Density Functional Study of the Ground and Excited State Potential Energy Surfaces of a Light-Driven Rotary Molecular Motor (3R,3′R)-(P,P)-trans-1,1′,2,2′,3,3′,4,4′-Octahydro-3,3′-dimethyl-4,4′-biphenanthrylidene

Potential energy surfaces of the ground and the first excited singlet states of the (3R,3'R)-(P,P)-trans-1,1',2,2',3,3',4,4'-octahydro-3,3'-dimethyl-4,4'-biphenanthrylidene rotary molecular motor have been investigated along the central C-4=C-4, double bond twisting mode starting from the (P,P)-trans and from the (P,P)-cis conformations occurring in the photoisomerization cycle of this compound. The potential energy profiles obtained with the help of the state average spin restricted ensemble-referenced Kohn-Sham (SA-REKS) method feature minima on the excited state surface, the positions of which are displaced with respect to the barriers on the ground state surface toward the isomerization products, the (M,M)-cis and the (M,M)-trans conformations, respectively. The origin of these minima is analyzed and explained. The results of the present study suggest that the experimentally observed unidirectionality of photoinduced rotation in the above compound can be corroborated by the obtained profiles of the ground and excited state potential energy surfaces.