Vapor-induced miscibility switching and optical response in a functional molecular liquid–pillar[5]arene system

Stimulus-responsive control of intermolecular interactions in multicomponent systems is a grand challenge in supramolecular chemistry and materials science. Herein, we demonstrate a stimulus-responsive miscibility switching in a system composed of a functional molecular liquid (FML) and a pillar[5]arene (P5A). Our strategy exploits alkyl chains, which are ubiquitously incorporated in FMLs as a liquifying group, to control the miscibility. P5A should be an ideal counterpart because it is known to encapsulate a linear alkane even in the solid state. An FML bearing two alkyl chains spontaneously forms a complex with P5A, resulting in solidification, accompanied by drastic chromism from yellow to red due to charge-transfer interactions. Moreover, while the FML exhibits room-temperature phosphorescence (RTP) in its liquid state, the emission is quenched upon complexation. Remarkably, exposing the solid to linear alkane vapor triggers the reversal: the red color fades, and RTP is turned on. The real-time microscopic observation reveals the abrupt movement and reshaping of crystals, accompanied by liquid seepage, indicating a vapor-induced crystal transition and concomitant solid-liquid phase separation. This reversible phase mixing/separation process, controlled by competitive host-guest chemistry of cyclic host and FMLs, represents a promising approach for designing stimulus-responsive multicomponent systems with switchable optical and physical properties.