Bis-Butoxyphenyl Dipyridine Cation With High Affinity for PF6−: Enhancement of solid fluorescence and inhibits Escherichia coli and Staphylococcus aureus activity through anion-π+ interactions

The contamination of anions and the overuse of antibiotics pose significant hazards to human health. Excess anions can cause serious damage to organisms and the natural environment, while the misuse of antibiotics has led to the emergence of multi-drug resistant bacteria. Therefore, it is important to develop multifunctional fluorescent sensing devices that integrate the rapid and sensitive detection of specific anions and that exhibit excellent antimicrobial activity against multi-resistant bacteria. This study describes the synthesis of three water-soluble cationic fluorescent molecules, DPPT-R (R = 5C, 6C, 7C). Their structures were characterized using 1H NMR spectroscopy, HRMS, and single crystal X-ray diffraction. Spectroscopic properties of the probes were recorded using UV-visible absorption and fluorescence spectroscopy. The recognition mode and mechanism of the fluorescent probes towards PF6− were investigated. Under aqueous solution conditions involves the PF6− induced aggregation of the probe via strong electrostatic interactions, leading to fluorescence quenching. Adjusting the alkyl chain length of the probe can enhance the sensitivity for the detection of the target anions. The detection limits are 0.892 μM, 0.786 μM and 0.444 μM for R = 5C, 6C, 7C, respectively. In addition, DPPT-R and DPPT-R@PF6 exhibit good inhibitory activity and fluorescence imaging capability against methicillin-resistant Staphylococcus aureus and multidrug-resistant Escherichia coli. Among them, the side chain of DPPT-R@PF6 with an almost 90° bend may be more advantageous for membrane insertion, thus enhancing its antimicrobial activity. In particular, after DPPT-R and PF6− form the DPPT-R@PF6 complex, the complex enhances its solid-state fluorescence, intensity of bacterial fluorescence imaging, and antibacterial activity through anion-π+ interactions. Therefore, this study can provide a new strategy and theoretical basis for the design and application of multifunctional fluorescent sensing materials and antibacterial molecules based on electrostatic interactions and anion-π+ interactions.