Electron-acceptor groups

Donors a.ndAcceptors. Table 3 Hsts common electron—donor groups, and electron—acceptor groups selected from a large number capable of evoking fluorescence. 

The conjugated chromophore (color-causing) system can be extended by electron-donor groups such as —NH2 and —OH and by electron-acceptor groups such as —NO2 and —COOH, often used at opposite ends of the molecule. An example is the aromatic compound alizarin [72-48-0], also known since antiquity as the ted dye madder. 

Dyes, polymethine used for dyes having at least one electron donor and one electron acceptor group linked by methine groups or aza analogues aUopolar cyanine, dye bases, complex cyanine, hemicyanine, merocyanine, oxonol, streptocyanine, and styryl. Supersensitization has been reported for these types—18 cites for cyanines, 3 for merocyanine, and 6 for all other polymethine types. 

It is of interest that these reactions exhibit stronger activation of the substituent at C-5 by a neighboring electron-acceptor group compared with the corresponding ortho-substituted benzene. This is possibly owing either to the effect of the nitrogen hetero atom or to a weaker delocalization of multiple bonds in the heterocyclic nucleus. 

Polyene and polymethine dyes are two structurally related groups of dyes which contain as their essential structural feature one or more methine (-CH=) groups. Polyene dyes contain a series of conjugated double bonds, usually terminating in aliphatic or alicyclic groups. They owe their colour therefore simply to the presence of the conjugated system. In polymethine dyes, electron-donor and electron-acceptor groups terminate either end of the polymethine chain, so that they may be considered as typical donor-acceptor dyes. 

The addition of an electron acceptor group into the D-ti-D can further enhance the 2PA cross sections. A series of dyes, cationic PD 2630, and two neutral dyes, SD 2243 and TD 2765, are shown in Fig. 18a. Compared with the polymethine dye PD 2630, SD 2243 has a squaraine fragment and a strong electron acceptor inserted into the main conjugation chain, while TD 2765 contains a tetraone fragment and an... 

Molecular rotors have in common that fluorescent excitation leads to an ICT, in the case of DMABN from the nitrogen in the dimethylamino electron donor group to the nitrile electron acceptor group (Fig. 1). The ICT leads to a highly polar... 

EDG electron donor group EAG electron acceptor group... 

These interactions (dd, di, ii) are a function of dipole moment and polarizability. It has been shown that the dipole moment cannot be replaced entirely by the use of electrical effect substituent constants as parameters52. This is because the dipole moment has no sign. Either an overall electron donor group or an overall electron acceptor group may have the same value of /x. It has also been shown that the bond moment rather than the molecular dipole moment is the parameter of choice. The dipole moments of MeX and PhX were taken as measures of the bond moments of substituents bonded to sp3- and sp2-hybridized carbon atoms, respectively, of a skeletal group. Application to substituents bonded to sp-hybridized carbon atoms should require a set of dipole moments for substituted ethynes. 

A - electron acceptor group D - electron donor group... 

Mechanical movements in supramolecular structures rely on modulation of noncovalent-bonding interactions. Such changes occur when charge-transfer interactions between electron-donor and electron-acceptor groups are weakened. 

This initial attack of the ozone molecule leads first to the formation of ortho- and para-hydroxylated by-products. These hydroxylated compounds are highly susceptible to further ozonation. The compounds lead to the formation of quinoid and, due to the opening of the aromatic cycle, to the formation of aliphatic products with carbonyl and carboxyl functions. The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron acceptor groups. In summary, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups. 

Both mono- and difluorination of aromatic thioethers can be achieved for compounds bearing an electron acceptor group, not only at the a-methylene carbon but also in the aromatic ring (EWG = CN, NO2, RSO2) (Scheme 19) [79]. 

It is worth noting, however, that the primary halide-containing anion-radicals may be somewhat stable if an aromatic molecule has another electron-acceptor group as a substituent such as the... 

More vigorous conditions were required for the formylation of thienothiophene 3 with an electron acceptor group in the molecule. Thus, refluxing methyl thieno[3,4-6]thiophene-2-carboxylate, iV-methyl-formanilide and phosphorus oxychloride for 1 hour gave a mixture (1 1)... 

SCHEME 5. Dioxirane oxidation of alkenes with both electron-donor and electron-acceptor groups... 

In the absence of conjugation N3 absorption bands of organic azides occur in the 2100-2110 cm 1 region (Ref 9). Conjugation or presence of electron acceptor groups shift the bands to 2135-2166 cm"1. It is claimed that for organic azide measurement of band intensity is a more sensitive indication of compound structure than measurement of band position. Electron donor groups in the molecule increase band intensity and electron acceptors lower it... 

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