Anthracene, dimerization

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,... 

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. 

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

Similar formations of bisanthracenes have been studied with a variety of substituents. An interesting example of a cation-assisted intramolecular anthracene dimerization where the bisanthracene formation includes cycli-zation of a polyether chain to a crown ether is illustrated in (4.27). In the absence of Li+ the dimer reverts back to the open chein compound, but in the presence of Li+ the crown ether is stabilized so that the product itself also becomes more stable 430). 

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. 

Anthracene undergoes a photochemical 9,10,9, 10 -cycloaddition which goes through the excimer as intermediate. Many aromatic molecules follow similar cycloaddition paths. The close approach of the molecules in the excimer is essential for bond formation, and steric hindrance can prevent the reaction unsubstituted anthracene dimerizes so fast that no excimer fluorescence can be detected, 9,10-dimethylanthracene shows both excimer fluorescence and photodimerization, but 9,10-diphenylanthracene shows neither excimer emission nor photodimerization (Figure 4.52). 

Although la undergoes the usual anthracene dimerization, lb does not. Explain. 

The fluorescence of anthracene in benzene is efficiently quenched by N,N-dimethylaniline and a strong exciplex emission appears in a longer wavelength than the emission of anthracene [382-384], However, the addition product was not obtained at all, except the (4 + 4) anthracene dimer (Scheme 114). In contrast, the addition product and reductive dimerization product of dimethylaniline to the anthracene ring are produced via photoinduced electron transfer, which was first reported by Pac and Davidson [385-387], In the case of V-mcthylaniline, some addition products are obtained both in nonpolar and polar solvents [386-389],... 

Figure 14 Schematic representation of anthracene dimers with no parallel molecular planes (a) 55° dimer (see Ref. 88) (b) zero overlap twisted intramolecular charge-transfer type pair and (c) large overlap excimer-type conformation of anthracene units in l,l-di(9-anthryl)alkane (see Ref. 89).

Figure 12. Atomic models and structural diagrams of a sandwich pair of anthracene molecules (left) and an anthracene dimer (right) (28).

Anthracene dimerization is the photochemical reaction having the longest published history, and this process continues to draw attention. Saltiel and... 

However, if 9,10-anthracene dimer (dianthracene) or bicumene is used as the aromatic donor, the simple reaction scheme in Eq. 15 does not apply any longer, but a follow-up reaction is observed that efficiently competes with back electron transfer. Various examples for this case are described in the following section. 

A well-studied example of a photoreaction involving excimers is anthracene dimerization (Charlton et at., 1983). Figure 6.7 shows part of the potential energy surfaces of the supermolecule consisting of two anthracene molecules. Singlet excited anthracene A -l- A) can either fluoresce (monomer fluores-... 

From Example 6.4 it can be seen that the molecule may also end up in a minimum or funnel in S, or T, that is further away from the geometry of the starting species. This then corresponds to a nonspectroscopic minimum or funnel (Figure 6.3, minimum 0 such as the pericyclic funnel of the anthracene dimerization in Figure 6.7, or even to a spectroscopic minimum of another molecule or another conformer of the same molecule (Figure 6.3, minimum i). Reactions of the latter kind can sometimes be detected by product emission (Figure 6.3, path j). (Cf. Example 6.5.)... 

Much information about the detailed mechanism of anthracene dimerization was gained in the study of intramolecular photoreactions of linked anthracenes such as a,(d-bis(9-anthryl)aikanes (66). It was shown that luminescence and cycloaddition are competing pathways for the deactivation of excimers. In compounds with sterically demanding substituents R and R that impair the cycloaddition reaction, the radiative deactivation is enhanced (H.-D. Becker, 1982). 

The photoreactions of anthracene in the presence of trans trans hexa-2,4-diene have been re-examined. The products from the reaction are the anthracene dimer, the two isomers (88) and (89) of [4+4]cycloaddition to the 9,10-positions of the arene, and a [2+4] adduct (90) as well as several unidentified minor adducts. Dimerization of the arene is reduced by the diene rather than enhanced, and a Diels-Alder reaction of the strained trans... 

Hammond and collaborators as part of their extensive studies on photosensitized reactions. Examples include (a) cis-trans isomerization of olefins sensitized by biacetyl 42,64,65,176 2,3-pentanedione 64>65>, benzil 64,65,89)> a. and /3-naphil 64>, and phenanthrenequinone 30-65> (b) diene and anthracene dimerizations sensitized by biacetyl 9>45>94>, 2,3-pentanedione 4,162)j camphorquinone 04>, benzil 84,162) and / -naphthil 94>, and (c) the quadricyclene-norbomadiene interconversion sensitized 66> by benzil. 

Bohning and Weiss 3°) have suggested that cis-trans isomerization of stilbenes may proceed via an intermediate addition product (vide infra) and not by energy transfer. The results obtained in the biacetyl-anthracene system are not completely clear Backstrom and Sandros reported isolation of anthracene dimer while Dubois and Behrens 45> suggested that a 1 1 adduct of unspecified structure is formed. 

The studies reported last year on the stereoisomeric control of the photo-induced Diels Alder reaction of maleic anhydride with homo-chiral anthracene derivatives such as (46) have been extended using 320-400 nm radiation, and this gives the head-to-tail anthracene dimer as well as the previously reported adduct (47) with excellent diastereoselectivity. The thermal and photochemical retroaddition process has been examined and the results suggest that this facile process may promote the anthracene as a new chiral auxiliary. 

---