Luminescence intensity electronics

Electron-transfer reactions producing triplet excited states can be diagnosed by a substantial increase in luminescence intensity produced by a magnetic field (170). The intensity increases because the magnetic field reduces quenching of the triplet by radical ions (157). 

Figure 2. Principles of reversible luminescence sensing using photochemical quenching processes (electron, energy or proton transfer). Dye = luminescent indicator Q = quencher species dotted arrow non-radiative deactivation processes. The luminescence intensity (and excited state lifetime) of the indicator dye decreases in the presence of the quencher. The indicator dye is typically supported onto a polymer material in contact with the sample. The quencher may he the analyte itself or a third partner species that interacts with the analyte (see text).

Comparison of this luminescence intensity in different samples reveals that any correlation is absent any impurity concentration. Thus it was supposed that the mostly probable luminescence center is Ti, which presence is quite natural in Ti bearing benitoite. The wide occurrence of Ti " minor impurities in minerals was detected by EPR. Like the other d ions (V, Mo ), Ti ions occur often in minerals as electron center (Marfunin 1979). It may be realized in benitoite, which does have some natural exposure to gamma rays in its natural setting. There could be radiation centers, such as, for example, Ti + gamma ray + electron donor Ti + electron hole. Benitoite color does not change with gamma irradiation to quite high doses (Rossman 1997) but luminescence is much more sensitive compared to optical absorption and can occur from centers at such low concentration that they do not impact the color of a benitoite. 

The PET systems of the aminoalkyl aromatic type discussed so far display a very simple behavior in that luminescence intensity (or quantum yield) is the only variable. Such systems are very user-friendly as a result and tolerate a wide variety of communication wavelengths. However these simple systems could be adapted to include an additional absorptiometric sensing channel which can confirm the results of ion density (pH say) obtained via luminescence. Of course, such increased user-confidence is only attained with a proportionate reduction in simplicity. Now excitation needs to be done at the isosbestic wavelength. These systems, e.g. 11 and 12, use a push-pull fluorophore with electron donor and acceptor substituents which give rise to internal charge transfer (ICT) excited states. In contrast, the simple PET systems employed aromatic hydrocarbon fluorophores with essentially pure nn excited states. The charge separation in ICT states can cause electrostatic... 

Fig. 36. Electrogenerated luminescence spectra for semiconductor electrodes. is luminescence intensity energies corresponding to maxima on the curves are shown in the figure (in electron-volts). [From Pettinger et al. (1976).]...

From photoinduced absorption, luminescence and electron spin resonance observations, the dominant photocarriers generated in the polymer were shown to be polarons and bipolarons [189-191]. It was found that the magnitude of photoinduced absorption is rather independent of the condition of sample preparation whereas the photoluminescence intensity is strongly influenced. The results suggest that the luminescent exciton does not play a primary role in the photogeneration of polaronic species. 

When an ion-pair recombines, it may form an excited state which can luminesce. The intensity of luminescence is a direct monitor of the competing ratio of recombination and luminescence. With steady-state conditions, the luminescence intensity is proportional to the rate of recombination. For instance, Morrow et al. [380] have radio lysed solutions of pyrene in cyclohexane. Solvated electrons and pyrene cations are produced. On recombination, an excited singlet state is produced which can fluoresce. If two pyrene molecules are in (or near) contact when one or other molecule is in the excited singlet state, then excimer fluorescence may be observed. The intensity of fluorescence can be decreased by application of an electric field, since fewer ion-pairs recombine to form the excited state. Jarnagin [381] and Holroyd and Russell [382] have photoionised iVjA iV. iV -tetramethyl-p-phenylenedia-mine (TMPD) with light (of photon energy 5.5—6 eV) in hydrocarbon solvents and measured the photocurrent at various electric field strengths. 

Often, excitation only takes place with photons of higher energy. The absorption that is effective for excitation of luminescence is given by the excitation spectrum, which shows the dependence of the luminescence intensity on the photon energy. Strong luminescence occurs when the exciting photon has sufficient energy to promote an electron from the valence band to the conduction band. 

The luminescence spectra of all receptors in CH3CN were found to be dramatically affected by the addition of acetate or chloride. While compound 19 exhibits an emission decrease, the other receptors 17,18 and 20 show a remarkable intensity increase (up to 500%) with a slight concomitant blue shift of the emission maximum (660 nm for 17). The anion-induced enhancement of luminescence intensity in the case of 17 is clearly due to the decrease of the electron transfer between the ruthenium(II) bipyridyl centre and the quinone moieties. Alternatively, receptors bearing ruthenium or rhenium complexes on the upper rim were also described [20]. 

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