Elucidation of the key role of isomerization in the self-assembly and luminescence properties of AIEgens

Due to the hierarchical nature of the self-assembly process, it is effective to control assembled nanostructures by tuning the spatial configurations of the building blocks through Z-/E-isomerization. A pair of AIE stereoisomers termed (Z)-/(E)-TPE-UPy was reported with different self-assembly mechanisms, morphologies and luminescence properties. In this study, we present a multiscale modeling combining MD simulations, hybrid QM/MM calculations and the PCM model, to systematically clarify the molecular configuration-molecular assembly-photophysical property relationship of (Z)-/(E)-TPE-UPy. Our study shows that (Z)-TPE-UPy follows a concentration-dependent ring-chain polymerization mechanism. At low concentration, (Z)-TPE-UPy tends to form ring-like (Z)-close-dimers with all H-bond sites occupied, while at high concentration, the H-bond backbone in the chain-like structures is more planar and stronger, making the zig-zag chain-like conformations more favorable. For the (E)-isomer, the H-bond backbone is quite planar and rigid, which makes it linearly elongate one-by-one at the whole range of concentrations via the isodesmic polymerization mechanism. (Z)-TPE-UPy oligomers exhibit large flexibility and diverse conformations, leading to sharply enhanced viscosity at high concentration in experiments. Moreover, the fluorescence spectrum of (Z)-/(E)-TPE-UPy aggregate is conformation-dependent and the enhanced emission in the aggregated state is attributed to the restriction of the low-frequency intramolecular rotations of the phenyl rings and the distortion of the C = C plane, as well as the reduction of electron-vibration couplings. Our work not only offers valuable insights into the key role of stereoisomerism in assembled morphologies and luminescence properties, but also provides a theoretical basis for the rational design of new building blocks based on stereoisomers.