A new insight into the luminescence mechanism of a FRET-ICT-based ratiometric and colorimetric pH probe

Abstract

Recently, a ratio fluorescent probe molecule BP was designed and synthesized to detect pH in living cells. However, few have conducted in-depth theoretical exploration of the detailed transfer process. In this paper, the luminescence mechanisms of BP and BPH were studied with the results of geometric optimization structures and molecular orbitals at the CAM-B3LYP/TZVP computational level. It was found that a fluorescence resonance energy transfer (FRET) process occurs in the excited state of BP molecules, and the final fluorescence emission is attributed to the intramolecular charge transfer (ICT) of the acceptor styryl pyridine ring. Meanwhile, the fluorescence emission of BPH molecules is due to the ICT process caused by the local excitation process of the acceptor group protonated styryl pyridine ring itself. Therefore, this luminescence is a FRET “ON-OFF” combined ICT process, not like the FRET “OFF-ON” mechanism which the experiment suspected. Our work illustrates a specific FRET combined ICT process of probes BP and BPH from a theoretical perspective which could provide theoretical guidance for the future design of similar fluorescent probes.