DYNAMIC ELECTRON-TRANSFER IN AQUO-SIO2 AND ALCO-SIO2 GELS

The photophysical properties of ruthenium tris(bipyridine) dichloride, Ru(bpy)(3)(Cl)(2), entrapped within monolithic SiO2 gels in the absence and presence of methyl viologen, MV(2+), are reported. Intense visible excitation (>20 mW/cm(2)) of RU(bpy)(3)(2+) doped gels results in the appearance of a uniform red color and a slightly asymmetric EPR signal, g = 2.010. Complex photoluminescence decays from Ru(bpy)(3)(2+)* in the presence and absence of MV(2+) are well modeled by two normal distributions of relaxation rates. The faster distribution is dynamically quenched by electron transfer to MV(2+). Charge-separated pairs [Ru(bpy)(3)(3+), MV(.+)] from photonic gels with Ru(bpy)(3)(2+) incorporated prior to gelation and MV(2+) adsorbed postgelation recombine with a rate constant of (2.7 +/- 0.2) x 10(9) M(-1) s(-1). The kinetics and cage escape efficiencies are similar to those observed in fluid solution.