The effect of chemical substituents on the functionality of a molecular switch system: a theoretical study of several quinoline compounds.

This study investigates the effect of chemical substituents on the functional properties of a molecular photoswitch (Phys. Chem. Chem. Phys., 2008, 10, 1243) by means of theoretical tools. Molecular switches are known to consist of so-called frame and crane components. Several functional groups are substituted to the 7-hydroxyquinoline molecular frame at position 8 as crane fragments. The impact of π-electron donating NH2 groups attached to the frame is also investigated. Excited state intramolecular hydrogen transfer mediated by the frame-crane torsion has been considered as a possible reaction mechanism. For all the investigated systems, we present the resulting potential energy profiles of the ground and first excited states. Vertical excitation energies and oscillator strengths of the 5 lowest-lying excited electronic states calculated at the two terminal points of the reaction path are also presented. Single point calculations were carried out at the CC2 level, while the presence of conical intersections between the ground and first excited states near perpendicular twisted geometries was demonstrated using the CASSCF method. Our results undoubtedly reveal the fulfillment of several molecular switch properties of the studied quinoline compounds. Comparisons between the different substituted systems have also been made.