The rotational profiles of 9-(2,3-dihydro-2-phenyl-1H-benz[e]inden-1-ylidene)-9H-fluorene, a recently developed light-driven molecular motor, have been determined in the gas phase and chloroform solution for both the singlet and the triplet electronic states. Calculations in the gas phase were performed at the (U)B3LYP/6-31G(d) and (U)MP2/6-31G(d) levels, while the effects of the solvent were examined considering both implicit and explicit solvation models through self-consistent reaction-field and quantum mechanically/molecular mechanics schemes, respectively. The free energy barriers for the thermal helix inversion obtained in the gas phase (17.0 and 13.6 kcal mol~(-1) at the UB3LYP and UMP2 levels, respectively) are in good agreement with the experimental estimation (21 kcal mol~(-1)). On the other hand, the implicit solvation model led to a reduction of the barrier, while application of umbrella sampling with explicit chloroform molecules yielded a free energy barrier of 21.3 kcal mol~(-1) for the ground state. The geometrical changes produced during the rotation, which are very significant, are discussed in detail.
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