Time-dependent Stokes Shift Research Articles

Dynamic fluorescence Stokes shift of an acetylene-bridged donor–acceptor compound in alcohols at low temperatures

The time-dependent fluorescence Stokes shift of p,p’-cyano thiomethyl diphenylacetylene in three different alcohols (1-propanol, 1-butanol, and 1-octanol) at room temperature was measured using time-correlated single photon counting and the spectral reconstruction method. These measurements were also performed with the probe in 1-propanol at low temperatures. The observed behavior is consistent with the dynamics of solvation of a polar excited state. As expected, the average solvation time increases with decreasing temperature and is correlated with solvent bulk viscosity down to 180 K. Apparent deviations observed at lower temperatures are attributed to the finite emission lifetime (≊1 ns) of the probe. A significant fast component (<100 ps) is observed in the dynamic Stokes shift even at temperatures close to the freezing point of 1-propanol. Different mechanisms that could result in such a fast component are discussed.

Configurational relaxation of ?Van Der Waals complexes? in solution

Novel measurements on the microscopic solvation dynamics of 3-amino-N-methylphthalimide (3ANMP) in nonpolar decalin (decahydronaphthalene) solutions with small concentrations of ethanol and other polar additives have been made, using the time-dependent fluorescence Stokes shift (TDFSS) technique. It is found that the experimentally measured TDFSS slightly differs from that predicted by the theory of the generalized Smoluchowski-Vlasov equation (SVE). The essentially slower character of the TDFSS in binary solutions than given by the SVE approach may be explained by the control of the solvation behavior by the large-wavevector processes involving nearest-neighbor molecules.

Photochemistry of an unsymmetrical polymethine-cyanine dye; solute—solvent interactions and relaxation dynamics of LDS 751

The photophysical and photochemical properties of the LDS 751 or styryl 8 molecule have been investigated in various solvents. Our results indicate that the photostability of this laser dye is mostly due to an efficient S1 → S0 non-radiative internal conversion process which is faster than the radiative process. This radiationless relaxation is governed by a viscosity-dependent process, tentatively assigned to the free rotor effect between the polymethinic chain and the end groups due to the single bond character of the dominant benzenoid structure. Moreover, intersystem crossing to the triplet state and trans-cis photoisomerization are totally inefficient. The shape (relatively small molar extinction coefficient and large bandwidth) and the blue shift of the absorption spectrum in polar solvents show that this unsymmetrical polymethine-cyanine dye is highly polar in the ground state with the cationic positive charge mainly localized on the benzothiazole nitrogen and inducing a large orientation polarization of the surrounding solvent molecules. The absorption transition involves a much less polar Franck—Condon excited state and a charge transfer from the dimethylanilino nucleus to the benzothiazole nucleus. The shape of the fluorescence spectrum, similar to that of symmetrical polymethine-cyanines and almost independent of solvent polarity, confirms the unpolar character of the first singlet excited state. The sub-picosecond analysis of the time-dependent fluorescence Stokes shift thus probes the relaxation of solvent molecules freed from solute—solvent interactions after the Franck—Condon absorption transition.

Solvation dynamics in methanol: Solute and perturbation dependence

We present the results of a molecular dynamics (MD) simulation study of solvation dynamics in response to instantaneous changes in the charge distribution of neutral diatomic solutes in methanol at room temperature and atmospheric pressure. Our simulations are designed to model the time dependent Stokes shift in the fluorescence spectrum of probe molecules in this solvent and focus on the solute and perturbation dependence of CΔE(t), the response function of ΔE the difference in the solute-solvent interaction energy between the excited and ground state solutes. The solute dependence is investigated by comparing the solvation dynamics for a given change in the charge distribution of solutes with different Lennard-Jones site diameters and the perturbation dependence by comparing the responses to dipole creation and reversal in each of these solutes. We find that CΔE(t) is strongly dependent on both the solute and the perturbation, especially in the early (< 200 fs) portions of its time-evolution. By examining the time-evolution of the structure around the solutes and the effects of deuteration of the hydroxyl hydrogen, we show that early solvation dynamics is determined by the proximity of the OH groups to the solute and by the ability of the hydroxyl hydrogens in the solute vicinity to rotate freely about the CO bonds. We also demonstrate that changes in the first few solvation shells occur on all the time scales relevant to CΔE(t) relaxation, with the slowest step corresponding to the most sharply-defined structural feature, the solute-solvent hydrogen bond. Comparison of CΔE(t), obtained by nonequilibrium MD simulation of dipole reversal, to the time correlation of equilibrium fluctuations in ΔE confirms our earlier finding, based on the results for dipole creation, that the linear response approximation has a very limited range of validity for solvation dynamics in methanol.

Sub-picosecond fluorescence study of the LDS 751 dye molecule in ethanol

A time-dependent fluorescence Stokes shift study of the LDS 751 (styryl 8 ) dye molecule in ethanol solution is presented. Fluorescence intensity curves were recorded with sub-picosecond time resolution at various wavelengths using the fluorescence upconversion method. From the experimental curves, time-resolved fluorescence spectra have been reconstructed, allowing the calculation of the spectral shift correlation function c( t)=[ ν ( t)- ν (∞)]/[ ν (0)- ν (∞)]. We find c( t) of LDS 751 in ethanol at room temperature to be well described by a monoexponential function with a 1/ e time of 5±1 ps.

Dynamic salt effect on excited large dipole solvation

A time-resolved fluorescence technique is employed to measure the time-dependent Stokes shift of coumarins 102 and 153 in non-aqueous electrolyte solutions. The electrolyte relaxation time exhibits a linear dependence on the electrolyte concentration in the range 10 mM–1 M.

Ultrafast solvent dynamics: Connection between time resolved fluorescence and optical Kerr measurements

The vibrational characteristics of liquid dynamics are used to describe the ultrafast relaxations observed in time-dependent fluorescence Stokes shift [J. Chem. Phys. 95, 4715 (1991)] and heterodyne detected optical Kerr effect measurements on acetonitrile, via a Brownian oscillator model. Introducing a frequency distribution of vibrational modes makes it possible to compare the two experiments. The ultrafast decays observed in the fluorescence Stokes shift and optical Kerr signals are produced by destructive superposition of the high frequency, underdamped modes.

Microscopic expression for dielectric friction on a moving ion

A theoretical study of the dielectric friction on an ion moving through a dipolar liquid is presented. A microscopic expression for the dielectric friction is derived from linear response theory for a Brownian ion. This expression supports the picture of a ‘‘dynamic solventberg’’ model in the sense that much of the contribution to dielectric friction (ζDF) comes from the nearest-neighbor molecules. The translational modes of the solvent are found to have a very strong influence on the dielectric friction, in agreement with the observation of Colonomos and Wolynes [J. Chem. Phys. 71, 2644 (1979)]. In fact, except in the limit of small ion size, the microscopic ζDF is significantly larger than the continuum ζDF in the absence of the translational motion of the solvent molecules, but the reverse is obtained in the presence of a substantial translational contribution. It is found that the recovery of the continuum limit results from the molecular expression requires serious assumptions, some of which are hard to justify. It is also found that the point dipole approximation for the dipolar solvent molecule leads to improper results for ζDF because this approximation gives a wrong wave vector (k) dependence of the wave vector dependent dielectric function (ε(k)) of the liquid at large k (kσ≫2π, where σ is the solvent molecular diameter). We show that within a linear equilibrium theory for dipolar liquids, the cross correlations between the short ranged hard force and the long ranged dipolar force is zero so the calculations of Colonomos and Wolynes are internally consistent. However, this cross correlation can be quite important if the soft force also contains a spherically symmetric part. The similarity between the solvent role in ζDF and in the time-dependent fluorescence Stokes shift is discussed. The limitations of the present theory are also pointed out.

Nonexponential solvation dynamics of simple liquids and mixtures

Novel measurements on the microscopic solvation dynamics of coumarin probes in several simple polar solvents and solvent mixtures have been made using the time dependent fluorescence Stokes shift technique. The microscopic solvent relaxation function, C( t), is observed to be poorly modeled by a single exponential decay in many cases. The average experimental solvation times,<τ s >, for pure solvents and binary solvent mixtures are close to values predicted by dielectric continuum theory, but in many cases, the observed C( t) shape does not agree with that predicted by dielectric continuum theory. The results suggest that molecular motion of solvent molecules near the solute can be responsible for microscopic solvation components of C( t) that are not predicted using bulk dielectric data of the neat solvent and the dielectric continuum theory. In addition to the solvation dynamics results, some potential sources of probe molecule non-ideality are examined, and it is shown that these effects are not significant contributors to experimental error for these C( t) measurements.

Impulsive pump-probe and photon-echo spectroscopies of dye molecules in condensed phases.

A theory for impulsive resonant pump-probe and photon-echo spectroscopies of polyatomic dyes in solution is developed. A multimode calculation in which the intramolecular and solvation modes are modeled as Brownian oscillators is presented. Electronic dephasing, the time-dependent Stokes shift, and quantum beats are analyzed.

Relationship between energy gap time correlation and fluorescence stokes shift correlation functions in solvation dynamics

The relationship between the energy gap time correlation function (EGTCF), measured in optical lineshape experiments, for a charged particle in a polar liquid, and the time correlation function obtained from time-dependent fluorescence Stokes shift measurements is explored. We show that if the distortion of the host solvent by the polar solute molecule is neglected, then both provide the same dynamical information. In the presence of a sizeable solvent distortion, emission and absorption lineshapes and Stokes shift measurements are all influenced differently by the solvent dynamics.

Nonlocal electrostatic effects on polar solvation dynamics

A phenomenological theory of polar solvation dynamics in electron transfer that accounts for the spatial- and frequency-dependent dielectric function of the solvent is developed and described in a format appropriate to time-dependent fluorescence Stokes shifts. The basic features of the relaxation dynamics are explored by using various analytical expressions for the dielectric function. The presence of spatial correlations persisting to frequencies higher than those corresponding to longitudinal solvent relaxation, τ−1L, yields significant or even substantial decay components with relaxation times shorter than τL. These are associated with motions of individual molecules within the solvent structural network. The implications of these predictions for solvation dynamics in activated charge-transfer processes are noted.

Polar Solvent Dynamics and Electron-Transfer Reactions

Polar solvents often exert a dramatic influence on reactions in solution. Equilibrium aspects of this influence involve differential solvation of reactants compared to the transition state that lead to alteration of the free-energy barrier to reaction. Such effects are well known, and often give rise changes in reaction rates of many orders of magnitude. Less well understood are effects arising from non-equilibrium, dynamical aspects of solvation. During the course of reaction, charge is rapidly redistributed among reactants. How the reaction couples to its solvent environment depends critically on how fast the solvent can respond to these changes in reactant charge distribution. In this article the dynamics of solvation in polar liquids and the influence of this dynamics on electron-transfer reactions are discussed. A molecular picture suggests that polar solvation occurs on multiple time scales as a result of the involvement of different types of solvent motion. A hierarchy of models from a homogeneous continuum model to one incorporating molecular aspects of solvation, combined with computer simulations, gives insight into the underlying dynamics. Experimental measures of solvation dynamics from picosecond and subpicosecond time-dependent Stokes shift studies are compared with the predictions of theoretical models. The implication of these results for electron-transfer reactions in solution are then briefly considered.

Solvent relaxation in thermal electron-transfer reactions: Some comparisons with real-time measurements of solvation dynamics

The role of solvent dielectric relaxation on the barrier-crossing dynamics for outer-sphere electron transfer (ET), as evaluated from the solvent-dependent kinetics of metallocene self-exchange reactions, is compared with recent real-time measurements of polar solvation dynamics obtained from time-dependent fluorescence Stokes shifts (TDFS) for suitable charge-transfer excited states. While the solvent-dependent kinetics obtained in some aprotic media are consistent with the TDFS measurements, the barrier-crossing dynamics in several associated and/or highly polar liquids are indicative for much faster relaxation than inferred from TDFS. The possible nature, and implications, of these rapid modes are discussed.