77406-15-6Relevant articles and documents
Time-resolved charge-transfer spectroscopy of aromatic EDA complexes with nitrosonium. Inner-sphere mechanism for electron transfer in the isoergonic region
Bockman,Karpinski,Sankararaman,Kochi
, p. 1970 - 1985 (2007/10/02)
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 (k1) 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 k1 ～ 108 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 (ks) to form Ar?+ and NO as kinetically separate entities. Microsecond kinetics (kII) 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 kII. 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 (ka) 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.
