Beyond [N]Phenylenes: Synthesis and Investigation of Linear [4] and [5]Naphthylenes Incorporating Multiple Four-Membered Rings.

Ladder-type polycyclic conjugated hydrocarbons (PCHs) incorporating four-membered rings represent a unique class of organic materials with local antiaromaticity and dearomatization induced by the four-membered rings. [N]Phenylenes represent the only systematically synthesized series of PCHs containing multiple four-membered rings or biphenylene units. Linear [N]phenylenes become increasingly unstable as the conjugation length extends. Here, we report the regioselective synthesis of linear [4] and [5]naphthylenes, representing the longest PCHs containing multiple four-membered rings synthesized in solution to date. These structures were accessed through sequential catalytic arene oxanorbornene annulation reactions followed by aromatization. Raman spectroscopy and computation revealed significant dearomatization of naphthalenoid units and enhanced bond localization with increasing the conjugation length, contrasting with the typical bond delocalization trend in conventional conjugated molecules. These [N]naphthylenes exhibited high stability, markedly narrow blue emission profiles with very small Stokes shifts, high fluorescence quantum yields, as well as small shifts in the energy levels of frontier molecular orbitals upon increasing conjugation length. Skeletal flexibility was also observed in [4] and [5]naphthylene, but absent in [3]naphthylene, suggesting dynamic structural behaviors of longer [N]naphthylenes.