Chromium(III)-trisoxalate,[Cr(ox)3]3- (ox = C2O42-), incorporated into polymeric networks of composition [NaCr(ox)3][MII(bpy)3] and [NaCr(ox)3][MIII(bpy)3]ClO4 (bpy= 2,2'-bipyridine, MII = Zn, Fe, Ru; MIII = Rh, Cr), results in interesting features ranging from phonon-assisted and resonant energy migration within the R1 line the 2E state to persistent spectral side-hole burning via the latter, and manifestations of specific nearest-neighbour π–π interactions between bipyridine and oxalate.
Resonant excitation energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ in the doped 3D oxalate networks [Rh1-xCrx(bpy)3][NaMIII1-yCry(ox)3]ClO4 (ox=C2O4-, bpy=2,2’-bipyridine, M=Al,Rh) is due to two types of interaction, namely super exchange coupling and electric dipole–dipole interaction. The energy transfer probability for both mechanisms is proportional to the spectral overlap of the 2E→4A2 emission of the [Cr(ox)3]3- donor and the 4A2→2T1 absorption of the [Cr(bpy)3]3+ acceptor.The spin-flip transitions of (pseudo-)octahedral Cr3+ are known to shift to lower energy with increasing pressure. Because the shift rates of the two transitions in question differ, the spectral overlap between the donor emission and the acceptor absorption is a function of applied pressure. For [Rh1-xCrx(bpy)3][Na-M1-yCry(ox)3]ClO4 the spectral overlap is thus substantially reduced on increasing pressure from 0 to 2.5 GPa. As a result, the energy transfer probability decreases with increasing pressure as evidenced by a decrease in the relative emission intensity from the [Cr(bpy)3]3+ acceptor.
In the 3D network [Rh(bpy)3][NaCr(ox)3]ClO4 (ox = oxalate, bpy = 2,2'-bipyridine) phonon-assisted as well as resonant energy migration within the R1 line of the 4A2 → 2E transition of Cr3+ has been identified. The latter is dominant below 4.2 K, and in a fluorescence line narrowing spectrum, it manifests itself in a multiline pattern across the inhomogeneous line width with spacings corresponding to the zero-field splitting of the 4A2 ground state (Milos, M.; Kairouani, S.; Rabaste, S.; Hauser, A. Coord. Chem. Rev. 2008, 252, 2540). H. Riesen demonstrated efficient spectral hole burning within the R1 line of Cr3+ doped at low concentrations into partially deuterated NaMg[Al(ox)3]·9H2O (Riesen, H. Coord. Chem. Rev. 2006 250, 1737). Here we show that at higher Cr3+ concentrations in the same host, both phenomena can be observed simultaneously, the resonant energy migration thus creating an additional series of persistent side holes.
In the mixed crystal series of the cubic three-dimensional networks of composition [Zn1−xRux(bpy)3][NaCr(ox)3] (0 ≤ x ≤1, ox = C2O42−, bpy = 2,2′-bipyridine), high-resolution absorption spectroscopy in the region of the 4A2→2E transition (R-lines) reveals the creation of five specific spectroscopic sites for the [Cr(ox)3]3− complex. The concentration of these spectroscopic sites follows a binomial distribution of [Zn(bpy)3]2+ and [Ru(bpy)3]2+ among the four nearest neighbors of a given [Cr(ox)3]3− complex within the network. The tris-bipyridine complexes occupying those positions have an optimal π−π interaction with the oxalate ligands of the tris-oxalate chromophore. The energy of each spectroscopic [Cr(ox)3]3− site depends on the total concentration of [Ru(bpy)3]2+ in the mixed crystal and on its specific distribution among the four nearest neighbors. Single crystal X-ray diffraction indicates a reduction of the unit cell volume when [Zn(bpy)3]2+ (a = 15.6365(18) Å) is substituted by [Ru(bpy)3]2+ (a = 15.5098(6) Å). This alone would lead to a red-shift of the R lines in analogy to the red-shift of 25.2 cm−1/GPa due to the decrease of the metal ligand Cr−O bond length as observed in high-pressure luminescence experiments. However, specific π−π interactions with the nearest neighbors have the opposite effect and shift the transition in discrete jumps to higher energies with increasing [Ru(bpy)3]2+ mole fraction.
In the 3D oxalate networks [NaCr(ox)3][Rh(bpy)3]ClO4 and [NaCr(ox)3][Ru(bpy)3] (ox=oxalate, bpy=2,2′-bipyridine) three different types of energy migration within the 4A2→2E transition can be identified. One is a resonant process between spectral members spaced by the ground-state zero-field splitting (ZFS). This leads to the sequential appearance of additional sharp lines spaced by the ground-state ZFS in the fluorescence line narrowing spectrum across the inhomogeneous line. The second one is a quasi-resonant process between spectral neighbours and manifests itself by rapid spectral diffusion. The third one is the well-known phonon-assisted process setting in at higher temperature.
Excitation energy migration is an important phenomenon at high concentration of luminescent chromophores. In crystalline solids it results in a quenching of the intrinsic luminescence of the chromophore as the excitation energy migrates to impurity centres or other forms of trap sites. As concluded from the extensively studied systems where Cr3+ is doped as the active chromophore into inert host lattices, energy migration in crystalline solids is usually a phonon-assisted process, in which the simultaneous creation or annihilation of phonons helps to bridge the energy miss-match in the energy levels of two neighbouring chromophores within a inhomogeneously broadened absorption band. However, in the three-dimensional network systems [Ru(bpy)3][NaCr(ox)3] and [Rh(bpy)3][NaCr(ox)3]ClO4, it proved possible to unambiguously identify three different mechanisms for energy migration within the R1 line of the 4A2 → 2E transition of Cr3+. In addition to the common temperature dependant phonon-assisted process, a resonant process between the zero-field split components of the 4A2 ground state leading to a multi-line pattern in a fluorescence line narrowing spectrum and a quasi-resonant process within the same component leading to fast spectral diffusion can be identified at very low temperature. The parameters governing these processes are discussed and the behaviour of the model systems is compared to more conventional doped oxides and related systems.