1,720,975 research outputs found
Classical description of activated conformational processes in molecular systems coupled to solvent degrees of freedom
No full textExtended Fokker-Planck (FP) equations are generalized stochastic equations which describe the evolution of a set of coordinates, loosely referred to as solute, coupled with a relevant set of solvent variables. They are useful for the analysis of molecular dynamics in liquids, when a time-scale separation between probe motions and relaxation times of interactions with surroundings particles cannot be performed, because of persistence of slowly fluctuating components. In this article, we focus attention on a model system, made up of an angular coordinate and its conjugate momentum, submitted to a bistable potential, and coupled to a dissipative harmonic mode, to investigate the influence of polar solvents on reactive dynamics. The results are appropriate to describe dielectric effects, solvent-controlled conformational changes involving charge transfer which occur in photophysical processes, and the dynamic Stokes shifts observed in time-resolved fluorescence experiments
Diffusion-model for a reactive coordinate coupled to a solvent variable of different timescale
No full textA Smoluchowsky stochastic equation for a system made by a reactive coordinate x coupled to a solvent coordinate y is used to investigate the effect of additional degrees of freedom on the dynamics of reactive events. The eigenvalue spectrum is evaluated by representing the time evolution operator over basis sets of both orthonormal and optimized nonorthogonal functions, and the advantage of the latter choice in terms of computation time and memory requirement is demonstrated. Correlation functions for the observable f(x, y) = x, sensitive to the kinetic process, are evaluated over a large range of diffusional anisotropies, i.e., ratios of the principal values of the diffusion tensor of the system. A rationalization is made of the different physical regimes observed by varying both hydrodynamic and energy parameters of the model. Only in asymptotic cases of very slow solvent motions do adiabatic approximations lead to monodimensional diffusion equations with a sink term
A diffusive model for interpreting solvation dynamics in isotropic and ordered liquid phases
No full textSolutions are discussed for a two-body diffusive model in which a rotating probe is coupled to a solvent polarisation field, in the case of anisotropic diffusion. The model describes the diffusional rotational behaviour of a rigid molecule, carrying a permanent electric dipole and coupled, via a dipole-field term, to a polarisation vector or reaction field coordinate, which is also relaxing diffusively. When the solvent relaxation coordinate relaxes faster or slower than the probe rotation, a semi-analytical treatment of the system is possible, based on the separation of timescales. This treatment can be applied in the presence of a mean field potential acting on the probe orientation, thus allowing to consider liquid crystalline phases. We specialize our treatment to the case of solvation dynamics: a theoretical transient Stokes shift (TSS) correlation function is defined and its behaviour is discussed
Flow patterns of the nematic director in a rotating cylindrical sample
No full textThe nematic liquid crystal dynamics in a rotating cylindrical container, subjected to a constant magnetic field, has been considered in the past assuming a spatially homogeneous response of the director. In this communication, the formation and dynamics of flow-induced patterns of the director orientation field are investigated by numerical techniques, within different degrees of approximation. Nematodynamics equations are simplified by constraining the director to lie in a planar section of the cylinder, and the time and space dependence of the velocity field in the plane is taken into account. Patterns of the director orientation are calculated and discussed for different velocity profiles
Nematodynamics equations in two dimensions
No full textA discussion is provided of the combined influence of the elastic torques, boundary conditions and magnetic torque on the hydrodynamical behaviour of a nematic liquid crystal in two dimensions. A partial differential equation for the dependence in time and space of the director orientation is obtained from the constitutive hydrodynamical equations of a nematic phase (Leslie-Ericksen equations). A numerical solution is developed to calculate spatial distributions of the director for a sample in a rotating cylindrical vessel, subjected to a constant magnetic field. Boundary conditions are considered assuming a perfect alignment of the director to the internal walls of the cylinder. From the stationary and transient distributions, we simulate electron spin resonance spectra of a paramagnetic probe immersed in the liquid crystalline phase
Solvation dynamics of Coumarin 503 in the liquid-crystal mixture ZLI 1167
No full textThe solvation dynamics of the dye Coumarin 503 in the nematic mixture ZLI 1167 has been investigated by means of time-resolved fluorescence spectroscopy, both in the nematic and isotropic phases of the sample. No alignment was imposed on the micro-domains of the nematic phase, so that a complete depolarization of the emitted light was obtained. For the isotropic phase the usual set-up with vertically polarized excitation light and magic-angle detection was used. A time-dependent frequency shift of the maximum of the fluorescence band, caused by solvent reorganization after pulse excitation of the fluorescent probe, was observed. The analysis of the transient Stokes shift correlation function clearly shows biexponential behavior in the nematic phase, the slowest time constant varying from 1670 ps at 311.5 K to 230 ps at 373 K. The decay of the correlation function appears to be largely unaffected by the nematic–isotropic transition, suggesting that the local environment, rather than long-range ordering, determines Stokes shift dynamics. A theoretical model, which takes into account probe reorientations in the presence of a nematic field, and fluctuation of the local solvent polarization, has been developed to interpret the experimental findings
Interpretation of transient Stokes shift experiments in liquid crystals
No full textWe have extensively investigated the effect of the nematic barrier on the transient Stokes shift dynamics of a fluorescent dye with the aid of a stochastic model. A theoretical transient Stokes shift correlation function has been defined and its characteristic decay time has been determined for a wide range of dynamical regimes and dipolar coupling of the solute with the polar environment. Recent experimental data have been reviewed and discussed in relation with the newly available theoretical results
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