10206
J. Phys. Chem. 1996, 100, 10206-10209
Electron Transfer Rates from Vibrational Quantum States Kenneth G. Spears,* Xiaoning Wen, and Ruihua Zhang Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: February 13, 1996X
Electron transfer involves changes in molecular geometry that are important for controlling rates. In this work we report the first clear effects of vibrational quantum state on solution phase electron transfer rates. The spontaneous electron transfer rates for the recovery of an ion pair [Co(Cp)2+/V(CO)6-] are studied with picosecond infrared spectroscopy following optical excitation into its charge transfer band. The rates increase about 2-fold for each additional vibrational quantum in the CO stretching mode. These results allow new tests of electron transfer theory.
The understanding of molecular electron transfer processes has made enormous progress in the last 20 years.1 Electron transfer is very diverse, and such processes are important in molecular electronics devices, solar energy conversion, and biological processes. The experimental electron transfer rate normally is given as a single experimental number for a complex system, at some specific temperature and environment. The availability of a single rate has allowed only indirect tests of electron transfer models by averaging over the vibrational details in a quantum mechanical treatment of rate. Our recent experimental work has demonstrated, for the first time, that solution phase electron transfer rates can be measured with vibrational state resolution.2 In addition, theoretical models3 predict significant rate changes for some molecules when excited vibrational levels are populated. The earlier molecule we selected for the study of vibrational effects2 had some complexities in the observed kinetics that made it difficult to demonstrate both the quantum-resolved electron transfer rates and the correlated vibrational activity in the molecule after the electron transfer. Therefore, we report data for a new molecule that resolves these key experimental points and which we believe can serve as a major test of theoretical electron transfer models in the future. The less informative case of vibrational activity following electron transfer should be observable in many molecules, and such a case was reported for ultrafast electron transfer.4 The idea that multiple electron transfer rates, rather than a single rate, can be observed in the liquid phase was not previously recognized as possible because it is usually thought that complex molecules redistribute vibrational energy much faster than they transfer electrons. The processes of intramolecular vibrational redistribution (IVR) and vibrational relaxation (VR) are indeed quite fast; however, electron transfer also can be fast, and in certain molecules some vibrational modes can have lifetimes in the hundreds of picoseconds due to their high frequencies and wide separation from other frequencies. One such class of molecules includes the metal carbonyls, as shown by direct lifetime measurements of vibrational lifetimes.5 The molecule of interest is a molecular ion pair, specifically associated in a low dielectric solvent. The ion pair has sufficient electronic interaction to form a weak charge transfer absorption band. Optical excitation in the charge transfer band creates a full electron transfer to form a neutral pair. The neutral pair then returns to the ion pair by spontaneous electron transfer. X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00444-3 CCC: $12.00
Figure 1. Schematic of vibrational coordinates between the ion pair [A+D-] and the neutral pair [A D]. Horizontal lines are for vibrational quantum states in the CO stretching mode, where optical excitation in a charge transfer absorption at 600 nm directly populates vibrational levels of [A D] which returns to [A+D-] via spontaneous electron transfer.
The ion pair is [Co(Cp)2+|V(CO)6-] denoted as [A+D-], and its charge transfer band peaking at 630 nm has been reported previously in a study of the properties and photochemistry of carbonyl metalate compounds.6 The broad charge transfer absorption has no vibrationally resolved features, and any excitation wavelength must excite many vibrational modes. Therefore, the method of specific vibrational population is indirect; we rely on a complex vibrational excitation to relax in all vibrational modes except the long-lived CO stretching modes in V(CO)6. The initial population probability in the long-lived modes reflects a Franck-Condon absorption probability hidden under the broad absorption. As shown by our experiments, the fast (
