External electric field effects on state energy and photoexcitation dynamics of diphenylpolyenes

External electric field effects on state energy and photoexcitation dynamics have been examined for para-substituted and unsubstituted all-trans-diphenylpolyenes doped in a film, based on the steady-state and picosecond time-resolved measurements of the field effects on absorption and fluorescence. The substitution dependence of the electroabsorption spectra shows that the dipole moment of the substituted stilbene in the Franck-Condon excited state becomes larger with increasing difference between the Hammet constants of the substituents. Fluorescence quantum yields of 4-(dimethylamino)-4'-nitrostilbene and 4-(dimethylamino)-4'-nitrodiphenylbutadiene are markedly reduced by an electric field, suggesting that the rates of the intramolecular charge transfer (CT) from the fluorescent state to the nonradiative CT state are accelerated by an external electric field. The magnitude of the field-induced decrease in fluorescence lifetime has been evaluated. The isomerization of the unsubstituted all-trans-diphenylpolyenes to the cis forms is shown to be a significant nonradiative pathway even in a film. Field-induced quenching of their fluorescence as well as field-induced decrease in fluorescence lifetime suggests that the trans to cis photoisomerization is enhanced by an electric field.     

Reactivity dynamics of a confined molecule in presence of an external magnetic field

Reactivity dynamics and stability of a confined hydrogen molecule in presence of an external magnetic field has been studied using quantum fluid density functional theory. Dynamic profiles of various reactivity parameters such as hardness, electrophilicity, magnetizability, phase volume, entropy, etc. have been studied within a confined environment. Responses in the reactivity parameters as well as the associated electronic structure principles validate the stability of the confined H₂ molecule in ground and excited states in presence of an external magnetic field. Confinement to the system has been imposed by the Dirichlet type boundary condition. Confinement and excitation act in opposite directions. Ground state type dynamics is obtained on simultaneous electronic excitation and confinement. © 2014 Wiley Periodicals, Inc.     

Magnetic properties of a molecule in non-uniform magnetic field

Summary The potentials of the electromagnetic field in the Bloch gauge are used to obtain definitions for the multipole moment operators and for the operators expressing the electric and magnetic field of electrons acting on the nuclei of a molecule. Perturbation theory is employed to determine induced electronic moments and total electromagnetic field at the nuclei. A series of response tensors is defined to describe the contributions arising in non-uniform magnetic field and their origin dependence is studied.This paper is dedicated to Professor Werner Kutzelnigg on the occasion of his 60th birthday     

Theoretical study of magnetic properties of ammonia molecule in nonuniform magnetic field

The interaction Hamiltonian within the Bloch gauge for the potentials of the electromagnetic field has been used to define magnetic multipole moment operators and operators for the magnetic field of electrons acting on the nuclei of a molecule in the presence of nonhomogeneous external magnetic field. Perturbation theory has been applied to evaluate the induced electronic moments and magnetic field at the nuclei. Multipole magnetic susceptibility and nuclear magnetic shielding tensors have been introduced to describe the contributions arising in nonuniform fields, and their origin dependence has been analyzed. Extended numerical tests on the ammonia molecule in a static, nonuniform magnetic field have been carried out, using the random-phase approximation within the framework of accurate Hartree-Fock zero-order wavefunctions, and allowing for both angular momentum and torque formalisms in the calculation of paramagnetic contributions.     

Theoretical study of the magnetic properties of an SF6 molecule in non-uniform magnetic field

Perturbation theory has been applied to evaluate the induced electronic moments and magnetic field at the nuclei of an SF₆ molecule interacting with an external magnetic field with a spatially uniform gradient. Contributions to magnetic susceptibility and magnetic shielding of the nuclei have been evaluated using the random phase approximation within the framework of accurate Hartree-Fock zero order wavefuctions. The quality of the calculations has been judged by the fulfillment of sum rules for the origin dependence of the response properties.     

Femtosecond photoexcitation dynamics in polydiacetylene 4BCMU film

Femtosecond pulses were used to study photoexcitation dynamics in polydiacetylene 4BCMU film. Ultrafast photoinduced dichroism and optical Kerr gate responses were observed. These measurements allowed estimation of the diffusion constant (D ∼ 0.1 cm²/s), the nonlinear index of refraction (n₂ ∼ 10⁻⁸ esu), and third-order susceptibility (δχ⁽³⁾ ∼ 6 × 10⁻¹⁰ esu), in the film. We also found that the sign of the photoinduced transmission signal was dependent on the pump energy at the low energy absorption edge of the film. Possible explanations are proposed.     

A molecular dynamics study of Lys-Trp-Lys: structure and dynamics in solution following photoexcitation

We report studies of the structure and dynamics of a tripeptide Lys-Trp-Lys (KWK) in aqueous solution following photoexcitation by molecular dynamics simulations. For ground-state KWK, we observe three stable conformations with free energy differences of less than 5.2 kJ/mol. Each conformer is stabilized by a pi-cation interaction between one of three protonated amino groups and the indole moiety. For the excited state of tryptophan in KWK, the simulated molecular dynamics of the three isomers are similar, all in good agreement with recent femtosecond experiments (J. Phys. Chem. B 2005, 109, 16901). Specifically, we observe: (1) the fluorescence anisotropy is dominated by a single-exponential component and decays in approximately 130 ps, (2) the total dynamic Stokes shift reaches approximately 2700 cm(-1), and (3) the excited state relaxation dynamics occurs on several time scales ranging from femtoseconds to tens of picoseconds. The relaxation dynamics involve rapid initial response of neighboring water, followed by local motions of flexible peptide chains. These processes drive global restructuring of the tripeptide on a rather flat energy surface, inducing slower dynamics evident in both the water and protein contributions to the stabilization energy of the photoexcited chromophore. The water and protein dynamics are strongly correlated. On a still longer time scale, we observe isomerization of two excited state conformers to the other most stable one, an analogue for evolution of trajectories along the funnel on the rugged free energy landscape to the final "native" state. Our studies suggest new experiments to detect this unique dynamics.     

Intramolecular electron transfer within a covalent, fixed-distance donor-acceptor molecule in an ionic liquid

Intramolecular photoinduced charge separation and recombination within the donor-acceptor molecule 4-(N-pyrrolidino)naphthalene-1,8-imide-pyromellitimide, 5ANI-PI, are studied using ultrafast transient absorption spectroscopy in the room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [EMIM][Tf2N]. The rate constants of both photoinduced charge separation and charge recombination for 5ANI-PI in [EMIM][Tf2N] are comparable to those observed in pyridine, which has a static dielectric constant similar to that of [EMIM][Tf2N] but a viscosity that is nearly 2 orders of magnitude lower than that of [EMIM][Tf2N]. The electron-transfer dynamics of 5ANI-PI in [EMIM][Tf2N] are compared to those in pyridine as a function of temperature and are discussed in the context of recently reported ionic liquid solvation studies.     

Intramolecular relaxation dynamics in semiflexible dendrimers

The intramolecular relaxation dynamics of semiflexible dendrimers in dilute solutions are theoretically investigated in the framework of optimized Rouse-Zimm formalism. Semiflexibility is implemented by modeling topological restrictions on the bond directions and orientations of the respective bond-vectors. Based on our recently developed approach for semiflexible dendrimers [A. Kumar and P. Biswas, Macromolecules 43, 7378 (2010)], the mechanical and dielectric relaxation moduli are studied as functions of local flexibility parameters and branching topology. It is rather interesting to observe that semiflexibility affects the local modes of G'(ω) and Δε'(ω), which have lower relaxation rate with increasing bond restrictions, while the collective modes with small relaxation rate remain almost constant. The relaxation dynamics of the flexible dendrimer is similar to that of the semiflexible dendrimer with unrestricted bond orientations (Φ = 0) and is flanked by the compressed (Φ = 30°) and expanded (Φ = 150°) conformations, respectively. The effect of semiflexibility is typically reflected in the intermediate frequency regime. The expanded conformations of semiflexible dendrimers display a power-law behavior in the intermediate frequency regime for both loss and storage modulus resembling fractal structures, while the compressed and unrestricted bond orientation conformations exhibit an approximately logarithmic dependence. The power-law exponent is found to be similar to the flexible dendrimers with excluded volume interactions. Thus, by tuning Φ, a spectrum of dynamic relaxation pattern is obtained spanning a broad range of conformations from a power-law fractal network to a non-fractal one. In certain limits, this highly generalized model captures the characteristics of flexible dendrimers and also resembles La Ferla's model semiflexible dendrimers. The influence of hydrodynamic interactions reduces the dynamical range and the width of the intermediate domain by decreasing the smaller relaxation rates and increasing the higher relaxation rates correspondingly.     

Influence of an external magnetic field on the dynamics of magnetite reduction by hydrogen

Russian Chemical Bulletin -     

Superfluorescent transitions in an external magnetic field

Polarisation properties of the superfluorescence in the near-infrared regime have been investigated between high-lying levels of Sr and Ba under the influence of a static homogeneous external magnetic field. In some transitions the time-resolved measurements show a change of the polarisation of the superfluorescence depending on the magnetic field strength. In suitable experimental conditions intensity modulations were observed. These were assigned as Zeeman quantum beats or indirectly observed Zeeman superfluorescent beats. The experimental findings of superfluorescence in two-level, three-level, or multi-level configurations in dependence on the magnetic field strength can be explained well in a semiclassical model of multi-level superfluorescence.     

Transistors from a conjugated macrocycle molecule: field and photo effects

This study explores a conjugated macrocycle molecule and details its synthesis, molecular structure, assemblies in the solid state and application in phototransistors.     

Ultrafast dynamics of photoactive yellow protein via the photoexcitation and emission processes

Pump-dump fluorescence spectroscopy was performed for photoactive yellow protein (PYP) at room temperature. The effect of the dump pulse on the population of the potential energy surface of the electronic excited state was examined as depletion in the stationary fluorescence intensity. The dynamic behavior of the population in the electronic excited state was successfully probed in the various combinations of the pump-dump delay, the dump-pulse wavelength, the dump-pulse energy and the observation wavelength. The experimental results were compared with the results obtained by the femtosecond time-resolved fluorescence spectroscopy.     

Molecular dynamics simulation of the nematic liquid crystal phase in the presence of an intense magnetic field

The influence of an intense external field on the dynamics of the nematic liquid crystal phase is investigated using a molecular dynamics simulation for the Gay-Berne nematogen under isobaric-isothermal conditions. The molecular dynamics as a function of the second-rank orientational order parameter P<2> for a system consisting of a nematic liquid crystal in the presence of an intense magnetic field is compared with that of a similar system without the field. The translational motion of molecules is determined as a function of the translational diffusion coefficient tensor and the anisotropy and compared with the values predicted theoretically. The rotational dynamics of molecules is analyzed using the first- and the second-rank orientational time correlation functions. The translational diffusion coefficient parallel with respect to the director is constrained by the intense field, although the perpendicular one is decreased as the P<2> is increased, just as it is in the system without the field. However, no essential effect of the strong magnetic field is observed in the rotational molecular dynamics. Further, the rotational diffusion coefficient parallel with respect to the director obtained from the first-rank orientational time correlation function in the simulation is qualitatively in agreement with that in the real nematic liquid crystalline molecules. The P<2> dependence of the rotational diffusion coefficient for the system with the intense magnetic field shows a tendency similar to that for the system without the field.     

A theoretical study of spin level crossing induced by an external magnetic field of ring molecule magnet models

The dependence of the magnetization of model systems on an external magnetic field has been investigated. An ab initio path integral Monte Carlo method is used to study the spin level crossing phenomena of molecules with ring structures such as those in the ferric wheel [Fe(OMe)₂(O₂CCH₂Cl)]₁₀. The ab initio treatment is essential to calculate the magnetization in a system with a large contribution from next-neighbor interactions. A possible use as a molecular device for switching or molecular recognition is suggested.     

Spin dynamics of the radical pair in a low magnetic field studied by the transient absorption detected magnetic field effect on the reaction yield and switched external magnetic field

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.     

Intramolecular charge-transfer dynamics in covalently linked perylene-dimethylaniline and cyanoperylene-dimethylaniline

The excited-state dynamics of covalently linked electron donor-acceptor systems consisting of N, N-dimethylaniline (DMA) as electron donor and either perylene (Pe) or cyanoperylene (CNPe) as acceptor has been investigated in a large variety of solvents, including a room-temperature ionic liquid, by using femtosecond time-resolved fluorescence and absorption spectroscopy. The negligibly small solvent dependence of the absorption spectrum of both compounds and the strong solvatochromism of the fluorescence are interpreted by a model where optical excitation results in the population of a locally excited state (LES) and emission takes place from a charge-separated state (CSS). This interpretation is supported by the fluorescence up-conversion and the transient absorption measurements that reveal substantial spectral dynamics in polar solvents only, occurring on time scales going from a few hundreds of femtoseconds in acetonitrile to several tens of picoseconds in the ionic liquid. The early transient absorption spectra are similar to those found in nonpolar solvents and are ascribed to the LES absorption. The late spectra due to CSS absorption show bands that are red-shifted relative to those of the radical anion of the acceptor moiety by an amount that depends on solvent polarity, pointing to partial charge separation. Global analysis of the time-resolved data indicates that the charge separation dynamics in PeDMA is essentially solvent controlled, whereas that in CNPeDMA is faster than diffusive solvation, this difference being accounted for by a larger driving force for charge separation in the latter. On the other hand, the CSS lifetime of PeDMA is of the order of a few nanoseconds independently of the solvent, whereas that of CNPeDMA decreases with increasing solvent polarity from a few nanoseconds to a few hundreds of picoseconds. Comparison of these results with previously published data on the fluorescence quenching of Pe and CNPe in pure DMA shows that the charge separation and the ensuing charge recombination occur on similar time scales independently of whether these processes are intra- or intermolecular.     

Intramolecular hydrogen bonds and conformations of the 1,4-butanediol molecule

IR and Raman spectra of 1,4-butanediol (BD) versus variations in the medium (CCl₄, CH₃CN), concentration, temperature, and phase state were obtained. The observed changes attest to the conformational variety of BD molecules under the experimental conditions. On the basis of the analysis of the v(OH) region it is concluded that both in the gas phase and in CCl₄ solution conformers of BD with free OH groups coexist with conformers with O-H...O intramolecular hydrogen bonds. The difference in enthalpies, H, for the groups of conformers with and without intramolecular hydrogen bonds was found from the temperature dependence of the v(OH)_free and v(OH)_intra band intensities. The structures and energies for 70 possible spectrally and energetically distinguishable conformers of BD that do not take into account intramolecular hydrogen bonds were calculated by molecular mechanics with account of electron lone pairs of oxygen atoms. Using the experimental values of H and the calculated relative conformer energies,E, the intramolecular hydrogen bond energyE _intra= 3.7 kcal mol^–1 was found.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1572–1577, September, 1993.