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 4 n π electron periphery, dibenzopentalenes have a planar structure and are fairly stable. Thus, appropriate modifications should provide desirable electronic properties. Recently, we have discovered novel N i‐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.