Photodimerization of Arenes

The [4+4]photodimerization of anthracene has already been reported more than 115 years ago (4.26)429). 

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). 

Analogous [4+4]photodimers are obtained in irradiating naphthalene derivatives (4.28)431). 

While benzene and substituted benzenes usually do not dimerize, pyridine derivatives undergo [4+4]photodimerizations432) (4.29). 

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