Mechanical flexibility and pressure-activated emission in room-temperature phosphorescent organic crystals

The development of organic flexible materials with advanced functionalities, particularly those exhibiting stimuli-responsive photophysical behaviour, is crucial for next-generation intelligent photonic technologies. However, achieving mechanical flexibility and pressure-activated emission in room-temperature phosphorescent organic crystals remains a significant challenge. Here we show a crystal of 4,4’-sulfonylbis(bromobenzene) that exhibits both mechanical flexibility and a 4.8-fold enhancement of room-temperature phosphorescence under high pressure. Its elastic deformability originates from a folded molecular conformation stabilized by synergistic dipole–dipole and Br···Br interactions. The pressure-activated emission arises from the synergistic interplay of strengthened spin–orbit coupling and a reduced singlet–triplet energy gap, together with suppressed exciton–vibrational coupling. This work not only establishes a strategy of folded molecular geometry for designing organic crystals with mechanical flexibility and pressure-activated emission but also provides in-depth insight into how the proportion of 1(n, π*) character influences spin–orbit coupling coefficients. Folded molecular geometry enables mechanically flexible organic crystals with pressure-activated phosphorescence and reveals how the proportion of ¹(n, π*) character affects spin–orbit coupling.