Bathochromic Shifts in Rhenium Carbonyl Dyes ... - ACS Publications

Feb 9, 2018 - Department of Chemistry, Eckerd College, St. Petersburg, Florida 33711, United States. •S Supporting Information. ABSTRACT: A series o...
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Article Cite This: Inorg. Chem. 2018, 57, 5389−5399

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Bathochromic Shifts in Rhenium Carbonyl Dyes Induced through Destabilization of Occupied Orbitals Daniel A. Kurtz,† Kelsey R. Brereton,† Kevin P. Ruoff,† Hui Min Tang,†,‡ Greg A. N. Felton,§ Alexander J. M. Miller,*,† and Jillian L. Dempsey*,† †

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore § Department of Chemistry, Eckerd College, St. Petersburg, Florida 33711, United States ‡

S Supporting Information *

ABSTRACT: A series of rhenium diimine carbonyl complexes was prepared and characterized in order to examine the influence of axial ligands on electronic structure. Systematic substitution of the axial carbonyl and acetonitrile ligands of [Re(deeb)(CO)3(NCCH3)]+ (deeb = 4,4′-diethylester-2,2′bipyridine) with trimethylphosphine and chloride, respectively, gives rise to red-shifted absorbance features. These bathochromic shifts result from destabilization of the occupied dorbitals involved in metal-to-ligand charge-transfer transitions. Time-Dependent Density Functional Theory identified the orbitals involved in each transition and provided support for the changes in orbital energies induced by ligand substitution.

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INTRODUCTION Rhenium(I) carbonyl complexes are versatile dyes (chromophores) that have been extensively used in applications such as photosensitization of light-driven CO2 reduction,1−5 long-range electron transfer through proteins,6−10 and singlet oxygen generation.11,12 Most rhenium dyes do not exhibit strong light absorption across the visible spectrum, with transitions from filled Re d-orbitals to empty ligand π* orbitals generally requiring ultraviolet or blue light. Efforts to red-shift the absorbance of rhenium carbonyl complexes have focused on modifying the diimine ligand with electron-withdrawing groups or introducing additional conjugation, which lowers the energy of the metal-to-ligand charge transfer (MLCT) transition by stabilizing the π* orbitals (Figure 1A).13,14 These modifications can have moderate effects on the energy of the relevant MLCT transition, such as a red-shift of ∼100 nm when switching from -NH2 groups to -NO2 groups on the 4 and 4′ positions on bipyridine.15 Although extensive synthetic modification can be used to access bidentate ligands with widely varied electronic properties, there are practical limits to the extent of LUMO stabilization before the ligand will no longer effectively bind the metal center. An alternative, underexplored strategy to tune the absorption profile of rhenium chromophores is to modify the axial ligands cis to the diimine ligand in order to destabilize the d-orbitals (Figure 1B). In the parent complex, the occupied d-character orbitals are highly stabilized through backbonding interactions with the facially coordinated carbonyl ligands;16 destabilization of these orbitals thus lowers the energy of the MLCT transition. While previous reports have demonstrated that the © 2018 American Chemical Society

Figure 1. Simplified orbital energy diagram illustrating the strategy of red-shifting absorption by (A) stabilizing π* LUMO levels with electron-withdrawing groups or (B) destabilizing HOMO levels with modification of axial ligands.

axial ligand identities affect the energy of the MLCT transitions,17 a detailed study of the relationship between ligand electronic attributes and experimental optical properties has not been reported. We report herein a hybrid approach to absorbance tuning of rhenium dyes involving destabilization of the metal d-orbitals and stabilization of the bipyridine π* orbitals. A series of estersubstituted bipyridine rhenium complexes incorporating axial ligands with different donating strength (CO, CH3CN, Cl−, and PMe3) provides and opportunity to correlate changes in ligand bonding properties with the electronic structure of the complex. Received: February 9, 2018 Published: April 12, 2018 5389

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Inorganic Chemistry

unwanted trimethylamine complex.22 Workup and recrystallization provided [Re(deeb)(CO)2(PMe3)(NCCH3)][PF6] (3) as a deep red solid in 60% yield. The neutral dicarbonyl complex Re(deeb)(CO)2(PMe3)Cl (4) was obtained by ligand substitution of cationic 3. Excess benzyltrimethylammonium chloride ([BnMe3N]Cl) was added to a solution of 3 in CH2Cl2, which was then refluxed for 24 h. Removal of the excess salts by an aqueous workup and precipitation with pentane yielded 4 in 65% yield as a dark purple powder. Structural Characterization in Solution and Solid State. The structures of the four new complexes reported here were characterized by multinuclear NMR spectroscopy, infrared (IR) spectroscopy, and single-crystal X-ray diffraction. NMR spectroscopy in CD3CN provides insight into the solution symmetry and geometry. IR spectroscopy reports on the number of carbonyl ligands and provides a measure of the electron-richness of the metal center. IR spectra were recorded in CH2Cl2, which has fewer solvent features in the region of interest (2100−1800 cm−1) than CH3CN. The 1H NMR spectrum of 1 (Figure S1) features three aromatic resonances and two aliphatic resonances of the deeb ligand, consistent with a Cs-symmetric complex. The bound CH3CN ligand is initially observed in both the 1H NMR (δ 2.02) and in the 13C NMR (Figure S2, δ 123.8, 3.9) spectra; however, within 7 days of dissolution in CD3CN, the bound CH3CN ligand resonance diminishes in intensity as a resonance that corresponds to free CH3CN grows in (Figure S17). This indicates a slow ligand substitution reaction in which the large excess of CD3CN slowly replaces the bound CH3CN. The carbonyl region in the IR spectrum of 1 (Figure 2) shows one

The ester-substituted bipyridine ligand, in addition to facilitating attachment of these complexes on metal oxide thin films in future studies, lowers the π* orbital energy to redshift the absorbance profile.18,19 Through electrochemical and spectroscopic measurements, we quantified the relationship between the ligand donor strength and the HOMO/LUMO energetics. We utilized computational methods to further elucidate the electronic structure of these complexes and rationalize the bathochromic shifts in the MLCT transitions as a function of axial ligand identity.

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RESULTS Synthesis. The syntheses of the four target complexes are summarized in Scheme 1. [Re(deeb)(CO)3(NCCH3)][PF6] Scheme 1. Synthesis of complexes 1−4a

a Reagents and conditions: (i) 3 equiv AgOTf in CH3CN at 295 K, [NH4][PF6] metathesis; (ii) 3 equiv AgOTf in refluxing acetone, 30 equiv PMe3 in refluxing acetone, [NH4][PF6] metathesis; (iii) 1 equiv TMNO in refluxing CH3CN; (iv) 15 equiv [BnMe3N]Cl in refluxing CH2Cl2.

(1, deeb = 4,4′-diethylester-2,2′-bipyridine) was obtained as a yellow solid via halide abstraction from previously reported Re(deeb)(CO)3Cl (2)15 using silver triflate (AgOTf) in CH 3CN, followed by salt metathesis with ammonium hexafluorophosphate ([NH4][PF6]) (84% yield). While selective displacement of a single carbonyl ligand in rhenium(I) tricarbonyl complexes is often thermodynamically unfavorable,5,20 the presence of a tertiary phosphine ligand trans to a carbonyl ligand can enable substitution via photodissociation or oxidative decarbonylation.21−23 A suitable tertiary phosphine complex intermediate, [Re(deeb)(CO)3(PMe3)][PF6], was accessed in 76% yield by halide abstraction from 2 in acetone followed by addition of excess PMe3 and salt metathesis with [NH4][PF6]. When an orange solution of [Re(deeb)(CO)3(PMe3)][PF6] in CH3CN was treated with 1 equiv of trimethylamine N-oxide (TMNO), a color change to deep red was observed within minutes. In situ 1H NMR spectroscopy (Figures S15 and S16) indicated decarbonylation followed by coordination of CH3CN. The solution was then refluxed under N2 overnight to help avoid formation of an

Figure 2. Carbonyl stretching region of IR spectra of complexes 1−4, collected in CH2Cl2.

sharp absorption peak and one broad absorption peak attributed to two overlapping stretches, for a total of three CO stretching vibrational modes with A′(1), A″, and A′(2) symmetry. The 1H NMR spectrum of chloride tricarbonyl 2 (Figure S4) indicates retention of Cs symmetry. The IR spectrum of 2 exhibits three distinct peaks for the same three CO vibrational modes as 1 (Figure 2). The CO stretches of 2 are shifted to lower energy than those in 1, reflecting the reduced overall charge of the complex and increased electron density at the metal center. 5390

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Inorganic Chemistry The 1H NMR spectrum of complex 3 (Figure S9) exhibits a doublet for the methyl protons of the PMe3 ligand (δ 1.15, 2JPH = 9.5 Hz) along with the aromatic resonances assigned to the symmetric deeb ligand. Additionally, there is a 4JPH coupling of 0.7 Hz between PMe3 ligand and the two protons ortho to the nitrogen on the deeb ligand. Resonances for the bound CH3CN ligand are seen in both the 1H NMR (δ 2.04) and the 13C NMR (Figure S10, δ 124.7, 3.9) spectra. A single CO resonance in the 13C NMR appears as a doublet weakly coupled to the PMe3 ligand (2JPC = 7.2 Hz), confirming overall Cs symmetry with a cis-phosphinocarbonyl configuration. The IR spectrum of 3 shows two peaks in the carbonyl region for the cis CO ligands, assigned to a symmetric stretching mode with A′ symmetry and an asymmetric stretching mode with A″ symmetry. The CO stretches of 3 shift to lower energy relative to 1 and 2, as expected for a dicarbonyl complex with enhanced backbonding (due to less competition). The 1H NMR spectrum of complex 4 (Figure S12) supports assignment of Cs symmetry. Phosphorus−carbon coupling is observed between the carbonyl ligands and PMe3 (2JPC = 7.8 Hz), and a long-range phosphorus−hydrogen coupling is observed between PMe3 and the protons ortho to the nitrogen on the deeb ligand (4JPH = 0.7 Hz). The IR spectrum of 4 exhibits the same pattern of peaks as 3, with both vibrational modes lower in energy due to both the charge neutrality and substitution by the more donating Cl− ligand. Dark purple block-shaped crystals of 4 suitable for X-ray diffraction study were grown from vapor diffusion of pentane into an acetone solution of 4. The solid-state structure of complex 4 (Figure 3) confirms the expected pseudo-Cs

Figure 4. Absorption spectra of complexes 1−4 (solid lines) in CH3CN at 295 K and emission spectra of complexes 1−3 (dashed lines) in 90/10 2-MeTHF/CH3CN at 77 K. For emission spectra, λexc = 365 nm for complexes 1 and 2 and λexc = 455 nm for complex 3.

energy shoulder is observed in both spectra. The absorbance features shift bathochromically from complex 1 to 4. The red shift leads complex 4 to absorb across the entire visible spectrum (ε406 = 4710 M−1 cm−1, ε533 = 5440 M−1 cm−1, Table 2). Full spectral assignments and factors contributing to this impressive absorption profile are discussed below. Such significant bathchromic shifts are rarely observed in rhenium(I) bipyridine carbonyl complexes.20,26 Photoluminescence spectra were recorded in two different solvents for complexes 1, 2, and 3. (No emission was observed for 4 at 295 or 77 K.) Complexes 1 and 2 both display strong emission in CH3CN at 295 K (Figures S18 and S19), while 3 displays extremely weak emission under the same conditions. Photoluminescence measurements at 77 K were recorded in a 90/10 2-MeTHF/CH3CN solution (2-MeTHF = 2-methyltetrahydrofuran) that combines the ideal glassing properties of 2MeTHF with the solubilizing properties of acetonitrile. The emission maxima blue-shift in frozen 2-MeTHF/CH3CN compared to the spectra at 295 K in either CH3CN or 2MeTHF/CH3CN (Figures S18 and S19) as predicted by the rigidochromic effect.27 The emission maximum shifts to longer wavelength moving from complex 1 to 3 (Figure 4). The excited-state lifetimes (τ) were measured using timeresolved photoluminescence spectroscopy at 295 K in CH3CN (1 and 2) and at 77 K in frozen 90/10 2-MeTHF/CH3CN (1, 2, and 3). The lifetimes for complexes 1 (τ = 150 ns) and 2 (τ = 11 ns) at 295 K are similar to previously reported complexes of the type [Re(deeb)(CO)3(L)]n+ (L = CN−, Cl−, Br−, I−, n = 0; L = pyridine, n = 1).15 At 77 K, the lifetimes of 1 and 2 increase by at least an order of magnitude (Figure 5), and comparisons to 3 (τ = 0.41 μs) become possible (Table 2, below). The lifetimes at 77 K decrease as the complex emission maximum red-shifts. Electrochemical Studies. Cyclic voltammetry was used to investigate the ground-state redox properties of 1−4. Each complex displays a one-electron reduction (E°′red) between −1.2 and −1.5 V vs Fc+/Fc in 0.25 M [NBu4][PF6] in CH3CN. The reduced species is proposed to involve electron delocalization onto the deeb ligand, in accordance with prior assignments of ligand-centered reductions in related complexes. Moving from complex 1 to 4, E°′red occurs at more negative potentials (Table 3, below). Cyclic voltammograms of 1−4 also contain a one-electron oxidation feature (E°′ox) assigned to a

Figure 3. Structural representation of 4 with thermal ellipsoids shown at the 50% probability level. Hydrogen atoms and a slightly disordered dichloromethane solvent molecule are omitted for clarity. Selected distances (Å): Re1−Cl1 2.4918(10), Re1−P1 2.3386(11), Re1−N1 2.180(3), Re1−N2 2.161(3), Re1−C1 1.903(4), Re1−C2 1.884(5).

symmetry with a trans configuration of the chloride and PMe3 ligands. The structure and bond lengths are in line with previously reported complexes of similar geometry.20,22,24,25 Photophysical Characterization. The electronic absorption and emission spectra of rhenium dyes 1-4 are shown in Figure 4. The UV/vis absorption spectra of 1 and 2 in CH3CN solution are dominated by a broad MLCT band (λmax = 365 nm for 1 and 410 nm for 2), typical of many reported rhenium(I) tricarbonyl complexes.15 In contrast, the absorption spectra of 3 and 4 each exhibit two features of similar intensity in the visible region (λmax = 365 and 465 nm for 3 and 406 and 533 nm for 4). In addition to these two major transitions, a distinct, low5391

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Density Functional Theory (TD-DFT) calculations to best reflect experimental spectra collected in that solvent. The functional B3LYP provided electronic transition energies for complexes 1 and 4 that agreed most closely with the experimentally observed UV/vis absorption spectra in acetonitrile (see below). The B3LYP geometry optimization of complex 4 also converged at a geometry that closely resembles the crystallographic parameters, so this functional was used going forward. The vibrational frequencies for complexes 1−4 were calculated using a dichloromethane implicit solvation model. Table 1 shows the experimental values for the carbonyl Table 1. Experimental and Calculated Vibrational Frequencies for Complexes 1−4 IR stretches (in CH2Cl2, cm−1)

Figure 5. Time-resolved photoluminescence measurements of 1 at 295 K (left) and at 77 K (right). Measurements recorded in 90/10 2MeTHF/CH3CN.

complex 1 2 3 4

rhenium-based oxidation (Figure 6 and Table 3, below). The cathodic shift of E°′ox (1.13 V difference between 1 and 4) a

experimental 2042, 2025, 1943, 1920,

1945a 1926, 1904 1873 1848

calculated 2026, 2009, 1930, 1912,

1937, 1932 1917, 1900 1867 1846

Two overlapping stretches give rise to single, broad absorption.

stretching frequencies alongside calculated values that have been corrected using the reported empirical scaling factor of 0.9614 for B3LYP.30,31 All of the experimental trends are nicely reproduced in the calculated CO stretching frequencies. TD-DFT was used to calculate the first 50 excitations for complexes 1−4 using the B3LYP functional in acetonitrile solvent. Figure 7 shows the calculated excitation energies (black vertical lines) and depicts the HOMO−1 and LUMO orbitals for all four complexes. The oscillator strength (right axis) calculated for a transition represents the probability of the occurrence of that transition and is proportional to the molar extinction coefficient (left axis). For all complexes, the dominant low-energy feature observed in the experimental spectra (colored lines) can be attributed to excitation from the Re-based HOMO−1 to the bipyridyl π*-based LUMO. For complex 1, excitation from the HOMO−2 to the LUMO also contributes to the dominant absorption feature. The analogous transition has a negligible oscillator strength in complexes 2−4. Less intense transitions from the HOMO (another Re-centered d-character orbital) to the LUMO account for the shoulder observed on the low-energy edge of the absorption spectra.

Figure 6. Cyclic voltammograms of 1 mM 1−4 in 0.25 M [NBu4][PF6] in CH3CN. For complexes 2−4, 1 mM Fc was added and the internal reference Fc+/Fc couple (0 V) is shown with a dashed box. Due to an interaction with oxidized 1 and Fc+, ferrocene was not added to the solution when scanning oxidatively; the reduction feature was referenced to Fc+/Fc in a separate experiment. All scans were recorded using a Pt working electrode, glassy carbon counter electrode, Ag wire pseudoreference electrode, and 3 V/s scan rate.

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DISCUSSION The present series of complexes can be compared in pairs organized by apical ligand: acetonitrile vs chloride (1 vs 2, and 3 vs 4), and carbonyl vs PMe3 (1 vs 3, and 2 vs 4). These different ligands lead to relatively large spectroscopic and electrochemical changes. Redox Properties and Electrochemically Induced Reactivity. The deeb-based reduction features for complexes 1−3 are reversible at all scan rates studied (0.025−50 V/s, Figures S23, S25, and S27). For complex 4, however, the primary reduction is not chemically reversible at slow scan rates, only becoming fully reversible at scan rates above ∼0.5 V/s (Figure S29). The anodic feature observed at fast scan rates diminishes as the scan rate is lowered and a new oxidation peak grows in. The new anodic feature aligns with the reoxidation feature of reduced 3 (Figure S28). This observation suggests

across the four complexes is substantially larger in magnitude than the shifts in deeb-based E°′red (0.25 V shift from 1 to 4). Computational Studies. Density Functional Theory (DFT) was employed to further probe the structural and spectroscopic features of complexes 1−4. The optimal computational protocol for the ground-state singlet (S0) geometries was developed for complex 4 by comparing three different functionals (M06, B3LYP, and PBE), each coupled with the LANL2DZ-ECP basis set for Re and 6-31G* for all other atoms. These three functionals were chosen based on literature precedent for calculations on similar rhenium dyes.28,29 All calculations were performed using an implicit SMD solvation model. The SMD model for dichloromethane was used for the calculation of IR stretching frequencies in order to compare with experimental spectra. Similarly, the SMD model for acetonitrile was used for Time-Dependent 5392

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Figure 7. Calculated excitations (black vertical lines) and experimental UV/vis absorption spectra (colored lines) for complexes 1−4 (A−D, respectively) along with pictures of the calculated HOMO−1 and LUMO orbitals involved in the most intense low-energy transition for each complex.

indicated by the peak-to-peak separation, which increases from 100 mV at a scan rate of 25 mV/s to 360 mV at 50 V/s. The quasi-reversibility is attributed to a relatively slow heterogeneous electron transfer rate constant for the ReII/I couple. Interestingly, when an equimolar amount of Fc is present in solution, current passed at the onset of the ReII/I couple (∼1.4 V) increases, and the waveform becomes irreversible (Figure S31). In a separate experiment, an irreversible oxidation of ferrocene was detected with an onset of ∼1.6 V, presumably decomposing upon oxidation in accordance to previous reports in other solvents (AlCl3/1-butylpyridinium chloride mixtures33 and SO234). The catalytic current observed for mixtures of 1 and Fc suggest that 1+ mediates the oxidative decomposition of Fc+. The oxidation of 2 (E°′ox ≈ 1.03 V) approaches reversibility at very fast scan rates (∼100 V/s, Figure S26), but is chemically

that chloride dissociation occurs after the one-electron reduction of 4, generating the CH3CN-ligated species 3. This EC mechanism stands in contrast to the reported reactivity of Re(dmbpy)(CO)3Cl (dmbpy = 4,4′-dimethyl-2,2′-bipyridine), which only dissociates chloride on this time scale upon a twoelectron reduction.32 Indeed, we also observe no evidence for chloride loss upon one-electron reduction of the tricarbonyl chloride complex 2. Therefore, our observation of oxidation of [Re(deeb)(CO)2(PMe3)(NCCH3)]• on the return trace after the one-electron reduction of 4 indicates that chloride loss from reduced 4 is more facile and suggests that replacing the πaccepting CO in 2 with the σ-donating PMe3 ligand in 4 leads to a more electron rich metal center that facilitates chloride dissociation upon reduction. The oxidation of 1 is chemically reversible at all scan rates studied, but electrochemically quasi-reversible (Figure S24), as 5393

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Inorganic Chemistry Table 2. Photophysical Properties of Complexes 1−4 ΔGST (eV)

a

τ (ns)

λmax (nm)

complex

expt

calc

295 K

77 K

1 2 3

2.62 2.35 2.05

2.44 2.20 1.76

150 13 −

4600 2700 410

4

1.80a

1.42

−

−

abs (ε, M−1 cm−1) 365 410 365 465 406 533

emission (77 K)

Φr

525 585 663

0.049 0.013b −

−

−

(4750) (4820) (5270) (6150) (4710) (5440)

Value extrapolated, see Figure 8 bReference 18.

irreversible at slower scan rates. Upon oxidation to the Re(II) species, a number of tricarbonyl chloride rhenium complexes have been reported to undergo oxidative disproportionation via a chloride-bridged inner-sphere electron transfer mechanism; similar reactivity likely gives rise to the irreversibility observed for 2 at lower scan rates.35 The oxidation of 3 at 0.76 V is chemically reversible at all scan rates studied. Similar to 1, it is electrochemically quasireversible at higher scan rates, likely due to a slow heterogeneous electron transfer rate constant (Figure S28). Interestingly, the oxidation of chloride complex 4 is chemically reversible at all scan rates studied, and the redox couple shows no increased peak-to-peak separation at fast scan rates (Figure S30). This is in contrast to the established followup chemical steps noted above for rhenium chloride complexes, including 2. This differing reactivity suggests that the steric hindrance of the PMe3 may prevent the formation of the chloride-bridged dimer needed for inner sphere electron transfer. Alternatively, as the ReII/I reduction potential of 4 is 700 mV more cathodic than that of 2, the oxidized species may be more stable and not as reactive toward the aforementioned inner sphere disproportionation pathway. Photophysics and Excited-State Redox Properties. The Stokes shift and luminescence lifetimes for these rhenium complexes (Table 2) are consistent with prior assignments for related complexes of the emissive excited state as a MLCT triplet state.20,36 The potentials for oxidation and reduction of the excited state can be calculated using a thermochemical cycle involving the ground-state reduction potentials and the excitedstate energy, ΔGST, defined here as the energy difference between the ground vibrational states of the singlet ground state and the triplet excited state. Values of ΔGST were obtained for complexes 1, 2, and 3 based on the x-intercept of a linear, tangential fit to the high-energy side of the low-temperature photoluminescence spectra (Figures S19−S21).37−40 The measured ΔGST values of 2.44, 2.20, and 1.76 eV for 1, 2, and 3, respectively, are consistent with a decrease in the energy of the HOMO/LUMO gap as the substituents become more donating (Table 2). A linear correlation between the measured ΔGST values and the lowest energy absorption λmax values is observed for complexes 1−3 (Figure 8). Although no emission was observed for complex 4, a ΔGST value can be predicted by extrapolation from the linear correlation of Figure 8 (1.80 eV). TD-DFT calculated ΔGST values exhibit the same trends within the series of complexes, with reasonable 0.2−0.4 eV agreement with the experimental values. The excited-state photophysical data and electrochemical properties were used to calculate the excited-state redox properties (eqs 1 and 2), summarized in Table 3.41 These values indicate that these complexes are potent excited-state reductants and oxidants. Whereas the ReII/ReI* potential

Figure 8. Linear correlation between experimentally determined ΔGST values and lowest energy absorption λmax of 1−3 used to estimate ΔGST of 4.

Table 3. Summary of Ground State and Excited State Redox Properties of Complexes 1−4 E°′ (V vs Fc+/Fc) I

I

•−

complex

Re /Re (deeb )

1 2 3 4

−1.22 −1.34 −1.33 −1.47

Re

E°′* (V vs Fc+/Fc) II/I

Re */ReI(deeb•−)

ReII/ReI*

1.40 1.01 0.72 0.33

−1.17 −1.32 −1.29 −1.50

I

1.45 1.03 0.76 0.30

remains relatively constant for 1−4, the ReI*/ReI(deeb•−) potential varies by more than 1 V as a function of the ancillary ligand, with complex 1 being an extremely potent photooxidant. Excited-state quenching studies are the focus of current work on the photochemistry of these complexes. E°′(Re I */Re I(deeb•−)) = E°′(Re I /Re I(deeb•−)) + ΔGST (1) II

I

II

I

E°′(Re /Re *) = E°′(Re /Re ) − ΔGST

(2)

Influence of Axial Ligands on Electronic Structure. The spectroscopic and electrochemical data presented above clearly demonstrate that the axial ligands strongly influence the ground-state electronic structure of the complexes. Computational methods provide insight into the impact of PMe3 ligation on electronic structure. A detailed orbital energy diagram can be built from the calculated transitions and the relative energies of orbitals relevant to the visible transitions. All the transitions discussed below are comprised of charge transfers from Re dcharacter orbitals (HOMO, HOMO−1, and HOMO−2) to π*5394

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Figure 9. (Left) Energy diagram calculated using TD-DFT highlighting the relevant visible transitions and (right) calculated molecular orbitals for the HOMO, HOMO−1, and HOMO−2 orbitals for complexes 2 and 4 showing the degree of backbonding to the carbonyl ligands in each.

orbital destabilization leads to significant perturbation of the HOMO, HOMO−1, and HOMO−2 energies across the series 1-4, while the LUMO and LUMO−1 energies remain about the same. Another interesting observation gleaned from the orbital energies displayed in Figure 9 (left) is the differing extent of destabilization of the individual d-orbitals across the series of complexes. The three lowest-energy d-orbitals in complex 2 are all (a) lower in energy than the corresponding orbitals in complex 4 and (b) closer in energy to each other than the same orbitals in complex 4. The HOMO and HOMO−1 have appropriate symmetry to engage in π-backbonding with all three CO ligands in complexes 1 and 2 (1 in Figure S40; 2 in Figure 9, right). Therefore, when a phosphine ligand is substituted for the CO, the amount of π-backbonding is substantially decreased and the HOMO and HOMO−1 are not stabilized to the same extent in 4 as they are in 2. In contrast, the symmetry of HOMO−2 only allows for backbonding interactions with the two equatorial CO ligands. The HOMO− 2 is thus less perturbed when the axial CO ligand is replaced with PMe3, giving rise to a larger net spacing between HOMO and HOMO−2 in 4. The molecular orbital picture of Figure 9 also helps rationalize the electrochemical properties of complexes 1-4 (Table 3). The metal-based d-orbitals are strongly affected by ligand substitution, so the ReII/I potentials are dramatically affected by ligand substitution. On the other hand, the LUMO is not greatly affected by ligand substitution, leading to the deeb-based reduction potentials remaining roughly the same over the series.

character orbitals on the deeb ligand (LUMO and LUMO+1). The TD-DFT calculations reveal that the visible region transitions with the highest oscillator strength for all complexes involve electronic excitation from the HOMO−1 to the LUMO. The HOMO → LUMO transitions are calculated to be at slightly lower energy than this prominent band and with much lower oscillator strengths, corresponding to small shoulders on the low-energy side of the main absorbance bands of all four complexes (Figure 4). The main absorption band in the UV/vis absorption spectrum of tricarbonyl chloride complex 2 (λmax = 410 nm) is red-shifted with respect to the analogous absorption band in the spectrum of tricarbonyl acetonitrile complex 1 (λmax = 365 nm), consistent with other studies comparing analogous neutral chloride and cationic acetonitrile complexes.15 The magnitude of the red-shift (∼0.4 eV) is reproduced nicely by TD-DFT. The energetic basis for the red-shift is found in ground-state DFT calculations that show a 0.4 eV destabilization of the HOMO−1 levels and only 0.1 eV change in energy of the LUMO orbitals upon replacing chloride with acetonitrile. The π-donation by the Cl− ligand into the metal-based orbitals in 2 (absent in 1), is thought to be the origin of the HOMO−1 destabilization. The same acetonitrile/chloride substitution leads to the absorbance features of 4 being red-shifted with respect to those of 3 by ∼0.35 eV. DFT calculations again indicate a 0.4 eV destabilization of HOMO−1 levels and negligible changes to the LUMO energy upon replacing acetonitrile in 3 with chloride in 4. The substitution of a CO ligand for a PMe3 ligand also impacts the electronic structure properties, as evidenced by the absorbance differences observed for 1 vs 3 and 2 vs 4. The UV/ vis absorption spectrum of 3 is significantly red-shifted from that of 1. Replacing the strongly π-acidic CO ligand with a PMe3 ligand destabilizes the Re d-orbitals significantly, leading to a 0.8 eV increase in the HOMO−1 level of 3 relative to 1. This HOMO−1 destabilization is consistent with the ∼0.75 eV difference in the lower energy λmax values in the UV/vis absorption spectra of 1 and 3. Similarly, a 0.6 eV HOMO−1 destabilization is calculated upon moving from 2 to 4, corresponding nicely to the ∼0.7 eV difference between the lower energy λmax values. The combined data provides an explanation for the improved red light absorption properties of 3 and 4 relative to 1 and 2: as shown in Figure 9 (left), Re d-

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CONCLUSION A series of rhenium diimine carbonyl complexes with axial carbonyl, chloride, acetonitrile, or phosphine ligands was prepared and characterized by multinuclear NMR spectroscopy, infrared spectroscopy, and X-ray diffraction crystallography (for complex 4). Cyclic voltammetry was used to determine both the one-electron, metal-based oxidation potentials and oneelectron, deeb-based reduction potentials of each complex. The axial ligand is observed to have a greater influence on the oxidation feature than the reduction feature. The electronic absorption spectra of the complexes show an increased visibleregion absorption profile as acetonitrile is substituted by 5395

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was submerged in a glass tube containing electrolyte (0.25 M [NBu4][PF6] in acetonitrile, CH3CN) and separated from the solution with a porous glass Vycor tip. The working electrode was pretreated with cyclical scans from approximately 2 to −2 V (the exact value varied in accordance with the silver wire pseudoreference) at 250 mV/ s in 0.25 M [NBu4][PF6] until cycles were superimposable (typically achieved within three cycles). Time-Resolved Photoluminescence. Samples for time-resolved photoluminescence experiments were prepared in a N2-filled glovebox. ∼2−5 mg of each sample was dissolved in ∼1 mL of a 90/10 2MeTHF/CH3CN solution and transferred to a NMR tube. The cap was secured with electrical tape and parafilm. A coldfinger Dewar with a quartz cavity at the bottom was used for the 295 K lifetime experiments, and was filled with liquid N2 for the lifetime experiments at 77 K. Time-resolved photoluminescence experiments were performed using a custom-built laser flash photolysis system.44 Laser excitation (5−7 ns fwhm, 10 Hz, Q-switched) was provided by the third harmonic of a Nd:YAG laser (Spectra-Physics, Inc., model Quanta-Ray LAB-170-10) that pumped an OPO (basiScan, GWU Lasertechnik) to access tunable excitation (415−800 nm). Laser power at the sample cuvette was attenuated by the use of a half waveplate (WPMH10M355, ThorLabs) and polarizer (GL10-A, ThorLabs). A glass window was used to deflect a small portion of excitation beam to a Si diode detector (DET10A, ThorLabs), triggering the oscilloscope to start data collection. Timing of the laser was controlled by a digital delay generator (9514+ Pulse Generator, Quantum Composers). Single wavelength emission data (monitored near the λem,max of each complex) was obtained using a double slit monochromator (Spectral Products CM112) outfitted with a Hamamatsu R928 photomultiplier tube (PMT). The signal intensity was attenuated with a neutral density filter, and scattered excitation light was filtered with a color filter wheel containing various long pass and short pass filters. The signal was amplified by a 200 MHz wideband voltage amplifier (DHPVA-200, Electro Optical Components) and processed using a digitizer (CompuScope 12502, GaGeScope) controlled by custom software (MATLAB). A dark current (detection without laser excitation) was subtracted from the raw luminescence data, and further analyzed using Igor Pro 6.37 (Wavemetrics). Single-Crystal X-ray Diffraction. Single-crystal X-ray diffraction data were collected on a Bruker APEX-II CCD diffractometer at 100 K with Cu Kα radiation (λ = 1.54175 Å). The structures were solved using Olex245 with the ShelXT46 structure solution program using intrinsic phasing and refined with the ShelXL47 refinement program using least-squares minimization. A disordered solvent molecule of CH2Cl2 was refined over two positions, each with 50% occupancy. SADABS-2016/2 (Bruker,2016/2) was used for absorption correction with Abs T max = 0.7536 and Abs T min = 0.5210. wR2(int) was 0.0792 before and 0.0529 after correction. Computational Details. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations were done using Gaussian 09.47 Geometry optimizations were performed using the B3LYP hybrid functional with the LANL2DZ ECP48,49 basis set for Re and 6-31G* basis set for all heteroatoms. The implicit SMD solvation model was employed for all calculations (acetonitrile or dichloromethane solvent). The M06 and PBE functionals were also tested for select species, but B3LYP generally showed the best agreement with experimental parameters. Synthesis of Re(deeb)(CO)3Cl (2). Re(CO)5Cl (335 mg, 0.93 mmol) and deeb (274 mg, 0.92 mmol) were combined in 50 mL of toluene. The mixture was heated to reflux and stirred for 2 h, resulting in a color change from white to dark orange. Once cooled to 295 K, 100 mL of pentane was added, and an orange precipitate was collected. The solid was washed with pentane and dried under reduced pressure to yield a fine orange powder in 84% yield. 1H NMR (600 MHz, CD3CN) δ 9.19 (dd, J = 5.7, 0.8 Hz, 2H), 8.95 (dd, J = 1.7, 0.8 Hz, 2H), 8.07 (dd, J = 5.6, 1.7 Hz, 2H), 4.48 (q, J = 7.1 Hz, 4H), 1.44 (t, J = 7.1 Hz, 7H). 13C{1H} NMR (151 MHz, CD3CN) δ 198.4, 189.9, 164.1, 157.1, 155.0, 142.0, 127.6, 124.5, 63.6, 14.3. Elemental Analysis,

chloride and as carbonyl is substituted by PMe3. These bathochromic shifts are also apparent in the photoluminescence spectra of the complexes and are accompanied by a decrease in excited-state lifetime. Computational methods were used to assign each of the MLCT transitions observed in the UV/vis absorption spectra. These DFT results reveal that the aforementioned red-shifting in electronic absorption and emission spectra is a consequence of destabilization of Re dorbitals. The destabilization could arise from changes in donor properties, such as the substitution of a strongly σ-donating phosphine for a π-accepting carbonyl ligand, or from changes in overall charge leading to energy lowering of the d-orbitals. While rhenium carbonyl complexes are generally used as photo-oxidants, the excited-state reduction potential of 4 (E°′*(ReII/ReI*) = −1.5 V vs Fc+/Fc) and its broad absorption profile in the visible region provides potential avenues for new reactivity. Collectively, the excited-state and ground-state redox properties and wide spectral differences across this series of chromophores encourage more detailed studies of their rich photochemistry in order to reveal how Re photosensitizers can be harnessed for new applications.

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EXPERIMENTAL SECTION

General Considerations. Syntheses were performed under N2 using standard Schlenk line techniques unless otherwise noted. All solvents used for synthetic procedures were purchased from Thermo Fischer Scientific and were used without purification. Rhenium pentacarbonyl chloride (ACROS Organics, 98%), silver trifluoromethanesulfonate (AgOTf), ammonium hexafluorophosphate ([NH4][PF6], Oakland Chemical, 99%), trimethylphosphine (PMe3) (Aldrich, 1.0 M in toluene), trimethylamine N-oxide dihydrate (TMNO, Alfa Aesar, 98+%), and benzyltrimethylammonium chloride ([BnMe3N]Cl, Aldrich, 97%) were used without further purification. 4,4′-diethylester2,2′-bipyridine (deeb) was synthesized according to previous procedures.42 Acetonitrile-d3 (99.8% D) was purchased from Cambridge Isotope Laboratories. 2-Methyltetrahydrofuran (99+%, Extra Dry) was purchased from Acros Organics. 1 H, 13C{1H}, and 31P{1H} NMR spectra were collected on a Bruker 600 MHz spectrometer at 295 K. Chemical shifts are reported relative to residual protio solvent signals. UV/vis absorption spectra were collected using an Agilent Cary 60 UV/vis absorbance spectrophotometer. Photoluminescence measurements were recorded using an LED excitation source (Ocean Optics LLS, either 365 or 455 nm), fiber coupled to a 90° sample compartment (Ocean Optics CUV-UVFL), fiber coupled to an Ocean Optics USB2000+ spectrophotometer detector. Electrochemical Measurements. Electrochemistry was performed in a N2-filled glovebox with a WaveDriver (Pine Research) potentiostat using a 3 mm diameter platinum working electrode, a 3 mm diameter glassy carbon counter electrode, and a silver wire pseudoreference electrode. A 20 mL scintillation vial was used as an electrochemical cell, fitted with a custom-made Teflon cap to hold the three electrodes. The electrode leads in the glovebox were connected to the potentiostat with a custom shielded electrode cable feedthrough. All scans were referenced to the ferrocenium/ferrocene couple at 0 V. Acetonitrile (Fisher Scientific, HPLC grade, >99.9%) for electrochemical experiments was dried and degassed using a Pure Process Technology solvent purification system. Ferrocene was present in each scan unless otherwise noted. Ohmic drop was minimized using a high electrolyte concentration (0.25 M tetrabutylammonium hexafluorophosphate, [NBu4][PF6]), through minimization of the distance between the working and reference electrodes, and through manual iR compensation.43 Platinum electrodes (CH Instruments, 3 mm diameter disk) were polished with 0.05 μm alumina powder (CH Instruments, contained no agglomerating agents) Milli-Q water slurries, rinsed, and ultrasonicated briefly in Milli-Q water to remove residual polishing powder. The silver wire pseudoreference electrode 5396

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color change from dark red to dark purple. The excess [BnMe3N][Cl] was removed by filtration, and the solvent was removed by rotary evaporation. The residue was dissolved in 20 mL CH2Cl2 and impurities were extracted with H2O (3 × 100 mL). The CH2Cl2 layer was then dried with anhydrous MgSO4 and filtered, and the solvent was removed. The residue was dissolved in minimal (5 mL) of CH2Cl2, to which 100 mL of pentane was added, resulting in the precipitation of a dark purple solid. The solid was filtered, washed with pentane and dried under reduced pressure to give 65% yield. Single dark purple block crystals of 4 suitable for X-ray diffraction study were grown from vapor diffusion of pentane into an acetone solution of 4. 1 H NMR (600 MHz, CD3CN) δ 9.15 (ddd, J = 5.7, 0.7, 0.7 Hz, 2H), 8.88 (dd, J = 1.7, 0.8 Hz, 2H), 7.97 (dd, J = 5.7, 1.7 Hz, 2H), 4.46 (q, J = 7.1 Hz, 4H), 1.43 (t, J = 7.1 Hz, 6H), 1.19 (d, J = 9.3 Hz, 9H). 13 C{1H} NMR (151 MHz, CD3CN) δ 207.3 (d, J = 7.8 Hz), 164.4, 157.1, 153.5, 140.4, 127.0, 124.0, 63.4, 18.3 (d, J = 35.3 Hz), 14.36. 31 1 P{ H} NMR (243 MHz, CD3CN) δ −23.11.

Calcd for C19H16ClN2O7Re: C, 37.66; H, 2.66; N, 4.62; Found C, 37.39; H, 2.57; N, 4.65. Synthesis of [Re(deeb)(CO)3(NCCH3)][PF6] (1). Re(deeb)(CO)3Cl (50.3 mg, 0.083 mmol) was dissolved in 15 mL of degassed CH3CN. AgOTf (82.5 mg, 0.32 mmol, 3.9 equiv) was then added under positive N2 pressure. The solution was protected from light by foil and stirred at reflux for 3.5 h. The solution changed color from orange to yellow during the reflux, and a AgCl precipitate formed. The AgCl was removed by filtration and the solvent was removed from the filtrate by rotary evaporation. The yellow-orange residue was dissolved in 2 mL of CH3CN, to which was added 15 mL of a saturated, aqueous solution of [NH4][PF6] followed by 40 mL of H2O. A yellow solid precipitated from solution and was collected on a frit, washed with copious H2O and diethyl ether, and dried in vacuo (84% yield). (600 MHz, CD3CN) δ 9.20 (dd, J = 5.7, 0.8 Hz, 2H), 9.01 (dd, J = 1.7, 0.8 Hz, 2H), 8.16 (dd, J = 5.7, 1.6 Hz, 2H), 4.50 (q, J = 7.1 Hz, 4H), 2.02 (s, 3H), 1.45 (t, J = 7.1 Hz, 6H). 13C{1H} NMR (151 MHz, CD3CN) δ 194.4, 190.6, 164.0, 157.7, 156.1, 142.9, 128.2, 124.8, 123.8, 63.9, 14.3, 3.9. Elemental Analysis, Calcd for C21H19F6N3O7PRe: C, 33.34; H, 2.53; N, 5.55; Found C, 33.64; H, 2.46; N, 5.62. Synthesis of [Re(deeb)(CO)3(PMe3)][PF6]. Re(deeb)(CO)3Cl (298 mg, 0.49 mmol) was dissolved in 50 mL of degassed acetone. AgOTf (379 mg, 1.47 mmol, 3.0 equiv) was then added under positive N2 pressure. The solution was protected from light by foil and stirred at reflux for 2 h, accompanied by the precipitation of AgCl and a solution color change from orange to yellow. After the reaction, the AgCl was removed by filtration using a fine frit and to the filtrate was added 1.0 M PMe3 in toluene (14.8 mL, 14.8 mmol, 30 equiv) via syringe. The solution was sparged with N2, and stirred at reflux under N2 for 16 h. The orange solution was concentrated and to the residue was added 5 mL of MeOH. The solution was filtered and added dropwise to a solution of ∼1 g of [NH4][PF6] in 100 mL of H2O while vigorously stirring. The mixture was sonicated for 10 min, and the resulting yellow-orange powder was collected by filtration. After rinsing with copious H2O and diethyl ether, the solid was suspended in 5 mL of MeOH and sonicated for 10 min. The solid was filtered again and dried under reduced pressure to give the dark yellow product (76% yield). 1H NMR (600 MHz, CD3CN) δ 9.20 (ddd, J = 5.7, 0.9, 0.9 Hz, 2H), 9.03 (dd, J = 1.7, 0.8 Hz, 2H), 8.12 (dd, J = 5.7, 1.7 Hz, 2H), 4.50 (q, J = 7.1 Hz, 4H), 1.45 (t, J = 7.1 Hz, 6H), 1.10 (d, J = 9.4 Hz, 9H). 13 C{1H} NMR (151 MHz, CD3CN) δ 195.4 (d, J = 7.7 Hz), 188.5 (d, J = 59.6 Hz), 163.9, 157.0, 155.9, 142.3, 128.3, 125.3, 63.8, 14.3, 13.2 (d, J = 32.5) Hz. 31P{1H} NMR (243 MHz, CD3CN) δ −28.55 (s, 1P), −144.66 (hept, J = 706 Hz, 1P, PF6). Elemental Analysis, Calcd for C22H25F6N2O7P2Re: C, 33.38; H, 3.18; N, 3.54; Found C, 33.54; H, 31.0; N, 3.54. Synthesis of [Re(deeb)(CO)2(PMe3)(NCCH3)][PF6] (3). [Re(deeb)(CO)3(PMe3)][PF6] (147 mg, 0.19 mmol) was dissolved in 20 mL of degassed CH3CN. TMNO (22.0 mg, 0.39 mmol, 1.1 equiv) was dissolved in 5 mL of CH3CN and 1 mL of MeOH, and then injected into the reaction mixture. Within 5 min of injection, the solution color changed from yellow-orange to dark red. The solution was then stirred at reflux for 2 h. After removing the solvent, the residue was dissolved in minimal CH3CN; addition of H2O and sonication resulted in the precipitation of a dark red solid. The solid was collected, washed with excess H2O and dried under reduced pressure to give 3 in 60% yield. 1 H NMR (600 MHz, CD3CN) δ 9.17 (ddd, J = 5.7, 0.7, 0.7 Hz, 2H), 8.96 (dd, J = 1.7, 0.8 Hz, 2H), 8.08 (dd, J = 5.7, 1.6 Hz, 2H), 4.49 (q, J = 7.1 Hz, 4H), 2.04 (s, 3H), 1.44 (t, J = 7.2 Hz, 6H), 1.15 (d, J = 9.5 Hz, 9H). 13C{1H} NMR (151 MHz, CD3CN) δ 202.4 (d, J = 7.2 Hz), 164.2, 157.33, 154.7, 141.6, 127.9, 124.6, 63.7, 16.7 (d, J = 36.3 Hz), 14.35, 3.92. 31P{1H} NMR (243 MHz, CD3CN) δ −22.25 (s, 1P), −144.66 (hept, J = 706 Hz, 1P, PF6). Elemental Analysis, Calcd for C23H28F6N3O6P2Re: C, 34.33; H, 3.51; N, 5.22; Found C, 34.61; H, 3.54; N, 5.16. Synthesis of Re(deeb)(CO)2(PMe3)Cl (4). [Re(deeb)(CO)2(PMe3)(NCCH3)][PF6] (121 mg, 0.15 mmol) was dissolved in 100 mL of dichloromethane (CH2Cl2), to which [BnMe3N][Cl] (734 mg, 4.5 mmol, 30 equiv) was added. The solution was sparged with N2 for 20 min, and then stirred at reflux for 24 h which was accompanied by a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00360. Figures S1−S40 and Tables S1−S5, additional experimental details, NMR spectra, cyclic voltammograms, UV−vis and emission spectra, computational details including input files, optimized coordinates for all complexes, calculated electronic spectra of complexes 1−4, orbital compositions of excited states, and crystallographic details (PDF) Accession Codes

CCDC 1822704 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kelsey R. Brereton: 0000-0003-4189-7064 Alexander J. M. Miller: 0000-0001-9390-3951 Jillian L. Dempsey: 0000-0002-9459-4166 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was solely supported by the UNC Energy Frontier Research Center (EFRC): Center for Solar Fuels, an EFRC funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DESC0001011. J.L.D. acknowledges support from a Packard Fellowship for Science and Engineering and a Sloan Research Fellowship. A.J.M.M. acknowledges support from a Sloan Research Fellowship. K.R.B. thanks Professor Thomas Cundari for insightful discussions. Quinton J. Bruch assisted with crystallographic data collection and Carolyn L. Hartley assisted with quantum yield determination measurements. 5397

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Quenching of Molecular Excited States. Langmuir 1999, 15 (3), 650−653. (20) Sato, S.; Sekine, A.; Ohashi, Y.; Ishitani, O.; Blanco-Rodríguez, A. M.; Vlček, A.; Unno, T.; Koike, K. Photochemical Ligand Substitution Reactions of fac-[Re(bpy)(CO)3Cl] and Derivatives. Inorg. Chem. 2007, 46 (9), 3531−3540. (21) Koike, K.; Tanabe, J.; Toyama, S.; Tsubaki, H.; Sakamoto, K.; Westwell, J. R.; Johnson, F. P. A.; Hori, H.; Saitoh, H.; Ishitani, O. New Synthetic Routes to Biscarbonylbipyridinerhenium(I) Complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+ (X2bpy = 4,4′-X2-2,2′-bipyridine) via Photochemical Ligand Substitution Reactions, and Their Photophysical and Electrochemical Properties. Inorg. Chem. 2000, 39 (13), 2777−2783. (22) Kurtz, D. A.; Dhakal, B.; Donovan, E. S.; Nichol, G. S.; Felton, G. A. N. Non-Photochemical Synthesis of Re(diimine)(CO)2(L)Cl (L = Phosphine or Phosphite) Compounds. Inorg. Chem. Commun. 2015, 59, 80−83. (23) Ko, C.-C.; Ng, C.-O.; Yiu, S.-M. Luminescent Rhenium(I) Phenanthroline Complexes with a Benzoxazol-2-ylidene Ligand: Synthesis, Characterization, and Photophysical Study. Organometallics 2012, 31 (20), 7074−7084. (24) Morimoto, T.; Ito, M.; Koike, K.; Kojima, T.; Ozeki, T.; Ishitani, O. Dual Emission from Rhenium(I) Complexes Induced by an Interligand Aromatic Interaction. Chem. - Eur. J. 2012, 18 (11), 3292− 3304. (25) Smithback, J. L.; Helms, J. B.; Schutte, E.; Woessner, S. M.; Sullivan, B. P. Preparative Routes to Luminescent Mixed-Ligand Rhenium(I) Dicarbonyl Complexes. Inorg. Chem. 2006, 45 (5), 2163− 2174. (26) Kurtz, D. A.; Dhakal, B.; Hulme, R. J.; Nichol, G. S.; Felton, G. A. N. Correlations between Photophysical and Electrochemical Properties for a Series of New Mn Carbonyl Complexes Containing Substituted Phenanthroline Ligands. Inorg. Chim. Acta 2015, 427 (1), 22−26. (27) Lees, A. J. The Luminescence Rigidochromic Effect Exhibited by Organometallic Complexes: Rationale and Applications. Comments Inorg. Chem. 1995, 17 (6), 319−346. (28) Zhao, F.; Wang, J.; Liu, W.; Wang, Y. Electronic Structures and Spectral Properties of Rhenium(I) Tricarbonyl Diimine Complexes with Phosphine Ligands: DFT/TDDFT Theoretical Investigations. Comput. Theor. Chem. 2012, 985, 90−96. (29) Veronese, L.; Procopio, E. Q.; De Rossi, F.; Brown, T. M.; Mercandelli, P.; Mussini, P.; D’Alfonso, G.; Panigati, M. New Dinuclear Hydrido-Carbonyl Rhenium Complexes Designed as Photosensitizers in Dye-Sensitized Solar Cells. New J. Chem. 2016, 40 (3), 2910−2919. (30) Halim, M. A.; Shaw, D. M.; Poirier, R. A. Medium Effect on the Equilibrium Geometries, Vibrational Frequencies and Solvation Energies of Sulfanilamide. J. Mol. Struct.: THEOCHEM 2010, 960 (1−3), 63−72. (31) Merrick, J. P.; Moran, D.; Radom, L. An Evaluation of Harmonic Vibrational Frequency Scale Factors. J. Phys. Chem. A 2007, 111 (45), 11683−11700. (32) Breikss, A. I.; Abruña, H. D. Electrochemical and Mechanistic Studies of [Re(CO)3(dmbpy)Cl] and Their Relation to the Catalytic Reduction of CO2. J. Electroanal. Chem. Interfacial Electrochem. 1986, 201 (2), 347−358. (33) Gale, R. J.; Singh, P.; Job, R. Metallocene Electrochemistry. I: Evidence for electronic stabilization with alkylated cyclopentadiene: electrochemical synthesis of decamethylferricinium dication. J. Organomet. Chem. 1980, 199 (2), C44−C46. (34) Sharp, P. R.; Bard, A. J. Electrochemistry in Liquid Sulfur Dioxide. 4. Electrochemical Production of Highly Oxidized Forms of Ferrocene, Decamethylferrocene and Iron bis(tris(1-pyrazolyl)borate. Inorg. Chem. 1983, 22 (19), 2689−2693. (35) Bullock, J. P.; Carter, E.; Johnson, R.; Kennedy, A. T.; Key, S. E.; Kraft, B. J.; Saxon, D.; Underwood, P. Reactivity of Electrochemically Generated Rhenium (II) Tricarbonyl α-Diimine Complexes: A Reinvestigation of the Oxidation of Luminescent Re(CO)3(α-

REFERENCES

(1) Hori, H.; Johnson, F. P. A.; Koike, K.; Ishitani, O.; Ibusuki, T. Efficient photocatalytic CO2 reduction using [Re(bpy)(CO)3{P(OEt)3}]+. J. Photochem. Photobiol., A 1996, 96 (1−3), 171−174. (2) Koike, K.; Hori, H.; Ishizuka, M.; Westwell, J. R.; Takeuchi, K.; Ibusuki, T.; Enjouji, K.; Konno, H.; Sakamoto, K.; Ishitani, O. Key Process of the Photocatalytic Reduction of CO2 Using [Re(4,4′-X2bipyridine)(CO)3PR3]+ (X = CH3, H, CF3; PR3 = Phosphorus Ligands): Dark Reaction of the One-Electron-Reduced Complexes with CO2. Organometallics 1997, 16 (26), 5724−5729. (3) Ishitani, O.; George, M. W.; Ibusuki, T.; Johnson, F. P. A.; Koike, K.; Nozaki, K.; Pac, C.; Turner, J. J.; Westwell, J. R. Photophysical Behavior of a New CO2 Reduction Catalyst, Re(CO)2(bpy){P(OEt)3}2+. Inorg. Chem. 1994, 33 (21), 4712−4717. (4) Morimoto, T.; Tanabe, J.; Sakamoto, K.; Koike, K.; Ishitani, O. Selective H2 and CO production with rhenium(I) biscarbonyl complexes as photocatalyst. Res. Chem. Intermed. 2013, 39 (1), 437− 447. (5) Vlček, A. Ultrafast Excited-State Processes in Re(I) CarbonylDiimine Complexes: From Excitation to Photochemistry. Top. Organomet. Chem. 2009, 29, 115−158. (6) Shih, C.; Museth, A. K.; Abrahamsson, M.; Blanco-Rodriguez, A. M.; Di Bilio, A. J.; Sudhamsu, J.; Crane, B. R.; Ronayne, K. L.; Towrie, M.; Vlcek, A.; Richards, J. H.; Winkler, J. R.; Gray, H. B. TryptophanAccelerated Electron Flow Through Proteins. Science (Washington, DC, U. S.) 2008, 320 (5884), 1760−1762. (7) Pizano, A. A.; Olshansky, L.; Holder, P. G.; Stubbe, J.; Nocera, D. G. Modulation of Y 356 Photooxidation in E. coli Class Ia Ribonucleotide Reductase by Y731 Across the α2:β2 Interface. J. Am. Chem. Soc. 2013, 135 (36), 13250−13253. (8) Pizano, A. A.; Lutterman, D. A.; Holder, P. G.; Teets, T. S.; Stubbe, J.; Nocera, D. G. Photo-ribonucleotide reductase β2 by selective cysteine labeling with a radical phototrigger. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (1), 39. (9) Olshansky, L.; Stubbe, J.; Nocera, D. G. Charge-Transfer Dynamics at the α/β Subunit Interface of a Photochemical Ribonucleotide Reductase. J. Am. Chem. Soc. 2016, 138 (4), 1196− 1205. (10) Dempsey, J. L.; Winkler, J. R.; Gray, H. B. Proton-Coupled Electron Flow in Protein Redox Machines. Chem. Rev. 2010, 110 (12), 7024−7039. (11) Abdel-Shafi, A. A.; Bourdelande, J. L.; Ali, S. S. Photosensitized generation of singlet oxygen from rhenium(I) and iridium(III) complexes. Dalton Trans. 2007, 84 (24), 2510. (12) Yi, X.; Zhao, J.; Wu, W.; Huang, D.; Ji, S.; Sun, J. Rhenium(I) Tricarbonyl Polypyridine Complexes Showing Strong Absorption of Visible Light and Long-Lived Triplet Excited States as a Triplet Photosensitizer for Triplet−triplet Annihilation Upconversion. Dalton Trans. 2012, 41 (29), 8931. (13) Ley, K. D.; Schanze, K. S. Photophysics of Metal-Organic πConjugated Polymers. Coord. Chem. Rev. 1998, 171, 287−307. (14) Zarkadoulas, A.; Koutsouri, E.; Kefalidi, C.; Mitsopoulou, C. A. Rhenium Complexes in Homogeneous Hydrogen Evolution. Coord. Chem. Rev. 2015, 304−305, 55−72. (15) Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. D.; Meyer, T. J. Photophysical Properties of Polypyridyl Carbonyl Complexes of Rhenium(I). J. Chem. Soc., Dalton Trans. 1991, 849. (16) Caspar, J. V.; Meyer, T. J. Application of the Energy Gap Law to Nonradiative, Excited-State Decay. J. Phys. Chem. 1983, 87 (6), 952− 957. (17) Velmurugan, G.; Ramamoorthi, B. K.; Venuvanalingam, P. Are Re(I) Phenanthroline Complexes Suitable Candidates for OLEDs? Answers from DFT and TD-DFT Investigations. Phys. Chem. Chem. Phys. 2014, 16 (39), 21157−21171. (18) Hasselmann, G. M.; Meyer, G. J. Diffusion-Limited Interfacial Electron Transfer with Large Apparent Driving Forces. J. Phys. Chem. B 1999, 103 (36), 7671−7675. (19) Thompson, D. W.; Kelly, C. a; Farzad, F.; Meyer, G. J. Sensitization of Nanocrystalline TiO 2 Initiated by Reductive 5398

DOI: 10.1021/acs.inorgchem.8b00360 Inorg. Chem. 2018, 57, 5389−5399

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Inorganic Chemistry Diimine)Cl and Related Compounds. Inorg. Chem. 2008, 47 (17), 7880−7887. (36) Sato, S.; Ishitani, O. Photochemical Reactions of fac-rhenium(I) Tricarbonyl Complexes and Their Application for Synthesis. Coord. Chem. Rev. 2015, 282−283, 50−59. (37) Yanagida, M.; Yamaguchi, T.; Kurashige, M.; Hara, K.; Katoh, R.; Sugihara, H.; Arakawa, H. Panchromatic Sensitization of Nanocrystalline TiO 2 with cis-Bis(4-car boxy-2-[2′-(4′carboxypyridyl)]quinoline)bis(thiocyanato-N)ruthenium(II). Inorg. Chem. 2003, 42 (24), 7921−7931. (38) Onozawa-Komatsuzaki, N.; Kitao, O.; Yanagida, M.; Himeda, Y.; Sugihara, H.; Kasuga, K. Molecular and Electronic Ground and Excited Structures of Heteroleptic Ruthenium Polypyridyl Dyes for Nanocrystalline TiO2 Solar Cells. New J. Chem. 2006, 30 (5), 689. (39) Garakyaraghi, S.; Crapps, P. D.; McCusker, C. E.; Castellano, F. N. Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics. Inorg. Chem. 2016, 55 (20), 10628− 10636. (40) Han, Y.; Dillon, R. J.; Flynn, C. J.; Rountree, E. S.; Alibabaei, L.; Cahoon, J. F.; Papanikolas, J. M.; Dempsey, J. L. Interfacial Electron Transfer Yields in Dye-Sensitized NiO Photocathodes Correlated to Excited-State Dipole Orientation of Ruthenium Chromophores. Can. J. Chem. 2017, 10, 1−10. (41) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. Estimation of Excited-State Redox Potentials by Electron-Transfer Quenching. Application of Electron-Transfer Theory to Excited-State Redox Processes. J. Am. Chem. Soc. 1979, 101 (17), 4815−4824. (42) Gillaizeau-Gauthier, I.; Odobel, F.; Alebbi, M.; Argazzi, R.; Costa, E.; Bignozzi, C. A.; Qu, P.; Meyer, G. J. Phosphonate-Based Bipyridine Dyes for Stable Photovoltaic Devices. Inorg. Chem. 2001, 40 (23), 6073−6079. (43) Elgrishi, N.; Rountree, K. J.; McCarthy, B. D.; Rountree, E. S.; Eisenhart, T. T.; Dempsey, J. L. A Practical Beginner’s Guide to Cyclic Voltammetry. J. Chem. Educ. 2018, 95, 197. (44) Lennox, J. C.; Dempsey, J. L. Influence of Proton Acceptors on the Proton-Coupled Electron Transfer Reaction Kinetics of a Ruthenium−Tyrosine Complex. J. Phys. Chem. B 2017, 121 (46), 10530−10542. (45) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42 (2), 339−341. (46) Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64 (1), 112−122. (47) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (48) Feller, D. The Role of Databases in Support of Computational Chemistry Calculations. J. Comput. Chem. 1996, 17 (13), 1571−1586. (49) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. Basis Set Exchange: A Community Database for Computational Sciences. J. Chem. Inf. Model. 2007, 47 (3), 1045−1052.

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DOI: 10.1021/acs.inorgchem.8b00360 Inorg. Chem. 2018, 57, 5389−5399