charge-separated state (CS, τ1). Finally, charge recombination (CR, τ5) returns the system to the ground state. The correlations between τ3 and τ5 help us establish a model describing vectorial charge transfer across the CuHETPHEN dyads, and further evidence appears in the dynamics of (bLNDI)Cu(dmp) and (bL-NDI)Cu(phen). If the MLCT process is indeed occurring nearly unidirectionally toward bL, these dyads should not exhibit a slow rise in the amplitude of the NDI·- component, as ILCT would be unnecessary for charge separation. Indeed, for both dyads in CH3CN, formation of the long-lived charge separated state occurs with a time constant of ~6.5 ps, while in CH2Cl2, a second negative component is not even observed. In addition to demonstrating a means for quantifying the directionality of charge transfer in heteroleptic complexes – an incredibly important consideration when designing charge relays and photocatalytic assemblies – our results highlight how solvent may be used to modulate such directionality. It is important to note that a difference in thermodynamic driving force cannot account for the variable rates of charge separation observed. The dyad pair (bL)Cu(dmp-NDI) and (bLNDI)Cu(dmp) have nearly identical redox properties, yet we observe a thirty-fold difference in charge separation rate between these complexes. Furthermore, (bL)Cu(dmp-NDI) actually exhibits slightly faster charge separation than (bL)Cu(phen-NDI) in CH2Cl2, while (bLNDI)Cu(dmp) and (bL-NDI)Cu(phen) have nearly identical charge separation rates in CH3CN. Finally, we find that the charge recombination times of both phen-bearing dyads are exceptionally long in CH3CN. This is particularly noteworthy, because coordinating solvents such as CH3CN have previously been found to accelerate ground state recovery for Cu(I)diimine complexes, as observed in the model compounds. Perhaps surprisingly, the recovery times of


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both phen-bearing dyads are unchanged relative to that of the model compound in CH2Cl2, while the recovery times of both dmp-bearing dyads in either solvent are an order of magnitude faster! To explain these remarkable observations, we note that relatively sterically unencumbered phen-bearing Cu(I)diimines can undergo significant structural flattening, from pseudotetrahedral in the ground state to roughly square planar in the Cu(II)* MLCT state.6,19 However, the additional methyl groups of dmp-bearing Cu(I)diimines reduce such a dramatic flattening distortion in the MLCT state, and the ground- and excited-state geometries of the dmp-bearing complexes are nearly the same.19,21,39,46-49,50 Furthermore, in all dyad/solvent combinations, the charge-separated state is lower in energy than the MLCT state. This means that for dmp-bearing dyads, relaxation to the ground state may occur through a Franck-Condon transition from the charge-separated state at a faster rate than in the model complex in accordance with the energy gap law.51 The higher dielectric constant CH3CN media stabilizes the charge-separated state more effectively, but the same two-fold acceleration of ground state recovery in CH3CN vs. CH2Cl2 observed for the model complex is found for charge recombination in both dmp-bearing dyads. In the phen-bearing dyads, on the other hand, the charge-separated state is stabilized by CH3CN, and the complex is “locked” into the excited state geometry from which it cannot relax directly to the ground state. This mechanism of excited state stabilization via solvent coordination was previously reported by Scaltrito et al. for a different Cu(I)diimine donoracceptor complex, where the charge recombination rate increased by over two orders of magnitude in going from dichloroethane to DMSO.52 We note that solvent plays a strikingly different role in modulating the photochemical dynamics of the model complexes, where it can quench the Cu(II)* excited state; this cannot occur in the dyads because charge separation occurs on a much faster timescale. Thus, while the


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longer lifetime of the charge-separated species observed for the phen-bearing dyads in CH3CN can be satisfactorily explained, it is not clear a priori how the competing effects of dielectric environment, solvent polarity, and structural relaxation will resolve in any given dyad. Indeed, the distinct roles of the solvent in the dyads vs. the model complexes highlight the importance of designing appropriate models and evaluating their behavior each step of the way when constructing increasingly complex multifunctional assemblies. In conclusion, we have synthesized and thoroughly analyzed the excited state dynamics of four molecular chromophore—acceptor dyads based on heteroleptic Cu(I)diimine complexes with NDI linked directly to one of the phenanthroline ligands. The location of the NDI electron acceptor does not change the electron donor-acceptor thermodynamics but has significant influence on the kinetics of both charge separation and recombination. We find that the initial charge transfer occurs with strong directional preference toward bL, and consequently, the fastest charge separation is found when NDI is linked directly to bL. In contrast, formation of the NDI radical anion can take hundreds of picoseconds in dyads with the opposite arrangement, because the electron must first migrate from bL to the secondary ligand before charge separation can occur. Further, while the conventional wisdom for maximizing the excited state lifetime of Cu(I)diimine complexes in polar solvents tells us to incorporate large substituents at the 2,9phenanthroline positions, in these linked dyads we find that the opposite is true: the longest charge-separated decay times are those of the 2,9-unsubstituted complexes, in which the high dielectric solvent environment locks the flattened geometry into place. Therefore, this work using model electron acceptors provides critical insight into the design principles for directly linking more challenging electron acceptors to molecular chromophores based on the


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CuHETPHEN platform and highlights the need to consider such an assembly as much more than the sum of its parts. ASSOCIATED CONTENT •

General methods, synthesis scheme and details, ground state characterization (1H NMR, UV-Vis absorbance spectra, cyclic voltammetry) of dyads, ultrafast and nanosecond TA spectra of dyads, details of SVD fitting and kinetic analysis (PDF)

AUTHOR INFORMATION The authors declare no competing financial interests. ACKNOWLEDGMENT The authors gratefully acknowledge support by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, through Argonne National Laboratory (ANL) under Contract No. DE-AC0206CH11357. D.H. acknowledges support from the Joseph J. Katz Fellowship from ANL. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The authors would also like to thank Dr. Ryan G. Hadt for helpful discussions. REFERENCES 1. 2.

3.

Concepcion, J. J.; House, R. L.; Papanikolas, J. M.; Meyer, T. J. Chemical Approaches to Artificial Photosynthesis. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15560-15564. Ashford, D. L.; Gish, M. K.; Vannucci, A. K.; Brennaman, M. K.; Templeton, J. L.; Papanikolas, J. M.; Meyer, T. J. Molecular Chromophore–Catalyst Assemblies for Solar Fuel Applications. Chem. Rev. 2015, 115, 13006-13049. Mulfort, K. L.; Utschig, L. M. Modular Homogeneous Chromophore–Catalyst Assemblies. Acc. Chem. Res. 2016, 49, 835-843.


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