Explanation of pH probe mechanism in borondipyrromethene-benzimidazole compound using ultrafast spectroscopy technique

New borondipyrromethene (BODIPY) compounds containing protective, formyl and benzimidazole groups at meso position were designed and synthesized targeting pH probe application. Absorption and emission properties of these three compounds were investigated as a function of solvent and H+ concentration. The novel BODIPY compound with a benzimidazole moiety showed drastic fluorescence quenching upon increasing protonation. The fluorescence quenching of this compound under acidic conditions associated with photoinduced electron transfer (PET) mechanism was proven with ultrafast pump probe spectroscopy experiments, as well as density functional theory (DFT) calculations. These results revealed the benzimidazolium cation formed as a result of protonation of the pyridinic nitrogen of the benzimidazole moiety to act as an electron accepting group, whereas BODIPY tended to act as the electron donating group. Experimental results proved this novel BODIPY compound with the benzimidazole moiety to be a good candidate for pH probe applications. Revealing the underlying mechanism of fluorescence quenching upon protonation using ultrafast spectroscopy techniques may open a new path to the design and preparation of novel fluorescence pH sensors.