Solvent and counter-ion effects on intramolecular electron transfer rates of an organic mixed-valence compound

The intramolecular electron transfer (ET) rates of a radical anion of 4b,9b-dihydro-4b,9b-dimethyl-indeno[2,1-a]indene-5,10-dione in N,N-dimethylformamide (DMF), acetonitrile (AN) and tetrahydrofuran (THF) were determined at various temperatures using an electron spin resonance line-broadening method. Cryptand[2.2.2] (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) was added to the solutions in order to eliminate the counter-ion effect on the ET rates caused by the presence of the sodium cation produced in the chemical reduction for the radical preparation. The obtained rate constant in the DMF solution was 1.5×1010 s−1 at 253 K, a value about 10 times higher than that in the DMF solution with 0.4 mol l−1 (C2H5)4NClO4 as a supporting electrolyte. A significant contribution of solvent dielectric friction to the barrier crossing dynamics was shown in the DMF solution. In the THF solution, much slower electron transfer rates were observed (8.0×108 s−1 at 253 K). This retardation was attributed to the slow relaxation time of the effective electric field by the nearby Na[cryptate]+ ion.