Anthracene photodimer

Interest in the monomerization of anthracene photodimers continues the head-to-tail dimer (290) (see p. 361) produced by irradiation of 9-cyanoanthracene has been investigated by two groups. " From results of differential scanning calorimetry and emission spectroscopy of cleaved surfaces of partially monomerized single crystals, it is suggested that the first exothermic peak (and the subsequent ones) arises from monomer crystallization in the photodimer host matrix. " Theocharis and Jones have also examined this retroprocess and using single crystal X-ray techniques they have shown that the first step in the solid-state thermal monomerization of this dimer is a... 

Anthracene photodimers have been investigated extensively. A study of the photodissociation of the dimer of 9-methylanthracene, under both steady-state and flash conditions, suggests that the process occurs through an excited singlet... 

Applequist DE, Little RL, Friedrich EC, Wall RE (1959) Anthracene photodimers. 

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). 

Although the quinolizinium ion (1), like naphthalene, does not undergo photodimerization, its linear benzo derivative, the acridizinium ion, like anthracene, does so readily (Scheme 27) (57JOC1740). The photodimer dissociates when heated in ethanol. It has been reported that both the dimerization and dissociation in methanol are light-catalyzed and that the quantum yields for the two reactions are 0.23 and 0.49 respectively (78JPR739). 

PAHs photooxidized by the type II singlet oxygen mediated mechanism include acenaphthylene, whose oxidized products and yields are shown in Fig. 10.29B (Barbas et al., 1994), phenanthrene (Barbas et al., 1996), anthracene (Dabestini et al., 1995), and tetracene (Dabestini et al., 1996). An additional photochemical process, the formation of photodimers, is also observed for acenaphthylene, anthracene, and tetracene. 

Chandross and Ferguson64 find that the absorption spectra of dimers, produced65 by photolytic cleavage of photodimers of anthracene and mono-derivatives in a rigid methylcyclohexane glass at 77°K, are consistent with a symmetrical sandwich configuration these dimers also emit the characteristic excimer fluorescence. On the other hand, it is necessary to assume a 60° rotation of one component about the intermolecular axis of the 9,10-di-chloroanthracene dimer (as in the crystalline compound) to account for the observed resonance splittings of both absorption bands.64... 

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

Dimerization. Many polycyclic aromatic hydrocarbons form photodimers via 4 + 4 cycloadditions.58-59 Anthracene (11), for example, dimerizes with a limiting quantum yield of 0.3 when irradiated in benzene.59-60 The reaction takes place from the singlet state and competes with fluorescence, which drops to zero at high anthracene concentrations. More detailed coverage of these reactions is found in the reviews by Bowen59 and by Trecker.58... 

The structures of the photodimers which are similar products to those of anthracenes involve linkage together of two nuclei through 9,10-positions with the same sort of head-to-head structure as in the 9-sub-stituted anthracene dimerization, presumably for the same reason, i.e., maximum resonance stabilization of the intermediate dimer diradical (A).1188 Such a process is consistent with a process in which an excited azanthracene molecule, for example, exists as a diradical with the odd electrons localized at the 9,10-positions, and reacts on collision with another unexcited molecule. 

In some molecules, the interaction can develop into a stronger force and the interplanar distance further reduced to form stable photodimers through covalent bonds. For example, anthracene forms a photodimer and no excimer emission is observed, whereas some of its derivatives with bulky substituents which hinder close approach give excimer fluorescence. In 9-methylanthracene both photodimer formation and excimer emission is observed. 9, 10-diphenylanthracene neither forms a photodimer nor emits excimer fluorescence due to steric hindrance. These observations are tabulated in the Table 6.3, which shows that the nature of the excited state is also important. 

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215], Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218], Irradiation of crystals of 7-... 

Anthracene is well known to undergo photodimerization. Tamaki investigated photodimerization of anthracene-2-sulfonic acid (2-AS) in aqueous solution [77], The 2-AS forms a 2 1 complex with y-CI), but the ratio of the four photodimers was not affected by the presence of y-CD. In contrast, 2-AS was supposed to form a 2 2 complex and photodimerization gave single photodimer of anh -head-tail structure [78],... 

Photodimerization of 1-substituted anthracene usually give four photodimers. Ueno et al. prepared y-CD derivatives bearing two 1-carbonylan-... 

Fritzsche also used charge transfer complexes with picric add to characterize the photodimer of anthracene [72] and he also observed colored complexes between aromatic hydrocarbons and anthraquinones and nitroanthraquinones. He had obtained these substances, without recognizing their nature, in an attempt to nitrate anthracene. Ten years after Fritzsche s publications, Berthelot carried out detailed investigations on numerous picrates of aromatic hydrocarbons and delineated applications of these complexes for the identification and separation of aromatic hydrocarbons [73,74]. 

Stilbenes offer both advantages and disadvantages compared with photodimers. The absorption band which is changed by conversion of the more stable trans isomer to the cis form is much weaker than the strong band of the anthracene system that moves into the ultraviolet after dimerization. The stilbene band is somewhat stronger than the near-uv anthracene band. Trans-4-methoxy-4 -nitrostilbene absorbs at ca. 360 nm (e = 6 x 1(P). ... 

Ultienne116,117 reported a few photodimers of 1-aza- and 2-aza-anthracene the structure of the dimer from the former was assumed to be 95. 

Photodimers have been reported as the principal products from irradiation of numerous arene substrates. While there is little selectivity to be considered in most of these reactions, it has been reported that almost all photolyses of 9-substituted anthracenes give head-to-head dimers as depicted in U10). ° An intramolecular version of these photodimerizations has also been reported. Irradiation of di(a-2-naphthylmethyl) ether gave a mixture of the endo cyclomer (111) and the exo cyclomer (112). ° The photodimerization of 2-pyridones has been the subject of extensive investigation for a number of years." "" In most instances the head-to-tail or trans-anti isomer was the major or exclusive product in these reactions. Irradiation of A -methyl-2-pyridone for 15 h provided the trans-anti dimer (113) in 51% yield. This material was accompanied by much smaller quantities of other dimeric species. Vari-... 

---