Superexchange and Dipole−Dipole Energy Transfer from the [Cr(ox)₃]^3- of 3D Oxalate Networks to Encapsulated [Cr(bpy)₃]³⁺

Abstract

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 [NaAl1 - x Cr x (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 - y Cr y (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 > 106 s-1) is attributed to superexchange coupling between the Cr3+ 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.

Luminescence and Energy Transfer of [Ru(bpy)3]2+, [Cr(ox)3]3-, and [Os(bpy)3]2+in Three-Dimensional Oxalato-Networks

“…Together with our previous studies 3, 4 we have now demonstrated the versatility of these systems for quantitative energy transfer studies, resulting in unambiguous conclusions with regard to interaction mechanisms and to the nature of the various processes.…”

“…In 4 namely, the dipole-dipole mechanism with R c ) 10.6 Å and a super exchange mechanism due to a nearest neighbor interaction originating from π-overlap of the oxalate and bipyridine ligands. The ligands of the acceptor shells 1 and 2 have, in fact, very good π-overlap with the donor ligands, the ligand plane separation being only 3.7 Å, and also those in shell 3 still have nonnegligible overlap.…”

“…The class of cubic, three-dimensional metal-tris-oxalate network structures with tris-bipyridine complexes as counter ions of general compositions [ 4 2-) first synthesized by Decurtins et al 1 have interesting structural, magnetic, and photophysical properties. 2 Chemical variation and combination of metal ions of different valencies in the oxalate backbone as well as in the tris-bipyridine complexes offer unique opportunities for investigating a large variety of photophysical processes, from straight forward luminescence to light-induced electron transfer, and excitation energy transfer.…”

A kinetically inert and optically active CrIII partner in thermodynamically self-assembled heterodimetallic non-covalent d–f podates

“…S5, ESI) and the associated lifetime observed at 295 K (0.2-0.3 ms) involves the existence of thermally-activated non-radiative vibrational quenching processes combined with energy migration processes as previously reported for Cr III -containing coordination networks in which a temperature of 1.5 K is required to 'freeze' such processes. 31 As the maximum of the emission band shifts from 13301 cm Ϫ1 at 10 K to 13347 cm Ϫ1 at 295 K (Fig. S3, ESI), we suspect that the long-lived luminescence observed at low temperature and originating from isolated pseudo-octahedral CrN 6 chromophores in [GdCrL 3 ] 6ϩ is mainly quenched at room temperature leading to a faint residual emission arising from crystal defects or 'killer sites' produced by minor photochemical degradation.…”

Optical properties of thin films of [Ru(bpy)3][NaCr(ox)3] polymeric networks

“…The fact that the intensity of the emission from ͓Cr͑ox͒ 3 ͔ 3− decreases and the one of ͓Cr͑bpy͒ 3 ͔ 3+ increases with increasing temperature indicates a temperature dependent energy migration within the R 1 line of ͓Cr͑ox͒ 3 ͔ 3− followed by energy transfer to ͓Cr͑bpy͒ 3 ͔ 3+ acting as a final trap. 8 Based on the value for the limiting total quantum efficiency of this energy transfer of 0.28, the effective ͓Cr͑bpy͒ 3 ͔ 3+ concentration can be estimated to be below 0.05 mol %. 9 Figure 3 shows a comparison of the PL of ͓Cr͑ox͒ 3 ͔ 3− at 1.4 K from the thin film and from a polycrystalline sample using the non site selective excitation of the He-Ne laser.…”