Energy transfer up-conversion

ETU energy transfer up-conversion mgl 1 -deoxy- l-(methylamino)glucitol... 

Aromatic polycarboxylates easily form 2D or 3D networks, for instance [Nd2(122)3(dmf)4]-H2O which present a 2D structure in which the 1,4-naphthalenedicarboxylate anions link Ndm ions of two adjacent double chains keeping them at a short distance of about 4.1 A (J. Yang et al., 2006). This allows up-conversion to take place, albeit with very low efficiency a blue emission is seen at 449.5 nm upon excitation at 580 nm (corresponding to the 4Gs/2 magnetic properties an energy-transfer up-conversion mechanism involving no excited state absorption is more likely. 

Up-conversion relies on sequential absorption and luminescence with intermediate steps to generate shorter wavelengths. Hence, the presence of more than one metastable excited state is required the intermediate metastable states act as excitation reservoirs. One typical example is ground-state absorption followed by inter-mediate-state excitation, excited-state absorption, and final-state excitation to give the up-conversion (the intermediate states and final states are real states) [1, 35], There are many types of up-conversion mechanisms such as excited-state absorption, energy transfer up-conversion and cooperative up-conversion. All these up-conversion processes can be differentiated by studying the energy dependence, lifetime decay curve, power dependence, and concentration dependence by experimental measurements [36-39]. 

Fig. 16.16 Principal UC processes for lanthanide-doped crystals a excited state absorption, b energy-transfer up-conversion, and c photo avalanche. The dashed/dotted, and full arrows represent photon excitation, energy transfer, and the emission process, respectively. Reproduced from Ref. [34] by permission of The Royal Society of Chemistry...

Auzel s chapter on coherent emission is different from many reviews on the subject, which are concerned with the laser effect itself, in that he concentrates on the broader issues. The emphasis of chapter 151 is on superradiance, superfluorescence, amplification of spontaneous emission by other stimulated emission than the laser effect, and coherent spontaneous emission. Also discussed are up-conversion by energy transfer, up-conversion by the avalanche effect, and recent advances in lanthanide lasers and amplifiers. 

Fig. 9.lld). The up-conversion spectrum consists of three major peaks (Fig. 9.18). [All up-conversion spectra from Er3+ (including those using energy transfer, below) are similar, but the relative intensities of the three peaks vary with concentration of defects and the host matrix.]... 

Figure 9.19 Simplified energy level diagram for the Er3+/Yb3+ couple showing the important up-conversion and energy transfer transitions a) GSA, ET, and relaxation (/>) CR, ESA, ET, and relaxation...

The up-conversion efficiency is low and varies with the concentration of the activator and sensitisor ions. A maximum efficiency is observed with concentrations of about 1-3% of the active center. Above this value increasing back transfer from Er3+ to Yb3+ and increasing interactions between both lanthanide ions, leading to cluster formation and Yb3+-Yb3+ energy transfer, limits the efficiency. 

The second example of the application of fluorescence up-conversion microscope is imaging of organic microcrystals based on ultrafast fluorescence dynamics (femtosecond fluorescence dynamics imaging) (Fujino et al. 2005a). In this measurement, the site-specific energy transfer rate in a tetracene-doped anthracene microcrystal was measured, and the crystal was visualized based on the observed local ultrafast dynamics. 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

Up-conversion is a process by which two photons of lower energy are subsequently converted into a luminescence photon of higher energy (typically, two IR photons giving rise to one visible photon, e.g. in Er111-containing compounds). This anti-Stokes process is usually observed for ions embedded in solids and is made possible by various mechanisms, such as the now classical excited state absorption mechanism (ESA), or sequential energy transfers (ETU for... 

One can determine the number of cation dopants associated with a given defect structure by measuring the two body energy transfer rates between probe ions, the efficiency of three body up-conversion, and the effect of adding a second dopant ion in higher concentration on the splittings of the first dopant ion. 

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