| In this dissertation, we made continuous efforts on exploring the working mechanisms of nitrogen-containing artificial light-driven molecular rotary motor, and designing of novel electron-tunable nitrogen-containing systems with potential application as molecular motor and switch. The research objects include 1,2-dicyanoethylene,1,2-dicyano-1,2-dithienylethene and an aryl amine. The important findings are summarized as follows.1. We carried out mechanistic studies on the cis-trans isomerization processes of 1, 2-dicyanoethylene in its neutral, cationic and anionic forms-at the density-functional (TD)BH&HLYP, multireference ab initio CASSCF and MS-CASPT2 level. The results confirmed the importance role of electron-induction in reducing the reaction barrier, and more importantly, revealed the different nonadiabatic channels tuned by electron attachment/detachment. In neutral dicyanoethylene, the Si?S0 decay need overcome a mild barrier to reach a H-migration-type(Si/So-CI), which is away from the C=C torsional coordinates and may slow down the speed of C=C rotation and hurt its directionality; while in cationic and anionic isomerization processes, the D1 and Do PESs intersect along the rotary path, therefore, the nonadiabatic D1 ? D0 decay is barrierless, as result, the speed and directionality of C=C rotation is maintained. The current study revealed the role of electron induction in tuning the cis-trans photoisomerization, and shed light on the design of novel light-driven molecular switches.2. We further applied the idea of electron-tunning photoisomerization on a widely-used photochromic compound, namely,1,2-dicyano-1,2-dithienylethene(CTE). The results at CASSCF and MS-CASPT2 level showed that the thienyl substituents systemally lower the thermal isomerization barriers for neutral, cationic and anionic CTE by about 30?90 kJ·mol-1, with respect to those of 1,2-dithienylethene. For photo-induced cis-trans isomerization, the S1?S0 decay of neutral CTE need to readjust its geometry to reach the S1/S0 conical intersections that results from the carbon-atom pyramidalization mode and is therefore away from the C=C torsional coordinates. Consquently, it slows down the speed of C=C rotation and hurt its directionality. While in cationic and anionic isomerization processes, the D1 and Do PESs intersect along the rotary path, therefore, the nonadiabatic D1?>D0 decay is barrierless, thus the speed and directionality of C=C rotation is maintained. Again, this study demonstrated that the electron can effectively tune the photoisomerization.3. We carried out DFT and TDDFT mechanistic studies on the reaction mechanism of a synthetic imine rotary motor. The working mechanisms are rationalized and some key findings can be outline. In photoisomerization step, both the bright(?,?) and dark (n,?*) states are involved. First, the helical isomer (P)-cis is excited to S2(?,?*), then relaxes to S1(n,?*) along C=N ratary path-on (n,?*) state the rotation is accelerated until the molecule reach S1 minimum at torsion angle about 90°. Then, the molecule needs repopulate to a high-energy S1/S0-CI, and non-radiative relaxation to So state. Finally reaction goes on along the ground-state path and generates (M)-trans. In this process, the high-energy S1/S0-CI acts as the barrier and hindered the speed of rotation. However, compared with the Cis in polyene motors, the S1/S0-CI in current imine motor was found geometrically near the C=N rotation path, therefore it is beneficial to keep the rotation directionality. In thermal isomerization step, we confirmed that the (M)-trans?(P)-trans isomerzation by the ring-inversion of stator, but the influence of reversed process of previous step is nonnegligible. The current study suggested that in further design of imine rotary motor, two key problems have to be solved:One is to reduce the barrier caused by high-energy conical intersection in photoisomerization step, the other is to prevent the unwanted reverse reation in thermal rotation. |
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