<scp>ESIPT</scp> mechanism of the <scp>HPQRB</scp> fluorescent probe and its ratiometric detection of <scp>SO₂</scp>

Abstract A large amount of SO 2 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 HSO 3 − 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‐HSO 3 both have planar structures in the ground state (S 0 ) and the first excited (S 1 ) 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 C 9 (shown in Figure 1) of HPQRB is the reaction site of HSO 3 − 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 S 1 state is the reason why HPQRB‐HSO 3 only has one emission peak. Natural transition orbitals (NTOs) and hole–electron show that both HPQRB and HPQRB‐HSO 3 are local excitation (LE) and exhibit ππ* properties. Compared with HPQRB, the conjugated structure of HPQRB‐HSO 3 after Michael addition is disrupted, causing a weaker electron transfer after photoexcitation, which leads to the blue shift of the emission peaks.