TIME-RESOLVED CHARGE-TRANSFER SPECTROSCOPY OF AROMATIC EDA COMPLEXES WITH NITROSONIUM - INNER-SPHERE MECHANISM FOR ELECTRON-TRANSFER IN THE ISOERGONIC REGION

Photoinduced electron transfer in various 1:1 aromatic EDA complexes with nitrosonium by the direct laser-pulse (20-ps and 10-ns fwhm) excitation of the charge-transfer bands leads to the spontaneous generation of the redox pair Ar.+ and NO. Temporal relaxation by back electron transfer to regenerate the EDA complex [Ar,NO+] is measured by following the spectral decay of Ar.+ with the aid of time-resolved spectroscopy over the two separate time domains I and II. Picosecond kinetics (k(I)) are associated with the first-order collapse of the geminate ion radical [Ar.+,NO] by inner-sphere electron transfer back to the EDA complex-the relatively slow rates with k(I) approximately 10(8) s-1 arising from driving forces that approach the isoergonic region, coupled with the rather high reorganization energy of nitric oxide. These allow effective competition from diffusive separation (k(s)) to form Ar.+ and NO as kinetically separate entities. Microsecond kinetics (k(II)) are thus associated with the second-order (back) electron transfer from the freely diffusing Ar.+ and NO. However, the comparison of the second-order rate constants calculated from Marcus theory shows that outer-sphere electron transfer is too slow to account for the experimental values of k(II). The second-order process is unambiguously identified (by the use of the thermochemical cycle in Scheme IV) as the alternative, more circuitous inner-sphere pathway involving the (re)association (k(a)) of Ar.+ and NO to afford the cation radical pair [Ar.+NO] followed by its collapse to the EDA complex. The general implications of inner-sphere complexes as reactive intermediates in electron transfer mechanisms in the isoergonic and endergonic regions are presented.