We present the first high-resolution rotational spectrum of an artificial molecular rotary motor.Bycombining chirped-pulse Fourier transform microwave spectroscopya nd supersonic expansions,w ec aptured the vibronic ground-state conformation of as econd-generation motor based on chiral, overcrowded alkenes.The rotational constants were accurately determined by fitting more than 200 rotational transitions in the 2-4 GHz frequency range.E vidence for dissociation products allowed for the unambiguous identification and characterization of the isolated motor components.E xperiment and complementary quantum-chemical calculations providea ccurate geometrical parameters for the C 27 H 20 molecular motor,t he largest molecule investigated by highresolution microwave spectroscopytod ate.InspiredbyNaturesabilitytoperformmotorfunctionsatthe molecular level, chemists have engaged in the design of synthetic nanomachines that can perform molecular motion in ac ontrolled manner and mimic their biological counterparts using simpler models. [1,2] An elegant design of asynthetic rotary molecular motor based on chiral, overcrowded alkenes was introduced by Feringa and co-workers.[3] Key features of this design include 1) alight-activated power stroke in which excited-state cis-trans isomerization converts photon energy into mechanical motion and 2) ac hiral center that imposes unidirectional motion departing from conventional molecular photoswitching.T he operation mechanism of such amotor is illustrated in Figure 1. Thes ystem is comprised of a" stator" fluorene unit connected to an upper "rotor" via aC =C"axle". An ultraviolet trigger results in photoisomerization of the axle,l eading to ar otation of the rotor with respect to the stator.This motion yields isomer 1-B.The methyl group at the chiral center now adopts ap seudoequatorial conformation while that of 1-A is pseudoaxial. At hermally activated helix inversion returns the methyl group to the more energetically favorable pseudoaxial orientation, 1-C.This step reintroduces the steric hindrance,l ocks the rotor,a nd ensures unidirectional rotation in the forward direction.Thes ynthesis of nanomachines,s uch as the one investigated here,m arks an era where small artificial molecular constructs are able to perform mechanical work. Rotaxanebased systems [4,5] and unidirectional rotary molecular motors [6,7] are among the systems designed to perform translational and rotary motion, respectively.T he functional performance of these nanomachines clearly emerges from their unique structural properties.Further understanding and optimizing such molecular machinery are therefore largely dependent on the ability to get detailed information on the molecular conformations of the key mechanical steps and their structural evolution, preferably under conditions where the system is not perturbed by external influences.E xperimental techniques that have thus far been employed for the structure elucidation of such molecular machines include NMR, [8] time-resolved IR, [9] fluorescence...
