Remarkable acceleration of template-directed photodimerisation of 9-phenylethynylanthracene derivatives assisted by complementary salt bridge formation
The photoirradiation of 9-phenylethynylanthracene in degassed chloroform and benzene afforded not only a [4 + 2]-anti Diels–Alder addition dimer, but also a [4 + 4]-anti-dimer as a minor product for the first time as revealed by single-crystal X-ray analysis, while the anthracene residue was quantitatively oxidised in undegassed dilute chloroform, giving the corresponding endoperoxides. The photochemical reactions of carboxylic acid monomers bearing a 9-phenylethynylanthracene unit at one and both ends were further investigated in the presence and absence of the complementary amidine dimer as the template. It was found that a similar photooxidation reaction of the monomers was significantly suppressed in the presence of the template even in undegassed chloroform. In addition, the template-directed photodimerisation of the mono- and di-9-phenylethynylanthracene-bound monomers was remarkably accelerated 30- or 61-fold in the degassed chloroform, giving the [4 + 2]-anti- and [4 + 4]-anti-dimers as major and minor products, respectively, whereas the di-9-phenylethynylanthracene-bound monomer was preferentially photo-polymerised in the absence of the template.