Theoretical insights into excited-state intramolecular and multiple intermolecular hydrogen bonds in...

The photophysical properties and photochemistry reactions of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ) system in different solutions are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Our theoretical investigation explores that an ultrafast barrier-free excited state intramolecular proton transfer (ESIPT) process occurs and the configuration twisting is found in the electronic excited state. In the polar protic methanol solution, the hydrogen-bonded complex composed by HPQ and two methanol molecules (HPQ-2M) could exist stably in the ground state. Upon photoexcitation the isolated HPQ is initially excited to the first excited state, while the HPQ-2M system is firstly excited to the S₃ state and undergoes internal conversion (IC) to the S₁ state. The intermolecular hydrogen bonds are strengthened in the excited state. The simulated electronic spectra agree well with the experimental results. The strengthening of the intermolecular hydrogen bonds is also confirmed by the calculated vibrational spectra. In addition, the intramolecular charge transfer happens in both HPQ and HPQ-2M systems from the frontier molecular orbital analysis.