Femtosecond and nanosecond laser flash photolysis was used to determine the photophysical and photochemical processes in aqueous solutions of Fe(III) ion and 5-sulfosalicylic acid (SSA) containing the FeSSA complex and the free ligand. Excitation of the FeSSA complex in the charge transfer band (λ_max = 505 nm) is followed by an ultrafast relaxation to the ground electronic state with two characteristic times of 0.26 and 1.8 ps. The shorter time constant is ascribed to internal conversion to the vibrationally hot electronic ground state of FeSSA and the 1.8 ps time constant is assigned to the vibrational cooling of the ground state. The UV irradiation of the solution (308 nm) leads to the excitation of both the free ligand and the FeSSA complex. The latter relaxes rapidly and the free ligand undergoes intersystem crossing to the triplet state. This system undergoes an irreversible photochemical reaction originating from an electron transfer (k = (9 ± 2) × 10⁸ M^−1 s^−1) from the free ligand in the triplet state to the FeSSA complex. This electron transfer is accompanied by an energy transfer between these species (k = (1.3 ± 0.2) × 10⁹ M^−1 s^−1).

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe^•+, generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump−pump−probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe^•+ was found to depend on the time delay between the first and second pump pulses, Δt₁₂, i.e., on the “ageâ€� of the ion. At short Δt₁₂, the GSR dynamics is biphasic, and at Δt₁₂ larger than about500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe^•+ remains biphasic independently of Δt₁₂. The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.

The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donor−acceptor 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.

Visible pump−probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [LCo^IIINCM^II(CN)₅]^-, where L is a pentadentate macrocyclic pentaamine (L¹⁴) or triamine-dithiaether (L^14S) and M is Fe or Ru. Nanosecond pump−probe spectroscopy on frozen solutions of [L¹⁴Co^IIINCFe^II(CN)₅]^- and [L^14SCo^IIINCFe^II(CN)₅]^- at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350−400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [L¹⁴Co^IIINCRu^II(CN)₅]^- was too short-lived to allow its detection. Femtosecond pump−probe spectroscopy on aqueous solutions of [L¹⁴Co^IIINCFe^II(CN)₅]^- and [L^14SCo^IIINCFe^II(CN)₅]^- at room temperature enabled the lifetimes of their Co^II−Fe^III MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.

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 fluorescence dynamics of perylene in the presence of tetracyanoethylene in acetonitrile was studied experimentally and theoretically, taking into consideration that the quenching is carried out by remote electron transfer in the Marcus inverted region. The initial stage was understood as a convolution of the pumping pulse with the system response accounting for the fastest (kinetic) electron transfer accompanied by vibrational relaxation. The subsequent development of the process was analyzed with differential encounter theory using different models of transfer rates distinguished by their mean square values. The single channel transfer having a bell-shaped rate with a maximum shifted far from the contact produces the ground state ion pair. It was recognized as inappropriate for fitting the quenching kinetics at moderate and long times equally well. A good fit was reached when an additional near contact quenching is switched on, to account for the parallel electron transfer to the electronically excited state of the same pair. The concentration dependence of the fluorescence quantum yield is well fitted using the same rates of distant transfer as for quenching kinetics while the contact approximation applied to the same data was shown to be inadequate.

The charge recombination dynamics of the ion pairs formed upon electron-transfer quenching of perylene by tetracyanoethylene in acetonitrile has been investigated using ultrafast fluorescence upconversion, transient absorption, and transient grating techniques. For this donor/acceptor pair, charge separation is highly exergonic (ΔG_CS= −2.2 eV), but charge recombination is weakly exergonic (ΔG_CR = −0.6 eV). It was found that for more than 90% of the ion pair population, charge recombination is ultrafast and occurs in less than 10 ps. This decay component could not be observed in a previous investigation with a lower time resolution. The results indicate that the primary quenching product is a contact ion pair and not a solvent-separated ion pair as generally assumed for highly exergonic electron-transfer quenching processes. A possible explanation for this apparent divergence is that the contact ion pair is initially formed in an electronic excited state. Only a very minor fraction of the ion pair population undergoes the slow charge recombination predicted by Marcus theory for weakly exergonic charge-transfer processes (normal region).

Several aspects of ultrafast photochemistry in the condensed phase are discussed and illustrated by three examples from our laboratory.