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 two-component material 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 and material functions. As a counterpart, we select P5A, which is known to encapsulate 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. In addition, 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 of the vapor response 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.