Steady-state Spectra Research Articles

Solvent Effect on Excited-State Intramolecular Proton-Coupled Charge Transfer Reaction in Two Seven-Membered Ring Pyrrole-Indole Hydrogen Bond Systems

In the past decades, tremendous efforts have been invested into organic molecules involved in the excited-state intramolecular proton transfer (ESIPT) reaction due to their enormously Stokes-shifted fluorescence and distinctive photophysical properties. The alterations of the environmental medium can effectively adjust the luminous performance of ESIPT molecules, which inspires us to unravel the solvent effect on the ESIPT mechanism. Here, we report the solvent-dependent excited-state properties of two new seven-membered ring pyrrole-indole ESIPT molecules, g-PPDBI and e-PPDBI, by steady-state spectra, picosecond transient fluorescence spectra, femtosecond transient absorption spectra, and theoretical calculations. The bathochromic-shifted normal fluorescence and the negligibly shifted tautomer fluorescence suggest the occurrence of an excited-state intramolecular proton-coupled charge transfer reaction. Thus, the solvent effect plays a vital role in stabilizing the intramolecular charge transferred state, resulting in a higher ESIPT reaction barrier in more polar solvents. Additionally, the observation of the slight dynamic difference between PPDBIs with different π-conjugation positions provides a new strategy to adjust the performance of ESIPT molecules.

Cosmic rays and non-thermal emission in simulated galaxies – III. Probing cosmic-ray calorimetry with radio spectra and the FIR–radio correlation

ABSTRACT An extinction-free estimator of the star formation rate (SFR) of galaxies is critical for understanding the high-redshift universe. To this end, the nearly linear, tight correlation of far-infrared (FIR), and radio luminosity of star-forming galaxies is widely used. While the FIR is linked to massive star formation, which also generates shock-accelerated cosmic-ray (CR) electrons and radio synchrotron emission, a detailed understanding of the underlying physics is still lacking. Hence, we perform three-dimensional magnetohydrodynamical (MHD) simulations of isolated galaxies over a broad range of halo masses and SFRs using the moving-mesh code arepo, and evolve the CR proton energy density self-consistently. In post-processing, we calculate the steady-state spectra of primary, shock-accelerated and secondary CR electrons, which result from hadronic CR proton interactions with the interstellar medium. The resulting total radio luminosities correlate with the FIR luminosities as observed and are dominated by primary CR electrons if we account for anisotropic CR diffusion. The increasing contribution of secondary emission up to 30 per cent in starbursts is compensated by the larger bremsstrahlung and Coulomb losses. CR electrons are in the calorimetric limit and lose most of their energy through inverse Compton interactions with star light and cosmic microwave background (CMB) photons while less energy is converted into synchrotron emission. This implies steep steady-state synchrotron spectra in starbursts. Interestingly, we find that thermal free–free emission flattens the total radio spectra at high radio frequencies and reconciles calorimetric theory with observations while free–free absorption explains the observed low-frequency flattening towards the central regions of starbursts.

Operando Spectroscopic Monitoring of Active Species in CO2 Hydrogenation at Elevated Pressure and Temperature: Steady-State versus Transient Analysis

Operando infrared spectroscopy is an invaluable tool to provide a deep insight into underlying mechanisms in heterogeneous catalysis. Recent advances in spectroscopic techniques enabled operando analysis under elevated pressure and temperature. In this study we compared a conventional steady-state IR analysis with a transient IR analysis by modulation excitation spectroscopy (MES) using a model reaction, CO2 hydrogenation to methanol at 30 bar and 250 °C. The steady-state IR spectra provide information about surface properties of catalysts while the transient IR spectra are a powerful tool to unveil the kinetics of surface species involved in catalytic cycles and therefore to determine rate-limiting step. Operando MES-IR spectroscopy evidenced that under the reaction conditions (30 bar, 250 °C) hydrogenation of surface methoxy species (CH3O−) is the rate-limiting step for the Cu–Zn based catalyst. We herein demonstrate that comparative analysis of both steady-state and transient spectra at elevated pressure provides a solid understanding of surface processes, allowing a rational catalyst design.

The Critical Number of Gold Atoms for a Metallic State Nanocluster: Resolving a Decades-Long Question.

Probing the transition from a metallic state to a molecular state in gold nanoparticles is fundamentally important for understanding the origin of surface plasmon resonance and the nature of the metallic bond. Atomically precise gold nanoclusters are desired for probing such a transition based upon a series of precise sizes with X-ray structures. While the definition of the metallic state in nanoclusters is simple, that is, when the HOMO-LUMO gap (Eg) becomes negligibly small (Eg < kBT, where kB is the Boltzmann constant and T the temperature), the experimental determination of ultrasmall Eg (e.g., of kBT level) is difficult, and the thermal excitation of valence electrons apparently comes into play in ultrasmall Eg nanoclusters. Although a sharp transition from nonmetallic Au246(SR)80 to metallic Au279(SR)84 (SR: thiolate) has been observed, there is still uncertainty about the transition region. Here, we summarize several criteria on determining the metallic state versus the molecular (or nonmetallic) state in gold nanoclusters, including (1) Eg determined by optical and electrochemical methods, (2) steady-state absorption spectra, (3) cryogenic optical spectra, (4) transient absorption spectra, (5) excited-state lifetime and power dependence, and (6) coherent oscillations in ultrafast electron dynamics. We emphasize that multiple analyses should be performed and cross-checked in practice because no single criterion is definitive. We also review the photophysics of several gold nanoclusters with nascent surface plasmon resonance. These criteria are expected to deepen the understanding of the metallic to molecular state transition of gold and other metal nanoclusters and also promote the design of functional nanomaterials and their applications.

Novel Fluorescence Guanidine Molecules for Selective Sulfate Anion Detection in Water Complex Samples over a Wide pH Range.

Quantitative analysis of sulfate anions in water still remains an important challenge for the society. Among all the methodologies, the most successful one is based on optical supramolecular receptors because the presence of small concentrations of sulfate anion modifies the photophysical properties of the receptor. In this case, fluorescence anion sensors have been designed by the incorporation of guanidine motifs into fluorenyl cores. The photophysical behaviors of the new mono- (M) and bis-guanidine (B) derivatives were studied through pH dependence, solvent effects, and ion sensing on steady-state spectra and time-resolved fluorescence spectroscopy. In more detail, the results demonstrate that M is a highly selective and sensitive sulfate ion receptor in real water samples and, even more importantly, its function remains unchanged at different ranges of pH. The reason behind this resides on the fluorescence quenching produced by an internal charge-transfer process when the sulfate anion is complexed with M. It is worth noting that the global and partial affinity constants (1010 M-2 and 105 M-1, respectively) of complex formation are far above from the current sulfate sensors in water (104 M-1) which give an LOD of 0.10 μM in water with an analytical range of 2.5-10 μM. On the other hand, although it would seem, at first sight, that the B derivate will be the most promising one, the possibility of having two simultaneous protonation states reduces the complex formation and, therefore, its sensitivity to sulfate anions. The results presented here offer the possibility of using a new molecule in water environments, which opens the door to infinite applications such as the detection of trace amounts of sulfate ions in food or water.

Vibratile Dihydrophenazines with Controllable Luminescence Enabled by Precise Regulation of π-Conjugated Wings

Vibratile Dihydrophenazines with Controllable Luminescence Enabled by Precise Regulation of π-Conjugated Wings

Interrogating the mechanism of the solvation dynamics in BmimBF4/PC mixtures: A cooperative study employing time-resolved fluorescence and molecular dynamics

In this article, the solvation in ionic liquid/molecular solvent (IL/MS) mixtures is investigated. The steady-state and time-resolved measurements of the solvation of coumarin 153 (C153) in BmimBF4/propylene carbonate are presented. A slight bathochromic shift was observed in the steady-state spectra upon increasing the IL molar fraction xIL. The time correlated photon counting TCSPC and fluorescence upconversion techniques were used to obtain the experimental solvation response functions (SRFs). The SRFs were found to be multi-exponential and the average solvation times increased with xIL. The MD simulations were performed to help the interpretation of the experimental data. An unprecedentedly good agreement between the measured and the calculated SRFs was achieved. The partition of the coulombic energy of interaction between the atoms of C153 and cations, anions and PC molecules provides a deep understanding of the time signatures of the mixture components participation in the solvation response. Indeed, the slow portion of the SRFs was found to be dominated by the IL. These time signatures change in function of xIL and represent the previously established regimes of IL/MS mixtures: electrolyte solution (xIL< 0.2) and lubricated IL (xIL> 0.2). These results will be of crucial interest for the understanding of the molecular rearrangements occurring in IL/MS electrolyte models for dye sensitized solar cells (DSSCs) after photo-excitation.

Effect of energy bandgap and sacrificial agents of cyclopentadithiophene-based polymers for enhanced photocatalytic hydrogen evolution

A library of donor-acceptor system consisting of cyclopentadithiophene-based polymer photocatalysts have been designed and synthesized. Among all photocatalysts, the active PCPDTBSO achieved hydrogen evolution rates of 24.6 mmol h–1 g–1 with apparent quantum yields of 8.7 % at 500 nm. More importantly, combined the results of photocatalytic efficiency, apparent quantum yield, the time-resolved fluorescence decay spectra, the steady-state photoluminescence spectra, and the transient absorption spectroscopy, and the oxidation potentials of sacrificial donors and protons reduction potentials in different pH values, we confirmed the concept that ascorbic acid is a suitable sacrificial donor for narrow bandgap polymers and triethylamine is a suitable sacrificial donor for wide bandgap polymers owing to the existence of the optimal thermodynamic driving force. We believed this study would be advantageous for the selection of photocatalysts and sacrificial donors for hydrogen production.

Weak magnetohydrodynamic turbulence

Low-frequency hydromagnetic turbulence is thought to play an important role in charged particle energization in space and astrophysical environments. For understanding large-scale turbulence in magnetized plasmas, low-frequency electromagnetic turbulence has been widely investigated within the theoretical framework of incompressible magnetohydrodynamic (MHD) theory. Among the existing works is the weak turbulence formalism of incompressible MHD turbulence. The present paper revisits the existing formalism under the assumption of zero residual energy. Under the strict assumption of turbulence taking place in a two-dimensional plane, which can be interpolated to a three-dimensional situation with azimuthal symmetry, the well-known steady-state turbulent spectrum of k⊥−2 is recovered, where k⊥ denotes the wave number perpendicular to the ambient magnetic field.

A novel Eu3+-doped ScCaO(BO3) red phosphor for tricolor-composited high color rendering white light

Exploration of novel red emitting phosphors containing Eu3+ ions is an everlasting topic in the field of luminescent materials due to the application requirement of warm white light-emitting diodes. In this work, the Eu3+-doped ScCaO(BO3) phosphors were synthesized though the high temperature solid-state reaction method. The phase purity and detail information on crystal structure were obtained by X-ray diffraction (XRD) and the technology of Rietveld XRD refinement. Scanning electron microscopy (SEM) was applied to check the particle morphology and EDS mapping was taken to analyze element distributions. Optical properties of the phosphors were studied by measuring the steady-state excitation and emission spectra and also the transient luminescence decays. The emission spectra of ScCaO(BO3):Eu3+ majorly consisted of 5D0-7F1, 7F2 and 7F4 transitions, covering the orange (582–605 nm), red (605–635 nm) and deep red (690–720 nm) spectral regions. The doping concentration of Eu3+ ions was optimized to 12% to maximize the optical performance of the red emitting phosphors, whose CIE coordinate and fluorescent lifetime were (0.6458, 0.3537) and 1.516 ms, respectively. The investigation of the thermal quenching behavior of the ScCaO(BO3):0.12Eu3+ phosphor reveals that the emission intensity of Eu3+ at 400 K can maintain 68.98% of that at room temperature due to the existence of the thermal activation energy of 0.258 eV. Using such a red-emitting material, the prototype warm white LED device with the CIE coordinate of (0.3262, 0.3363), color rendering index of 86.1 and correlated color temperature of 5871 K can be obtained by combining with the blue emitting BaMgAl10O17:Eu2+ and green emitting Zn2SiO4:Mn2+ phosphors and coating all three on a near-UV LED chip. All the results demonstrate that the ScCaO(BO3):0.12Eu3+ phosphor can sever as a potential red emitting candidate for near-UV excited warm WLEDs.

The effect of cavity size on ruthenium (II) tris-(2,2-bipyridine) photophysics encapsulated within zirconium based metal organic frameworks

Metal organic frameworks (MOFs) are well known for their exceptionally large surface areas and high porosity making them ideal candidates for gas storage and adsorption, drug delivery systems, and light harvesting platforms. A class of MOF’s that have gained much attention for their exceptional hydrothermal stability are the zirconium oxide MOF’s. One in particular is Uio-66 which is composed of Zr6O8 clusters connected by benzene dicarboxylate linkers. One important characteristic of the Uio-66 series of MOFs is that extension of organic linkers results in a systematic increase in cavity size while retaining the same overall framework topology. This property affords the opportunity to examine the effects of cavity size on the photophysics of encapsulated guests. Here a comparison of the photophysics of ruthenium (II) tris-(2,2′-bipyridine) (RuBpy) encapsulated within three Zr-based MOF’s Uio-66, Uio-67, and Uio-68 is presented. All three RuBpy@Uio-xx (xx = 66, 67 or 68) materials display a bathochromic shift in the steady state spectra and an emission lifetimes that fit to a biexponential decay function. Both RuBpy@Uio-66 and RuBpy@Uio-67 exhibit extended emission lifetimes which is characteristic of deactivation of the higher energy triplet ligand field state (3LF) and depopulation from a higher energy metal-to-ligand charge transfer state (3MLCT). Interestingly, RuBpy@Uio-68 exhibits a solution like emission lifetime that would be consistent with the larger cavity providing sufficient volume for access to the 3LF state. However, closer examination of the photophysics reveals that even the larger cavity imposes a significant level of confinement although lesser than that observed for the RuBpy@Uio-66 and RuBpy@Uio-67 systems.

Boosting the Energy Migration Upconversion through Inter-Shell Energy Transfer in Tb 3+ -Doped Sandwich Structured Nanocrystals

Boosting the Energy Migration Upconversion through Inter-Shell Energy Transfer in Tb ³⁺ -Doped Sandwich Structured Nanocrystals

Excimer Formation of Perylene Bisimide Dyes within Stacking-Restrained Folda-Dimers: Insight into Anomalous Temperature Responsive Dual Fluorescence

Excimer Formation of Perylene Bisimide Dyes within Stacking-Restrained Folda-Dimers: Insight into Anomalous Temperature Responsive Dual Fluorescence

Can di-4-ANEPPDHQ reveal the structural differences between nanodiscs and liposomes?

The potential-sensitive di-4-ANEPPDHQ dye is presently gaining popularity in structural studies of the lipid bilayer. Within the bilayer, dye environmental sensitivity originates from the excitation induced charge redistribution and is usually attributed to solvent relaxation. Here, di-4-ANEPPDHQ is utilized to compare the structure of neutral and negatively charged lipid bilayers between two model systems: the nanodiscs and the liposomes. Using the well-established approach of measuring solvatochromic shifts of the steady-state spectra to study the bilayer structural changes has proved insufficient in this case. By applying an in-depth analysis of time-resolved fluorescence decays and emission spectra, we distinguished and characterized two and three distinct emissive di-4-ANEPPDHQ species in the liposomes and the nanodiscs, respectively. These emissive species were ascribed to the dual emission of the dye rather than to solvent relaxation. An additional, long-lived component present in the nanodiscs was associated with a unique domain of high order, postulated recently. Our results reveal that the di-4-ANEPPDHQ steady-state fluorescence should be interpreted with caution. With the experimental approach presented here, the di-4-ANEPPDHQ sensitivity was improved. We confirmed that the bilayer structure is, indeed, altered in the nanodiscs. Moreover, molecular dynamic simulations showed a distribution of the probe in the nanodiscs plane, which is sensitive to lipid composition. In POPC nanodiscs, probe frequently interacts with MSP, while in POPC-POPG nanodiscs, such interactions are rare. We did not observe, however, any impact of those interactions on the probe fluorescence.

Cosmic rays and non-thermal emission in simulated galaxies − I. Electron and proton spectra compared to Voyager-1 data

ABSTRACT Current-day cosmic ray (CR) propagation studies use static Milky Way models and fit parametrized source distributions to data. Instead, we use three-dimensional magnetohydrodynamic (MHD) simulations of isolated galaxies with the moving-mesh code arepo that self-consistently accounts for hydrodynamic effects of CR protons. In post-processing, we calculate their steady-state spectra, taking into account all relevant loss processes. We show that this steady-state assumption is well justified in the disc and generally for regions that emit non-thermal radio and gamma rays. Additionally, we model the spectra of primary electrons, accelerated by supernova remnants, and secondary electrons and positrons produced in hadronic CR proton interactions with the gas. We find that proton spectra above 10 GeV only weakly depend on galactic radius, while they acquire a radial dependence at lower energies due to Coulomb interactions. Radiative losses steepen the spectra of primary CR electrons in the central galactic regions, while diffusive losses dominate in the outskirts. Secondary electrons exhibit a steeper spectrum than primaries because they originate from the transported steeper CR proton spectra. Consistent with Voyager-1 and AMS-02 data, our models (i) show a turnover of proton spectra below GeV energies due to Coulomb interactions so that electrons start to dominate the total particle spectra and (ii) match the shape of the positron fraction up to 10 GeV. We conclude that our steady-state CR modelling in MHD CR galaxy simulations is sufficiently realistic to capture the dominant transport effects shaping their spectra, arguing for a full MHD treatment to accurately model CR transport in the future.

Cosmic rays and non-thermal emission in simulated galaxies – II. γ-ray maps, spectra, and the far-infrared–γ-ray relation

ABSTRACT The γ-ray emission of star-forming (SF) galaxies is attributed to hadronic interactions of cosmic ray (CR) protons with the interstellar gas and contributions from CR electrons via bremsstrahlung and inverse Compton (IC) scattering. The relative importance of these processes in different galaxy types is still unclear. We model these processes in three-dimensional magnetohydrodynamical (MHD) simulations of the formation of isolated galactic discs using the moving-mesh code arepo, including dynamically coupled CR protons and adopting different CR transport models. We calculate steady-state CR spectra and also account for the emergence of secondary electrons and positrons. This allows us to produce detailed γ-ray maps, luminosities, and spectra of our simulated galaxies at different evolutionary stages. Our simulations with anisotropic CR diffusion and a low CR injection efficiency at supernovae (SNe; $\zeta_\mathrm{SN}=0.05$) can successfully reproduce the observed far-infrared (FIR)–γ-ray relation. Starburst galaxies are close to the calorimetric limit, where CR protons lose most of their energy due to hadronic interactions and hence, their γ-ray emission is dominated by neutral pion decay. However, in low SF galaxies, the increasing diffusive losses soften the CR proton spectra due to energy-dependent diffusion, and likewise steepen the pionic γ-ray spectra. In turn, IC emission hardens the total spectra and can contribute up to ∼40 per cent of the total luminosity in low SF galaxies. Furthermore, in order to match the observed γ-ray spectra of starburst galaxies, we require a weaker energy dependence of the CR diffusion coefficient, $D\propto E^{0.3}$, in comparison to Milky Way-like galaxies.

UV–Vis, Raman spectroscopic and density functional theoretical studies on microsolvation 1, 2, 4-triazole-3-thione clusters

Photophysical and photochemical reactions of microsolvation clusters are attracting an increasing attention due to their wide applications in materials science and biology. In this paper, 1, 2, 4-triazole-3-thione (3TT) is investigated in solid, protic, and aprotic solvents using FT-Raman, resonance Raman and electronic absorption spectroscopic experiments. The structures of microsolvation clusters in solvents were confirmed by 488 nm Raman spectroscopy combining with density functional theory (DFT) calculation. Steady-state absorption and resonance Raman spectra of 3TT in different environments indicate that the intermolecular hydrogen bonding may reveal important insights in the photophysical and photochemical process. With the aid of DFT and time-dependent density functional theory (TDDFT) calculations, we assigned the observed Raman spectra to the microsolvation clusters in acetonitrile, water and methanol, and carried out preliminary investigations on spectrum shifts of UV and Raman spectra due to the hydrogen bonding with the solvent molecules. The intermolecular >NH···O and >=S···H hydrogen bonding interactions, which are the key constituents of stable thione structure of 3TT, revealed the obvious spectrum shifts of 3TT, including Raman and absorption shifts in CH3CN, CH3OH and H2O. The hydrogen bond sites were further confirmed to be located on the functional group SCNH of 3TT with CH3CN, H2O and CH3OH.

Binding studies of known molecules with acetylcholinesterase and bovine serum albumin: A comparative view

The interactions between selected molecules (piperine, tacrine, curcumin and silibinin) and proteins (acetylcholinesterase and bovine serum albumin) were investigated by Fluorescence spectroscopy, molecular docking, molecular dynamics, free energy calculation and non-covalent interaction analysis. These binding characteristics are of huge interest for understanding pharmacokinetic mechanism of the target molecules. The steady-state emission spectrum results showed that presence of static quenching mode for piperine, tacrine, curcumin, silibinin molecules with BSA and AChE complexes separately and this excitation-emission matrix analysis suggest that formation of ground-state complex between piperine, tacrine, curcumin, silibinin drugs and both BSA, AChE protein molecules. And, the binding model from molecular docking analysis of both BSA and AChE with these molecules clearly displayed non-covalent interactions (hydrogen bonding and hydrophobic interactions) which played a significant role in the binding mechanism. Further, the protein-ligand complexes are subjected to molecular dynamics and binding free energy calculation to confirm the stability of the molecule in the active site of BSA and AChE. The NCI (non-covalent interaction) approach supports to visualize the iso-surface of the reduced density gradient of such interactions between protein and ligands.

3D hollow MXene@ZnIn2S4 heterojunction with rich zinc vacancies for highly efficient visible-light photocatalytic reduction

Well-designed heterojunction semicounductor coupled with high-conductive cocatalyst can obtain boosted photocatalytic activity. Herein, a novel three-dimensional (3D) hollow heterojunction was prepared by coating the indium zinc sulfide (ZnIn2S4) nanosheets with rich-zinc vacancies (VZn) on 3D hollow titanium carbide (Ti3C2). The obtained 3D hollow heterojunction (Ti3C2@ZnIn2S4) achieved effective optical collection and promoted the separation and transmission of photogenerated carriers as well as the surface reaction of spatial separation. In addition, time-resolved photoluminescence and steady-state photoluminescence spectra indicated that the existence of VZn and the introduction of hollow Ti3C2 spherical shell effectively inhibited the recombination of photogenerated carriers and accelerated their separation and transmission, thus further enhancing the photocatalytic activity. In addition, the introduction of 3D hollow Ti3C2 benefited a larger specific surface area for heavy metal adsorption. Due to the unique structural and compositional characteristics, the heterojunction showed high efficiency of Cr(VI) reduction under visible light. In particular, the optimal Ti3C2@ZnIn2S4 heterojunction (1%-Ti3C2@ZnIn2S4) achieved 100% removal of Cr(VI) within 25 min, with a reaction rate constant of 0.225, which was 8.5 times higher than that of the pristine ZnIn2S4. The superior reusability and structural stability further indicated the MXene-based novel photocatalyst is promising for application in environmental remediation.

Multistimuli-Responsive Fluorescent Organometallic Assemblies Based on Mesoionic Carbene-Decorated Tetraphenylethene Ligands and Their Applications in Cell Imaging

Multistimuli-Responsive Fluorescent Organometallic Assemblies Based on Mesoionic Carbene-Decorated Tetraphenylethene Ligands and Their Applications in Cell Imaging