Crystallization-Induced Reversal from Dark to Bright Excited States for Construction of Solid-Emission-Tunable Squaraines.

Squaraines (SQs) with tunable emission in the solid state is of great importance for various demands; however a remaining challenge is emission quenching upon aggregation. Herein, a unique SQ, named as CIEE-SQ, is designed to exhibit strong emission in crystal, undergoing crystallization-induced reverse from dark 1 (n+σ,π*) to bright 1 (π,π*) excited states. Such an excited state of CIEE-SQ can be subtly tuned by molecular conformation changes during the unexpected temperature-triggered single-crystal to single-crystal (SCSC) reversible transformation. Furthermore, co-crystallization between CIEE-SQ and chloroform largely stabilize the 1 (π,π*) state, enhancing the transition dipole moment and decreasing the reorganization energy to boost the fluorescence, which is promising in data encryption and decryption.