Superexchange and dipole-dipole energy transfer from the [Cr(ox)(3)](3-) of 3D oxalate networks to encapsulated [Cr(bpy)(3)](3+)

Electronic energy transfer from [Cr(ox)(3)](3-) (ox = oxalate) in three-dim ensional (3D) anionic oxalate networks to encapsulated [Cr(bpy)(3)](3+) (bp y = 2,2'-bipyridine) cations at 1.5 K was investigated by time-resolved lum inescence spectroscopy. Two series of mixed crystals of nominal composition s [NaAl1-xCrx(ox)3][Rh0.99Cr0.01(bpy)(3)]ClO4 (x = 0, 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1) and [NaAl0.99Cr0.01(ox)(3)][Rh1-xCrx(bpy)(3)]ClO4 (y = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) were utilized. Energy transfer from [Cr(ox)(3)](3-) to [Cr(bpy)(3)](3+) occurs by two mechanisms. Rapid, short -range transfer (k(et) > 10(6) s(-1)) is attributed to superexchange coupli ng between the Cr3+ ions via pi overlap of the oxalate and bipyridine ligan ds. In addition, at low [Cr(ox)(3)](3-) concentrations (nominally x = 0.01) a very much slower process with a maximum k(et) approximate to 200 s(-1) i s identified in the time-resolved spectra and attributed to a dipole-dipole mechanism. Furthermore, the resonant [Cr(ox)(3)](3-) to [Cr(ox)(3)](3-) en ergy migration previously reported by von Arx et al. (Phys. Rev. (1996), B5 4, 15800) assists [Cr(ox)(3)](3-) to [Cr(bpy)(3)](3+) transfer as the [Cr(o x)(3)](3-) concentration increases.