The use of visible light to produce highly selective and potent drugs through photodynamic therapy (PDT) holds much potential in the treatment of cancer. PDT agents can be designed to follow an O2-dependent mechanism by producing highly reactive species such as 1O2 and/or an O2 independent mechanism through processes such as excited state electron transfer, covalent binding to DNA or photoinduced drug delivery. Ru(II)-polypyridyl and Rh2(II,II) complexes represent an important class of compounds that can be tailored to exhibit desired photophysical properties and photochemical reactivity by judicious selection of the ligand set. Complexes with relatively long-lived excited states and planar, intercalating ligands localize on the DNA strand and photocleave DNA through 1O2 production or guanine oxidation by the excited state of the chromophore. Photoinduced ligand substitution occurs through the population of triplet metal centered (3MC) excited states and facilitates covalent binding of the metal complex to DNA in a mode similar to cisplatin. Ligand photodissociation also provides a route to selective drug delivery. The ability to construct metal complexes with desired light absorbing and excited state properties by ligand variation enables the design of PDT agents that can potentially provide combination therapy from a single metal complex.

CONSPECTUS Uncovering the factors that govern the electronic structure of Ru(II)–polypyridyl complexes is critical in designing new compounds for desired photochemical reactions, and strategies to tune excited states for ligand dissociation and 1O2 production are discussed herein. The generally accepted mechanism for photoinduced ligand dissociation proposes that population of the dissociative triplet ligand field (3LF) state proceeds through thermal population from the vibrationally cooled triplet metal-to-ligand charge transfer (3MLCT) state; however, temperature-dependent emission spectroscopy provides varied activation energies using the emission and ligand exchange quantum yields for [Ru(bpy)2(L)2]2+ (bpy = 2,2′-bipyridine; L = CH3CN or py). This suggests that population of the 3LF state proceeds from the vibrationally excited 3MLCT state. Because the quantum yield of ligand dissociation for nitriles is much more efficient than that for py, steric bulk was introduced into the ligand set to distort the pseudo-octahedral geometry and lower the energy of the 3LF state. The py dissociation quantum yield with 500 nm irradiation in a series of [Ru(tpy)(NN)(py)]2+ complexes (tpy = 2,2′:6′,2″-terpyridine; NN = bpy, 6,6′-dimethyl-2,2′-bipyridine (Me2bpy), 2,2′-biquinoline (biq)) increases by 2–3 orders of magnitude with the sterically bulky Me2bpy and biq ligands relative to bpy. Ultrafast transient absorption spectroscopy reveals population of the 3LF state within 3–7 ps when NN is bulky, and density functional theory calculations support stabilized 3LF states. Dual activity via ligand dissociation and 1O2 production can be achieved by careful selection of the ligand set to tune the excited-state dynamics. Incorporation of an extended π system in Ru(II) complexes such as [Ru(bpy)(dppn)(CH3CN)2]2+ (dppn = benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine) and [Ru(tpy)(Me2dppn)(py)]2+ (Me2dppn = 3,6-dimethylbenzo[i]dipyrido[3,2-a:2′,3′-c]phenazine) introduces low-lying, long-lived dppn/Me2dppn 3ππ* excited states that generate 1O2. Similar to [Ru(bpy)2(CH3CN)2]2+, photodissociation of CH3CN occurs upon irradiation of [Ru(bpy)(dppn)(CH3CN)2]2+, although with lower efficiency because of the presence of the 3ππ* state. The steric bulk in [Ru(tpy)(Me2dppn)(py)]2+ is critical in facilitating the photoinduced py dissociation, as the analogous complex [Ru(tpy)(dppn)(py)]2+ produces 1O2 with near-unit efficiency. The ability to tune the relative energies of the excited states provides a means to design potentially more active drugs for photochemotherapy because the photorelease of drugs can be coupled to the therapeutic action of reactive oxygen species, effecting cell death via two different mechanisms. The lessons learned about tuning of the excited-state properties can be applied to the use of Ru(II)–polypyridyl compounds in a variety of applications, such as solar energy conversion, sensors and switches, and molecular machines.

The introduction of steric bulk to the bidentate ligand in [Ru(tpy)(bpy)(py)]2+ (1; tpy = 2,2′:2′,6″-terpyridine; bpy = 2,2′-bipyridine; py = pyridine) to provide [Ru(tpy)(Me2bpy)(py)]2+ (2; Me2bpy = 6,6′-dimethyl-2,2′-bipyridine) and [Ru(tpy)(biq)(py)]2+ (3; biq = 2,2′-biquinoline) facilitates photoinduced dissociation of pyridine with visible light. Upon irradiation of 2 and 3 in CH3CN (λirr = 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 2 and 3, respectively. These values represent an increase in efficiency of the reaction by 2–3 orders of magnitude as compared to that of 1, Φ500 < 0.0001, under similar experimental conditions. The photolysis of 2 and 3 in H2O with low energy light to produce [Ru(tpy)(NN)(OH2)]2+ (NN = Me2bpy, biq) also proceeds rapidly (λirr > 590 nm). Complexes 1–3 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 2 and 3 relative to that of 1. The crystallographic dihedral angles defined by the bidentate ligand, Me2bpy in 2 and biq in 3, 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 1, 83.34°. The steric bulk afforded by Me2bpy and biq also result in major distortions of the pyridine ligand in 2 and 3, respectively, relative to 1, 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 2 and 3 to undergo ligand exchange with λirr > 590 nm makes them potential candidates to build photochemotherapeutic agents for the delivery of drugs with pyridine binding groups.

The new complex [Ru(tpy)(Me2dppn)(py)]2+ efficiently photodissociates py in CH3CN with Φ500 = 0.053(1) induced by steric bulk from methyl substituents and produces 1O2 with ΦΔ = 0.69(9) from its long-lived 3ππ* excited state. The unique excited state processes that result in dual reactivity were investigated using ultrafast transient absorption spectroscopy.

Light-activated inhibition of cathepsin activity was demonstrated with in a cell-based assay. Inhibitors of cathepsin K, Cbz-Leu-NHCH2CN (2) and Cbz-Leu-Ser(OBn)-CN (3), were caged within the complexes cis-[Ru(bpy)2(2)2]Cl2 (4) and cis-[Ru(bpy)2(3)2](BF4)2 (5), where bpy = 2,2′-bipyridine, as 1:1 mixtures of Δ- and Λ stereoisomers. Complexes 4 and 5 were characterized by 1H NMR, IR and UV-vis spectroscopies and electrospray mass spectrometry. Photochemical experiments confirm that 4 releases two molecules of 2 upon exposure to visible light for 15 min, whereas release of 3 by 5 requires longer irradiation times. IC50 determinations against purified cathepsin K under light and dark conditions with 4 and 5 confirm that inhibition is enhanced from 35 to 88-fold, respectively, upon irradiation with visible light. No apparent toxicity was observed for 4 in the absence or presence of irradiation in bone marrow macrophage (BMM) or PC-3 cells, as judged by the MTT assay, at concentrations up to 10 μM. Compound 5 is well tolerated at lower concentrations (<1 μM) but does show growth inhibitory effects at higher concentrations. Confocal microscopy experiments show that 4 reduces intracellular cathepsin activity in osteoclasts with light activation. These results support further development of caged nitrile-based inhibitors as chemical tools for investigating spatial aspects of proteolysis within living systems.

Ruthenium(II) tris(2-pyridylmethyl)amine (TPA) is an effective caging group for nitriles that provides high levels of control over the enzyme activity with light. Two caged nitriles were prepared, [Ru(TPA)(MeCN)2](PF6)2 (1) and [Ru(TPA)(3)2](PF6)2 (2), where 3 is the cathepsin K inhibitor Cbz-Leu-NHCH2CN, and characterized by various spectroscopic techniques and mass spectrometry. Both 1 and 2 show the release of a single nitrile within 20 min of irradiation with 365 nm light. Complex 2 acts as a potent, photoactivated inhibitor of human cathepsin K. IC50 values were determined for 2 and 3. Enzyme inhibition for 2 was enhanced by a factor of 89 upon exposure to light, with IC50 values of 63 nM (light) and 5.6 μM (dark).

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)n]m+ (n = 1–3, m = 1–2) were generated from these ligands on solid phase, 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.

Coupling a reactive metal to light absorbers affords molecular devices for photoinitiated electron collection and photocatalytic conversion of substrates to fuels. A new Ru(II),Pt(II) tetrametallic supramolecule, [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](PF(6))(6), and the trimetallic precursors, [{(phen)(2)Ru(dpp)}(2)RuCl(2)](PF(6))(4) and [{(phen)(2)Ru(dpp)}(2)Ru(dpq)](PF(6))(6), have been synthesized, and their redox, spectroscopic, spectroelectrochemical, photophysical and photocatalytic properties studied. They efficiently absorb UV and visible light. The electrochemistry of [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](PF(6))(6) suggests a lowest-lying terminal Ru→dpq charge-separated state that quenches the emission of the parent complex with non-unity population of the emissive (3)MLCT excited state. Photolysis of [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](6+) at 470 nm with DMA gives multielectron reduction, storing electrons in a new manner on the central (dpp)(2)Ru(II)(dpq) moiety. Addition of H(2)O to the photolysis system produces 21 μmol of H(2) in 5 h, with 115 turnovers of the tetrametallic photocatalyst.