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Extremely solvent-enhanced absorption and fluorescence spectra of carbazole were investigated by performing a generalized multi-set damped Franck–Condon spectral simulation. Experimental absorption and fluorescence spectra of carbazole in the gas phase were first well reproduced by performing an un-damped Franck–Condon simulation, but a one-set scaling damped Franck–Condon simulation severely underestimated the intensities of the peaks of experimental absorption and fluorescence spectra of carbazole in n-hexane. Then, a multi-set scaling damped Franck–Condon simulation was proposed and carried out for simulating the extremely solvent-enhanced absorbance and fluorescence, and here, the simulated spectra agreed well with the experimental ones. Five (four) representative solvent-enhanced normal modes corresponding to the combination of ring stretching and ring breathing vibrational motions were determined to be responsible for enhanced absorbance (fluorescence) in n-hexane solution. Furthermore, different scalings were applied to the ground and first-excited states, resulting in different enhancement of absorbance and fluorescence, and this analysis revealed atoms in the carbazole interacting with n-hexane solvent molecules and, hence, leading to different normal-mode vibrational vector patterns in the ground and first-excited states, respectively. Basically, the same conclusion was drawn from a simulation with HF-CIS and the three functionals (TD)B3LYP, (TD)B3LYP-35, and (TD)BHandHLYP. The present multi-set scaling damped Franck–Condon simulation scheme was demonstrated to successfully interpret extremely solvent-enhanced absorbance and fluorescence of carbazole in n-hexane-solvent. [ABSTRACT FROM AUTHOR]

Since the concept of aggregation‐induced emission (AIE) was proposed by Benzhong Tang's research group in 2001, the exploration of the mechanism of AIE and the development of new high‐performance AIE materials have been the focus and goal of this field. On the basis of a large number of experiment results, AIE mechanism has been well explained by lots of works, such as restricted intramolecular motion (RIM), J‐aggregate et al. As tetraphenylethlene (TPE) molecules are stacked, the rotation of the benzene ring rotor is blocked, and the energy attenuation is released in the form of radiation, showing the AIE effect. In order to further explore the AIE effect of TPE, we performed electronic structure, spectrum simulation, and AIE mechanism calculations of the anthryl‐tetraphenylethene (TPE‐an) monomer and dimer in the gas phase, tetrahydrofuran (THF), and aqueous solutions at the B3LYP/6‐31G** level. The calculation results show that TPE‐an molecule is in a propeller‐like configuration, and its fluorescence intensity is weak; compared with the monomer, the fluorescence intensity of the dimer increases by 87% in aqueous solution; the fluorescence intensity in the gas phase, THF solution, and aqueous solution gradually enhances with the increase of the degree of aggregation, which are consistent with the experimental results. The enhancement of fluorescence intensity is caused by the change of molecular structure caused by aggregation. This detailed AIE luminescence mechanism will provide theoretical guidance for AIE material design. [ABSTRACT FROM AUTHOR]

In this work, starting from the general theory of sum-frequency generation (SFG), we proposed a computational strategy utilizing density functional theory with periodic boundary conditions to simulate the vibrational SFG of molecules/solid surface adsorption system. The method has been applied to the CH3OH/TiO2(110) system successfully. Compared with the isolated molecule model, our theoretical calculations showed that the TiO2 substrate can significantly alter the second-order susceptibilities of a methanol molecule which is directly related to the SFG intensity. In addition, the SFG spectra have obvious changes while the methanol coverage increases, especially for the OH vibration peaks. Our theoretical spectra agree reasonably well with experimental measurements at 1 ML coverage, and an interesting peak which is absent in the theoretical spectra is tentatively assigned to some CH3 stretch vibration of methanol adsorbed on the oxygen vacancy of TiO2. [ABSTRACT FROM AUTHOR]

A novel double-input pulsewidth-modulation (PWM) dc/dc converter for high-/low-voltage sources is proposed in this paper. With a PWM control scheme, the proposed double-input dc/dc converter can draw power from two different voltage sources simultaneously or individually. The operation modes and the steady-state analysis of the proposed double-input dc/dc converter are introduced in detail. The PWM control scheme for the power flow balancing is also presented. By using a single passive loss-less soft-switching cell, switching losses of all power switches can be reduced significantly. Finally, experimental measurements are demonstrated to verify the performance of the proposed converter. Index Terms--Double-input converter, pulse width-modulation (PWM) control.

Local interactions of different structures of G-rich DNA were determined by employing the high-resolution and low-frequency satellite holes (SH) together with vibrations of 3,3'-diethyloxadicarbocyanine cation (DODC+). In addition, a binding structure of dimeric hairpin quadruplex:DODC+ is suggested. The SH findings agree with groove binding model and remove possible ring intercalation in the base pairs of guanine quartet. The implications of the results are discussed.

Multi-reference configuration interaction with single and double excitation method has been utilized to calculate the potential energy surfaces of the five low-lying electronic states 1A1, 1A2, 3A2, 1B2, and 3B2 of carbon dioxide molecule. Topology of intersections among these five states has been fully analyzed and is associated with double-well potential energy structure for every electronic state. The analytical potential energy surfaces based on the reproducing kernel Hilbert space method have been utilized for illustrating topology of surface crossings. Double surface seam lines between 1A1 and 3B2 states have been found inside which the 3B2 state is always lower in potential energy than the 1A1 state, and thus it leads to an angle bias collision dynamics. Several conical/surface intersections among these five low-lying states have been found to enrich dissociation pathways, and predissociation can even prefer bent-geometry channels. Especially, the dissociation of O(3P) + CO can take place through the intersection between 3B2 and 1B2 states, and the intersection between 3A2 and 1B2 states. [ABSTRACT FROM AUTHOR]

Partially deuterated protonated water dimers, H2O·H+·D2O, H2O·D+·HDO, and HDO·H+·HDO, as important intermediates of isotopic labeled reaction of H3O+ + D2O, undergo direct dissociation and indirect dissociation, i.e., isomerization before the dissociation. With Rice-Ramsperger-Kassel-Marcus theory and ab initio calculations, we have computed their dissociation and isomerization rate constants separately under the harmonic and anharmonic oscillator models. On the basis of the dissociation and isomerization rate constants, branching ratios of two primary products, [HD2O+]/[H2DO+], are predicted under various kinetics models with the harmonic or anharmonic approximation included. The feasible kinetics model accounting for experimental results is shown to include anharmonic effect in describing dissociation, while adopting harmonic approximation for isomerization. Thus, the anharmonic effect is found to play important roles affecting the dissociation reaction, while isomerization rates are shown to be insensitive to whether the anharmonic or harmonic oscillator model is being applied. [ABSTRACT FROM AUTHOR]

The absorption and fluorescence spectra of 4-(3-methoxybenzylidene)-2-methyl-oxazalone (m-MeOBDI) dissolved in neutral, acidic, and basic solvent environments have been investigated and assigned by using Franck–Condon (FC) simulations at the quantum TDDFT level. Four different structures of m-MeOBDI in the ground and excited states are optimized and are found to be responsible for the observed absorption and fluorescence spectra. The (absorption) fluorescence of m-MeOBDI in pure methanol and neutral/basic methanol/water (1/9 vol) mixed solvent is found to arise from the (ground neutral N-I) excited neutral N-I* and cationic C-III* species, respectively. In acidic solvent, the absorption is found to arise from ground acidic C-II species, and the excited divalent cation DC-IV* is found to be formed in its excited state due to the excess H+ in the solution, and then it emits ∼560 nm fluorescence. FC simulations have also been employed to confirm our assignments as well as interpret the vibronic band profiles. As expected, the simulated emission spectrum of the divalent cationic species is in good agreement with the experimental observation. Therefore, within the present FC simulation, the observed absorption and fluorescence spectra have been reasonably interpreted and novel fluorescence mechanisms of m-MeOBDI in various pH solvent environments have been proposed. [ABSTRACT FROM AUTHOR]

High resolved absorption and fluorescence spectra of zinc complexes of phthalocyanine (ZnPc) and tetrabenzoporphyrin (ZnTBP) in the region of Q states were reported. Few theoretical investigations were performed to simulate the well-resolved spectra and assigned the vibrational bands of the large molecules, especially for high symmetrical characteristic molecules, on account of the difficulties to optimize the excited states and analyze a large number of final vibrational-normal modes. In the present work, the S0 ↔ S1 absorption and fluorescence spectra (that is, the Q band) of ZnPc and ZnTBP were simulated using time-dependent density functional theory with the inclusions of Duschinsky and Herzberg-Teller contributions to the electronic transition dipole moments. The theoretical results provide a good description of the optical spectra and are proved to be in excellent agreement with experimental spectra in inert-gas matrices or in supersonic expansion. This study focused attentions on the optical spectral similarities and contrasts between ZnPc and ZnTBP, in particular the noticeable Duschinsky and Herzberg-Teller effects on the high-resolved absorption and fluorescence spectra were considered. Substitution of meso-tetraaza on the porphyrin macrocycle framework could affect the ground state geometry and alter the electron density distributions, the orbital energies that accessible in the Q band region of the spectrum. The results were used to help interpret both the nature of the electronic transitions in Q band region, and the spectral discrepancies between phthalocyanine and porphyrin systems. [ABSTRACT FROM AUTHOR]

By using three different hybrid exchange-correlation functionals containing 20%, 35%, and 50% of exact Hartree-Fock (HF) exchange of the density functional theory and its time-dependent extension plus the Hartree-Fock and the configuration interaction of single excitation methods, equilibrium geometries, and their 30 vibrational-normal-mode frequencies of the ground S0(1A1) and the first excited S1(1B2) states of fluorobenzene (FB) were calculated. The dispersed fluorescence spectrum and internal conversion (IC) rate of the A 1B2 → X 1A1 transition were simulated by Franck-Condon (FC) calculations within the displaced harmonic oscillator approximation plus anharmonic and distorted corrections. The simulated spectral profile is primarily described by the Franck-Condon progression from the ring-breathing modes v9 and v10 which belong to totally symmetry modes. Anharmonic corrections simultaneously improve the intensity order of 910 and 1010 bands and diminish 110 transition that is fairly strong in harmonic simulations. It is concluded that the amount of Hartree-Fock exchange does impact the geometries and vibrational frequencies of FB molecule, but not the relative intensities of the transitions. It is anharmonic corrections that make the relative intensities of the transitions in good agreement with experimental results. Distorted corrections could assign most of the dominant overtones of out-of-plane nontotally symmetry modes, and the results agree well with the experimental assignments. Furthermore, it was found that the internal conversion rate is dominated by three promoting modes that are computed with lowing symmetry to C1. By choosing dephasing width as 10 cm-1 that is consistent with spectral simulation, we obtained the lifetimes of the A 1B2 → X 1A1 de-excitation as 11 and 19 ns, respectively, from TD(B3LYP) and HF/CIS calculations in comparison with the experimental value 14.75 ns. [ABSTRACT FROM AUTHOR]

Using high-pressure infrared methods, we have investigated close interactions of charge-enhanced C-H-O type in ionic liquid/dimethyl sulfoxide (DMSO) mixtures. The solvation and association of the 1-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF4-) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BMM+BF4-) in DMSO-d6 were examined by analysis of C-H spectral features. Based on our concentration-dependent results, the imidazolium C-H groups are more sensitive sites for C-H-O than the alkyl C-H groups and the dominant imidazolium C-H species in dilute ionic liquid/DMSO-d6 should be assigned to the isolated (or dissociated) structures. As the dilute mixtures were compressed by high pressures, the loss in intensity of the bands attributed to the isolated structures was observed. In other words, high pressure can be used to perturb the association-dissociation equilibrium in the polar region. This result is remarkably different from what is revealed for the imidazolium C-H in the BMM+BF4-/D2O mixtures. DFT-calculations are in agreement with our experimental results indicating that C4-H-O and C5-H-O interactions seem to play non-negligible roles for BMM+BF4-/DMSO mixtures. [ABSTRACT FROM AUTHOR]

We consider a system of two coupled Brownian particles fluctuating between two states. The fluctuations are produced by both equilibrium thermal and external nonthermal noise, the transition rates depending on the interparticle distance. An externally induced modulation of the transition rates acts on the internal degree of freedom (the interparticle distance) and generates reciprocating motion along this coordinate. The system moves unidirectionally due to rectification of the internal motion by asymmetric friction fluctuations and thus operates as a dimeric motor that converts input energy into net movement. The properties of the motor are primarily determined by the properties of the reciprocating engine, represented by the interparticle distance dynamics. Two main mechanisms are recognized by which the engine operates: energetic and informational. In the physically important cases where only one of the motion-inducing mechanisms is operative, exact solutions can be found for the model with linearly coupled particles. We focus on the informational mechanism, in which thermal noise is involved as a vital component and the reciprocating velocity exhibits a rich behavior as a function of the model parameters. An efficient rectification method for the reciprocating motion is also discussed. [ABSTRACT FROM AUTHOR]

The performance of an analytical expression for algorithmic decoherence time is investigated for non–Born–Oppenheimer molecular dynamics. There are two terms in the function that represents the dependence of the decoherence time on the system parameters; one represents decoherence due to the quantum time-energy uncertainty principle and the other represents a back reaction from the decoherent force on the classical trajectory. We particularly examine the question of whether the first term should dominate. Five one-dimensional two-state model systems that represent limits of multidimensional nonadiabatic dynamics are designed for testing mixed quantum-classical methods and for comparing semiclassical calculations with exact quantum calculations. Simulations are carried out with the semiclassical Ehrenfest method (SE), Tully’s fewest switch version (TFS) of the trajectory surface hopping method, and the decay-of-mixing method with natural switching, coherent switching (CSDM), and coherent switching with reinitiation (CSDM-D). The CSDM method is demonstrated to be the most accurate method, and it has several desirable features: (i) It behaves like the representation-independent SE method in the strong nonadiabatic coupling regions; (ii) it behaves physically like the TFS method in noninteractive region; and (iii) the trajectories are continuous with continuous momenta. The CSDM method is also demonstrated to balance coherence well with decoherence, and the results are nearly independent of whether one uses the adiabatic or diabatic representation. The present results provide new insight into the formulation of a physically correct decoherence time to be used with the CSDM method for non–Born–Oppenheimer molecular dynamic simulations. [ABSTRACT FROM AUTHOR]

Population transfers in degenerate (or almost degenerate) two-level systems interacting with the few-cycle laser pulse are investigated. A simple and analytical formula of nonadiabatic transition probability is derived with completely degenerate condition, demonstrating the sensitive dependence of the transition probability on the phase of the few-cycle pulses. As one of the applications of this formula, a new way of controlling the nuclear wave packet dynamics at a potential curve crossing by 1 cycle laser pulse is proposed. [ABSTRACT FROM AUTHOR]

In studying ultrafast electron transfer from a dye molecule to a nanosized semiconductor particle, pump-probe experiments are commonly used. In this system the electron transfer (ET) rate is faster than vibrational relaxation so that the ET rate should be described by a single-level rate constant and the probing signal (often in the form of time-resolved spectra) contains the contribution from the dynamics of both population and coherence (i.e., wave packet). In this paper, we shall present the theoretical treatments for femtosecond time-resolved pump-probe experiment and the dynamics of population and coherence by the density matrix method, and the calculation of single-level ET rate constant involved in a pump-probe experiment. As an application, we show the theoretical results using parameters extracted from experiments on a specific dye/semiconductor system. [ABSTRACT FROM AUTHOR]

The hydrophobic hydration of dimethylsulfoxide (DMSO)/D[sub 2]O was explored using a combination of the high-pressure method and ab initio calculations. The frequencies of the C-H stretching vibration of DMSO increase as the mole fraction of D[sub 2]O increases, while no appreciable changes in spectral shapes are observed upon dilution. Interestingly, the infrared spectra of DMSO/D[sub 2]O observed under high-pressure exhibits dramatic changes, while the new spectral features locating at ∼2942 and ∼3033 cm[sup -1] appear. The spectral changes were attributed to the combined effect of C-H-O hydrogen bonding between C-H in DMSO and oxygen atom in D[sub 2]O, a phase change, pressure increase, etc. Ab initio calculation results, forecasting the frequency shift of the C-H stretching vibration as C-H-O is interacting via hydrogen bonding, are discussed. The reorganization of the hydrogen-bond network or geometry may be responsible for spectral changes as the pressure was elevated. [ABSTRACT FROM AUTHOR]

This study provides the evidence for hydrophobic hydration of CH[sub 3]COOD as its aqueous solution was compressed by high pressure. On going from neat CH[sub 3]COOD to diluted CH[sub 3]COOD/D[sub 2]O mixtures, the C-H vibration absorption bands do not reveal the solvent shift in frequency upon dilution, while the C-H absorption bands are altered into one prominent band at ∼2955 cm[sup -1] as the pressure was applied above 0.3 GPa. The possible explanation is the enhancement of C-H-O interactions between C-H and D[sub 2]O under the condition of high pressure. Ab initio calculations predict that the CH[sub 3] stretching frequencies and intensities of acetic acid are hardly affected by the presence of water, if the methyl group does not participate in the complex formation. Nevertheless, the strongest absorption peak at ∼2928 cm[sup -1] is predicted in several low-energy (acetic acid)-(water)[sub χ]complexes with the participation of C-H-O hydrogen bond. The reorganization of hydrogen-bond network or geometry is suggested to be responsible for the formation of C-H-O hydrogen bond. [ABSTRACT FROM AUTHOR]

Brownian-particle survival is considered in the case when killing traps are gathered in spherical clusters distributed in space in a noncorrelated manner. Recently, we [J. Chem. Phys. 111, 711 (1999)] have proposed a mean-field theory of trapping by identical clusters of traps, in which the cluster radius R and the number of traps in a cluster n are the same for all of the clusters. Here we focus on a more realistic model, where the parameters R and n vary from one cluster to another, according to a given probabilistic law. For the model of polydisperse clusters, general expressions are obtained for the particle survival probability and the time-dependent rate coefficient. To exhibit the effect of polydispersity, we compare the trapping kinetics for polydisperse clusters with that for the reference system of identical clusters. It is shown, in particular, that fluctuations in n always lead to a process slowdown. An explicit solution is derived for slightly polydisperse nonoverlapping clusters. [ABSTRACT FROM AUTHOR]

Photodissociation of jet-cooled ethylbenzene at 248 nm was studied using VUV photoionization/multimass ion imaging techniques. The photofragment translational energy distribution obtained at 248 nm showed that after the excitation 75% of the ethylbenzene molecules dissociate from electronic excited state, and the rest 25% of the molecules dissociate through a hot molecule mechanism. This is the first experimental evidence which proves that the dissociation of alkyl-substituted benzenes can occur not only from hot molecule mechanism in this UV region. [ABSTRACT FROM AUTHOR]

The infrared spectroscopic profiles of HCOOD/D[sub 2]O mixtures were measured as a function of pressure and concentration. The C–H bond of HCOOD shortens as the pressure is elevated, while the increase in C–H bond length upon diluting HCOOD with D[sub 2]O was observed. Based on the experimental results, the shift in frequency of C–H stretching band is concluded to relate to the mechanism of the hydration of the C–H group and the water structure in the vicinity of the C–H group. The pressure-dependent results can be attributed to the strengthening of C–H---O electrostatic/dispersion interaction upon increasing pressure. The observations are in accord with ab initio calculation forecasting a blueshift of the C–H stretching mode via C–H---O interaction in HCOOD-water/(HCOOD)[sub 2]-(D[sub 2]O) complexes relative to the noninteracting monomer/dimer. Hydrogen-bonding nonadditivity and the size of water clusters are suggested to be responsible to cause the redshift in C–H stretching mode upon dilution HCOOD with D[sub 2]O. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The vibronic spectra of ethylene have been studied using ab initio molecular orbital methods. Geometries of the singlet π–π*, π–3s, and π–3p excited electronic states have been optimized at the CIS and CASSCF levels of theory with the 6-311(2+)G* basis set. Vertical and adiabatic excitation energies, calculated by the multireference configuration interaction (MRCI) and equation-of-motion coupled cluster (EOM-CCSD) methods are in quantitative agreement with experiment. Vibrational frequencies and normal coordinates for the ground and excited states are used for the calculations of vibrational overlap integrals and Franck–Condon factors, taking into account distortion, displacement, and normal mode mixing (up to four modes). Major features of the observed absorption spectrum of ethylene have been interpreted on the basis of the computed Franck–Condon factors. The role of each electronic state in the spectra has been clarified; the π–3s transition corresponds to the distinct intensive peaks in the 57 000–61 000 cm-1 energy region, the less intensive distinct bands in the interval of 62 000–65 000 cm-1 are due to the π–3pσ states and the π–π* peaks constitute the continuum underlying the spectrum. The theoretical vibronic spectrum is in qualitative agreement with the experimental one, except of some details. Possible reasons for the discrepancies between theory and experiment are also discussed. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Vibrational dephasing of the 1385 cm-1 vibron of host molecules (naphthalene) perturbed by electronically excited guest molecules (pentacene) was studied by the time-resolved coherent Stokes Raman spectroscopy (CSRS) in the temperature region of 6–26 K. The decay time was faster when the excitation frequency was tuned from the off-resonance to the resonance of the electronic transition of pentacene. For the resonance case, longer decay times were observed at higher temperatures (∼26 K) than at low temperatures (∼6 K). Two possible mechanisms were considered for the inverse temperature behavior. The shorter decay time in the impurity perturbed domains may be attributed to the increase of a coupling strength on the decay channel from 1385 cm-1 mode to 1365 cm-1 mode by stimulating phonon emission. [ABSTRACT FROM AUTHOR]