Cryoelastic and cryochromic organic crystals

Cryoelastic and cryochromic luminescent organic crystals offer a powerful platform for adaptive photonics and sensing technologies, yet the structural origins of their temperature-dependent color changes remain unclear. This study reveals that flexible naphthalene-based Schiff base single crystals maintain reversible elasticity and crystallinity across the 298-77 K temperature range while exhibiting distinct thermofluorochromic responses: some crystals undergo progressive blue shifts, while others display pronounced red shifts (up to ~92 nm) accompanied by extended excited-state lifetimes. Single-crystal X-ray diffraction reveals fully reversible lattice contraction without disorder, and identifies a correlation between emission shift direction and π⋯π contact area changes. Enhanced overlap strengthens exciton coupling and drives red shifts, while the lack of overlap maintains monomeric blue emission. This structure-property correlation provides geometric descriptors for predicting low-temperature color-changing behavior, enabling temperature-gated optical encryption and low-temperature luminescent sensing technologies based on mechanically robust organic crystals.