Energy transfer, inefficient

Stable white emission with CIE coordinates of (0.3519, 0.3785) was obtained in such a rare-earth-based OLED device. The authors mentioned that the QE of the device was not good, possibly due to the inefficient energy transfer process between the ligand and the rare-earth metal. A suitable choice of the ligand may improve this type of device performance. 

Several routes are possible to populate the triplet state. The triplet excited state can, in principle, be directly excited from the ground state, but a low extinction coefficient associated with the S0 to T, transition (reflected in the long lifetime) makes direct excitation an inefficient process for tryptophan. The triplet state can be thermally populated, but for tryptophan the large energy gap between the ground state and the triplet state makes this process unfavorable. Energy transfer from a higher state can also populate the... 

Nonradiative energy transfer is very often used in practical applications, such as to enhance the efficiency of phosphors and lasers. A nice example is the commercial phosphor Cas(P04)3 (FCl), which is doubly activated by Sb + and Mn + ions. When the phosphor is singly activated by Mn + ions, it turns out to be very inefficient, due to the weak absorption bands of the divalent manganese ion. However, coactivation with Sb + ions produces a very intense emission from the Mn + ions, because the Sb + ions (the donor centers) efficiently absorb the ultraviolet emission (253.6 nm) of... 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

Resonances with pure excitation of the CO stretching mode (0, U2, 0) (an example with Vi - 8 is shown in Fig. 5) have the smallest rate and therefore the longest lifetime energy transfer from r to R is rather inefficient, and therefore the system needs a long time before enough eneigy is accumulated in the dissociation coordinate to permit dissociation. On the other extreme, direct excitation in R allows a rather rapid bond rupture, and therefore the resonances (t>i, 0, 0) have the shortest lifetimes. Excitation of the bending mode (0,0, U3) leads to lifetimes that are between C-O excitation as the lower limit and H-C excitation as the upper extreme. This mode specificity is further elucidated in Fig. 8, where we show the widths for several... 

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

The UV photon may be assumed to have an energy less than the band gap of the phosphor host so the absorption by A, S, and P takes place at the centers. Many activators, particularly Mn2+, have a low absorption cross-section so that direct pumping of the activator is inefficient. Sensitizers, such as Pb2+, are selected to have large absorption cross-sections at UV wavelengths. They then transfer the excitation to the activator which re-emits it as luminescent radiation. Most co-activators have a luminescent emission of their own so that the efficiency of the system depends on the kinetics of energy transfer compared with direct emission of the co-activator. In many mineral systems, one observes both and thus the activator radiation Ra and the sensitizer radiation Rg are... 

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