New Insights into the Excited State Dynamics of Quinoline-Pyrazole Isomerism

In this article, we propose a new excited state dynamics mechanism for quinoline-pyrazole isomers (QP-1 and QP-2) in the liquid phase based on the time-dependent density functional theory (TD-DFT) method. The calculated potential energy curve shows that QP-2 is more likely to occur as an excited state intramolecular proton-transfer (ESIPT) process than QP-1, whereas QP-2 will occur as a twisted intramolecular charge transfer (TICT) process, which is also the main decay pathway of QP-2 fluorescence quenching. The TICT process is not involved in QP-1 due to the high energy barrier. Based on excited state energy decomposition and charge decomposition analysis, it is determined that the energy gap and hole-electron interaction are the main driving forces that can cause the TICT process to occur. These results are inconsistent with the conclusions of recent theoretical reports [J. Phys. Chem. A 2015, 119, 6269-6247], in which the authors proposed that QP-2 will not twist after proton transfer (PT-2) in the lowest singlet excited state, and its fluorescence quenching pathway is the rapid intersystem crossing. We found that the reason for this difference was that the author did not consider the solvent effects in the calculations. Moreover, the rapid intersystem crossing of QP-2 is invalidated by computing the spin-orbit coupling between singlet and triplet excited states. Considering the fact that the spectral data of QP isomers were obtained in solution, our calculation results should be more in line with the experimental requirements, so as to further provide help for the reasonable explanation of the experimental mechanism and the detailed description of the theoretical calculation.