We compare the bimolecular quenching rate constant (k2) of luminescent tris(2,2′-bipyridine)ruthenium(II) by oxygen in water, ethylene glycol and glycerol as a function of temperature and viscosity to several theoretical models. The Smoluchowski equation with experimentally determined diffusion coefficients produced calculated values that were in the best agreement with experiment. For the less viscous solvent, water, this equation produced a value that was approximately an order of magnitude larger than the experimental value. With an increase in solvent viscosity, the Smoluchowski value approached the experimental value. Using the Smoluchowski equation with calculated diffusion coefficients based on the known radii of the reacting species produced deviations an order of magnitude larger in water and a factor of two or three lower in ethylene glycol and glycerol. If an assumption is made that the radii of both molecules are equal, we have the Stokes Einstein equation, and the only parameters become temperature and viscosity. Using this relationship, the calculated values for water are about a factor of two larger and with ethylene glycol and glycerol about a factor of 6 smaller than experimental data. These results show that bimolecular quenching is a more complex process affected by many parameters such as solvent cage effects in addition to viscosity and temperature.

The synthesis of some heteroleptic, cyclometalated iridium(III) complexes is described. The utility of these [Ir(ppy)2(N-N)]Cl (ppy = 2-phenylpyridine and N-N = substituted bipyridine, biquinoline, or phenanthroline) complexes as luminescence-based sensors is assessed. The emission intensity of an Ir(III) complex featuring the 3,3′-Hndcbpy ligand (Hndcbpy = dicarboxylic acid-2,2′-bipyridine; n = 0,1,2 to indicate deprotonated, mono- and diprotonated species, respectively) is seen to increase in the presence of Pb(II). Insight into the structure and analyte-sensing capability is achieved by X-ray crystallography in conjunction with computational modeling. Complexes incorporating carboxylic acid-functionalized bipyridine and biquinoline as the polypyridyl ligand show pH sensitivity while similar phenanthroline complexes do not.

Luminescence lifetimes are widely used as an analysis tool. Since decays in analytical systems are frequently complex decays rather than single exponentials, apparent lifetime methods based on the rapid lifetime determination (RLD) method or single frequency phase shift (SFPS) measurements are frequently used to reduce cost and simplify data analysis. It is demonstrated here that these methods can produce large errors under the right conditions. Both methods can give unexpected and uncharacteristic Stern-Volmer quenching plots (SVQPs) in two-component systems. Behaviors include bimodal quenching curves as well as "anti-quenching" curves. These phenomena are exacerbated by small fractions of long unquenched components.

The inclusion of a series of luminescent Re(I) complexes in α- and β-cyclodextrin in water is described. The complex's general structure is [L2Re(CO)3(4-R-pyridine)][ClO4] where L2 represents the α-diimine ligands 2,2′-bipyridine or 1,10 phenanthroline. The R groups were selected to span a range of hydrophobicity from –H to –(CH2)12CH3. These R's showed a variety of binding constants of the complexes to the hydrophobic interior of the β-cyclodextrin. Binding to β-cyclodextrin was accompanied by shifts in the emission λmax to shorter wavelengths, and increases in both the excited state lifetime and luminescence quantum yield. No such effects were observed for α-cyclodextrin. The binding data could be fit by a simple two state model and binding constants ranged from 0.2 to 6.5mM−1. In contrast to previous studies, the mechanism responsible for the changes in photophysics is attributed to shielding the complexes from solvent interaction rather than from quenching by O2. Solvent exposure experiments suggest that binding effectively blocks about half the solvent access to the chromophore.

The luminescence properties of Re(I) complexes incorporating the dcbpy ligand (dcbpy = n,n'-dicarboxylic acid-2,2'-bipyridine; n = 3, 4) were investigated as well as their utility as Pb(2+) sensors. An unusual binuclear complex of the 3,3'- species was isolated. The emission intensity and lifetime for all complexes were found to be highly temperature-dependent, with quantum yields and lifetimes dramatically greater at 77 K than at room temperature. The monomeric 3,3'-dcbpy Re(I) complex demonstrates nearly 1:1 binding with Pb(2+). The effect of this lead binding on the emission intensity is great, but the low quantum yields allow only for detection of the metal at the micromolar level. The binding of Pb(2+) to the 4,4'-dcbpy complex is modeled and the interaction is demonstrated to involve two binding sites.

Luminescence properties and the x-ray structures of the fluorescent crown ether, 16-anthracen-ylmethyl-1,4,7,10,13-pentaoxa-16-aza-cyclooctadecane (CEA) and its complex with potassium hexafluorophosphate (CEAK) have been obtained. In the solid state CEAK gives a structured blue emission and CEA gives a broad structureless green emission. The differences in luminescence behavior are explained on the basis of crystal packing. X-ray analysis shows that every two adjacent anthracene moieties in CEA form a sandwich-like anti-parallel dimer; the green-structureless emission then arises from the π-π stack of the aromatic rings. In CEAK, disruption of the π-π stacking structure forces a large separation between the anthracene rings, which yields an anthracene monomer emission. Luminescence lifetime data support the assignments.

Water quenching of luminescent [Ru(phen) 2 dppz]Cl 2 , [Ru(phen) 2 dppn]Cl 2 , and [Ru(4,7-Ph 2 phen) 2 dppz]Cl 2 (phen = 1,10-phenanthroline; 4,7-Ph 2 phen = 4,7-diphenyl-1,10-phenanthroline; dppz = dipyrido[3, 2-a:2′3′-c]phenazine; dppn = benzodipyrido(a:3,2-h:2′,3′-j)phenazine) complexes was studied in acetonitrile and in polymers. The polymers contained hydrophobic and hydrophilic components to control mechanical properties and were designed to absorb water with changing humidity and, thus, affect the emission intensity and lifetime. Quenching by water in mixed solvents and in polymers was shown to arise from a combination of diffusional and static ground-state associational quenching. The factors controlling polymer properties are discussed. The systems can be tailored to give a wide range of responses or function as a binary sensor at a fixed humidity level.

A rapid and reproducible method for determining the temperature dependence of luminescence lifetimes has been developed. With the use of this method, a set of standards for the excited-state lifetime oxygen quenching of several ruthenium(II) transition metal complexes was established. With the use of three solvents of different viscosities and two metal complexes with widely different lifetimes, an overlapping range of ca. 100 ns to 6 micros was obtained. The decays are pure single exponentials, which means that they can be used reliably with both phase and pulsed lifetime instruments. For a pure single-exponential decay, a properly operating phase shift instrument will give the same lifetime as a time domain instrument. With the use of a thermal deactivation model and a three-parameter temperature-dependent oxygen quenching constant, the lifetime temperature-dependent data was well fit by a simple six-parameter equation that covers the temperature range of 10-50 degrees C and oxygen pressures from 0 to 1 atm of oxygen with excellent precision (ca. <1%). This permits both laboratory and field calibration of instruments.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTQuantum Dots: An Experiment for Physical or Materials ChemistryL. D. Winkler , J. F. Arceo , W. C. Hughes , B. A. DeGraff , and B. H. Augustine View Author Information Departments of Chemistry and Physics, James Madison University, Harrisonburg, VA 22807Cite this: J. Chem. Educ. 2005, 82, 11, 1700Publication Date (Web):November 1, 2005Publication History Received3 August 2009Published online1 November 2005Published inissue 1 November 2005https://pubs.acs.org/doi/10.1021/ed082p1700https://doi.org/10.1021/ed082p1700research-articleACS PublicationsRequest reuse permissionsArticle Views4058Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Absorption,Emission spectroscopy,Quantum dots,Semiconductors,Theoretical calculations Get e-Alerts

A series of luminescent transition metal complexes using the pH-sensitive ligand 5-carboxy-1,10-phenanthroline has been synthesized and characterized. The complexes, based on Ru(II) and Re(I), show monotonic changes in both luminescent intensity and lifetime with pH values over the range 2 < pH < 9. The impact of various structural features on both the range of pH sensitivity and dynamic response was studied using both intensity and lifetime measurements. It was possible to predictably tune the pH sensitivity range over about 1.5 pK(a) units. While significant variation in the dynamic response range was observed, the correlation with structural features needs further study.

Data related to the pH-dependent photophysics of a class of rhenium complexes containing the hydroxypyridine ligand are presented. Data include ground-state pKa values, emission energies, and lifetimes. The complexes all have ground-state pKa values near 7.0 and exhibit a dramatic change in emission intensity near this pH. The lifetimes of these complexes, however, are constant over this pH range. A model is presented to account for the observed photophysical behavior. The pH-dependent emission properties of these species make them good candidates for luminescence-based pH probes, especially in the environmental and biomedical fields.

For evaluating exponential luminescence decays, there are a variety of computational rapid integral methods based on the areas of the decay under different binned intervals. Using both Monte Carlo methods and experimental photon counting data, we compare the standard rapid lifetime determination method (SRLD), optimized rapid lifetime determination methods (ORLD), maximum likelihood estimator method (MLE), and the phase plane method (PPM). The different techniques are compared with respect to precision, accuracy, sensitivity to binning range, and the effect of baseline interference. The MLE provides the best overall precision, but requires 10 bins and is sensitive to very small uncorrected baselines. The ORLD provides nearly as good precision using only two bins and is much more immune to uncompensated baselines. The PPM requires more bins than the MLE and has systematic errors, but is largely resistant to baseline issues. Therefore, depending on the data acquisition method and the number of bins that can be readily employed, the ORLD and MLE are the preferred methods for reasonable signal-to-noise ratios.

An adaptation of square-wave gated phase-modulation (GPM) fluorimetry allows for self-referenced intensity measurements without the complexity of dual excitation or dual emission wavelengths. This AC technique utilizes square-wave excitation, gated detection, a reference emitter, and a sensor molecule. The theory and experimental data demonstrating the effectiveness and advantages of the adapted GPM scheme are presented. One component must have an extremely short lifetime relative to the other. Both components are affected identically by changes in intensity of the excitation source, but the sensor intensity also depends on the concentration of the analyte. The fluctuations of the excitation source and any optical transmission changes are eliminated by ratioing the sensor emission to the reference emission. As the concentration of the analyte changes, the corresponding sensor intensity changes can be quantified through several schemes including digitization of the signal and digital integration or AC methods. To measure pH, digital methods are used with Na 3 [Tb(dpa) 3 ] (dpa = 2,6-pyridinedicarboxylic acid) as the long-lived reference molecule and fluorescein as the short-lived sensor molecule. Measurements from the adapted GPM scheme are directly compared to conventional ratiometric measurements. Good agreement between the data collection methods is demonstrated through the apparent pK a . For the adapted GPM measurements, conventional measurements, and a global fit the apparent pK a values agree within less than 2%. A key element of the adapted GPM method is its insensitivity to fluctuations in the source intensity. For a roughly 8-fold change in the excitation intensity, the signal ratio changes by less than 3%.

The effects of oxygen on the photochemical properties of ruthenium(II) complexes in solution and in polymers are reported. In solution, the complex is actually protected from decomposition by the presence of oxygen as a result of deactivation of the complex by oxygen quenching before it can undergo ligand loss by monomolecular dissociation; however, in polymers, the presence of oxygen increases photochemical decomposition. Singlet molecular oxygen, a product of the oxygen quenching process, may attack the ground state complex or triplet oxygen may directly attack the excited state of the complex. Both mechanisms may be involved in the photodestruction of the complex. The role of oxygen in the photodecomposition was examined by monitoring the photochemical decomposition of various complexes of different singlet oxygen reactivity, as well as absorption and mass spectroscopy studies. It is suggested that in polymers, unlike in solutions, the newly formed reactive singlet oxygen is not able to diffuse away from the complex. The singlet oxygen, trapped in close proximity to the metal complex, has an enhanced opportunity to attack it. This cage effect is supported by studies using tris(1,10-phenanthroline)ruthenium(II) in poly(ethylene glycol) of increasing molecular weight to create an increasingly constraining cage around the complex. Increased poly(ethylene glycol) molecular weight leads to increased oxygen attack of the complex, supporting the cage effect.

A multicomponent luminescent sensor system is described that uses probe species with constant lifetimes to generate an analyte-dependent change in the apparent sensor lifetime. This new sensing scheme not only allows for lifetime-based measurement techniques to be applied to sensors that employ static quenching interactions but also provides the ability to vary the sensitivity of the sensor system with simple changes in instrumental parameters. A model for the multicomponent sensor is presented, followed by data measured using a prototype pH sensor based on the model.

Oxygen and copper(II) ion quenching studies were conducted on a model sensor system to elucidate the significant, yet not fully characterized, role of polymer supports in sensor performance. The system consisted of [Ru(Ph2phen)2(4-cyclamCH2(4′-Me)bpy)]Cl2 (Ph2phen = 4,7-diphenyl-1,10-phenanthroline; bpy = 2,2′-bipyridine; cyclam = 1,4,8,11-tetraazacyclotetradecane) as the sensor molecule, copper(II) ions as the quenching analyte, and two polymer supports. One polymer used was a cyclic siloxane cross-linked with a hydrophilic poly(ethylene oxide); the other was a ternary polymer with poly(ethylene oxide), poly(dimethylsiloxane), and 2-hydroxyethylmethacrylate. Both polymers contain a hydrophobic binding region for the sensor molecule and a hydrated hydrophilic region for transport of the ions to the sensor. Luminescence intensity and lifetime measurements show the polymer supports to be equally effective at shielding the sensor molecule from oxygen quenching but not copper(II) ion quenching. Unlike oxygen, the copper(II) ions quench the sensor molecule in solution and in the polymer supports through a combination of static and dynamic quenching. The unquenched excited state lifetimes, bimolecular rate constants, and equilibrium constants are presented, and their differences are interpreted to provide information about the local environment of the sensor molecule immobilized in the polymer supports.

To ascertain the relationship between the physical properties of polymer supports and the observed response of luminescence-based oxygen sensors, a quenching-based method was developed to measure oxygen diffusion in polymers. The method offers advantages over existing quenching-based techniques since it allows a simple correction for films of high optical density, and the computations do not assume uniform oxygen concentration throughout the film. Diffusion coefficients (D) were measured for a series of sensors with [Ru(Ph2phen)3]Cl2 (Ph2phen = 4,7-diphenyl-1,10-phenanthroline) asthe luminophore and polystyrene, poly(trimethylsilylmethyl methacrylate), poly(butyl methacrylate), poly(trimethylsilylmethyl methacrylate-co-butyl methacrylate), or poly(trimethylsilylmethyl methacrylate-co-1H, 1H-heptafluorobutyl methacrylate) as the support. The solvent from which the films were cast was varied, and filler materials such as hydrophobic, amorphous silica or tributyl phosphate plasticizer were added. Results are interpreted by a domain model in which the local environment of the sensor, rather than the bulk properties of the polymer, is the most critical parameter in sensor design.

A comprehensive study of the binding interactions between ruthenium complexes with multiple binding sites and monomeric and polymeric beta-cyclodextrins is presented. A variety of different binding modes involving single and multiple binding arrangements are found. Binding is critically dependent on the geometry of the guest and host and hydrophobicity effects. There is an optimal spacing of the cyclodextrin units in order to exploit the geometry of the multiple attachment points of the guest. On CD binding, the complexes are shielded from oxygen quenching. Quenching can be reduced by from 3 to greater than 10 times compared to the free complex even though only a small portion of the surface of the complex is protected. The greatly enhanced shielding, can be attributed to the excitation being localized in the protected region. With the polymeric hosts, the associated linker can also assist in shielding. Implications of these results to polymer supported quenchometric oxygen sensors are discussed.