Photoinduced electron transfer reactions across rigid linear spacer groups of high symmetry

A series of highly symmetrical dienes 1–3 were prepared through [2+2] cyclo-addition of norbornadiene (NBD) in an exo–trans–exo geometry. These molecules can be utilized as spacer groups in donor(D)–spacer(S)–acceptor(A) types of electronic dyads possessing strictly linear rigidity. The electronic coupling between D and A was promoted effectively by the σ-bonds through the all-trans spacer groups so that a highly efficient photoinduced electron transfer process takes place. The even–odd NBD dependent alternation of the D/A orientation is intriguing, which leads to an unusual correlation in the spacer tuning electron transfer process.