Fluorescence electron donating

Fluorescence is greatly affected by the structure of a molecule. Usually only aromatic compounds fluoresce although some aliphatic and alicyclic molecules are known to fluoresce. Electron-donating groups such as -OH and -OCH3, that can increase the electron flow of an aromatic system usually increase the fluorescence while other groups that contain hetero atoms with n-electrons that can absorb the emitted energy, will usually quench the fluorescence. However, it is always difficult to predict whether or not, or to what extent, a compound will fluoresce. 

In two other studies, it was observed that C60 in LB films can quench the fluorescence of pyrene [293] and of 16-(9-anthroyloxy)palmitic acid [294] by photoinduced electron transfer. In these studies, both C60 and the electron-donating fluorophore were incorporated into a tricosanoic acid LB film in different ratios. 

All of these molecules have an electron withdrawing group in the center and two electron-donating arylamine groups attached at each end. They all demonstrate strong red fluorescence... 

Substituent groups have a marked effect on the fluorescence quantum yield of many compounds. Electron-donating groups such as -OH, -NH2 and -NR.2 enhance the fluorescence efficiency, whereas electron-withdrawing groups such as -CHO, -C02H and -N02 reduce the fluorescence quantum yield, as shown by naphthalene and its derivatives in Table 4.3. 

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

Different metal ions have been known to induce spectral changes in NIR dyes depending on the electron donating or electron withdrawing capacity of the metal ions. Since fluorescence is directly proportional to the excitation of the electrons, the disturbance of the electron cloud may affect the fluorescence intensity as the metal ion forms a complex with the NIR dye. To illustrate this phenomena fluorescence spectral changes of an NIR dye in methanol in the presence of varying potassium concentrations are shown in Figure 7.10. 

In 09NJC1320, an original method was developed for the preparation of fluorescent photochrome 112, which contains both electron-donating and electron-withdrawing groups and bipyridine-bridged dithienylethene fragments, from 110 with bisphosphonate bipyridine 111 in the presence of a base (Scheme 36). 

Ferrocene has been widely investigated as an electron donor and its electron donating ability can be tuned by redox reactions. As anticipated, when a ferrocene unit is covalently connected to an electron acceptor moiety that shows intrinsic fluorescence, the fluorescence of the acceptor moiety would be largely quenched because of the photoinduced electron transfer between ferrocene and the fluorescent acceptor. For instance, triad 15 that contains perylene diimide flanked by two ferrocene moieties, shows rather weak fluorescence due to the photoinduced electron transfer between perylene diimide and ferrocene units. Either chemical or electrochemical oxidation of ferrocene unit lead to fluorescence enhancement. This is simply because the electron donating ability of ferrocene is reduced after oxidation and accordingly the photoinduced electron transfer is prohibited. In this way, the fluorescence intensity of 15 can be reversibly modulated by sequential electrochemical oxidation and reduction. Therefore, a new redox fluorescence switch can be established with triad 15.25... 

Similarly, the fluorescence intensity of the 1,4-disubstituted azine with ferrocene and pyrene units (17) can be reversibly modulated by sequential redox reactions of ferrocene moiety. In the neutral state, compound 17 displays weak fluorescence owing to the electron transfer from the ferrocenyl group to the excited pyrene unit or by energy transfer from the excited pyrene unit to the ferrocenyl unit. Oxidation of the ferrocenyl unit, however, leads to remarkable fluorescence enhancement. This is because the ferrocenium cation shows weak electron donating ability and also the corresponding spectral overlap becomes small.27... 

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