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.     

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.     

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.     

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.     

Determination of diphenhydramine by capillary electrophoresis with tris(2,2'-bipyridyl)ruthenium(II) electrochemiluminescence detection

Tris(2,2′-bipyridyl)ruthenium(II) electrochemiluminescence detection in a capillary electrophoresis separation system was used for the determination of diphenhydramine. In this study, platinum disk electrode (300 μm in diameter) was used as a working electrode and the influence of applied potential and buffer conditions were investigated. Under optimal conditions: 1.2 V applied potential, pH 8.50, 15 kV separation voltage and 10 mmol l⁻¹ running buffer, the calibration curve of diphenhydramine was linear over the range of 4×10⁻⁸ to 1×10⁻⁵ mol l⁻¹. This technique gave satisfactory precision, and relative standard deviations of migration times and chemiluminescence peak intensities were less than 1 and 6%, respectively. The technique was applied to animal studies for determination of diphenhydramine extracted from rabbit plasma and urine samples, and the extraction efficiency were between 92 and 98.5%.     

Direct tris(2,2'-bipyridyl)ruthenium (II) electrochemiluminescence detection of polyamines separated by capillary electrophoresis

Capillary electrophoresis (CE) with tris(2,2'-bipyridyl) ruthenium (II) (Ru(bpy)3(2+)) electrochemiluminescence (ECL) detection technique was developed for the analysis of four polyamines (putrescine (Put), cadaverine (Cad), spermidine (Spd), and spermine (Spm)) analysis. The four polyamines contain different amine groups, which have different ECL activity. There are several parameters which influence the resolution and ECL peak intensities, including the buffer pH and concentrations, separation voltage, sample injection, electrode materials, and Ru(bpy)3(2+) concentrations. Polyamines are separated by capillary zone electrophoresis in an uncoated fused-silica capillary (50 cmx25 micro m (ID) filled with acidic phosphate buffer (200 mmol/L phosphate, pH 2.0) - 1mol/L phosphoric acid (9:1 v/v) and a separation voltage of 5 kV (25 micro A), with end-column Ru(bpy)3(2+) ECL detection. A 5 mmol/L Ru(bpy)3(2+) solution plus 200 mmol/L phosphate buffer (pH 11.0) is added into the reagent reservoir. The calibration curve is linear over a concentration range of two or three orders of magnitude for the polyamines. The analysis time is less than 25 min. Detection limits for Put and Cad are 1.9x10(-7) mol/L and 7.6x10(-9) mol/L for Spd and Spm, respectively. Intraday and interday relative standard deviations of ECL peak intensities are less than 8%. The main advantages of this CE-ECL detection technique for polyamines analysis presented herein are the omission of chemical derivatization of the analytes and the high selectivity.     

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.     

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.     

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.     

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.     

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.     

Determination of carbamazepine by chemiluminescence detection using chemically prepared tris(2,2'-bipyridine)ruthenium(III) as oxidant.

A new chemiluminescence method for the determination of carbamazepine (CBZ) has been developed. The method is based on the chemiluminescence produced in the reaction of tris(2,2'-bipyridine)ruthenium(III) and CBZ in an acidic medium. The chemiluminescence intensity was enhanced by organic solvents in the reaction system. Under the optimum experimental conditions, the calibration curve was linear over the range 4.0 x 10(-3)-8.6 x 10(-7) mol/L for CBZ. The detection limit (S/N = 3) was 2.5 x 10(-7) mol/L and the relative standard deviation of six replicate measurements was 2.6% for 4.0 x 10(-4) mol/L of CBZ. The possible reaction mechanism were also discussed. The chemiluminescence method was successfully applied to assay the CBZ contents in pharmaceutical tablets.     

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.     

Ultrafast fluorescence detection in tris(2,2'-bipyridine)ruthenium(II) complex in solution: relaxation dynamics involving higher excited states

The excited-state dynamics of a transition metal complex, tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck-Condon singlet excited state in this molecule was missing. In this study, emission from the Franck-Condon excited singlet state of [Ru(bpy)(3)](2+) has been observed for the first time, and its lifetime has been estimated to be 40 +/- 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited (1)(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 +/- 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin-orbit coupling is proposed. This enhances the radiative rate constant, k(r), of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56-1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.     

Sensitive determination of metal ions by liquid chromatography with tris(2,2'-bipyridine) ruthenium (II) complex electrogenerated chemiluminescence detection.

Emetine dithiocarbamate metal complex, which is prepared from emetine, carbon disulfide, and metal (II), was found to indicate a large chemiluminescence intensity on the electrogenerated chemiluminescence of tris(2,2'-bipyridine)ruthenium(II). Liquid chromatography equipped with the chemiluminescence detection was developed for analyzing trace metal ions by use of the metal complex formation. The mixture of the Cu(II) and Co(II) complexes as a model sample was injected into the LC system. The two metal complexes and an excess emetine were successfully separated. The Cu(II) and Co(II) complexes were determined over the range 1-300 nM (the detection limit of 650 fg) and 30-5000 nM (the detection limit of 17 pg), respectively.     

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.     

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.     

Time-resolved photoacoustics study of the ruthenium(II) bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine) complex

The formation and the decay of the triplet metal to ligand charge transfer state ((3)MLCT) of ruthenium(II) bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine) (Ru(bpy)(2)(dcbpy)) were characterized using photoacoustic calorimetry. At pH 6 and 2, the (3)MLCT state formation leads to a volume change of -8 mL mol(-1) and enthalpy changes of 17 kcal mol(-1) and 13 kcal mol(-1), respectively. We attribute the volume contraction to structural changes and to solvent electrostriction. At pH 4, the photoexcitation of the complex leads to an expansion of 14 mL mol(-1) and an enthalpy change of approximately 119 kcal mol(-1) due to protonation of the carboxyl group in the excited state.