“Seeing” and Controlling Photoisomerization by (<i>Z</i>)-/(<i>E</i>)-Isomers with Aggregation-Induced Emission Characteristics

Efficient photoisomerization of chromophores is important in living systems, and structural constraints of protein pocket on chromophores are the probable reason for moving their dynamic reaction equilibrium forward. On the other hand, photochemical reaction to switch a molecule from one isomer to the other with different geometry and property in a high yield will continue to play a vital role in the synthetic chemistry and material science. Because of the important role of efficient photoisomerization, a biomimetic approach for "seeing" and controlling the photoisomerization is developed by using the technology of aggregation-induced emission (AIE) with supramolecular chemistry. It is revealed that a (<i>Z</i>)-isomer of a 2-ureido-4[1<i>H</i>]-pyrimidinone-containing tetraphenylethene (TPE-UPy) can be photoisomerized into supramolecular polymer form of its (<i>E</i>)-counterpart in chloroform in a high reaction yield of 68.1%. The yield is further enhanced to 100% in THF as aggregates of supramolecular polymers of (<i>E</i>)-TPE-UPy are formed, which completely inhibits the reverse photoreaction to form (<i>Z</i>)-TPE-UPy. In chloroform with organic acid, a mixture of equal amounts of (<i>E</i>)- and (<i>Z</i>)-isomers was obtained due to the disruption of the formation of intermolecular hydrogen bonds. The AIE characteristics of the isomers allow us to directly "see" the "turn-on" photoisomerization process by distinct fluorescence color changes, and the photoisomerization observed here may enable the development of a promising generation of optical power limiting materials.