B2N2-Embedded Polycyclic Aromatic Hydrocarbons with Furan and Thiophene Derivatives Functionalized in Crossed Directions

A series of polycyclic aromatic hydrocarbons (PAHs), consisting of two pairs of BN units, have been designed and their synthesis has been achieved by electrophilic C?H borylation. Two conjugation extension directions can be found in these B₂N₂-embedded PAHs. The B₂N₂-containing backbone with shorter effective conjugation length is isoelectronic with diaryl-fused anthracene, whereas the second derivative, with longer effective conjugation length, is isoelectronic with bis(trans-arylvinyl)benzene. By incorporating different aryl groups, i.e., furyl, thienyl, benzob]furyl, and benzob]thienyl groups, into the two crossed directions of the B₂N₂-embedded PAHs, their electronic and optical properties have been comparatively investigated by photophysical, electrochemical, and theoretical approaches. It is found that both the substituents and their conjugation extension directions have significant effects on the aromatic and photophysical properties of the B₂N₂-embedded PAHs. The conjugation extension in the shorter backbone is more pronounced on the effective conjugation length than the longer backbone. Moreover, all the B₂N₂-embedded PAHs behave as both Lewis acids and Lewis bases, and reversible photoluminescence switching can be observed by simply neutralizing the added Lewis acid or Lewis base.