Ultrafast Photoinduced Electron Transfer from Peroxide Dianion

The encapsulation of peroxide dianion
by hexacarboxamide cryptand
provides a platform for the study of electron transfer of isolated
peroxide anion. Photoinitiated electron transfer (ET) between freely
diffusing Ru­(bpy)32+ and the peroxide dianion
occurs with a rate constant of 2.0 × 1010 M–1 s–1. A competing electron transfer quenching pathway
is observed within an ion pair. Picosecond transient spectroscopy
furnishes a rate constant of 1.1 × 1010 s–1 for this first-order process. A driving force dependence for the
ET rate within the ion pair using a series of Ru­(bpy)32+ derivatives allows for the electronic coupling and reorganization
energies to be assessed. The ET reaction is nonadiabatic and dominated
by a large inner-sphere reorganization energy, in accordance with
that expected for the change in bond distance accompanying the conversion
of peroxide dianion to superoxide anion.