Stilbazoles dimerization

Another series of trimers was reported (15-n,m) that consisted of two stilbazoles halogen bonded to a dimeric iodotetrafluorobenzene unit [43]. Examples were reported for m = 2,4, 6 and 8 and n = 6, 8,10 and 12. In characterising the halogen bonds in these complexes, XPS data were examined and it was reported that the binding energy of the N Is level increased by around 0.9 to 1.1 eV on complexation, while small hypsochromic shifts (typ-... 

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.,... 

Irradiation of 4-stilbazole (25) in aqueous HC1 gave predominantly the syn head-to-tail dimer 21 and minor amounts of dimer 22. The syn head-to-tail structure was also assumed for the solid state photodimer of 4-stilbazole methiodide 26 (58). 

Hydrogen bonded dimers of 4-RF-substituted benzoic acids [119, 120, 122, 126], /rcyclohexane carboxylic acids [120], and hydrogen bonded complexes of benzoic acids with 4,4 -bipyridine and stilbazole [166, 243-246] can be regarded as supramolecular rod-like mesogens with two fluorinated chains. Figures 33 and 34 show comparisons of selected benzoic acid dimers and their bipyridine and stilbazole complexes. Comparison of the benzoic acid dimers 122 and 123 with -alkyl chains and F-terminated -alkyl chains, respectively, indicates that a single F-atom at the end of the alkyl chain reduces the mesophase stability... 

Subsequent to the discovery of the polymerization of DSP crystals, another monomer, l,4-bis(3-pyridyl-2-vinyl)benzene (P2VB), which belongs to the DSP series (Ar -CH=CH-Ar-CH=CH-Ar ), was found to photopolymerize to the same type of polymer in a similar manner. However, P2VB polymerizes much more slowly than DSP131. Polymerization of the former can be regarded as an extension of the dimerization of stilbazol in the crystalline state141. 

The most probable cause of the predominant a dimer formation in concentrated homogeneous solutions of cations 1 and 2 is that this dimer has the least repulsion between the two positively charged pyridinium rings the p, or head-to-head dimer has the most electrostatic repulsion. The difference in the distribution of photoproducts for cations 1 and 2 in concentrated homogeneous solution indicates that the position of the nitrogen in the pyridinium ring has a profound effect on the photochemistry of stilbazole. Results in concentrated aqueous acid... 

Addition.—Intermolecular [ 2 + w2] cycloadditions in nitrogen-containing heterocycles continue to arouse interest, particularly in systems which are related to constituents of nucleic acids. Cyclobutane dimers are readily obtained on direct irradiation of furo[3,2-Z>]pyridin-2(4 )-ones and the pyrrolo[3,2-Z>]-pyridin-2-one (151), although it is not possible on the basis of the evidence available to distinguish between the head-to-head (152) and the head-to-tail (153) structures.118 Cyclobutane dimers have also been obtained from alkyl 4-stilbazole salts on irradiation in solution and in the solid state.118 Examples of analogous intramolecular [ 2 + 2] cycloadditions have also been reported.120-121 The [ 2 + 2] cycloaddition of quinol-2-one to furan has been described,122 but... 

Dimeric stilbazole complexes of Ir(i) 73 were reported and a very strong odd-even effect was noticed such that mesophases were observed when n = odd, but not when n = even. 

Photoreaction of stilbazoles 13a-f (Scheme 1.4) has also been investigated in the presence of SCA[n]. Upon irradiation in dilute aqueous HCl solution without a host, trans-stilbazoles 13a-f gave predominantly c/x-isomers 16a-f, along with only small amounts of anti-head-to-tail (HT) dimers 14a-f and anti-head-to-head (HH) dimers 15a-f. In aqueous solution, stilbazoles were found to form 2 1 complexes with SCA[8] and SCA[6], irradiation of which afforded the corresponding anti-in dimer in 66-86% yield in the presence of SCA[8] and in 60-76% yield in the presence of SCA[6]. The anti-HH dimer was given in only 16% yield, which was attributed to the electrostatic repulsion between two pyridinium components. The hydrophobic nature of SCA cavity should be responsible for the inclusion of stilbazoles. It is thus concluded that SCAs control the orientation of stilbazoles through complex formation to eventually enhance the distribution of HT dimer. 

For polymers formed between Ihe diiodo componnd and the three simple difunctional pyiidines, liquid-crystalline behavior was not observed, but the polymers formed using the dimeric stilbazoles did show nematic phases. 

Iridium complexes of dimeric stilbazole, 9, have been synthesized recently. These materials show a very laige odd-even effect in their melting behavior, and are called metal-containing Siamese Twin liquid crystals [18]. 

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