Tuning the Photophysical Properties of Ru(II) Monometallic and Ru(II


Jun 8, 2015 - Feixiang Cheng , Shiwen Yu , Pinhua Liu , Mingli Ren , Chixian He. Journal of Coordination Chemistry 2016 69 (23), 3458-3471 ...
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Article pubs.acs.org/JPCA

Tuning the Photophysical Properties of Ru(II) Monometallic and Ru(II),Rh(III) Bimetallic Supramolecular Complexes by Selective Ligand Deuteration Alec T. Wagner,* Rongwei Zhou, Kevan S. Quinn, Travis A. White, Jing Wang, and Karen J. Brewer† Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212, United States S Supporting Information *

ABSTRACT: A series of three new complexes of the design [(TL)2Ru(BL)]2+, two new complexes of the design [(TL)2Ru(BL)Ru(TL)2]4+, and three new complexes of the design [(TL)2Ru(BL)RhCl2(TL)]3+ (TL = bpy or d8-bpy; BL = dpp or d10-dpp; TL = terminal ligand; BL = bridging ligand; bpy = 2,2′-bipyridine; dpp = 2,3-bis(2-pyridyl)pyrazine) were synthesized and the 1H NMR spectroscopy, electrochemistry, electronic absorbance spectroscopy, and photophysical properties studied. Incorporation of deuterated ligands into the molecular architecture simplifies the 1H NMR spectra, allowing for complete 1H assignment of [(d8bpy)2Ru(dpp)](PF6)2 and partial assignment of [(bpy)2Ru(d10-dpp)](PF6)2. The electrochemistry for the deuterated and nondeuterated species showed nearly identical redox properties. Electronic absorption spectroscopy of the deuterated and nondeuterated complexes are superimposable with the lowest energy transition being Ru(dπ) → BL(π*) charge transfer in nature (BL = dpp or d10-dpp). Ligand deuteration impacts the excited-state properties with an observed increase in the quantum yield of emission (Φem) and excited-state lifetime (τ) of the Ru(dπ) → d10-dpp(π*) triplet metal-to-ligand charge transfer (3MLCT) excited state when dpp is deuterated, and a decrease in the rate constant for nonradiative decay (knr). Choice of ligand deuteration between bpy and dpp strongly impacts the observed photophysical properties with BL = d10-dpp complexes showing an enhanced Φem and τ, providing further support that the lowest electronic excited state populated via UV or visible excitation is the photoactive Ru(dπ) → dpp(π*) CT excited state. The Ru(II),Rh(III) complex incorporating the deuterated BL shows increased hydrogen production compared to the variants incorporating the protiated BL, while demonstrating identical dynamic quenching behaviors in the presence of sacrificial electron donor.



INTRODUCTION Converting solar energy to chemical energy is of considerable interest and importance in the development of renewable energy resources.1−3 An attractive approach is solar H2O splitting to produce H2 and O2 as this process is carbon neutral. This thermodynamically unfavorable, multielectron process can be achieved with solar photons, which provide sufficient energy to drive this uphill reaction.4−7 Given the low absorptivity of H2O within the required range for H2O splitting (E > 1.23 eV), Photocatalytic systems must be designed to efficiently absorb solar energy and being capable of delivering electrons to water for this multielectron catalysis. The prototypical light absorber [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) undergoes visible light excitation to populate a metal-to-ligand charge transfer (MLCT) excited state that is both a stronger oxidizing and reducing agent than the ground-state species. This long-lived, highly energetic state is capable of delivering the harnessed solar energy to water with an appropriate catalyst and electron relay.8−12 Coupling multiple subunits, whose individual properties contribute to the properties of the newly formed species, produces supramolecular complexes capable of function-specific design.13 Photochemical molecular devices, which incorporate light absorber (LA) units, are supramolecular complexes that utilize light energy as the thermodynamic driving force to © XXXX American Chemical Society

initiate a process such as directional electron transfer, charge separationm or energy transfer. Connecting Ru(II) light absorbing (LA) units to reactive metals (RM) through a polyazine bridging ligand (BL) offers a way to expand the supramolecular architecture and develop active photocatalysts to achieve solar water splitting. Use of polyazine BLs allows for electronic coupling, promoting electron or energy transfer in this architecture.14−18 Incorporating RM centers (Rh,19−23 Pt,24−27 Pd,24 Co24,28−30) to form bimetallic complexes have recently been shown to photocatalytically reduce water to produce hydrogen. Structural assignment of mono- and multimetallic complexes containing bidentate polypyridyl ligands using 1H NMR spectroscopy is difficult given the large number of nearly equivalent aromatic protons, the number of structural isomers and low symmetry of the complexes. Deuteration of select terminal ligands (TL) or BL permits assignment of the 1H NMR spectrum of nondeuterated ligands within the supramolecular assembly.31−35 This was demonstrated with the complex [(bpy)2Ru(L)]2+ (L = 5,6,10,11-tetrahydro-16,18diazadipyrido[2,3-a:3′,2′-n]pentacene).32,33 Substitution of Received: March 24, 2015 Revised: June 7, 2015

A

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Figure 1. Monometallic and bimetallic complexes employed to study selective deuteration of polyazine ligands. D indicates complete ligand deuteration.

bpy with d8-bpy reduces the number of nonequivalent aromatic proton signals associated with bpy and L from 26 to 10, leaving proton signals unique to L. This clarifies the 1H NMR spectrum and allows for full assignment of previously indistinguishable signals from L.32,33 Simplification of the 1H NMR using deuterated TLs or BLs allows for enhanced structural characterization while also offering a method to study H2O reducing photocatalysts during reduction or photocatalysis to gain insight into the photocatalytic functioning. Ligand deuteration has also been shown to influence the excited-state properties of complexes.31,36−46 Ligand deuteration modifies the stretching frequency and amplitude associated with the nonradiative deactivation (knr) modes of the excited states of inorganic complexes. In deuterated species, nonradiative deactivation is decreased due to the increased mass of deuterium dampening the nonradiative deactivation modes associated with C−D bonds compared to equivalent C−H bonds. When the deuterated ligand is associated with the emitting excited state, the lower stretching frequency decreases the overlap between the excited and ground states, causing a decrease in the rate constant for nonradiative deactivation and an increase in the observed excited-state lifetime.36,41,47−50 This effect was demonstrated with complexes of the type [Ru(bpy)2(L3)]+, [Ru(bpy)2(L4)]+, and [Ru(bpy)2(HL4)]2+, where L3 = 3-(pyridin-2-yl)-1,2,4-triazole, L4 = 3-(pyrazin-2-yl)-1,2,4triazole, and HL4 = 2-(4H-1,2,4-triazole-3-yl)pyrazine. The excited-state lifetimes were measured with and without select deuteration of the ligands to determine the nature of the emitting state. Upon deuteration of the bpy ligand in both [(bpy)2Ru(L3)]+ and [(bpy)2Ru(L4)]+ the lifetime increases from 145 to 250 ns and 230 to 290 ns, respectively, whereas deuteration of the L3 and L4 ligand has no appreciable effect on

the lifetime. This demonstrates the emitting state for both of these complexes is formally Ru(dπ) → bpy(π*) 3MLCT.41,42 In [(bpy)2Ru (HL4)] 2+, bpy deuteration has no effect on the excited-state lifetime, whereas deuteration of HL4 caused the lifetime to increase from 230 to 470 ns, indicating that the emitting state is formally Ru(dπ) → HL4(π*) 3MLCT.41,42 Reported herein are the synthesis and 1H NMR spectroscopy, electrochemistry, electronic absorbance spectroscopy, and photophysical properties of three new Ru(II)-polyazine monometallic complexes, [(d 8 -bpy) 2 Ru(dpp)](PF 6 ) 2 , [(bpy)2Ru(d10-dpp)](PF6)2 and [(d8-bpy)2Ru(d10-dpp)](PF6)2 two new Ru(II),Ru(II) polyazine-bridged bimetallic complexes, [(d8-bpy)2Ru(dpp)Ru(d8-bpy)2](PF6)4 and [(bpy)2Ru(d10dpp)Ru(bpy)2](PF6)4, and three new polyazine-bridged Ru(II),Rh(III) bimetallic complexes, [(bpy) 2 Ru(dpp)RhCl2(bpy)](PF6)3, [(d8-bpy)2Ru(dpp)RhCl2(d8-bpy)](PF6)3, and [(bpy)2Ru(d10-dpp)RhCl2(bpy)](PF6)3 (Figure 1). Related Ru(II),Rh(III) bimetallics form a new class of single component H2O reduction photocatalysts.51−53



RESULTS AND DISCUSSION Synthesis. Deuterated polypyridine ligands, prepared using the method of Browne et al.,39 were incorporated into the building block approach to design Ru(II) monometallic, Ru(II),Ru(II) bimetallic and Ru(II),Rh(III) bimetallic complexes.13,41,54 The Ru(II)-based LA was synthesized by coordinating two bpy TL and dpp to the Ru(II) metal center in two steps.55,14 The Rh(III)-based reactive metal (RM) subunit was synthesized by coordinating one bpy TL to the Rh(III) metal center.56 Deuterated bpy TL (d8-bpy) and dpp BL (d10-dpp) were selectively substituted for the protiated analogues to develop the desired Ru(II) or Rh(III) B

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Scheme 1. Schematic Depicting the Building Block Approach in the Synthesis of [(bpy)2Ru(d10-dpp)]2+, [(bpy)2Ru(d10dpp)Ru2(bpy)]4+, and [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ a

a

bpy = 2,2′-bipyridine; dpp = ,3-bis(2-pyridyl)pyrazine; D indicates ligand deuteration.14,19,56

monometallic precursors. The Ru(II),Ru(II) and Ru(II),Rh(III) bimetallic complexes were synthesized in moderate yield by covalently coupling one LA and one RM subunit or another Ru(II) unit using careful control of reaction stoichiometry, Scheme 1.19,56 Structural characterization of the bimetallic complexes through the use of 1H NMR spectroscopy was complicated by isomerization at the metal centers, making this characterization technique impractical in the protiated bimetallics. However, characterization of the monometallic complexes by 1H NMR spectroscopy was greatly simplified by deuteration of the ligands compared to characterization of the fully protiated species. Both monometallic and bimetallic complexes were characterized by ESI-MS, electrochemistry, electronic absorption spectroscopy, and steady-state and timeresolved luminescence spectroscopy. Nuclear Magnetic Resonance Spectroscopy. 1-D 1H NMR and 2-D 1H−1H correlation spectroscopy (COSY) NMR experiments were used to characterize the seven new molecules discussed in this report. Inclusion of deuterated ligands into the supramolecular architecture permits detailed analysis of the unsymmetrical monometallic complexes. In the monometallic complexes, d10-dpp substitution results in the complex exhibiting only 1H signals from the remaining bpy ligands and vice versa when d8-bpy is substituted. Figure 2 shows the 1 H NMR spectra for [(bpy)2Ru(dpp)]2+, [(d8-bpy)2Ru(dpp)] 2+, and [(bpy) 2Ru(d10-dpp)]2+. [(d8-bpy)2Ru(d10dpp)]2+ shows no appreciable 1H NMR signals due to deuteration of all ligands in the complex. Substitution of deuterated ligands into the bimetallic architecture should yield similar results that were observed in the monometallic complexes. However, this trend was not

Figure 2. Downfield (aromatic) region of the 1H NMR spectra of [(d8-bpy)2Ru(dpp)]2+ (top), [(bpy)2Ru(dpp)]2+ (middle), and [(bpy)2Ru(d10-dpp)]2+ (bottom) recorded at 400 MHz in CD3CN. bpy = 2,2′-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine.

observed to the same extent due to the number of structural isomers present and the lack of diagnostic resonances in both the Ru(II),Ru(II) and Ru(II),Rh(III) bimetallic complexes (Figures S1 and S2, Supporting Information, respectively). The 1 H NMR spectra of the selectively deuterated Ru(II),Rh(III) bimetallics do show qualitative differences and allow general assignment of the proton resonances as originating from either bpy or dpp. The Ru(II),Ru(II) bimetallic complexes show a large number of signals in their 1H NMR spectra (Figure S1, Supporting Information) due to the inequivalence of the two C

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Electrochemistry. Electrochemical analysis of Ru(II) monometallic, Ru(II),Ru(II) bimetallic, and Ru(II),Rh(III) bimetallic complexes, Table S1 (Supporting Information), provides insight into the orbital energetics within this structural motif given the rich cathodic and anodic electrochemistry. Reversible, one-electron RuII/III oxidations are observed for monometallic (+1.48 V vs Ag/AgCl), Ru(II),Ru(II) bimetallic (+1.44 V vs Ag/AgCl), and Ru(II),Rh(III) bimetallic (+1.61 V vs Ag/AgCl) complexes, indicating the highest occupied molecular orbital (HOMO) within this architecture is Rubased. Reductively, the Ru(II) monometallic complexes display a one-electron dpp0/− reduction (−0.98 V vs Ag/AgCl) prior to the bpy0/− reductions (−1.40 and −1.69 V vs Ag/AgCl), which is in agreement with the previous synthesized protiated analogue.14 The Ru(II),Ru(II) bimetallics show a dpp0/− reduction (−0.61 V vs Ag/AgCl) prior to a second dpp couple, dpp−/2− reduction (−1.12 V vs Ag/AgCl), and the third ligand-based reduction inside the electrochemical solvent window (−1.41 V vs Ag/AgCl) is 2 bpy0/− reductions corresponding to one bpy on each Ru(II) center.57 The reductive properties of the Ru(II),Rh(III) bimetallics (Figure 4) are considerably more complex than the Ru(II) mono-

bpy ligands on each Ru(II) center induced by the nonsymmetrical bridging ligand dpp. On the contrary, the Ru(II),Rh(III) bimetallic complexes show complicated 1H NMR spectra due to the number of structural isomers generated during synthesis. These isomers arise from the use of the dpp bridging ligand, an AB chelating ligand, connecting the Ru(II) and Rh(III) metal centers. However, through the use of 1-D 1H NMR and 2-D 1H−1H COSY and 1H−13C HSQC NMR techniques, many of the protons were able to be assigned and the number of isomers was verified. Figure 3 shows the effect of coordination of a

Figure 3. Downfield (aromatic) region of the 1H NMR spectra of [(d8-bpy)2Ru(dpp)RhCl2(d8-bpy)]3+ (top) and [(d8-bpy)2Ru(dpp)]2+ (bottom) recorded at 400 MHz in acetone-d6. bpy = 2,2′-bipyridine and dpp = 2,3-bis(2-pyridyl)pyrazine.

Rh(III) metal center to the unbound side of the dpp bridging ligand in a Ru(II) monometallic complex to form a Ru(II),Rh(III) bimetallic complex. The bpy ligands used in the complex were deuterated to reduce the complexity of the NMR spectra and follow the 10 distinct dpp resonances before and after coordination. Upon coordination of the Rh(III) metal center, the dpp proton resonances shift further downfield and are split into two distinct resonances (Figure 3) due to the presence of two different ligand types on the Rh(III) center: two identical chloride ligands and a bpy ligand (TL′). The splitting of the dpp proton resonances is due to the location of their ring with respect to the chloride ligands and the bpy ligand (TL′) on the Rh(III) center, generating two geometric isomers for the dpp protons. Isomer 1 has dpp pyrazine (ring A) cis to chloride and trans to bpy (TL′) and dpp pyridine (ring C) trans to chloride and cis to bpy (TL′). Isomer 2 has dpp pyrazine cis to bpy (TL′) and trans to chloride while the dpp pyridine (ring C) is trans to bpy (TL′) and cis to chloride. Using a different NMR spectroscopic handle, the bpy attached to the Rh(III) metal center, it is possible to determine the total number of geometric isomers in the Ru(II),Rh(III) bimetallic complex. The complex using deuterated bpy terminal ligands on the Ru(II) center and a deuterated dpp bridging ligand, [(d8-bpy)2Ru(d10-dpp)RhCl2(bpy)]3+, shows only proton signals from the bpy attached to the Rh(III) center (Figure S4, Supporting Information). The integration of the resonances in the 1H NMR spectrum adds up to 34.5, approximately 4 times the number of protons found on a bpy ligand. This indicates that there are four different magnetic environments for each bpy proton, each arising from a different geometric isomer of the Ru(II),Rh(III) complex. The integrations of resonances between 9.6 and 9.9 ppm also indicate that the four isomers occur in approximately a 1:1:1:1 population distribution. A distribution of isomers similar to this is expected for multimetallic complexes utilizing asymmetrical bridging ligands.

Figure 4. Cyclic voltammogram of [(bpy)2Ru(dpp)RhCl2(bpy)]3+ (solid line), [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ (dashed line), and [(d8-bpy)2Ru(dpp)RhCl2(d8-bpy)]3+ (dotted line) using 0.1 M Bu4NPF6 in CH3CN at RT and referenced against Ag/AgCl E1/2 (ferrocene/ferrocene+) = 0.46 V vs Ag/AgCl). bpy = 2,2′-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine.

metallic counterparts, expected to possess dpp0/− and RhIII/II and RhII/I reductions prior to bpy0/− reductions. Three separate reduction processes, −0.38, −0.72, and −0.99 V vs Ag/AgCl, are observed and attributed to RhIII/II, RhII/I, and dpp0/− reductions, demonstrating the energetic proximity of the Rh(dσ*) and dpp(π*) orbitals in this bimetallic motif.58−60 To confirm the assignment of these reduction processes, a study was performed by White et al. using a set of nearly identical Ru(II),Rh(III) bimetallics, differing only in the terminal ligands.61 It was found that the first two reductions displayed electrochemical irreversibility, indicating an electrochemical reduction followed by rapid halide loss. The irreversibility of these RhIII/II and RhII/I also varies with potential scan rate, providing mechanistic insight into the systems. The third reduction is assigned as dpp0/− due to its reversibility, characteristic of organic ligands in Ru(II) polyazine complexes. Substitution for deuterium does not significantly affect the redox properties of the monometallic and bimetallic complexes as the electron density on the metal or ligand π* aromatic system is not impacted. Electronic Absorption Spectroscopy. Ru(II)−polyazine complexes are efficient light absorbers throughout the UV and visible regions, with intraligand (IL) π → π* transitions in the UV and metal-to-ligand charge transfer (MLCT) transitions D

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to 525 and 508 nm for Ru(II),Ru(II) and Ru(II),Rh(III) bimetallics, respectively. Also, there is an increase in molar absorption coefficients (approximately double) in the Ru(II),Ru(II) bimetallics compared to those of the Ru(II) monometallics and the Ru(II),Rh(III) bimetallics due to the presence of an additional light absorbing unit. The Ru(II),Rh(III) bimetallics are also expected to have Rh based transitions centered around 310 nm, but they are expected to be weak compared to the Ru(II) based transitions and to be masked by the bpy and dpp π → π* transitions. The electronic absorption data indicates that exchange of deuterium for hydrogen on the TL and BL does not greatly affect the light absorbing properties of Ru(II)-polypyridyl complexes. Steady-State Luminescence Spectroscopy. Steady-state luminescence spectroscopy provides insight into the excitedstate dynamics of the Ru(II) monometallic, Ru(II),Ru(II) bimetallic and Ru(II),Rh(III) bimetallic complexes. The luminescence spectra of the monometallic and bimetallic complexes were measured at room temperature in CH3CN and at 77 K in a 4:1 EtOH/MeOH rigid glass matrix to understand the deuteration effect on Ru(II) monometallics and Ru(II),Rh(III) bimetallics. Table 1 summarizes the excitedstate properties. Photoexcitation into the Ru(dπ) → dpp(π*) 1 MLCT excited state is followed by unity population of the Ru(dπ) → dpp(π*) 3MLCT excited state by intersystem crossing (kisc), Figure 6.8,14,19,52 A broad emission centered at λem = 670 nm from this Ru(dπ) → dpp(π*) 3MLCT state was observed for each of the monometallic complexes at RT (Figure 7), with an increased quantum yield of emission for [(d8-bpy)2Ru(d10-dpp)]2+ (Φem = (1.7 ± 0.05) × 10−2) and [(bpy)2Ru(d10-dpp)]2+ (Φem = (1.5 ± 0.05) × 10−2) when compared to those for [(d8-bpy)2Ru(dpp)]2+ (Φem = (1.3 ± 0.05) × 10−2) and [(bpy)2Ru(dpp)]2+ (Φem = (1.2 ± 0.05) × 10−2). In alcoholic glass at 77 K, the emission remained broad but shifted to higher energy for all four Ru(II) monometallic complexes. The RT emission intensity (Figure 7) of the Ru(II),Rh(III) bimetallic complexes is substantially weaker (Φem = (1.3−1.6) × 10−4) than those of the corresponding Ru(II) monometallic complexes (Φem = (1.2−1.6) × 10−2), and the model Ru(II),Ru(II) bimetallic systems, which lack Rh RM centers (Φem = (1.4−1.7) × 10−3). The Ru(II),Ru(II) bimetallics, [(bpy) 2 Ru(dpp)Ru(bpy) 2 ] 4+ , [(d 8 -bpy) 2 Ru(dpp)Ru(d 8 bpy)2]4+, and [(bpy)2Ru(d10-dpp)Ru(bpy)2]4+, are used as models for the nature and energy, knr and kr of the Ru(dπ) → μ-BL(π*) 3MLCT excited state. All three Ru(II),Rh(III) bimetallic complexes display broad emission from the Ru(dπ) → μ-dpp(π*) 3MLCT excited state centered around 780 nm. A decrease in Φem for this 3MLCT state attributed to efficient intramolecular electron transfer (ket) from the 3MLCT state to populate a Ru(dπ) → Rh(dσ*) metal-to-metal charge transfer (3MMCT) excited state, Figure 7. Under the assumption that the rate constants for radiative (kr) and nonradiative (knr) decay are the same for model Ru(II),Ru(II) bimetallic and Ru(II),Rh(III) bimetallic complexes, the rate constant for intramolecular electron transfer (ket) to populate the nonemissive 3MMCT excited state was calculated to be (1.7−2.0) × 107 s−1. While Φem for the mixed-metal bimetallic complexes is reduced by 2 orders of magnitude compared to the case for monometallic complexes, the dpp BL deuteration effect remains. Similar to [(bpy)2Ru(d10-dpp)]2+, the bimetallic complex [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ shows a small, but consistent, enhancement of the quantum yield emission

dominating the visible region. The observed transitions are summarized in Table S2 (Supporting Information). Exchange of deuterium for proton in the TLs and BLs does not significantly affect the light absorbing properties (Figure 5) of

Figure 5. Electronic absorption spectra of (A) [(bpy)2Ru(dpp)]2+ (purple line), [(bpy)2Ru(d10-dpp)]2+ (green line), and [(d8-bpy)2Ru(dpp)]2+ (red line); (B) [(bpy)2Ru(dpp)Ru(bpy)2]4+ (purple line), [(bpy)2Ru(d10-dpp)Ru(bpy)2]4+ (green line), and [(d8-bpy2Ru(dpp)Ru(d8-bpy)2]4+ (red line); and (C) [(bpy)2Ru(dpp)RhCl2(bpy)]3+ (purple line), [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ (green line), and [(d8-bpy)2Ru(dpp)RhCl2(d8-bpy)]3+ (red line) in RT CH3CN. bpy = 2,2′-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine.

these Ru(II)−polypyridyl complexes as the electronic configuration of the complexes are not altered upon incorporation of deuterium. The monometallic complexes display intense bpy (λabs = 282 nm; ε = 6.8 × 104 M−1 cm−1) and dpp (λabs = 310 nm; ε = 2.9 × 104 M−1 cm−1) based π → π* transitions and broad MLCT transitions at lower energy corresponding to the Ru(dπ) → bpy(π*) (λabs = 420 nm; ε = 1.2 × 104 M−1 cm−1) and Ru(dπ) → dpp(π*) (λabs = 460 nm; ε = 1.3 × 104 M−1 cm−1) CT transitions, respectively. Coupling the Ru(II)-based LA and a second metal center to generate Ru(II),Ru(II) or Ru(II),Rh(III) bimetallic complexes shifts the dpp-based transitions to lower energy, indicative of dpp(π*) stabilization upon coordination of two electropositive metal-based centers. The dpp-based IL π → π* transitions shift to ca. 335 nm and the Ru(dπ) → dpp(π*) CT transitions shift E

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The Journal of Physical Chemistry A Table 1. Photophysical Properties of Monometallic and Bimetallic Complexes room tempa complex 2+

[(bpy)2Ru(dpp)] [(d8-bpy)2Ru(dpp)]2+ [(bpy)2Ru(d10-dpp)]2+ [(d8-bpy)2Ru(d10-dpp)]2+ [(bpy)2Ru(dpp)Ru(bpy)2]4+ [(d8-bpy)2Ru(dpp)Ru(d8-bpy)2]4+ [(bpy)2Ru(d10-dpp)Ru(bpy)2]4+ [(bpy)2Ru(dpp)RhCl2(bpy)]3+ [(d8-bpy)2Ru(dpp)RhCl2(d8-bpy)]3+ [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+

λmax

em

(nm)

670 670 670 670 746 746 744 780 780 780

Φ

em

−4

(×10 )

120 ± 5 130 ± 5 150 ± 5 170 ± 5 14 ± 1 14 ± 1 17 ± 1 1.3 ± 0.1 1.3 ± 0.2 1.6 ± 0.1

−1

τ (ns)

kr (×10 s )

410 ± 10 460 ± 10 520 ± 10 610 ± 10 130 ± 7 150 ± 7 170 ± 7 30 ± 2 31 ± 2 38 ± 3

2.9 ± 1 2.8 ± 1 2.8 ± 1 2.8 ± 1 1.1 ± 0.2 0.93 ± 0.1 0.98 ± 0.1 1.1 ± 0.2c 0.93 ± 0.1c 0.98 ± 0.1c

4

77 Kb 6

−1

knr (×10 s ) 2.4 2.1 1.9 1.6 7.7 6.6 5.9 7.7 6.6 5.9

± ± ± ± ± ± ± ± ± ±

0.2 0.1 0.1 0.1 0.4 0.4 0.4 0.4c 0.4c 0.4c

7

−1

ket (×10 s )

2.6 ± 0.2 2.6 ± 0.1 2.0 ± 0.1

λmax

em

(nm)

τ (μs) ± ± ± ±

616 618 616 618

6.0 6.6 7.7 7.9

0.01 0.01 0.01 0.01

710 710 714

1.8 ± 0.01 2.2 ± 0.01 2.7 ± 0.01

Measured at room temperature in CH3CN. Emission spectra corrected for PMT response. Φem of monometallics measured using absorbance matched samples with [(bpy)2Ru(dpp)]2+ (Φem = 1.2 × 10−2).53 Φem of bimetallics measured using absorbance matched samples with [Os(bpy)3]2+ (4.6 × 10−3).62 bMeasured at 77 K in 4:1 v/v EtOH/MeOH rigid glass matrix. cValues correspond to kr and knr for analogous Ru(II),Ru(II) bimetallic complexes used as models to calculate ket. Emission spectra corrected for PMT response. bpy = 2,2′-bipyridine; dpp =2,3-bis(2pyridyl)pyrazine. a

Figure 6. State diagram for Ru(II) monometallic and Ru(II),Ru(II) bimetallic (left) and Ru(II),Rh(III) bimetallic (right) complexes. Abbreviations: GS = ground state; MLCT = metal-to-ligand charge transfer; MMCT = metal-to-metal charge transfer; kr = rate constant for radiative decay; knr = rate constant for nonradiative decay; kisc = rate constant for intersystem crossing; ket = rate constant for intramolecular electron transfer.

(Φem = (1.6 ± 0.1) × 10−4) compared to [(d8-bpy)2Ru(dpp)RhCl2(d8-bpy)]3+ (Φem = (1.3 ± 0.2) × 10−4) and [(bpy)2Ru(dpp)RhCl2(bpy)]3+ (Φem = (1.3 ± 0.1) × 10−4). In both the monometallic and bimetallic complexes exchange of the heavier deuterium in the bridging ligand architecture leads to an increase in emission quantum yield. These results suggest that the use of deuterated dpp in this supramolecular architecture enhances their excited-state properties. Time-Resolved Luminescence Spectroscopy. Timeresolved luminescence spectroscopy provides the excited-state lifetime (τ) of the observed emissive state. Ru(II)−polyazine complexes typically emit from the 3MLCT excited state, giving rise to lifetimes ranging from picoseconds to microseconds.63 The decay of the 3MLCT excited state of both the monometallic and bimetallic complexes fit well to a singleexponential decay. In RT CH3CN, τ values for [(bpy)2Ru(dpp)]2+ and [(d8-bpy)2Ru(dpp)]2+ are 410 and 460 ns, respectively, whereas values for [(bpy)2Ru(d10-dpp)]2+ and [(d8-bpy)2Ru(d10-dpp)]2+ are lengthened to 520 and 610 ns, respectively. In alcoholic glass at 77K, τ values were 6.0, 6.6, and 7.7 μs for [(bpy)2Ru(dpp)]2+, [(d8-bpy)2Ru(dpp)]2+, and [(bpy)2Ru(d10-dpp)]2+, respectively. Both the Ru(II),Ru(II) and Ru(II),Rh(III) bimetallic complexes display a trend similar to that of their monometallic synthons upon incorporation of the deuterated dpp bridging ligand. At room temperature the τ values of the Ru(II),Ru(II) complexes [(bpy)2Ru(dpp)Ru(bpy)2]4+, [(d8-bpy)2Ru(dpp)Ru(d8-bpy)2]4+, and [(bpy)2Ru(d10-dpp)Ru(bpy)2]4+ are 130, 150, and 170 ns, respectively. At room temperature the τ values of the Ru(II),Rh(III) complexes [(bpy) 2 Ru(dpp)RhCl 2 (bpy)] 3+ and [(d 8 -bpy) 2 Ru(dpp)RhCl2(d8-bpy)]3+ are 30 and 31 ns, respectively, whereas

[(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ was 38 ns. In alcoholic glass at 77 K, the τ values of the Ru(II),Rh(III) complexes were 1.8, 2.2, and 3.5 μs for [(bpy)2Ru(dpp)RhCl2(bpy)]3+, [(d8bpy)2Ru(dpp)RhCl2(d8-bpy)]3+, and [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+, respectively. Incorporation of the deuterated dpp BL in this supramolecular architecture leads to a 48% increase in the excitedstate lifetime of the fully deuterated Ru(II) monometallic, 31% increase for the Ru(II),Ru(II) bimetallic complex, and a 27% increase in the Ru(II),Rh(III) bimetallic complex. The Ru(II),Rh(III) bimetallic complex does not show as large an increase in excited-state lifetime compared to the Ru(II),Ru(II) model bimetallic complex because there is competing depopulation of the excited state through formation of the Ru(dπ) → Rh(dσ*) 3MMCT state via the ket pathway. This is attributed to the increased mass of deuterium decreasing the stretching frequency and amplitude of C−D vs C−H bonds, resulting in a decreased rate constant for nonradiative deactivation of excited state to the ground state.41 Decreasing frequency of the nonradiative deactivating vibrational modes decreases the overlap between the excited-state and groundstate potential energy surfaces, interfering with the resonant transfer of energy from the excited state to the ground state. In the monometallic complexes, coordination of the d10-dpp leads to a knr = (1.9 ± 0.1) × 106 s−1 and (1.6 ± 0.1) × 106 s−1 in [(bpy)2Ru(d10-dpp)]2+ and [(d8-bpy)2Ru(d10-dpp)]2+ compared to knr = (2.4 ± 0.2) × 106 s−1 and (2.1 ± 0.1) × 10−6 s−1 for [(bpy)2Ru(dpp)]2+ and [(d8-bpy)2Ru(dpp)]2+, respectively. Deuteration of ligands has a minimal effect on kr in the monometallic complexes, showing that the increased τ and Φem of [(bpy)2Ru(d10-dpp)]2+ is dominated by the decrease in knr. A F

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Rh(dσ*) 3MMCT in nature. The Ru(II),Ru(II) model gives kr and knr for the Ru(II),Rh(III) systems. Interestingly, changes in excited-state lifetimes upon deuteration of the μ-dpp acceptor for the 3MLCT state are evident both at room temperature and at 77 K. Excited-State Quenching. The emissive nature of the 3 MLCT excited state provides a handle to study the excitedstate dynamics. This probe was used to study the rate of intramolecular electron transfer (ket) as described above. Addition of the electron donor (ED) N,N-dimethylaniline (DMA) provides a means to assay the kinetics of quenching of the 3MLCT state by this ED. The sacrificial electron donor DMA has been shown to quench the 3MLCT emissive excited state of Ru−polyazine complexes and through bimolecular interactions.64 A Stern−Volmer quenching analysis was performed by monitoring the 3MLCT excited-state lifetime decay of the bimetallic complexes in the presence of DMA in deoxygenated spectral grade acetonitrile. All complexes showed a linear relationship with the reduction of the 3MLCT excitedstate lifetime varying linearly with increasing [DMA] (Figures S8 and S9, Supporting Information). The excited-state lifetime data were fitted using eq 1, which relates the ratio of excitedstate lifetime in the absence of DMA (τ0) and excited-state lifetime in the presence of DMA (τ) to the concentration of DMA.65 A Stern−Volmer analysis allows for the calculation of the quantity (kq + k2), the bimolecular quenching rate constant, which is the sum of the bimolecular reduction (kq) and bimolecular deactivation (k2) pathways. (kq + k 2)[DMA] τ0 + 1 = τ(kq + k 2)[DMA] + 1 = (k r + k nr + ket) τ (1)

From these experiments, values for the deactivation of the 3 MLCT excited state through bimolecular collisions with DMA were obtained and were (4.9 ± 0.3) × 109 and (5.0 ± 0.2) × 109 M−1 s−1, for the fully protiated and deuterated bridging ligand only complexes, respectively, indicating efficient quenching of the 3MLCT excited state by DMA. Because the bimolecular deactivation rates were identical, within error, for the two selectively deuterated complexes, this indicates that the complexes behave identically in solution and proceed through the same set of photon absorption to the 1MLCT state, intersystem crossing to the 3MLCT state, and reductive quenching steps, leading to efficient the formation of the reactive 3MMCT state. Because excited-state quenching behavior is the same for Ru(II),Rh(III) bimetallics regardless of ligand deuteration, the escape DMA radical cations formed through quenching events into the bulk solution is the same for all studied Ru(II),Rh(III) bimetallics. Lifetime quenching was corroborated with steady-state emission intensity quenching measurements performed on the same solutions (Figure S7, Supporting Information). Photocatalytic Hydrogen Production. The title Ru(II),Rh(III) bimetallic complexes function as homogeneous water reduction catalysts in organic solutions in the presence of the sacrificial electron donor DMA by funneling electrons from the Ru(II) light absorber through the dpp bridging ligand to the Rh(III) center reducing the metal to Rh(I) and liberating two equivalents of chloride ion. However, compared to other recently developed water reduction catalysts, the title Ru(II),Rh(III) complexes utilizing bpy terminal ligands are relatively inefficient due to their propensity to form Rh(I)−

Figure 7. Steady-state luminescence spectra of (A) [(bpy)2Ru(dpp)]2+ (purple line), [(bpy)2Ru(d10-dpp)]2+ (green line), [(d8-bpy)2Ru(dpp)]2+ (red line), and [(d8-bpy)2Ru(d10-dpp)]2+ (blue line); (B) [(bpy)2Ru(dpp)Ru(bpy)2]4+ (purple line), [(bpy)2Ru(d10-dpp)Ru(bpy)2]4+ (green line), and [(d8-bpy2Ru(dpp)Ru(d8-bpy)2]4+ (red line); and (C) [(bpy) 2 Ru(dpp)RhCl2 (bpy)] 3+ (purple line), [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ (green line), and [(d8-bpy2Ru(dpp)RhCl2(d8-bpy)]3+ (red line) recorded in CH3CN at RT (bpy = 2,2′-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine).

nearly identical trend is observed for the Ru(II),Ru(II) bimetallic complexes. In the Ru(II),Rh(III) bimetallic complexes, the increase in τ for [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ again is due to the decrease in knr. Given the presence of a lowlying 3MMCT excited state that competitively deactivates the 3 MLCT state through the ket pathway, accurate calculation of the knr and kr values becomes difficult due to this increased complexity of the bimetallic excited-state dynamics. However, incorporation of a d10-dpp BL into both the monometallic and bimetallic complexes causes an increase in the excited-state lifetimes, whereas incorporation of d8-bpy had no significant effect on the excited-state properties. This further suggests that the photoactive states within the Ru(II),Rh(III) bimetallic architecture are Ru(dπ) → μ-dpp(π*) 3MLCT and Ru(dπ) → G

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Rh(I) dimers in solution under previously reported catalytic conditions.19,52 The title Ru(II),Rh(III) catalysts produce small, but detectable amounts of hydrogen reproducibly using these reaction conditions in degassed acetonitrile: [bimetallic] = 130 μM; [DMA] = 1.5 M; [H2O] = 0.62 M; [DMAH+][CF3SO3−] = 0.11 mM; solution volume = 4.5 mL; headspace = 5.5 mL; λirrad = 470 nm. The fully protiated complex, [(bpy)2Ru(dpp)RhCl2(bpy)]3+, underwent photoreduction in the presence of DMA and produced an average of 44 μL of H2 gas after 40 h with ΦH2 = 2.4 × 10−4. Variation of the bridging ligand to utilize the deuterated variant, [(bpy) 2 Ru(d 10 -dpp)RhCl2(bpy)]3+, increased the amount of H2 produced to 80 μL over 40 h (Figure 8) with ΦH2 = 4.5 × 10−4. Hydrogen

MLCT states. Simplification of the NMR spectrum also allows for potential mechanistic information regarding the mode of H2O reduction to H2 using Ru(II),Rh(III) bimetallic photocatalysts. Incorporation of a deuterated bridging ligand into the Ru(II),Rh(III) bimetallic architecture leads to increased water reduction activity while maintaining the desired homogeneous catalytic behavior.



ASSOCIATED CONTENT

* Supporting Information S

Materials, synthetic methods, instrumentatl methods, tabulated electrochemical and electronic absorbance data, additional NMR spectra, geometric isomers, and Stern−Volmer quenching experiment plots. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b02836.



AUTHOR INFORMATION

Corresponding Author

*A. T. Wagner. E-mail: *[email protected] Notes

The authors declare no competing financial interest. † Karen J. Brewer is deceased (October 24, 2014).



Figure 8. Hydrogen production for [(bpy)2Ru(d10-dpp)RhCl2(bpy)]3+ in deoxygenated acetonitrile. Data are the average of three individual runs performed at room temperature with error of one standard deviation (bpy =2,2′-bipyridine, dpp =2,3-bis(2-pyridyl)pyrazine).

ACKNOWLEDGMENTS The authors acknowledge the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Offices of Sciences, U.S. Department of Energy DE FG02-05ER15751 for their generous financial support of this research.

production was monitored in real time using a HY-OPTIMA 700 in-line process solid-state hydrogen sensor from the H2 scan attached to each individual reaction vessel. The increased H2 production is consistent with the increase in excited-state lifetime and concurrent decrease in knr observed upon bridging ligand deuteration. Identical kq values for all Ru(II),Rh(III) bimetallics and increased excited-state lifetime with deuterated dpp bridging ligand lead to a greater fraction of quenched molecules and increased H2 production.



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CONCLUSIONS A series of eight new complexes of the design [(TL)2Ru(BL)]2+, [(TL)2Ru(BL)Ru(TL)2]4+, and [(TL)2Ru(BL)RhCl2(TL)]3+ (TL = bpy or d8-bpy; BL = dpp or d10-dpp) were synthesized using the building block method and 1H NMR spectroscopic, electrochemical, electronic absorbance spectroscopic, and photophysical properties were investigated. Substitution of deuterated ligands in the monometallics allows for significant clarification of the aromatic region of the 1H NMR spectrum allowing for full assignment of the 10 nonequivalent protons in [(d8-bpy)2Ru(dpp)]2+ and assignment of the 16 nonequivalent protons in [(bpy)2Ru(d10dpp)]2+. Minimal 1H NMR clarification of the bimetallic complexes was observed as the number of structural isomers and chemical environments present inhibits complete assignment of a 1H NMR spectrum. Incorporation of the deuterated ligands does not significantly change the redox or light absorbing properties of both the monometallic and bimetallic complexes. Substituting d 10 -dpp for dpp in both the monometallic and bimetallic complexes shows enhanced excited-state properties with increased Φem and excited-state lifetimes due to a decrease in the value of knr. The enhanced excited-state lifetimes and emission quantum yield allow assignment of the emitting state as Ru(dπ) → dpp(π*) H

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DOI: 10.1021/acs.jpca.5b02836 J. Phys. Chem. A XXXX, XXX, XXX−XXX