Lighting Up the Invisible Twisted Intramolecular Charge Transfer State by High Pressure.

The twisted intramolecular charge transfer (TICT) state plays an important role in determining the performance of optoelectronic devices. However, for some nonfluorescent TICT molecules, the "invisible" TICT state could only be visualized by modifying the molecular structure. Here, we introduce a new facile pressure-induced approach to light up the TICT state through the use of a pressure-related liquid-solid phase transition of the surrounding solvent. Combining ultrafast spectroscopy and quantum chemical calculations, it reveals that the "invisible" TICT state can emit fluorescence when the rotation of a donor group is restricted by the frozen acetonitrile solution. Furthermore, the TICT process can even be effectively regulated by the external pressure. Our study offers a unique strategy to achieve dual fluorescence behavior in charge transfer molecules and is of significance for optoelectronic and biomedical applications.