A solvatochromic Pd(II) complex with AIEE property: Structural characteristics, chromotropic mechanism and its application in solvent detection

In this work, we investigate a palladium complex exhibiting aggregation-induced emission enhancement (AIEE) properties and remarkable solvatochromism behavior driven by structural changes arising from solvent-solute interactions. Specifically, in polar aprotic solvents, the color of the Pd complex changes from blue to purple as solvent polarity increases, accompanied by a blue shift in the UV–Vis spectrum. Notably, this blue shift is even more pronounced in polar protic solvents, which also show a significant increase in fluorescence emission. These effects are likely driven by hydrogen-bond interaction between the protic solvents and the Pd complex. Density Functional Theory (DFT) calculations show that increased solvent polarity significantly alters the electronic distribution of the Pd complex. Higher solvent polarity leads to a higher orbital transition energy, resulting in a blue shift in the UV–Vis absorption spectrum. X-ray single-crystal structure analysis further reveals that hydrogen-bonding interactions between the Pd complex and methanol stabilize its planar structure. When water is introduced, the Pd complex undergoes aggregation through non-covalent interactions, which modify the stacking arrangement. This change weakens the π-π interactions between adjacent molecules, thereby minimizing energy loss in the system. Consequently, the Pd complex effectively utilizes the AIEE mechanism to detect water content in methanol.