Structure Of Excited States Research Articles

Antivitamins B12 in a Microdrop: The Excited-State Structure of a Precious Sample Using Transient Polarized X-ray Absorption Near-Edge Structure.

Polarized transient X-ray absorption near-edge structure (XANES) was used to probe the excited-state structure of a photostable B12 antivitamin (Coβ-2-(2,4-difluorophenyl)-ethynylcobalamin, F2PhEtyCbl). A drop-on-demand delivery system synchronized to the LCLS X-ray free electron laser pulses was implemented and used to measure the XANES difference spectrum 12 ps following excitation, exposing only ∼45 μL of sample. Unlike cyanocobalamin (CNCbl), where the Co-C bond expands 15-20%, the excited state of F2PhEtyCbl is characterized by little change in the Co-C bond, suggesting that the acetylide linkage raises the barrier for expansion of the Co-C bond. In contrast, the lower axial Co-NDMB bond is elongated in the excited state of F2PhEtyCbl by ca. 10% or more, comparable to the 10% elongation observed for Co-NDMB in CNCbl.

Adsorption, Chemical Enhancement, and Low-Lying Excited States of p-Methylbenzenethiol on Silver and Gold Nanoparticle Surfaces: A Surface Enhanced Raman Spectroscopy and Density Functional Theory Study

Adsorption and chemical enhancement of p-methylbenzenethiol (PMBT) on silver and gold nanoparticle surfaces have been studied using surface enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculations. In normal Raman spectra, the Raman intensity of the molecule is sensitive to methyl substitution at the para position. DFT calculations for the Raman spectrum of PMBT reproduces well the Raman spectrum in nonpolar solution relative to PMBT in powder. This accords with the order of the PMBT molecules in the solid. The SERS results of PMBT adsorbed on Au and Ag nanoparticles indicate that the Raman intensity in the low-wavenumber region increases with increasing excitation wavelength. The electronic structures of low-lying excited states have been explored for this increase in different PMBT-S-metal cluster complexes. DFT results indicate that low-energy excited states are in fact present and originate from two types of excitations, one localized at the sulfur–silver/gold bonding region and another one from a charge transfer state excited from PMBT to the silver and gold surfaces. Both interfacial excited states contribute significantly to the chemical enhancement mechanism and change relative Raman intensities of adsorbed PMBT. The chemical bonding interaction and the interfacial energy level alignment are therefore important to understand SERS processes of PMBT adsorbed on noble metal surfaces of nanostructures.

Simulation of the Inelastic Alpha-Particle Scattering Reaction for Investigating the Cluster Structure of Excited States of 9Be

It is proposed to investigate inelastic scattering of alpha particles on the clusters of the $$^{9}{\text{Be}}$$ nucleus for determining the contribution from the cluster configurations ( $$^{8}{\text{Be}} + n$$ , $$\alpha + \alpha + n$$ , $$^{5}{\text{He}} + \alpha $$ ) to the $$^{9}{\text{Be}}$$ structure. The kinematic simulation of the $$^{9}{\text{Be}}(\alpha ,\alpha {\kern 1pt} 'n)X$$ and $$^{9}{\text{Be}}(\alpha ,\alpha {\kern 1pt} '\alpha )X$$ reactions shows that detection of the scattered alpha particle in coincidence with the particle from the breakup of a certain excited state of $$^{9}{\text{Be}}$$ allows determining the contribution from different cluster configurations to the structure of those states.

Radiative and non-radiative decay kinetics of (CdSe)N (N = 3 and 4) clusters

Photoluminescence intermittency in semiconductor quantum dots (QDs) has limited their applications in single QD process. Understanding the origin of intermittency and its correlation with microstructure is crucial for the design and preparation of QDs with high fluorescence quantum yield. The small-sized (CdSe)3 and (CdSe)4 clusters provide a typical model for studying the effect of size, structure, surface defect, etc., on the radiative and nonradiative rates of II–VI QDs. The rate constants of radiative and nonradiative processes, kr and knr, are computed for the model systems using first-principles calculations, Marcus theory, and Fermi’s golden rule. Although kr and knr vary in a complicated way from cluster to cluster, their correlation with the geometrical and electronic properties of ground- and excited-state structures are revealed. Structure distortion in excitation, spatial overlap between the Kohn-Sham orbitals occupied by excited electron, reorganization energy, Huang-Rhys factor of dominant vibrational modes, as well as the unsaturation of surface atoms exert influences on kr and knr. Moreover, these quantities correlate with each other and their roles may change in the radiative/nonradiative process. While some quantities, such as reorganization energy and atomic unsaturation, have been used solely to estimate the photoluminescence yield, a combination of these factors is suggested to give reliable predictions. In addition, our calculations indicate that kr and knr can be tuned by designing the QDs with specific geometrical and electronic structures.

Excitation energy-dependent photocurrent switching in a single-molecule photodiode

The direction of electron flow in molecular optoelectronic devices is dictated by charge transfer between a molecular excited state and an underlying conductor or semiconductor. For those devices, controlling the direction and reversibility of electron flow is a major challenge. We describe here a single-molecule photodiode. It is based on an internally conjugated, bichromophoric dyad with chemically linked (porphyrinato)zinc(II) and bis(terpyridyl)ruthenium(II) groups. On nanocrystalline, degenerately doped indium tin oxide electrodes, the dyad exhibits distinct frequency-dependent, charge-transfer characters. Variations in the light source between red-light (∼1.9 eV) and blue-light (∼2.7 eV) excitation for the integrated photodiode result in switching of photocurrents between cathodic and anodic. The origin of the excitation frequency-dependent photocurrents lies in the electronic structure of the chromophore excited states, as shown by the results of theoretical calculations, laser flash photolysis, and steady-state spectrophotometric measurements.

Extending Phenomenological Crystal-Field Methods to C_{1} Point-Group Symmetry: Characterization of the Optically Excited Hyperfine Structure of ^{167}Er^{3+}:Y_{2}SiO_{5}.

We show that crystal-field calculations for C_{1} point-group symmetry are possible, and that such calculations can be performed with sufficient accuracy to have substantial utility for rare-earth based quantum information applications. In particular, we perform crystal-field fitting for a C_{1}-symmetry site in ^{167}Er^{3+}:Y_{2}SiO_{5}. The calculation simultaneously includes site-selective spectroscopic data up to 20 000 cm^{-1}, rotational Zeeman data, and ground- and excited-state hyperfine structure determined from high-resolution Raman-heterodyne spectroscopy on the 1.5 μm telecom transition. We achieve an agreement of better than 50MHz for assigned hyperfine transitions. The success of this analysis opens the possibility of systematically evaluating the coherence properties, as well as transition energies and intensities, of any rare-earth ion doped into Y_{2}SiO_{5}.

Synthesis, structural/photophysical characterization and theoretical investigations with new β-pyridinium/quinolinium and β-bromine substituted bis(1,3-dimethylbarbituric acid) trimethine oxonol dyes that display large Stokes shifts

An efficient 3-step procedure for the synthesis of novel β-pyridinium/quinolinium/bromine-substituted bis(1,3-dimethylbarbituric acid)trimethine oxonol dyes was developed that involves a simple triethylamine-promoted condensation of 1,3-dimethylbarbituric acid with preformed vinamidinium salts. 1H NMR, 13C NMR, IR and mass spectral data confirmed the structures of the dyes. Solutions of the dyes in CH3CN exhibited absorption and fluorescence emission maxima in the range of 467–480 nm and 531–712 nm, respectively, identifying that the dyes show unusually large Stokes shifts (~230 nm). Investigating the photophysical properties of dye 3a in a series of organic solvents with different polarity confirmed intramolecular charge transfer (ICT). Optimized geometries of the synthesized dyes in the ground state calculated using density functional theory (DFT) at the B3LYP/6-311G(d,p) theory level identified that planar cis and trans configurations were preferred for the pyridinium/quinolinium and bromine substituted dyes, respectively. TD-DFT calculations on the dye excited state structures identified that rotation of the bulky pyridinium substituents during relaxation is responsible for the large Stokes shift. The unique properties of the dyes suggest they could find use as fluorescent visualizers for intracellular imaging.

Excited States with Selected Configuration Interaction-Quantum Monte Carlo: Chemically Accurate Excitation Energies and Geometries.

We employ quantum Monte Carlo to obtain chemically accurate vertical and adiabatic excitation energies, and equilibrium excited-state structures for the small, yet challenging, formaldehyde and thioformaldehyde molecules. A key ingredient is a robust protocol to obtain balanced ground- and excited-state Jastrow–Slater wave functions at a given geometry, and to maintain such a balanced description as we relax the structure in the excited state. We use determinantal components generated via a selected configuration interaction scheme which targets the same second-order perturbation energy correction for all states of interest at different geometries, and fully optimize all variational parameters in the resultant Jastrow–Slater wave functions. Importantly, the excitation energies as well as the structural parameters in the ground and excited states are converged with very compact wave functions comprising few thousand determinants in a minimally augmented double-ζ basis set. These results are obtained already at the variational Monte Carlo level, the more accurate diffusion Monte Carlo method yielding only a small improvement in the adiabatic excitation energies. We find that matching Jastrow–Slater wave functions with similar variances can yield excitation energies compatible with our best estimates; however, the variance-matching procedure requires somewhat larger determinantal expansions to achieve the same accuracy, and it is less straightforward to adapt during structural optimization in the excited state.

Optical Spectra and Fluorescence Quenching in Azaacenes Bearing Five‐Membered Rings

AbstractOptical absorption and emission properties as well as fluorescence quenching via introduction of an additional unsaturated five‐membered ring in some azaacenes are investigated computationally. It is shown that non‐radiative decay via conical intersections or intersystem crossing is rather unlikely. Instead, the additional double bond alters the electronic structure of the first excited singlet state, turning it into a dark state with long lifetime and hence low fluorescence rate, that cannot be seen anymore with the naked eye.

Optical Control of Non-Equilibrium Phonon Dynamics.

The light-induced selective population of short-lived far-from-equilibrium vibration modes is a promising approach for controlling ultrafast and irreversible structural changes in functional nanomaterials. However, this requires a detailed understanding of the dynamics and evolution of these phonon modes and their coupling to the excited-state electronic structure. Here, we combine femtosecond mega-electronvolt electron diffraction experiments on a prototypical layered material, MoTe2, with non-adiabatic quantum molecular dynamics simulations and ab initio electronic structure calculations to show how non-radiative energy relaxation pathways for excited electrons can be tuned by controlling the optical excitation energy. We show how the dominant intravalley and intervalley scattering mechanisms for hot and band-edge electrons leads to markedly different transient phonon populations evident in electron diffraction patterns. This understanding of how tuning optical excitations affect phonon populations and atomic motion is critical for efficiently controlling light-induced structural transitions of optoelectronic devices.

Toward an Accurate Description of Thermally Activated Delayed Fluorescence: Equal Importance of Electronic and Geometric Factors

Obtaining reasonable geometric and electronic structures of excited states is essential for accurately predicting the thermally activated delayed fluorescence (TADF) for the application in organic ...

Predicting Phosphorescence Quantum Yield for Pt(II)-Based OLED Emitters from Correlation Function Approach

We report a new formulation for Golden Rule-based predictions of photoluminescence quantum yields (PLQY) of phosphorescent emitters containing a heavy element, and its implementation compatible with first-principles computation frameworks. The main components of PLQY (i.e., phosphorescent rate and intersystem crossing rate from the lowest triplet state to the ground singlet state) are obtained through correlation functions in time domain, and the relativistic effects are also considered using the relativistic effective core potentials. The spin–orbit coupling is treated in a perturbative manner to generate spin–orbit-corrected, two-component T1 substates within single-excitation theory, where the electronic transition dipole moments and the non-Born–Oppenheimer coupling matrix elements to the S0 state are computed. We applied this new approach to the photophysical properties of 34 Pt(II) complexes designed for the organic light-emitting diode (OLED) applications and observed a good agreement between predictions and experiments over diverse scaffolds. For the two representative complexes, further analysis on the nonradiative characteristics was performed based on the decomposition of the non-Born–Oppenheimer coupling into contributions from the nuclear vibrations and from the excited-state electronic structures.

Photodissociation Dynamics of Ortho-Substituted Thiophenols at 243 nm.

The photoinduced S-H (D) bond fission dynamics of four ortho-substituted thiophenols, 2-fluoro, 2-chloro, 2-bromo, and 2-methoxythiophenol at a pump wavelength of 243 nm, have been investigated by velocity-map imaging and high-level electronic structure calculations. The D atom images of the deuterated ortho-substituted thiophenols show much reduced X̃/Ã branching ratios of the cofragment radicals over that of bare thiophenol. The angular distributions of the D fragment display negative anisotropies, indicating that transition dipole moments are perpendicular to the fast dissociating S-D bond axis. Initial excitation at 243 nm occurs directly to the 1πσ* state or to the 21ππ* state followed by efficient coupling to the 1πσ* state. The calculated potential energy curves for the 1πσ* or 21ππ* excited states of the ortho-substituted thiophenols along the CCS-D torsion angle (ϕ) display minima at the nonplanar structures, whereas all of the states for bare thiophenol present minima at the planar geometries. This different topology of the ortho-substituted thiophenols in the excited states induces the wide spread of the reactive flux along the ϕ coordinate on the repulsive surface as it should experience significant torque with respect to ϕ during the fragmentation. This encourages the dissociating molecules to follow the adiabatic path at the conical intersection between the ground and the 1πσ* states at extended S-D bond lengths, giving rise to decreased X̃/Ã branching ratios, demonstrating that the excited-state molecular structure dictates the nonadiabatic transition probability.

Structural Dynamics of Bismuth Triiodide in Solution Triggered by Photoinduced Ligand-to-Metal Charge Transfer.

Bismuth triiodide, BiI3, is one of the simplest bismuth halides, which have recently attracted considerable attention because of their promising properties. Here, we investigate the structural dynamics of a photoinduced reaction of BiI3 in solution phase using time-resolved X-ray liquidography (TRXL) and density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The photoreaction was initiated by excitation at 400 nm, which corresponds to the ligand-to-metal charge-transfer transition. The detailed structures and kinetic profiles of all relevant intermediate species from the TRXL data show that the trigonal planar structure of BiI3, which is predicted to be the most stable structure of the lowest excited state by TDDFT calculation, was not observed, and the photoreaction proceeds via two parallel pathways within the time resolution of 100 ps: (i) isomer formation to produce iso-BiI2-I, which relaxes back to the ground-state structure, and (ii) dissociation into BiI2· and I· radicals, which nongeminately recombine to generate ground-state BiI3 and I2.

Prediction of the Efficiency of Phosphorescent Emitters: A Theoretical Analysis of Triplet States in Platinum Blue Emitters.

DFT methods are routinely used to predict the excited-state structure of phosphorescent triplet emitters. However, sometimes they fail: different functionals predict diverse lowest adiabatic emissive states. An evaluation is undertaken to determine whether it is possible to use DFT methods to investigate the triplet emitter's hypersurfaces and to explain the experimental observation that similar ligands lead to remarkably diverse phosphorescence quantum yields.

Wave Function Control of Charge-Separated Excited-State Lifetimes.

Control of excited-state processes is crucial to an increasing number of important device technologies that include displays, photocatalysts, solar energy conversion devices, photovoltaics, and photonics. However, the manipulation and control of electronic excited-state lifetimes and properties continue to be a challenge for molecular scientists. Herein, we present the results of ground-state and transient absorption spectroscopies as they relate to magnetic exchange control of excited-state lifetimes. We describe a novel mechanism for controlling these excited-state lifetimes that involves varying the magnetic exchange interaction between a stable organic radical and the unpaired electrons present in the open-shell configuration of a charge-separated excited state. Specifically, we show that the excited-state lifetime can be controlled in a predictable manner based on an a priori knowledge of the pairwise magnetic exchange interactions between excited-state spins. These magnetic exchange couplings affect the excited-state electronic structure in a manner that introduces variable degrees of spin forbiddenness into the nonradiative decay channel between the excited state and the electronic ground state.

Electronic structure in the transition metal block and its implications for light harvesting.

Transition metal-based chromophores play a central role in a variety of light-enabled chemical processes ranging from artificial solar energy conversion to photoredox catalysis. The most commonly used compounds include elements from the second and third transition series (e.g., ruthenium and iridium), but their Earth-abundant first-row analogs fail to engage in photoinduced electron transfer chemistry despite having virtually identical absorptive properties. This disparate behavior stems from fundamental differences in the nature of 3d versus 4d and 5d orbitals, resulting in an inversion in the compounds' excited-state electronic structure and undermining the ability of compounds with first-row elements to engage in photoinduced electron transfer. This Review will survey the key experimental observations establishing this difference in behavior, discuss the underlying reasons for this phenomenon, and briefly summarize efforts that are currently under way to alter this paradigm and open the door to new opportunities for using Earth-abundant materials for photoinduced electron transfer chemistries.

Analytic Gradients for the Spin-Flip ORMAS-CI Method: Optimizing Minima, Saddle Points, and Conical Intersections.

Analytic nuclear gradients are derived and implemented for the recently introduced SF-ORMAS-CI (spin-flip occupation restricted multiple active space CI) method. Like most SF methods, SF-ORMAS-CI successfully describes bond breaking, diradical systems, transition states, and low-lying excited states, without suffering from spin contamination. The availability of analytic gradients now enables the efficient optimization of equilibrium structures in both ground and excited electronic states, as well as the computation of seminumerical Hessians. Therefore, it is now possible to determine minima, transition states, and conical intersections using the SF-ORMAS-CI method without the need for numerical differentiation. In the present study the SF-ORMAS method and gradient are applied to optimize structures for several organic molecules, such as ethylene, azomethane, and trimethylmethylene. In most cases, structures optimized with SF-ORMAS are almost identical to those obtained using other multireference methods, despite the lack of dynamic correlation in SF-ORMAS.

Investigation of spatial manifestation of α clusters in 16O via α-transfer reactions

Recently, we have determined surface distributions of α clusters in the ground state of Ne20 from α-transfer cross sections, without investigating the properties of its excited states. In this paper we extend our comprehension of α-cluster structures in excited states of nuclei through reaction studies. In particular we focus on O16, for which attention has been paid to advances of structure theory and assignment regarding 4+-resonance states. We study the surface manifestation of the α-cluster states in both the ground and excited states of O16 from the analysis of the α-transfer reaction C12(Li6,d)16O. The α-transfer reaction is described by the distorted-wave Born approximation. We test two microscopic wave functions as an input of reaction calculations. Then a phenomenological potential model is introduced to clarify the correspondence between cluster-wave functions and transfer-cross sections. Surface peaks of the α-wave function of O16(0+) are sensitively probed by transfer-cross sections at forward angles, while it remains unclear how we trace the surface behavior of O16(4+) from the cross sections. From inspection of the cross sections at forward angles, we are able to specify that the α-cluster structure in the 01+ and 02+ states prominently manifests itself at the radii ∼4 and ∼4.5 fm, respectively. It is remarkable that the 41+ state has the C12+α-cluster component with the surface peak at the radius ∼4 or outer, whereas the C12+α-cluster component in the 42+ state is found not to be dominant. The 42+ state is difficult to be interpreted by a simple potential model assuming the C12+α configuration only.

Ultrafast stimulated Raman study of the complete reactive potential energy surface of a photomolecular rotor

Femtosecond Stimulated Raman Spectroscopy (FSRS) reports on structure and dynamics of excited states. Here we record FSRS for a photomolecular motor in its stable and metastable states, and thus map the entire excited state PES.