Sensitization upconversion

Fig. (23). Schematic representation of quantum dot sensitized upconversion of IR to Visible light via triplet-triplet annihilation. Reproduced with permission from [224] 2015 Macmillan Publishers Limited.

The integration of Nd ions to nanomaterials containing Yb allows for Nd Yb ET to conventional Yb -sensitized upconversion processes as a versatile strategy to extend the single NIR excitation bands for Yb to shorter wavelengths. 

Broadband dye-sensitized upconversion of near-infrared light Nat Photon., 6. 560-564. 

If an ion possesses two excited states with approximately same energy separation, energy transfer can occm between two ions in the first excited state (Figme 4c). One ion returns uomadiatively to the ground state while the second is promoted to the second excited state, then decays with emission of a photon whose energy is about twice that of incident photons. There are other upconversion processes two-step absorption, cooperative sensitization, cooperative luminescence. Upconversion by energy transfer is the most efficient process. 

Wavelength Sensitivity. The overall wavelength sensitivity of emission spectrometers plays a central role in determining the accuracy of the recorded data and the range of emission wavelengths that can effectively be studied by a particular apparatus. The upconversion method adds a source of wavelength dependent sensitivity that is both useful and problematic. 

On the positive side, upconversion allows efficient detection of red wavelengths (> 650 nm) where direct detection spectrometers are insensitive. For example, for our spectrometer 700 nm fluorescence is upconverted to 422 nm. This latter wavelength is in a region where our pmt and array detectors are quite sensitive. 

The concept of upconversion 32) in which higher-lying states of an activator are excited by successive energy transfers from a less energetic sensitizer has also been applied to lanthanide lasers (33). 

Comparisons between these two metals have been carried out on complexes with donor-acceptor Schiff bases. The new species show efficient absorption in the orange-red part of the spectrum and room-temperature near-infrared (NIR) phosphorescence. Particularly, Pt(ii) complexes possess phosphorescence quantum yields (O) of 0.1, but the emission of the respective Pd(ii) complexes is less efficient (OaO.Ol). The Pd(ii) and Pt(ii) complexes are demonstrated to be efficient sensitizers in triplet-triplet annihilation-based upconversion systems. 

Figure 4.8 Simplified energy level scheme of a sensitizer-acceptor system used for photon energy upconversion (sensitized delayed fluorescence). The first step is absorption (Abs) of a photon by the sensitizer followed by efficient intersystem crossing (ISC) In the sensitizer molecules. The triplet Is then...

---