ESIPT mechanism of the HPQRB fluorescent probe and its ratiometric detection of SO2

A large amount of SO₂ produced by large ships during fuel burning is a serious threat to people's health. It is urgently required to develop a method of detection with rapid response time and low detection limits. HPQRB has great fluorescent properties; it has a low detection limit and rapid response time. In this article, the detection mechanism of fluorescent probe HPQRB for HSO₃⁻ and the excited-state intramolecular proton transfer (ESIPT) process have been unveiled by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). HPQRB and HPQRB-HSO₃ both have planar structures in the ground state (S₀) and the first excited (S₁) state. Combining structural parameters and infrared vibrations, the hydrogen bond has been strengthened upon photoexcitation, providing the driving force for the ESIPT process. Orbital-weighted Fukui function and dual descriptor confirm that C₉ (shown in Figure 1) of HPQRB is the reaction site of HSO₃⁻ attacking. The calculated absorption and emission are consistent with the experiment, indicating that our calculations are reliable. By building potential energy curves (PECs), we find that the high reaction barrier from keto form to enol form in the S₁ state is the reason why HPQRB-HSO₃ only has one emission peak. Natural transition orbitals (NTOs) and hole–electron show that both HPQRB and HPQRB-HSO₃ are local excitation (LE) and exhibit ππ* properties. Compared with HPQRB, the conjugated structure of HPQRB-HSO₃ after Michael addition is disrupted, causing a weaker electron transfer after photoexcitation, which leads to the blue shift of the emission peaks.