Stilbenes Nitrostilbenes

Miscellaneous Dyes. Other classes of dyes that stiU have some importance are the stilbene dyes and the forma2an dyes. Stilbene ( es are in most cases mixtures of dyes of indeterminate constitution that are formed from the condensation of sulfonated nitroaromatic compounds ia aqueous caustic alkah either alone or with other aromatic compounds, typically arylarnines (5). The sulfonated nitrostilbene [128-42-7] (79) is the most important nitroaromatic, and the aminoa2oben2enes are the most important arylarnines. Cl Direct Orange 34 [2222-37-6] (Cl 40215-40220), the condensation product(s) of (79) and the aminoa2oben2ene [104-23-4] (80), is a typical stilbene dye. 

Nitro substituted stilbenes have been used as models in stUbene cis trans-isomerization studies. It had been reported previously, that cis -v benzene solution, and a photostationary state of 99.5% trans isomer had been reached 132) ... 

Laser flash photolysis (30 ns, 50 mj, 347 nm) of 4-nitrostilbene, 4,4 -dinitro-stilbene, 4-nitro-4 -methoxystilbene and 4-dimethyl-amino-4 -nitrostilbene permits the observation of transients 133) por the latter two compounds, the lifetime and the absorption spectra of the transients vary strongly with the polarity of the solvent used. First order decay rate constants are given in Table 4. 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

Dinitro-p-azostilbene or Azobis-(9-nitrostilbene, called in Beil Bis-[4 -nitro-stilbenyl-(4)]-diimid or [4 -Nitro-stilben]-<4azo4>-[4,-nitro- stilben], (OjN KAf- CH CH— C6H4-N N-C6H4-CH CH-CsH4(N01) yel-red Crystspmp 263°. It is listed in Beil 16, 84 without reference to its source was not found in CA through 1956... 

The p for stilbene derivatives are much higher than their benzene analogs (JL). To determine the effect of substitution on the nonlinearity, 4-methoxy-41-nitrostilbene... 

The optical spectra of the stilbene and azobenzene derivatives were studied in different solvents to establish the solvatochromic shifts of the new sulfonyl-containing chromophores, and to compare them to those found for the nitro analogues. Figure 3 shows the spectra in toluene and in DMF of 4-dibutylamino-4 -nitrostilbene, and of 4-dibutylamino-4 -methylsulfonylstilbene, and Figure 4 shows the spectra in the same solvents, but of 4-dibutylamino-4 -nitroazobenzene, and 4-dibutylamino-4 -methylsulfonylazobenzene. Table II summarizes the results. 

The aldehydes 3 were synthesized either by Vilsmeyer formyladon of 2 or by reaction of 4-fluorobenzaldehyde with a secondary amine (20). The stilbenes 4 were formed from the corresponding 4-dialkylamino-benzaldehydes either by the Homer-Emmons reaction with 4-nitrobenzyl-(diethyl)phosphonate (prepared by the Arbuzov reaction of a-bromo-4-nitrotoluene) or with 4-methylsulfonylbenzyl(diethyl)phosphonate (prepared in three steps from 4-methylthiobenzylaicohol) (21). A few nitrostilbene compounds were synthesized by heating aldehyde 3 with 4-nitrophenylacetic acid in the presence of piperidine. 

A New Molecular Crystal DEANST. Another example of a low molecular weight compound with a large third-order NLO property is a styrene derivative. By selecting an appropriate -conjugation length and chemical structures of donor and acceptor, several new organic molecular crystals such as stilbene and azo benzene derivatives which exhibit %(3) larger than 10 12 esu can be developed. The %(3) of 4-(N,N-diethylamino)-4 -nitrostilbene (DEANS), an intramolecular CT compound, has been reported in a solution, in a polymer matrix and in a crystalline state(27.29.30). 

Upon one-electron reduction followed by one-electron oxidation, cis isomers of 2-and 4-nitro stilbenes turn into the neutral nitrostilbene molecules, but in the trans forms. Upon oxidation of the naked anion radical, the neutral trans forms are the only products (cis —> trans conversion degrees were 100%). In case of the coordination complexes, the trans isomers are formed only up to 40% (Todres 1992). Scheme 3-39 describes these transformations. 

Stilbenes offer both advantages and disadvantages compared with photodimers. The absorption band which is changed by conversion of the more stable trans isomer to the cis form is much weaker than the strong band of the anthracene system that moves into the ultraviolet after dimerization. The stilbene band is somewhat stronger than the near-uv anthracene band. Trans-4-methoxy-4 -nitrostilbene absorbs at ca. 360 nm (e = 6 x 1(P). ... 

The influence of the rigidity of the backbone on the mesophases of p-nitro-stilbene-containing polymers was the subject of a study by Maughon et al. [64]. The rigid poly(norbornene) backbone was compared with the flexible poly-(butadiene) chain. For this purpose, p-nitrostilbene was linked by an ester group to the norbornene bicycle, IX-n (n=6,8,10,12), and to the cyclobutene, X-n, n=6y 8,10,12, respectively (Fig. 10). Polymers were synthesized with initiator 6 SEC and DSC data of the resulting polymers are summarized in Table 8. 

As a preparative method the direct decarboxylation of olefinic acids is almost limited to the formation of styrenes and stilbenes from substituted cinnamic acids. Thermal decomposition of cinnamic acid gives styrene (41%). The yield is nearly quantitative if the reaction is carried out in quinoline at 220° in the presence of a copper catalyst. The yields of substituted styrenes where the aryl radical contains halo, methoxyl, aldehyde, cyano, and nitro groups are in the range of 30-76%. cis-Stilbene and cis-p-nitrostilbene are prepared in this way from the corresponding a-phenylcinnamic acids (65%). One aliphatic compound worthy of mention is 2-ethoxypropene, prepared by heating -ethoxycro-tonic acid at 165° (91% yield). The mechanism of acid-catalyzed decarboxylations of this type has been studied. Isomerization of the double bond from the a,/5- to the /5, y-position before decarboxylation very likely occurs in many instances. ... 

Mononitrodiphenylethylene or Nitrostilbene, G6H6.CH CH.C6H.4.N02 mw 225.24, N 6.22%. The following isomers are described in Beil 2-Nitro, 3-Nitro, 4-Nitro a-Nitro-stilbene, CgHg.CH C(N02).CgH5 (Ref)... 

Fig. 3 cttpa variations with the number of excited states involved in the SOS expression (3) for the 4-dimethylamino-4-nitrostilbene and the dimethylamino)- -stilbene... 

In most cases studied, the cis isomer predominates in the photoequilibrium. There exist, however, some cases in the series of sterically hindered stilbenes where the cis isomers are thermodynamically the more stable isomers. For example, oc,/ -dichloro- and a-carboxystilbene contain larger amounts of cis than of trans isomers [12, 53, 55, 56] and only the cis isomer is known for a-nitrostilbene [58], Maciaszek and co-workers [58] were also interested in the photochemical preparation of the less stable isomer of several stilbenes bearing bulky substituents in the a-position Photochemical isomerization was found impracticable for cis-a-nitro- and ot-CONH2-4,4 -dimethylstilbene. With a-carboxy- and a-methoxycarbonyl-4,4 -dimethylstil-bene in benzene, however, a considerable amount of less stable trans isomer has been observed under steady state conditions. 

For nitrostilbenes (Table 5d) in nonpolar solvents at ambient temperature the quantum yields of isomerization are comparable to those of stilbene itself. However, 4-nitro-4 -R-stilbenes show a different dependence on solvent properties and temperature. On increasing the solvent polarity, decreases strongly for polar substituents for example, R = OCH3, NH2, and N(CH3)2, 4,4 -NMS, 4,4 -NAS, and 4,4 -NDS, respectively. nonpolar solvents (cyclohexane and benzene) are... 

Photostationary cis/trans ratios [2] and triplet transfer rate constants [211] as a function of ET obtained with a-methylstilbene [cf. 219] are analogous to those observed with stilbene. For several nitrostilbenes it has been shown that the position of the photostationary state depends only slightly on substituent and medium [201]. A solvent dependence however, was found for two polar nitrostilbenes 4,4 -NMS and 4,4 -NDS. The shift of the photostationary state to the tram isomer with increasing solvent polarity accounts partly for the decrease of d> c. A small decrease of d>f-.c with increasing polarity has also been found for several azastilbenes (Table 9c). 

Whitten and co-workers [230] have amply investigated stilbene isomerization induced by excited porphyrins. Electron-deficient nitrostilbenes quench excited Zn or Mg etioporphyrins through exciplex formation. Light absorbed by the porphyrin affects isomerization of 4-nitro- or 4,4 -dinitro-stilbene with low quantum yields ((%trans)sens = 99.5), possibly via a stilbene exchange chain mechanism [230]. Azastilbenes coordinate to me-talloporphyrins in both the ground and the excited state (but do not quench it), and isomerize efficiently upon selective excitation of the porphyrin [231]. Complexed 1,4 -NPE gives d , = 6.6, = 0.2, and (%trans)sens = 96,... 

With ferrocene as quencher and triphenylene as sensitizer the cis trans isomerization of several 4-nitrostilbenes has been examined [201], From linear plots of ([t]/[c])sens versus [Q], larger slope/intercept ratios were found as compared to direct excitation conditions (Table 8). A cis -> trans pathway partly bypassing the triplet state accounts for this difference (Section VI). Plots of ([t]/[c])scns as a function of the azulene or ferrocene concentrations are shown in Figure 10 for three stilbenes. 

Fluorescence Quantum Yields. For tram-stilbcne 4>f is independent of lexc and the concentration (typically < 10 4 M) and only slightly reduced by the presence of c/s-stilbene [13, 81, 99, 240, 241, 294]. The liquid solutions at room temperature f is about 0.04 for trons-stilbene and 4-bromostilbene, and larger for 4-chlorostilbene and several cyanostilbenes. For nitrostilbenes very small, but there are exceptions in the presence of an electron... 

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