title: Intersystem Crossing Pathway in Quinoline-Pyrazole Isomerism: A Time-Dependent Density Functional Theory Study on Excited-State Intramolecular Proton Transfer.
abstract: The dynamics of the excited-state intramolecular proton-transfer (ESIPT) reaction of quinoline-pyrazole (QP) isomers, designated as QP-I and QP-II, has been investigated by means of time-dependent density functional theory (TDDFT). A lower barrier has been found in the potential energy curve for the lowest singlet excited state (S1) along the proton-transfer coordinate of QP-II compared with that of QP-I; however, this is at variance with a recent experimental report [J. Phys. Chem. A 2010, 114, 7886-7891], in which the authors proposed that the ESIPT reaction would only proceed in QP-I due to the absence of a PT emission for QP-II. Therefore, several deactivating pathways have been investigated to determine whether fluorescence quenching occurs in the PT form of QP-II (PT-II). The S1 state of PT-II has nπ* character, which is a well-known dark state. Moreover, the energy gap between the S1 and T2 states is only 0.29 eV, implying that an intersystem crossing (ISC) process would occur rapidly following the ESIPT reaction. Therefore, it is demonstrated that the ESIPT could successfully proceed in QP-II and that the PT emission would be quenched by the ISC process.
authors.affiliation: †Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China.|‡Department of Applied Chemistry, Institute of Molecular Science and Center for Interdisciplinary Molecular Science, National Chiao-Tung University, Hsinchu 30050, Taiwan.
authors.forename: Yu-Hui
authors.initials: YH
authors.lastname: Liu
authors.affiliation: †Department of Physics, College of Mathematics and Physics, Bohai University, Jinzhou 121013, China.
authors.forename: Sheng-Cheng
authors.initials: SC
authors.lastname: Lan
authors.affiliation: ‡Department of Applied Chemistry, Institute of Molecular Science and Center for Interdisciplinary Molecular Science, National Chiao-Tung University, Hsinchu 30050, Taiwan.
authors.forename: Chaoyuan
authors.initials: C
authors.lastname: Zhu
authors.affiliation: ‡Department of Applied Chemistry, Institute of Molecular Science and Center for Interdisciplinary Molecular Science, National Chiao-Tung University, Hsinchu 30050, Taiwan.
authors.forename: Sheng-Hsien
authors.initials: SH
authors.lastname: Lin
mesh_terms: D005453:Fluorescence; D007536:Isomerism; D008956:Models, Chemical; D011522:Protons; D011720:Pyrazoles / Q000737:chemistry*; D011804:Quinolines / Q000737:chemistry*
chemical_list: D011522:Protons; D011720:Pyrazoles; D011804:Quinolines; C031280:pyrazole; C037219:quinoline