Photoinduced electron transfer kinetics of linked Ru-Co photocatalyst dyads

Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 59–65

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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Photoinduced electron transfer kinetics of linked Ru-Co photocatalyst dyads Lars Kohler, Karen L. Mulfort

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Division of Chemical Sciences and Engineering, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL, 60439, United States

A R T I C LE I N FO

A B S T R A C T

Keywords: Photocatalyst dyads Modular assembly Ruthenium Cobalt Photoinduced electron transfer

Two new supramolecular photocatalyst dyads based on the [Ru(2,2′-bipyridine)3]2+ photosensitizer linked to a macrocyclic Co(II)tetra(pyridyl) catalyst for proton reduction are reported. The dyads differ primarily in the bridging ligand which links the molecular modules; the first being a short and flexible linker, and the second a longer and electronically conjugated linker. Ultrafast transient optical spectroscopy was used to monitor the photoinduced kinetics of the dyads following visible excitation of the photosensitizer module. Direct comparison of transient spectra and kinetics indicates that there are indeed substantial differences between the ultrafast transient optical spectroscopy of the dyads, but there is no indication of oxidative quenching of the photosensitizer module by the catalyst module. These initial design and characterization studies of the linked Ru (II)—Co(II) dyads provide an important foundation for advanced designs of systems for efficient solar energy conversion by molecular architectures.

1. Introduction Molecular complexes and supramolecular assemblies designed for electron donor-acceptor interactions have been critical to the progress of several research areas, including molecular recognition, molecular electronics, and molecular photovoltaics [1–6]. Such progress has been driven by the ability to tune molecular structure and higher-order molecular interactions with atomic-level precision. The control over molecular structure one atom, one bond, or one functional group at a time has enabled detailed evaluations of the influence of chemical and electronic structure on a system’s photophysical properties and photochemical outcomes, which in turn leads to the derivation of design principles for advanced molecular systems. The development of molecular architectures that accomplish artificial photosynthesis requires the integration of components that can effectively absorb visible light, efficiently generate light-initiated excited states, stabilize long-lived charge separation, and can accumulate multiple redox equivalents [7–10]. Recognizing that in natural photosynthesis these functions are accomplished by molecular cofactors which are precisely positioned by the reaction center protein environment [11], several groups have pursued the design and development of linked molecular and supramolecular assemblies composed of complementary photosensitizer and catalyst modules. [12,13] In particular, the molecular photosensitizer [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) has been a remarkably versatile module for building linked photocatalyst dyads to perform light-driven water splitting. Indeed, because

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of its relative strength as both an excited state reductant and an excited state oxidant, [14] it has been linked to catalyst centers based on Pt [15,16], Pd [17], Rh [18,19], and Co [20–22] for catalytic proton and CO2 reduction, and catalyst modules based on Ru [23–26] and Mn [27,28] for catalytic water oxidation. Here we describe the design, synthesis, and photophysical characterization of two new linked photocatalyst dyads (D1, D2) based on the benchmark [Ru(bpy)3]2+ molecular photosensitizer module (Ru) linked to a highly active and stable molecular [Co(bpy)2]2+ catalyst module for H2 generation from water (Co) (Fig. 1). The key difference between these dyads is the bridging link between Ru and Co, from a short methylene link in D1 to a longer conjugated phenanthrolinebased bridge in D2. The driving force for photoinduced electron transfer from the visible light-excited state of Ru to Co is similar for D1 and D2, but we find that the link between the two molecular modules plays an important role in the photoinduced electron transfer kinetics, with likely implications for applications in molecular photocatalysis. 2. Materials and methods 2.1. General methods 1

H NMR was performed on a Bruker DMX 500 and referenced to TMS or residual solvent peak. ESI-MS was collected on a ThermoFisher LCQ Fleet, from dilute methanol solutions in positive ionization mode. UV–vis absorption measurements were performed on a Beckman

Corresponding author. E-mail address: [email protected] (K.L. Mulfort).

https://doi.org/10.1016/j.jphotochem.2018.12.025 Received 7 November 2018; Received in revised form 20 December 2018; Accepted 23 December 2018 Available online 27 December 2018 1010-6030/ © 2018 Published by Elsevier B.V.

Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 59–65

L. Kohler, K.L. Mulfort

Fig. 1. Chemical structures of catalyst module (Co), photosensitizer module (Ru), and linked dyads D1 and D2.

a depolarizer, synchronously chopped at 2500 Hz, focused at the sample position to a spot size of 400 μm, and attenuated to a pulse energy of 0.5 μJ. Transient absorption spectra were collected using the Helios control software. The data were corrected for temporal chirp in the probe beam using the separately collected nonresonant response of the blank solvent. The nonresonant response gave an instrument response time of approximately 300 fs. All experiments were performed at room temperature with constant stirring with samples in 2 mm quartz cuvettes that had been bubbled with nitrogen. For longer time-scale processes, the probe light comes from a continuum light source (Ultrafast Systems, EOS). In this case, the system operates at 1 kHz and has a time resolution of 200 ps per point. Decay times of several hundred microseconds can be measured.

Coulter DU800 spectrophotometer. Steady state emission spectra were measured on a Quantamaster spectrophotometer from Photon Technology International; each sample was dissolved in anhydrous CH3CN and thoroughly de-aerated with N2. Emission lifetimes were measured at the maximum emission wavelength for each complex following 460 nm LED excitation using the time-correlated single photon counting (TCSPC) module on the Quantamaster spectrophotometer (PTI). Elemental analysis was performed by Midwest Micro Lab, LLC (Indianapolis, IN, USA). 2.2. Electrochemistry Cyclic voltammetry was performed using a standard three-electrode cell on a BioAnalytical Systems (BAS) 100B potentiostat and cell stand with a 3 mm-diameter glassy carbon working electrode, a Pt wire auxiliary electrode, a pseudo Ag/AgCl reference electrode (1.5 mm diameter Ag wire coated with AgCl), and at a scan rate of 50 mV/s. Solutions were prepared in anhydrous acetonitrile with tetrabutylammonium hexafluorophosphate (0.1 M) added as supporting electrolyte, purged with N2 prior to measurement, and subsequently maintained under a blanket of N2. Ferrocene (purified by sublimation) was added to each solution as an internal standard and redox potentials were referenced to SCE using the ferrocene/ferrocenium couple (0.40 V vs. SCE (CH3CN) [29]).

2.4. Synthesis of precursors and dyads Synthesis details are presented in the Supporting Information. 3. Results and discussion 3.1. Design and synthesis of linked dyads The photocatalyst dyads D1 and D2 were constructed using a modular design approach. [30] In this approach, molecular modules are structurally tuned and mechanistically investigated separately, prior to linking with a complementary module. Subsequently, the knowledge gained from studying the modules independently provides a foundation for achieving a deep understanding of the activity observed in the modular assemblies. Here we selected to use the well-known molecular photosensitizer module [Ru(bpy)3]2+ (Ru) for its robust structure, relatively long excited state lifetime, and fairly negative excited state reduction potential. [14] Additionally, the bpy chelating ligands have well-developed synthetic chemistry and are readily functionalized with a variety of linking groups. The catalyst module, Co, is a highly active and stable molecular catalyst for aqueous proton reduction that we have been developing in our group [31] and is closely related to a number of other molecular photocatalysts based on tetrapyridyl coordination of Co(II) [32–35]. The linked dyads D1 and D2 were also designed to investigate the effect of two distinctly different types of bridging ligands on the photoinduced electron transfer kinetics between the Ru photosensitizer module and the Co catalyst module. D1 links Ru and Co through a short

2.3. Transient absorption spectroscopy Femtosecond transient absorption spectroscopy was measured at the Center for Nanoscale Materials at Argonne National Laboratory using an amplified Ti:Sapphire laser system (Spectra Physics, Spitfire Pro) and an automated data acquisition system (Ultrafast Systems, Helios). The amplifier was seeded with the 100 fs output from the oscillator (Spectra Physics, Tsunami) and was operated at 5.0 kHz, giving 0.6 mJ pulses centered at 790 nm. The beam was split 90/10, with the weaker beam being used to generate the white light continuum (440 to 760 nm) probe after traversing a motorized delay stage by being focused into a sapphire plate. The continuum probe was focused to a spot size of 200 μm at the sample and subsequently focused into a fiber optic coupled to a multichannel spectrometer and CMOS sensor. The other 790 nm beam was used to pump an optical parametric amplifier (OPA, Light Conversion, TOPAS). The OPA was tuned to 1660 nm and its output was quadrupled, giving 415 nm pump beam. This beam was passed through 60

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Scheme 1. Synthesis of D1.

and flexible methylene spacer that does not support electronic interaction between the two modules, but positions them in close spatial proximity. By contrast, the link between Ru and Co in D2 is a modified tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2″’,3″’-j]phenazine (tpphz) bridging ligand that has been used in numerous examples to create homo- and hetero-metallic dinuclear assemblies. [36–41] The tpphz bridging ligand positions the metal centers 12–13 Å away from each other (estimated from crystal structures of related heterometallic complexes[41]), but provides a conjugated link for multiple metal-metal and metal-ligand charge transfer pathways. Another key difference between D1 and D2 is the location of the dyad connection to Co, which may have additional influence on the catalytic activity since we propose that the ligand plays an important role in catalytic proton reduction. [31] We hypothesize that the link directly to the bpy-bridging amine in D1 may impact the proton relay activity to the cobalt site. [42,43] Conversely, the bridging ligand of D2 connects the extended π-system of tpphz to the catalyst macrocycle, likely influencing the ability of the bpy components of Co to participate in distributing the multiple electron transfer steps required for catalytic proton reduction. The synthesis for D1 is presented in Scheme 1, and complete details are described in the Supporting Information. The bridging ligand bL1 was obtained by deprotonation of the bis(bipyridine) precursor (1) followed by nucleophilic substitution to 4-bromomethyl-4′-methyl-2,2′bipyridine. Because of its steric bulk around the Ru(II) metal center, [Ru(bpy)2Cl2] selectively bound to the methyl-appended bpy to yield dyad precursor Ru-bL1. The heterometallic dyad D1 was then obtained by introducing Co(II) to a solution of Ru-bL1 dissolved in acetonitrile. The synthesis of D2 was more complicated than that for D1, requiring six steps to achieve the asymmetric bridging ligand bL2, and eight steps overall to isolate D2 (Scheme 2). Complete procedures are described in the Supporting Information, but briefly 2-chloro-1,10phenanthroline (4) was first oxidized to diketone (5) using a mixture of concentrated sulfuric and nitric acid in the presence of potassium bromide. The diketone was protected using an acetonide group and reacted with 6-(N-butylamino)-2,2′-bipyridine (3) using a palladium catalyzed Buchwald-Hartwig amination to form the protected tetrapyridyl intermediate (7). After deprotection with aqueous trifluoroacetic acid the resulting diketone (8) was condensed with 1,10phenanthrolin-5,6-diamine to obtain the final phenazine containing bridging ligand (bL2). Similar to Ru-bL1, [Ru(dtbbpy)2Cl2] (where dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) selectively reacted with bL2 on the sterically more accessible phenanthroline side and the corresponding Ru-bL2 was treated with cobalt(II) tetrafluoroborate to obtain the final dyad D2. We note that tert-butyl groups were appended to the auxiliary ligands of the Ru module on this dyad, which increased solubility in organic solvents and resulted in improved yields and product purity following isolation by chromatography.

3.2. Ground state characterization The absorbance and emission spectra of D1, D2 and their modules were measured in CH3CN and are shown in Fig. 2. The summary of these spectra in Table 1 also includes related minimalist [Ru(bpy)3]2+type modules to compare with Ru-bL1 and Ru-bL2 which include the full bridging ligand and Co(II)-binding site. In general, the Ru modules and dyads are characterized by a metal-to-ligand charge transfer (MLCT) band around 450–460 nm, typical for [Ru(bpy)3]2+ and its analogs, and stronger π→π* transitions below 400 nm. [14] We observe that the wavelength and magnitude of the MLCT band of Ru-bL1 and its minimal analog [Ru(bpy)2(dmb)]2+ (where dmb = 4,4′-dimethyl-2,2′bipyridine) is largely unchanged, suggesting that the Co(II)-binding ligand has little to no interaction on the MLCT state. We also observe that the visible spectra of D1 is quite similar to that of Ru-bL1, confirming that the two modules are electronically decoupled in this dyad. By contrast, we observe dramatic differences in the absorbance spectra between the minimalist analog [Ru(dtbbpy)2(phen)]2+ (where phen = 1,10-phenanthroline), the dyad precursor Ru-bL2, and the heterometallic dyad D2. Increasing the conjugation from phen to tpphz results in a 230% increase in the extinction coefficient of the MLCT band, and similar effects have previously described. [39,44] However, when Co(II) is bound to Ru-bL2 to generate D2, we observe blue shifts in both the MLCT and ligand-centered bands in the near UV and visible region of the spectrum. We assign these differences to changes in the electronic structure of the tpphz ligand following Co(II) complexation based on similar changes observed in previous literature that confirmed that protonation of the phenanthroline nitrogens distal to Ru(II) has a similar effect as metalation of the distal phenanthroline [45]. The emission spectra of the dyad precursors and dyads in CH3CN are presented in Fig. 2B. Complexes Ru-bL1 and Ru-bL2 display a strong emission peak at 613 and 629 nm, respectively, following excitation at their peak MLCT absorbance, which is assigned to 3MLCT decay. However, under the same conditions of solvent, concentration, and excitation wavelength, the dyads display a very weak response with roughly the same emission maximum. The intensity of the peak for D1 is only 4% of that observed for Ru-bL1, and the intensity of the response for D2 is less than 1% of that observed for Ru-bL2. The quenching of the emissive 3MLCT state of the Ru modules in the dyads could be a result of either electron or energy transfer, although the efficiency for intramolecular energy transfer between the modules of these dyads is likely to be quite low since there is no spectral overlap between the Ru* donor and Co acceptor. [46] The cyclic voltammetry of Co, Ru-bL1, D1, Ru-bL2, and D2 in CH3CN are presented in Figures S32 and S33 and is summarized in Table 1. The Co(III/II) oxidation potential of Co appears at approximately 1 V vs. SCE and is only quasi-reversible, which is commonly observed in related Co(II) macrocycles. [47] The Co(II/I) reduction potential is well-behaved, reversible, and appears at -0.69 V vs. SCE. Reduction of the metal center is followed by two one-electron bpy-

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Scheme 2. Synthesis of D2.

measurement. From the optical and electrochemical data, we can estimate the thermodynamic driving force for electron transfer from the photoexcited Ru module to the Co catalyst module in the dyads using the Rehm-Weller equation. [48] Interestingly, despite the structural differences between D1 and D2 and their associated shifts in the ground state response of each module, we calculate effectively the same ΔG0ET for electron transfer from Ru* to Co for each dyad, -0.32 V. These ground state measurements suggest that there is sufficient driving force in both dyads for the Co catalyst module to oxidatively quench Ru* in assemblies for photocatalytic proton reduction.

3.3. Transient absorption spectroscopy Ultrafast and nanosecond transient absorption spectroscopy were used to investigate the influence of bridging ligand between D1 and D2. Following excitation of the MLCT band of D1, we observe a large ground state bleach between the pump wavelength and approximately 500 nm (Fig. 3A). A broad excited state feature extends throughout the rest of the visible region, which is consistent with the response obtained following ultrafast excitation of [Ru(bpy)3]2+ and its closely related analogs. [51] The kinetics of the ground state bleach at 468 nm and excited state absorption at 601 nm in the ultrafast region were modeled using a global fit to an exponential decay equation containing a longlived amplitude and are summarized in Table 2. The complete decay of the D1 excited state was measured using a long time-scale experiment (Figure S36). The overall kinetics have three components: an initial relatively fast decay component with τ = 1.7 ns (Fig. 3C, red traces), followed by a bi-exponential decay in the nanosecond regime with τdecay2 = 30 ns and τdecay3 = 489 ns (Figure S37). The longest decay component, τdecay3 = 489 ns, is in reasonable agreement with excited state lifetimes measured for related [Ru(bpy)3]2+ modules, [52] although the observation of multiple components in the photoinduced kinetics signal unusual behavior. Bi-exponential kinetics observed in molecular photocatalyst dyads composed of Ru and a Mn(II) water oxidation catalyst module were attributed to degradation of the catalyst module, [53] but here we confirmed the stability of D1 by mass spectrometry following the optical experiments. Our group’s previous work on dyads composed of Ru linked to a cobaloxime catalyst module via axial ligation directly to the Co(II) center suggests that bi-exponential kinetics on the picosecond timescale arise from a more efficient charge

Fig. 2. (A) Absorbance and (B) emission spectra for Ru-bL1, D1, Ru-bL2, and D2 in CH3CN. Excitation wavelength of 450 nm for Ru-bL1 and D1, 442 nm for Ru-bL2 and D2.

based reductive waves at approximately -1.2 and -1.5 V vs. SCE. The Ru (II) center in all of the complexes compared here display a Ru(III/II) oxidation potential near 1.2 V vs. SCE, suggesting only nominal influence of the auxiliary and bridging ligands on the Ru(II) electronic structure. The cyclic voltammograms measured for D1 and D2 are approximately the sum of their modules, although there is additional irreversibility in the Co(II/I) potential and a large irreversible peak at negative potentials, suggesting an electrode adsorption event during the 62

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Table 1 Summary of optical and electrochemical characterization of molecular modules and dyads in CH3CN.

Co [Ru(bpy)2(dmb)]2+ Ru-bL1 D1 [Ru(tbbpy)2(phen)]2+ Ru-bL2 D2

λabs / nm (ε / M−1 cm−1)

λem / nm

Ru(III/II) (V vs. SCE)

Co(II/I) (V vs. SCE)

ΔGRu*→Co (V)

ref.

346 451 452 452 454 442 396 442

n/a 629 613 613 610 629 631

– +1.25 +1.26 +1.26 +1.14 +1.23 +1.22

−0.69 – – −0.67 – – −0.64

– – – −0.32 – – −0.32

this work [49] this work this work [50] this work this work

(14,700) (14,000) (16,000) (16,100) (16,000) (36,900) (36,500) (27,200)

recombination pathway than that for charge separation. [21,54,55] However, in this case inspection of the transient spectra in the visible region does not reveal any spectral features that might be attributed to the Co(I) reduced state of Co [56–58], even at very early delay times. The ultrafast transient spectra of D2 in CH3CN are presented in Fig. 3B and show striking differences in comparison to D1. The kinetics of the ground state bleach at 484 nm and the excited state absorption at 606 nm were modeled using a global fit to a three-phase exponential decay equation. Both features display a non-impulsive rise component with a time constant of 7.2 ps (Table 2). Following this initial rise, both the ground state bleach and excited state absorption recover following a two-exponential decay model with a component of 230 ps and a longer 6.8 ns component (Fig. 3C, blue trace). The large excited state absorption feature observed following excitation of D2 is distinct from that observed with D1, and correlates well with the excited state spectra observed for related multimetallic complexes based on coordination to [Ru(bpy)2(tpphz)]2+. [40,41] Comparison of the transient spectra and kinetics for the dyads shows that the bridging ligand does indeed have a profound influence on the excited state lifetime of the Ru module. However, neither dyad appears to support intramolecular photoinduced charge transfer from Ru* to Co. The transient spectra and kinetics of D1 suggest that Ru acts like a completely independent unit, much like [Ru(bpy)3]2+ alone, and that the flexible methylene bridging ligand does not support efficient electron transfer to Co. Conversely, the tpphz bridging ligand of D2 is an excellent electron acceptor for the photoexcited Ru module, but subsequent formation of the reduced Co module is not observed. Emission lifetime measurements of the precursors and dyads show that the 3 MLCT lifetime is quenched for both D1 and D2 as compared to Ru-bL1 and Ru-bL2 (Figures S38-S40, Table S4), but without clear evidence of Co(I) formation this quenching is not a result of oxidative quenching but likely energy transfer mechanisms as proposed recently for similar chromophore—catalyst dyads. [59]

Table 2 Summary of photoinduced kinetics of linked photocatalyst dyads in CH3CN, λex = 415 nm. Kinetics for D1 at 468 nm measured by nanosecond transient absorption. Complete fitting details are found in the Supporting Information.

D1 D2

τrise (ps)

τdecay1 (ns)

τdecay2 (ns)

τdecay3 (ns)

– 7.2 ± 0.5

1.7 ± 0.1 0.23 ± 0.01

30 ± 1 6.8 ± 0.2

489 ± 52 –

4. Conclusions In summary, we have presented the design and synthesis of two new linked photocatalyst dyads using a modular design approach. These dyads were assembled using the prototype [Ru(bpy)3]2+ photosensitizer module (Ru) and a newly developed and highly active macrocyclic Co(II)bis(bpy) catalyst module (Co). However, probing the photoinduced electron transfer behavior using transient optical spectroscopy, we do not observe significant accumulation of the one-electron reduced Co module which may then go on to promote photocatalytic activity for proton reduction. [22] Indeed, attempts to detect aqueous H2 photocatalysis were unsuccessful using conditions that yield impressive activity and stability from the analogous intermolecular system. [31] From these studies we conclude that these linked assemblies in particular suffer from either ineffective forward electron transfer from Ru* to Co, or highly efficient back electron transfer, [54] precluding homogeneous, solution phase, photocatalytic activity. However, linked molecular dyads such as these may be better deployed as electrode-immobilized photoelectrocatalysts [60,61] which would provide a strategy for fast regeneration of the photosensitizer ground state at relatively mild applied potentials and promote efficient, long-lived charge accumulation at the catalyst module. Immobilized molecular architectures of this nature combine the atomic-level precision enabled by molecular synthesis and characterization with the ability to achieve highly stable and active photocatalysts whose overall kinetics are not dictated by diffusion of sacrificial electron donors or acceptors. Therefore, ongoing efforts with these dyad architectures are

Fig. 3. Comparison of ultrafast transient optical spectroscopy of D1 and D2 in CH3CN. Transient spectra of D1 (A) and D2 (B), delay time noted in legend. C) Kinetic traces of excited state decay and ground state bleach for D1 (red) and D2 (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 63

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focused on investigating the effect of surface tethering on the photoinduced charge transfer kinetics between the molecular modules.

[16]

Author information [17]

The authors declare no competing financial interests. Acknowledgments

[18]

This work was supported 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 under Contract No. DE-AC02-06CH11357. 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. We thank Dr. David J. Gosztola for his expert assistance at the transient absorption facility at CNM.

[19]

[20]

[21]

[22] [23]

Appendix A. Supplementary data [24]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.12. 025. Details of synthesis for new compounds and dyads, 1H NMR spectra of new compounds, cyclic voltammetry of modules and dyads, nanosecond transient absorption of D1. (PDF).

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