A TD-DFT study on the excited-state hydrogen-bonding interactions in the 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone-(H2O)3 cluster

In the present work, both the intramolecular and intermolecular hydrogen bonding interactions in 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone monomer and the hydrogen-bonded 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone-(H2O)3 cluster in the electronically excited states are theoretically investigated using the time-dependent density functional theory (TD-DFT) method. The calculated bond lengths of the intramolecular hydrogen bond CO⋯H1O1 in the 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone monomer and the intermolecular hydrogen bonds HB-II, HB-III and HB-IV in the hydrogen-bonded 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone-(H2O)3 cluster are shorter in the excited state S1 than those in the ground state S0. However, the distance between the carbonyl oxygen atom O and the hydroxyl hydrogen atom H1 is further lengthened in the 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone-(H2O)3 cluster, and the bond length of the intermolecular hydrogen bond HB-V is slightly lengthened upon electronic excitation to the S1 state. The calculated stretching vibrational modes of the CO and OH of the intramolecular hydrogen bond CO⋯H1O1 and the intermolecular hydrogen bonds HB-II, HB-III and HB-IV in the excited states are all red-shifted compared with those predicted for the ground state. Therefore, it has been demonstrated that the intramolecular hydrogen bond CO⋯H1O1 in the 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone monomer and the intermolecular hydrogen bonds HB-II, HB-III and HB-IV in the 2-(2-thienyl)-3-hydroxy-4(1H)-quinolone-(H2O)3 cluster are all strengthened in the electronically excited state. Moreover, the two emission bands at the short wavelength observed in the experiments have been assigned by our TD-DFT calculations.