PHOTOFRAGMENTATION IN LINKED DONOR-ACCEPTOR MOLECULES - INTRAMOLECULAR SINGLE-ELECTRON TRANSFER INDUCED CLEAVAGE OF A 1,2-DIAMINE

Two intramolecular donor-acceptor molecules which fragment by a single electron transfer initiated cation radical carbon-carbon bond cleavage have been synthesized and their photoreactivity studied. The intramolecular ''diads'' consist of a 1,2- diamine linked via an ester bond to either an anthraquinone or a 9,10 dicyanoanthracene electron-acceptor chromophore. As the covalent linkage between the donor and acceptor chromophores prevents solvent separation of the photogenerated radical ion pair, these systems provide a ''clock'' to examine directly competition between fragmentation and back electron transfer. The linked anthraquinone molecule fragments efficiently, with quantum yields approaching 80%, despite the inability of the photoproduced radical ions to separate. These high yields may be attributed to a slow, spin-forbidden back electron transfer and a rapid fragmentation. In contrast, the quantum yields for the dicyanoanthracene diad (reactive singlet) are markedly lower, less than 0.001 in benzene. The reactivity of comparable intermolecular donor-acceptor combinations is also reported.