Sensitised delayed fluorescence

One such method is sensitised fluorescence (see additional remarks on this subject in Chap. 6). Here, the detection limit for fluorescing compounds lies at less than 10" molecules/host molecule or ca. 10 impurity molecules per cm. Fig. 3.6 shows as an example of such a measurement the detection of -methyl-naphthalene in naphthalene. Still more sensitive is the method of sensitised delayed fluorescence (Sect. 6.9). Here, the detection limit is at ca. 10 molecules/host molecule or 10 impurity molecules per cm [5,6]. 

Fig. 6.22 Sensitised delayed fluorescence, schematically, a The mixed crystal H + C is excited into the triplet band T] of the host (naphthalene green light). The delayed fluorescence contains a relatively large proportion of guest light (anthracene violet light) and little host light (naphthalene ultraviolet), b The ratio of guest intensity Qc to host intensity Qh iti the delayed fluorescence is plotted against the guest...

Sensitised delayed fluorescence Energy transport Reaction velodty rate constant kf c- Diffusion coefficient D, Trapping velodty as well as triplet lifetime T Host and guest delayed fluorescence... 

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

Table 6.5 Dynamic interaction parameters, from measurements of sensitised and delayed fluorescence. Other results for the triplet states can be found in Chap. 7.

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

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