P-Nitrostilbene

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. ... 

Various catalyst types were tested with a wide variety of substrates. For example, p-iodonitrobenzene and styrene react according to the Heck cross coupling to p-nitrostilbene with a yield of 99%. This situation is similar for many other reactions of styrene and methylacrylate with other reactants containing iodine and bromine. On the other hand, chlorobenzene reacts with styrene only in trace amounts. 

Stereospecific synthesis. A soln. of butyllithium in hexane added dropwise at — 70° under Ng to a stirred slurry of diphenylbenzylsulfonium tetrafluoro-borate in anhydrous tetrahydrofuran, then a soln. of p-nitrobenzaldehyde in tetrahydrofuran added dropwise, stirred 2 hrs. at —70°, allowed to warm to room temp, during 2 hrs., and hnally stirred 1 hr. at room temp. trans-p-nitrostilbene oxide. Y 72%. F. e. s. A. W. Johnson, V. J. Hruby, and J. L. Williams, Am. Soc. 86, 918 (1964) with tert-butyl-Li s. E. J. Corey and W. Oppolzer, Am. Soc. 86, 1899 (1964). 

LC Poly(butadiene) +p-nitrostilbene LC Poly(hydroquinone terephthalate), f-butyl LC Poly(norborene) +p-nitrostilbene LC Polylster... 

It should be possible in principle to determine the orientation of chromophores in a single monolayer on an OWG by the absorption of transverse electric (TE, s-polarized) and transverse magnetic (TM, p-polarized) modes laser. Swalen et al. [109] reported that much stronger absorption was observed for a thin evaporated film of 4-dimethylamino-4 -nitrostilbene with the TM mode and for seven monolayers of cyanine dyes with the TE mode. These results corresponded... 

If the nitro group is located at the ethylene fragment, one-electron transfer initiates dimerization of the developing anion-radicals. a-Nitrostilbene, w-methyl-co-nitrostyrene, and a-nitro-p-ferrocenylethylene give anion-radicals, which dimerize spontaneously. It is interesting to compare reactions of cyclooctatetraene dipotassium (C8HgK2) with a-nitro and a-cyano ferrocenylethylenes (Todres and Tsvetkova 1987, Todres and Ermekov 1989 Scheme 3.4). 

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... 

Even though the high carbon basicity of thiolate ion nucleophiles is a major reason why their nucleophilic reactivity is much higher than that of oxyanions or amines of comparable pATa, there is an added effect that comes from a reduced intrinsic barrier. For example, intrinsic rate constants for thiolate ion addition to a-nitrostilbene or p-nitrostyrene are up to 100-fold higher than for amine addition. This has been explained in terms of the soft soft interaction... 

The intrinsic rate constants for thiolate ion addition to 76-Cr and 76-W are substantially larger than those for alkoxide ion addition. This is similar to the previously mentioned higher intrinsic reactivity of thiolate ions compared to amine nucleophiles for the addition to a-nitrostilbene and p-nitrostyrene. It can be understood in terms of the soft-soft interaction of the thiolate ion with the carbene complex which is more advanced than C S bond formation at the transition state.184... 

The prototypical organic material with high P (all values of P quoted here are for 1.06 1 light unless noted) is p-nitroaniline (PNA). In electrostatic units (esu), P for PNA is 34.5 x 10 30 esu. (37) It has a dipole moment of 6.8 D. Extension of the chromophore by insertion of a single double bond to produce 4-amino-4 -nitrostilbene increases P dramatically 248 x 10"30 esu. (38) However, both crystalline materials are centrosymmetric, and so no SHG is possible. Most achiral molecules crystallize in centrosymmetric space groups, but some do not. Several materials have been discovered during the last few years which have combinations of hyperpolarizability, crystal growth and habit, and linear spectral properties which make them candiates for useful NLO materials. As will be obvious from the comparison, considerable compromise must be made to yield a practical material. 

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... 

Molecules such as 4 -dimethylamino-4-nitrostilbene ( DANS , 1) possess the necessary electronic structure, and have very large p... 

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 rate constant for the nonradiative deactivation (knj) of the trans triplet configuration is much larger than that for the radiative transition (kr). An estimation for 4-nitrostilbene in ethanol shows that kt is at least 2000 times smaller than fcnr, using 4>jsc = 0.7 [407] and 4>p = 3 x 10-4. This places kr at 0.05s 1 since knr(zs 1/tt) is about 102s. The more than 30-fold decrease in d>p for stilbenes not carrying carbonyl or nitrogen substituents (n, n states) can be explained by a comparable kt value and the corresponding decrease in [Pg.65]

The rate constant for the individual transitions may depend on a variety of features, such as substitution, medium and temperature. One important parameter is the height of the activation barrier between t and p. If the rate constant for the step 11 — 1 p is of the size comparable to that for the step t -> 3t, both processes may contribute to tram-> cis photoisomerization. For most nitrostilbenes examined, is substantial and for 4-nitro-, 4,4 -dinitrostilbene, and 4,4 -NMS a singlet pathway has been ruled out. However, for 4,4 -NDS in nonpolar solvents [160] and for quaternary salts of 4-nitrostyrylquinolinium [229], a contribution of the singlet mechanism has been concluded. 

In view of the well-documented ability of Oj to function generally as an acceptor of triplet energy, it had been expected that its presence should also increase ([t]/[c])3 ratios for the triplet sensitized photoisomerization of stilbene-like olefins. However, as was first noted for nitrostilbenes, the quenching of the olefin triplets by O2 does not alter ([t]/[c])s (42,43). For the parent stilbene it was shown that when azulene is used as a quencher of stilbene triplets the slope of the ([t]/[c])s-vs. -[Az] plot is strongly attenuated by O2 see Figure 5 (41,44). Since the intercept of the azulene plot was not influenced by O2, it was reasoned that O2 deactivates p without changing the decay firaction 8 as predicted by Schemes 1 and 2 (44). To account for stilbene triplet deactivation by O2 without change in 8 two possibilities were considered ... 

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