Effect of external pressure on the excitation energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ in [Rh(1-x)Cr(x)(bpy)3][NaM(1-y)Cr(y)(ox)3]ClO4.

Resonant excitation energy transfer from [Cr(ox)(3)](3-) to [Cr(bpy)(3)](3+) in the doped 3D oxalate networks [Rh(1-x)Cr(x)(bpy)(3)][NaM(III) (1-y)Cr(y)(ox)(3)]ClO(4) (ox=C(2)O(4) (-), 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 (2)E→(4)A(2) emission of the [Cr(ox)(3)](3-) donor and the (4)A(2)→(2)T(1) absorption of the [Cr(bpy)(3)](3+) acceptor. The spin-flip transitions of (pseudo-)octahedral Cr(3+) 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 [Rh(1-x)Cr(x)(bpy)(3)][NaM(1-y)Cr(y)(ox)(3)]ClO(4) 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.

Persistent spectral hole-burning in the R(1) line of Cr(3+)-doped NaMg[Al(oxalate)(3)].9(H(2)O/D(2)O) induced by resonant energy transfer.

In the 3D network [Rh(bpy)(3)][NaCr(ox)(3)]ClO(4) (ox = oxalate, bpy = 2,2'-bipyridine) phonon-assisted as well as resonant energy migration within the R(1) line of the (4)A(2) --> (2)E transition of Cr(3+) 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 (4)A(2) 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 R(1) line of Cr(3+) doped at low concentrations into partially deuterated NaMg[Al(ox)(3)].9H(2)O (Riesen, H. Coord. Chem. Rev. 2006 250, 1737). Here we show that at higher Cr(3+) concentrations in the same host, both phenomena can be observed simultaneously, the resonant energy migration thus creating an additional series of persistent side holes.

Structural study of mixed crystals of [Zn(1-x)Ru(x)(bpy)(3)][NaCr(ox)(3)] probed by high-resolution absorption spectroscopy and high-pressure experiments.

In the mixed crystal series of the cubic three-dimensional networks of composition [Zn(1-x)Ru(x)(bpy)(3)][NaCr(ox)(3)] (0 (2)E 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 pi-pi 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) A) is substituted by [Ru(bpy)(3)](2+) (a = 15.5098(6) A). 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 pi-pi 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 vitro acetylcholinesterase inhibitory properties of thymol, carvacrol and their derivatives thymoquinone and thymohydroquinone.

The aim of this study was to examine in vitro the inhibitory activity exerted by the main constituents of essential oil obtained from the aromatic plant Thymus vulgaris L. on acetylcholinesterase (AChE). The total essential oil and selected compounds, specifically linalool and thymol, carvacrol and their derivatives thymoquinone and thymohydroquinone, were tested for AChE inhibition. Thymohydroquinone exhibited the strongest AChE inhibitory effect over the range of concentrations. The AChE inhibitory potential decreased in the following order: thymohydroquinone > carvacrol > thymoquinone > essential oil > thymol > linalool. It is interesting that the AChE inhibitory effect exerted by carvacrol was 10 times stronger than that exerted by its isomer thymol, although thymol and carvacrol have a very similar structure.