Simultaneous control of spectroscopic and electrochemical properties in functionalised electrochemiluminescent tris(2,2'-bipyridine)ruthenium(II) complexes

Using a combination of electrochemical, spectroscopic and computational techniques, we have explored the fundamental properties of a series of ruthenium diimine complexes designed for coupling with other molecules or surfaces for electrochemiluminescence (ECL) sensing applications. With appropriate choice of ligand functionality, it is possible to manipulate emission wavelengths while keeping the redox ability of the complex relatively constant. DFT calculations show that in the case of electron withdrawing substituents such as ester or amide, the excited state is located on the substituted bipyridine ligand whereas in the case of alkyl functionality it is localised on a bipyridine. The factors that dictate annihilation ECL efficiency are interrelated. For example, the same factors that determine ΔG for the annihilation reaction (i.e. the relative energies of the HOMO and LUMO) have a corresponding effect on the energy of the excited state product. As a result, most of the complexes populate the excited state with an efficiency (Φ(ex)) of close to 80% despite the relatively wide range of emission maxima. The quantum yield of emission (Φ(p)) and the possibility of competing side reactions are found to be the main determinants of ECL intensity.     

Syntheses, characterization, and photo-hydrogen-evolving properties of tris(2,2'-bipyridine)ruthenium(II) derivatives tethered to a cis-Pt(II)Cl2 unit: insights into the structure-activity relationship

The photo-hydrogen-evolving activity (activity to enhance the photochemical EDTA-reduction of water into molecular hydrogen) was evaluated for three different Ru(II)Pt(II) dimers with a general formula of [(bpy)2Ru(micro-bridge)PtCl2]2+(bpy = 2,2'-bipyridine; bridge = 4,4'-bis(N-(3-aminopropyl)carbamoyl)-2,2'-bipyridine (L1), 2,3-bis(2-pyridyl)pyrazine (L2), and 4,4'-bis(N-(4-pyridyl)methylcarbamoyl)-2,2'-bipyridine (L3); EDTA = ethylenediaminetetraacetic acid disodium salt). A new Ru(II)Pt(II) complex, [(bpy)2Ru(micro-L3)PtCl2]2+, was synthesized and characterized. It was confirmed that all three compounds are ineffective towards photochemical H2 production. In each case, an acetate-buffer solution (pH = 5) containing the Ru(II)Pt(II) dimer and EDTA was photolysed using a 350-W Xe lamp under an Ar atmosphere, during which the amount of H2 evolved was analysed by gas chromatography. Additional photolysis experiments were carried out by adding [Ru(bpy)3]2+ and methylviologen (N,N'-dimethyl-4,4'-bipyridinium) to the photolysis solutions described above to test the H2-evolving activity of the Pt(II) unit involved in these Ru(II)Pt(II) dimers. As a result, the Pt(II) units involved in the L1 and L2 compounds were found to be active as an H2-evolving catalyst, while that of the L3 compound was found to show no activity at all. The extent of intramolecular electron-transfer quenching from the 3MLCT excited state of the [Ru(bpy)3]2+ derivative to the tethering Pt(II) catalyst centre was investigated by comparison of the luminescence spectra of these compounds, together with the related compounds. The results showed that the quenching of the 3MLCT luminescence is not at all enhanced in either the L1 or the L3 compounds. On the other hand, the L2 compound is strongly quenched as previously reported. In addition to the above studies, the H2-evolving activity of some Pt(II) monomers, cis-PtCl2(NH3)2, PtCl2(en)(en = ethylenediamine), cis-PtCl2(4-methylpyridine)2, PtCl2(2,2'-bipyrimidine), PtCl2(4,4'-dicarboxy-2,2'-bipyridine), and [PtCl(terpy)]+(terpy = 2,2':6',2'-terpyridine), were similarly investigated in the presence of EDTA, [Ru(bpy)3]2+ and methylviologen, since they were regarded as structural analogues of the Pt(II) units involved in the L1-L3 compounds. The compounds having a cis-Pt(II)Cl2 unit were generally found to show high H2-evolving activity. This was interpreted in terms of the ligation of negatively charged chloride anions leading to the destabilization of the Pt(II) dz2 orbital responsible for the hydrogenic activation. Importantly, cis-PtCl2(4-methylpyridine)2 exhibited relatively high activity as an H2-evolving catalyst, suggesting the importance of the flexible rotation of the pyridyl ligands for efficient hydrogenic activation at the axial site of the Pt(II) ion. The DFT calculations also showed the validity of the structure-activity relationship discussed above for the L3 compound.     

Photophysical properties of ruthenium(II) tris(2,2'-bipyridine) complexes bearing conjugated thiophene appendages

A small series of ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized in which ethynylated thiophene residues are attached to one of the 2,2'-bipyridine ligands. The photophysical properties depend on the conjugation length of the thiophene-based ligand, and in each case, dual emission is observed. The two emitting states reside in thermal equilibrium at ambient temperature and can be resolved by emission spectral curve-fitting routines. This allows the properties of the two states to be evaluated in both fluid butyronitrile solution and a transparent KBr disk. It is concluded that both emitting states are of metal-to-ligand charge-transfer (MLCT) character, and despite the presence of conjugated thiophene residues, there is no indication for a low-lying pi,pi*-triplet state that promotes nonradiative decay of the excited-state manifold. A key feature of these systems is that the conjugation length imposed by the thiophene-based ligand helps to control the rate constants for both radiative and nonradiative decay from the two MLCT triplet states.     

Luminescent properties of tris(2,2'-bipyridine)ruthenium(II) in sol-gel-processed dip-coated thin films

The luminescence decay and spectral behavior of ruthenium(II)-tris-1,2-bipyridine dichloride dissolved in different organically modified silicate gel matrixes were investigated. Dip-coated thin films were synthesized from tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS), ethyltriethoxysilane (ETEOS), and methyl- trimethoxysilane (MTMOS). A blue shift in the ruthenium complex emission spectrum with respect to the aqueous solution was observed for all the films on the sol to gel conversion. This spectral shift was slightly dependent on the precursor used to obtain the films and independent of the reaction pH to prepare the "sol". In the data treatment of the time-resolved luminescence measurements, it was assumed that the distribution of the luminophore in the films was nonhomogeneous. The analysis of the luminescence decay profiles was based on a multisite model. All decay curves are best described by a double-exponential model. The parameters of the decay components depended principally on the thermal treatment used in the processing of the films. The lifetimes decreased and the emission espectra showed a red shift with the increase in the drying temperature. A luminescence quenching of the ruthenium complex in the films by dissolved oxygen in aqueous solution was also observed. The quenching rate constant obtained from the preexponential amplitude-weighted mean lifetimes (tau(M)) was in the order of 10(9) M(-1) s(-1). When a phenolic derivative was used as quencher the process rate was greatly reduced compared to the quenching in water. It would seem that the metallic complex sequestered within the film is placed either into a higher microviscosity microenvironment or in a location which the phenolic quencher cannot access. In both cases, the quenching plot based on tau(o)(M)/tau(M) could be fitted satisfactorily by a sum of two terms of Stern-Volmer. This fact is indicative of the matrix microheterogeneity for the films and is fully consistent with the biexponential nature of the luminescence intensity decay profiles.     

Thin-film solid-state electroluminescent devices based on tris(2,2'-bipyridine)ruthenium(II) complexes

The behavior of light-emitting electrochemical cells (LEC) based on solid films ( approximately 100 nm) of tris(2,2'-bipyridine)ruthenium(II) between an ITO anode and a Ga-In cathode was investigated. The response times were strongly influenced by the nature of the counterion: small anions (BF(4)(-) and ClO(4)(-)) led to relatively fast transients, while large anions (PF(6)(-), AsF(6)(-)) produced a slow time-response. From comparative experiments of cells prepared and tested in a glovebox to those in ambient, mobility of the anions in these films appears to be related to the presence of traces of water from atmospheric moisture. An electrochemical model is proposed to describe the behavior of these LECs. The simulation results agreed well with experimental transients of current and light emission as a function of time and show that the charge injection is asymmetric at the two electrodes. At a small bias, electrons are the major carriers, while for a larger bias the conduction becomes bipolar.     

Toward a photochemical and thermal molecular machine: reversible ligand dissociation and binding in a ruthenium(II)-2,2'-bipyridine complex with tris(2-pyridylmethyl)amine

A Ru(II) complex having tris(2-pyridylmethl)amine (TPA) and 2,2'-bipyridine (bpy), [Ru(TPA)(bpy)]X(2) (X = ClO(4), PF(6)), exhibited a severe distortion of the coordination of the axial pyridine moiety of TPA due to steric hindrance. The complex showed interesting dissociation-binding behavior of the axial pyridine arm to form a solvent adduct with TPA ligation in a unique meridional tridentate fashion. The complex undergoes thermal dissociation to form solvent-coordinated species via an S(N)2-like mechanism with activation energy of 117 kJ/mol. In contrast, the complex showed reversible photochemical dissociation and rebinding via an S(N)1-like mechanism by MLCT irradiation. The photochemical dissociation was accelerated approximately 200-fold faster than the thermal process. The dissociation process involves selective binding behavior toward external ligands (solvents) with pi-acceptor character, which is indispensable, and no sigma-donating molecules could bind to the Ru(II) center. The guest molecule can be released upon photoirradiation after its thermal binding.     

Vitamin C derivatives as new coreactants for tris(2,2'-bipyridine)ruthenium(II) electrochemiluminescence

Vitamin C derivatives (VCDs) have been widely used as the alternative and stable sources of vitamin C, and accordingly exhibit many new applications, such as anti-tumor and central nervous system drug delivery. In this study, their Ru(bpy)₃²⁺ electrochemiluminescence (ECL) properties have been investigated for the first time using well-known ascorbyl phosphate and ascorbyl palmitate as representative VCDs. Ascorbyl phosphate and ascorbyl palmitate are VCDs with different substituted positions. Both of them increase Ru(bpy)₃²⁺ ECL, indicating that other VCDs may also enhance Ru(bpy)₃²⁺ ECL signal. The calibration plot for ascorbyl phosphate is linear from 3 × 10⁻⁶ to 1.0 × 10⁻³ M with a detection limit of 1.4 × 10⁻⁶ M at a signal-to-noise ratio of 3. The relative standard deviation is 3.6% for six replicate measurements of 0.01 mM ascorbyl 2-phosphate solution. The proposed method is about one order of magnitude more sensitive than electrochemical and UV–vis methods for the determination of ascorbyl phosphate, and is used successfully for the determination of ascorbyl phosphate in whitening and moisturising body wash.     

Quenching of the electrochemiluminescence of tris(2,2'-bipyridine)ruthenium(II) by ferrocene and its potential application to quantitative DNA detection

Efficient and stable quenching of electrochemiluminescence (ECL) of tris(2,2'-bipyridine)ruthenium(II) by oxidizing ferrocene methanol (FcMeOH) at the electrode is reported. Bimolecular energy or electron transfer between Ru(bpy)(3)(2+*) and ferrocenium (Fc(+)), the oxidized species of Fc, along with suppression of radical reactions is suggested as the mechanism for quenching ECL. Fc shows more efficient quenching of ECL compared with the known quenchers phenol and 1,1-dimethyl-4,4'-bipyridine dication (MV(2+)). The ECL quenching rate constant was 5.6 x 10(10) M(-)(1) s(-)(1). Using Fc as a quencher label on a complementary DNA sequence, an intramolecular ECL quenching in hybridized oligonucleotide strands has been realized. With essentially complete quenching efficiency, this system has the potential for application to sequence-specific DNA detection.     

Effect of the parent ligand on the photophysical properties of closely-coupled, binuclear ruthenium(II) tris(2,2'-bipyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) tris(2,2'-bipyridine) complexes via an alkynylene group differ significantly from those of the relevant mononuclear complex. In particular, the energy of the first triplet excited state is lowered relative to the parent complex, because of the presence of the alkynylene substituent, while the triplet lifetime is prolonged, in part, because of extended electron delocalization. We now report that the triplet lifetime is also affected by the nature of the spectator 2,2'-bipyridyl ligands. Thus, replacing the parent 2,2'-bipyridine ligands with the corresponding 4,4'-dinitro-substituted ligands serves to decrease the luminescence yield and lifetime. With the corresponding carboxylate ester, the luminescence yield and lifetime are increased. Perdeuteration of the parent 2,2'-bipyridine ligands also leads to a modest increase in the luminescence yield. Such observations are indicative of electronic coupling between the various metal-to-ligand, charge-transfer excited triplet states. Temperature dependence studies confirm that these excited states are closely spaced and thermally accessible at ambient temperature. For some of the binuclear complexes, the quantum yield for formation of the lowest-energy triplet state is significantly less than unity.     

Stability of thin-film solid-state electroluminescent devices based on tris(2,2'-bipyridine)ruthenium(II) complexes

The factors affecting the operating life of the light-emitting electrochemical cells (LECs) based on films of tris(2,2'-bipyridine)ruthenium(II) both in sandwich (using an ITO anode and a Ga:Sn cathode) and planar (using interdigitated electrode arrays (IDAs)) configurations were investigated. Stability of these devices is greatly improved when they are produced and operated under drybox conditions. The proposed mechanism of the LEC degradation involves formation of a quencher in a small fraction of tris(2,2'-bipyridine)ruthenium(II) film adjacent to the cathode, where light generation occurs, as follows from the observed electroluminescence profile in the LECs constructed on IDAs, showing that the charge injection in such devices is highly asymmetric, favoring hole injection. Bis(2,2'-bipyridine)diaquoruthenium(II) is presumed to be the quencher responsible for the device degradation. A microscopic study of photo- and electroluminescence profiles of planar light-emitting electrochemical cells was shown as a useful approach for studies of charge carrier injection into organic films.     

Direct ion exchange of tris(2,2'-bipyridine)ruthenium(II) into an alpha-zirconium phosphate framework

The first direct ion exchange of a luminescent metal complex into an alpha-zirconium phosphate framework has been accomplished. A hydrated form of alpha-ZrP, with an expanded 10.3 A interlayer distance, has been used for the intercalation of Ru(bpy)(3)(2+), resulting in further expansion to 15.2 A. The Ru(bpy)(3)(2+) luminescence band is slightly blue-shifted. High Ru(bpy)(3)(2+) loadings lead to luminescence self-quenching.     

Direct observation of the fourth MLCT triplet state in ruthenium(II) tris(2,2'-bipyridine)

The luminescence properties of ruthenium(ii) tris(2,2'-bipyridine) have been recorded in butyronitrile solution and in a transparent KBr disk over a reasonable temperature range. In solution, spectral curve fitting routines indicate that emission arises solely from an ensemble of triplet states, each of which is of Metal-to-Ligand, Charge-Transfer (MLCT) character and of closely comparable energy. At ambient temperature, dual emission is observed for the KBr disk and interpreted in terms of luminescence from both the ensemble and the fourth MLCT triplet state that lies at slightly higher energy. Relative reorganisation energies, energies, Huang-Rhys factors and radiative rate constants have been calculated for the two emissive states. It is confirmed that the fourth MLCT triplet state possesses more singlet character than the ensemble.     

Novel tris(2,2'-bipyridine)ruthenium(II) cathodic electrochemiluminescence in aqueous solution at a glassy carbon electrode

A novel approach of generating cathodic electrochemiluminescence of Ru(bpy)3(2+) at -0.4 V triggered by reactive oxygen species is reported for detecting alkylamines and some organic acids.     

Solid-supported chemiluminescence and electrogenerated chemiluminescence based on a tris(2,2'-bipyridyl)ruthenium(II) derivative

We report a simple and efficient technique for the covalent immobilisation of a tris(2,2'-bipyridyl)ruthenium(II) derivative suitable for both chemiluminescence and electrochemiluminescence detection.     

Emission of tris(2,2'-bipyridine)ruthenium(II) by coreactant electrogenerated chemiluminescence: from O2-insensitive to highly O2-sensitive

We describe the influence of dissolved oxygen on the emission of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) by electrogenerated chemiluminescence (ECL) with tertiary amine as coreactant in aqueous solutions. The significance of the reactions between molecular oxygen and the ECL intermediate reducing radicals has been demonstrated for the first time. By varying the experimental conditions, the oxygen effect on different ECL routes of the Ru(bpy)3(2+)/tri-n-propylamine (TPrA) system was examined. When coreactant direct oxidation played a predominant role in producing ECL, the maximum emission intensity, especially that of the low-oxidation-potential (LOP) ECL, could change from O2-insensitive to highly O2-sensitive with decreasing TPrA concentration. This behavior can be interpreted as follows: A large excess of intermediate reducing radicals was produced at high [TPrA], and the dissolved oxygen within the ECL reaction layer was completely reduced by these radicals and exerted no quenching effect on the emission. At low [TPrA], however, coreactant oxidation generated a relatively small amount of reducing intermediates, and molecular oxygen acted as an interceptor, destroying the intermediates before they participated in the ECL pathways, which led to the obvious reduction of the emission intensity. In the latter case, the less efficient LOP ECL route was more remarkably affected. When ECL was generated primarily via the catalytic route at high [Ru(bpy)3(2+)], the reactions consuming the intermediate radicals by O2 became insignificant, and he drop of emission intensity in the presence of oxygen could mainly be ascribed to the excited-state quenching. A similar oxygen effect was also observed for the Ru(bpy)3(2+)/triethylamine (TEA) system.     

Isotope effects on the picosecond time-resolved emission spectroscopy of tris(2,2'-bipyridine)ruthenium (II)

The excited-state properties of the transition metal complexes tris(2,2'-bipyridine) ruthenium(II) and tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) are examined using picosecond time-resolved luminescence spectroscopy. For both complexes, direct observation of a short-lived high-energy emission with a lifetime of less than 4 ps is reported. Upon deuteriation of the complexes the lifetime of the high-energy emission shows a marked increase with a biexponential decay (20 and approximately 300 ps components). Examination by time-resolved excited-state resonance Raman shows that for the perprotio complexes features attributable to the 3MLCT excited state are formed within 4 ps, while for the perdeuterio a rise time of approximately 20 ps is observed in the 3MLCT features. The results indicate that the emission in both cases may be 1MLCT in origin and are discussed with respect to heterogeneous electron transfer.     

Competing through-space and through-bond, intramolecular triplet-energy transfer in a supposedly rigid ruthenium(II) tris(2,2'-bipyridine)--fullerene molecular dyad

A ditopic ruthenium(II) tris(2,2'-bipyridyl)-based fullerene conjugate has been synthesized so as to separate the photoactive terminals by way of a short ethynylene spacer group that is expected to act as a rigid rod. Intramolecular triplet-energy transfer from the metal complex to the fullerene is quantitative at all temperatures and there is no indication for competing electron transfer. Temperature dependence studies indicate two pathways for triplet-energy transfer. An activationless route dominates at low temperature and is attributed to through-bond electron exchange that takes place via super-exchange interactions. The triplet energy of the bridging unit lies well above that of the metal complex. An activated process is switched-on at high temperatures and is believed to involve through-space electron exchange within closed conformations. Molecular dynamics simulations predict that, in addition to an extended conformation, the linker can distort in such a way that the terminals come into orbital contact. In fact, the resultant closed conformation possesses an idealised geometry for fast electron exchange.     

Biosensing based on NADH detection coupled to electrogenerated chemiluminescence from ruthenium tris(2,2'-bipyridine)

A flow injection analysis system for a reduced form of nicotinamide adenine dinucleotide (NADH) was constructed by the electrogenerated chemiluminescence using ruthenium tris(2,2'-bipyridine) complex, and this technique was applied to the determination of ethanol concentration by using an immobilized alcohol dehydrogenase (ADH). The NADH determination was optimum under the following conditions. The flow rate was 1.5 to 2.0 ml/min, and the applied voltage was 1.6 V. The NADH calibration plot showed the linear correlation in the concentration range 10 to 250muM, and the lower detection limit was 10muM. Ethanol was successfully measured in the concentration range 10 ppm to 5% by using ADH-immobilized glass beads.     

Selective determination of acenaphthylene by flow injection analysis with tris(2,2'-bipyridine)ruthenium(II) chemiluminescence detection

A simple, rapid flow injection chemiluminescence (FI-CL) method has been developed for selective determination of acenaphthylene (ACY), based on the CL produced in the reaction of tris(2,2′-bipyridine)ruthenium(III) (Ru(bipy)₃³⁺) and ACY in an acidic buffer solution. Under the optimum experimental conditions, the calibration curve was linear over the range 5.0 × 10⁻³ to 4.0 × 10⁻⁷ mol L⁻¹ for ACY. The detection limit (S/N = 3) was 2.0 × 10⁻⁷ mol L⁻¹ and the relative standard deviation of 10 replicate measurements was 2.3% for 5.0 × 10⁻⁵ mol L⁻¹ of ACY. Selectivity of CL reaction of ACY from other 15 polycyclic aromatic hydrocarbons (PAHs) was investigated by flow injection method. The method was applied to determine the ACY content in soil.     

Preparation and multicolor electrochromic performance of a WO3/tris(2,2'-bipyridine)ruthenium(II)/polymer hybrid film

A tungsten trioxide (WO(3))/tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)(3)](2+); bpy=2,2'-bipyridine)/poly(sodium 4-styrenesulfonate) (PSS) hybrid film was prepared by electrodeposition from a colloidal triad solution containing peroxotungstic acid (PTA), [Ru(bpy)(3)](2+), and PSS. A binary solution of [Ru(bpy)(3)](2+) and PTA (30 vol % ethanol in water) gradually gave an orange precipitate, possibly caused by the electrostatic interaction between the cationic [Ru(bpy)(3)](2+) and the anionic PTA. The addition of PSS to the binary PTA/[Ru(bpy)(3)](2+) solution remarkably suppressed this precipitation and caused a stable, colloidal triad solution to form. The spectrophotometric measurements and lifetime analyses of the photoluminescence from the excited [Ru(bpy)(3)](2+) ion in the colloidal triad solution suggested that the [Ru(bpy)(3)](2+) ion is partially shielded from electrostatic interaction with anionic PTA by the anionic PSS polymer chain. The formation of the colloidal triad made the ternary [Ru(bpy)(3)](2+)/PTA/PSS solution much more redox active. Consequently, the rate of electrodeposition of WO(3) from PTA increased appreciably by the formation of the colloidal triad, and fast electrodeposition is required for the unique preparation of this hybrid film. The absorption spectrum of the [Ru(bpy)(3)](2+) ion in the film was close to its spectrum in water, but the photoexcited state of the [Ru(bpy)(3)](2+) ion was found to be quenched completely by the presence of WO(3) in the hybrid film. The cyclic voltammogram (CV) of the hybrid film suggested that the [Ru(bpy)(3)](2+) ion performs as it is adsorbed onto WO(3) during the electrochemical oxidation. An ohmic contact between the [Ru(bpy)(3)](2+) ion and the WO(3) surface could allow the electrochemical reaction of adsorbed [Ru(bpy)(3)](2+). The composition of the hybrid film, analyzed by electron probe microanalysis (EPMA), suggested that the positive charge of the [Ru(bpy)(3)](2+) ion could be neutralized by partially reduced WO(3)(-) ions, in addition to Cl(-) and PSS units, based on the charge balance in the film. The electrostatic interaction between the WO(3)(-) ion and the [Ru(bpy)(3)](2+) ion might be responsible for forming the electron transfer channel that causes the complete quenching of the photoexcited [Ru(bpy)(3)](2+) ion, as well as the formation of the ohmic contact between the [Ru(bpy)(3)](2+) ion and WO(3). A multicolor electrochromic performance of the WO(3)/[Ru(bpy)(3)](2+)/PSS hybrid film was observed, in which transmittances at 459 and 800 nm could be changed, either individually or at once, by the selection of a potential switch. Fast responses, of within a few seconds, to these potential switches were exhibited by the electrochromic hybrid film.