Zwitterions of the excited 4-([2,2′-bipyridine]-4-yl) phenol photoacid molecules: Formation and fluorescence

Based on the density functional theory and time-dependent density functional theory, we have investigated the zwitterionic structure of the excited 4-([2,2′-bipyridine]-4-yl) phenol (bpy-phenol) photoacid molecules. The zwitterion can be formed stepwisely via a proton-coupled electron transfer (PCET) reaction in a H-bonded bpy-phenol…F− complex and an intermolecular excited state proton transfer (ESPT) reaction between HF molecules and excited deprotonated anions of bpy-phenol. Supported by the results of Hirshfeld population analysis and electrostatic potential, a basic site was generated in the bpy residue due to the PCET process. With a proper F− ions concentration, the ESPT reaction can be occurred with little barrier between the HF molecules and the bpy-phenol anions, leading to the generation of zwitterions. Moreover, the zwitterion fluoresces at 550 nm, which is longer than that of bpy-phenol anions at 490 nm. Our finding not only discovers the zwitterionic process of photoacid molecules, but also pioneers a frontier field of activity study after PCET reactions.