Push-pull stilbenes

Liquid crystals may be obtained from suitably modified molecules presenting NLO properties. Thus, various push-pull stilbene 8 and diacetylene 9 derivatives bearing long chains R have been shown to display nematic and smectic mesophases (22). 

Structures of type 15 to 20 can also be considered aza analogues of ( )-stilbene compounds 17 and 18 are an unusual type of push-pull ( )-stilbene that should show a spectrum of properties ranging between those of ( )-stilbenes and betaines. The 19 types of compounds selected, 10-28, and their precursors, 29-47, outlined in Scheme 3 and Table I, have been ordered by (a) the nature of the interannular linkage (-Q-) (b) the nature of the two atoms linking the 7r-deficient nucleus and the interannular group (C—N bond type and C—C bond type) (c) the substitution pattern between the 7r-deficient nucleus and the interannular group. 

The effect of Si02 and Si02-Ti02 surfaces (in the powder form) on the fluorescence and lifetimes of the adsorbed two push-pull stilbenes, , -l-(4-cyanophenyl)-4-(4-N,N-dimethylaminophenyl)-l,3-butadiene and -9-(4-cyanostyryl)-2,3,6,7-tetrahy-dro-lH,5H-pyrido[3,2,l-i,j]-quinoline was observed [56]. The stilbene fluorescence was quenched effectively by mechanism of electron injection from excited stilbene... 

At present, the accessible physico-chemical properties have been studied mainly in liquid solution and the overall results provide evidence of their intrinsic high dipolar character. Moreover, compounds of type 1 may be ideal substrates for the study of their photophysical and other physical properties, especially for unconventional extended rr-systems 15-20, which are push-pull aza analogues of (E)-stilbene. Their capacity for specific physical behavior merits further exploration. 

As concerns photochromes in a solid matrix, a question that immediately arises is to what extent the nature of the matrix impedes the photochromic reaction. This problem has been studied in detaih but it is beyond the scope of this review. There is a general rule that states photochromic reactions are sluggish in polymer matrices compared to fluid solutions. This statement is true for some stilbene derivatives, but it is not true for azo derivatives, especially for push-pull azobenzene derivatives like DRl, for which the trans->cis quantum yield equals 0.11 in PMMA at 20°C compared to 0.24 in a liquid hydrocarbon mixture at -110°C. Photochromism of spiropyrans shows an important matrix effect as the quantum yield for the conversion between the spiropyran and the photomerocyanin is equal to 0.8 in ethyl acetate and decreases to 0.102 in PMMA at room temperature. The same decrease is observed for the back photochemical reaction efficiency 0.6 in ethyl acetate, compared with 0.02 in PMMA at room temperature. Conversely, the matrix effect is much less for furylfulgides the quantum yields are almost the same in solutions as in polymer matrices. Although most of photochromic molecules exhibit photochromism in polymers and sol-gels, few of them exhibit this property in the crystalline state, due to topochemical reasons. However, some anils and dithienylethenes are known to be photochromic in the crystalline state. 

The photoisomerization of the push-pull azobenzene derivative disperse red 1 (DRl) has also been shown recently to occur efficiently in poly-(methyl metacrylate) (PMMA) thin films at room temperature [23,24]. DRl is a pseudo-stilbene type azobenzene molecule, as classified by Rau. This means that the high-energy n-n transition is overlapping the low energy n-n transition, which leads to a large structureless band in the trans isomer and... 

The UV/Vis absorption of conjugated chromophores is strongly influenced by push-pull effects. Table 11.5 summarizes the colors and the absorption maxima of the trans-stilbenes 52a-f (n = 1). Increasing acceptor strength causes increasing... 

The nature of the azo oroup The most important structural feature of an azobenzene group is the type of substituents it possesses. If there is no amino substituent, or if there are no electron-donor -electron-acceptor substituents to confer a push-pull electron movement, the cis isomer is relatively long lived. If such substituents are present (aminoazobenzenes or stilbene-like azobenzenes), the cis isomer reverts back to trans in a few seconds at room temperature, which means that its absorbance and other characteristics cannot be directly measured. Typically, the maximum absorbances of cis and trans isomers of an aminoazobenzene or of a stilbene-like azobenzene are... 

H. Shinmori, M. Takeuchi, S. Shinkai, Spectroscopic sugar sensing by a stilbene derivative with push (Me2N)-pull ((Ho)2B-)-type substituents, Tetrahedron 1995,51, 1893. 

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