**Toshikazu Ono \* and Yoshio Hisaeda \***

Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

**\*** Correspondence: tono@mail.cstm.kyushu-u.ac.jp (T.O.); yhisatcm@mail.cstm.kyushu-u.ac.jp (Y.H.)

Received: 28 October 2020; Accepted: 18 November 2020; Published: 19 November 2020

**Abstract:** Vapochromic materials, which change color and luminescence when exposed to specific vapors and gases, have attracted considerable attention in recent years owing to their potential applications in a wide range of fields such as chemical sensors and environmental monitors. Although the mechanism of vapochromism is still unclear, several studies have elucidated it from the viewpoint of crystal engineering. In this mini-review, we investigate recent advances in the vapochromism of organic crystals. Among them, macrocyclic molecules and inclusion complexes, which have apparent host–guest interactions with analyte molecules (specific vapors and gases), are described. When the host compound is properly designed, its cavity size and symmetry change in response to guest molecules, influencing the optical properties by changing the molecular inclusion and recognition abilities. This information highlights the importance of structure–property relationships resulting from the molecular recognition at the solid–vapor interface.

**Keywords:** vapochromism; fluorescence; macrocycles; inclusion crystals; host–guest chemistry
