Chemosensors, also known as chemical and synthetic probes, exhibit notable optical signals such as colorimetric, fluorometric, and electrochemical responses when subjected to variations in temperature, pH, pressure, and mechanochemical properties (such as tension and stress). Notably, chemosensors that demonstrate selectivity and sensitivity in their signals are more suitable for ratiometric signal-readout systems than simple on/off signaling. Ratiometric sensing/imaging approaches offer advantages in terms of reducing background noises and signal interferences, making them particularly superior in these aspects. For example, biological systems often employ the fluorescence resonance energy transfer (FRET) mechanism, where fluorescence responses in donor and acceptor chromophores mutually change. Therefore, developing functional ratiometric signaling systems poses a significant challenge in multidisciplinary chemistry. In recent years, there has been significant progress in the synthesis of dynamically controllable chemosensors and smart materials that respond to external stimuli such as temperature, pressure, chemical additives, and photoirradiation. In particular, hydrostatic pressure has gained attention in the field of "mechano" science, including mechanochemistry and mechanobiology. Although the effects of hydrostatic pressure in the solution state have been studied since the 1960s, this excludes high-pressure solid chemistry using diamond anvil cells. Indeed, we have successively developed the hydrostatic pressure-control concept by inducing pressureresponsive (chir)optical changes on the functional materials. Among these advancements, notable examples include the mechanochromophore in thermally activated delayed fluorescence (TADF) and the dynamic polymers in aggregation-induced emission (AIE), both of which exhibit ratiometric responses under the influence of hydrostatic pressure. These discoveries have motivated us to develop a practical ratiometric chemosensor that can be controlled by hydrostatic pressure.