Mário N. Berberan-Santos

Dual Fluorescence Sensor for Trace Oxygen and Temperature with Unmatched Range and Sensitivity

An optical dual sensor for oxygen and temperature is presented that is highly oxygen sensitive and covers a broad temperature range. Dual sensing is based on luminescence lifetime measurements. The novel sensor contains two luminescent compounds incorporated into polymer films. The temperature-sensitive dye (ruthenium tris-1,10-phenanthroline) has a highly temperature-dependent luminescence and is incorporated in poly(acrylonitrile) to avoid cross-sensitivity to oxygen. Fullerene C70 was used as the oxygen-sensitive probe owing to its strong thermally activated delayed fluorescence at elevated temperatures that is extremely oxygen sensitive. The cross-sensitivity of C70 to temperature is accounted for by means of the temperature sensor. C70 is incorporated into a highly oxygen-permeable polymer, either ethyl cellulose or organosilica. The two luminescent probes have different emission spectra and decay times, and their emissions can be discriminated using both parameters. Spatially resolved sensing is achieved by means of fluorescence lifetime imaging. The response times of the sensor to oxygen are short. The dual sensor exhibits a temperature operation range between at least 0 and 120 degrees C, and detection limits for oxygen in the ppbv range, operating for oxygen concentrations up to at least 50 ppmv. These ranges outperform all dual oxygen and temperature sensors reported so far. The dual sensor presented in this study is especially appropriate for measurements under extreme conditions such as high temperatures and ultralow oxygen levels. This dual sensor is a key step forward in a number of scientifically or commercially important applications including food packaging, for monitoring of hyperthermophilic microorganisms, in space technology, and safety and security applications in terms of detection of oxygen leaks.

The Role of Local Triplet Excited States and D-A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices

Here, a comprehensive photophysical investigation of a the emitter molecule DPTZ‐DBTO2, showing thermally activated delayed fluorescence (TADF), with near‐orthogonal electron donor (D) and acceptor (A) units is reported. It is shown that DPTZ‐DBTO2 has minimal singlet–triplet energy splitting due to its near‐rigid molecular geometry. However, the electronic coupling between the local triplet (3LE) and the charge transfer states, singlet and triplet, (1CT, 3CT), and the effect of dynamic rocking of the D–A units about the orthogonal geometry are crucial for efficient TADF to be achieved. In solvents with low polarity, the guest emissive singlet 1CT state couples directly to the near‐degenerate 3LE, efficiently harvesting the triplet states by a spin orbit coupling charge transfer mechanism (SOCT). However, in solvents with higher polarity the emissive CT state in DPTZ‐DBTO2 shifts below (the static) 3LE, leading to decreased TADF efficiencies. The relatively large energy difference between the 1CT and 3LE states and the extremely low efficiency of the 1CT to 3CT hyperfine coupling is responsible for the reduction in TADF efficiency. Both the electronic coupling between 1CT and 3LE, and the (dynamic) orientation of the D–A units are thus critical elements that dictate reverse intersystem crossing processes and thus high efficiency in TADF.

Optoelectronic and structural properties of amorphous silicon–carbon alloys deposited by low-power electron-cyclotron resonance plasma-enhanced chemical-vapor deposition

The optoelectronic and structural properties of hydrogenated amorphous silicon-carbon alloys ~a-SiC:H! are studied over the entire compositional range of carbon content. The films are prepared using low-power electron-cyclotron resonance ~ECR! plasma-enhanced chemical vapor deposition. The carbon content was varied by using different methane ~or ethylene-!-to-silane gas phase ratios and by introducing the methane ~or ethylene! either remotely into the plasma stream or directly through the ECR source, together with the excitation gas ~hydrogen!. Regardless of the deposition conditions and source gases used, the optical, structural and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The deep defect density from photothermal deflection spectroscopy, the ESR spin density, the steady state and the transient photoluminescence, the dark and photoconductivity, the temperature of the hydrogen evolution peaks and the bonding from infrared spectroscopy are correlated to the Urbach tail energy, the B factor of the Tauc plot and E04 ~defined as the energy at which the absorption coefficient is equal to 10 4 cm 21 !. Silicon-rich and carbon-rich regions with very different properties, corresponding approximately to carbon fractions below and above 0.5, respectively, can be distinguished. The properties of the ECR a-SiC:H alloys are compared with those of alloys deposited by rf glow discharge.

On the barometric formula

The barometric formula, relating the pressure p(z) of an isothermal, ideal gas of molecular mass m at some height z to its pressure p(0) at height z=0, is discussed. After a brief historical review, several derivations are given. Generalizations of the barometric formula for a nonuniform gravitational field and for a vertical temperature gradient are also presented.

Fluorescence depolarization by electronic energy transfer in donor–acceptor pairs of like and unlike chromophores

Fluorescence depolarization by energy transfer resulting from dipole–dipole interaction (Forster type) is studied in donor–acceptor pairs of like and unlike chromophores at a fixed distance and with random and uncorrelated static orientations. For unlike chromophores, the acceptor anisotropy decay is shown to display three different extreme types of behavior. When the intrinsic decay rate of the acceptor is much faster than both the transfer rate and the donor intrinsic decay rate, the acceptor anisotropy decays from a positive value, then rises and passes through a maximum, and finally tends to a negative limiting value, yielding a zero steady‐state value. The existence of a maximum is shown to be due to the peculiar relation between the orientation factor and the average angle formed by the donor and acceptor transition moments. For pairs of like chromophores, the exact anisotropy is calculated and compared with that given by an approximate treatment. It is also shown that the anisotropy of the indirect...

Thermally activated delayed fluorescence as a cycling process between excited singlet and triplet states: application to the fullerenes.

In efficient thermally activated delayed fluorescence (TADF) the excited chromophore alternates randomly between the singlet and triplet manifolds a large number of times before emission occurs. In this work, the average number of cycles n is obtained and is shown to have a simple experimental meaning: n+1 is the intensification factor of the prompt fluorescence intensity, owing to the occurrence of TADF. A new method of data analysis for the determination of the quantum yield of triplet formation, combining steady-state and time-resolved data in a single plot, is also presented. Application of the theoretical results to the TADF of [70]fullerenes shows a general good agreement between different methods of fluorescence analysis and allows the determination of several photophysical parameters.

Fluorescence study of the location and dynamics of α-tocopherol in phospholipid vesicles

The intrinsic fluorescence of α-tocopherol has been used as a tool to study the location and dynamics of the molecule in phospholipid vesicles made of egg yolk phosphatidylcholine using steady-state and time-resolved techniques. From absorption spectra it was concluded that most α-tocopherol molecules are hydrogen bonded, although the aggregates formed are fluorescent. By calculating several fluorescence parameters in different solvents it was concluded that α-tocopherol should be situated in a polar region of the membrane. From the results obtained in measurements of fluorescence quenching and resonance energy transfer it was deduced that th chromanol moiety of the molecule is located in a position close to that occupied by the probes 7-(9-anthroxyloxy)stearic acid (7-AS) and 5-(N-oxy-4,4-dimethyloxazolidin-2-yl)stearic acid (5-NS) in the membrane. The lateral diffusion coefficient of α-tocopherol in phospholipid vesicles was calculated through quenching of its fluorescence by the spin probe 5-NS, and a value of 4.8 · 10−6 cm2 · s−1 was found, indicating a very high lateral diffusion of α-tocopherol.

A new family of luminescent compounds: platinum(II) imidoylamidinates exhibiting pH-dependent room temperature luminescence

The imidoylamidinate platinum(II) compounds [Pt{NH=C(R)NC(Ph)NPh}2] (R = CH2Ph 2, p-ClC6H43, Ph 4) were prepared by the reaction of the appropriate trans-[PtCl2(RCN)2] with 4 equiv of the amidine PhC(NH)NHPh giving 2-4 and 2 equivs of the salt PhC(=NH)NHPh.HCl. We also synthesized, by the double alkylation of 4 with MeOSO2CF3, complex [Pt{NH=C(Ph)N(Me)C(Ph)=NPh}2][CF3SO3]2 (5) which models the bis-protonated form of 4. The complexes were characterized by 1H, 13C NMR, and IR spectroscopies, FAB-MS and by C, H, N elemental analysis. The X-ray crystallography of 4.2CH2Cl2 enables the confirmation of the square planar coordination geometry of the metal center with almost planar imidoylamidine ligands, while in 5.2CHCl3 the planarity of the metallacycles is lost and and the central N atom is sp3-hybridized. The imidoylamidinate complexes represent a new family of Pt(II)-based luminescent complexes and they are emissive at room temperature both in solution and in the solid state, with an emission quantum yield ranging from 3.7 x 10(-4) to 6.2 x 10(-2) in methanol solution; the emission intensity is pH-dependent, being quenched at low pH. UV-visible and luminescence spectroscopies indicate that the lowest excited state of these compounds is 3MLCT or 3IL with significant MLCT character, with emission lifetimes of a few micros. A blue shift of both the absorption and emission with increasing solvent polarity and with decreasing pi-electron withdrawing properties of the ligand substituent was observed.

Fluorescence quenching with exponential distance dependence: Application to the external heavy-atom effect

A model for fluorescence quenching with exponential distance dependence is developed and applied to external heavy-atom quenching. The systems studied are C70-bromobenzene and phenanthrene-iodide, in liquid solutions at room temperature and in rigid glasses at 77 K. The predicted parameter correlation is observed in the two systems, which correspond to two extreme and opposite situations, strong and weak quenching. A detailed analysis of the fitted parameters allows the determination of the effective Bohr radii L and of the intrinsic unimolecular rate constants for quenching at contact, k0. The unusually high value of L for the C70-bromobenzene pair is tentatively attributed to the extended size of the external part of the π orbitals of the fullerene. The room temperature quenching rate constant computed with the determined low-temperature parameters is shown to be in good agreement with the experimental one for both systems studied.

Energy transfer in spherical geometry. Application to micelles

A model for long-range dipole–dipole energy transfer in spherical geometry is presented which, from steady-state or time-resolved fluorescence measurements, enables the determination of the relative positions of donor and acceptor inside the sphere. Its application to micelles (sodium dodecyl sulphate, SDS, and Triton X-100) did not lead to a quantitative determination of the position of the probes [n-(9-anthroyloxy) stearic acids, n-AS, (n= 2, 3, 6, 9, 12) and functional rhodamine and cyanine dyes], and this fact is attributed to probe-induced perturbations on the micellar structure. The results obtained lead to the following conclusions: (i) functional probes in micelles have specific radial positions and (ii) the perturbed region is a preferential solubilization site for a second probe.