Selective Photoinduced Ligand Exchange in a New Tris–Heteroleptic Ru(II) Complex

The complex <i>cis</i>-[Ru­(biq)­(phen)­(CH₃CN)₂]²⁺ (<b>1</b>, biq = 2,2′-biquinoline, phen = 1,10-phenathroline) displays selective photosubstitution of only one CH₃CN ligand with a solvent molecule upon irradiation with low energy light (λ_irr ≥ 550 nm), whereas both ligands exchange with λ_irr ≥ 420 nm. In contrast, [Ru­(phen)₂(CH₃CN)₂]²⁺ (<b>2</b>) and [Ru­(biq)₂(CH₃CN)₂]²⁺ (<b>3</b>) exchange both CH₃CN ligands with similar rates upon irradiation with a broad range of wavelengths. The photolysis of <b>1</b> in the presence of pyridine (py) results in the formation of the intermediate <i>cis</i>-[Ru­(biq)­(phen)­(py)­(MeCN)]²⁺, which was isolated and characterized by X-ray crystallography, revealing that the CH₃CN positioned <i>trans</i> to the phen ligand is more photolabile than that positioned <i>trans</i> to the biq ligand when irradiated with low energy light. These results are explained using the calculated stabilities of the two possible products, together with the molecular orbitals involved in the lowest energy excited state

New Ligand Design Provides Delocalization and Promotes Strong Absorption throughout the Visible Region in a Ru(II) Complex

The new Ru­(II)–anthraquinone complex [Ru­(bpy)₂(qdpq)]­(PF₆)₂ (<b>Ru-qdpq</b>; bpy = 2,2′-bipyridine; qdpq = 2,3-di­(2-pyridyl)­naphtho­[2,3-<i>f</i>]­quinoxaline-7,12-quinone) possesses a strong ¹MLCT Ru → qdpq absorption with a maximum at 546 nm that tails into the near-IR and is significantly red-shifted relative to that of the related complex [Ru­(bpy)₂(qdppz)]­(PF₆)₂ (<b>Ru-qdppz</b>; qdppz = naphtho­[2,3-<i>a</i>]­dipyrido­[3,2-h:2′,3′-f]­phenazine-5,18-dione), with λ_max = 450 nm. <b>Ru-qdppz</b> possesses electronically isolated proximal and distal qdppz-based excited states; the former is initially generated and decays to the latter, which repopulates the ground state with τ = 362 ps. In contrast, excitation of <b>Ru-qdpq</b> results in the population of a relatively long-lived (τ = 19 ns) Ru­(dπ) → qdpq­(π*) ³MLCT excited state where the promoted electron is delocalized throughout the qdpq ligand. Ultrafast spectroscopy, used together with steady-state absorption, electrochemistry, and DFT calculations, indicates that the unique coordination modes of the qdpq and qdppz ligands impart substantially different electronic communication throughout the quinone-containing ligand, affecting the excited state and electron transfer properties of these molecules. These observations create a pathway to synthesize complexes with red-shifted absorptions that possess long-lived, redox-active excited states that are useful for various applications, including solar energy conversion and photochemotherapy

Selective Photoinduced Ligand Exchange in a New Tris–Heteroleptic Ru(II) Complex

The complex <i>cis</i>-[Ru­(biq)­(phen)­(CH₃CN)₂]²⁺ (<b>1</b>, biq = 2,2′-biquinoline, phen = 1,10-phenathroline) displays selective photosubstitution of only one CH₃CN ligand with a solvent molecule upon irradiation with low energy light (λ_irr ≥ 550 nm), whereas both ligands exchange with λ_irr ≥ 420 nm. In contrast, [Ru­(phen)₂(CH₃CN)₂]²⁺ (<b>2</b>) and [Ru­(biq)₂(CH₃CN)₂]²⁺ (<b>3</b>) exchange both CH₃CN ligands with similar rates upon irradiation with a broad range of wavelengths. The photolysis of <b>1</b> in the presence of pyridine (py) results in the formation of the intermediate <i>cis</i>-[Ru­(biq)­(phen)­(py)­(MeCN)]²⁺, which was isolated and characterized by X-ray crystallography, revealing that the CH₃CN positioned <i>trans</i> to the phen ligand is more photolabile than that positioned <i>trans</i> to the biq ligand when irradiated with low energy light. These results are explained using the calculated stabilities of the two possible products, together with the molecular orbitals involved in the lowest energy excited state

Excited State Dynamics of Two New Ru(II) Cyclometallated Dyes: Relation to Cells for Solar Energy Conversion and Comparison to Conventional Systems

The preparation, characterization, and photophysical properties of a series of ruthenium­(II) complexes possessing the cyclometallating deprotonated 2-phenyl pyridine ligand, phpy^–, together with dppn (benzo­[<i>i</i>]­dipyrido­[3,2-<i>a</i>:2′,3′-<i>c</i>]­phenazine), a ligand with an extended π-system, are reported. Related complexes have been used as efficient dyes in dye-sensitized solar cells (DSSCs), and the Ru→dppn metal-to-ligand charge transfer (MLCT) absorption of the new complexes [Ru­(phpy)­(bpy)­(dppn)]⁺ (<b>4</b>) and [Ru­(phpy)­(dppn)₂]⁺ (<b>5</b>) is red-shifted relative to the Ru→bpy MLCT peak in [Ru­(phpy)­(bpy)₂]⁺ (<b>3</b>). These new compounds are compared to conventional complexes where phpy^– is replaced by 2,2′-bipyridine (bpy), including [Ru­(bpy)₃]²⁺, [Ru­(bpy)₂(dppn)]²⁺ (<b>1</b>), and [Ru­(bpy)­(dppn)₂]²⁺ (<b>2</b>). Unlike <b>1</b> and <b>2</b>, with long-lived dppn-centered ³ππ* excited states (τ ∼ 20 μs), the corresponding cyclometallated complexes <b>4</b> and <b>5</b> exhibit weakly emissive Ru→dppn ³MLCT states with transient absorption lifetimes of 25 and 45 ps, respectively, which are significantly shorter than that of <b>3</b>, ∼9 ns. Although it is desirable to shift the absorption of ruthenium dyes used in DSSCs to lower energies, it is evident from this work, that for cyclometallated phpy^– complexes, lowering the energy of the ³MLCT state below that of <b>3</b> results in significant shortening of the excited state lifetime. The fast excited state decay, together with the lower energy of the ¹MLCT state, may result in lower charge injection efficiencies from these types of complexes

Unusually Efficient Pyridine Photodissociation from Ru(II) Complexes with Sterically Bulky Bidentate Ancillary Ligands

The introduction of steric bulk to the bidentate ligand in [Ru­(tpy)­(bpy)­(py)]²⁺ (<b>1</b>; tpy = 2,2′:2′,6″-terpyridine; bpy = 2,2′-bipyridine; py = pyridine) to provide [Ru­(tpy)­(Me₂bpy)­(py)]²⁺ (<b>2</b>; Me₂bpy = 6,6′-dimethyl-2,2′-bipyridine) and [Ru­(tpy)­(biq)­(py)]²⁺ (<b>3</b>; biq = 2,2′-biquinoline) facilitates photoinduced dissociation of pyridine with visible light. Upon irradiation of <b>2</b> and <b>3</b> in CH₃CN (λ_irr = 500 nm), ligand exchange occurs to produce the corresponding [Ru­(tpy)­(NN)­(NCCH₃)]²⁺ (NN = Me₂bpy, biq) complex with quantum yields, Φ₅₀₀, of 0.16(1) and 0.033(1) for <b>2</b> and <b>3</b>, respectively. These values represent an increase in efficiency of the reaction by 2–3 orders of magnitude as compared to that of <b>1</b>, Φ₅₀₀ < 0.0001, under similar experimental conditions. The photolysis of <b>2</b> and <b>3</b> in H₂O with low energy light to produce [Ru­(tpy)­(NN)­(OH₂)]²⁺ (NN = Me₂bpy, biq) also proceeds rapidly (λ_irr > 590 nm). Complexes <b>1</b>–<b>3</b> are stable in the dark in both CH₃CN and H₂O under similar experimental conditions. X-ray crystal structures and theoretical calculations highlight significant distortion of the planes of the bidentate ligands in <b>2</b> and <b>3</b> relative to that of <b>1</b>. The crystallographic dihedral angles defined by the bidentate ligand, Me₂bpy in <b>2</b> and biq in <b>3</b>, and the tpy ligand were determined to be 67.87° and 61.89°, respectively, whereas only a small distortion from the octahedral geometry is observed between bpy and tpy in <b>1</b>, 83.34°. The steric bulk afforded by Me₂bpy and biq also result in major distortions of the pyridine ligand in <b>2</b> and <b>3</b>, respectively, relative to <b>1</b>, which are believed to weaken its σ-bonding and π-back-bonding to the metal and play a crucial role in the efficiency of the photoinduced ligand exchange. The ability of <b>2</b> and <b>3</b> to undergo ligand exchange with λ_irr > 590 nm makes them potential candidates to build photochemotherapeutic agents for the delivery of drugs with pyridine binding groups

Photoinduced Ligand Exchange and Covalent DNA Binding by Two New Dirhodium Bis-Amidato Complexes

Two new dirhodium complexes, the head-to-tail (<i>H,T</i>) and head-to-head (<i>H,H</i>) isomers of <i>cis</i>-[Rh₂(HNOCCH₃)₂(CH₃CN)₆]²⁺, were synthesized, separated, and characterized following the reaction of Rh₂(HNOCCH₃)₄ with trimethyloxonium tetrafluoroborate in CH₃CN. The products were characterized by ¹H NMR spectroscopy, mass spectrometry, elemental analysis, and single crystal X-ray diffraction. Each bis-amidato isomer has a total of six CH₃CN ligands, two along the internuclear Rh–Rh axis, CH₃CN_<i>ax</i>, two in equatorial positions <i>trans</i> to the oxygen atoms of the bridging amidato groups, CH₃CN_<i>eq</i>^<i>O</i>, and two in equatorial positions <i>trans</i> to the amidato nitrogen atoms, CH₃CN_<i>eq</i>^<i>N</i>. When aqueous solutions of the complexes are irradiated with low energy light (λ_irr ≥ 495 nm, 60 min), both types of CH₃CN_<i>eq</i> ligands undergo efficient ligand exchange with solvent H₂O molecules to form monoaqua, followed by bis-aqua, adducts, releasing two CH₃CN_<i>eq</i> ligands in the process. The quantum yields, Φ_400nm, for the <i>H,T</i> and <i>H,H</i> isomers to form monoaqua adducts are 0.43 and 0.38, respectively, which are substantially greater than the 0.13 yield observed for <i>cis</i>-[Rh₂(O₂CCH₃)₂(CH₃CN)₆]²⁺; importantly, no ligand exchange is observed when the complexes are kept in the dark. Finally, low energy excitation (λ_irr ≥ 610 nm, 30 min) of the <i>H,T</i> isomer was shown to generate photoproducts that covalently bind to linearized DNA, making <b>2</b> a potential agent for photochemotherapy that does not require the formation of ¹O₂, as is typical of organic photodynamic therapy (PDT) agents

Electronic and Steric Effects on the Photoisomerization of Dimethylsulfoxide Complexes of Ru(II) Containing Picolinate

Calculations were performed on [Ru(tpy)(bpy)(dmso)]²⁺ (tpy = 2,2′:6′,2′′-terpyridine; bpy = 2,2′-bipyridine, dmso = dimethylsulfoxide, <b>1</b>), <i>cis</i>-[Ru(tpy)(Me-pic)(dmso)]⁺ (Me-pic = 6-methylpicolinate, <b>2</b>), <i>trans</i>-[Ru(tpy)(Me-pic)(dmso)]⁺ (<b>3</b>), and <i>trans</i>-[Ru(tpy)(pic)(dmso)]⁺ (pic = picolinate, <b>4</b>) to gain an understanding of the differences in their photoisomerization behavior. The results do not support a promoting role for the σ* ligand field (LF) states during excited-state S→O isomerization. Instead, the calculations show that the Ru−S bonding, the identity of the highest occupied molecular orbital, and steric interactions are important factors in dmso photoisomerization. Furthermore, the atom positioned trans to the S atom plays a critical role in promoting enhanced photoisomerizataion yields

DFT Investigation of Ligand Photodissociation in [Ru^II(tpy)(bpy)(py)]²⁺ and [Ru^II(tpy)(Me₂bpy)(py)]²⁺ Complexes

Photoinduced ligand dissociation of pyridine occurs much more readily in [Ru­(tpy)­(Me₂bpy)­(py)]²⁺ than in [Ru­(tpy)­(bpy)­(py)]²⁺ (tpy = 2,2′:6′,2″-terpyridine; bpy = 2,2′-bipyridine, Me₂bpy = 6,6′-dimethyl-2,2′-bipyridine; py = pyridine). The S₀ ground state and the ³MLCT and ³MC excited states of these complexes have been studied using BP86 density functional theory with the SDD basis set and effective core potential on Ru and the 6-31G­(d) basis set for the rest of the atoms. In both complexes, excitation by visible light and intersystem crossing leads to a ³MLCT state in which an electron from a Ru d orbital has been promoted to a π* orbital of terpyridine, followed by pyridine release after internal conversion to a dissociative ³MC state. Interaction between the methyl groups and the other ligands causes significantly more strain in [Ru­(tpy)­(Me₂bpy)­(py)]²⁺ than in [Ru­(tpy)­(bpy)­(py)]²⁺, in both the S₀ and ³MLCT states. Transition to the dissociative ³MC states releases this strain, resulting in lower barriers for ligand dissociation from [Ru­(tpy)­(Me₂bpy)­(py)]²⁺ than from [Ru­(tpy)­(bpy)­(py)]²⁺. Analysis of the molecular orbitals along relaxed scans for stretching the Ru–N bonds reveals that ligand photodissociation is promoted by orbital mixing between the ligand π* orbital of tpy in the ³MLCT state and the dσ* orbitals that characterize the dissociative ³MC states. Good overlap and strong mixing occur when the Ru–N bond of the leaving ligand is perpendicular to the π* orbital of terpyridine, favoring the release of pyridine positioned in a <i>cis</i> fashion to the terpyridine ligand

Confocal Fluorescence Microscopy Studies of a Fluorophore-Labeled Dirhodium Compound: Visualizing Metal–Metal Bonded Molecules in Lung Cancer (A549) Cells

The new dirhodium compound [Rh₂­(μ‑O₂CCH₃)₂­(η¹‑O₂CCH₃)­(phen­bodipy)­(H₂O)₃]­[O₂CCH₃] (<b>1</b>), which incorporates a bodipy fluorescent tag, was prepared and studied by confocal fluorescence microscopy in human lung adenocarcinoma (A549) cells. It was determined that <b>1</b> localizes mainly in lysosomes and mitochondria with no apparent nuclear localization in the 1–100 μM range. These results support the conclusion that cellular organelles rather than the nucleus can be targeted by modification of the ligands bound to the Rh₂⁴⁺ core. This is the first study of a fluorophore-labeled metal–metal bonded compound, work that opens up new venues for the study of intracellular distribution of dinuclear transition metal anticancer complexes

Solid-Phase Synthesis as a Platform for the Discovery of New Ruthenium Complexes for Efficient Release of Photocaged Ligands with Visible Light

Ruthenium-based photocaging groups have important applications as biological tools and show great potential as therapeutics. A method was developed to rapidly synthesize, screen, and identify ruthenium-based caging groups that release nitriles upon irradiation with visible light. A diverse library of tetra- and pentadentate ligands was synthesized on polystyrene resin. Ruthenium complexes of the general formula [Ru­(L)­(MeCN)_<i>n</i>]^<i>m</i>+ (<i>n</i> = 1–3, <i>m</i> = 1–2) were generated from these ligands on solid phase and then cleaved from resin for photochemical analysis. Data indicate a wide range of spectral tuning and reactivity with visible light. Three complexes that showed strong absorbance in the visible range were synthesized by solution phase for comparison. Photochemical behavior of solution- and solid-phase complexes was in good agreement, confirming that the library approach is useful in identifying candidates with desired photoreactivity in short order, avoiding time-consuming chromatography and compound purification