Back energy transfer

Fig. 20. Energy back-transfer model describing the energy transfer between the semiconductor host and the lanthanide ion R3+ (Culp et al., 1997 Takarabe et al., 1995).

The PL spectrum of a thin film of poly(3,6-dibenzosilole) 31 at 77 K exhibited a 0-0 transition at 3.5 eV and a second maximum at 3.3 eV (excitation at 4.4 eV) [41]. The phosphorescence emission spectrum at 77K consists of a broad band exhibiting vibronic structure (excitation at 3.9 eV). The polymer triplet energy level was taken to be the onset of triplet emission at 2.55 eV. This is considerably higher than the triplet energy of commonly used polyfluorenes (2.1 eV) [10,46] making it a host for phosphorescent emitters without the risk of energy back-transfer onto the polymer. 

The photon-echo method seems to be eminently suitable to provide additional information on energy transfer in mixed crystals and particular on the question to what extent energy back-transfer is important. 

Fig. 7 shows the excitation dependences of the relative number of the excited Er3+ ions estimated by using Eq. (2). The population inversion (Ni/NEr >0.5) for the samples of type 2 is achieved for 7ex>0.1 W/cnr. To achieve the population inversion for the samples of type 1 a stronger pump was needed. We note that at low temperatures the population inversion for the samples of both types could be achieved at lower pump intensities because of suppression of the non-radiative recombination and thermally activated energy back-transfer. However, despite of the population inversion there is no evidence of the optical gain in the structures investigated. It seems that the energy back-transfer is the main limiting factor to achieve the population inversion of Er3+. 

The obtained results demonstrate good perspectives of nc-Si/SiO2 Er for applications in light emitting devices. However, it is necessary to improve the structural, electronic and optical properties of the samples to suppress the non-radiative recombination processes and energy back-transfer from Er3+ to nc-Si. The contributions of the stimulated optical transitions in Er3+ ions can be obviously enhanced by optimizing the sample properties. 

Finally, both photo-induced electron transfer from the ligand to the metal ion, resulting in a reduction of Ln into Ln with a concomitant quenching of the metal-centered luminescence, and energy back transfer (see fig. 7) have to be avoided by an adequate ligand design positioning the LMCT and triplet states sufiiciently away from the emissive state. 

Similar to the design of blue light-emitting polymer, introducing sterically hindered groups to the phosphor or the polymer host is also an efficient approach to reduce the triplet energy back transfer. With this in mind, Yang and coworkers introduced the sterically... 

Gong, S., Yang, C., Qin, J., 2012. EfQcient phosphorescent polymer light-emitting diodes by suppressing triplet energy back transfer. Chem. Soc. Rev. 41,4797-4807. 

The triplet state of the hgand has to be located not too far in energy from one of the excited state of the lanthanide ions, in order to have an optimal rate of the energy transfer yet not too close to avoid energy back transfer from the lanthanide excited state to the triplet state, which would drastically decrease the luminescence intensity. 

Miyata K, Konno Y, Nakanishi T et al (2013) Chameleon luminophore for sensing temperatures control of metal-to-metal and energy back transfer in lanthanide coordination polymers. Angew Chem Int Ed 52 6413-6416... 

The efficiency of luminescence of RE complexes depends on several processes, such as light absorption by the complex, energy transfer from the ligand to the Ln(III) ion, multiphonon relaxation, and energy back transfer. 

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