Visible Light-Activated CO Release and 1O2 Photosensitizer

Aug 30, 2018 - Synopsis. Visible light irradiation of a Ru(II),Mn(I) bimetallic architecture releases three equivalents of CO from a [Ru(bpy)3]2+-deri...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Visible Light-Activated CO Release and 1O2 Photosensitizer Formation with Ru(II),Mn(I) Complexes Rachael N. Pickens, Bertrand J. Neyhouse, Demi T. Reed, Shanan T. Ashton, and Jessica K. White* Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/31/18. For personal use only.

S Supporting Information *

ABSTRACT: Two diimine-bridged Ru(II),Mn(I) complexes with a [(bpy)2Ru(BL)Mn(CO)3Br]2+ architecture, where bpy = 2,2′-bipyridine and BL = 2,3-bis(2-pyridyl)pyrazine (dpp; Ru(dpp)Mn) or 2,2′bipyrimidine (bpm; Ru(bpm)Mn), were designed to both dissociate multiple equivalents of CO and produce 1O2 when irradiated with visible light. Analysis of the complexes by Fourier transform infrared (FTIR) spectroscopy and cyclic voltammetry suggest a stronger π-accepting ability for bpm compared to that of dpp. Both complexes absorb light throughout the UV and visible regions with lowest energy absorption bands comprising overlapping Ru(dπ)→BL(π*) and Mn(dπ)→BL(π*) singlet metal-toligand charge transfer (1MLCT) and Br(p)→dpp(π*) singlet halide-toligand charge transfer (1XLCT) transitions. This lowest energy band is centered at 510 nm (ε = 12 000 M−1cm−1) for Ru(dpp)Mn and 553 nm (ε = 3240 M−1cm−1) for Ru(bpm)Mn, and the absorption band extends to nearly 700 nm in each case. Irradiation with visible light (both 470 and 627 nm) releases all three CO ligands, as observed by a combination of UV−vis, FTIR, and gas chromatography. The exchange of the first CO ligand with a solvent molecule occurs more efficiently for Ru(dpp)Mn (Φ470 = 0.22 ± 0.03 in H2O; 0.37 ± 0.06 in CH3CN) than for Ru(bpm)Mn (Φ470 = 0.049 ± 0.008 in H2O and 0.16 ± 0.03 in CH3CN), and the CO dissociation efficiency is unaffected by irradiation wavelength. The differences between Ru(dpp)Mn and Ru(bpm)Mn are proposed to result from the variation in electron density distribution across each formally reduced BL in the Mn(dπ)→BL(π*) 1MLCT excited state based on the nature of BL. Exhaustive photolysis causes the decomplexation of oxidized Mn(II), and the resulting [(bpy)2Ru(BL)]2+ complexes produce 1O2 with quantum yields (ΦΔ) of 0.37 ± 0.03 and 0.16 ± 0.01 for Ru(dpp) and Ru(bpm), respectively, with 460 nm irradiation. This bimetallic architecture presents the opportunity to use visible light to codeliver both CO and 1O2, both of which have biological relevance in photoactivated therapeutics, with spatiotemporal control.

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INTRODUCTION The utilization of UV and visible light to trigger molecular transformations is useful in biomolecule modification for targeting tumor and bacterial cells1−4 as well as in applications such as solar energy conversion,5,6 catalysis,7,8 and molecular switches and machines.9,10 Transition metal complexes are frequently utilized in such applications due to structural variability and rich, tunable redox and photophysical properties through ligand variation. In the realm of photoactivatable therapeutic molecules, common types of photoinduced reactions using transition metal complexes include the sensitization of cytotoxic 1O2 and the dissociation of ligands for the release of a bioactive molecule or promotion of covalent binding of the metal ion to DNA.11−13 Photoinduced singlet oxygen (1O2) production is the basis of traditional photodynamic therapy (PDT), which requires a photosensitizer (PS), a light source, and molecular oxygen to kill tumor cells.14−16 The PS is excited only at the tumor site and transfers its excited state energy to ground state triplet oxygen (3O2) to produce 1O2.14 The localized generation of 1 O2 causes oxidative damage to the tumor cell, providing control over cytotoxicity in space and time. Transition metal © XXXX American Chemical Society

PS are promising candidates for PDT due to their tunable photophysical properties through structural modifications.11,13,17 The prototypical inorganic PS, [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), produces 1O2 with a quantum yield (ΦΔ) of 0.86 in air-saturated methanol14 and has inspired the development of a host of Ru(II)−diimine complexes which produce 1O2 and selectively kill cells only when irradiated.12,18,19 The relatively long-lived excited states responsible for the production of 1O2 using Ru(II)−diimine complexes are typically either Ru(dπ)→ligand(π*) triplet metal-to-ligand charge transfer (3MLCT) or intraligand (IL) π→π* in nature.12,19,20 Light-activated transition metal complexes that combine the production of 1O2 with the activation of another therapeutic moiety in a single molecule are of great interest. Such multifunctional light-activated metal complexes that couple 1 O2 production with photoactivation of a metal center to form covalent adducts with DNA have been reported to enhance DNA photocleavage and improve photocytotoxicity compared Received: June 25, 2018

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DOI: 10.1021/acs.inorgchem.8b01759 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry to analogues that either only produce 1O2 or only covalently bind to DNA with light activation.21−24 Excited state dynamics must be considered in such complexes to evoke both reactions, as 1O2 generation results from a relatively long-lived 3MLCT or 3IL state, and ligand dissociation typically occurs due to either triplet ligand field (3LF) state population or a change in Lewis acidity in the excited state. The photosensitization of 1 O2 has also been coupled with the uncaging of a bioactive molecule from a transition metal complex. Recently, photoinduced dissociation of either a nitrile-containing cathepsin K inhibitor or the drug imatinib from a [Ru(tpy)(L)(drug)]2+ architecture, where tpy = 2,2′:6′,2″-terpyridine and L = benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine (dppn) or 2,6dimethylbenzo[i]dipyrido[3,2-a:2′,3′-c]phenazine (Me2dppn), was reported in conjunction with 1O2 generation with high ΦΔ.25,26 The excited state dynamics are complicated, as bifurcation of the 3MLCT state produces either the low-lying 3 IL state localized on dppn/Me2dppn or the thermally accessible 3LF state necessary for release of the drug and substitution with a solvent molecule. The photosensitization of 1 O2 was also coupled with the photoactivated release of CO from [Re(L)(CO)3(phosphine)]+ compounds (L = diimine ligand; phosphine = tris(hydroxymethyl)phosphine or 1,4diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane). Irradiation with 365 nm light releases one equivalent of CO and produces 1O2 in high yield, and the combination of these two events is credited for the complexes’ high photocytotoxicity toward a variety of cancer cells, including a cisplatin-resistant cell line.27 Strategies for the photoactivated release of CO from a metal complex using light were realized in the development of a class of compounds called photoCORMS (photoactivated CO releasing molecules).28−30 CO is anti-inflammatory and antiapoptotic in healthy cells at low concentrations yet proapoptotic and antiproliferative toward cancer cells at elevated concentrations.31 To this end, spatiotemporal control over the delivery of high concentrations of CO using a light trigger has shown promise for potential anticancer treatment. In some cases, CO sensitizes cancer cells to chemotherapy drugs such as doxorubicin and camptothecin by 1000-fold while protecting nearby healthy cells.32−34 PhotoCORMs containing Fe, Ru, Mo, Re, and Mn have been reported,29,30,35 with Mn(I) photoCORMs being the focus of many recent studies due to their ability to efficiently release CO from the MLCT state populated upon visible light irradiation.36 Mascharak and coworkers have attributed the photorelease of CO from fac-[Mn(L)(CO)3Br] compounds (L = bidentate Ndonor ligand) to the Mn(dπ)→L(π*) MLCT excited state, in which electron density is transferred to L away from the Mn center, thus weakening its π-backbonding to the CO ligands. This weakening of the Mn−C bond allows the exchange of one CO ligand for a coordinating solvent molecule.36,37 As promising results have been obtained by combining the photoactivation of more than one light-activated therapeutic event into one molecule, no molecules that combine CO release and singlet oxygen generation using visible light have been reported to the best of our knowledge. Herein, we report two Ru(II),Mn(I) bimetallic complexes, shown in Figure 1, designed to release three equivalents of CO from the Ru(II)based PS upon visible light irradiation by decomplexation of a bridging Mn center. The bidentate diimine bridging ligands 2,3-bis(2-pyridyl)pyrazine (dpp) and 2,2′-bipyrimidine (bpm),

Figure 1. Structural representations of the Ru(BL)Mn, Ru(BL), and Mn(BL) complexes and the BLs dpp (2,3-bis(2-pyridyl)pyrazine) and bpm (2,2′-bipyrimidine).

which exhibit different degrees of π-accepting ability when bridging two metal cations, were employed to covalently couple a Ru(II) PS and a Mn(I) photoCORM. The characterization and photoreactivity of [(bpy)2Ru(dpp)Mn(CO)3Br](PF6)2 and [(bpy)2Ru(bpm)Mn(CO)3Br](PF6)2 (Ru(dpp)Mn and Ru(bpm)Mn, respectively) were compared to the analogous [(bpy)2Ru(BL)](PF6)2 monometallics (Ru(BL), where BL = dpp or bpm) which serve as synthetic precursors as well as the 1O2-generating PS photoproducts following irradiation. The analogous Mn(I) complexes, fac[Mn(BL)(CO)3Br] (Mn(BL), where BL = dpp or bpm) were also compared for their redox properties and C−O stretching frequencies. As described herein, the varied degree of πaccepting ability and the difference in the symmetry of chelation between the two BL ligands influences the ground state properties and photochemical CO release.

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RESULTS AND DISCUSSION The bimetallic complexes Ru(dpp)Mn and Ru(bpm)Mn were prepared by reacting each of the previously reported Ru(dpp)38 and Ru(bpm),39 respectively, with an excess of Mn(CO)5Br in CH2Cl2 at reflux under Ar for 1 h (Scheme 1). Protecting the reaction flask from light is crucial to prevent photodecomposition of the Mn-containing precursor and product. It should be noted that Ru(bpm)Mn was previously reported as an electron relay in photocatalytic azide−alkyne click reactions, but its photochemical release of CO was not explored.40 The authors reported the reaction of cis-[Ru(bpy)2Cl2] with Mn(bpm) in EtOH under reflux for 12 h; however, we found these conditions to be too harsh to maintain the integrity of the MnI(CO)3Br moiety. The 1H NMR spectra (Figures S1 and S2) of the new compounds compared to the Ru(BL) precursors show complete conversion to the desired product by the absence of reactant peaks and new signals arising from coordination of the MnI(CO)3Br moiety. The 1H NMR spectrum of Ru(dpp)Mn is complicated by the presence of multiple isomers due to the asymmetric nature of dpp when bound to the RuII(bpy)2 moiety (described in Supporting Information, Figure S3). Electrospray ionization mass spectrometry data, Figure S4, and elemental analysis (C, H, N) are consistent with the proposed formula for each bimetallic compound. The carbonyl ligands on the Ru(BL)Mn complexes provide a convenient probe for characterization by Fourier transform B

DOI: 10.1021/acs.inorgchem.8b01759 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Approach for the Ru(BL)Mn Bimetallic Complexes (BL = dpp and bpm)

infrared (FTIR) spectroscopy. Solutions of the two compounds in CH3CN show three ν(CO) bands characteristic of Mn(I) tricarbonyl complexes with C1 symmetry (Figure 2).

Figure 3. Cyclic voltammograms of Ru(dpp)Mn (red) and Ru(bpm) Mn (blue) in rt CH3CN under a N2 atmosphere with 0.1 M Bu4NPF6 as the supporting electrolyte, a glassy carbon working electrode, Pt wire auxiliary electrode, Ag/AgCl reference electrode, and a scan rate of 200 mV/s. Potentials are referenced to the Fc+/Fc couple (0.44 V vs Ag/AgCl). Arrows indicate the scan direction.

Figure 2. FTIR spectra in the C−O stretching region of Mn(dpp) (red dotted), Ru(dpp)Mn (red solid), Ru(bpm)Mn (blue solid), and Mn(bpm) (blue dotted) in CH3CN.

Fc+/Fc, respectively, followed by a reversible RuIII/II wave at E1/2 = 1.01 V for both complexes. These assignments were based on a comparison to the Mn(BL) and Ru(BL) compounds presented in Figure S5. In these analogous monometallic compounds, the irreversible MnII/I wave occurs at Epa = 0.75 and 0.76 V for Mn(dpp) and Mn(bpm), respectively, and the reversible RuIII/II couples require higher potentials of E1/2 = 0.99 and 1.01 V for Ru(dpp) and Ru(dpp), respectively. The energy of the HOMO in the Ru(BL)Mn complexes, localized on the Mn(I) center, is sensitive to the nature of BL. The more positive MnII/I Epa for Ru(bpm)Mn compared to Ru(dpp)Mn suggests that μ-bpm is a stronger πacceptor than μ-dpp, and the relative oxidation potentials agree with the FTIR data which indicates a more electron-dense MnI center when BL = dpp. Cathodic scans reveal a BL-based LUMO as the first reduction is 160 mV more negative when BL = dpp vs bpm (E1/2 = −1.08 V and −0.92 V, respectively), in agreement with analogous Ru(BL)Ru bimetallic complexes reported in the literature.41,42 Isolation of the first reduction couple for each bimetallic compound (Figures S6) demonstrates the reversibility of the electrochemical process, which is consistent with the assignment of this couple as dpp0/− or bpm0/−. It is expected that MnI/0, bpy0/−, and BL−/2− reductions occur at potentials 2 h (Figure S11), and this process is greatly expedited with irradiation. A total loss of the CO stretches is also observed following further irradiation. We propose that the intermediate observed for Ru(dpp)Mn is [(bpy) 2 Ru(dpp)Mn(CO)2(CH3CN)2]3+, and that the Mn−Br bond dissociation occurs too rapidly to be observed under these experimental conditions. The enhanced Br− lability of Ru(dpp)Mn agrees with the increased electron density on the Mn center observed by comparing the redox potentials and the IR stretches of the two Ru(BL)Mn complexes. The formation of the Ru(BL) PS from the irradiation of the Ru(BL)Mn bimetallic compounds was probed using electronic absorption spectroscopy during the photolysis with 470 nm light in aqueous solutions (Figure 6). Irradiation of the chloride salt of Ru(dpp)Mn in H2O for 15 min with λirr = 470 nm causes a significant blue-shift in the visible region, forming a photoproduct spectrum that overlays well with the spectrum

Table 1. Quantum Yields for CO Release with 470 nm (Φ470) and 627 nm (Φ627) Irradiation Φ470a

compound Ru(dpp)Mn Ru(bpm)Mn Mn(dpp) Mn(bpm)

0.37 0.16 0.26 0.21

± ± ± ±

Φ627a

0.06 (0.22 ± 0.03) 0.03 (0.049 ± 0.008)b 0.02 0.02 b

0.38 ± 0.04 0.13 ± 0.01

In room temperature CH3CN with the complexes as PF6− salts, unless otherwise noted. bValues in parentheses were measured in room temperature H2O with the complexes as Cl− salts. a

each of the bimetallic compounds with λirr = 470 nm, ligand exchange is more efficient in CH3CN solution than in aqueous solution. This is consistent with H2O being a weaker ligand than CH3CN, more readily allowing dissociated CO to recoordinate before escaping the coordination sphere. To investigate whether the initially populated excited state affects the quantum yield for CO dissociation, the Φ470 values were compared to Φ627 in CH3CN solution. Irradiation with 470 nm light is expected to initially populate Ru→bpy and Ru→BL MLCT states primarily, while irradiation with 627 nm is expected to initially populate the lowest energy Mn→BL MLCT to a greater extent. The data demonstrate that the CO dissociation quantum yield for each compound does not depend on the irradiation wavelength, with Φ627 = 0.38 ± 0.04 for Ru(dpp)Mn and 0.13 ± 0.01 for Ru(bpm)Mn. This behavior suggests that the lowest energy Mn→BL MLCT state, which is presumed to be the excited state from which CO dissociation occurs, is populated with unit efficiency regardless of which excited state is initially populated. The nature of BL is clearly an important factor in the efficiency of the photochemical release of CO from Ru(BL)Mn bimetallic complexes. The photorelease of CO from Mn(L)(CO)3Br photoCORMs (L = bidentate N-donor ligand) was attributed to the Mn(dπ)→L(π*) MLCT excited state, in which the Mn−CO π-backbonding is weakened when electron density is moved from the Mn center to L.36,37 In the Ru(BL) Mn systems, the difference in CO ligand exchange efficiency may be a result of the difference in electron density distribution on the BLs upon Mn(dπ)→BL(π*) MLCT state population. Due to the symmetric nature of the [(bpy)2Ru(bpm)]2+ moiety, the transferred electron density is distributed

Figure 6. Electronic absorption spectra of chloride salts of Ru(dpp) Mn (A) and Ru(bpm)Mn (B) in H2O following irradiation with 470 nm for 0−15 min and 0−30 min, respectively. Inset: Overlaid absorption spectra of the photoproduct (purple) and the analogous Ru(BL) (black dashed) in H2O. E

DOI: 10.1021/acs.inorgchem.8b01759 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Ru(dpp) in air-saturated CH3CN.48 The greater ΦΔ value for Ru(dpp) compared to Ru(bpm) is related to their reported excited state lifetimes (τ = 210 and 40 ns in methanol, respectively).49 The 1H NMR spectra of the photolyzed Ru(BL)Mn samples in CH3CN show very broad signals, indicative of the formation of a paramagnetic species (Figure S17). In related Mn(I)-carbonyl complexes, similar photochemical behavior was observed upon visible light irradiation in which a paramagnetic species was identified as Mn(II) ion using electron paramagnetic resonance spectroscopy.50−52 As the Mn(I) center becomes oxidized to form Mn(II) following exhaustive photolysis, O2 is potentially reduced to form superoxide, O2−, representing another cytotoxic agent that may form upon visible light irradiation. Scheme 2 summarizes the proposed photochemical and thermal processes that occur with this Ru(BL)Mn architecture. In step 1, absorption of a photon causes the exchange of one CO ligand with a solvent molecule by population of the lowestlying Mn→BL MLCT state. In step 2, the Br− ligand is thermally exchanged with a solvent molecule in the dark, and this step is expedited under irradiation. In the dpp-bridged compound, Ru(dpp)Mn, this process appears to occur so rapidly that the [(bpy)2Ru(dpp)Mn(CO)2(CH3CN)Br]2+ intermediate could not be observed on the time scale of our FTIR experiment. For Ru(bpm)Mn, however, this analogous intermediate and the thermal conversion to afford [(bpy)2Ru(bpm)Mn(CO)2(CH3CN)2]3+ were observed. In step 3, further photolysis of the intermediate formed in step 2 releases the remaining two CO ligands, dissociates the oxidized Mn(II) ion, and evolves the Ru(BL) PS. The mechanistic details of this formation are unknown at this time, but we presume that O2 is reduced to superoxide upon formation of the oxidized Mn(II). Finally, in step 4, further irradiation of the resulting Ru(BL) PS produces 1O2 with an efficiency dictated by the identity of BL. It should be noted that while red light can efficiently dissociate the three CO ligands and dissociate the Mn ion to form the Ru(BL) PS, the resulting PS does not absorb enough red light to efficiently produce 1O2. Higher energy irradiation (ca. 470 nm) is necessary to both release CO and produce 1O2 efficiently with these molecules. However, this architecture provides a new paradigm for the visible light-activated delivery of CO and 1O2 in close proximity, and future modifications of the architecture will aim toward tuning the photophysics to drive both processes with lower energy light.

equally across both pyrimidine rings in the bpm ligand.46 Conversely, the asymmetric nature of the [(bpy)2Ru(dpp)]2+ moiety, in which dpp chelates Mn(I) through one pyrazine N and one pyridine N, is expected to cause the transferred electron density in the MLCT state to reside more significantly on the pyrazine moiety. Previous reports of the excited state basicity of [(bpy)2Ru(dpp)]2+ by Gafney and coworkers demonstrate the enhanced basicity of the pyrazine moiety upon Ru(dπ)→dpp(π*) MLCT state population, as the LUMO is localized more significantly on this portion of the ligand.47 As depicted in Figure 7, this asymmetric distribution

Figure 7. Simplified representation of the movement of electron density (illustrated with red circles) in the Mn→BL MLCT state for Ru(dpp)Mn and Ru(bpm)Mn.

of electron density in MLCT state of Ru(dpp)Mn is expected to greatly weaken the Mn−C bond trans to the pyridine ring compared to the Mn−C bond trans to the formally reduced pyrazine ring. In the case of Ru(bpm)Mn, both equatorial CO ligands are trans to the reduced bpm ligand. A similar trend with respect to the impact of the BL is observed between the two Mn(BL) analogues, where Φ470 = 0.26 ± 0.02 and 0.21 ± 0.02 for Mn(dpp) and Mn(bpm), respectively, in agreement with the difference in BL symmetry. It should be noted that while the Φ470 values for Ru(bpm)Mn and Mn(bpm) are quite similar within error (0.16 ± 0.03 and 0.21 ± 0.02, respectively), the CO dissociation for Ru(dpp) Mn (Φ470 = 0.37 ± 0.06) is more efficient than the analogous Mn(dpp) (Φ470 = 0.26 ± 0.02). This increased Φ470 for the bimetallic compound suggests that the stabilization of the pyrazine moiety on dpp is more significant than the stabilization of the pyridine moiety when bridging two metal ions, so the Mn−C bond trans to the pyridine ring is even more labile in Ru(dpp)Mn than it is in Mn(dpp). While the neutral Mn(BL) compounds exhibit quite large Φ470 values, their very low solubilities precluded the determination of their aqueous CO quantum yields. This low solubility suggests limitations in the viability of Mn(BL) as useful photoCORMs. Following photoinduced generation of the Ru(BL) PS, further irradiation of an air-saturated methanol solution of the photoproduct with 460 nm produces 1O2 with ΦΔ = 0.37 ± 0.03 and 0.16 ± 0.01 for Ru(dpp) and Ru(bpm), respectively. These values agree with the values measured using fresh samples of synthesized Ru(dpp) and Ru(bpm) in air-saturated methanol (ΦΔ = 0.36 ± 0.01 and 0.14 ± 0.01, respectively) and the previously reported value of ΦΔ = 0.31 ± 0.02 for

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CONCLUSION We reported the preparation, characterization, and photoreactivity of a bimetallic architecture combining a photoCORM unit with a Ru(II)−diimine PS designed to enable the localization of both CO and 1O2 upon irradiation with low energy visible light. The bimetallic compounds Ru(dpp)Mn and Ru(bpm)Mn absorb visible light extending to 700 nm, whereby irradiation is proposed to oxidize the Mn(I) center to Mn(II) upon population of the Mn(dπ)→dpp(π*) 1MLCT state, causing CO release and eventually decomplexation of Mn(II) from the Ru(BL) PS. Further irradiation of the photoproduct solution produces 1O2 with moderate efficiency, establishing the robust nature of the Ru(BL) PS even after undergoing photochemical reactions to release a Mn metal center and CO. The identity of BL is important in controlling the lability of the CO ligands as well as the efficiency with which 1O2 is produced following photoinduced formation of F

DOI: 10.1021/acs.inorgchem.8b01759 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 2. Proposed Mechanism for CO Release and 1O2 Production upon Visible Light Irradiation of Ru(BL)Mn Compounds (S = Solvent)

infused directly into the ESI source for ionization. The injection flow rate was 10 μL min−1, and the applied potential for ionization was 2.0 kV. FTIR. FTIR spectra were collected with a Shimadzu IRAffinity-1S Fourier transform infrared spectrophotometer in a CaF2 liquid IR cell. Samples (∼1 mM in CH3CN) were prepared in the dark, irradiated with 627 nm light using in-house built red LEDs purchased from Luxeon Star LEDs (Quadica Developments, Inc., Lethbridge, Alberta, Canada), and IR spectra were measured at time intervals until no further spectral changes were observed. Electronic Absorption Spectroscopy. Absorption spectra were recorded using an Agilent Cary 8454 Diode Array UV−visible spectrophotometer (1 nm resolution, 0.5 s integration time). Measurements were obtained in a 1 × 1 cm quartz cuvette at 25 °C. Extinction coefficient measurements were performed in triplicate in the dark. For photolysis experiments, CH3CN and H2O solutions were prepared in the dark and irradiated with 470 or 627 nm light (as described above), and the spectra were measured at time intervals until no further spectral changes were observed. Quantum Yield of Ligand Exchange. The quantum yields for CO release were determined in CH3CN solution using 470 and 627 nm LED irradiation. The decrease in the lowest energy MLCT absorption maximum was monitored as a function of time, and the extinction coefficient and solution volume were used to convert the absorbance values into moles of reactant. A plot of the moles of reactant vs time gives a linear trend considering only early times at which the absorbance changes by