Cold Snapshot of a Molecular Rotary Motor Captured by High‐Resolution Rotational Spectroscopy

Abstract We present the first high‐resolution rotational spectrum of an artificial molecular rotary motor. By combining chirped‐pulse Fourier transform microwave spectroscopy and supersonic expansions, we captured the vibronic ground‐state conformation of a second‐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. Evidence for dissociation products allowed for the unambiguous identification and characterization of the isolated motor components. Experiment and complementary quantum‐chemical calculations provide accurate geometrical parameters for the C27H20 molecular motor, the largest molecule investigated by high‐resolution microwave spectroscopy to date.


Cold Snapshot of aMolecular Rotary Motor Captured by High-Resolution Rotational Spectroscopy
SØrgio R. Domingos,Arjen Cnossen, Wybren J. Buma, Wesley R. Browne,Ben L. Feringa, and Melanie Schnell* Abstract: 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,a nd electronic [10] spectroscopies.T hey provide important information but do not meet up to the last requirement. High-resolution rotational spectroscopy on isolated nanomachines in the gas Figure 1. Structure of the molecularm otor.T he three components, namely rotor,a xle, and stator,a re indicated as well as the light-driven (power) and thermal strokes required for operation. Further photondriven and thermal isomerization events return the motor to its original configuration 1-A.
phase,o nt he other hand, is preeminently suited for this purpose but the largest molecular systems that have been studied with these techniques hardly come close to the molecular motor that is considered here with respect to the number of non-hydrogen atoms. [11][12][13] Herein, we report on the first high-resolution rotational spectrum of am olecular motor, which was obtained by combining microwave spectroscopy with the cold conditions of as upersonic jet. Microwave spectroscopy enables the unambiguous identification of molecular species and the determination of the thermal distribution of conformations.O wing to their unique moments of inertia, each conformation of aparticular molecule can be differentiated by its rotational spectrum. With the implementation of short and intense microwave chirps in broadband excitation schemes as in chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy,itis possible to record rotational spectra of complex, flexible molecules spanning several GHz in asingle acquisition. [14] Thecold molecular jet brings the molecules to rotational temperatures below 2K,which for amolecular system of this size (C 27 H 20 , M w = 344 gmol À1 )i mplies that the strongest rotational transitions are situated between 2and 4GHz. The broadband microwave spectrum of 1-A in this region is shown in Figure 2. Theexperimental spectrum is shown as the upper trace (in black). Thes pectrum shown below (in red) represents asimulation obtained from the fitted spectroscopic parameters reported in Table 1. Theright panel of this Figure displays as egment of the rotational spectrum, highlighting ab ranch of rotational transitions J K a K c J 0 K a 0 K c 0 denoted by the rotational quantum numbers J, K a ,and K c ,with J being the rotational angular momentum quantum number and K a and K c being the projections of J onto the principal axes at the prolate and oblate symmetric top limits,respectively.Atotal of 222 rotational transitions were assigned, and the primary rotational constants (A, B, C)w ere determined through ar ecurrent fit using the A-reduced semirigid rotor Hamiltonian as implemented in PGOPHER. [15] Quartic centrifugal distortion constants (D J and d J )w ere also determined. We note that the inclusion of distortion constants is not required to achieve agood fit. Thesmall magnitudes obtained for both D J and d J are astrong indicator of the rigidity of the molecule in spite of its size.Asummary of the fitted spectroscopic parameters is given in Table 1while acomplete list of all fitted rotational transitions is provided in the Supporting Information. We found neither evidence for internal dynamics arising from the methyl top nor for other large-amplitude motions, which would point to high barriers associated with these motions.Asthe methyl group is part of the ratchet during the operation of the motor, this is indeed what would be expected.
In our frequency range,w ec over mainly a-and b-type transitions.I nF igure 3, we show segments of the spectrum depicting ap rogression of a/b-type quartets over the range Following the progression from lower to higher frequencies (panels A!F), we observe an arrowing between transitions,r esulting in coalescence for 13 ! 12 further up the J levels.   shows the experimentalspectrum obtained using neon as the carrier gas.T he lower trace representss imulationso btained from the fitted spectroscopicparameters reported in Table 1. The marks the a/b quartet progressions that are shown in detail in Figure 3. Therotational transition marked with †c orrespondst othe dissociationstructure of the rotor as aconsequence of fragmentation (see main text). The spectroscopicparameters of the fragment are reported in Table 2.
In Table 1, we compare the results taken from our observations with as eries of quantum-chemical calculations at different levels of theory (see the Supporting Information for further details). We found avery good agreement between theoretical predictions and our experimental observations at all levels of theory.T he most impressive match between the experimental and calculated rotational constants was obtained at the M06-2X/6-311+ ++ +G** level of theory,f or which experiment and theory differ by less than 0.5 %for all three rotational constants.Inaddition, the magnitudes of the permanent dipole moment components are in good agreement with the observed intensities.A tt he same time,t he dispersion-corrected B3LYP-D3BJ level of theory predicts the experimental rotational constants equally well, in particular if one considers the deviations (ca. 1%)p redicted for vibrationally corrected rotational constants with respect to the equilibrium ones. [16] Theexcellent agreement between experiment and theory enabled us to determine key geometrical parameters of the molecular motor. The length of the C=Cb ond of the motor is 1.356 (M06-2X), which is very similar to the length determined from the crystal structure [6] (1.357 ). Comparison of this bond length with those of other non-sterically overcrowded alkenes,s uch as ethylene (C 2 H 4 ,1.325 ) and 2-butene (C 4 H 8 , 1.329 ), readily indicates that the C = Cl ink is substantially extended in the molecular motor. To evaluate the local geometry around the axle,w ed efined three planes (Figure 4), which comprise the planar part of the stator (in yellow), the axle and rotor (in blue), and the planar part of the rotor (in red), respectively.T he angles a = 50.88 8 (50.08 8), b = 42.08 8 (39.68 8), and g = 18.48 8 (22.88 8)d efine the relative twisting of the rotor with respect to the stator in the locked conformation (crystal structure values are given in parentheses). Comparison with the values obtained from the crystal structure shows that the structure of the motor is unmistakably affected by its environment. Theg as-phase dihedral angle at the axle coordinate differs by approximately 1.68 8 from the crystal structure,with D(2-3-4-5) = 13.58 8 (15.188 8).
Interestingly,w ea lso found evidence that under our experimental conditions,s ome fragmentation of the molecular motor occurs.I nt he rotational spectrum, we identified and fitted as eries of rotational lines that correspond to dissociation products of the rotor and the stator.F ragmentation occurs owing to preexpansion heating at the nozzle.One of the 23 rotational transitions belonging to the rotor moiety is shown in Figure 2( right panel). Thes pectroscopic parameters obtained from the fit to these transitions are given in Table 2. Quantum-chemical calculations on two tentative models for adissociation product of the rotor confirmed our expectations:f rom the comparison of rotational constants, dipole moment components,a nd asymmetry parameters,w e unambiguously identified r-B as the structure of the fragment. Dissociation thus results in ap lanar rotor fragment that no longer possesses ac hiral center as compared with the geometry of the rotor when it is coupled to the motor. The stator fragment was unambiguously assigned to fluorene based on af it using 18 rotational transitions and ad irect comparison with reported rotational constants. [17] Thes pectroscopic parameters are reported in the Supporting Information.
In summary,w eh ave presented the first high-resolution rotational spectrum of amolecular rotary motor and used it to determine the exact conformation of the motor in its vibronic ground state and to derive key structural parameters.R otational constants were determined with high accuracya nd provide an excellent basis for benchmarking the current levels of theory implemented in quantum-chemical methods for large molecular systems.T emperature-induced fragmentation  of the motor has allowed us to observe the motor components separately and analyze the structures of the dissociation products.T he unprecedented observation of am olecule of this size by microwave spectroscopy introduces exciting perspectives to the investigation of other molecules of similar and larger sizes by high-resolution spectroscopy.I nt he present study,w eh ave reported on the conformation of the motor in the absence of an external trigger and observed as ingle conformer corresponding to the thermally stable ground-state structure.T hese studies served to demonstrate the feasibility of high-resolution rotational spectroscopic studies on systems of this size.E xperiments that have now come within reach use UV photons to drive the initial power stroke of the motor and combine this activation of the motor with the high resolving power of rotational spectroscopy to determine the structure of intermediate metastable mechanical steps in the photocycle of these molecular machines.Such experiments are presently being set up in our laboratories.  [a] Rotational constants(A, B, C in MHz);type of spectrum observed (a-type, b-type, c-type) with ybeing observed and nbeing not observed; predicted dipole moments;number of lines used in the fit;s tandard error of the fit (in kHz);asymmetry parameter k = (2B-A-C)/(A-C). The experimental frequency accuracy is 25 kHz.