Synthesis and Properties of λ⁵-Phosphadibenzo[<i>a</i>,<i>e</i>]pentalenes: Zwitterionic Phosphonium- and Carbanion-Bridged Ladder Stilbene Analogues

We designed a Pd(0)-catalyzed one-pot tandem reaction to synthesize unprecedented zwitterionic λ⁵-phosphadibenzo[<i>a</i>,<i>e</i>]pentalenes <b>3a</b>-<b>3g</b> with 16 π electrons, of which antiaromaticity was efficiently modulated by the embedded π-polarized phosphorus ylide unit. Due to the dipole structure, these novel skeleton molecules exhibit versatile photophysical properties, including negative solvatochromism, cation-induced chromism, and broad emission range. Results from experimental and computational analyses indicated that incorporating polarized π-electron subunits could be an effective strategy for modifying antiaromatic distributions and, thus, triggering some interesting photophysical properties. This study provides a novel method for rationally designing new (anti)aromatic molecules.