Electron Tunneling through Water: Oxidative Quenching of Electronically Excited Ru(tpy)_2^(2+)(tpy = 2,2':6,2' '-terpyridine) by Ferric Ions in Aqueous Glasses at 77 K

The luminescence lifetime of Ru(tpy)_2^(2+) (tpy = 2,2':6,2' '-terpyridine) is 8.0 μs in H_2SO_4/H_2O and HSO_3F/H_2O glasses (25% v/v) at 77 K, and 10.2 μs in D_2SO_4/D_2O (25% v/v). Addition of moderate concentrations of the powerfully oxidizing Fe(OH_2)_6^(3+) ion to the glasses leads to accelerated and highly nonexponential *Ru(tpy)_2^(2+) decay kinetics. The quenching is attributed to electron transfer from *Ru(tpy)_2^(2+) to randomly dispersed Fe(OH_2)_6^(3+) complexes. The luminescence decay kinetics and quantum yields in the three aqueous glasses indicate that the electron-transfer rate constants decrease from ∼10^(13) s^(-1) at van der Waals contact with an exponential distance decay constant of 1.68 ± 0.07 Å^(-1).