Photoluminescence upconversion

Experimentally, it is necessary to distinguish the effects of IL-CT mixing from simultaneous population of both the 3IL and 3CT states that could be in fast thermal equilibrium. (Note that an energy difference of 0.5 eV would amount to ca. 5% population of the upper state at room temperature.) Simultaneous population of these two states can be observed in TRIR spectra, where the predominantly 3IL state is manifested by a weak band occurring below the ground-state A" +A (2) band while the lower-lying predominantly 3CT state shows the characteristic pattern of three up-shifted bands. This is, for example the case of [Re(Etpy)(CO)3(4,4 -(NH2)2-bpy)]+ [47] and [Re(py)(CO)3(dppz)]+ [48], Time-dependent simultaneous population of 3IL and 3MLCT states was detected by TRIR for [Re(Et)(CO)3(bpy)]+ in ionic liquids until a few hundreds of ps after excitation [76] and by photoluminescence upconversion for [Re(L)(CO)3(bpy)]B (L = Etpy, halide) in MeCN on a femtosecond timescale [10],... 

Time-resolved PL measurements were also performed using time-correlated single-photon counting (TCSPC) and photoluminescence upconversion (PLUC) spectroscopies. Descriptions of the setups can be found in refs. [14, 65], respectively. All measurements were taken in continuous-flow He cryostats (Oxford Instruments OptistatCF) under inert conditions. Finally, PL efficiency measurements were performed on simple polymer thin films spin coated on Spectrosil substrates using an integrating sphere coupled to an Oriel InstaSpec IV spectrograph and excitation with the same Ar+ laser as above. 

The interaction of RE with silver NPs was also used for increasing upconver-sion of infrared energy, which cannot be used for solar cells, to visible light where the solar cells are sensitive. Some examples can be foimd in Refs. [65,66] and in the recent paper by Sun et al. [67], where upconversion provides an additional example in this direction. Energy transfer enhanced by silver plasmons allowed a strong upconversion from infrared light absorbed by Yb to visible photoluminescence of Er. 

The second best case of Ln -based upconversion is based on the Tm /Yb duet, which can produce photoluminescence at 800 nm, through a two-photon process, and blue emission between 450 and 500 nm, through a three-photon process, and even ultraviolet emissions through multi-photon processes. 

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