Isolation of the Latent Precursor Complex in Electron-Transfer Dynamics. Intermolecular Association and Self-Exchange with Acceptor Anion Radicals

Abstract

Transient [1:1] complexes formed in the bimolecular interactions of electron acceptors (A) with their reduced anion radicals (A(-.)) are detected and characterized in solution for the first time. The recognition of such metastable intermediates as the heretofore elusive precursor complex (A(2)(-.)) in electron-transfer processes for self-exchange allows the principal parameters lambda (Marcus reorganization energy) and H(DA) (electronic coupling element) to be experimentally determined from the optical (charge-transfer) transitions inherent to these intermolecular complexes. The satisfactory correspondence of the theoretically predicted with the experimentally observed rate constants validates these ET parameters and the Marcus-Hush-Sutin methodology for strongly coupled redox systems lying in the (Robin-Day) Class II category. Most importantly, the marked intermolecular electronic interaction (H(DA)) within these precursor complexes must be explicitly recognized, since it dramatically affects the electron-transfer dynamics by effectively lowering the activation barrier. As such, the numerous calculations of the reorganization energy previously obtained from various self-exchange kinetics based on lambda = 4DeltaG must be reconsidered in the light of such a precursor complex, with the important result that ET rates can be substantially faster than otherwise predicted. On the basis of these studies, a new mechanistic criterion is proposed for various outer-sphere/inner-sphere ET processes based on the relative magnitudes of H(DA) and lambda.

Ultrafast Photoinduced Charge Separation in Naphthalene Diimide Based Multichromophoric Systems in Liquid Solutions and in a Lipid Membrane

“…The measurements were repeated with B1 in pure NB, which has the advantage over TCNE to be a substantially weaker electron acceptor (E red (NB) ) -1.15 V vs SCE, 63 E red (TCNE) ) +0.24 V vs SCE). 64 Therefore, CR of the resulting ion pair should be more exergonic and thus slower, as predicted for electron-transfer processes in the inverted regime. 65 As illustrated in Figure 7C, the transient spectrum of B1 in NB is almost the same as that in MeOH except for a larger relative intensity in the 550 nm region and a red shift of the negative band.…”

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

“…Although the IDU/TCNE complex exhibits some absorption at this wavelength, its absorption coefficient is too small compared to that of TCNE •-to contribute significantly to the transient signal. 41 With the stronger donors, the absorbance at 400 nm is negligibly small.…”

The origin of the two‐electron/four‐centers CC bond in π‐TCNE22− dimers: Electrostatic or dispersion?

“…[2][3][4][5][6][7][8][9] One example of crystal where this type of bond is found is the K 2 TCNE 2 glyme 2 crystal, 10 where one finds K 2 TCNE 2 aggregates that clearly show the presence of -TCNE 2 2À dimers (Fig. 1).…”

“…TCNE À repulsion is the attractive anion-solvent interaction). 13 Initial B3LYP calculations on TCNE 2 2À (dichloromethane) 6 Figure 1. Geometry of the (K þ ) 2 TCNE 2 2À aggregates used in this work: (a) Geometry found in the KIY-HIJ crystal (notice the equatorial position of the two K þ atoms, and the outward orientation of the CN groups).…”