Selective Host–Guest Co‐crystallization of Pyridine‐Functionalized Tetraphenylethylenes with Phthalic Acids and Multicolor Emission of the Co‐crystals

Tetraphenylethylene (TPE) and its derivatives are the most typical and most widely studied organic compounds showing aggregation‐induced emission (AIE). Due to their propeller‐like structures, V‐like clefts exist between the aryl rings, which make them promising host compounds. However, such a possibility is seldom explored. Herein, it is reported that TPE derivatives bearing two or four pyridine rings at the para positions of the phenyl rings (TPE‐Pys) can selectively include triangular (Δ‐like) m‐phthalic acid from a mixture of o‐, m‐, and p‐phthalic acids due to their shape complementary to form host–guest co‐crystals, which showed redder emission than the TPE‐Pys themselves. The emission of co‐crystals 1–5 could be reversibly switched between yellow and red by alternating exposure to HCl and ammonia vapor. The host–guest co‐crystals not only exhibited great potential for selectively recognizing and separating m‐phthalic acid and as multicolor emission materials, but are also suitable for use as secret ink due to their reversible color change on varying the host–guest interactions. Flying colors: Pyridyl‐functionalized tetraphenylethylene derivatives with aggregation‐induced emission characteristics selectively encapsulate triangular m‐phthalic acid from a mixture of o‐, m‐, and p‐isomers by host–guest interaction. The emission of the host–guest co‐crystals 1–5 could be reversibly changed between yellow and red by alternating fuming with HCl and ammonia vapor (see figure).