Aromatic difluoroboron ß-diketonate complexes: effects of π-conjugation and media on optical properties.

Aromatic difluoroboron ß-diketonate complexes (BF2bdks) are classic fluorescent molecules that have been explored as photochemical reagents, two-photon dyes, and oxygen sensors. To gain a better understanding of their emissive properties in both solution and polymer matrices, BF2bdks with varying aromatic groups were synthesized and their photophysical properties were investigated in both methylene chloride and poly(lactic acid) (PLA). Absorption spectra showed systematic variations that are well correlated with structural features, including the size of the aryl substituent and the presence of a para electron-donating methoxy substituent. Computational modeling of the absorption spectra with the TD-B3LYP/6-311+G(d)//B3LYP/6-31G(d) formulation of density functional theory and a polarizable continuum model of dichloromethane solvent shows that all systems show intense π-π* one-electron excitations, usually from one of the highest occupied molecular orbitals (HOMO - k, k = 0, 1, 2) to the lowest unoccupied molecular orbital (LUMO). Emission properties are sensitive to the dye structure and medium. Based on spectroscopic and lifetime studies, BF2bdks exhibit comparable fluorescence properties in both solutions and polymers when the diketonate group is functionalized with smaller aromatic ring systems such as benzene. For BF2bdks with larger arene ring systems, such as anthracene, emission from a strong intramolecular charge-transfer (ICT) state was also noted in both solution and in PLA. There are differences in relative intensities of peaks arising from π-π* and ICT excitations depending upon dye loading in PLA. Substituent effects were also observed. Electron-donating methoxyl groups on the aromatic rings lead to enhanced fluorescence quantum yields. For certain dyes, phosphorescence is detected at low temperature or under a nitrogen atmosphere in PLA matrices.

Viscosity and temperature effects on the rate of oxygen quenching of tris-(2,2'-bipyridine)ruthenium(II).

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.

Luminescence oxygen sensor based on a ruthenium(II) star polymer complex.

A novel quenchometric oxygen sensor based on a low polydispersity (PDI) star polymer [Ru(bpyPS(2))(3)](PF(6))(2) (bpy = 2,2'-bipyridine, PS = polystyrene) is reported. The synthesis, characterization, photophysics, and oxygen sensing properties are examined. Combining the polystyrene support with the oxygen sensing ruthenium complex provides much higher doping levels without microcrystallization of the complex than traditional two-component sensors. The single molecule approach also avoids sensor leaching. While the polydispersity was 1.10, indicating a very tight distribution of molecular weights, sensor heterogeneity was not completely eliminated, as the luminescence decays were still multiexponentials. The likely source of this heterogeneity and possible methods for generating more homogeneous materials are discussed.

Determining proton diffusion in polymer films by lifetimes of luminescent complexes measured in the frequency domain.

Polymer-supported luminescent metal complexes represent an important class of oxygen, pH, and ion sensors. The diffusion properties of the analyte into the sensing film are important for rational sensor and support design and development. We describe a technique using lifetime measurements in the frequency domain for determining the diffusion coefficient of hydrochloric acid through various polymeric pH sensor films. Two types of polymers are doped with [Ru(4,7-diphenyl-1,10-phenanthroline)2(4,4'-dicarboxy-2,2'-bipyridine)]Cl2. We monitor the phase shift of luminescence (from which we calculate the apparent lifetime, tau(app)) versus time after applying a step increase in the aqueous HCl concentration at the surfaces of the film. We model the decrease in tau(app) as a function of time using the diffusion coefficient of HCl in the polymer as the only adjustable parameter. The model accurately predicts the lifetime versus time curves, and the resulting diffusion coefficients are highly dependent on the polymer. Relative to bulk water, diffusion of protons within very hydrophilic hydrated D4 polymer (a polyethylene oxide cross-linked siloxane ring polymer) films is hindered approximately 4-fold, while within a more hydrophobic sol gel it is hindered by over 1 order of magnitude. The methodology is adaptable for measuring diffusion coefficients of a variety of analytes in different sensor films as long as the bound and unbound forms luminescence and the excited states have different lifetimes.

Luminescent donor-acceptor beta-diketones: modulation of emission by solvent polarity and group II metal binding.

A class of aryl trifluoromethyl-containing beta-diketones were synthesized via one step Claisen condensation. These pi-conjugated diketones exhibit strong solvatochromism from intramolecular donor-acceptor charge transfer (CT). In addition, fluorescence quantum yields (phi(f)) and lifetimes (tau(f)) were measured in different solvents. Diketones exhibit bathochromic shifts in emission spectra with increasing solvent polarity. Fluorescence changes upon Group II metal binding were also studied. Despite the relatively simple structure, the anthracene-CF(3) diketone, atm, has strong binding affinity for Mg2+. A 70 nm blue shift and sixfold increase in intensity were observed upon addition of only one equivalent MgCl2 in ethanol solution. It also shows selectivity for Mg2+ binding even in the presence of excess Ca2+. Association constant (Ka) calculations suggest atm has two orders of magnitude stronger chelation for divalent magnesium than for calcium. These findings make atm an attractive starting point for molecular probe and light emitting material design.

Abnormal behavior in Stern-Volmer luminescence quenching measurements via apparent lifetime methods.

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.

Ruthenium tris(bipyridine) complexes with sulfur substituents: model studies for PEG coupling.

Ruthenium polypyridyl complexes are incorporated into polymers for sensing and light emitting materials applications. Coupling reactions between metal complexes and polymers are one route to polymeric metal complexes. In an effort to increase conjugation efficiency, tune materials properties, and introduce a responsive crosslink, ruthenium tris(bipyridine) derivatives with sulfur substituents were synthesized and compared to oxygen analogues. Difunctional thiols, thioesters, thioethers, and disulfides, as well as hexafunctional nonpolymeric model systems, were explored. Upon exposure to oxygen, the thiol derivative was readily oxidized. These studies guided Ru(bpy)3 PEG coupling reactions with disulfide and thioether linkages, which proceeded to approximately 80% and approximately 60% yield, respectively. The luminescence properties of the Ru PEG derivatives and model systems were investigated. The emission spectra and lifetimes for all complexes in CH3CN under an inert atmosphere are comparable to [Ru(bpy)3]Cl2. Lifetime data for nonpolymeric analogues fit to a single exponential decay indicating heterogeneity, suggesting sample homogeneity, whereas data for polymers fit to a multiexponential decay. In contrast to certain [Ru(bpy)3](2+)/thiol mixtures, no intramolecular quenching by the sulfide is observed for [Ru(bpy)2{bpy(CH2SH)2}](PF6)2. Emission spectra red shift and multiexponential decay are noted for the oxidized Ru thiol product. The rates of oxygen quenching are slower for Ru PEG derivatives than those for nonpolymeric analogues, which may be attributed to shielding effects of the polymer chain.

Quenching of luminescent ruthenium(II) complexes by water and polymer-based relative humidity sensors.

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; 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.

Comparison of methods for rapid evaluation of lifetimes of exponential decays.

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.

pH-dependent photophysical behavior of rhenium complexes containing hydroxypyridine ligands.

Data related to the pH-dependent photophysics of a class of rhenium complexes containing the hydroxypyridine ligand are presented. Data include ground-state pK(a) values, emission energies, and lifetimes. The complexes all have ground-state pK(a) 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.

Self-referencing intensity measurements based on square-wave gated phase-modulation fluorimetry.

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 Na3[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 pKa. For the adapted GPM measurements, conventional measurements, and a global fit the apparent pKa 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%.

Structure and luminescence properties of monomeric and dimeric Re(I) complexes with dicarboxylic acid-2,2'-bipyridine ligands.

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.

Potassium ion induced changes of crystal structure and fluorescence of a crown ether.

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 pi-pi stack of the aromatic rings. In CEAK, disruption of the pi-pi stacking structure forces a large separation between the anthracene rings, which yields an anthracene monomer emission. Luminescence lifetime data support the assignments.

Luminescence lifetime standards for the nanosecond to microsecond range and oxygen quenching of ruthenium(II) complexes.

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.

Ruthenium(II) tris(bipyridine)-centered poly(ethylenimine) for gene delivery.

Ruthenium(II) tris(bipyridine)-centered poly(ethylenimine) (Ru PEI) was synthesized via acid hydrolysis of Ru tris(bipyridine)-centered poly(2-ethyl-2-oxazoline) (Ru PEOX), and the luminescence, DNA entrapment, and transfection efficiencies were evaluated. Emission maxima for Ru PEI samples are red-shifted compared to Ru PEOX precursors, and the luminescence lifetimes are shorter in both methanol and aqueous solutions. Slower oxygen quenching of Ru PEOX and Ru PEI luminescence versus [Ru(bpy)3]Cl2 (bpy = bipyridine) is attributed to polymer shielding effects. Ru PEI luminescence is similar in the presence and absence of DNA. Ru PEI (7900 Da) and linear PEI (L-PEI; 22,000 Da) fully entrapped DNA (5.4 kb; pcDNA) at an N/P ratio of 2. LNCaP prostate cancer cells were transfected with a plasmid encoding for green fluorescent protein using Ru PEI and L-PEI vectors for comparison. For N/P = 48, the transfection efficiency for Ru PEI was approximately 50% relative to that of L-PEI.

Luminescent transition metal complexes as sensors: structural effects on pH response.

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 pKa units. While significant variation in the dynamic response range was observed, the correlation with structural features needs further study.

On the Actinometric Measurement of Absolute Luminescence Quantum Yields.

The theory of the measurement of luminescent quantum yields using chemical actinometry is described. The sample's emission intensity is measured by nearly completely surrounding the sample with an actinometer solution, and the excitation intensity is directly measured with the same type of actinometer. The ratio of the measured sample emission intensity corrected for the fraction escaping through the excitation ports to the measured excitation intensity is the absolute luminescence yield. Equations, a suitable cell design, and computer calculated correction factors for different cell dimensions and optical densities are given. The absolute yield of the actinometer is not needed, only its relative response with wavelength. New quantum-flat actinometers which should greatly simplify the measurements are described.

Determining oxygen diffusion coefficients in polymer films by lifetimes of luminescent complexes measured in the frequency domain.

Polymer films doped with luminescent ruthenium complexes are proving to be important oxygen sensors. We describe a technique using lifetime measurements in the frequency domain for determining the diffusion coefficient of oxygen through various polymer supports. These fundamental measurements will allow for more rational design of improved sensors. Three types of polymers were doped with [Ru(4,7-diphenyl-1,10-phenanthroline)3]Cl2. We monitored the luminescence versus time after applying a step increase in the oxygen pressure at the surface of the film. We modeled the decrease in apparent lifetime as a function of time using the diffusion coefficient of oxygen in the polymer as the only adjustable parameter. The model accurately predicted the lifetime versus time curves, and diffusion coefficients agreed well with those obtained from intensity measurements. The advantages and disadvantages of the lifetime technique to those used earlier are discussed.

Absolute Quantum Efficiencies.

Recent developments in several areas of chemistry, laser technology, photodetector instrumentation, and calorimetry are surveyed, and their probable impact on the measurement of quantum yields is assessed. Chemical developments include: (a) synthesis and design of new luminescent molecules that could possibly serve as standards, (b) application of improved separation techniques to provide samples of extreme purity, and (c) advances in photochemistry that portend the development of wide-range chemical actinometers. The potential use of lasers in quantum-yield measurements and their advantages over conventional sources for application in both optical and calorimetric techniques are pointed out. New methods of quantum-yield measurements, based on the novel characteristics of laser pump sources, are suggested, including the feasibility of measuring yields under time-resolved conditions and of employing internal standards. The possible lifting of wavelength restrictions on both laser sources and detector devices and the implications of these developments for extending the spectral range of quantum-yield measurements are discussed. The current status of calorimetry for determining yields is surveyed, and the impact of recent technology on the feasibility of developing calorimetric methods competitive with optical methods is assessed.

Oxygen sensors based on luminescence quenching: interactions of metal complexes with the polymer supports.

Oxygen quenching of [Ru(Ph2phen)3]Cl2 (Ph2phen = 4,7-diphenyl-1,10-phenanthroline) has been studied in a diverse series of polymers, most with a common poly-(dimethylsiloxane) (PDMS) component. Systematic variations in the polymer properties have been made in order to delineate the structural features important for satisfactory use of supports for oxygen sensors. Most measurements were made using homo- or copolymers containing a PDMS region, although some measurements were made on small ring siloxane polymers. In particular, quenching behavior was examined as a function of polymer structure as well as the type of and amount of polar copolymer cross-linkers. Cross-linkers were added to enhance the solubility of oxygen probes in an otherwise nonpolar polymer. In addition, hydrophobic silica was added to alter quenching properties. Domain models are used to explain the variations in oxygen quenching properties as a function of additives and cross-linkers. These considerations have led to the most sensitive ruthenium-based sensor reported to date. The relative affinity of the different domains for the complex and the efficacy of the domains for oxygen quenching control the overall behavior of the sensing response. Guidelines for design of suitable polymer supports for oxygen sensors are proposed.