Preparation and photoluminescence characteristics of polysiloxane pendant tris(2,2′-bipyridine)ruthenium (II) complex

Polysiloxanes containing pendant tris(2,2′-bipyridine)ruthenium(II) complex (Ru(bpy)3²⁺) were prepared by reaction of polysiloxane-pendant 2,2′-bipyridine (PSiO-bpy) with cis-Ru(bpy)₂Cl₂. In methanol solution, the polymer pendant Ru(bpy)3²⁺ showed absorption maximum at 456nm and emission maximum at around 609nm, both of which are shifted to longer wavelength than the monomeric Ru(bpy)₃²⁺. The lifetime τ₀ of the excited polymer complex with low Ru(bpy)3²⁺ content was almost the same as that of the monomeric one in methanol (830ns), but τ₀ of the polymer with higher complex content was shorter because of a concentration quenching. In a solid state, τ₀ was much shorter (306–503ns) than that in a methanol solution contrary to the conventional polymeric system. Higher complex content in the polymer film caused higher glass transition temperature (Tg), but shorter τ₀. These results indicate concentration quenching in the polymer film. The excited polymer pendant Ru(bpy)3²⁺ was quenched by oxygen, and the relative emission intensity followed the Stern-Volmer equation. In a methanol solution the quenching rate constant (k_q) was the same order of magnitude as the monomeric complex, and independent of the complex content in the polymer. In a film, k_q was higher for the polymer with higher complex content.     

Preparation and luminescence properties of tris(bipyridine) ruthenium(II)-containing vinyl polymers: Ru(bpy)2 (poly-6-vinyl-2,2′-bipyridine)Cl2 and Ru(bpy)2(poly-4-methyl-4′-vinyl-,2,2′-bipyridine)Cl2

Ruthenium (II) complex-containing polymers were prepared and characterized by absorption and luminescence spectra, luminescence quantum yield, and luminescence lifetime. The polymers are Ru(bpy)₂(poly-6-vinyl-2,′2-bipyridine)CI₂ ( 1 ) and Ru(bpy)₂(poly-4-methyl-4′-methyl-4′ -vinyl-2,2′-bipyridine)CI₂ ( 2 ). The absorption spectra and luminescence spectra of polymers 1 and 2 were substantially the same as that of Ru(bpy)₃CI₂. The lifetime of polymers 1 and 2 was similar to that of the respective monomer model compounds. The lifetime of polymer 1 was very short (ca. 13 ns) in comparison to Ru(bpy)₃CI₂ (660 ns), whereas the lifetime of polymer 2 (660 ns) was similar to that of Ru(bpy)₃CI₂. The temperature-dependency of the lifetime was discussed in terms of Watts' model.     

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Photophysical properties in dilute MeCN solution are reported for seven Ru^II complexes containing two 2,2′‐bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridines, for one complex containing no 2,2′‐bipyridine, but 2 of these different ligands, for three multinuclear Ru^II complexes containing 2 or 4 [Ru(bpy)₂] moieties and also coordinated via 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridine ligands, and for the complex [(Ru(bpy)₂(L)]²⁺ where L is N,N′‐([2,2′‐bipyridine]‐4,4′‐diyl)bis[3‐methoxypropanamide]. Absorption maxima are red‐shifted with respect to [Ru(bpy)₃]²⁺, as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal‐to‐ligand charge transfer states ³MLCT in de‐aerated MeCN are equal to or longer than for [Ru(bpy)₃]²⁺ and vary considerably, i.e., from 0.86 to 1.71 μs. Rate constants k_q for quenching by O₂ of the ³MLCT states were measured and found to be well below diffusion‐controlled, ranging from 1.2 to 2.0⋅10⁹ dm³ mol⁻¹ s⁻¹. The efficiencies f of singlet‐oxygen formation during oxygen quenching of these ³MLCT states are relatively high, namely 0.53 – 0.89. The product of k_q and f gives the net rate constant k for quenching due to energy transfer to produce singlet oxygen, and k_q−k equals k, the net rate constant for quenching due to energy dissipation of the excited ³MLCT states without energy transfer. The quenching rate constants were both found to correlate with ΔG^CT, the free‐energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.     

具不同官能团钌(Ⅱ)邻菲啰啉配合物氧淬灭性能的比较研究

设计并制备具不同官能团的钌(Ⅱ)邻菲啰啉配合物——[Ru(bpy)2(phen-NO2)](PF6)2(1),[Ru(bpy)2(phen-Br)](PF6)2(2),[Ru(bpy)2(phen-NH2)](PF6)2(3),[Ru(bpy)2(bphen)](PF6)2(4),其中,bpy为2,2’-联吡啶,phen-NO2为5-硝基-邻菲啰啉,phen-Br为5-溴-1,10-菲啰啉,phen-NH2为5-氨基-邻菲啰啉,bphen为4,7-二苯基-邻菲啰啉.借助核磁共振(NMR)、红外光谱(FTIR)、元素分析、紫外可见光谱(UV-vis)和荧光光谱法对其进行了分析与表征.结果表明:配合物2～4在蓝-紫色可见光区域有较强吸收、可发射出明亮的橙红色荧光、具荧光量子效率(Φ)高、激发态寿命(τ)长、Stocks位移值较大(147～180 nm)和优良的氧致荧光淬灭性能(Ksv≥3.5,(I0/I)max≥4.0)等光物理性质.其中,具接枝型官能团氨基的配合物3的量子效率(Φ)=10%,τ=381.8 ns,Ksv=3.49,(I0/I)max=4.33;而溴修饰的配合物2以高达18%的荧光量子效率、634.7 ns的激发态寿命、180 nm的Stocks位移和Ksv=4.59,氧淬灭参数(I0/I)max=5.29,使之最有希望成为接枝型、较高性能的光学氧传感器的候选氧敏指示剂.     

Excited-State Dynamics of [Ru(bpy)3]2+ Thin Films on Sensitized TiO2 and ZrO2

The excited state dynamics of Tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate, [Ru(bpy)₃(PF₆)₂], was investigated on the surface of bare and sensitized TiO₂ and ZrO₂ films. The organic dyes LEG4 and MKA253 were selected as sensitizers. A Stern–Volmer plot of LEG4-sensitized TiO₂ substrates with a spin-coated [Ru(bpy)₃(PF₆)₂] layer on top shows considerable quenching of the emission of the latter. Interestingly, time-resolved emission spectroscopy reveals the presence of a fast-decay time component (25±5 ns), which is absent when the anatase TiO₂ semiconductor is replaced by ZrO₂. It should be specified that the positive redox potential of the ruthenium complex prevents electron transfer from the [Ru(bpy)₃(PF₆)₂] ground state into the oxidized sensitizer. Therefore, we speculate that the fast-decay time component observed stems from excited-state electron transfer from [Ru(bpy)₃(PF₆)₂] to the oxidized sensitizer. Solid-state dye sensitized solar cells (ssDSSCs) employing MKA253 and LEG4 dyes, with [Ru(bpy)₃(PF₆)₂] as a hole-transporting material (HTM), exhibit 1.2 % and 1.1 % power conversion efficiency, respectively. This result illustrates the possibility of the hypothesized excited-state electron transfer.     

Photophysical and Photochemical Properties of Ru(bpy)3 2+ in Si-Ti Mixed Oxides Xerogels Prepared by the Sol-Gel Process

The luminescence spectra and lifetime of tris(2,2-bipyridine)ruthenium(II), Ru(bpy)₃ ²⁺, were studied in sol-gel reaction systems of tetramethoxysilane (TMOS) and titanium(IV) isopropoxide (TTIP) with HCl. Luminescence lifetime in the TMOS system increased as the sol-gel reaction proceeded, because diffusion-controlled luminescence quenching such as oxygen and collisional quenching with solvent molecules were suppressed in the rigid matrices. On the other hand, luminescence lifetime in the TTIP system decreased during the sol-gel reaction. The decrease in lifetime was ascribed to electron transfer from photoexcited Ru(bpy)₃ ²⁺ to the conduction band of the TiO₂ xerogels. Extended X-ray absorption fine structure (EXAFS) measurements were done to associate lifetime in the Si-Ti xerogels with the structures of Ti⁴⁺ sites in the xerogels.     

Effects of Charge and Structure of Hyaluronic Acid on the Luminescence Quenching in Aqueous Solution

The quenching of electronically excited Ru(bpy)₃²⁺ (bpy = Tris-2,2′-bipyridine) by methylviologen (MV) and ferricyanide (FC) in aqueous solutions of hyaluronic acid (HA) was studied. The structural and viscosity changes occuring with increasing HA concentration were found to influence the photophysical and photochemical properties of the sensitizer. Different kinetic models had to be used for the quenchers studied. The kinetics of the quenching of *Ru(bpy)₃²⁺ by MV can be described by the pseudophase model, which indicates that the rate for the exchange of the quencher between the microdroplets is higher than that for the excited state decay of the Ruthenium complex. In contrast, the quenching by the negatively charged quencher, FC, can be described by the Infelta-Tachiya equation, which indicates that the distribution of this quencher on the aqueous microdroplets is of the Poisson type and there is no exchange of quencher molecules during the lifetime of the sensitizer. The lifetimes of the excited Ruthenium complex, the unimolecular constants for its quenching by FC and the average concentration of the aqueous microdroplets increase with increasing HA concentration, reflecting the change in the solution structure during the transition from semidilute to concentrated regions. For MV no significant dependence of the quenching constant on the HA content of the solution was found. The reaction behavior of charged reactants in HA solution depends strongly on the sign of the charge.     

Efficiency of polymer sensitizer in photosensitized reaction of tris(bipyridine)ruthenium(II) complex-containing polymer

Photochemical properties of Ru(bpy)₂(poly-4-methyl-4′-vinyl-2,2′-bipyridine)Cl₂ ( 2 ) were studied and compared with that of Ru(bpy)₃Cl₂. Continuous irradiation of a solution, which contains polymer 2 as a photosensitizer, methylviologen (MV²⁺) or 4,4′-bipyridinium-1,1′-bis(trimethylenesulfonate) (SPV) as an electron acceptor and triethanolamine (TEOA) as a sacrificial donor, resulted in the formation of viologen radical ion (MV⁺ or SPV^?). The rate of formation of MV⁺ or SPV^? for the polymer 2 system was smaller than that for the Ru(bpy)₃ Cl₂ systems. The reason for this fact was kinetically analyzed by quenching experiments of excited Ru(II) complexes by MV²⁺ or SPV, the photosensitized reactions of the TEOA–Ru(II) complex–MV²⁺ or -SPV systems, and the dye laser photolysis of the Ru(II) complex–MV²⁺ or -SPV systems.     

A New Heteroleptic Ruthenium(II) Polypyridyl Complex with Long‐Wavelength Absorption and High Singlet‐Oxygen Quantum Yield

Ruthenium(II) polypyridyl complexes with long‐wavelength absorption and high singlet‐oxygen quantum yield exhibit attractive potential in photodynamic therapy. A new heteroleptic Ru^II polypyridyl complex, [Ru(bpy)(dpb)(dppn)]²⁺ (bpy=2,2′‐bipyridine, dpb=2,3‐bis(2‐pyridyl)benzoquinoxaline, dppn=4,5,9,16‐tetraaza‐dibenzo[a,c]naphthacene), is reported, which exhibits a ¹MLCT (MLCT: metal‐to‐ligand charge transfer) maximum as long as 548 nm and a singlet‐oxygen quantum yield as high as 0.43. Steady/transient absorption/emission spectra indicate that the lowest‐energy MLCT state localizes on the dpb ligand, whereas the high singlet‐oxygen quantum yield results from the relatively long ³MLCT(Ru→dpb) lifetime, which in turn is the result of the equilibrium between nearly isoenergetic excited states of ³MLCT(Ru→dpb) and ³ππ*(dppn). The dppn ligand also ensures a high binding affinity of the complex towards DNA. Thus, the combination of dpb and dppn gives the complex promising photodynamic activity, fully demonstrating the modularity and versatility of heteroleptic Ru^II complexes. In contrast, [Ru(bpy)₂(dpb)]²⁺ shows a long‐wavelength ¹MLCT maximum (551 nm) but a very low singlet‐oxygen quantum yield (0.22), and [Ru(bpy)₂(dppn)]²⁺ shows a high singlet‐oxygen quantum yield (0.79) but a very short wavelength ¹MLCT maximum (442 nm).     

双核钌(II)多吡啶配合物与酵母RNA的相互作用研究

用紫外-可见吸收光谱和荧光光谱滴定、稳态荧光淬灭和反向盐滴定实验研究了双核钌(II)配合物[(bpy)₂Ru(ebipcH₂)Ru(bpy)₂](ClO₄)₄ {bpy＝2,2'-联吡啶; ebipcH₂＝N-乙基-4,7-二(咪唑-[4,5-f]-(1,10-邻菲啰啉)-2-基)咔唑}与酵母RNA的相互作用. 结果表明该双核配合物以插入方式与酵母RNA作用, 在生理盐浓度下(≈150 mmol/L NaCl)该配合物与RNA的相互作用明显强于DNA.     

Entrapment of ionic tris(2,2'-bipyridyl) ruthenium(II) in hydrophobic siliceous zeolite: O2 sensing in biological environments

Synthesis of the ionic dye, tris(2,2'-bipyridyl) ruthenium(II) chloride (Ru(bpy) 3 2+.2Cl (-)) within the supercages of a highly hydrophobic zeolite Y is reported. Use of the neutral precursor Ru(bpy)Cl 2(CO) 2 allowed for high loading levels of Ru(bpy) 3 2+ (1 per 7 and 25 supercages). The emission quenching of the Ru(bpy) 3 2+-zeolite crystals dispersed in polydimethoxysiloxane (PDMS) films by dissolved oxygen in water was examined. The quenching data as a function of oxygen concentration was fit to a linear Stern-Volmer plot ( R2 = 0.98). Using the Stern-Volmer plot as calibration, changes in concentration of dissolved oxygen due to reaction with glucose in the presence of glucose oxidase was monitored. Human monocyte-derived macrophages internalized the submicron-sized Ru(bpy) 3 2+-zeolite crystals, and intracellular oxygen concentrations initiated by zymosan-mediated oxidative burst could be monitored by measuring the emission from Ru(bpy) 3 2+ by confocal fluorescence microscopy.     

含长链不饱和酯基的三官能度溶胶-凝胶薄膜制备及发光氧气传感的研究

采用溶胶-凝胶法制备了含发光三吡啶钌[Ru(bpy)₃²⁺]分子的三官能度硅氧烷预聚液,并通过旋转涂敷及凝胶处理制备了高性能的薄膜,探讨了载体的微结构和发光分子的微环境效应。与传统的四官能度正硅酸乙酯(TEOS)/Ru(bpy)₃²⁺膜体系相比,含长链不饱和酯基的甲基丙烯酸丙酯基三甲氧基硅烷(PMA-TMS)/Ru(bpy)₃²⁺体系具有成膜性能好,气体的猝灭响应值高,响应时间短等特点。     

Control of Excitation and Quenching in Multi‐colour Electrogenerated Chemiluminescence Systems through Choice of Co‐reactant

We demonstrate a new approach to manipulate the selective emission in mixed electrogenerated chemiluminescence (ECL) systems, where subtle changes in co‐reactant properties are exploited to control the relative electron‐transfer processes of excitation and quenching. Two closely related tertiary‐amine co‐reactants, tri‐n‐propylamine and N,N‐diisopropylethylamine, generate remarkably different emission profiles: one provides distinct green and red ECL from [Ir(ppy)₃] (ppy=2‐phenylpyridinato‐C2,N) and a [Ru(bpy)₃]²⁺ (bpy=2,2′‐bipyridine) derivative at different applied potentials, whereas the other generates both emissions simultaneously across a wide potential range. These phenomena can be rationalized through the relative exergonicities of electron‐transfer quenching of the excited states, in conjunction with the change in concentration of the quenchers over the applied potential range.     

IrIII and RuII Complexes Containing Triazole‐Pyridine Ligands: Luminescence Enhancement upon Substitution with β‐Cyclodextrin

Novel 2‐(1‐substituted‐1H‐1,2,3‐triazol‐4‐yl)pyridine (pytl) ligands have been prepared by “click chemistry” and used in the preparation of heteroleptic complexes of Ru and Ir with bipyridine (bpy) and phenylpyridine (ppy) ligands, respectively, resulting in [Ru(bpy)₂(pytl‐R)]Cl₂ and [Ir(ppy)₂(pytl‐R)]Cl (R=methyl, adamantane (ada), β‐cyclodextrin (βCD)). The two diastereoisomers of the Ir complex with the appended β‐cyclodextrin, [Ir(ppy)₂(pytl‐βCD)]Cl, were separated. The [Ru(bpy)₂(pytl‐R)]Cl₂ (R=Me, ada or βCD) complexes have lower lifetimes and quantum yields than other polypyridine complexes. In contrast, the cyclometalated Ir complexes display rather long lifetimes and very high emission quantum yields. The emission quantum yield and lifetime (Φ=0.23, τ=1000 ns) of [Ir(ppy)₂(pytl‐ada)]Cl are surprisingly enhanced in [Ir(ppy)₂(pytl‐βCD)]Cl (Φ=0.54, τ=2800 ns). This behavior is unprecedented for a metal complex and is most likely due to its increased rigidity and protection from water molecules as well as from dioxygen quenching, because of the hydrophobic cavity of the βCD covalently attached to pytl. The emissive excited state is localized on these cyclometalating ligands, as underlined by the shift to the blue (450 nm) upon substitution with two electron‐withdrawing fluorine substituents on the phenyl unit. The significant differences between the quantum yields of the two separate diastereoisomers of [Ir(ppy)₂(pytl‐βCD)]Cl (0.49 vs. 0.70) are attributed to different interactions of the chiral cyclodextrin substituent with the Δ and Λ isomers of the metal complex.     

Surface confinement and its effects on the luminescence quenching of a ruthenium-containing metallopolymer

Luminescence quenching of the metallopolymers [Ru(bpy)(2)(PVP)(10)](2+) and [Ru(bpy)(2)(PVP)(10)Os(bpy)(2)](4+), both in solution and as thin films, is reported, where bpy is 2,2'-bipyridyl and PVP is poly(4-vinylpyridine). When the metallopolymer is dissolved in ethanol, quenching of the ruthenium excited state, Ru(2+*), within [Ru(bpy)(2)(PVP)(10)](2+) by [Os(bpy)(3)](2+) proceeds by a dynamic quenching mechanism and the rate constant is (1.1 +/- 0.1) x 10(11) M(-1) s(-1). This quenching rate is nearly two orders of magnitude larger than that found for quenching of monomeric [Ru(bpy)(3)](2+) under the same conditions. This observation is interpreted in terms of an energy transfer quenching mechanism in which the high local concentration of ruthenium luminophores leads to a single [Os(bpy)(3)](2+) centre quenching the emission of several ruthenium luminophores. Amplifications of this kind will lead to the development of more sensitive sensors based on emission quenching. Quenching by both [Os(bpy)(3)](2+) and molecular oxygen is significantly reduced within a thin film of the metallopolymer. Significantly, in both optically driven emission and electrogenerated chemiluminescence, emission is observed from both ruthenium and osmium centres within [Ru(bpy)(2)(PVP)(10)Os(bpy)(2)](4+) films, i.e. the ruthenium emission is not quenched by the coordinated [Os(bpy)(2)](2+) units. This observation opens up new possibilities in multi-analyte sensing since each luminophore can be used to detect separate analytes, e.g. guanine and oxoguanine.     

Experimental and density functional theory insights into the effect of withdrawing ligands on the fluorescence yield of Ru(II)‐based complexes

The quality of emission spectra of metal complexes gives good insights into their performance in many optoelectronic applications. Herein, the effect of the number and position of various ligand structures on the emission spectra of Ru bipyridine complexes was studied. Specifically, the use of a different number of withdrawing groups (COOH) was investigated in detail. The complexes were first investigated using density functional theory (DFT) and time‐dependent DFT calculations and then confirmed experimentally. The bandgap energy, reactivity, emission spectra and Stokes shift were found to depend on the number and position of the withdrawing groups attached to the Ru(bpy)₂²⁺ complexes. Upon increasing the number of withdrawing groups, the electrons were found to be withdrawn from the carbon orbitals and resonated to reach the metal, and accumulated around it, thus enhancing the metal‐to‐ligand charge transfer mechanism instead of the ligand‐to‐ligand charge transfer mechanism. The complexes with more withdrawing groups showed spectra with more intense emission peaks with shorter lifetime, indicating the enhancement in the photoactivity of the complexes. Ligands with ring nitrogens with two COOH groups showed the greatest effect on the enhancement of the emission spectra with a lifetime of 0.5359 ns. The resulting collective emission spectra covered a wide wavelength range, making the investigated complexes a good choice for many optoelectronic applications.     

Electrochemiluminescence Distance and Reactivity of Coreactants Determine the Sensitivity of Bead-Based Immunoassays

Herein we report the study of electrochemiluminescence (ECL) generation by tris(2,2′-bipyridyl)ruthenium (Ru(bpy)₃²⁺) and five tertiary amine coreactants. The ECL distance and lifetime of coreactant radical cations were measured by ECL self-interference spectroscopy. And the reactivity of coreactants was quantitatively evaluated in terms of integrated ECL intensity. By statistical analysis of ECL images of single Ru(bpy)₃²⁺-labeled microbeads, we propose that ECL distance and reactivity of coreactant codetermine the emission intensity and thus the sensitivity of immunoassay. 2,2-bis(hydroxymethyl)-2,2′,2′′-nitrilotriethanol (BIS-TRIS) can well balance ECL distance-reactivity trade-off and enhance the sensitivity by 236 % compared with tri-n-propylamine (TPrA) in the bead-based immunoassay of carcinoembryonic antigen. The study brings an insightful understanding of ECL generation in bead-based immunoassay and a way of maximizing the analytical sensitivity from the aspect of coreactant.     

TEMPO‐Mediated Oxidation of Primary Alcohols to Aldehydes under Visible Light and Air

A homogeneous visible light photoredox TEMPO‐mediated selective oxidation of primary alcohols to the corresponding carbonyl compounds was developed using molecular oxygen from air as the terminal oxidant. Ru(bpy)₃(PF₆)₂ (bpy: bipyridyl) and Ir(dtb‐bpy)(ppy)₂(PF₆) (dtb‐bpy: 4,4′‐di‐tert‐butyl‐2,2′‐bipyridyl; ppy: 2‐phenylpyridine) were used as the sensitizers.     

A New Luminescent Ruthenium(II) Polypyridine‐derived Dipicolylamine Complex as a Sensor for Cu2+ Ions

A chemo‐sensor [Ru(bpy)₂(bpy‐DPF)](PF₆)₂ ( 1 ) (bpy=2,2′‐bipyridine, bpy‐DPF=2,2′‐bipyridyl‐4,4′‐bis(N,N‐di(2‐picolyl))formylamide) for Cu²⁺ using di(2‐picolyl)amine (DPA) as the recognition group and a ruthenium(II) complex as the reporting group was synthesized and characterized successfully. It demonstrates a high selectivity and efficient signaling behavior only for Cu²⁺ with obvious red‐shifted MLCT (metal‐to‐ligand charge transfer transitions) absorptions and dramatic fluorescence quenching compared with Zn²⁺ and other metal ions.     

Structural analysis of photo-oxidized (ethylenediamine)bis(2,2′-bipyridine)ruthenium(II) complexes by using on-line electrospray mass spectrometry of labeled compounds

Photo-oxidation of Ru(bpy)₂(en)²⁺, where bpy = 2,2′-bipyridine, en = ethylenediamine, was studied in isotopic labeling experiments by using on-line electrospray mass spectrometry (ESMS). The complex was known to undergo photochemical dehydrogenation of a fourelectron oxidation, giving the α,α′-diimine complexes in a stepwise manner via a two-electron-oxidized intermediate that represents loss of two hydrogen atoms from the en ligand. On-line mass analysis after photoirradiation (λ > 420 nm) of Ru(bpy)₂(ed)²⁺ (ed = ethylene-d₄diamine) showed that the ligand of the intermediate with loss of two hydrogen atoms was not an enamine but had an imine structure. Also, a ligand-oxygenated complex that has mass 14 amu higher than the Ru(bpy)₂(en)²⁺ complex was observed in the ES mass spectra. The ligand of this complex was proposed to have a nitroso structure as a primary product in ¹⁸O₂ experiments. The oxygenated complex was not generated in a stepwise manner via the imine intermediate, but directly by loss of two amino hydrogen atoms and addition of an oxygen atom. The source of the oxygen atom would be from oxygen dissolved in solution rather than from water in solution. Another oxygenated complex Ru(bpy)₂(NO ₂ ^#x2212; )⁺ was produced by irradiation and the structure was identified in ¹⁸O₂ experiments.