A TDDFT study on the excited-state double proton transfer reaction of 8-hydroxyquinoline along a hydrogen-bonded bridge

The mechanism of the excited-state double proton transfer (ESDPT) reactions of 8-hydroxyquinoline (8HQ) along three types of hydrogen-bonded bridges-ammonia (NH3), water (H2O) and acetic acid (AcOH) has been investigated by using time-dependent density functional theory. Based on the analysis of hydrogen bond strengths and the excited-state potential energy surfaces (PES) along the proton transfer coordinates, it is concluded that the hydrogen bonds play the key role in the excited-state multiple proton transfer reaction. Moreover, three different concerted mechanisms have been found in 8HQ center dot NH3, 8HQ center dot H2O and 8HQ center dot AcOH complexes. Upon photoexcitation, the deprotonation of a hydroxyl group in 8HQ would occur first in 8HQ center dot NH3 due to the stronger hydrogen bond -OH center dot center dot center dot NH3. On the contrary, the hydrogen bond -OH center dot center dot center dot O-C-OH is much weaker in the 8HQ center dot AcOH complex, and this leads to a fast protonation of -N- in the 8HQ moiety. For the 8HQ center dot H2O complex, the hydrogen bond strengths are almost the same, so that both protons would transfer simultaneously in a symmetrical and concerted fashion.