Theoretical Study on Pentiptycene Molecular Brake: Photoinduced Isomerization and Photoinduced Electron Transfer

The isomerization of the double bond plays an important role in the braking and de-braking of the light-controlled molecular brake. Therefore, the pentiptycene-type (Pp-type) light-controlled molecular brake system ((E)- and (Z)-4'-pentiptycyl vinyl-[1,1'-biphenyl]-4-carbonitrile) containing the C = C double bond was theoretically studied. Combining the 6-31G(d) basis set, the ωB97XD functional with dispersion correction was applied to implement the (E)-configuration and (Z)-configuration initial optimization. Next, using the 6-311G(d,p) basis set, the relaxed potential energy surface scans of the rotation angle were operated, and then the optimization calculations of the transition states at the extremum high points. Analyzing the stagnation points and the rotational transition states on the potential energy profiles, the rotation mechanism and basic energy parameters of the molecular brake were obtained. Then, the DFT computations at ground states and the TD-DFT computations of vertical excitation energy were put into practice at the accuracy of the def-TZVP basis set for the two configurations, and using the natural transition orbital (NTO) analyses combining the excitation energies and absorption spectra, the electronic transition characteristics and electron transfer properties of light-controlled molecular brake were studied. Afterwards, in order to investigate the photoinduced isomerization reaction, the C = C double bond was scanned on the relaxed potential energy surface, and the intermediates of the isomerization reaction were searched and analyzed; thus, the braking mechanism of the light-controlled molecular brake was proposed.