Excitation Energy Transfer and Energy Migration

1) Excitation Energy Migration in Oxalate network structures

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 a phonon 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 an Fluorescence Line Narrowing spectrum and 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.

2) Effect of External Pressure on the Energy Transfer in Oxalate network structures

Excitation energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ in the three-dimensional oxalate network [Cr(bpy)3][NaCr(ox)3]ClO4 occurs via two types of interaction, the super exchange coupling and the dipole-dipole interaction. The energy transfer probability for both mechanisms is proportional to the spectral overlap, however the energy transfer rates are not of the same order of magnitude. The super exchange coupling correspond to the fast contribution to the energy transfer rate, with kETex > 106 s-1, while the dipole dipole interaction is the slow contribution of the energy transfer with a rate of kETdd~102 s-1. with increasing external pressure around [Cr(bpy)3][NaCr(ox)3]ClO4, the spectral overlap between the donor's emission and acceptor's absorption decrease, since the last two don't have the same shift rate in high pressure experiments. In the case of [Rh0.965Cr0.035(bpy)3][NaAl0.99Cr0.01(ox)3]ClO4 the spectral overlap is reduced about 6 times with increasing pressure from 0 GPa to about 2.5 GPa. The energy transfer probability decrease with increasing pressure, however, mainly the slow contribution based on the dipole dipole interaction is suppressed. When the rate of the fast contribution to the energy transfer is reduced 6 times, it is still faster than the initrinsic decay rate, thus the energy transfer takes place. However, when the slow contribution rate is reduced 6 times, it becomes slower than the intrinsic decay rate, consequently, the donor deactivate to its ground state before transferin its energy to an acceptor. Finally, with high pressure experiments we are showing that it is possible to dissociate the two contribution to the energy transfer in [Rh0.965Cr0.035(bpy)3][NaMIII0.99Cr0.01(ox)3]ClO4 with MIII = Al3+, Rh3+.

3) Persistent Hole Burning induced by Energy Transfer in Oxalate network structures

In the three-dimensional 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 multi-line pattern across the inhomogeneous line width with spacings corresponding to the zero-field splitting of the 4A2 ground state [ M. Milos, S. Kairouani, S. Rabaste, A. Hauser, Coord. Chem. Rev. 252 (2008) 2540]. On the other hand, H. Riesen demonstrated very efficient spectral hole burning within the R1 line of Cr3+ doped at very low concentrations into partially deuterated NaMg[Al(ox)3]·9H2O [H. Riesen, Coord. Chem. Rev. 250 (2006) 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.

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Last modified 2017/11/13 by ES