A new material concept of soft crystals is proposed. Soft crystals respond to gentle stimuli such as vapor exposure and rubbing but maintain their structural order and exhibit remarkable visual changes in their shape, color, and luminescence. Various interesting examples of soft crystals are introduced in the article. By exploring the interesting formation and phase‐transition phenomena of soft crystals through interdisciplinary collaboration, new materials having both the characteristics of ordered hard crystals and those of flexible soft matter are expected.

Square-planar platinum(II) complexes often exhibit intense color and luminescence when they are stacked via self-assembly or bridging by ligands. The origin of the color and luminescence are the metal–metal electronic interactions, and thus, remarkable changes in color (i.e., chromism) are expected by controlling the Pt···Pt interactions. The molecular arrangement of the assembled complex systems is sensitive to environmental factors such as temperature, pressure, solvent, and vapor. This account focuses on the sensing functionalities of luminescent platinum(II) complexes with 2,2′-bipyridine or its derivatives toward their environment. Especially, luminescence changes that are induced by the vapor molecules will be examined from the viewpoint of chemical sensing. The vapochromic behavior and structural aspects of the mononuclear and dinuclear platinum(II) complexes are discussed.