Water is one of the most important substances on our planet¹. It is ubiquitous in its solid, liquid and vaporous states and all known biological systems depend on its unique chemical and physical properties. Moreover, many materials exist as water adducts, chief among which are crystal hydrates (a specific class of inclusion compound), which usually retain water indefinitely at subambient temperatures². We describe a porous organic crystal that readily and reversibly adsorbs water into 1-nm-wide channels at more than 55% relative humidity. The water uptake/release is chromogenic, thus providing a convenient visual indication of the hydration state of the crystal over a wide temperature range. The complementary techniques of X-ray diffraction, optical microscopy, differential scanning calorimetry and molecular simulations were used to establish that the nanoconfined water is in a state of flux above −70 °C, thus allowing low-temperature dehydration to occur. We were able to determine the kinetics of dehydration over a wide temperature range, including well below 0 °C which, owing to the presence of atmospheric moisture, is usually challenging to accomplish. This discovery unlocks opportunities for designing materials that capture/release water over a range of temperatures that extend well below the freezing point of bulk water.

水是地球上最重要的物质之一¹。它以固态、液态和气态的形式无处不在，所有已知的生物系统都依赖于其独特的化学和物理特性。此外，许多材料以水加合物的形式存在，其中主要是晶体水合物（一类特定的包合物），它通常在低于环境温度下无限期地保留水²。我们描述了一种多孔有机晶体，在相对湿度超过 55% 的情况下，它可以轻松且可逆地将水吸附到 1 纳米宽的通道中。水的吸收/释放是显色的，因此可以在很宽的温度范围内方便地直观地指示晶体的水合状态。X射线衍射、光学显微镜、差示扫描量热法和分子模拟等互补技术被用来确定纳米约束水在-70°C以上处于流动状态，从而允许低温脱水发生。我们能够在很宽的温度范围内确定脱水动力学，包括远低于 0 °C 的温度，由于大气中存在水分，这通常很难实现。这一发现为设计在远低于散装水冰点的温度范围内捕获/释放水的材料提供了机会。