Excited-state intramolecular proton transfer in 10-hydroxybenzoquinoline, 1-hydroxyanthraquinone, methyl salicylate and 4-methyl-2,6-diformyl phenol: a QM/MM–MD study

The reaction dynamics of excited-state intramolecular proton transfer (ESIPT) tautomerism in 10-hydroxybenzoquinoline (HBQ), 1-hydroxyanthraquinone (HAQ), methyl salicylate (MS) and 4-methyl-2,6-diformyl phenol (MFOH) has been investigated by quantum mechanics/molecular mechanics–molecular dynamics (QM/MM–MD) simulation with four different solvents H2O, CH3OH, CH2Cl2 and CHCl3. The ESIPT of HBQ proceeds 20 ± 8 fs in water and methanol solvents; whereas, for other two solvents, CH2Cl2 and CHCl3, proton transfer (PT) occurs within 62 ± 6 fs and 76 ± 6 fs, respectively. Similarly, the ESIPT of HAQ proceeds 21 ± 6 fs in H2O, 33 ± 6 fs in CH3OH, 51 ± 6 fs in CH2Cl2 and 61 ± 6 fs in CHCl3. The ESIPT of MS proceeds in 23 ± 6 fs in H2O, 38 ± 6 fs in CH3OH, 60 ± 6 fs in CH2Cl2 and 73 ± 6 fs in CHCl3. The ESIPT of MFOH proceeds 20 ± 6, 22 ± 6, 35 ± 6 and 58 ± 6 fs in H2O, CH3OH, CH2Cl2 and CHCl3 solvent, respectively. Our QM/MM–MD simulations show that the rate of PT is inversely proportional to the polarity of the solvents. In addition, intramolecular hydrogen bonding capacity between the hydroxyl oxygen (proton donor) and the benzoquinolinic nitrogen (proton acceptor) plays an important role for the ESIPT reaction.