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Keywords:

  • molecular switches;
  • ab initio study;
  • nonadiabatic coupling;
  • conical intersection;
  • quinoline compounds

The present study is concerned with the theoretical study of possible molecular switch systems. The 7-hydroxyquinoline-8-carboxamide molecule and its single-, double-, and triple-substituted derivatives are investigated with the aim of revealing characteristic switch features. Molecular switches can be considered as composed of a frame and a crane component. According to a recent study, the 7-hydroxyquinoline double-ring system constitutes the frame moiety, while a carboxamide group at position 8 plays the role of the crane part (Csehi et al., Phys. Chem. Chem. Phys. 2013, 15, 18048). The effect of single 2-,4-,6-methyl, double 2,4-, 2,6-diamino, and triple 2,4,6-triamino substitutions to the molecular frame has been investigated using high level ab initio techniques. As a possible reaction mechanism, excited state intramolecular hydrogen transfer mediated by the frame-crane torsion has been considered. At the terminal structures of this pathway, second-order approximate coupled-cluster (CC2) quality vertical excitation energies and oscillator strengths have been calculated for the three lowest-lying singlet electronic excited states of all the studied systems. Single point calculations at selected geometries of the reaction path were carried out at the CC2 level as well, while conical intersections (CIs) between the ground and first excited states near perpendicular twisted geometries were optimized using the complete active space self-consistent field method. To confirm the presence of CIs, nonadiabatic coupling terms have been derived and applying the topological line integral technique, the topological (or Berry) phase has been calculated surrounding the point of CI. The results of this work clearly demonstrate the fulfillment of several molecular switch properties by the investigated quinoline derivatives. An extensive comparison between the different compounds is presented as well. © 2014 Wiley Periodicals, Inc.