Tetrasubstituted imidazole derivatives and their imidazolium: photophysical properties in the aggregate state and application for adenosine detection

Anion-π⁺ interaction as an important type of non-covalent interaction has great potential in the construction of aggregation-induced emission (AIE) molecules. In this work, tetraphenylimidazole (PI) and diphenylphenanthroimidazole (PPI) were used to reveal the synergistic effect of anion-π⁺ interaction and conjugation degree on the aggregate structure and photophysical properties. (Phenanthro)imidazole salts containing different counter ions such as iodine ion (I), fluoride ion (F), tetrafluoroboron (B), perchlorate (C) and hexafluorophosphate (P) were designed and synthesized through ionization reaction and anion displacement reactions. The experiments indicate that the anion-π⁺ interaction plays an important role in inducing tetraphenylimidazole salts (except for MPI-I) to be AIE active by enhancing the spatial configuration. However, the anion-π⁺ interaction for diphenylphenanthroimidazole salts induced the head-head arrangement between adjacent molecules in the lattice, resulting in enhancing the π-π stacking between the phenanthrene rings and reducing their solid-state fluorescence quantum yields. The photophysical properties can be effectively regulated by changing the counterions and the conjugation degree of the aromatic ring skeleton. In addition, MPPI-B has a good quenching fluorescence response to adenosine-containing biocompounds such as adenosine, AMP, ADP, ATP, and NAD⁺. The limit of detection to ADPNa₂ is 0.62 μg mL⁻¹ (1.22 × 10⁻⁶ mol L⁻¹). The responsive mechanism is attributed to the interaction between the imidazole cation and adenine. It also provided a new approach for future biological applications.