Room-temperature Quantum Yield Research Articles

Molecular Environment Effects That Modulate the Photophysical Properties of Novel 1,3-Phosphinoamines Based on 2,1,3-Benzothiadiazole.

We report synthesis, crystal structure, and photophysical properties of novel 1,3-phosphinoamines based on 4-amino-2,1,3-benzothiadiazole (NH2-btd): Ph2PCH(Ph)NH-btd (1) and Ph2P(E)CH(Ph)NH-btd, (E = O (2α and 2β·thf), S (3), Se (4)). Chalcogenides 2–4 exhibit bright emissions with a major band at 519–536 nm and a minor band at 840 nm. According to TD-DFT calculations, the first band is attributed to fluorescence, while the second band corresponds to phosphorescence. In the solid state, room temperature quantum yield reaches 93% in the case of the sulphide. The compounds under study feature effects of the molecular environment on the luminescent properties, which manifest themselves in fluorosolvatochromism as well as in a luminescent response to changes in crystal packing and in contributions to aggregation effects. Specifically, transformation of solid 2β·thf to solvate-free 2β either by aging or by grinding causes crystal packing changes, and, as a result, a hypsochromic shift of the emission band. Polystyrene films doped with 2 reveal a bathochromic shift upon increasing the mass fraction from 0.2 to 3.3%, which is caused by molecular aggregation effects.

Photo- and Electroluminescent Neutral Iridium(III) Complexes Bearing Imidoylamidinate Ligands.

Imidoylamidinate-based heteroleptic bis(2-phenylbenzothiazole)iridium(III) and -rhodium(III) complexes [(bt)2M(N∩N)] (bt = 2-phenylbenzothiazole, N∩N = N'-(benzo[d]thiazol-2-yl)acetimidamidyl (Ir1 and Rh1), N'-(6-fluorobenzo[d]thiazol-2-yl)acetimidamidyl (Ir2), N'-(benzo[d]oxazol-2-yl)acetimidamidyl (Ir3), N'-(1-methyl-1H-benzo[d]imidazol-2-yl)acetimidamidyl (Ir4); yields 70-84%) were obtained by the reaction of the in situ-generated solvento-complex [(bt)2M(NCMe)2]NO3 and benzo[d]thia/oxa/N-methylimidozol-2-amines in the presence of NaOMe. Complexes Ir1-4 exhibited intense orange photoluminescence, reaching 37% at room temperature quantum yields, being immobilized in a poly(methyl methacrylate) matrix. A photophysical study of these species in a CH2Cl2 solution, neat powder, and frozen (77 K) MeOC2H4OH-EtOH glass matrix─along with density-functional theory (DFT), ab initio methods, and spin-orbit coupling time-dependent DFT calculations─verified the effects of substitution in the imidoylamidinate ligands on the excited-state properties. Electrochemical (cyclic voltammetry and differential pulse voltammetry) and theoretical DFT studies demonstrated noninnocent behavior of the imidoylamidinate ligands in Ir1-4 and Rh1 complexes due to the significant contribution coming from these ligands in the HOMO of the complexes. The iridium(III) species exhibit a ligand (L, 2-phenylbenzothiazole)-centered (3LC), metal-to-ligand (L', imidoylamidinate) charge-transfer (3ML'CT,3MLCT) character of their emission. The imidoylamidinate-based iridium(III) species were proved to be effective as the emissive dopant in an organic light-emitting diode device, fabricated in the framework of this study.

Synthesis and Investigation of Macrocyclic CrIII Bis(alkynyl) Complexes: Structural and Spectroscopic Properties

The synthesis and characterization of four new CrIII–bis(alkynyl) complexes bearing the macrocyclic tetraaza ligand DMC (DMC = 5,12‐dimethyl‐1,4,8,11‐tetraazacyclotetradecane) are reported. Complexes trans‐[Cr(DMC)(C2R)2]X (R = Ph ([1]X), Fc ([2]X), X = Cl, ClO4. C2H ([3]X′); X′ = ClO4, BPh4) and cis‐[Cr(DMC)(C4TMS)2]Cl ([4]Cl) were studied using UV/Vis and FTIR spectroscopy, and their identities were verified with ESI‐MS and elemental analysis. The three trans complexes, [1]Cl, [2](ClO4), and [3](BPh4), were structurally characterized using single‐crystal X‐ray diffraction, which revealed a pseudo‐octahedral geometry around the Cr center with the nitrogen atoms occupying the equatorial plane and the alkynyl ligands residing in the apical positions. Spectroscopic analysis of [1]Cl, [3](BPh4) and [4]Cl shows highly structured d–d bands between 320 and 500 nm. All CrIII complexes reported herein are emissive, and detailed studies were performed for [1]Cl, [3](BPh4), and [4]Cl, yielding phosphorescence lifetimes (77 K) of 380, 358, and 160 µs, respectively, and room temperature quantum yields of 0.01 % for complex [1]Cl and 0.15 % for complex [4]Cl. Voltammetric studies of complex [2](ClO4) indicate a weak but discernible coupling between two ferrocenyl groups across the C2–Cr–C2 bridge.

Temperature and density dependence of exciton dynamics in IIa diamond: Experimental and theoretical study

We studied dependence of time‐resolved and time‐integrated photoluminescence of free excitons in IIa type monocrystalline diamond on sample temperature and carrier density. Numerical model based on rate equations for time evolution of excited carrier and occupied traps populations was used for theoretical modelling. The density dependent diffusion and free carrier/free exciton equilibrium were shown to be important factors in exciton recombination dynamics. Room temperature quantum yield of excitonic photoluminescence was found to be strongly dependent on the carrier density with the highest measured value of 13% at n = 1017 cm−3.

Efficient Photoluminescence via Metal–Ligand Alteration in a New MOFs Family

Here, we introduce a family of metal–organic frameworks (MOFs) whose photoluminescence is tunable through metal and organic ligand substitutions. The compounds in this family are composed of In, In–Eu, or Eu metal centers and organic ligand chromophores. Systematic variations in the metal and organic components resulted in materials with emissions ranging from white to red. The single-component white-light emitter material is made of In, 4,4′,4″-s-triazine-2,4,6-triyl-tribenzoic acid (TTB) and oxalic acid. Red-emitting MOFs composed of Eu metal centers and TTB ligands have a room temperature quantum yield (QY) of 50% and a 48% QY at 150 °C due to reversible thermal quenching. This is the highest quantum yield measured at elevated temperatures reported for this class of materials. The materials are thermally stable, retaining their high QY after heating at 150 °C for several hours. These thermal quenching/stability studies show the potential use of MOFs in devices that operate at elevated temperatures, suc...

Hydrothermal synthesis of ZnSe:Cu quantum dots and their luminescent mechanism study by first-principles

An one-pot synthesis of aqueous ZnSe:Cu nanocrystals (NCs) is realized in aqueous solution by a facile yet efficient hydrothermal technique. The dopant emission spectrum of the NCs is tunable, spanning a wide range from 438 to 543nm. Room-temperature quantum yield for the NCs prepared at the optimal conditions reaches as high as 20% without any post-treatment. The ZnSe:Cu NCs prepared in a neutral aqueous solution (pH=8) are remarkably stable and exhibit comparatively high photoluminescent quantum yield (PL QY) as high as 17%. First-principles pseudopotential calculations using plane-wave basis functions have been performed. The formation energies of copper ions occupied in the interstitial octahedron and substitutional tetrahedral Zn2+ sites have been calculated. The occupation of copper ions in the interstitial octahedral site is found to be more thermodynamics-facilitated by −0.98eV. The density of state analysis indicates that the Cu-related emission is primary dominated by the substitutional tetrahedral Cu ions, and the large dopant related emission width of ZnSe:Cu NCs originated from the corresponding Cu 3d impurity band.

Nickel-assisted growth and selective doping of spinel-like gallium oxide nanocrystals ingermano-silicate glasses for infrared broadband light emission

The target of taking advantage of the near-infrared light-emission properties of nickelions in crystals for the design of novel broadband optical amplifiers requires theidentification of suitable nanostructured glasses able to embed Ni-doped nanocrystalsand to preserve the workability of a glass. Here we show that Ni doping ofLi2O–Na2O–Ga2O3–GeO2–SiO2 glass (with composition 7.5:2.5:20:35:35 and melting temperature1480 °C, sensibly lower than in Ge-free silicates) enables the selective embedding of nickel ions inthermally grown nanocrystals of spinel-like gallium oxide. The analysis of transmission electronmicroscopy and x-ray diffraction data as a function of Ni-content (from 0.01 to 1 mol%) indicatesthat Ni ions promote the nanophase crystallization without affecting nanoparticle size (∼6 nm) andconcentration (∼4 × 1018 cm − 3). Importantly, as shown by optical absorption spectra, all nickel ions enter into the nanophase, with anumber of ions per nanocrystal that depends on the nanocrystal concentration and ranges from 1 to102. Photoluminescence data indicate that fast non-radiative decay processes become relevantonly at mean ion–ion distances shorter than 1.4 nm, which enables the incorporation of afew Ni ions per nanoparticle without too large a worsening of the light-emission efficiency.Indeed, at 0.1 mol% nickel, the room temperature quantum yield is 9%, with an effectivebandwidth of 320 nm.

Procedures for Controlling the Size, Structure and Optical Properties of CdS Quantum Dots during Synthesis in Aqueous Solution

We report an easy approach for the synthesis of CdS Quantum Dots (CdS QDs) with high luminescence and temporal stability through the reaction of Cd(2+) and S(2-) in the presence of mercaptoacetic acid (MAA) as a capping reagent in aqueous medium, under normal pressure and room temperature. The influence of several experimental variables, including temperature, pH, the Cd/S ratio and the Cd/MAA ratio, on the optical properties of the QDs obtained was studied systematically. The experimental results indicate that these variables play an important role in determining the size and state of the surface of the nanoparticles, and hence their luminescent properties and temporal stability. The general aspects of nanocrystal nucleation and growth in the synthesis of nanocrystals were studied. The best conditions for the synthesis of nanoparticles of high quality are also reported. The CdS nanocrystals obtained exhibited a narrow PL band, with reproducible room-temperature quantum yields.

Tunable, Ligand-Based Emission from Inorganic−Organic Frameworks: A New Approach to Phosphors for Solid State Lighting and Other Applications

Two dense inorganic−organic frameworks were prepared using the intrinsically luminescent organic ligand 9-fluorenone-2,7-dicarboxylic acid (H2FDC) in combination with the alkaline earth metals calcium and strontium. Ca(FDC)(H2O)2 (1) and Sr(FDC)(H2O)5·2H2O (2) were prepared hydrothermally and both adopt three-dimensional framework structures. They absorb strongly between 380 and 460 nm and show broad visible emission with peaks at 503 and 526 nm, respectively. Structure 1 shows a quantum yield at room temperature of 7.8% which increases to 15% at −196 °C, while structure 2 shows a room temperature quantum yield of 2.8%, increasing to 3.3% at −196 °C.

Synthesis of CdSe nanoparticles in the presence of aminodextran as stabilizing and capping agent

Water-dispersible Amdex–CdSe nanoparticle complexes with sufficient luminescence intensity were prepared at room temperature by rapidly mixing aqueous solutions of either sodium selenide or selenourea with those of cadmium chloride in the presence of amino-derivatized polysaccharides (Amdex) as stabilizing agent. It was shown that the size of CdSe crystallites decreased with increasing content of the polymer in the precipitation process. When present in a sufficient amount, Amdex was found to be an effective stabilizing and capping agent, producing CdSe nanocrystals of weak-to-medium luminescence intensity (maximum room temperature quantum yields of 15–16%). Furthermore, Amdex has proven to be an effective protective agent against photochemical degradation.

Photochemical Studies of CH3C(O)OONO2 (PAN) and CH3CH2C(O)OONO2 (PPN): NO3 Quantum Yields

The quantum yields for the production of NO3 in the gas-phase UV photolysis of peroxyacetyl nitrate (PAN, CH3C(O)OONO2) and peroxypropionyl nitrate (PPN, CH3CH2C(O)OONO2) at 248 and 308 nm were measured. We report room-temperature quantum yield values of 0.19 ± 0.04 and 0.41 ± 0.10 at 248 and 308 nm, respectively, for PAN and 0.22 ± 0.04 and 0.39 ± 0.04 at 248 and 308 nm, respectively, for PPN. UV absorption cross sections for PPN between 200 and 340 nm over the temperature range 296−253 K are also reported. The atmospheric implications of these results are discussed.

Colloidal YVO4:Eu and YP0.95V0.05O4:Eu Nanoparticles: Luminescence and Energy Transfer Processes

The luminescence of pure and europium-doped nanocrystalline YVO4 and YP0.95V0.05O4 and the energy transfer processes in such nanoparticles have been studied by temperature dependent luminescence spectroscopy and luminescence lifetime measurements. The results indicate thermally activated energy transfer between adjacent vanadate groups in YVO4 at temperatures above 100 K, but energy transfer to europium seems to take place from direct vanadate neighbors only. In contrast to the luminescence decay of europium, the kinetic of the vanadate luminescence strongly depends on the choice of surface capping and solvent, indicating partial quenching of the vanadate emission at surface sites. The strong competition with energy transfer to surface sites seems to be reason for the absence of energy transfer to europium from distant vanadate groups. The latter explains the low room-temperature quantum yield of 15% of YVO4:Eu colloids.

Synthesis and surface modification of amino-stabilized CdSe, CdTe and InP nanocrystals

Abstract CdSe, CdTe and InP nanocrystals were prepared by an organometallic synthesis using mixtures of highly boiling primary amines and trioctylphosphine (TOP) as the coordinating solvent, and were characterized by powder XRD, SAXS, HRTEM, absorption and luminescence spectroscopy. The use of amines allowed us to obtain small crystalline nanoparticles for all materials investigated. In all cases, as-prepared colloids show rather narrow particle size distributions which can be further improved by standard size selective precipitation. Amino-capped II–VI nanocrystals show strong size-dependent band edge photoluminescence (PL). CdSe nanocrystals with the mean particle size in the range of 1.2–3.0 nm exhibit emission from blue to green with room temperature quantum yields of 15–20%. CdTe nanocrystals (2.5–5.0 nm size range) show a PL tunable from green to red with quantum yields up to 65% at room temperature. InP nanocrystals (1.5–4.0 nm size range) possess a weak emission (

The near-uv photolysis of ozone: quantum yields of O(1D) between 305 and 329 nm at temperatures from 227–298 K, and the room temperature quantum yield of O(3P2) between 303 and 310 nm, measured by resonance enhanced multiphoton ionisation

Quantum yields Φ have been measured for the O(1D) fragment from the photolysis of ozone between 305 and 329 nm at temperatures of 298, 273, 247 and 227 K. Relative yield measurements of the atomic fragment were made by REMPI, and placed on an absolute scale by comparison with published quantum yields at 308 nm. The results confirm the existence of the long wavelength “tail” for O(1D) production beyond the 310 nm thermodynamic threshold for its formation in a spin allowed step with O2(a1Δg) as the cofragment. Contributions from both photodissociation of internally excited ozone and spin forbidden processes are apparent from the temperature and wavelength dependences of Φ. These results give support to the recently recommended values of the quantum yields in this wavelength and temperature region. Measurements over the structured Huggins band near 319 nm show evidence of an anticorrelation between the O(1D) yield and the absorption cross section, but the effect is not large enough to make significant error in the continuous function used to parametrise the quantum yield in this region. Measurements of the quantum yield of the O(3P2) fragment between 303 and 310 nm indicate that the short wavelength limiting value of the O(1D) quantum yield at room temperature lies between 0.85 and 0.9.

Synthesis and Characterization of Strongly Fluorescent CdTe Nanocrystal Colloids

ABSTRACTWe present a synthesis of colloidal CdTe nanocrystals whose absolute room temperature quantum yields are routinely above 60%. The preparation is based on the trioctylphosphine oxide (TOPO) method reported by Murray, with a more stalbe tellurium precursor now used as the chalcogenide source. The photoluminescence is continuously tunable over the range 590-760 nm and is as narrow as 135 meV (45 nm) FWHM. No deep trap luminescence is detected for the diameter range 4-11 nm. CdTe nanocrystals are characterized by UV/vis absorption, photoluminescence emission, transmission electron microscopy, and powder X-ray diffraction.

Photodegradation of .alpha.-olefin/carbon monoxide alternating copolymer

The anaerobic photodegradation behaviors of bulk alternating copolymers of propylene/carbon monoride (CO) and ethylene/CO have been studied by irradiation with 2537-A light. Photolysis of the former resulted in weight loss, molecular weight depression, cross-linking, and the formation of hydroxyl and terminal vinyl groups. The room temperature quantum yields of chain scission and Norrish type II reactions were found to be 0.015±0.002 and 0.011±0.002, respectively. Ethylene/CO alternating copolymer does not undergo Norrish type II photolysis because of the absence of γ-hydrogens. Even the Norrish type I processes occur with low probability probably because of the strong «cage effect» and reactivity of the primary radicals produced

Gas-phase organometallic catalysis: kinetics and mechanism of the hydrogenation of ethylene by Fe(CO)3(C2H4)2

Inert homogeneous gas-phase mixtures of ethylene and hydrogen plus a catalytic amount of Fe(CO)/sub 5/ are transformed into active ethylene hydrogenation systems upon irradiation by near-UV light from a pulsed nitrogen laser. Organometallic species present in the active catalytic mixture are identified and monitored by Fourier transform infrared spectroscopy. The catalyzed reaction is followed by gas chromatography, which provides a measure of ethylene and hydrogenated product concentrations. The catalytic process is efficient in its use of light, with typical room temperature quantum yields (product ethane molecules formed per photon absorbed) of 20 or more. The absorbed laser light generates a reservoir of Fe(CO)/sub 3/(C/sub 2/H/sub 4/)/sub 2/, which thermally dissociates by losing one highly labile ethylene to yield the active catalyst, Fe(CO)/sub 3/(C/sub 2/H/sub 4/). When the photolysis light is removed, catalytic activity is observed to decline as the catalyst combines with free CO to form stable Fe(CO)/sub 4/(C/sub 2/H/sub 4/). The rate of organic product formation is directly proportional to the catalysts reservoir concentration. Quantum efficiency of ethane production and the rate of Fe(CO)/sub 3/(C/sub 2/H/sub 4/)/sub 2/ decay are studied as functions of ethylene, hydrogen, CO and Fe(CO)/sub 5/ pressures. The results provide information on the mechanism ofmore » catalysis, as well as elementary rate parameters for many of the organometallic reactions.« less

ChemInform Abstract: INTERSYSTEM CROSSING IN COLD ISOLATED MOLECULES OF NAPHTHALENE

We report the results of an extensive study of the rate of radiationless decay in very cold, isolated, naphthalene molecules. The experimental method employed, namely, pulsed narrow band laser excitation of a pulsed seeded supersonic free jet, avoids the complications imposed by the sequence congestion generated by the thermal excitation characteristic of ordinary vapor samples. Our analysis of the fluorescence decay includes a method for transforming room temperature quantum yield data to quantum yields characteristic of the molecules in the supersonic free jet. It is concluded that (a) the out of plane vibration 2(b2g) is very likely a promoting mode for intersystem crossing; (b) there is no need to invoke near resonant coupling of levels in the prepared singlet and final triplet manifolds to explain features of the radiationless decay; and (c) in general, the broad features of radiationless decay in naphthalene are very much like those of the corresponding decay in benzene, and can be similarly explained.

Intersystem crossing in cold isolated molecules of naphthalene

We report the results of an extensive study of the rate of radiationless decay in very cold, isolated, naphthalene molecules. The experimental method employed, namely, pulsed narrow band laser excitation of a pulsed seeded supersonic free jet, avoids the complications imposed by the sequence congestion generated by the thermal excitation characteristic of ordinary vapor samples. Our analysis of the fluorescence decay includes a method for transforming room temperature quantum yield data to quantum yields characteristic of the molecules in the supersonic free jet. It is concluded that (a) the out of plane vibration 2(b2g) is very likely a promoting mode for intersystem crossing; (b) there is no need to invoke near resonant coupling of levels in the prepared singlet and final triplet manifolds to explain features of the radiationless decay; and (c) in general, the broad features of radiationless decay in naphthalene are very much like those of the corresponding decay in benzene, and can be similarly explained.

Photoionization of Ce3+ in Glass

The photoionization of Ce3+ is investigated by determining the optical absorption changes and the electron spin resonance changes that occur when cerium-containing silicate glasses are illuminated with ultraviolet light. The results are: (1) The room temperature quantum yield of photoelectrons from Ce3+ is 0.1. (2) The optical absorption and the electron spin resonance bands caused by trapped photoelectrons are found. (3) A Ce3+ ion from which a fourth electron has been removed by photoionization is different from a Ce4+ ion.