Direct Observation Of Sequential Electron And Proton Transfer In

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Direct Observation Of Sequential Electron And Proton Transfer In Excited State ETPT Reactions Kristina Martinez, Jacqueline Stash, Kaitlyn R. Benson, Jared J. Paul, and Russell H. Schmehl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09268 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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The Journal of Physical Chemistry

Direct Observation Of Sequential Electron And Proton Transfer In Excited State ETPT Reactions Kristina Martinez a, Jacqueline Stash, Kaitlyn R. Benson, Jared J. Paul b, Russell H. Schmehl a* a. Department of Chemistry, Tulane University, New Orleans, LA, 70118 b. Department of Chemistry, Villanova University, Philadelphia, PA, 19085

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ABSTRACT The prospect of a photoexcited chromophore undergoing a reaction that involves transfer of an electron and proton within an encounter complex with an appropriate acceptor is examined in this work, employing [(bpy)2Ru(4,4’-dhbpy)]2+ (4,4’-dhbpy = 4,4’-dihydroxy-2,2’-bipyridine, [Ru(II)OH]2+) as chromophore and MQ+ (N-methyl-4,4’-bipyridinium) as acceptor. The use of an aprotic solvent (CH3CN) allowed unambiguous evaluation of the photoproducts emerging from the encounter complex by using transient absorption spectroscopy in the visible. While the overall photoproducts reflect transfer of an electron and proton from [Ru(II)OH]2+ to MQ+, detailed analysis suggests that the process occurs via excited state electron transfer (kq = 7 x 107 M-1s-1) followed by proton transfer to MQ0 from [Ru(II)OH]2+ in solution. The kinetics of the proton transfer process are pseudo-first-order in the Ru complex and have second order rate constants that are near the diffusion limit. The results are discussed in terms of the free energies for the reactions involved.


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Introduction Mimicking the complex cascade of reactions that occur in photosynthesis is a difficult task, requiring the buildup of reducing equivalents while simultaneously managing multiple proton transfer interactions. Therefore, the design of chromophores that can participate in directed electron transfer and proton transfer reactions (proton-coupled electron transfer) involving their excited-states is a productive step toward the overarching goal of coupling photoredox reactions to catalytic processes yielding high energy products.1, 2 As an example, some chromophores can act as both reductants in their reactive excited-state and also serve as acids following reduction of substrates. If the substrate is a species that may be reduced by more than one electron, protonation of the one-electron reduced substrate will serve to lower the thermodynamic barrier to the second reduction. If the substrate is a catalyst, this feature, coupled with utilization of light energy to generate electron/proton equivalents has the potential to be applied to the catalytic reduction of small molecules such as protons and carbon dioxide to yield usable products such as H2, carbon monoxide.3 One early example of excited-state proton-coupled electron transfer (PCET*) in the literature can be seen in the work of Nocera and coworkers with Zinc porphyrin dyads linked through amdinium-carboxylate salt bridges. The observation of proton-coupled electron transfer (PCET) was shown to be greatly influenced by the dipole orientation of the salt bridge.4–8 Many transition metal complex chromophores have metal-to-ligand charge transfer (MLCT) excited states, some of which have ionizable protons as a part of the ligand structure. Some time ago Meyer’s group reported PCET* involving [(bpy)2Ru(bpz)]2+ (bpz = bipyrazine) complexes as electron and proton acceptors in reactions with hydroquinones,9,

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extending work of

Hoffman.11–13 The mechanism was determined to be PCET* based on the observed kinetic isotope effect and thermodynamic considerations, but the systems included secondary reactions that obscured direct observation and evolution of transient species. A significant potential advantage of exploring Ru(II) complexes with 3MLCT excited-states for the investigation of photoinduced ET/PT (electron transfer/ proton transfer), is the ability to prepare complexes having carefully tuned absorption /redox / acid-base characteristics that can be traced spectroscopically following pulsed laser excitation. With appropriate systems UV-vis transient absorption can be particularly useful in unambiguous separation of electron transfer


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(ET) only or proton transfer (PT) only intermediates from reactions that exhibit overall ET/PT following visible excitation. In 2010, Meyer and coworkers published a short review describing the PCET* events in photosystem II and inspired artificial photosynthetic schemes. Included was a report of PCET* from a complex, [(bpy)2Ru(5,6-dhphen)]2+ (5,6-dhphen = 5,6-dihydroxy-1,10-phenanthroline), to a proton/ electron acceptor N-methyl-4,4’-bipyridinium (MQ+).3 The detailed results of this work were never published. Since that time, MQ+ has been employed in various ways by other groups as an electron and/or proton acceptor in light induced reactions.14–16 More recently, PCET* from a [(bpy)2Ru(pyimH)]2+ (pyimH= 2-pyridyl-imidazole), to MQ+ was demonstrated by Wenger and coworkers.14 Photoexcitation of the complex in buffered water/acetonitrile solvent showed evidence for overall hydrogen atom transfer to MQ+. Upon further investigation, the mechanism was found to proceed through an initial electron transfer from the photoexcited Ru complex to the MQ+ followed by proton transfer that involved either the buffer or solvent. Here, transient spectroscopy was used effectively to illustrate the sequential reactions under certain conditions in buffered aqueous acetonitrile. We have recently been examining excited state proton-coupled electron transfer in a group of chromophores closely related to the 5,6-dhphen complex: [(LL)2Ru(4,4’-dhbpy)]2+ (4,4’-dhbpy = 4,4’-dihydroxy-2,2’-bipyridine and LL = diimine ligands). The utility of these complexes owes to the fact that the hydroxy moiety bound directly to a diimine ligand imparts unique electronic absorption spectra to all the species pertinent to reactions involving electron/proton transfer, single electron transfer, and single proton transfer. We began exploring the characteristics of the derivative in which LL = bpy (2,2’-bipyridine), investigating the ground-state acid-base chemistry in aqueous solution along with the electrochemistry and spectroelectrochemistry of

Chart 1. Structural representation of the Ru(II) complex used and the electron /proton acceptor and the acronyms employed.


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this complex and the analogous methoxy complex, [(bpy)2Ru(4,4’-dmbpy)]2+ (4,4’-dmbpy = 4,4’-dimethoxy-2,2’-bipyridine, (Chart 1).17–20 With MQ+ as the acceptor, which also has unique UV-vis spectra for the one electron reduced and the reduced protonated species, we are able to observe the electron and proton transfer independently and distinguish excited-state proton transfer (PT*) intermediates from those of excited state electron transfer (ET*) as well as the PCET* transient products. Pulsed laser excitation of solutions containing one of these chromophores along with MQ+ allows direct observation of the product as it emerges from the geminate reaction pair as the PT*, ET* or the PCET* species, as shown in Scheme 1.

Scheme 1. Possible products emerging from an encounter complex upon photolysis of the [Ru(II)OH]2+ / MQ+ system.

This work presents spectroscopic details for the reaction of photoexcited [Ru(II)OH]2+ with MQ+ in an aprotic solution. The results clearly illustrate that the final PCET* product forms via an initial electron transfer, followed by proton transfer after the photoproducts escape the successor complex cage. This system thus provides a rare opportunity to directly observe the evolution of reaction intermediates emanating from an encounter complex where both electron and proton transfer to an acceptor are possible.


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Experimental General Considerations. All experiments were carried out in acetonitrile that was dried over CaH 2 and distilled under nitrogen gas unless otherwise stated. 4,4’-bipyridine and methyl iodide were purchased from Sigma Aldrich and used without further purification. Deuterated acetonitrile was purchased from Cambridge Isotope Laboratories (CIL) and dried over 3 Å molecular sieves prior to freeze-pump-thaw degassing. Preparation

of

[(bpy)2Ru(LL)](PF6)2

complexes

(LL= 4,4’-dihydroxy-2,2’-bipyridine,

[Ru(II)OH] 2+, or 4,4’-dimethoxy-2,2’-bipyridine, [Ru(II)OMe] 2+). Synthesis of the complexes [Ru(II)OH](PF6)2 and [Ru(II)OMe](PF6)2 have been previously reported.17 The synthesis of [Ru(II)(OMe)OH](PF6)2 was prepared by the Paul group using methods directly analogous to those used in the synthesis of [Ru(II)OH](PF 6)2; the synthesis will be reported with other related complexes in a manuscript by Paul and coworkers. Preparation of N-methyl-4,4’-bipyridinium N-methyl-4,4’-bipyridinium was synthesized as previously reported.23 4,4’-bipyridine was refluxed in the presence of one equivalent of methyl iodide in dichloromethane overnight. The product precipitated out as a yellow powder. The iodide salt was isolated, and iodide exchanged for the hexafluorophosphate anion. The compound was then recrystallized from hot ethanol twice and dried in vacuo overnight. The resulting white powder was used without further purification. 1HNMR spectra were collected on a Varian 400 MHz NMR. For determination of pKa of MQ+, trichloroacetic acid was used after recrystallization from chloroform. General Electrochemical and Spectroelectrochemical Procedures Electrochemical and spectroelectrochemical measurements were carried out using a CH Instruments 630E Electrochemical Analyzer/Workstation. All measurements were done in acetonitrile

dried

over

CaH2

and

distilled

before

use.

Tetrabutylammonium

hexafluorophosphate (Alfa Aesar) was recrystallized and dried in vacuo before use as supporting electrolyte. Unless otherwise stated, cyclic voltammetric measurements were done using a glassy carbon working electrode, platinum wire counter electrode and Ag/AgCl reference electrode. Spectroelectrochemical measurements were carried out using


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a Pine Research Instruments Honeycomb electrode cell (platinum), with a Ag/AgCl reference, and Ocean Optics UV-visible source, cell and HR2000 spectrometer. General Nanosecond Transient Absorption Procedures Nanosecond transient absorption measurements were done on an Applied Photophysics LKS 60 Laser Flash Photolysis system with laser excitation from a Quantel Brilliant B Qswitched laser with second and third harmonic attachments and an OPO (OPOTEK) for visible light generation, data recorded using an Agilent Infinium digitizer. Laser excitation of sample was typically supplied at 450 nm, with a power output of 12 mJ/ pulse. In order to maintain constant ionic strength in quenching experiments, tetrabutylammonium hexafluorophosphate was added to all samples studied by transient absorption. Preparation of Samples for KIE studies Deuteration of the complex [Ru(II)OH]2+ was accomplished by first exchanging the PF6counterion for [BArF24]- (using sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) and then washing the complex in methanol-d4 (CIL) under N2 gas three times. Synthesis of Na[BArF24] is described elsewhere.21 The complex was rinsed with chloroform-d. The dry, deuterated complex was immediately transferred into a glovebox for sample preparation. Tetrabutylammonium hexafluorophosphate was used as electrolyte in solution for constant ionic strength. All materials were placed in the glove box and samples prepared and placed in sealed cuvettes prior to removal from glovebox. Upon removal, the samples were kept in a desiccator with a constant positive pressure of N2 gas supplied. Measurements were made immediately upon removal from the desiccator. Results and Discussion Thermodynamic Considerations: The synthesis and characterization of the Ru complex representing the focus of this work, along with partial spectroscopic and electrochemical characterization, has been published earlier.17 Since both proton and electron transfer reactions between the Ru complex chromophore and the MQ+ are possible in CH3CN, acquisition of the relevant redox potentials, absorption spectra and pKa values was necessary. The approaches to determination of the thermodynamic constants for the acid-base and redox chemistry of the MQ+ and [Ru(II)OH]2+ complex are described in detail in the supplementary information. Briefly, the acid dissociation constants for HMQ2+ and the Ru(II) hydroxybipyridine complex, [Ru(II)OH]2+, were obtained by spectrophotometric titration in the presence of an appropriate base of known


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pKa of its conjugate acid in CH3CN. The complex [Ru(II)OH]2+ has two titratable protons with pKa values close enough that a spectrophotometric method could not be used to obtain a reliable value for the first acid dissociation (see SI). As an alternative, the closely related complex, [(bpy)2Ru(4-OH,4’-OMe-bpy)]2+ ([Ru(II)(OMe)OH]2+) was used to obtain the single pKa using a spectrophotometric titration (SI). The pKa was estimated to be 17.7 ± 0.04.

The pKa values for

the one electron reduced species HMQ•+(cation radical) and [Ru(III)OH]3+ (oxidized complex) could not be obtained by direct measurement and were derived from thermodynamic cycles (SI). The electrochemistry and spectroelectrochemistry of MQ+ and HMQ2+ were previously reported by Wenger.22 We also carried out cyclic voltammetric analysis in CH3CN and the values reported in the SI agree well with those of Wenger. The redox potentials for [Ru(II)OH]2+ and [Ru(II)O-]+ were reported earlier by Paul et al.20 Of note, the Ru(III/II) potential for [Ru(II)O-]+ is irreversible and the value used is an approximation derived from the anodic peak potential, Ep,a. The acid dissociation constants and redox potentials for the reactants are summarized in Tables 2 and 3. This information provides the data to obtain the free energies for ground state reactions between MQ+ and [Ru(II)OH]2+ in CH3CN. The energies are given in Scheme 2. Each of the

Scheme 2. Free energies for ground state reaction between the Ru(II) hydroxybipyridine complex and MQ + in CH3CN at room temperature.

reactions are endergonic. In fact, solutions of the two reagents do not react in any reasonable combination of concentrations of the reagents in the absence of electronic excitation to create the 3

MLCT excited state of the Ru complex. In assessing possible excited state decay pathways for the Ru complex in the presence of

MQ+, the acid dissociation and redox behavior of [Ru(II)OH]2+* and [Ru(II)O-]+* must be evaluated. The protonated complex is luminescent in CH3CN solution at room temperature but [Ru(II)O-]+* is non-emissive. We found that CH3CN solutions of the complex at -410 C (232 K) exhibit luminescence from both species; further, the emission maximum and bandshape of the of


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the room temperature and 232 K emission of [Ru(II)OH]2+ were very similar. Thus, the 232 K emission spectra were used for determination of excited state energies (Figure S10). Table 1 shows the relevant ground and excited-state redox parameters for both [Ru(II)OH]2+ and [Ru(II)O-]+. Table 1. Ground and excited state Ru(III/II) potentials,

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MLCT energies, and excited-state lifetimes for

[Ru(II)OH]2+ , [Ru(II)O-]+ and [Ru(II)OMe]2+ in CH3CN.

Physical Property

[Ru(II)(OH)]2+

[Ru(II)O-]+

[Ru(II)OMe]2+

E(III/II), V vs. Fc+/Fc

0.85

0.26

0.87

*E(III/II), V vs. Fc+/Fc

-1.08

-1.56

-1.09

E0, eV (kcal/mole)

1.93 (44.5)

1.82 (42.0)

1.96 (45.2)

λmax (nm), -41 0C

642

680

---

τ0 (ns), 25 ̊C

640

Direct Observation Of Sequential Electron And Proton Transfer In

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