Role of Electronic Structure on DNA Light-Switch Behavior of Ru(II) Intercalators

Ru(II) Complexes with Absorption in the Photodynamic Therapy Window: 1O2 Sensitization, DNA Binding, and Plasmid DNA Photocleavage

Two Ru(II) complexes, [Ru(pydppn)(bim)(py)]2+ [2; pydppn = 3-(pyrid-2'-yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene; bim = 2,2'-bisimidazole; py = pyridine] and [Ru(pydppn)(Me4bim)(py)]2+ [3; Me4bim = 2,2'-bis(4,5-dimethylimidazole)], were synthesized and characterized, and their photophysical properties, DNA binding, and photocleavage were evaluated and compared to [Ru(pydppn)(bpy)(py)]2+ (1; bpy = 2,2'-bipyridine). Complexes 2 and 3 exhibit broad 1MLCT (metal-to-ligand charge transfer) transitions with maxima at ∼470 nm and shoulders at ∼525 and ∼600 nm that extend to ∼800 nm. These bands are red-shifted relative to those of 1, attributed to the π-donating ability of the bim and Me4bim ligands. A strong signal at 550 nm is observed in the transient absorption spectra of 1-3, previously assigned as arising from a pydppn-centered 3ππ* state, with lifetimes of ∼19 μs for 1 and 2 and ∼270 ns for 3. A number of methods were used to characterize the mode of binding of 1-3 to DNA, including absorption titrations, thermal denaturation, relative viscosity changes, and circular dichroism, all of which point to the intercalation of the pydpppn ligand between the nucleobases. The photocleavage of plasmid pUC19 DNA was observed upon the irradiation of 1-3 with visible and red light, attributed to the sensitized generation of 1O2 by the complexes. These findings indicate that the bim ligand, together with pydppn, serves to shift the absorption of Ru(II) complexes to the photodynamic therapy window, 600-900 nm, and also extend the excited state lifetimes for the efficient production of cytotoxic singlet oxygen.

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Good Vibrations Report on the DNA Quadruplex Binding of an Excited State Amplified Ruthenium Polypyridyl IR Probe

The nitrile containing Ru(II)polypyridyl complex [Ru(phen)2(11,12-dCN-dppz)]2+ (1) is shown to act as a sensitive infrared probe of G-quadruplex (G4) structures. UV-visible absorption spectroscopy reveals enantiomer sensitive binding for the hybrid htel(K) and antiparallel htel(Na) G4s formed by the human telomer sequence d[AG3(TTAG3)3]. Time-resolved infrared (TRIR) of 1 upon 400 nm excitation indicates dominant interactions with the guanine bases in the case of Λ-1/htel(K), Δ-1/htel(K), and Λ-1/htel(Na) binding, whereas Δ-1/htel(Na) binding is associated with interactions with thymine and adenine bases in the loop. The intense nitrile transient at 2232 cm-1 undergoes a linear shift to lower frequency as the solution hydrogen bonding environment decreases in DMSO/water mixtures. This shift is used as a sensitive reporter of the nitrile environment within the binding pocket. The lifetime of 1 in D2O (ca. 100 ps) is found to increase upon DNA binding, and monitoring of the nitrile and ligand transients as well as the diagnostic DNA bleach bands shows that this increase is related to greater protection from the solvent environment. Molecular dynamics simulations together with binding energy calculations identify the most favorable binding site for each system, which are in excellent agreement with the observed TRIR solution study. This study shows the power of combining the environmental sensitivity of an infrared (IR) probe in its excited state with the TRIR DNA "site effect" to gain important information about the binding site of photoactive agents and points to the potential of such amplified IR probes as sensitive reporters of biological environments.

New Tridentate Ligand Affords a Long-Lived 3MLCT Excited State in a Ru(II) Complex: DNA Photocleavage and 1O2 Production

Two new complexes, [Ru(tpy)(qdppz)](PF6)2 (1; qdppz = 2-(quinolin-8-yl)dipyrido[3,2-a:2',3'-c]phenazine, tpy = 2,2':6',2″-terpyridine) and [Ru(qdppz)2](PF6)2 (2), were investigated for their potential use as phototherapeutic agents through their ability to photosensitize the production of singlet oxygen, 1O2, upon irradiation with visible light. The complexes exhibit strong Ru(dπ) → qdppz(π*) metal-to-ligand charge transfer (MLCT) absorption with maxima at 485 and 495 nm for 1 and 2 in acetone, respectively, red-shifted from the Ru(dπ) → tpy(π*) absorption at 470 nm observed for [Ru(tpy)2]2+ (3) in the same solvent. Complexes 1 and 3 are not luminescent at room temperature, but 3MLCT emission is observed for 2 with maximum at 690 nm (λexc = 480 nm) in acetone. The lifetimes of the 3MLCT states of 1 and 2 were measured using transient absorption spectroscopy to be ∼9 and 310 ns in methanol, respectively, at room temperature (λexc = 490 nm). The bite angle of the qdppz ligand is closer to octahedral geometry than that of tpy, resulting in the longer lifetime of 2 as compared to those of 1 and 3. Arrhenius treatment of the temperature dependence of the luminescence results in similar activation energies, Ea, from the 3MLCT to the 3LF (ligand-field) state for the two complexes, 2520 cm-1 in 1 and 2400 cm-1 in 2. However, the pre-exponential factors differ by approximately two orders of magnitude, 2.3 × 1013 s-1 for 1 and 1.4 × 1011 s-1 for 2, which, together with differences in the Huang-Rhys factors, lead to markedly different 3MLCT lifetimes. Although both 1 and 2 intercalate between the DNA bases, only 2 is able to photocleave DNA owing to its 1O2 production upon irradiation with ΦΔ = 0.69. The present work highlights the profound effect of the ligand bite angle on the electronic structure, providing guidelines for extending the lifetime of 3MLCT Ru(II) complexes with tridentate ligands, a desired property for a number of applications.

Photophysical Properties and DNA Binding of Two Intercalating Osmium Polypyridyl Complexes Showing Light-Switch Effects

The synthesis and photophysical characterization of two osmium(II) polypyridyl complexes, [Os(TAP)₂dppz]²⁺ (1) and [Os(TAP)₂dppp2]²⁺ (2) containing dppz (dipyrido[3,2-a:2′,3′-c]phenazine) and dppp2 (pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline) intercalating ligands and TAP (1,4,5,8-tetraazaphenanthrene) ancillary ligands, are reported. The complexes exhibit complex electrochemistry with five distinct reductive redox couples, the first of which is assigned to a TAP-based process. The complexes emit in the near-IR (1 at 761 nm and 2 at 740 nm) with lifetimes of >35 ns with a low quantum yield of luminescence in aqueous solution (∼0.25%). The Δ and Λ enantiomers of 1 and 2 are found to bind to natural DNA and with AT and GC oligodeoxynucleotides with high affinities. In the presence of natural DNA, the visible absorption spectra are found to display significant hypochromic shifts, which is strongly evident for the ligand-centered π–π* dppp2 transition at 355 nm, which undergoes 46% hypochromism. The emission of both complexes increases upon DNA binding, which is observed to be sensitive to the Δ or Λ enantiomer and the DNA composition. A striking result is the sensitivity of Λ-2 to the presence of AT DNA, where a 6-fold enhancement of luminescence is observed and reflects the nature of the binding for the enantiomer and the protection from solution. Thermal denaturation studies show that both complexes are found to stabilize natural DNA. Finally, cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

Osmium(II) polypyridyl complexes comprising extended dipyrido[3,2-a:2′,3′-c]phenazine (1) and pyrido[2′,3′:5,6]pyrazino[2,3-f][1,10]phenanthroline (2) intercalating ligands are shown to be effective DNA binders accompanied by enhanced near-IR emission. The emission response to B-DNA is found to be sensitive to the enantiomer and the composition of DNA, with greater emission observed for AT-rich sequences. Thermal denaturation studies show that both complexes stabilize natural DNA. Cellular studies show that the complexes are internalized by cultured mammalian cells and localize in the nucleus.

Revealing the Photophysical Dynamics of Selected Rigid Donor-Acceptor Systems: From Ligands to Ruthenium(II) Complexes

Newly designed push-pull ligands (L1 and L2) with bithiophene (bth) as a donor and phenazine (phz) or quinoxalino[2,3-b]quinoxaline (qxq) as acceptors were synthesized and also incorporated with a bipyridyl Ru(II) complex to give Ru1 and Ru2, respectively. The ultrafast photophysical dynamics of the ligand and their respective Ru(II) complexes were well-characterized using time-resolved spectroscopies and quantum chemical calculations. Photoinduced charger transfer (CT) and intersystem crossing (ISC) processes were directly observed for L1 and L2. In addition, the interplay of three different triplet excited states was directly observed in the related Ru(II) complexes. The lowest-lying triplet excited states of the ligands and their respective Ru(II) complexes were both attributed to the CT transitions from donor (bth) to acceptor (phz or qxq) and result in ³ICT (intramolecular charge transfer) and ³ILCT (intraligand charge transfer) excited states, respectively. The lifetimes of the lowest-lying triplet excited states of L1, L2, Ru1, and Ru2 were measured to be 21.3, 50.4, 2.75, and 4.16 μs, respectively.

Influence of the Protonation State on the Excited-State Dynamics of Ruthenium(II) Complexes with Imidazole π-Extended Dipyridophenazine Ligands

Ruthenium(II) complexes, like [(tbbpy)₂Ru(dppz)]²⁺ (Ru-dppz; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, dppz = dipyrido-[3,2-a:2',3'-c]phenazine), have emerged as suitable photosensitizers in photoredox catalysis. Since then, there has been ongoing interest in the design of π-extended Ru-dppz systems with red-shifted visible absorption maxima and sufficiently long-lived excited states independent of the solvent or pH value. Herein, we explore the photophysical properties of protonation isomers of the linearly π-extended [(tbbpy)₂Ru(L)]²⁺-type complexes bearing a dppz ligand with directly fused imidazole (im) and methyl-imidazole units (mim) as L. Steady-state UV-vis absorption, resonance Raman, as well as time-resolved emission and transient absorption spectroscopy reveal that Ru-im and Ru-mim show desirable properties for the application in photocatalytic processes, i.e., strong visible absorbance and two long-lived excited states in the ³ILCT and ³MLCT manifold, at pH values between 3 and 12. However, protonation of the (methyl-)imidazole unit at pH ≤ 2 unit causes decreased excited-state lifetimes and an emission switch-off.

Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophysically characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent density functional theory calculations. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (¹MLCT) and thiazole (tz)-induced intraligand charge-transfer (¹ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered ³MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived ³ILCT state (τ ≈ 40-300 ns) with excess charge density localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τ_Em,1 ≈ 40-300 ns and τ_Em,2 ≈ 100-200 ns, which are attributed to emission from the ³ILCT and ³MLCT manifolds, respectively.

Photocatalytic hydrogen evolution using a Ru(ii)-bound heteroaromatic ligand as a reactive site

A Ru^II complex, [Ru^II(tpphz)(bpy)₂]²⁺ (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [Ru^II(tpphz-HH)(bpy)₂]²⁺ (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (³MLCT*) of 1 is firstly formed upon photoirradiation and the ³MLCT* state is reductively quenched with an electron donor to afford [Ru^II(tpphz˙^-)(bpy)₂]⁺, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H₂ evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its ³MLCT* state to react with a proton and aldehydes. In photocatalytic H₂ evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H₂ evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H₂ evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

Anticancer and antitrypanosomal activities of trinuclear ruthenium compounds with orthometalated phenazine ligands

Trinuclear ruthenium complexes with orthometalated phenazines of general formula [Ru3(μ3-O)(μ2-OAc)5(L)(py)2]PF6 (L = dppn, benzo[i]dipyrido[3,2-a:2',3'-c]phenazine, 1; dppz, dipyrido[3,2-a:2',3'-c]phenazine, 2; CH3-dppz, 7-methyldipyrido[3,2-a:2',3'-c]phenazine, 3; Cl-dppz, 7-chlorodipyrido[3,2-a:2',3'-c]phenazine, 4) were investigated for their cytotoxic activity toward the B16F10 murine melanoma and the L929 non-cancer cell lines and against Trypanosoma cruzi (2-4). This study also reports a multi-technique investigation into how complexes 1-4 interact with DNA and human serum albumin, HSA. At concentrations ranging from 2 to 50 μM, all the complexes reduced B16F10 murine melanoma cell viability by over 50%. Complex 4 had the highest cytotoxic effect in the series, diminishing B16F10 cell viability to 38% at 2 μM, with an overall order for anticancer activity of 4 > 2 > 3 > 1. Complexes 2-4 showed remarkable activity in inhibiting epimastigote and amastigote forms of T. cruzi. Complex 2 showed better antitrypanosomal activity than the reference drug (IC50 = 1.19 μM and IC50 = 0.25 μM for epimastigote and amastigotes forms, respectivily). Ethidium bromide (EB) displacement assays showed that DNA intercalation progressively increases with the extension of the π-conjugation of the cyclometalating ligand and the presence of substituents in the phenazinic portion (1 > 4-3 > 2), showing that complex 1 is a stronger intercalator than EB itself (Kapp > 107 M-1). Viscosity measurements followed the same trend. Cytotoxicity against cancer cells and antitrypanosomal activity follow the same order, which is different to the tendency of DNA intercalation, suggesting DNA is not the main target of these complexes. Compound 1-4 showed very high affinity with HSA (Kb ∼109 M-1). Circular dichroism results also showed that the complexes alter significantly the secondary structure of the HSA, lowering the α-helix % from 86.2 (pure protein) to less than 5% for compounds 1, 2 and 4 at 2.8 μM. These findings demonstrated the important role of phenazines for the biological activity of triruthenium compounds.

Increased Lipophilicity of Halogenated Ruthenium(II) Polypyridyl Complexes Leads to Decreased Phototoxicity in vitro when Used as Photosensitizers for Photodynamic Therapy

In the fight against cancer, photodynamic therapy is generating great interest thanks to its ability to selectively kill cancer cells without harming healthy tissues. In this field, ruthenium(II) polypyridyl complexes, and more specifically, complexes with dipyrido[3,2-a:2',3'-c]phenazine (dppz) as a ligand are of particular interest due to their DNA-binding and photocleaving properties. However, ruthenium(II) polypyridyl complexes can sometimes suffer from low lipophilicity, which hampers cellular internalisation through passive diffusion. In this study, four new [Ru(dppz-X₂ )₃ ]²⁺ complexes (X=H, F, Cl, Br, I) were synthesized and their lipophilicity (logP), cytotoxicity and phototoxicity on cancerous and noncancerous cell lines were assessed. This study shows that, counterintuitively, the phototoxicity of these complexes decreases as their lipophilicity increases; this could be due solely to the atomic radius of the halogen substituents.

Dual-Action Ru(II) Complexes with Bulky π-Expansive Ligands: Phototoxicity without DNA Intercalation

We report the synthesis and photochemical and biological characterization of Ru(II) complexes containing π-expansive ligands derived from dimethylbenzo[i]dipyrido[3,2-a:2',3'-c]phenazine (Me₂dppn) adorned with flanking aryl substituents. Late-stage Suzuki couplings produced Me₂dppn ligands substituted at the 10 and 15 positions with phenyl (5), 2,4-dimethylphenyl (6), and 2,4-dimethoxyphenyl (7) groups. Complexes of the general formula [Ru(tpy)(L)(py)](PF₆)₂ (8-10), where L = 4-7, were characterized and shown to have dual photochemotherapeutic (PCT) and photodynamic therapy (PDT) behavior. Quantum yields for photodissociation of monodentate pyridines from 8-10 were about 3 times higher than that of parent complex [Ru(tpy)(Me₂dppn)(py)](PF₆)₂ (1), whereas quantum yields for singlet oxygen (¹O₂) production were ∼10% lower than that of 1. Transient absorption spectroscopy indicates that 8-10 possess long excited state lifetimes (τ = 46-50 μs), consistent with efficient ¹O₂ production through population and subsequent decay of ligand-centered ³ππ* excited states. Complexes 8-10 displayed greater lipophilicity relative to 1 and association to DNA but do not intercalate between the duplex base pairs. Complexes 1 and 8-10 showed photoactivated toxicity in breast and prostate cancer cell lines with phototherapeutic indexes, PIs, as high as >56, where the majority of cell death was achieved 4 h after treatment with Ru(II) complexes and light. Flow cytometric data and rescue experiments were consistent with necrotic cell death mediated by the production of reactive oxygen species, especially ¹O₂. Collectively, this study confirms that DNA intercalation by Ru(II) complexes with π-expansive ligands is not required to achieve photoactivated cell death.

Binding of Fe(ii)-complex of phenanthroline appended glycoconjugate with DNA, plasmid and an agglutinin protein

A phenanthroline appended glycoconjugate and its Fe(ii) complex have been synthesized and characterized thoroughly. The Fe-complex interacts with DNA and WGA protein and alter their structures as studied by spectroscopy and microscopy.

An anthracene-pendant ruthenium(ii) complex conjugated to a biotin anchor, an essential handle for photo-induced anti-cancer activity

Efficient avidin binding and selective cancer cell response upon light irradiation of an enhanced ROS photogenerator biotinylated ruthenium complex.

Bimetallic Ru(ii) and Os(ii) complexes based on a pyrene-bisimidazole spacer: synthesis, photophysics, electrochemistry and multisignalling DNA binding studies in the near infrared region

We report in this paper the synthesis, characterization, photophysical and electrochemical properties, and detailed DNA binding affinities of two homobimetallic Ru(ii) and Os(ii) complexes derived from a new bridging ligand consisting of two pyridyl-imidazole coordinating units rigidly coupled with a central pyrene moiety. The structure of the diruthenium complex was confirmed by X-ray crystallography. Both complexes exhibit luminescence at room temperature from their ³MLCT states, with lifetimes of τ₁ = 12.6 ns and τ₂ = 48.8 ns for the Ru(ii) complex (1) and τ₁ = 23.7 ns for the Os(ii) complex (2). For 2, the luminescence maximum stretches to the NIR region, which is suitable for potential biological applications. Both complexes exhibit two successive one-electron reversible metal-centered oxidations in the positive potential window. Computational studies employing DFT and TD-DFT methods were also performed to assign the experimentally observed optical spectral bands in the complexes. The binding affinities of the complexes towards DNA were thoroughly investigated through a variety of techniques, viz. absorption, luminescence, excited state lifetime, circular dichroism, thermal denaturation, viscosity measurement, and relative DNA binding studies using ethidium bromide. Finally, molecular docking studies were also carried out to visualize the modes of interaction between the complexes and DNA.

Cu(i) vs. Ru(ii) photosensitizers: elucidation of electron transfer processes within a series of structurally related complexes containing an extended π-system

The charge transfer behavior of heteroleptic Cu(i) photosensitizers was investigated by spectroelectrochemistry and compared to their structurally related Ru(ii) complexes.

Sonochemical effect on two new Ruthenium(II) complexes with ligand (E)-N-((6-bromopyridin-2-yl)methylene)-4-(methylthio)aniline precursors for synthesis of RuO2 nanoparticles

Two novel Ru(II) complexes [(η⁶-p-cymene)RuCl(L2)]PF₆ (R2) and [(η⁶-C₆H₆)RuCl(L2)]PF₆ (R4), with ligand (E)-N-((6-bromopyridin-2-yl)methylene)-4-(methylthio)aniline (L2), were synthesized and characterized by elemental analysis, ¹H NMR, ¹³C NMR and IR spectroscopy. Based on X-ray crystallography studies, complexes R2 and R4 have coordination environments with formulated (η⁶-p-cymene)Ru(N₂Cl) and (η⁶-C₆H₆)Ru(N₂Cl), respectively. The thermal stabilities of compounds R2 and R4 were studied by thermal gravimetric (TG) and differential scanning calorimetry (DSC). Thermal decomposition of these complexes was at 280°C and 260°C under air atmosphere respectively. The interaction of these complexes with calf thymus DNA (CT-DNA) was explored through electronic absorption spectra, fluorescence and redox behavior studies. The results showed that the complexes bind to CT-DNA with electrostatic interactions. Nanoparticles of RuO₂ were prepared by calcination of R2 and R4. Also the role of the ultrasound waves on the characteristics of the RuO₂ nanoparticles was studied. The nanoparticles were characterized by IR spectroscopy and X-ray diffraction (XRD). Also size and morphology of nanoparticles were studied by scanning electron microscopy (SEM).

Long-Lived Polypyridyl Based Mononuclear Ruthenium Complexes: Synthesis, Structure, and Azo Dye Decomposition

Two mononuclear ruthenium complexes [(bpy)₂Ru^IIL₁/L₂](ClO₄)₂ ([1]²⁺/[2]²⁺) (bpy-2,2' bipyridine, L₁ = 2,3-di(pyridin-2-yl)pyrazino[2,3-f][1,10]phenanthroline) and L₂ = 2,3-di(thiophen-2-yl)pyrazino[2,3-f][1,10]phenanthroline have been synthesized. The complexes have been characterized using various analytical techniques. The complex [1]²⁺ has further been characterized by its single crystal X-ray structure suggesting ruthenium is coordinating through the N donors of phenanthroline end. Theoretical investigation suggests that the HOMOs of both complexes are composed of pyridine and pyrazine unit of ligands L₁ and L₂ whereas the LUMOs are formed by the contribution of bipyridine units. The low energy bands at ∼480 nm of the complexes can be assigned as MLCT with partial contribution from ligand transitions, whereas the rest are ligand centered. The complexes have shown Ru^II/Ru^III oxidation couples at E_1/2 at 1.26 (70 mV) V and 1.28 (62 mV) V for [1]²⁺ and [2]²⁺ vs Ag/AgCl, respectively, suggesting no significant role of distal thiophene or pyridine units of the ligands. The complexes are emissive and display solvent dependent emission properties. Both complexes have shown highest emission quantum yield and lifetime in DMSO (ϕ = 0.05 and τ_avg = 460 ns and λ_max^em at 620 nm for [1]²⁺; ϕ = 0.043 and τ_avg = 425 ns and λ_max^em at 635 nm for [2]²⁺). Further, the long luminescent lifetime of these complexes has been utilized to generate reactive oxygen species for efficient azo dye decomposition.

Precision targeted ruthenium(ii) luminophores; highly effective probes for cell imaging by stimulated emission depletion (STED) microscopy

Fluorescence microscopy has undergone a dramatic evolution over the past two decades with development of super-resolution far-field microscopy methods that break the light diffraction limited resolution of conventional microscopy, offering unprecedented opportunity to interrogate cellular processes at the nanoscale. However, these methods make special demands of the luminescent agents used for contrast and development of probes suited to super-resolution fluorescent methods is still relatively in its infancy. In spite of their many photophysical advantages, metal complex luminophores have not yet been considered as probes in this regard, where to date, only organic fluorophores have been applied. Here, we report the first examples of metal complex luminophores applied as probes for use in stimulated emission depletion (STED) microscopy. Exemplified with endoplasmic reticulum and nuclear targeting complexes we demonstrate that luminescent Ru(ii) polypyridyl complexes can, through signal peptide targeting, be precisely and selectively delivered to key cell organelles without the need for membrane permeabilization, to give high quality STED images of these organelles. Detailed features of the tubular ER structure are revealed and in the case of the nuclear targeting probe we exploit the molecular light switch properties of a dipyrido[3,2-a:2',3'-c]phenazine containing complex which emits only on DNA/RNA binding to give outstanding STED contrast and resolution of the chromosomes within the nucleus. Comparing performance with a member of the AlexaFluor family commonly recommended for STED, we find that the performance of the ruthenium complexes is superior across both CW and gated STED microscopy methods in terms of image resolution and photostability. The large Stokes shifts of the Ru probes permit excellent matching of the stimulating depletion laser with their emission whilst avoiding anti-Stokes excitation. Their long lifetimes make them particularly amenable to gated STED, giving a much wider window for gating than traditional probes. Our findings indicate that ruthenium polypyridyl peptide targeted probes are a powerful new partner to STED microscopy, opening up new approaches to probe design for STED microscopy.

Effect of Bridge Alteration on Ground- and Excited-State Properties of Ruthenium(II) Complexes with Electron-Donor-Substituted Dipyrido[3,2-a:2',3'-c]phenazine Ligands

A series of Ru(II) 2,2'-bipyridine (bpy) complexes with an electron-accepting dipyrido[3,2-a:2',3'-c]phenazine (dppz) ligand coupled to an electron-donating triarylamine (TAA) group have been investigated. Systematic alteration of a bridging unit between the dppz and TAA allowed exploration into how communication between the donor and acceptor is perturbed by distance, as well as by steric and electronic effects. The effect of the bridging group on the electronic properties of the systems was characterized using a variety of spectroscopic methods, including Fourier transform-Raman (FT-Raman) spectroscopy, resonance Raman spectroscopy, and transient resonance Raman (TR²) spectroscopy. These methods were used in conjunction with ground- and excited-state absorption spectroscopy, electrochemical studies, and DFT calculations. The ground-state electronic absorption spectra show distinct variation with the bridging group, with the wavelength observed for the lowest energy electronic transition ranging from 449 nm to 522 nm, accompanied by large changes in the molar absorptivity. The lowest-energy Franck-Condon state was determined to be intra-ligand charge transfer (ILCT) in nature for most compounds. The presence of higher-energy metal-to-ligand charge transfer (MLCT) Ru(II) → bpy and Ru(II) → dppz transitions was also confirmed via resonance Raman spectroscopy. The TR² spectra showed characteristic dppz^• - and TAA^• + vibrations, indicating that the THEXI state formed was also ILCT in nature. Excited-state lifetime measurements reveal that the rate of decay is in accordance with the energy gap law and is not otherwise affected by the nature of the bridging unit.

Homo- and Heterobimetallic Ruthenium(II) and Osmium(II) Complexes Based on a Pyrene-Biimidazolate Spacer as Efficient DNA-Binding Probes in the Near-Infrared Domain

We report in this work a new family of homo- and heterobimetallic complexes of the type [(bpy)2M(Py-Biimz)M'(II)(bpy)2](2+) (M = M' = Ru(II) or Os(II); M = Ru(II) and M' = Os(II)) derived from a pyrenyl-biimidazole-based bridge, 2-imidazolylpyreno[4,5-d]imidazole (Py-BiimzH2). The homobimetallic Ru(II) and Os(II) complexes were found to crystallize in monoclinic form with space group P21/n. All the complexes exhibit strong absorptions throughout the entire UV-vis region and also exhibit luminescence at room temperature. For osmium-containing complexes (2 and 3) both the absorption and emission band stretched up to the NIR region and thus afford more biofriendly conditions for probable applications in infrared imaging and phototherapeutic studies. Detailed luminescence studies indicate that the emission originates from the respective (3)MLCT excited state mainly centered in the [M(bpy)2](2+) moiety of the complexes and is only slightly affected by the pyrene moiety. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations in the positive potential window and several reduction processes in the negative potential window. An efficient intramolecular electronic energy transfer is found to occur from the Ru center to the Os-based component in the heterometallic dyad. The binding studies of the complexes with DNA were thoroughly studied through different spectroscopic techniques such as UV-vis absorption, steady-state and time-resolved emission, circular dichroism, and relative DNA binding study using ethidium bromide. The intercalative mode of binding was suggested to be operative in all cases. Finally, computational studies employing DFT and TD-DFT were also carried out to interpret the experimentally observed absorption and emission bands of the complexes.

Photochemical Properties and Reactivity of a Ru Compound Containing an NAD/NADH-Functionalized 1,10-Phenanthroline Ligand

An NAD/NADH-functionalized ligand, benzo[b]pyrido[3,2-f][1,7]-phenanthroline (bpp), was newly synthesized. A Ru compound containing the bpp ligand, [Ru(bpp)(bpy)2](2+), underwent 2e(-) and 2H(+) reduction, generating the NADH form of the compound, [Ru(bppHH)(bpy)2](2+), in response to visible light irradiation in CH3CN/TEA/H2O (8/1/1). The UV-vis and fluorescent spectra of both [Ru(bpp)(bpy)2](2+) and [Ru(bppHH)(bpy)2](2+) resembled the spectra of [Ru(bpy)3](2+). Both complexes exhibited strong emission, with quantum yields of 0.086 and 0.031, respectively; values that are much higher than those obtained from the NAD/NADH-functionalized complexes [Ru(pbn)(bpy)2](2+) and [Ru(pbnHH)(bpy)2](2+) (pbn = (2-(2-pyridyl)benzo[b]-1.5-naphthyridine, pbnHH = hydrogenated form of pbn). The reduction potential of the bpp ligand in [Ru(bpp)(bpy)2](2+) (-1.28 V vs SCE) is much more negative than that of the pbn ligand in [Ru(pbn)(bpy)2](2+) (-0.74 V), although the oxidation potentials of bppHH and pbnHH are essentially equal (0.95 V). These results indicate that the electrochemical oxidation of the dihydropyridine moiety in the NADH-type ligand was independent of the π system, including the Ru polypyridyl framework. [Ru(bppHH)(bpy)2](2+) allowed the photoreduction of oxygen, generating H2O2 in 92% yield based on [Ru(bppHH)(bpy)2](2+). H2O2 production took place via singlet oxygen generated by the energy transfer from excited [Ru(bppHH)(bpy)2](2+) to triplet oxygen.

[Ru(phen)2dppz]2+ luminescence reveals nanoscale variation of polarity in the cyclodextrin cavity

Insertion of dppz with phosphorylated β-cyclodextrin results in multi-exponential [Ru(phen)₂dppz]²⁺ emission; binding is weaker than [Ru(phen)₃]²⁺, but shows stereoselectivity.

Label-free electrochemiluminescent detection of transcription factors with hybridization chain reaction amplification

Label-free and efficient ECL strategy for detection of NF-κB based on the HCR signal amplification.

Optimized synthesis of a tert-butyl-phenyl-substituted tetrapyridophenazine ligand and its Ru(ii) complexes and determination of dimerization behaviour of the complexes through supramolecular “Fingerhakel”

A high dimerization constant of a ruthenium complex is observed with the aid of ¹H-NMR spectroscopy. The solid state molecular structure indicates that multiple π-interactions are the reason for strong dimerization.

New cyclometallated Ru(II) complex for potential application in photochemotherapy?

In an effort to create a molecule that absorbs further into the optimum window for photochemotherapy (PCT), the new cyclometallated complex [Ru(biq)2(phpy)](PF6) (1, biq = 2,2'-biquinoline, phpy(-) = deprotonated 2-phenylpyridine) was synthesized, characterized and compared to the known photoactive complexes [Ru(biq)2(bpy)](PF6)2 (2, bpy = 2,2'-bipyridine) and [Ru(biq)2(phen)](PF6)2 (3, phen = 1,10-phenanthroline), both of which undergo exchange of one biq ligand when irradiated with red light in coordinating solvents. Excited state ligand dissociation in 2 and 3 is believed to be related to the steric hindrance afforded by the presence of two coordinated biq ligands. The ligand exchange quantum yield of 2 is ~2-fold greater than that of 3, which was shown to be cytotoxic when irradiated with visible light. Cyclometallation results in a red shift of the MLCT absorption maximum of ` by ~100 nm relative to those of 2 and 3, but, although 1 exhibits a distorted octahedral geometry, photoinduced ligand exchange does not occur. DFT calculations were used to aid in our understanding of the lack of photochemistry of 1 which is explained by the destabilization of the eg(σ*) orbitals upon cyclometallation.

Ruthenium polypyridine complexes combined with oligonucleotides for bioanalysis: a review

Ruthenium complexes are among the most interesting coordination complexes and they have attracted great attention over the past decades due to their appealing biological, catalytic, electronic and optical properties. Ruthenium complexes have found a unique niche in bioanalysis, as demonstrated by the substantial progress made in the field. In this review, the applications of ruthenium complexes coordinated with polypyridine ligands (and analogues) in bioanalysis are discussed. Three main detection methods based on electrochemistry, electrochemiluminescence, and photoluminscence are covered. The important targets, including DNA and other biologically important targets, are detected by specific biorecognition with the corresponding oligonucleotides as the biorecognition elements (i.e., DNA is probed by its complementary strand and other targets are detected by functional nucleic acids, respectively). Selected examples are provided and thoroughly discussed to highlight the substantial progress made so far. Finally, a brief summary with perspectives is included.

Benzimidazole derivatives: synthesis, physical properties, and n-type semiconducting properties

A series of new benzimidazole derivatives were synthesized by the solid-state condensation and direct sublimation (SSC-DS) method and their physical properties were investigated. The reaction yields and product stability were significantly affected by the identity of the diamine and anhydride substituents. On the other hand, the substituents of the benzimidazole ring allowed fine tuning of the emission maxima, fluorescence quantum yields, and redox potentials. The HOMO-LUMO levels were estimated by cyclic voltammetry in film on indium tin oxide (ITO) and compared with values obtained by other methods. The described benzimidazoles showed high crystallinity, which is attributed to a high planarity and interactions between carbon and heteroatoms. These compounds showed n-type semiconducting behavior in organic field-effect transistors (OFETs). Optimized devices for fluorinated NTCBI (naphthalene tetracarboxylic bisbenzimidazole) showed respectable electron mobilities of ∼10(-2)  cm(2)  V(-1)  s(-1) .

A colorimetric and luminescent dual-modal assay for Cu(II) ion detection using an iridium(III) complex

A novel iridium(III) complex-based chemosensor bearing the 5,6-bis(salicylideneimino)-1,10-phenanthroline ligand receptor was developed, which exhibited a highly sensitive and selective color change from colorless to yellow and a visible turn-off luminescence response upon the addition of Cu(II) ions. The interactions of this iridium(III) complex with Cu2+ ions and thirteen other cations have been investigated by UV-Vis absorption titration, emission titration, and 1H NMR titration.

RutheniumII complexes bearing fused polycyclic ligands: from fundamental aspects to potential applications

In this review, we first discuss the photophysics reported in the literature for mononuclear ruthenium complexes bearing ligands with extended aromaticity such as dipyrido[3,2-a:2',3'-c]phenazine (DPPZ), tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]-phenazine (TPPHZ),  tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]acridine (TPAC), 1,10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene (PHEHAT) 9,11,20,22-tetraaza- tetrapyrido[3,2-a:2',3'-c:3'',2''-l:2''',3'''-n]pentacene (TATPP), etc. Photophysical properties of binuclear and polynuclear complexes based on these extended ligands are then reported. We finally develop the use of binuclear complexes with extended π-systems for applications such as photocatalysis.

Key role of ancillary ligands in imparting blue shift in electroluminescence wavelength in ruthenium polypyridyl light-emitting diodes

Ancillary ligand substitution proves to be an effective way to produce the blue shift of electroluminescence peak wavelength.