A new class of highly efficient and stable, blue-phosphorescent PtIIcomplexes based on a tetradentate chelating framework has been found to exhibit highly sensitive and reversible responses to multiple external stimuli including temperature, pressure, and UV irradiation with distinct phosphorescent color switching—from blue to red or white. Intermolecular excimer formation is the main origin of this intriguing multi-response phenomenon. Highly efficient singlet-oxygen sensitization by the PtIIcompounds yields UV-light-induced phosphorescence enhancement and color switching.