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-dimensional (3D) anionic oxalate networks to encapsulated [Cr(bpy) 3 ] 3+ (bpy = 2,2‘-bipyridine) cations at 1.5 K was investigated by time-resolved luminescence spectroscopy. Two series of mixed crystals of nominal compositions [NaAl 1 - x Cr x (ox) 3 ][Rh 0.99 Cr 0.01 (bpy) 3 ]ClO 4 ( x = 0, 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1) and [NaAl 0.99 Cr 0.01 (ox) 3 ][Rh 1 - y Cr y (bpy) 3 ]ClO 4 ( 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 coupling between the Cr 3+ ions via π overlap of the oxalate and bipyridine ligands. In addition, at low [Cr(ox) 3 ] 3- concentrations (nominally x = 0.01) a very much slower process with a maximum k et ≈ 200 s -1 is 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- energy migration previously reported by von Arx et al. ( Phys. Rev. ( 1996 ), B54, 15800) assists [Cr(ox) 3 ] 3- to [Cr(bpy) 3 ] 3+ transfer as the [Cr(ox) 3 ] 3- concentration increases.