Lanthanide triplet complex

Stelmakh and Tsvirko have also proposed the mechanism [b] for the production of S2 state fluorescence in Mg, Zn, and lanthanide-porphyrins complexes (15), and showed the possibility of another mechanism of pumping by triplet-triplet annihilation (16)(mechanism [a]). The latter process was considered to predominate at relatively low values of the excitation densities. However, the contribution of the mecahnism [C] was not determined experimentally. The experimental results in the present work are summarized as follows ... 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

Multilayer devices with lanthanide chelate complexes. In these complexes, efficient energy transfer from the singlet or triplet exciton on the ligand of the complex to the lanthanide atom at its center results in efficient, atomic-like line emission spectra from the latter. By adjusting the identity and concentration of the different lanthanide complex dopants, a line spectrum with white CIE coordinates was achieved.77... 

Figure 2.18 Schematic representation of photophysical processes in lanthanide(III) complexes (antenna effect). A = absorption, F = fluorescence, P = phosphorescence, L = lanthanide-centred luminescence, ISC = intersystem crossing, ET = energy transfer S = singlet, T = triplet. Full vertical lines radiative transitions dotted vertical lines nonradiative transitions...

Solvatochromism is important for lanthanide coordination complexes becanse it may tune the energy of the triplet state of the ligand relative to the electronic level of the lanthanide ion that is, as already pointed earlier, quite independent on the chemical environment. 

Fig. 7.1 Simplified energy diagram of the lanthanide organic complex system. Abs. absorption, Fluor, fluorescence, Phosph. phosphorescence, EM lanthanide (Ln ) ion emission, ISC intersystem crossing, ET energy transfer, S singlet, T triplet. Non-rad. nonradiative transitions (Reproduced from Ref. [18] by permission of the Royal Society of Chemistry)...

Figure 5 Triplet-mediated ligand-to-metal energy transfer in lanthanide complexes.

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

Paramagnetic molecules K)a and 2NO are very efficient quenchers of singlet and triplet states. Oxygen may form CT complex and lead to peroxide formation in the triplet state. Paramagnetic ions of the transition series and lanthanides also quench the triplet states. 

The structurally similar, but magnetically distinct, lanthanide(III) texaphyrin complexes, MLu and MGd both generate reactive oxygen species (ROS), albeit via different mechanisms. Photoirradiation of MLu causes excitation from the singlet ground state to the triplet state (Fig. 3). 

Parker and Williams recently reported NAND logic action in the terbium complex 16.[S8] The delayed emission of the lanthanide ion is switched off when H+ and 02 are present simultaneously. Protonation of the phenanthridine side chain causes its triplet excited state to approach theTb(m) 5D4 excited state energetically. This leads to equilibration of these two excited states and sharing of their properties. Thus, the metal-centered state displays the 02 sensitivity usually only found in organic triplets. 

---