Effect of the Excitation Wavelength on the Ultrafast Charge Recombination Dynamics of Donor−Acceptor Complexes in Polar Solvents

Abstract

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.

Solvent and Spectral Effects in the Ultrafast Charge Recombination Dynamics of Excited Donor−Acceptor Complexes

“…An example of such a fit for IDU/TCNE in VaCN is illustrated in Figure 3. The most important parameters obtained in the simulations are listed in Table 3 together with the experimental data 17 for comparison. The nonexponentiality parameter s was calculated from the fit of the decay of the DAC excited-state population, P(t), to the trial function For s < 1, this function is known as the stretch exponential function.…”

“…17 The effective CR time constants of all DACs have been shown to decrease markedly with the diffusive relaxation time of the solvent. For the sake of brevity, this dependence will be called hereafter the solvent effect.…”

“…17 The last experimental fact is an indication that the medium reorganization energy is practically independent of the solvent and, hence, the solvent effect is mainly due to dynamic solvent properties. In spite of the fact that very detailed experimental and theoretical explorations of the CR kinetics have been performed with a large number of DACs, there are still several unresolved issues regarding the mechanism of ultrafast CR of excited DACs.…”

Charge recombination in excited donor-acceptor complexes with two absorption bands

“…The dynamics of charge recombination in the complex between 1,2,4-trimethoxybenzene (TMB) and tetracyanoethylene (TCNE) in several polar solvents was studied in [17]. This complex is characterized by two charge-transfer absorption bands (CT1 and CT2) with maxima at 655 and 434 nm, respectively.…”

Physical Chemistry at the University of Geneva

“…Beside this, the group is investigating ultrafast charge transfer reactions that take place on timescales similar to those of vibrational and solvent relaxations and whose dynamics depends on the excitation wavelength. [33] Such 'non-Kasha' photochemistry opens interesting perspective for a coherent control of chemical reactions in liquids.…”