Incorporation of ESIPT into Pillar[5]arene for Solid-State Dual Emission Responsive to n-Hexane Vapor

Abstract Pillar[5]arene is a promising macrocyclic receptor of chemical sensor showing shape-selective encapsulation of neutral molecules into the cavity, but the poor fluorescence properties remain as a challenge. Herein, we report a pillar[5]arene coupled with 2-(2-hydroxyphenyl)benzoxazole (HBO), which displays bright fluorescence in both solution and the solid state. Owing to the excited-state intramolecular proton transfer (ESIPT) in the HBO moiety, greatly improved fluorescence is observed for the pillar[5]arene derivative in CHCl3 (Φlum = 11%) and powder form (Φlum = 25%). Moreover, the emission color changes from light green to blue by exposing the powder sample to n-hexane vapor. The color change derives from variable dual emission via ESIPT and excimer-forming pathways, as suggested by fluorescence lifetime measurements at different wavelengths. Powder X-ray diffraction indicates that increased crystallinity and small alteration of the solid-state structure leads to the visible fluorescent chromism upon vapor encapsulation.