Delayed photoluminescence

In the case of radiative channels, we assume that the spin-carrying species themselves are radiative and can photoluminesce. This emission is in the form of fluorescence and delayed photoluminescence for spin 1/2 species and phosphorescence for spin triplets. Therefore, we separate our discussion into spin 1/2 and spin 1, respectively. 

Schweitzer B, Arkhipov VI, Bassler H (1999) Field-induced delayed photoluminescence in a conjugated polymer. Chem Phys Lett 304 365... 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

The first observations of P-type delayed fluorescence arose from the photoluminescence of organic vapors.<15) It was reported that phenanthrene, anthracene, perylene, and pyrene vapors all exhibited two-component emission spectra. One of these was found to have a short lifetime characteristic of prompt fluorescence while the other was much longer lived. For phenanthrene it was observed that the ratio of the intensity of the longer lived emission to that of the total emission increased with increasing phenanthrene vapor... 

Fluorescence and phosphorescence are particular cases of luminescence (Table 1.1). The mode of excitation is absorption of a photon, which brings the absorbing species into an electronic excited state. The emission of photons accompanying deexcitation is then called photoluminescence (fluorescence, phosphorescence or delayed fluorescence), which is one of the possible physical effects resulting from interaction of light with matter, as shown in Figure 1.1. 

The most common application of photoluminescence is found in fluorescence spectroscopy. Fluorescence is the immediate release of electromagnetic energy from an excited molecule or release of the energy from the singlet state. If the emitted energy arises from the triplet state or is delayed, the process is referred to as phosphorescence. 

Figure 25. Excitation and photoluminescence (solid and dashed lines) of x% Mn2+ CdS nanocrystals, where. v = 0 (a), 0.8 (fe), 2.5 (c), and 4.8 (d). The solid luminescence spectra were collected in CW mode, and the dashed luminescence spectra were collected with a pulsed excitation source and a 2-ms delay between excitation and emission detection. Note that the intensities of (b)-(d) are referenced to that of (a). [Adapted from (82).]...

Figure 9. Picosecond photoluminescence maps of opal CT (left pannel) and opal A (right pannel). The PL intensity z is plotted in logarithmic scale and increases from blue colors to red colors. Zero-time delay corresponds to the beginning of the streak camera sweep. Opal CT "common" is from Mapimi, Mexico opal A "noble" is from Lightning Ridge, Australia.

Figure 11. Transient photoluminescence of a para-hexaphenyle layer with molecules lying on the substrate, for increasing time delays. The dotted curve corresponds to the ultrafast response of a film with molecules standing almost perpendicular to the substrate.

The modem silicon-based microelectronics led to the miniaturization of electronic devices. However, delays caused by metallic intercoimec-tions became a bottleneck for the improvement of their performances. One possible solution of this problem is to use optical intercoimections for the transfer of information, and, therefore, silicon compatible materials and devices that are able to generate, guide, amplify, switch, modulate, and detect light are needed. Rare earth silicates with luminescent rare earths and compatibility with silicon may be a good choice for these applications (Miritello et al., 2007). Miritello et al. presented the study on nanocrystalline erbium silicate thin films fabricated on silicon/silica substrates. The obtained films exhibit strong photoluminescence emission around 1540 nm with room temperature excitation by 488 ran Ar laser. 

Figure 24-6 Photoluminescence methods (fluorescence and phosphorescence). Fluorescence and phosphorescence result from absorption of electromagnetic radiation and then dissipation of the energy by emission of radiation (a). In (b), the absorption can cause excitation of the analyte to state 1 or state 2. Once excited, the excess energy can be lost by emission of a photon (luminescence, shown as solid line) or by nonradiative processes (dashed lines). The emission occurs over all angles, and the wavelengths emitted (c) correspond to energy differences between levels. The major distinction between fluorescence and phosphorescence is the time scale of emission, with fluorescence being prompt and phosphorescence being delayed.

Exciplexes have charge-transfer character and hence a small electron-hole overlap. This reduces the oscillator strength of the exciplex and, if nonradiative processes are not dominant, lengthens its photoluminescence time constant. Figure 2.11 shows the PL decay measured from a PFB F8BT blend together with those of the pure polymers. In the blend a delayed emission is visible that is absent in the pure polymers and has a time constant of 45 ns (when fitted between 30 and 90 ns). This is the long-lived exciplex state. 

We show that the exciplex states that form at the interface between F8 and PFB can be thermally excited to form bulk excitons. This is observed as delayed PFB exciton emission in time-resolved photoluminescence measurements. These findings are analogous to those presented for PFB F8BT and TFB F8BT interfaces in the previous section and therefore support the generality of the phenomenon of endothermic exciplex-to-exciton energy transfer in polyfluorene blends. 

Fig. 2.30 Photoluminescence decays at 650 nm of the PFB/F8BT bilayer LED and the F8BT-only LED from Fig. 2.29 as well as the PFB F8BT blend LED from Fig. 2.31. The long-lived exciplex emission is clearly visible in both the blend and the bilayer data but its relative intensity is much lower in the bilayer device. An exponential fit of the delayed emission between 30-90 ns yields 51.5 and 38.7 ns for the blend and the bilayer device, respectively.

Fig. 2.47 Photoluminescence decays (measured at 620 nm) of pure F8BTand of PFB F8BT blends for different weight ratios as indicated in the figure. Fitting the delayed emission from the blends between 30-90 ns yields the time constants Tdeiayed given in Table 2.2. The decays have been normalized to their peak intensity.

Fig. 2.48 Time-resolved photoluminescence spectra of PFB F8BT blends of different weight ratios, (a), (b) and (c), and of films of pure PFB and F8BT (d). The spectrum with the highest intensity represents the PL emission integrated over the first 10 ns. The less-intense spectra are the delayed PL integrated over subsequent 10-ns time windows, i.e. over 10-20 ns, 20-30 ns,..., 80-90 ns.

Phosphorescence can be observed without interference from fluorescence by a process called time resolution. Instruments for measuring phosphorescence are very similar to those used for fluorescence but a mechanism that allows the sample to be irradiated and then, after a time delay, allows measurement of phosphorescent intensity (phosphoroscope) is required as an extra component. The instrument should also have the capability of keeping samples at very low temperatures. Another type of long-lived photoluminescence is time-delayed fluorescence, where the electrons in the molecule obtain enough energy to be excited from a special excited state to the normal excited state and then fluoresce. 

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