Substituted trans-stilbene derivatives

A series of substituted trans-stilbene derivatives have been prepared and immobilized on a quartz surface [19]. Several immobilization methods have been tried including the silanization technique, cross-linking with cyanuric chloride, surface activation with cyanogen bromide, and surface smoothing with coating proteins. 

Brown s result was supported by later experiments in which bromonium ions were generated by bubbling gaseous hydrobromic acid through a solution of bromohydrins in halogenated solvents. Under these conditions, bromine is eliminated as it is formed, so that the resulting alkene is observed directly (Scheme 15). This method has been applied to the bromohydrins derived from cis- and trans-stilbenes (Scheme 16) and from 5//-dibenzo[a,d]-cycloheptene and -azepine systems ([29a] and [29b] respectively Scheme 17), in which steric constraints should favour elimination (path a) as against substitution (path b). 

The fluorescence of trans-stilbene and four methoxy-substituted stilbene derivatives... 

When the central linkage does not provide a linear molecule, however, mesophase stability suffers. Mesophases exhibited by derivatives of phenyl benzoate, for example, are generally some 35° less stable than those of correspondingly substituted azobenzenes. The effect of linearity, or at least planarity, of molecules on mesophase stability is further evidenced on comparison of tmns-4-alkoxycinnamic acids and trans-stilbenes with their corresponding cis isomers, which demonstrates that severely nonlinear molecules show no mesomorphic properties. Again, derivatives of epicholestanol and caprostanol (Fig. 5) are not known to be mesogenic. 

Dihydroxylation of the stilbene double bond in the trans isomers of Combretastatin A-1 and A-4 produced diols which by treatment with boron trifluoride in ethyl ether [44] or with trifluoroacetic acid [17] resulted in pinacolic rearrangement to produce an aldehyde. The aldehyde was converted in a variety of derivatives, as illustrated in the Scheme 20, via the following reaction sequence reduction with sodium borohydride to primary alcohol which was derivatized to the corresponding mesylate or tosylate, substitution with sodium azide and final reduction to amine with lithium aluminum hydride. Alternatively the aldehyde was converted to oxime which was catalitically hydrogenated to amine [17]. 

Up to now, photochemical studies are still confined to the reactions of simple azobenzene compounds, such as those derived by coupling a phenol or aromatic amine with a diazotised aniline. Azobenzene is isoelectronic with stilbene, its derivatives undergo facile trans cis photo isomerisation reactions in a manner analogous to that of stilbene. Azo dyes which possess a hydroxyl or amino substitutent ortho or para to the azo group... 

This chapter deals with the properties of the excited states of 1,2-diary lethylenes in general, with the effects caused by specific variation of reaction conditions on photochemical cis trans isomerization in condensed phase, and with the reaction mechanisms. In particular, the influence of substitution on the properties of excited states involved in cis trans isomerization in solution is examined. Besides various substituted stilbenes (which have been most extensively studied), styrylpyridines (StPs, azastilbenes) including some of their positively charged derivatives (quaternary stilbazolium salts), dipyridylethylenes (DPEs), styrylnaphthalenes (StNs), their pyridine analogues (NPEs), and some related compounds, such as dinaphthylethylenes (DNEs), are surveyed. Results on photochemical cis ttrans isomerization of stilbenes and other 1,2-diarylethylenes under direct (Section II) and sensitized (Section III) irradiation conditions are summarized, as well as their photophysical excited singlet and triplet state properties (Section IV) and some selected side reactions (Section V). The mechanistic section (Section VI) describes several photochemical isomerization routes. Characteristic photophysical and photochemical aspects of specific classes of substituted stilbenes are discussed and mechanistic schemes are critically examined with reference to their experimental basis. 

Two DHPs were obtained from tetramethoxystilbene [279]. Substitution by a nitro group in the meta or para position reduces distinctly [82], Saltiel et al. have questioned whether the values may be erroneous [105], On the basis of quenching measurements with azulene they proposed additional routes for bromostilbenes from c via excited states of DHP which may relax back to 3c or c. A consequence of a higher value of for the mechanism of cis -> tram isomerization is that the ratio of c decaying to the trans isomer may have to be reexamined. For trans-ct-bromostilbene and the / -phenyl substituted derivatives several photoreactions (e.g., debromination) compete with photocyclization [475]. Interestingly, no evidence for photocyclization could be found for several fluorinated stilbenes [481]. Rotamers can be distinguished in the cyclization of c/s-2,2 -DNE [482],... 

The fluorescence of Irons-stilbene and four methoxy-substituted stUbene derivatives has been detected in a variety of solvents [37]. Compared to other stUbene derivatives, trons-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. 

A series of aminostilbenes (lA-C, 2A-C) have been synthesized to test the effect of substitution of the amino group upon the photophysics and photochemistry of stilbenes [45]. This study indicated that the photophysics properties of trans-2-aminostilbene, lA, and trans-3-aminostilbene, IB, were similar. as-2-Aminostilbene, 2A, and ds-3-aminostilbene, 2B, showed similar fluorescent lifetimes. Anomalous behavior of emission anisotropy for short-living derivatives of stilbenes was described [46]. The absorption, fluorescence and polarization excitation, and emission spectra... 

The electronic spectroscopy of trans-isomers of 3-(N-phenylamino)stilbene (mlc), 3-(N-methyl-N-phenylamino)stilbene (mid), 3-(N,N-diphenylamino)stilbene (mle), and 3-(N-(2,6-dimethylphenyl)amino)stilbene (mlf) and their double-bond constrained analogues, m2a-m2c and m2e, were studied [71]. When compared with trans-3-aminostilbene (mla), mlc-mle displayed a redshift of the So —> Si absorption and fluorescence spectra, lower oscillator strength and fluorescence rate constants, and more effldent Si —> Ti intersystem crossing. The N-Ph derivatives mlc-mle had lower fluorescence quantum yields and higher photoisomerization quantum yields. The role of Si —> Ti transition in the amino-substituted stilbenes as the predominant nonradiative decay pathway was discussed. The excited triplet (Ti) state formation of stilbene dendrimers (tetramethoxystilbene (generation G) GO, Gl,... 

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