Fluorescence electron withdrawing

Sulfonation increases the water solubiUty of dyes. A sulfonic group is electron withdrawing but has a weakening effect on the fluorescence only if it is ortho to the donor group. For example, in Brilliant Sulfoflavine FF [2391-30-2] (17), where R = p-tolyl, —it has a weakening effect, although the cleanness of the fluorescent color is somewhat improved compared to that of the unsulfonated compound. 

It would be preferable to incorporate both fluorescent and electron transport properties in the same material so as to dispense entirely with the need for electron-transport layers in LEDs. Raising the affinity of the polymer facilitates the use of metal electrodes other than calcium, thus avoiding the need to encapsulate the cathode. It has been shown computationally [76] that the presence of a cyano substituent on the aromatic ring or on the vinylene portion of PPV lowers both the HOMO and LUMO of the material. The barrier for electron injection in the material is lowered considerably as a result. However, the Wessling route is incompatible with strongly electron-withdrawing substituents, and an alternative synthetic route to this class of materials must be employed. The Knoevenagel condensation... 

Substitutions in position 3 with electron-withdrawing groups such as -CN, -COR, -COOR, and -N02 or with alkyl groups increase the fluorescence intensity of the basic coumarin structure [29, 30],... 

Linking the macrocyclic receptor l,7-diaza-15-crown-5 to the fluorophore 7-nitrobenzofurazane leads to the Hg(ll) selective fluorescence probe NBO-crown <1997JFL231S>. The cryptand was functionalized with an electron-withdrawing fluorophore (NBD) which behaves as a fluorescence on/off signaling system by translocating Cd(ll) inside and outside the cryptand cavity <2004IC4626>. 

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. 

Figure 2.10. Example of a donor-acceptor fluoroionophore in which the electron-withdrawing character of the acceptor (carbonyl group of the coumarine) is cation-controlled. Absorption and fluorescence spectra of ClS3-crown(Oj) and its complexes with perchlorate salts in acetonitrile. (Adapted from Ref. SO.)...

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). 

Electron donor groups in the 4-position of naphthalimides increase electron transfer to the electron-withdrawing carbonyl groups in the 1,8-positions, giving products that are strongly fluorescent. Commercially the most important products are based on N-substituted-4-aminonaphthalimides (3.6), as illustrated by Cl Disperse Yellow 11 (3.6a) and Cl Solvent Yellow 43 (3.6b). 

On the other hand, electron-withdrawing substituents tend to diminish or inhibit fluorescence completely. In dilute solutions in water, aniline is about 40 times more fluorescent than benzene whereas benzoic acid is nonfluorescent. 

The last three entries in Table 13 reflect a marked preference for a-methylene versus methyl proton transfer for electron-withdrawing a substituents. These amines are also unusual in that they react with t in nonpolar solvents and do not display exciplex fluorescence. While this unusual behavior was initially attributed to a free radical hydrogen atom transfer mechanism leading to the formation of exceptionally stable "merostabilized" a-aminoallyl radicals (115), our current view is that the high kinetic acidity of the a-C-H bond of these amines when complexed with t is responsible for their behavior. 

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