Boosting Ultrafast Trans-Cis Photoisomerization and Intersystem Crossing in Nanocrystals of Double-Bond Photoswitching Molecules

Double-bond photoswitching molecules typically can undergo trans -> cis photoisomerization, but their photoisomerization mechanisms in the solid state have been rarely reported. Herein, the excited-state evolutions of trans-azobenzene (AB), trans-stilbene (TS), and N-benzylideneaniline (NBA) are studied to unveil the relationship between the double-bond photoswitching molecules and the photoisomerization kinetics in the solution phase and in nanocrystal suspensions (NCS). The photoisomerization rate of NBA is the fastest among these three molecules in the solution phase due to its most apparent single-bond feature of the central double bond in the S-1 state. The free space ordered crystal configuration boosts the photoisomerization rates of AB and TS in NCS to be faster than that of the solution phase because the hindrance of the wrapping solvent is eliminated in the crystals. In contrast, when NBA is prepared into NCS, it becomes distorted into a nonplanar structure because of the asymmetrical C-H center dot center dot center dot pi interactions, resulting in NBA having a large spin-orbital coupling (SOC) value which opens the intersystem crossing channel to generate the triplet state instead of undergoing photoisomerization in solution. Therefore, this work reveals that the crystal configuration and the molecular structure may critically determine the photophysical properties of photoswitching materials.