The dynamics of charge recombination (CR) of excited donor-acceptor complexes composed of methoxysubstituted benzenes and pyromellitic dianhydride were investigated in four different solvents using both the multiplex transient grating and the transient absorption techniques. At constant driving force, the CR dynamics are substantially faster than those with methyl-substituted benzenes as donors. In acetonitrile (ACN), the CR time constant decreases from 3.5 ps with anisole down to 240 fs with tetramethoxybenzene. In valeronitrile, the CR is always slower than in ACN but is, in most cases, faster than diffusional solvation. The free energy, the solvent, and the temperature dependence of the CR dynamics can be qualitatively well reproduced using the hybrid model of Barbara and co-workers after incorporation of the contribution of inertial motion to solvation. The ability of this model to account for the absence of normal region at small driving force is also examined.

The influence of the excitation pulse carrier frequency on the ultrafast charge recombination dynamics of excited donor-acceptor complexes has been explored both theoretically and experimentally. The theoretical description involves the explicit treatment of both the optical formation of the nuclear wave packet on the excited free energy surface and its ensuing dynamics. The wave packet motion and the electronic transition are described within the framework of the stochastic point-transition approach. It is shown that the variation of the pulse carrier frequency within the absorption band can significantly change the effective charge recombination dynamics. The mechanism of this phenomenon is analyzed and a semiquantitative interpretation is suggested. The role of the vibrational coherence in the recombination dynamics is discussed. An experimental investigation of the ultrafast charge recombination dynamics of two donor-acceptor complexes in valeronitrile also is presented. The decays of the excited state population were found to be highly nonexponential, the degree of non-exponentiality depending on the excitation frequency. For one complex, the charge recombination dynamics was found to slow down upon increasing the excitation frequency, while the opposite behavior was observed with the other complex. These experimental observations follow qualitatively the predictions of the simulations.

The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donoracceptor complexes (DACs) composed of benzene derivatives as donors and of tetracyanoethylene or pyromellitic dianhydride as acceptors has been investigated in polar solvents using ultrafast time-resolved spectroscopy. Three different wavelength effects have been observed. (1) With complexes exhibiting two well-separated charge-transfer bands, the CR dynamics was found to be slower by a factor of about 1.5 upon excitation in the high-energy band. This effect was measured in both fast and slow relaxing solvents and was discussed in terms of different DAC geometries. (2) When the CR is faster than diffusive solvation, a slowing down of the CR with increasing excitation wavelength accompanied by an increase of the nonexponential character of the dynamics was measured. This effect appears only when exciting on the red edge of the charge-transfer absorption band. (3) When the driving force for CR is small, both nonequilibrium (hot) and thermally activated CR pathways can be operative. The results obtained with such a complex indicate that the relative contribution of these two paths depends on the excitation wavelength.

A single pulse interferometric coherent anti-Stokes Raman (CARS) spectroscopy method is used to obtain broadband CARS spectra and microscopy images of liquid and polymer samples. The pump, Stokes, and probe pulses are all selected inside a single broadband ultrafast pulse by a phase- and polarization-controlled pulse shaping technique and used to generate two spectral interference CARS signals simultaneously. The normalized difference of these two signals provides an amplified background-free broadband resonant CARS spectrum over the 400-1500 cm(-1) range with 35 cm(-1) spectral resolution. Chemically selective microscopy images of multicomponent polymer and liquid samples are investigated with this new CARS method. Multiplex CARS spectra at 10,000 spatial points are measured within a few minutes, and used to construct chemically selective microscopy images with a spatial resolution of 400 nm. The spectral bandwidth limits, sensitivity, homodyne amplification advantages, spatial resolution, depolarization, chromatic aberration, and chemical imaging aspects of this new technique are discussed in detail.

The solvation dynamics of an organic dye, IR140, in methanol, ethanol, and in a series of six alkanenitriles has been investigated using the transient grating technique. In all solvents, the dynamics exhibit ultrafast, almost solvent-independent, components ascribed to inertial solvation, and a slower viscosity-dependent component, due to diffusive solvation. The relative amplitudes of these components depend on both the solvent and on the wavelength at which the experiment is performed. The contribution of inertial motion increases with decreasing size of the solvent molecules and with decreasing wavelength. It appears that diffusive motion is associated with a loose solvent shell, while inertial motion dominates when the solvation layer is dense.

The charge recombination dynamics of geminate ion pairs formed by electron transfer quenching of cyanoanthracene derivatives by bromo- and iodo-anisole in acetonitrile was investigated using various ultrafast spectroscopic techniques. Without a heavy atom, the only charge recombination pathway is that leading to the neutral ground state. With heavy atom substituted anisoles, charge recombination to the local triplet state of the excited precursor is observed. Time constants for triplet charge recombination ranging from 400 ps to less than 10 ps, depending on the heavy atom and on the energy gap between the ion pair and the triplet state, have been measured. This heavy atom effect was observed with ion pairs formed upon electron-transfer quenching with driving force going from −0.15 to −0.6 eV, suggesting that these intermediates are in fact exciplexes. A new scheme for producing free ions with a high yield using this effect and a secondary electron donor is also demonstrated

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The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.