Dynamic Modulation of the Diradical Nature of an Overcrowded Phosphaquinoid

Molecules capable of reversibly transitioning between istinct states in response to multiple stimuli are highly sought after for responsive materials, enabling precise control over electronic structure at the molecular level. While overcrowded alkene (OCA) diradicaloids have shown promise in this context, research has largely centered on carbon-based systems, leaving main-group alternatives underexplored. Herein, we present the synthesis of an air stable, phosphaalkene substituted hybrid OCA diradicaloid, obtained by replacing one exocyclic C=C bond of an anthraquinodimethane core with a low-valent phosphaalkene unit (–C=P–Mes*, Mes*= 2,4,6-tri-tert butylphenyl) and appending a fluorenyl group at the opposite terminus. This unique molecular framework enables distinct responses to external stimuli, as demonstrated by comprehensive spectroscopic analysis and supported by theoretical studies. The molecule undergoes a temperature-induced, reversible conformational change from a folded quinoidal to a twisted diradicaloid state via rotation of the fluorenyl unit about the central overcrowded alkene bond. Furthermore, reversible Lewis acid–base interactions at the phosphorus center dynamically modulate the diradical character, illustrating a rare example of chemical control in a main-group-based OCA diradicaloid. These findings open new avenues for the design of main-group diradicaloids with multi-stimulus responsiveness.