π-Extended Pentalenes: The Revival of the Old Compound from New Standpoints

Polycyclic conjugated systems bearing carbocyclic five-membered rings have attracted much attention due to their novel electronic properties and utility in organic electronic devices. Despite possessing a 4nπ electron periphery, dibenzopentalenes have a planar structure and are fairly stable. Thus, appropriate modifications should provide desirable electronic properties. Recently, we have discovered novel Ni-catalyzed coupling reactions that constitute a versatile new route to dibenzopentalene derivatives from readily available o-bromoethynylbenzenes. Moreover, we developed another dibenzopentalene synthesis using indenone derivatives as important intermediates. These methods are applicable to π-extended pentalene derivatives possessing various functional groups and aromatic ring systems. Among them, a dinaphthylpentalene derivative shows a high hole mobility (1.8 × 10(-3) cm(2) V(-1) s(-1) ) for an amorphous material. Furthermore, this is the first pentalene derivative that can be employed as a p-type material for organic heterojunction photovoltaic cells (PCE = 0.94%, V(OC)  = 0.96 V) and as organic thin-film semiconductors.