Solvent relaxation method

Keywords Block copolymer micelles Fluorescence anisotropy Fluorescence correlation spectroscopy Molecular dynamics simulations Monte Carlo simulations Solvent relaxation method Time-resolved fluorescence... 

The principle of the solvent relaxation method is depicted in Fig. 5 for a fluorophore immersed in a polar solvent (with equilibrium ground-state relative... 

Monitoring of the half-width of the emission band provides information about whether the entire process, or just a part of it, was included within the time window of the experiment. If only a decrease is observed, the early part of the relaxation process is beyond the time resolution of the relevant apparatus. In contrast, if only the rising part is observed, the process is slow and the fluorescence lifetime is too short and does not allow monitoring of the entire relaxation process. The following chapters give some examples of studies of self-assembling polymer systems by the solvent relaxation method. 

A major advance in the investigation of the intramolecular dynamics of spin equilibria was the development of the Raman laser temperature-jump technique (43). This uses the power of a laser to heat a solution within the time of the laser pulse width. If the relaxation time of the spin equilibrium is longer than this pulse width the dynamics of the equilibrium can be observed spectroscopically. At the time of its development only two lasers had sufficient power to cause an adequate temperature rise, the ruby laser at 694 nm and the neodymium laser at 1060 nm. Neither of these wavelengths is absorbed by solvents. Various methods were used in attempts to absorb the laser power, with partial success for microsecond relaxation times. 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

At a given computational level, the solvent relaxation contribution to the excitation energy may be approximated by using two basically different methods, the state-specific method (SS) and the linear response method (LR), depending on the QM methodology used. This directly involves the problem of extending the PCM basic model to a QM description proper for excited states. 

Jaume et al. (1984) studied the contribution of solvent relaxation to the reaction coordinate of the F (H20) + CH3F(H20)SN2 reaction. Potential energy calculations were performed using the ab initio MO method with the 3-21G basis set. The authors found large variation of the solvation parameters along the reaction path and concluded that solvent coordinates are an important part of the reaction coordinate. They showed that the solvent acts not only as a medium for the reaction but also as a rectant. Thus, the solvent does not adjust its position to the changing chemical system but rather takes part in it. 

Solvation dynamics are measured using the more reliable energy relaxation method after a local perturbation [83-85], typically using a femtosecond-resolved fluorescence technique. Experimentally, the wavelength-resolved transients are obtained using the fluorescence upconversion method [85], The observed fluorescence dynamics, decay at the blue side and rise at the red side (Fig. 3a), reflecting typical solvation processes. The molecular mechanism is schematically shown in Fig. 5. Typically, by following the standard procedures [35], we can construct the femtosecond-resolved emission spectra (FRES, Stokes shifts with time) and then the correlation function (solvent response curve) ... 

A novel pump-damp-probe method (PDPM), which allows the characterization of solvation dynamics of a fluorescence probe not only in excited but also in the ground states has been recently developed (Changenet-Barret, 2000 and references therein). In PDPM, a pump produces a nonequilibrium population of the probe excited, which, after media relaxation, is simulated back to the ground states. The solvent relaxation of the nonequlibrium ground state is probed by monitoring with absorption technique. The inramolecular protein dynamics in a solvent-inaccessible region of calmodulin labeled with coumarin 343 peptide was examined by PDPM. In the pump-dump-probe experiments, part of a series of laser output pulses was frequency-doubled and softer beams were used as the probe. The delay of the probe with respect to the pump was fixed at 500 ps. 

An elegant alternative is to use the NMR relaxation rate of the solvent. The method relies on the fact that, when there is rapid exchange between solvent molecules on the solid surface and In the bulk solution, the contributions of these two populations to the (average) relaxation rate are additive. In a colloidal dispersion, mobile solvent In the bulk (b) and less mobile adsorbed solvent (a) have widely different relaxation rates, indicated by and T. respectively. The average solvent relaxation time is then given by )... 

In order to understand the dynamics of the solvent fluctuation, many experimental as well as theoretical efforts have been made intensively in the last decade. One of the most convenient methods to observe solvent reorganization relaxation processes within the excited state molecule is time resolved fluorescence spectroscopy. By using time resolved techniques a time dependent fluorescence peak shift, so ( ed dynamic Stokes shift, has been detected in nanosecond picosecond >, and femtosecond time regions. Another method to observe solvent relaxation processes is time resolved absorption spectroscopy. This method is suitable for the observation of the ground state recovery of the solvent orientational distribution surrounding a solute molecule. 

In the present study, the rate constants of the thermal-induced intramolecular ET reaction of pico-second order were first determined for mixed-valence biferrocene monocation (Fe(II),Fe(III)) in seven solvents at various temperatures by the NMR relaxation method. The procedure of the determination of the rate constants from the measured H spin-lattice relaxation times (Ti) is shortly mentioned and the effect of dynamical property of solvent on the ET rate is discussed. 

The two most important nonradiative relaxation methods that compete with fluorescence are illustrated in Figure 27-lb. Vibrational relaxation, depicted by the short wavy arrows between vibrational energy levels, takes place during collisions between excited molecules and molecules of the solvent. Nonradiative relaxation between the lower vibrational levels of an excited electronic state and the higher vibrational levels of another electronic state can also occur. This type of relaxation, sometimes called internal conversion, is depicted by the two longer wavy arrows in Figure 27-lb. Internal conversion is much less efficient than vibrational relaxation, so that the average lifetime of an electronic excited state is between 10 and 10 s. The exact mechanism by which these two relaxational processes occur is currently under study, but the net result is a tiny increase in the temperature of the medium. 

Following photo excitation a solution sample returns to thermal equilibrium by a variety of photochemical and photophysical processes. The faster processes, e.g. vibrational relaxation and solvent relaxation, have only recently begun to be studied by direct kinetic methods (1-5). Picosecond emission spectroscopy has been especially useful in elucidating these ultrafast processes (1,/3, 5). As electronically excited molecules relax, their fluorescence spectrum shows time dependence that reflects the relaxation processes. 

Method B. Very few molecules exhibit dramatic fluoresence dynamics of the type observed for 3HF. In most cases the observed effects are more subtle, involving a slight time-dependent variation in the fluorescence band profile. Effects of this type can be due to vibrational relaxation, solvent relaxation and torsional relaxation. 

Of the relaxation methods only the temperature-jump and pressure-jump methods have been adapted for high pressure application, and of these two only the former (hptj) has been used in many systems for volume of activation determinations. Despite the flurry of activity in developing hptj,84 88 the method has not found application in organometallic chemistry, although in principle it could be employed if the system properties and solvent were suitable. 

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