Synthetic molecular motors that are capable of delivering controlled movement upon energy input are one of the key building blocks in nanomachinery. [1] The major energy sources of molecular motors are from chemical reactions, [1a] photon beams, [1b] or electric current, [1f] which are converted into mechanical forces through the excitation of the electronic states of the molecule. The energy scale of the electronic excitation is normally two orders of magnitude larger than the molecular vibrational frequencies. To reduce the heat dissipation and increase the energy utilization efficiency, a motor running purely on the electronic ground-state (GS) potential energy surfaces is highly desirable.Nanoclusters, which are neither molecules nor fragments of solids, are unique chemical species with remarkable novel structures, chemical bonding, and unexpected reactivity. [2] Recently, two planar boron clusters, B 19 À and B 13 + , have been demonstrated to undergo a rare fluxional behavior: the outer ring rotates almost freely with respect to the inner ring. [3][4][5] These clusters are anomalously stable species of low reactivity, [3,6,7] similar to their prototypical organic aromatic congeners: benzene and coronene. [2b,c, 3, 5, 8] The low energy barrier that made the in-plane rotation possible was due to the delocalized bonding, which rendered concentric double aromaticity of both equilibrium and transition states of the clusters. [4,5] This peculiar fluxionality of B 19 À and B 13 + at room temperature has gained them the name of "molecular Wankel motors". [4,5] In equilibrium, driven by thermal fluctuations, the outerring rotation is bidirectional, and has a timescale of the order of tens of picoseconds at 300 K. To utilize B 13 + as an engine for nanomachinery, an energy source is needed to drive the molecule out of equilibrium. Herein, we show that a unidirectional rotation of the outer ring of B 13 + can be achieved with a circularly-polarized infrared (IR) laser, rendering a photodriven molecular Wankel motor running on the electronic GS potential energy surfaces with a rotational period of a few picoseconds.The ground-state (GS) geometry of B 13 + , which exhibits C 2v symmetry, is shown in Figure 1 a. In equilibrium, B 13 + has a small intrinsic dipole moment (0.4 Debye) along the C 2v axis, through which we define the square ring as the head of the molecule. Without an electric field, a complete 3608 rotation of the outer ring is decomposed into 30 identical elementary moves (see Figure 3 of Ref.[5] for a detailed description). Each elementary move induces a slight relative movement between the inner and the outer ring. In Figure 1 a, through the sequence 1!2!3!4, all atoms in the outer ring B 10 are rotated counterclockwise by one atomic position. A 368 rotation is thus completed through three consecutive elementary moves. During each move, a reorientation of the molecule takes place and rotates the head counterclockwise by 1208. This discretized reorientation of the molecule is critical to the guided...