Unprecedentedly Ultrafast Dynamics of Excited States of C═C Photoswitching Molecules in Nanocrystals and Microcrystals.

The C═C photoswitching molecules [1,2-di(4-pyridyl)ethylene (DPE), 4-styrylpyridine (SP), and trans-1,2-stilbene (TS)] show favorable photoisomerization characteristics. Although the solid states of photoswitching molecules are usually used in optical devices, their excited state's evolution has been little explored. Here, the excited state's relaxation of DPE, SP, and TS in nanocrystal/microcrystal suspensions as well as in solution phase was studied to uncover the early events of their excited states. The dynamics of nanocrystal/microcrystal suspensions was tremendously accelerated in comparison to the kinetics obtained in the solution for these molecules under excitation. DPE exhibits the slowest decay rate, while SP shows the fastest decay rate in nanocrystal suspensions or solution, suggesting SP may be the best candidate for the photoswitching device. The intermolecular interactions and space restriction of the crystal lead to the acceleration of the excited state's evolution for DPE, SP, and TS. This provides new insight into the design of optical materials.