Emission Spectral Shapes Research Articles

Blue Light-Emitting Diodes Based on Pure Bromide Perovskites.

Blue perovskite light-emitting diodes (LEDs) are essential for the creation of full-color displays and white-light illumination, and some significant progress is made in recent years. However, most high-performance blue perovskite LEDs are currently based on mixed-halide perovskites and suffer from unstable spectra due to inevitable halide phase segregation, which is unfavorable for the application of blue perovskite LEDs. In contrast, blue emissions from pure bromide perovskites generally exhibit stable spectra (consistent emission peak positions and spectral shapes) and are worthy of attention. In this review, the recent advances in blue LEDs based on pure bromide perovskites according to different strategies are classified and summarized. Moreover, the challenges related to poor charge injection, high defect-state density, lack of high-performance in the deeper blue region, and inferior operational stability are addressed. Finally, an outlook is provided on feasible future research directions for highly bright, efficient, and stable blue perovskite LEDs.

Explicit Modelling of Spectral Bandshapes by a Mixed Quantum-Classical Approach: Solvent Order and Temperature Effects in the Optical Spectra of Distyrylbenzene.

The absorption and emission spectral shapes of a flexible organic probe, the distyrylbenzene (DSB) dye, are simulated accounting for the effect of different environments of increasing complexity, ranging from a homogeneous, low-molecular- weight solvent, to a long-chain alkane, and, eventually, a channel-forming organic matrix. Each embedding is treated explicitly, adopting a mixed quantum-classical approach, the Adiabatic Molecular Dynamics - generalized vertical Hessian (Ad-MD|gVH) model, which allows a direct simulation of the environment-induced constraining effects on the vibronic spectral shapes. In such a theoretical framework, the stiff modes of the dye are described at a quantum level within the harmonic approximation, including Duschinsky mixing effects, while flexible degrees of freedom of the solute (e. g. torsions) and those of the solvent are treated classically by means of molecular dynamics sampling. Such a setup is shown to reproduce the distinct effects exerted by the different environments in varied thermodynamic conditions. Besides allowing for a first-principles rationale on the supramolecular mechanism leading to the experimental spectral features, this result represents the first successful application of the Ad-MD|gVH method to complex embeddings and supports its potential application to other heterogeneous environments, such as for instance, pigment-protein complexes or organic dyes adsorbed into metal-organic frameworks.

Non-Phenomenological Description of the Time-Resolved Emission in Solution with Quantum-Classical Vibronic Approaches-Application to Coumarin C153 in Methanol.

We report a joint experimental and theoretical work on the steady-state spectroscopy and time-resolved emission of the coumarin C153 dye in methanol. The lowest energy excited state of this molecule is characterized by an intramolecular charge transfer thus leading to remarkable shifts of the time-resolved emission spectra, dictated by the methanol reorganization dynamics. We selected this system as a prototypical test case for the first application of a novel computational protocol aimed at the prediction of transient emission spectral shapes, including both vibronic and solvent effects, without applying any phenomenological broadening. It combines a recently developed quantum-classical approach, the adiabatic molecular dynamics generalized vertical Hessian method (Ad-MD|gVH), with nonequilibrium molecular dynamics simulations. For the steady-state spectra we show that the Ad-MD|gVH approach is able to reproduce quite accurately the spectral shapes and the Stokes shift, while a ∼0.15 eV error is found on the prediction of the solvent shift going from gas phase to methanol. The spectral shape of the time-resolved emission signals is, overall, well reproduced, although the simulated spectra are slightly too broad and asymmetric at low energies with respect to experiments. As far as the spectral shift is concerned, the calculated spectra from 4 ps to 100 ps are in excellent agreement with experiments, correctly predicting the end of the solvent reorganization after about 20 ps. On the other hand, before 4 ps solvent dynamics is predicted to be too fast in the simulations and, in the sub-ps timescale, the uncertainty due to the experimental time resolution (300 fs) makes the comparison less straightforward. Finally, analysis of the reorganization of the first solvation shell surrounding the excited solute, based on atomic radial distribution functions and orientational correlations, indicates a fast solvent response (≈100 fs) characterized by the strengthening of the carbonyl-methanol hydrogen bond interactions, followed by the solvent reorientation, occurring on the ps timescale, to maximize local dipolar interactions.

Electronic and Optical Properties of Eu2+-Activated Narrow-Band Phosphors for Phosphor-Converted Light-Emitting Diode Applications: Insights from a Theoretical Spectroscopy Perspective.

In this work, we present a computational protocol that is able to predict the experimental absorption and emission spectral shapes of Eu2+-doped phosphors. The protocol is based on time-dependent density functional theory and operates in conjunction with an excited-state dynamics approach. It is demonstrated that across the study set consisting of representative examples of nitride, oxo-nitride, and oxide Eu2+-doped phosphors, the energy distribution and the band shape of the emission spectrum are related to the nature of the 4f–5d transitions that are probed in the absorption process. Since the 4f orbitals are very nearly nonbonding, the decisive quantity is the covalency of the 5d acceptor orbitals that become populated in the electronically excited state that leads to emission. The stronger the (anti) bonding interaction between the lanthanide and the ligands is in the excited state, the larger will be the excited state distortion. Consequently, the corresponding emission will get broader due to the vibronic progression that is induced by the structural distortion. In addition, the energy separation of the absorption bands that are dominated by states with valence 4f–5d and a metal to ligand charge transfer character defines a measure for the thermal quenching of the studied Eu2+-doped phosphors. Based on this analysis, simple descriptors are identified that show a strong correlation with the energy position and bandwidth of the experimental emission bands without the need for elaborate calculations. Overall, we believe that this study serves as an important reference for designing new Eu2+-doped phosphors with desired photoluminescence properties.

Nitrogen-Doped Carbon Dot and CdTe Quantum Dot Dual-Color Multifunctional Fluorescent Sensing Platform: Sensing Behavior and Glucose and pH Detection.

A novel fluorescent probe based on a nitrogen-doped carbon dot (N-CD) and CdTe quantum dot (CdTe QD) platform has been constructed for H2O2/glucose detection and pH sensing. In this work, H2O2-tolerant blue fluorescence N-CDs were added to the H2O2-mediated yellow fluorescence quenching of CdTe QDs to construct a dual-color ratiometric fluorescent H2O2 probe. H2O2-induced passivated group detachment and action on deep nanocrystals promoted CdTe QD fluorescence quenching. Meanwhile, the addition of the blue fluorescent background of N-CDs sharply reflected the color change in CdTe QDs. Under the optimized experimental conditions, the platform was effectively applied to the detection of H2O2 produced by the enzymatic reaction of glucose, showing high sensitivity (limit of detection 7.86 μM) and wide linear range (26-900 μM) for glucose detection. The pH-sensing behavior of CdTe QDs and N-CDs was attributed to the displacement of a weak acid (3-mercaptopropionic acid) by a strong acid (HCl) and the acid titration process of two coexisting bases (N-CDs and NH3·H2O), respectively. The loss of passivation and doping effects led to a decrease in the fluorescence intensity of CdTe QDs and N-CDs. Moreover, utilizing the ability of bimaterial system fluorescence to pH sensing, a semiquantitative pH detection based on the linear response was developed. The pH range was analyzed by three kinds of N-CD (Fex = 440 nm) and CdTe QD (Fex = 548 nm) typical emission spectral shapes. In addition, the recovery results showed that the bimaterial system was proved to be appropriate for the assay of glucose in spiked serum samples.

Vibrationally resolved absorption and emission spectral shapes of one 5-carbohelicene derivative: A theoretical study

In this work, we present a theoretical study on the vibrationally resolved absorption (ABS) and emission (EMI) spectra of a methoxy-substituted 5-carbohelicene (MeO-HeliIm). To analyze the optical properties of MeO-HeliIm from theoretical perspective, herein, we adopt different computational methods and theoretical models, accounting for Franck-Condon (FC), Herzberg-Teller (HT), temperature effects and Duschinsky mixing. Therefore, time-independent (TI) and time-dependent (TD) approaches have been used to compute the vibronic spectra, Adiabatic Hessian (AH) and Vertical Hessian (VH) models have been applied to build the PESs, HTi and HTf have been compared to check the accuracy of linear approximation of dipole moment. The different methods and models present with similar results, all in nice agreement with the experiment. This confirms the reliability and accuracy of our theory and results. We show that the temperature effect leads to a reduced resolution and a broader spectral width. FC plays a dominant role in reproducing the spectral shape, relative height of different bands and spectral width. However, HT causes a broader spectral width to EMI. Detailed analysis by comparing among the results obtained by AH, Adiabatic Shift and Frequencies (ASF), and Adiabatic Shift (AS) models, we find that the slight overestimation of Duschinsky enlarges the spectral width.

Optimizing Model Calibrations for Natural Product Chemical Compositions with Absorbance-Transmittance Excitation-Emission (A-TEEM) Spectroscopy

The food, beverage and natural supplements industries face many challenges relating to quality control as influenced by natural product variation, formulation errors and product adulteration. Conventional chromatographic and spectrophotometric assays can be time-consuming, costly and insensitive to key quality parameters and adulterants. Thus, there is a recognized need for a rapid, sensitive, non-destructive optical technique. This study investigates optimization of multivariate calibrations of chemical product compositions analyzed using A-TEEM spectroscopy. The A-TEEM method provides rapid (s-min) analyses of all chromophoric and fluorescent compounds in the UV-VIS range with micro- to sub-microgram/L sensitivity to many aromatic compounds. Conventional fluorescence EEM analyses primarily focus only on the inner-filter-effect corrected fluorescence data. In this paper, we systematically investigated the statistical significance of evaluating the separate absorbance, EEM and combined multi-block absorbance and EEM data variable sets. We report that there is a consistent improvement for the majority of A-TEEM models using ‘multi-block’ data organization compared to the separate absorbance or EEM variable data sets. We attribute the increased statistical advantage to the fact that most chemicals, even with similar structures, in a given solvent exhibit unique molar extinction coefficients, fluorescence quantum efficiencies as well as absorbance- and fluorescence excitation and emission spectral shapes. The A-TEEM data analyses further compared the Extreme Gradient Boosting (XGB) algorithm for both discrimination and regression to other comparable methods including Partial-Least Squares and Support Vector Machine. The conclusion, based on a multi-instrument comparison, was that the XGB analyses of the multi-block A-TEEM data lead to the most effective validation for both discrimination and regression models of food, beverage and natural supplements chemical composition and adulteration.

Studies of Thallium Line Spectra in Thallium – Mercury Discharge

In this paper, we present a study of broadening of thallium emission spectral line shapes in the Tl-Hg discharge. The spectral lines were emitted from high frequency electrodeless lamps, containing Tl, Hg, Ar mixtures and measured by means of Fourier transform spectrometer. The deconvolution procedure, by means of ill posed inverse task solution, was performed to obtain the real (without instrumental function) profiles for further analyze. The solution was implemented using Tikhonov regularization algorithm. The Tl 276.8 nm, 351.9 nm, 352.9 nm spectral lines were analyzed in detail in dependence on the discharge power. The additional broadening of Tl 276.8 nm and 351.9 nm lines were observed due to the excitation transfer in collisions of ground state Tl atoms with excited Hg atoms.

Phase evolution and photoluminescence in Er3+ doped glass ceramics containing lutetium oxyfluoride nanocrystals

Crystalline phase evolution through merely adjusting composition was achieved in silicate glass ceramics containing LunOn-1Fn+2 (n = 5–10) nanocrystals. Orthorhombic or cubic phase nanocrystals were precipitated in the aluminosilicate glass matrix after thermal treatment together with varying the Na2O/NaF ratio. Oxyfluoride nanocrystals with quasi-spherical shape show homogenous and dense distribution in glass matrix by transmission electron microscopy measurement. Intense upconversion and mid-infrared emissions were realized in these glass ceramics compared to the precursor glass, and the emission spectral shapes, relative emission intensity and fluorescence decay curves of Er3+ in cubic LuOF embedded samples exhibit remarkable differences due to the crystal phase dependent effect in glass ceramics. These results indicate that the crystallization and luminescence properties of oxyfluoride glass ceramics could be modified through the alteration of glass composition, which could be used for the development of novel glass ceramics and design of luminescent properties.

(Invited) High-Yield and Long-Lived Triplet Excited States of Pentacene Alkanethiolate Monolayer Protected Gold Nanoparticles By Singlet Fission

Photochemical conversion to high-yield and log-lived triplet excited states of organic molecules is an interesting research content. For example, three-dimensional self-assembled monolayers (SAMs) of organic dyes on metal nanoparticles (MNP) are highly promising as photofunctional nanomaterials. A major research topic is light energy conversion via photoinduced processes such as electron and energy transfer. The other application is optical molecular rulers (nanoruler) in the biological fields. In these cases, a serious problem of organic dye-modified SAMs on MNP is the significant deactivation pathway of the excited states of dyes caused mainly by the fast nanosurface energy transfer (NSET). Actually, more than 90% of fluorescence is quenched and formation of the excited triplet states is negligible. Therefore, the formation of high-yield and long-lived triplet excited states on MNP is favorable for various applications relaying on photoinduced processes. Generation of efficient singlet fission (SF) on gold nanoparticles (GNP) is a promising method to overcome the above-mentioned problem. SF is described as a spin-allowed process in which one singlet exciton splits into two triplet excitons.1 In our study, TIPS-pentacene-alkanethiolate monolayer-protected gold nanoparticles with different chain lengths and particle sizes were successfully synthesized to efficiently generate excited triplet states by singlet fission (Fig. 1). Time-resolved spectroscopy revealed high-yield excited triplet states by suppressing ET to the gold surface.2 The mean diameters (R CORE) of TP-C11-S-MPC and TP-C11-L-MPC are 1.65 ± 0.30 nm and 2.13 ± 0.33 nm, respectively. The core of TP-C11-S-MPC and TP-C11-L-MPC contains 158 and 338 Au atoms, respectively. Moreover, based on the values of elemental analysis, there are 51 and 75 TP molecules on one GNP. In addition, the coverage ratios of TP units (γ) to surface Au atoms in TP-C11-S-MPC and TP-C11-L-MPC are 63% and 51%, respectively. Steady-state absorption and fluorescence spectra of TP-Cn-X-MPCs and TP-Ref were measured in toluene. In absorption spectra (Fig. 2A), the spectral shape of TP-C11-S-MPC (spectrum a) matches well with that of TP-Ref (spectrum b).The fluorescence of TP-C11-S-MPC was strongly quenched relative to TP-Ref. To compare the emission spectral shapes, the fluorescence spectra of TP-C11-S-MPC (spectrum a) and TP-Ref (spectrum b) were normalized in Fig. 2B. The fluorescence excitation spectrum was observed (spectrum c in Fig. 2A) to carefully check the fluorescence species of TP-C11-S-MPC. Then, Fig. 2C shows the femtosecond transient spectra of TP-C11-S-MPC. After laser pulse excitation, the triplet–triplet absorption of TP units at ca. 520 nm develops, whereas the singlet–singlet absorption at ca. 630 nm decays. The maximum Φ Τ values attained 172 ± 26% and 157 ± 17% in TP-C11-S-MPC and TP-C7-S-MPC, respectively. [1] Hasobe, T. et al, J. Phys. Chem. A 2016, 120, 1867. [2] Hasobe, T. et al, Angew. Chem. Int. Ed. 2016, 55, 5230. Figure 1

Free Carrier Distribution Criterion in Quantum Dot Lasers

The spontaneous emission spectra of a 1.28μm InAs/GaAs QD (Quantum Dot) Fabry-Pérot laser device has been measured under continuous wave operation at a fixed junction temperature of 300K. At low carrier densities, empirically observed static peak wavelength position and a fixed spectral shape of the spontaneous emission spectra are indicative of the random-like population distribution rather than a global Fermi level in the system. A theoretical model based on the Monte-Carlo method has been shown to have good agreement with the empirical results. In addition the evolutions of spontaneous emission spectral shapes are also explained in terms of many body effects.

Absorption and emission spectral shapes of a prototype dye in water by combining classical/dynamical and quantum/static approaches.

We study the absorption and emission electronic spectra in an aqueous solution of N-methyl-6-oxyquinolinium betaine (MQ), an interesting dye characterized by a large change of polarity and H-bond ability between the ground (S0) and the excited (S1) states. To that end we compare alternative approaches based either on explicit solvent models and density functional theory (DFT)/molecular-mechanics (MM) calculations or on DFT calculations on clusters models embedded in a polarizable continuum (PCM). In the first approach (ClMD), the spectrum is computed according to the classical Franck-Condon principle, from the dispersion of the time-dependent (TD)-DFT vertical transitions at selected snapshots of molecular dynamics (MD) on the initial state. In the cluster model (Qst) the spectrum is simulated by computing the quantum vibronic structure, estimating the inhomogeneous broadening from state-specific TD-DFT/PCM solvent reorganization energies. While both approaches provide absorption and emission spectral shapes in nice agreement with experiment, the Stokes shift is perfectly reproduced by Qst calculations if S0 and S1 clusters are selected on the grounds of the MD trajectory. Furthermore, Qst spectra better fit the experimental line shape, mostly in absorption. Comparison of the predictions of the two approaches is very instructive: the positions of Qst and ClMD spectra are shifted due to the different solvent models and the ClMD spectra are narrower than the Qst ones, because MD underestimates the width of the vibrational density of states of the high-frequency modes coupled to the electronic transition. On the other hand, both Qst and ClMD approaches highlight that the solvent has multiple and potentially opposite effects on the spectral width, so that the broadening due to solute-solvent vibrations and electrostatic interaction with bulk solvent is (partially) counterbalanced by a narrowing of the contribution due to the solute vibrational modes. Qst analysis evidences a pure quantum broadening effect of the spectra in water due to vibronic progressions along the solute/solvent H-bonds.

Quantum-classical calculation of the absorption and emission spectral shapes of oligothiophenes at low and room temperature by first-principle calculations.

We report a thorough computational characterization of the low- and room-temperature absorption and emission spectra of a series of oligothiophenes that contain between three and seven thiophene units. Our computational approach is based on time-dependent (TD) density functional calculations with the CAM-B3LYP functional. The effect of vibrations is included without resorting to any empirical parameters either at a fully quantum level or with a hybrid quantum-classical protocol. This latter approach is introduced to describe the relevant broadening effects in absorption at room temperature and is based on the partition of the vibrational modes into two sets: the inter-ring torsions treated at the anharmonic level in a classical way and the remaining modes described at the quantum level. The contribution of the quantum modes to the spectrum is computed by using a harmonic approximation, which accounts for Duschinsky mixing and changes in the vibrational frequencies associated with the electronic transition; a path-integral TD approach is adopted to account for the effect of temperature. The spectra simulated at low temperatures are in very good agreement with their experimental counterparts, which indicates that our calculations can quantitatively reproduce the effect of chain lengthening on the position and the shape of the spectra. Good agreement is also obtained at room temperature, for which we show that the classical description of the broadening, owing to the inter-ring torsions, reproduces the loss of the vibronic structure observed in the experiment and introduces only a slight overestimation of the spectral width.

Excited-State Characteristics of Tetracyanidonitridorhenium(V) and -technetium(V) Complexes with N-Heteroaromatic Ligands

Six-coordinate tetracyanidonitridorhenium(V) and -technetium(V) with axial N-heteroaromatic ligands, (PPh4)2[MN(CN)4L] [M = Re, L = 4-(dimethylamino)pyridine (dmap), 3,5-lutidine (lut), 4-picoline (pic), 4-phenylpyridine (ppy), pyridine (py), 3-benzoylpyridine (3bzpy), 4,4'-bipyridine (bpy), pyrazine (pz), 4-cyanopyridine (cpy), or 4-benzoylpyridine (4bzpy); M = Tc, L = dmap, lut, pic, py, pz, or cpy] were synthesized and characterized. The crystal structures of 11 complexes were determined by single-crystal X-ray analysis. All of the complexes showed photoluminescence in the crystalline phase at room temperature. The emission maximum wavelengths (λ(em)) of the rhenium complexes with dmap, lut, pic, ppy, or py were similar to one another with a quite high emission quantum yield (Φ(em)): λ(em) = 539-545 nm, Φ(em) = 0.39-0.93, and emission lifetime (τ(em)) = 10-45 μs at 296 K. The emission spectra at 77 K exhibited vibronic progressions, and the emissive excited state is characterized as (3)[(d(xy))(1)(dπ*)(1)] (dπ* = d(xz), d(yz)). On the other hand, the emission maximum wavelength of the rhenium complex with 3bzpy, bpy, pz, cpy, or 4bzpy was significantly dependent on the nature of the axial ligand in the crystalline phase: λ(em) = 564-669 nm, Φ(em) ≤ 0.01-0.36, and τ(em) = 0.03-13.3 μs at 296 K. The emission spectra at 77 K in the crystalline phase did not show vibronic progressions. The emissive excited state of the rhenium complex with bpy, pz, cpy, or 4bzpy is assignable to originate from the metal-to-N-heteroaromatic ligand charge-transfer (MLCT)-type emission with a spin-triplet type. The change in the excited-state characteristics of rhenium complexes by the N-heteroaromatic ligand is a result of stabilization of the π* orbital of the N- heteroaromatic ligand to a lower energy level than the dπ* orbitals. The emission spectral shapes of technetium complexes were almost independent of the nature of the N-heteroaromatic ligand with λ(em) = 574-581 nm at room temperature. The different emission characteristics between the pz and cpy coordinate rhenium complexes and the technetium analogues would be due to stabilization of technetium-centered orbitals compared with the rhenium ones in energy.

Computational Modeling of the Triplet Metal-to-Ligand Charge-Transfer Excited-State Structures of Mono-Bipyridine–Ruthenium(II) Complexes and Comparisons to their 77 K Emission Band Shapes

A computational approach for calculating the distortions in the lowest energy triplet metal to ligand charge-transfer ((3)MLCT = T(0)) excited states of ruthenium(II)-bipyridine (Ru-bpy) complexes is used to account for the patterns of large variations in vibronic sideband amplitudes found in the experimental 77 K emission spectra of complexes with different ancillary ligands (L). Monobipyridine, [Ru(L)(4)bpy](m+) complexes are targeted to simplify analysis. The range of known emission energies for this class of complexes is expanded with the 77 K spectra of the complexes with (L)(4) = bis-acetonylacetonate (emission onset at about 12,000 cm(-1)) and 1,4,8,11-tetrathiacyclotetradecane and tetrakis-acetonitrile (emission onsets at about 21,000 cm(-1)); no vibronic sidebands are resolved for the first of these, but they dominate the spectra of the last two. The computational modeling of excited-state distortions within a Franck-Condon approximation indicates that there are more than a dozen important distortion modes including metal-ligand modes (low frequency; lf) as well as predominately bpy modes (medium frequency; mf), and it simulates the observed 77 K emission spectral band shapes of selected complexes very well. This modeling shows that the relative importance of the mf modes increases very strongly as the T(0) energy increases. Furthermore, the calculated metal-centered SOMOs show a substantial bpy-π-orbital contribution for the complexes with the highest energy T(0). These features are attributed to configurational mixing between the diabatic MLCT and the bpy (3)ππ* excited states at the highest T(0) energies.

Observation of the chemiluminescent NO + O → NO2 + hν reaction in the upper mesospheric dark polar regions by OSIRIS on Odin

The visible and near infrared continuum spectrum produced by the NO + O → NO2 + hv chemiluminescent reaction has been detected in the upper mesospheric dark polar regions by OSIRIS on the Odin spacecraft. Averaged observed NO2 emission spectral shapes are obtained by spectrally resolving the NO + O continuum from the blended strong upper mesospheric OH vibration–rotation airglow bands. The observed continuum spectral shape when compared with laboratory measurements is shifted to lower wavelengths by approximately 20 nm in the steeply sloped 400 to 500 nm region. The observed laboratory continuum spectral shape for upper mesospheric ambient pressure is presented for reference over the 400 to 800 nm region. An example of an NO2 continuum volume emission-rate altitude profile derived from a single OSIRIS limb scan is also included. Limb radiances up to 3 × 109 photons cm–2 nm–1 s–1 are observed at the peak of the NO2 continuum corresponding to total volume emission rates of approximately 2 × 104 photons cm–3 s–1. Data extracted from numerous single-volume emission-rate altitude profiles obtained over approximately a 24 h period are assembled into a Southern Hemisphere polar map of the 90 km NO2 continuum emission. The map illustrates the considerable spatial brightness variation typically observed in the dark Antarctic polar region throughout the OSIRIS mission dataset. After further analysis these measurements will assist in quantifying the role of thermospheric formed NOx in the catalytic removal of ozone in the upper stratosphere.

Enhanced Lasing Properties of Dissymmetric Eu(III) Complex with Bidentate Phosphine Ligands

The luminescent and lasing properties of Eu(III) complexes were enhanced by using an dissymmetric Eu(III) complex. The photophysical properties (the emission spectral shapes, the emission lifetimes, the emission quantum yields, and the stimulated emission cross section (SEC)) were found to be dependent on the geometrical structures of Eu(III) complexes. The geometrical structures of Eu(III) complexes were determined by X-ray single crystal analyses. The symmetrical group of Eu(hfa)3(BIPHEPO) (tris(hexafluoroacetylacetonato)europium(III) 1,1'-biphenyl-2,2'-diylbis(diphenylphosphine oxide)) was found to be C1, which was more dissymmetric than Eu(hfa)3(TPPO)2 (tris(hexafluoroacetylacetonato)europium(III) 1,2-phenylenebis(diphenylphosphine oxide): C2 symmetry) and Eu(hfa)3(OPPO)2 (tris(hexafluoroacetylacetonato)europium(III) 1,2-phenylenebis(diphenylphosphine oxide): C2 symmetry). The analytical data were supported by Judd-Ofelt analysis. The most dissymmetrical Eu(III) complex, Eu(hfa)3(BIPHEPO), showed large electron transition probability and large SEC (4.64 x 10(-20) cm2). The SEC of Eu(hfa)3(BIPHEPO) was superior to even the values of Nd-glass laser for practical use (1.6-4.5 x 10(-20) cm2). The lasing properties of Eu(III) complexes in polymer thin film were measured by photopumping of a Nd:YAG laser (355 nm). The threshold energy of lasing oscillation was found to be 0.05 mJ. The increasing rate of the lasing intensity of Eu(hfa)3(BIPHEPO) as a function of the excitation energy was much larger than that of Eu(hfa)3(TPPO)2 and Eu(hfa)3(OPPO)2. The dissymmetrical structure of Eu(hfa)3(BIPHEPO) promoted the enhancement of the lasing property.

Inverse Compton e pair cascade model for the -ray production in massive binary LSI +61 303

We apply an inverse Compton e ± pair cascade model for γ-ray production in the massive binary system LSI +61° 303 assuming that electrons are accelerated already inside the inner part of the jet launched by the compact object. γ-ray spectra, affected by the cascade process, and lower energy spectra, from the synchrotron cooling of the highest energy electrons in the jet, are calculated as a function of the phase of this binary system. γ-ray spectra expected in such a model have different shape than those ones produced by electrons in the jet directly to observer. Moreover, the model predicts clear anticorrelation between γ-ray fluxes in the GeV (1-10 GeV) and TeV (>200 GeV) energy ranges with the peak of the TeV emission at the phase ∼0.5 (the peak half-width ranges between the phases ∼0.4-0.9, for the inclination of the binary system equal to 60°, and ∼0.4-0.1 for 30°). The fine features of TeV γ-ray emission (fluxes and spectral shapes) as a function of the phase of the binary system are consistent with recent observations reported by the MAGIC collaboration. Future simultaneous observations in the GeV energies (by the GLAST and AGILE telescopes) and in the TeV energies (by the MAGIC and VERITAS telescopes) should test other predictions of the considered model supporting or disproving the hypothesis of acceleration of electrons already in the inner part of the microquasar jets.

Observations concerning light promoted electronic delocalization in covalently linked transition metal complexes

The emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that Ru(II)/Ru(III) electronic coupling is weaker in their lowest energy metal to ligand charge transfer (MLCT) excited states than in their corresponding mixed valence ground states. In general, the amplitudes of the vibronic contributions to emission band shapes decrease markedly with the excited state-ground state energy differences, and it is expected that complexes with degenerate, or mixed valence excited states will have very weak vibronic side bands if configurational mixing of the degenerate MLCT excited states is substantial. However, the bimetallic PP-bridged ruthenium complexes emit at significantly lower energy than their monometallic analogs, but the vibronic contributions to their 77 K emission spectra are very similar to those of their monometallic complexes analogs. This indicates that the mixed valence excited states of the bimetallic complexes are electronically localized.

Emission band shape probes of the mixed-valence excited state properties of polypyridyl-bridged bis-ruthenium(II) complexes

The absorption spectra and emission spectral band shapes of several polypyridine-ligand (PP) bridged bis-ruthenium(II) complexes imply that the Ru(II)/Ru(III) electronic coupling is weak in their lowest energy metal to ligand charge transfer (MLCT) excited states. Many of these PP-bridging ligands contain pyrazine moieties and the weak electronic coupling of the excited states contrasts to the strong electronic coupling inferred for the correlated mixed-valence ground states. Although the bimetallic complexes emit at significantly lower energy than their monometallic analogs, the vibronic contributions to their 77 K emission spectra are much stronger than expected based on comparison to the monometallic analogs (around twofold in some complexes) and this feature is characteristic of bimetallic complexes in which the mixed-valence excited states are electronically localized. The weaker excited state than ground state donor/acceptor electronic coupling in this class of complexes is attributed to PP-mediated super-exchange coupling in which the mediating orbital of the bridging ligand (PP-LUMO) is partly occupied in the MLCT excited states, but is unoccupied in the ground states; therefore, the vertical Ru(III)-PP − (MLCT) energy is larger and the mixing coefficient smaller in these excited states than is found for Ru(II)-PP in the corresponding ground states.