Stilbazoles isomerization

The L-B films offer some advantages over aqueous-hydrocarbon interfaces of micelles and the related assemblies discussed above in terms of the magnitude of their orienting ability and the ease of interpretation of selectivity in photoreactions conducted in them. Molecules in the films have very little freedom of motion (stiff reaction cavities), their interfaces are very well defined, and therefore the alignment of reactant molecules can be readily expressed in the products. Photodimerization of stilbazole derivatives 62, N-octadecyl-l-(4-pyridyl)-4-(phenyl)-l,3-butadiene, (63), surfactant styrene derivatives 64 and 65, and cinnamic acids have been carried out in L-B films [18, 196-200], In all cases, single isomeric head-head dimers are obtained. Geometric isomerization of olefins has not been observed in competition with photodimerization. Independent of the location of the chromophore (i.e.,... 

Metal perturbation of IL photochemistry appears to be less important in [Ru(bipy)2(h- n.s-4-stilbazole)2]2+ and [Ru(bipy)2(cw-4-stilbazole)2]2+. Both complexes undergo wavelength-dependent isomerization of the coordinated stilbazole ligands as the only important photoreaction."4 This wavelength dependence has beep attributed to the presence of two different types of reactive excited states, aRu-t stilbazole CT state which favors formation of the trans isomer and a higher energy IL state localized on stilbazole which decays with nearly equal probability to the two isomers. The latter pathway is qualitatively similar to the isomerization process that obtains upon direct photolysis of the free ligand. Thus coordination to Ru has not altered the characteristic photoreaction mode of the stilbazole molecule. 

Photocontrol of chemical and physical functions could be used in this system (Fig. 6.21). Stilbazole, a compound related to natural photoresponsive molecules, was used as the photoresponsive switch [136]. Because stilbazole undergoes isomerization from fhe trans form to fhe cis form on UV irradiation, and fhe reverse in visible hght, via complexation of the pyridine group to fhe metal center of metalloporphyrins, fhe (TPP)Al-OMe-stilbazole system on irradiation wifh UV light serves as an on-switch to speed up the reaction whereas in visible hght fhe system worked as an off-switch to reduce the rate of the once accelerated reaction. 

The behavior is typical of relatively simple dipolar materials where a N phase was found at short chain lengths, giving way to a SmA phase in higher homologs. While the iridium complexes could be cycled in and out of the isotropic phase with few problems of decomposition, the rhodium complexes began to decompose rapidly on melting in air. Lateral fluorination of air-iridium stilbazole complexes strongly destabilized the mesophase the 2-fluorinated 4-alkoxystilbazole systems exhibited only monotropic N and SmA phases, whereas the structural isomeric 3-fluori-nated compounds were not mesomorphic. " ... 

More recently, 4-alkoxy-2- and 4-alkoxy-3-stilbazoles have been reported (Figure 85) and their silver complexes with AgDOS and AgOTf have been synthesized. While the ligands themselves are non-mesomorphic, the eomplexes show only SmA phases, although complexes of the 2-stilbazoles do not form readily. While the new complexes are Uquid crystalline, the use of the isomeric stilbazoles suppresses the polymorphism of the 4-stilbazole isomers. 

The only photoreaction which the 4-styrylpyridine (4-stilbazole) complexes [Ru(bipy)(cw-4-stilbazole)] + and [Ru(bipy)(/ ra 5-4-stilbazole)] + undergo is that of cis trans isomerization of the stilbazole ligands there is no significant photodissociation. The anion [Pt(NH3)4(NH2)(N02)] " photoisomerizes to [Pt(NH3)4(NHa)(ONO)]. The quantum yield has been determined for this process, but no kinetic results are available. ... 

It has been mentioned that cis-trans isomerism has been observed in the stilbazoles (pp. 122, 334, 336). Examples are fairly common3i2-3 Hydrogenation over Lindlar s catalyst of aryl-pyridyl- or dipyridyl-acetylenes gives the m-stilbazoles or rw-l,2-dipyridylethylenes i4 5. rw-Stilbazoles readily form the /m -isomers when heated or irradiated with iodine, or heated with palladized charcoaU a, 3i3, 3i5 ... 

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