The development of high‐contrast stimulus‐responsive materials with excited triplet emission is of great significance for anti‐counterfeiting, sensor and memory applications, but remains a challenge. Here, we report a strategy for the rational design of stimulus‐responsive phenothiazine derivatives with triplet‐related dual emissions and high‐contrast mechanochromism guided by Polymorph Prediction. The designed phenothiazine derivatives have the characters of simple structures, a facile synthetic procedure, and a good crystalline nature. We found that the crystals of those derivatives with the potential to form both quasi‐axial (ax) and quasi‐equatorial (eq) conformations could undergo conformation transition and show significant emission difference (Δλem>100 nm) under mechanical force. Meanwhile, all these phenothiazine derivatives exhibit aggregation‐induced emission and emit room‐temperature phosphorescence or thermally activated delayed fluorescence. The significant luminescent change of these materials under different stimuli gives them promise for applications in encryption and anti‐counterfeiting.

The potential conformations of phenthiazine derivatives have been predicted by Polymorph Prediction. The crystals of those derivatives with the potential to form both quasi‐axial and quasi‐equatorial conformations can undergo conformation transition and show high‐contrast mechanochromism from local emission (LE) to intramolecular or intermolecular charge transfer (CT) emission. All the derivatives also exhibit aggregation‐induced emission and triplet‐related dual emission characters. More information can be found in the Research Article by S. Xiao, T. Yi, and co‐workers (DOI: 10.1002/chem.202200458).