Dimeric anthracene

To realize a photoregulated ion-binding system in calixarenes we introduced two anthracenes near the metal-binding site of calix[4]arene [37-40]. Compound 17 having a podand-type cavity showed poor ion affinity whereas the photochemically produced isomer 18 with a dimeric anthracene-cap showed much improved ion affinity and sharp Na+ selectivity [38,39], Interestingly, 17 immobilized in the PVC membrane plasticized with di(2-ethylhexyl)sebacate underwent ring closure to 18 when it was photoirradiated at 381 nm [40], The... 

Use of a less bulky guest such as ethyl acetate induces a dimeric lattice pattern 65, which exclusively emits excimer fluorescence from the face-to-face dimeric anthracene units [53]. Segregated anthracene (A) and anthraquinone (Q) columns (66) are found in the charge-transfer molecular crystals of adduct 29 2 (anthraquinone) [66]. This is also the case for the 1 1 complex of the diresorcinol derivative of anthracene (29) and the dipyrimidine derivative of anthraquinone (67) as a specific hydrogen-bond donor and an acceptor, respectively [66]. 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

Due to the steric requirement of these substituents the formation of a columnar structure with infinite M M interactions is inhibited, and only the association of pairs of molecular units is allowed. The Ni Ni distance is 3.21 A [164]. If the same compound is crystallized in the presence of benzimidazole, the [Ni(dmg-BF2)2]2 dimer units are sandwiched between sheets of benzimidazole molecules due to n-n interactions resulting in an increased Ni Ni separation of 3.358 A [165]. With anthracene the n-n interactions seem to be stronger, because in this case the parent dimer molecule is cleaved. Each monomer now has a conformation of type B (Fig. 32) and is sandwiched by anthracene molecules [166]. Compound 121 has the same configuration [163d]. 

Liu HY, Abdalmuhdi I, Chang CK, Anson FC. 1985. Catalysis of the electroreduction of dioxygen and hydrogen peroxide by an anthracene-linked dimeric cobalt porphyrin. J Phys Chem 89 665. 

Derivatives of anthracene bearing substituents on the 1 or 2 position can be photodimerized with efficiencies comparable to that for the unsubstituted molecule. However, with substituents at the 9 meso) or 9, 10 dimeso) positions a very interesting photochemical problem results. Since dimerization occurs across the 9, 10 positions, substituents at these positions exert a first-order effect on the photochemical reaction. The mero-substituted anthracenes examined include the following as 19>... 

In general, symmetrically substituted diwero-anthracenes produce only very low yields of dimers upon irradiation. On the other hand, asymmetrically substituted derivatives dimerize, although these products are less stable than those formed from the mono-substituted anthracenes. 

Each of the following aromatic compounds is known to undergo an anthracene-like dimerization ... 

We now address ourselves to the problem of accounting for the observations noted in Section 2.2a by building a mechanism for the dimerization of anthracene and related compounds. 

Since anthracene dimers split to form anthracene monomers upon melting, it may be possible that if the excess vibrational energy in the hot dimer is not dissipated by solvent collisions<88> rapidly enough the bonds may rupture and monomers may be produced,... 

In the first mechanism the excimer is represented as a common intermediate for the formation of dimer and the deactivation of the excited anthracene. In the second, excimer formation is totally nonproductive with regard to dimer formation. Again as in paragraph (b) one can think of the excimer in the second mechanism as having a structure that, if dimerization proceeded, would yield the unobserved head-to-head product ... 

By examining any correlation between excimer formation (as evidenced by characteristic excimer fluorescence) and dimerization quantum yield, one could perhaps determine whether dimerization is dependent upon prior excimer formation. Excimer fluorescence from anthracene solutions at room temperature is negligible although it has been observed in the solid state at low temperature.<75) Unfortunately, the data for substituted anthracenes allow no firm conclusions to be drawn. Some derivatives dimerize but do not exhibit excimer fluorescence. Others both dimerize and show excimer fluorescence. Still others show excimer fluorescence but do not dimerize and finally, some neither dimerize nor show excimer fluorescence. Hopefully, further work will determine what role excimer formation plays in this photodimerization. 

Attempts to produce dimers of naphthalene similar to those observed for anthracene (Chapter 2) have been unsuccessful, although three naphthalene derivatives have been reported to produce dimer (65) upon photolysis<78) (the structure of these dimers have been the object of some debate, however) ... 

Further investigation of this interesting reaction carried out much later by other workers indicated that the photoproduct was not an anthracene polymer but was dimeric in nature. This conclusion was supported by the following data ... 

It can be seen that a large variety of 9-substituted anthracenes dimerize upon irradiation. There are a few, however, from which no dimers have yet been isolated. Although at this point it is difficult to say what determines whether a particular derivative will dimerize or not, it would appear from the above lists that the controlling factor cannot be steric in nature since the relatively crowded 9-cyclohexyl anthracene dimerizes whereas 9-phenyl-anthracene does not. This result would tend to indicate that electronic effects of the substituents may influence the dimerization. 

In all of the cases in which dimerization has been observed, the yield of dimer formed, ranging from 40 to 70% under similar conditions, is less than that observed for anthracene. It should be recalled, however, from Chapter 1 that the yield is not a direct measure of the reactivity since it is dependent upon all other competing processes. 

For meso-substituted anthracenes there are two possible stereoisomeric products, the head-to-head dimer and the head-to-tail dimer. All known... 

The stereochemistry of the dimers produced from the dimaso-anthracenes is assumed to be trans, analogous to the head-to-tail dimers observed for the meso derivatives ... 

A number of other aromatic compounds have been observed to undergo photoaddition to anthracene to yield products similar in structure to the anthracene dimer ... 

In the case of the photoaddition of anthracene derivatives and tetracene, the cross-dimer is the major product isolated in all cases even when anthracene itself is used. 

Table 10.2. Photosensitization of Butadiene Dimerization Using Anthracene Derivatives as Sensitizersar>...

The oxygen molecule, a paramagnetic species with an unpaired electron on each atom, has already been referred to as biradical, albeit an unreactive one. The photochemical excitation of an anthracene to a biradical, or to something rather like one, has also been mentioned (p. 331) if this excitation is carried out in the absence of air or oxygen, instead of the trans-annular peroxide—(104)—a photo-dimer (130) is obtained ... 

In a similar vein, the cleavage of the photodimer of anthracene occurs in the dark in the presence of nitrosonium cation to afford the anthracene cation radical as its 7t-dimer via a thermal electron transfer196 (equation 66). 

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