Excited state intramolecular proton transfer and substituent effect of 10-hydroxybenzo[h]quinoline: A time-dependent density functional theory study

The excited state intramolecular proton transfer (ESIPT) and the substituent effect of 10-hydroxybenzo[h]quinoline (HBQ) compounds are investigated using the time-dependent density functional theory (TDDFT) method. With the spectra and potential energy curve calculations we have demonstrated the occurrence of an ultrafast excited state intramolecular proton transfer reaction in HBQ compounds. The HBQ-a and HBQ-b in the enol conformations can convert into the keto tautomers in the excited state S1. The significant Stokes shift of HBQ-a is observed, as large as 300 nm, which is much larger than that of HBQ-b. The calculated molecular orbital gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) for HBQ-a is 3.33 eV, smaller than the 3.79 eV for HBQ-b. The calculations demonstrate that it is much easier to take place the ESIPT reaction for HBQ-a than HBQ-b. The reaction mechanism of ESIPT is analyzed with theoretical potential energy curves and the ESIPT reaction energy barriers of 3.73 kcal/mol for HBQ-a and 17.75 kcal/mol for HBQ-b are obtained. These results clearly indicate that the substituent with electron-withdrawing groups in the hydrogen-accepting moiety in HBQ-a facilitate the proton transfer in the excited state.