Effect of the Excitation Wavelength on the Ultrafast Charge Recombination Dynamics of Donor-Acceptor Complexes in Polar Solvents Olivier Nicolet, # Natalie Banerji, Ste ´ phane Page ` s, and Eric Vauthey* Department of Physical Chemistry, UniVersity of GeneVa, 30 quai Ernest-Ansermet, CH-1211 GeneVa, Switzerland ReceiVed: June 15, 2005; In Final Form: July 19, 2005 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. Introduction For many practical applications, the charge recombination (CR) of ion pairs formed upon photoinduced electron-transfer (ET) reactions is an unwanted energy-wasting process. For this reason, the factors influencing its dynamics have been inves- tigated in detail. 1-9 There are two major difficulties when studying the CR dynamics of an ion pair formed upon bimolecular ET. The first, which is still debated, concerns the exact nature of this ion pair, contact ion pair (CIP), or solvent- separated ion pair. 8-13 The second is that the time resolution of the measurements is limited to the time scale of ion pair formation. Thus, the CR dynamics is no longer experimentally accessible as soon as it is faster than quenching. Both problems can be eliminated when working with a donor-acceptor complex (DAC). Indeed, excitation in its charge-transfer (CT) band results in the population of an excited state that is essentially a CIP. The investigation of the CR dynamics of CIPs generated by CT excitation has been pioneered by Mataga and co-workers, 14-16 who have shown that its driving force depen- dence deviates substantially from the predictions of the semi- classical theory of nonadiabatic ET reaction, especially in the weakly exergonic region. While this theory predicts an increase of the rate constant with the driving force (normal regime) followed by a decrease at higher exergonicity (inverted re- gime), 17 only the inverted regime is observed with the CR of CIPs. 14-16,18 Indeed, CR becomes faster as the driving force decreases and takes place in the subpicosecond time scale at ΔG CR > ∼ -1 eV. Several hypotheses have been proposed to explain this effect. 18-22 We have recently shown that the driving force, the solvent and the temperature dependence of the CR dynamics of a series of excited DACs composed of methoxy- benzenes and pyromellitic dianhydride, could be very well reproduced with the hybrid model of Barbara and co-work- ers, 23,24 after incorporation of the contribution of inertial motion to solvation. 25 The basic idea of this model is illustrated in Figure 1: upon optical excitation, the excited DAC population is formed away from equilibrium and, therefore, CR can take place while the population is still relaxing as soon as the Franck-Condon factor is large enough. Of course, such a “hot” CR requires a sufficiently large electronic coupling constant, V. For weakly exergonic processes, CR can completely occur before the excited population has equilibrated, and the normal region where CR is a thermally activated process is not observed. An important signature of the nonequilibrium char- # Present address: Chemistry Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720-8198. * Corresponding author: eric.vauthey@chiphy.unige.ch. Figure 1. Cuts in the free energy surface of the ground and excited states of a DAC along the solvation coordinate illustrating the dependence of the nonequilibrium CR dynamics on the excitation wavelength. The thin parabolas represent vibrational excited states. 8236 J. Phys. Chem. A 2005, 109, 8236-8245 10.1021/jp0532216 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/27/2005