one of the key challenges 
 in the development of novel materials 
 is the ability to tune and control their macroscopic physical and 
 chemical properties on a molecular level by external stimuli . such a degree of control is challenging but crucial for a wide range 
 of applications ranging from catalysis to pharmacological and medical 
 applications . in photoactive materials light absorbed by chromophores can be converted 
 into directed functionality , typically by means of photoinduced isomerization 
 of nitrogen or carbon containing double bonds and pericylic reactions . indeed , e  z isomerization 
 results in large amplitude changes in structure as well as chemical 
 and physical properties . molecular motors based on overcrowded alkenes are particularly 
 attractive due to the synthetically tunable properties , such as thermal 
 stability , absorption spectra , etc . , and therefore are readily applied 
 to a wide range of functional systems . however , although the thermally 
 activated step in the unidirectional rotary cycle is well understood 
 and is now largely predictable , the photochemical properties and especially 
 photochemical quantum yields are less well understood . hence both 
 from a fundamental perspective and in achieving the goal of complete 
 molecular design , gaining insight into the excited - state properties 
 of light driven molecular - sized machines is essential . the overcrowded alkene - based molecular 
 rotary motors are composed 
 of a central carbon  carbon double bond ( the axle ) embedded 
 in an intrinsically chiral environment . the unidirectional rotational 
 cycle starts with a photoinduced e  z isomerization around the central c = c bond , generating 
 a metastable isomer p * ( figure 1 ) , which relaxes thermally to the stable isomer p , which in 
 the case of a symmetric stator unit is identical to i. this isomer 
 marks half of the rotation cycle and is the starting point for a second 
 photoinduced isomerization step that leads again to a metastable isomer . thermally activated conformational isomerization of this isomer brings 
 the system back to its initial state . overall , the rotor part has 
 then performed a complete rotation with respect to the stator unit 
 of the molecular motor . although the rotation includes both 
 photochemical and thermal steps 
 that involve different parts of the motor , the structural evolution 
 is largely governed by the excited - state properties of the central 
 c = c group , which , in turn , are dictated by the potential energy 
 surfaces of the excited states that are accessed upon electronic excitation . 
 despite the large number of theoretical and experimental studies that have focused on characterizing 
 this process , there are several key questions outstanding including 
 identification of the main structural coordinates and the role of 
 various electronic states involved in the excited - state dynamics . time - resolved studies are essential to elucidate these dynamics but 
 have thus far focused on probing the change in the electronic structure 
 after photoexcitation through uv / vis absorption and fluorescence studies . these techniques offer 
 a high time resolution but provide only indirect information as to 
 how the structure evolves spatially over time . in the present contribution 
 we show that this information can be obtained through a combination 
 of picosecond time - resolved vibrational spectroscopy of the electronically 
 excited states together with quantum - mechanical calculations . the setup for the time - resolved infrared measurements has been 
 described in detail previously . mid - ir 
 probe pulses were generated using an amplified ti : sapphire system 
 ( spectra - physics hurricane , 600 j , repetition rate 1 khz ) pumping 
 a commercial bbo - based opa ( spectra - physics opa-800c ) , the signal 
 and idler output of which were difference - frequency - mixed in aggas2 . uv pump pulses ( 400 nm , 1 j ) were generated 
 by doubling part of the output of the ti : sapphire laser . the diameter 
 of the uv pump focus was 180200 m ; that of 
 the ir probe focus , 150 m . the sample cell with caf2 windows spaced by 500 m was placed in the ir focus . 
 to avoid accumulation of the thermally unstable conformer p * during 
 the measurements , the pump  probe delay time was scanned by mechanically 
 adjusting the beam path of the uv pump . the temporal resolution of 
 200 fs has been determined from the full width at half - maximum of 
 the pump probe cross - correlation function . the pump pulse was chopped 
 at 500 hz , and transient spectra were obtained by subtracting nonpumped 
 absorption spectra from the pumped absorption spectra , both of which 
 were recorded by spectrally dispersing the probe pulses using an oriel 
 ms260i spectrograph and measuring the frequency - dependent ir intensity 
 using a 2  32 hgcdte detector array ( infrared associates ) . compound 
 i was available from earlier studies ( see ref ( 44 ) ) . in all experiments we 
 investigate a 10 mm solution of compound i in deuterated cyclohexane 
 ( aldrich , 99.6 atom% d ) . for the investigation of the ground - state 
 conformers ( including 
 vertical excitation energies and forces acting at the franck condon 
 point ) quantum mechanical calculations were performed using the turbomole 
 suite of programs . geometry optimization 
 and normal - mode analysis were performed by ( time - dependent ) density 
 functional theory ( ( td)-dft ) using the hybrid functional b3lyp . for the geometry optimization the d3 correction 
 method of grimme et al . the cc - pvdz or aug - cc - pvdz basis 
 sets were employed , respectively . post - hartree  fock calculations 
 were performed on the basis of spin - component scaled second - order 
 mller  plesset perturbation theory and the approximate coupled - cluster singles - and - doubles 
 model cc2 . all perturbation method calculations were performed using 
 the resolution of identity ( ri ) approximation and the efficient ricc2 module implemented 
 in the turbomole package together with the standard auxiliary basis 
 sets . the gaussian09 program package was employed to study the excited - state properties 
 in more detail and in particular to optimize the structure of the 
 dark - state species . geometry optimization and normal - mode analysis 
 were performed in the framework of td - dft at the b3lyp / cc - pvdz level . the structural evolution from the 
 initial state i to isomer p * 
 ( figure 1 ) was considered first in terms of the spectral properties 
 of the initial state and the first metastable product ( p * ) . the steady - state 
 uv / vis absorption spectra of the two isomers are distinct , reflecting 
 a change in the conjugated  system ( figure 1a ) . the ftir spectra of the two isomers ( figure 1c ) show that the 
 carbon  carbon stretch and ring vibrations couple to a large 
 degree as a consequence of the extended  system ; however , dft 
 calculations enable us to assign the bands in the 15501650 
 cm range as being mainly associated with 
 modes involving the central c = c double bond ( supporting information ) . the metastable isomer p * shows a 
 shift of these modes to lower frequencies confirming a decrease in 
 bond strength . comparison of the optimized geometries of the two isomers 
 ( i ( p * ) , cc bond length 1.369 ( 1.377 )  ; cc = cc dihedral 
 angle 14.1 ( 29.5 ) ) evidence the corresponding lower  
 bond character as well . unidirectional motion of the unidirectional 
 motor starting from 
 the thermally stable conformer ( i ) via the product of the photoinitiated 
 step ( p * ) to the thermally stable conformer ( p ) ( optimized geometries 
 on the b3lyp+d / aug - cc - pvdz level ) : ( a ) experimental and calculated 
 ( light color and dotted line ) uv  vis spectra of conformer i 
 and p * ; ( b ) molecular orbitals of the initial conformer ; ( c ) experimental 
 and calculated ir spectra ( b3lyp+d / aug - cc - pvdz ) of the initial state 
 ( blue ) and final state ( red ) . in both the measured uv / vis absorption 
 and ftir spectra at the photostationary state ( pss ) , i and p * are 
 present in a 25:75 ratio . time - resolved vibrational spectroscopy was employed to achieve the required temporal resolution as well 
 as structural sensitivity to elucidate the structural evolution that 
 follows photoexcitation with a short uv - pump pulse ( center wavelength 
 400 nm ) . the resulting structural evolution was followed by time - resolved 
 differential absorption spectra in cyclohexane - d12 with a time resolution of 200 fs ( figure 2 ) . the transient ir spectra show , as expected , 
 pronounced differences between the ir absorption spectrum of the molecule 
 in the initial and excited states . a first point of note is that the 
 spectrum does not evolve further after ca . 30 ps , and the difference 
 spectrum corresponds perfectly with the difference spectrum obtained 
 upon continuous wave excitation ( figure 1 ) ; i.e. , the metastable isomer p * has formed 
 fully within the 30 ps after excitation . ftir and time - resolved 
 infrared spectra of the molecular motor 
 in cyclohexane - d12 : ( a ) ftir spectra of 
 the thermal stable conformer ( initial , solvent corrected , top ) , uv - ump / ir - probe 
 transient infrared spectra ( uv = 400 nm ) ( middle ) , 
 and difference ftir spectra between the two ground - state conformers 
 of the molecular motor ; ( b ) kinetics of selected bands and corresponding 
 results of the global fit ; ( c ) species associated spectra ( sas ) of 
 species 1 and calculated ir spectra ( top ) and sas of the pss ( sp3 ) 
 together with ftir spectra ( bottom ) . negative bands indicate ground - state 
 bleaching , and positive bands , species created after excitation . the time - resolved ir spectra obtained 
 within the first 30 ps exhibit 
 several remarkable features . the first and most striking is a broad 
 and unstructured absorption over the entire mid - ir range ( 12003000 
 cm ) . this absorption , which decays rapidly , is 
 particularly prominent at early time delays ( < 5 ps ) . individual 
 absorption bands with remarkably high integrated intensities in the 
 14001600 cm range are superimposed on 
 the broad spectral feature . notably , these bands are significantly 
 broader than the bleached bands in the transient spectra and bands 
 in the ftir spectra . the solvent response and the dependence of the 
 uv pump intensity ( supporting information ) confirm that these features are intrinsic to the compound and are 
 not due to solvent or two - photon processes . the width and intensity 
 of the broad excited - state absorption bands 
 excludes that they arise from vibrational transitions . indeed , the 
 absorbance is closer to that expected for electronic transitions , 
 albeit these typically occur at much higher energies ( in the uv / vis 
 range ) . the initial state of the compound 
 ( i ) shows an absorption band at 390 nm ( figure 1 ) , which is reproduced well by b3lyp / cc - pvdz 
 calculations in the gas phase ( 403 nm ; f = 0.428 ) . the s1  s0 transition has predominantly 
 homo  lumo character ( figure 1 ) . in contrast to previous assignments , however , we find that this electronic transition 
 is not localized on the stator but involves to a significant extent 
 excitation from the bonding  orbital of the central double 
 bond to the antibonding  * orbital ( figure 1 ) . as a consequence , the double bond character 
 of this bond is reduced as well as the barrier to rotation around 
 this bond . in line with calculations on ethylene and its derivatives , we find a dark state ( s2 ) associated with the homo1 
  lumo excitation at slightly higher excitation energies that 
 is not directly accessible from the ground state ( figure 1 , table 1 ) . because the homo1 orbital is localized 
 on the aromatic stator unit , s2 has a significantly larger 
 dipole moment than s1 ( 4.3 vs 2.2 d at the scs - cc2/cc - pvdz 
 level ) . it should be emphasized that the description of the lower - lying 
 electronic states in the compound studied here is thus entirely different 
 from ethylene and its derivatives where , as discussed by salem and 
 others , for torsion angles of around 90 the two low - lying electronic 
 states have either a biradical or zwitterionic character depending 
 on the occupation of the two central 2p - orbitals and require multireference 
 methods for a proper description . in the present study , in contrast , 
 we investigate the dynamics occurring at small torsional angles ( vide infra ) for which the employed single - reference methods 
 not only have been validated . in addition 
 we employ the results of multireference calculations on smaller motor 
 molecules as reported by liu and morokuma to make a further comparison with our experimental results . for s0 the optimized 
 structure on the b3lyp+d / aug - cc - pvdz level was used to calculate the 
 vertical excitation energies . for vertical excitation they thus strongly suggest that the broad 
 and structureless absorption feature observed in the time - resolved 
 ir spectra is associated with the electronic s1  
 s2 transition . the presence of two close - lying states also 
 explains why the induced absorption bands have such high intensities . our calculations show that the energy gap between the two states strongly 
 depends on structural parameters such as the c = c bond length 
 that are involved in the 14001600 cm normal 
 modes . for these modes one therefore expects that their transition 
 moments will be enhanced by vibronic coupling as is indeed observed . the conclusion that the excited - state dynamics of the molecular rotor 
 should be described in a multistate model is further supported by 
 recent multireference calculations on smaller rotor molecules that 
 also find very small energy differences between the s1 and 
 s2 states . the excited - state 
 dynamics were elucidated further using a global 
 analysis of the transient spectra , which shows that the spectra can 
 be described well with two exponential decay terms with time constants 
 of 1.53  0.03 and 12  1 ps . we note that our experiments 
 have a time resolution of 150200 fs and therefore do not allow 
 observation and confirmation of the ultrafast dynamics reported previously 
 in fluorescence - upconversion studies . species - associated spectra ( sas ) ( supplementary 
 figure 2 ) were derived by using a two - step sequential kinetic 
 model . the sas of the first species shows an overall background 
 signal 
 with broad and intense features superimposed on it that indicate vibronic 
 coupling . furthermore , the increase ( decrease ) of absorbance in the 
 1550 ( 1600 ) cm region indicates a substantial 
 change in bond character of the central double bond ( figure 2c ) . this spectrum was related 
 to the structure of this excited - state species using excited - state 
 geometry optimizations . state without any structural constrains associated with the homo1 
  lumo excitation leads to a stable minimum that can be reached 
 from the franck  condon excited geometry by relatively small 
 structural changes ( supplementary figure 4 ) , and an excited - state ir absorption spectrum that is in good agreement 
 with the sas of species 1 ( figure 2 ) . it is notable that for a proper comparison with 
 the experimental spectrum , the theoretically predicted stick spectrum 
 needs to be convolved with 20 cm gaussian line 
 shapes , suggesting that in the  dark  state a wide range 
 of structures is sampled . our experiments and calculations thus demonstrate 
 that excitation and excited - state decay involve two different electronic 
 states . in earlier studies , time - resolved fluorescence and visible 
 absorption data were interpreted in terms of a decay from a single , 
 isolated electronic state decaying to the ground state after structural 
 relaxation in the excited state . the present data show that the s1 state actually decays to an intermediate , dark electronic 
 state , and from there to the ground state . in the sas of the 
 second species ( supplementary 
 figure 2 ) the broad background is absent , and bands have widths 
 comparable to those in the s0 state . the absence of the 
 broad background indicates that this species is no longer in an electronically 
 excited state but back in the ( vibrationally hot ) electronic ground 
 state . indeed , the vibrational activity observed in the 1550 cm region follows closely the ir absorption spectrum 
 of p*. in addition , repopulation of the original state ( i ) is observed 
 also ( i.e. the quantum yield for isomerization is less than unity ) , and the sas of the second species contains contributions 
 from both vibrationally hot ground - state i and metastable p * isomers . vibrational cooling of these isomers leads to the sas of the final 
 species , which is in excellent agreement with the steady - state ftir 
 difference spectrum of room - temperature i and p*. the present 
 experimental and theoretical data allow a complete 
 picture of the mechanism by which the molecular rotary motor undergoes 
 isomerization after electronic excitation ( figure 3 ) . photon absorption brings the motor to 
 a  bright  state from which it decays on an ultrafast 
 time scale and within the time resolution of the present experiments 
 to a different electronically excited  dark  state . 
 in line with previous calculations on similar systems , our calculations 
 indicate that this is a barrierless process proceeding via a conical 
 intersection . previously , it was assumed 
 that structural evolution on the homo  lumo potential energy 
 surface is responsible for the observed ultrafast decay of the fluorescence , 
 and ( following commonly accepted models on photoisomerization of carbon carbon 
 double bonds ) that it primarily proceeds along torsional and pyramidalization 
 coordinates . the present study , in contrast , leads to the conclusion 
 that the fluorescence decay is mediated by the passage through the 
 conical intersection of the homo  lumo and homo1  
 lumo potential energy surfaces . inspection of the structural changes 
 that occur upon geometry relaxation to the minimum on the homo1 
  lumo potential energy surface show that the central c = c 
 bond length increases from 1.381 to 1.461  and that only relatively 
 small changes occur in the torsional and pyramidalization coordinates . 
 in combination with the forces that act on the molecule at the franck condon 
 excited  bright  state ( supplementary 
 figure 3 ) , this suggests that motion along the c = c stretch 
 coordinate needs to be taken into account as well for understanding 
 the decay of the fluorescence . proposed multistate dynamics of the photoinduced 
 unidirectional 
 rotation of the molecular motor starting from conformer i to conformer 
 p * ( projected into one dimension ) : dotted arrow , ultrafast process 
 reported in previous work ; fast ( red arrow ) and slow process ( blue 
 arrow ) observed in the present time - resolved infrared study . ( a ) calculated 
 energy profile along the torsion coordinate showing how the energies 
 of the electronic states change as the molecules evolves to conformer 
 p*. ( b ) schematic representation of the proposed model . hence , it can be concluded that 
 internal conversion to the electronic 
 ground state takes place from the  dark  state . the 
 decay rate for this process indicates that it is mediated by conical 
 intersections between the two states as has indeed been proposed in 
 previous theoretical studies . these studies 
 concluded , however , as well that to reach these intersections a barrier 
 needs to be overcome that results from the steric repulsion of the 
 methyl group of the rotor unit with the hydrogen of the stator unit . previous time - resolved fluorescence measurements reported an ultrafast 
 damping mode at 113 cm that was speculated to 
 arise from motion along a coordinate involving inversion / pyramidalization 
 although no direct support could be found for such an assignment . interestingly , our calculations find in the 
  dark  state a mode at 119 cm ( b3lyp / cc - pvdz ; 
 not scaled , supporting information ) that 
 involves motion of the methyl group with respect to the hydrogen of 
 the stator unit . one might therefore wonder to what extent this mode 
 is related to the oscillatory behavior of the fluorescence . our observed 
 s2 decay constant of 1.5 ps matches the slow component 
 of the overall fluorescence decay ( 0.91.5 ps ) . our calculation of the excitation energies of 
 the  bright  and  dark  states along the 
 119 cm normal mode did not show a conical intersection 
 between the two states , supporting the previous explanation of the 
 oscillations observed in the fluorescence measurements by the movement of wave packets and suggesting 
 a scenario in which a coherent oscillation in the dark s2 state periodically partly repopulates the fluorescent s1 state . however , our experimental methods do not provide the time 
 resolution to confirm this . the photodynamics 
 of a unidirectional molecular motor has been 
 studied by time - resolved ir spectroscopy in which both vibrational 
 and electronic structures are probed . in combination with quantum 
 mechanical calculations , the high structural sensitivity and real - time 
 character of the technique allows us to understand fundamental aspects 
 of the operation mechanism . the results show that conversion of photon 
 energy into directed motion proceeds on the potential energy surfaces 
 of two different electronic states , which is the more important as 
 it implies that the conical intersection between the  dark  
 electronically excited state and the ground state determines the efficiency 
 of the motor . to rationally tune the performance of photoinitiated 
 unidirectional rotation thus involves a multicoupled - state optimization 
 of nuclear motion .
controlling the excited - state properties 
 of light driven molecular 
 machines is crucial to achieving high efficiency and directed functionality . 
 a key challenge in achieving control lies in unravelling the complex 
 photodynamics and especially in identifying the role played by dark 
 states . 
 here we use the structure sensitivity and high time resolution 
 of uv - pump / ir - probe spectroscopy to build a detailed and comprehensive 
 model of the structural evolution of light driven molecular rotors . 
 the photodynamics of these chiral overcrowded alkene derivatives are 
 determined by two close - lying excited electronic states . 
 the potential 
 energy landscape of these  bright  and  dark 
 states gives rise to a broad excited - state electronic absorption band 
 over the entire mid - ir range that is probed for the first time and 
 modeled by quantum mechanical calculations . 
 the transient ir vibrational 
 fingerprints observed in our studies allow for an unambiguous identification 
 of the identity of the  dark  electronic excited state 
 from which the photon s energy is converted into motion , and 
 thereby pave the way for tuning the quantum yield of future molecular 
 rotors based on this structural motif .