Non-classical S-Heteroacenes with o-Quinoidal Conjugation and Open-Shell Diradical Character

A series of non-classical S-heteroacenes were synthesized and exhibited intriguing physical properties and chemical reactivities that are very different from classical acenes. X-ray crystallographic analyses revealed that all acenothiophene derivatives Ph-AT-1-Ph-AT-3 had an o-quinoidal π-conjugation with large bond-length alternation, whereas the acenodithiophene derivative Ph-ADT-3 easily dimerized or reacted with oxygen to form a peroxy-bridged dimer. The long acenothiophene Ph-AT-4 was also highly reactive. The origin of these unique properties was investigated carefully by both experiments and theoretical calculations. The high reactivity of the long non-classical S-heteroacenes can be explained by their intrinsic open-shell diradical character as well as the o-quinoidal conjugation.