Effect of benzene ring on the excited-state intramolecular proton transfer mechanisms of hydroxyquinoline derivatives

The proton transfer process of 8-hydroxyquinoline (HQ) can occur from the hydroxy oxygen to the pyridine nitrogen through a five-membered ring. For 10-hydroxybenzo[h]quinolone (HBQ), the proton transfer process occurs through a six-membered ring due to the insertion of a benzene ring between phenol and pyridine. In this work, the proton transfer processes of the five-membered (HQ) and six-membered (HBQ) ring hydroxyquinoline hydrogen bond systems were studied through theoretical methods. The geometric parameters and infrared (IR) vibrational spectra analysis show that the insertion of a benzene ring between phenol and pyridine affect the strength of the intramolecular hydrogen bond (OH center dot center dot center dot N). Upon photoexcitation, the electron density is redistributed, which can provide driving force for the proton transfer processes. The potential barrier of the excited-state intramolecular proton transfer (ESIPT) process for HQ is 3.05 kcal/mol, whereas the ESIPT process of HBQ is barrierless. It is demonstrated that the proton transfer process of HBQ is easier than that of HQ in S-1 state. The proton transfer process of HQ can only occur in S-1 state, whereas that of HBQ can occur in the ground state (S-0) and S-1 state. In addition, the analysis of the nonadiabatic dynamics simulations reveals that the ESIPT process of HBQ (26 fs) is faster than that of HQ (53 fs). To sum up, the insertion of a benzene ring between phenol and pyridine affect the strength of the intramolecular hydrogen bond (OH center dot center dot center dot N) and then affect the proton transfer processes to some extent.