various essential biological 
 tasks that involve motion and mechanical 
 work are performed by molecular motor proteins .
the activity of these motors is modulated dynamically through 
 complex biological pathways in response to environmental stimuli .
allosteric regulation plays a key role in biological 
 systems in order to switch between active and inactive states . since 
 allosteric effects involve conformational changes ( cf .
dynamic motion ) , 
 the operating principles of allosteric systems are very important 
 in the design of artificial molecular motors and machines .
furthermore , allosteric control has been applied to various supramolecular 
 systems to inhibit or activate , for example , catalysis and substrate 
 binding .
different successful approaches 
 have been taken to develop synthetic 
 motors and machinery that mimic their biological counterparts . in this respect ,
the use of light as the energy source has proven 
 to be particularly attractive for nanotechnology applications .
however , activity regulation of synthetic nanomotors by multiple 
 external stimuli , as is observed in biological systems , still remains a formidable challenge .
overcrowded 
 alkene - based rotary motors , developed by our group , are 
 among the most promising candidates to perform work at the nanoscale .
they have been applied to control a wide variety 
 of processes , such as self - assembly , substrate 
 binding , wettability , and gel formation . in these 
 motors ,
repetitive unidirectional rotation is accomplished through 
 sequential photochemical and thermal isomerization steps ( scheme 1a ) .
that is , light 
 excitation can induce a double bond isomerization giving a higher 
 energy ( unstable ) isomer .
this isomer 
 then thermally relaxes to the energetically most favored ( stable ) 
 isomer via a helix inversion process . in the latter thermal helix 
 inversion ( thi )
step , the aromatic unit in the upper half of the molecule 
 flips along the lower half .
when these photochemical and thermal isomerization 
 steps are repeated , one - half of the motor completes a full 360 
 rotation with respect to the other half .
the thi process is the rate - determining 
 step in the overall rotary cycle and , hence , is what determines the 
 speed of rotation at infinite photon flux .
please note that the two line 
 drawings for stable l1 , as well as those for unstable l1 , represent identical molecular structures with different 
 viewpoints .
the development of strategies 
 that allow for regulation of the 
 rotational speed of molecular motors is crucial for bringing nanoscale 
 machinery to a higher level of sophistication and complexity . within 
 our group ,
considerable effort has gone into adjusting the frequency 
 of rotation via structural modifications , which required tedious synthetic work .
in addition , we have reported 
 on the supramolecular locking of rotation using an acid  base - responsive 
 self - complexing pseudorotaxane , as well 
 as reversing the direction of rotation by base - catalyzed epimerization .
up until now , however , we have not been able 
 to control the speed of rotation in a dynamic way .
recently , 
 the molecular motor l1 was developed ( see scheme 1a ) , which incorporates a 4,5-diazafluorenyl lower half that 
 can bind metal ions .
our initial studies focused on ruthenium bipyridine 
 complex formation with the goal of shifting the excitation wavelength 
 to the visible region , but interestingly , also an effect on the energy 
 barrier for thi was observed .
reversible metal complexation has been 
 applied in the past to control free rotation around a single bond 
 using a  molecular brake  approach as described by the 
 group of kelly .
we envisioned that metal 
 complexation to l1 could also offer an extremely powerful 
 method to postmodify the rotational behavior of light - driven molecular 
 motors in a reversible manner ( scheme 1b ) .
herein , we describe the synthesis , characterization , 
 and isomerization 
 behavior of different metal dichloride complexes of l1 , that is , zn(cl)2(l1 ) , pd(cl)2(l1 ) , and pt(cl)2(l1 ) .
predictions 
 on the relative rates of rotation can be made using dft optimized 
 molecular structures , which reveal a correlation between the energy 
 barrier for thi and certain structural parameters .
we demonstrate 
 that the steric crowding in the  fjord  region , and 
 hence the rotational speed , can be controlled through metal ion binding . 

thus , the metal ion can act as a kind of allosteric effector , which 
 induces a structural change .
importantly , the metal complexes can 
 be prepared in situ and metal decomplexation occurs 
 upon addition of a competing ligand .
this opens up new opportunities 
 to dynamically modulate the properties of light - driven molecular motors 
 and represents a significant step forward toward higher levels of 
 control in nanomechanical devices .
dft calculations have proven 
 to provide accurate predictions for the structural parameters and 
 the barrier for thi of sterically overcrowded alkene - based molecular 
 motors .
it was previously found that 
 the tpsstpss functional along with the 6 - 31g+(d , p):lanl2dz mixed basis 
 set gives the best approximation of the thermodynamic properties of 
 transition metal complexes .
therefore , this method 
 was chosen together with an iefpcm , ch2cl2 solvation 
 model to calculate the structures and relative free energies ( g ) of the stable and unstable isomers , in addition to the 
 transition states ( ts ) for thi .
the coordination of zinc , palladium , 
 and platinum to 4,5-diazafluorenyl ligands has been studied before 
 and is known to afford stable metal  ligand complexes . for that reason
furthermore , 
 the resulting metal  ligand complexes are diamagnetic so that h nmr can be used as a convenient tool to study the photochemical 
 and thermal isomerization processes ( vide infra ) . in the earlier reported x - ray crystal structures of palladium and 
 platinum 4,5-diazafluoren-9-one dichloride complexes ,
a square planar 
 coordination geometry was observed , whereas 
 the zinc dichloride complex of a structurally related 4,5-diazafluorene 
 derivative revealed a tetrahedral coordination environment .
the input geometries for dft energy minimization 
 were adapted as such , and the obtained molecular structures and relative 
 g values are presented in figure 1 .
plots of the relative 
 gibbs free energies ( 20 c ) between 
 the stable , unstable , and transition state ( ts ) geometries of ( a ) 
 zn(cl)2(l1 ) , ( b ) pd(cl)2(l1 ) , and ( c ) pt(cl)2(l1 ) obtained 
 by dft using the tpsstpss/6 - 31g+(d , p):lanl2dz level of theory . for 

similar to what has been found for uncomplexed l1 , the metal  motor complexes exist 
 in a 
 twisted conformation , which is known to be preferred over anti folding when the lower half of these structures is 
 sufficiently rigid .
furthermore , inherent 
 to this type of molecular motors , the 
 methyl group is oriented pseudoaxially in the thermodynamically most 
 stable form and equatorially in the higher energy unstable isomer . 

the metal - to - nitrogen bond lengths and diazafluorenyl bite angles 
 in the dft optimized complexes of stable l1 closely resemble 
 those found in the crystal structures of related metal 
n bond lengths in 
 the calculated zinc , palladium , and platinum complexes of the stable 
 ligand are about 2.19 , 2.09 , and 2.07  , respectively ( cf . 2.13 , 
 2.08 , and 2.06  in the reported solid state structures of 4,5-diazafluorene 
 complexes ) .
in addition , the observed diazafluorenyl 
 n  m  n bite angles range from 83.2 for zinc and 
 84.0 for palladium to 83.6 for platinum ( cf . 84.8 , 
 83.7 , and 83.6 in the solid state structures ) .
the ts geometries were searched via a dihedral scan over the 
 central 
 olefinic bond ( see the supporting information for details ) . for the palladium and platinum complexes , two different 
 ts structures were found and energy minimized , whereas for the zinc 
 analogue only one ts could be optimized ( see figure 1 ) . the first and lowest energy transition 
 state ( ts1 )
has a planar diazafluorenyl moiety and the c(2)h 
 and c(2)ch3 protons are located on the same side 
 of the lower half ( see scheme 1 for the atom numbering ) .
the second transition state ( ts2 ) , 
 found for pd(cl)2(l1 ) and pt(cl)2(l1 ) , is slightly higher in energy ( 34 kj mol ) and is structurally similar to that calculated earlier 
 for the uncomplexed ligand l1 .
it has a slightly bent diazafluorenyl lower half with c(2)h 
 and c(2)ch3 positioned on opposite sides of this 
 unit .
the most important structural parameters of all the transition 
 states obtained are summarized in table 1 .
see scheme 1 for the atom numbering . from these structural parameters , 
 it can be derived that metal 
 complexation leads to a contraction of the lower diazafluorenyl half 
 of the molecular motor , which is reflected in the shortened n 
the angle 
 enlargement implies a reduction of the steric crowding in the  fjord  
 region .
it has been well - established by our group that a decrease 
 of the steric hindrance in this region lowers the barrier to thi and 
 hence , enhances the speed of rotation of the motor . based on these structural features , it is anticipated that 
 the rotary speed of metal - complexed l1 will increase 
 in the order zn(cl)2(l1 ) <
the gibbs 
 free energy barriers [ g(20 c ) ] for thi , as calculated by dft ( see figure 1 ) , decrease from zinc to palladium 
 and platinum , which is also expected based on the structural features .
the molecular 
 motor l1 was obtained by following a recently reported 
 procedure , and it was used as a racemate .
subsequent stirring with zncl2 in etoh at room temperature 
 afforded zn(cl)2(l1 ) as a precipitate , which 
 was collected by filtration .
likewise , the addition of l1 to a solution of na2[pdcl4 ] in ch3oh / ch2cl2 at room temperature led to precipitation 
 of pd(cl)2(l1 ) , whereas the corresponding 
 platinum complex pt(cl)2(l1 ) was synthesized 
 by heating a solution of l1 with k2[ptcl4 ] in h2o / mecn under reflux .
the photochemical 
 and thermal isomerization behavior of the metal - complexed motors was 
 studied in degassed ch2cl2 by low temperature 
 uv  vis and h nmr spectroscopy . in the uv  vis spectrum ( figure 2 ) , the absorption maxima of zn(cl)2(l1 ) and pd(cl)2(l1 )
are situated 
 around  = 405 nm , whereas pt(cl)2(l1 ) has a maximum absorption at  = 422 nm .
these maxima are 
 all bathochromically shifted relative to the parent uncomplexed l1 ( max = 390 nm ) .
irradiation of the uv  vis samples with 365 nm light at 20 
 c resulted in a further bathochromic shift of the major absorption 
 bands to  = 426 nm for zn(cl)2(l1 ) 
 and pd(cl)2(l1 ) and to  = 431 nm for 
 pt(cl)2(l1 ) ( see figure 2 ) .
uv  vis 
 absorption spectra of ( a ) zn(cl)2(l1 ) , ( b ) 
 pd(cl)2(l1 ) , and ( c ) pt(cl)2(l1 ) in degassed ch2cl2 ( 2  10 m ) before ( solid line ) and after 
 ( dashed line ) irradiation with 365 nm light at 20 c . irradiation was continued until 
 no further changes were observed , 
 meaning that the photostationary state ( pss ) had been reached . during 
 the course of irradiation , clear isosbestic points were observed indicating 
 that the conversion from stable to unstable isomer is a unimolecular 
 process . upon warming of the uv  vis samples to room temperature , 
 the original absorption spectra were obtained in all cases , which 
 is consistent with thermal isomerization from the unstable back to 
 the thermodynamically most stable form . likewise ,
h nmr analysis of solutions in cd2cl2 , irradiated with 365 nm light at 75 c , 
 showed a new set of peaks that can be ascribed to the unstable isomer 
 ( see figures s16s18 in the supporting information ) . by integration of either the c(2)ch3 or c(3)ch signals ,
which are shifted downfield for 
 the photogenerated isomer , the unstable / stable isomer ratios at the 
 pss were determined ( table 2 ) .
pleasingly , comparison of these pss365 ratios 
 with that of the uncomplexed l1 reveals that they do 
 not substantially decrease upon metal complexation . when the h nmr samples were allowed to warm 
 to room temperature , full conversion to the stable isomer
furthermore , upon the repetition of this irradiation / warming cycle , 
 no degradation products could be detected ( figures s16s18 
 in the supporting information ) .
half - life , t1/2 = ln(2)/k. maximum rotation rate ,  
 = 1/2k. the quantum yields for the photoisomerization 
 process were determined 
 by irradiation of concentrated solutions of the stable isomers in 
 ch2cl2 .
the absorption increase at  = 
 450 was monitored over time by uv  vis spectroscopy and was 
 used to calculate the rates of formation of the photogenerated , unstable 
 isomers ( see figures s12s15 in the supporting information ) .
comparison of these rates to the one measured 
 for the formation of fe ions from ferrioxalate under 
 identical conditions afforded the photochemical quantum yields , which 
 are given in table 2 .
these quantum yields range between 3.9% and 6.4% and point out 
 that metal binding has only limited impact on the photoconversion 
 efficiency .
furthermore , the smallest value found for pd(cl)2(l1 ) is in line with the lower pss365 ratio , 
 whereas the decreased pss365 ratio for pt(cl)2(l1 ) may be ascribed to a higher quantum yield for the 
  reverse  reaction ( i.e. , photoconversion from the unstable 
 to the stable isomer ) .
the rates of the thermal isomerization 
 step were determined over 
 a range of temperatures ( 30 to 0 c , see figures s8s11 
 in the supporting information for details ) 
 for each of the metal  motor complexes by following the decrease 
 in absorption at  = 450 nm in the uv  vis spectrum .
application 
 of the eyring equation afforded the thermodynamic parameters ( see 
 figures s8s10 in the supporting information ) of which the gibbs free energies of activation ( g ) are given in table 2 .
the corresponding rate constants ( k ) were obtained by extrapolation to 20 c and used 
 to calculate the half - lives ( t1/2 ) at 
 this temperature .
since the thi step is the rate - determining step 
 in the rotary cycle , the rate constant can also be 
 used to define a maximum rotation rate (  ) at 20 c given 
 infinite photon flux ( table 2 ) . from this data
, it is evident that regulation of the rotational 
 speed is possible through metal complexation to l1 . the experimentally obtained g values for thi compare well with the dft calculated ones ( see figure 1 ) .
n distances , the c(8)c(8)c(9 ) angles 
 (  , see table 1 ) , and the experimentally determined g and k values correlate with each 
 other . in agreement with the predictions based on the dft
optimized 
 molecular structures ( vide supra ) , the speed of rotation 
 increases in the order : l1 < zn(cl)2(l1 ) <
 
 region of l1 and can be used as a convenient tool to 
 regulate the rotational speed .
previous approaches to achieve such 
 changes in the speed of rotation required a new motor design and synthesis .
the metal  ligand coordination strategy 
 presented here offers significant benefits over existing methods .
when the metal complexes 
 were dissolved in a coordinating solvent ( dmso - d6 ) or when a strongly coordinating ligand was added ( pyridine - d5 ) , the h nmr spectrum of uncomplexed l1 was recovered ( see figures s19 and s20 in the supporting information ) .
this observation triggered 
 us to investigate the possibility to switch between a metal associated 
 and dissociated state via the addition of chemical stimuli .
in that 
 way , the rotary speed of the molecular motor could be reversibly tuned in situ , which is important toward the development of more 
 complex molecular motor and machine - like systems .
the in situ metal complex formation and metal decomplexation 
 were studied by h nmr and uv  vis spectroscopy . 

zinc and palladium were used because the corresponding metal  motor 
 complexes are formed at room temperature .
addition of zncl2 or na2[pdcl4 ] to ligand l1 dissolved 
 in cd2cl2/cd3od afforded h nmr spectra consistent with those of the previously isolated zn(cl)2(l1 ) and pd(cl)2(l1 ) 
 complexes ( figure 3 ) .
subsequent addition of pyridine - d5 induced metal decomplexation as evidenced by the regeneration of 
 the h nmr spectrum of l1 ( see figures s21 
 and s22 in the supporting information ) .
( a ) h nmr spectrum of l1 ( 2  10 m solution in cd2cl2/cd3od ) and h nmr spectrum after ( b ) the addition 
 of either zncl2 to the solution of l1 , which 
 generates zn(cl)2(l1 ) , or ( c ) the addition 
 of na2[pdcl4 ] to l1 , which generates 
 pd(cl)2(l1 ) . during these h nmr experiments significant color 
 changes 
 were observed upon addition of the metal salts , which prompted us 
 to additionally monitor this in situ reversible complexation 
 by uv  vis spectroscopy .
adding either zncl2 or na2[pdcl4 ] to a solution of l1 in ch2cl2/ch3oh resulted in the bathochromic 
 shifting of the absorption maxima , which is consistent with formation 
 of the metal complex ( figure 4 ) .
again , metal dissociation was successfully achieved through 
 the addition of pyridine as is clear from the hypsochromic shift back 
 to the original absorption maximum . in the case of zinc
, these metal 
 complexation and decomplexation steps were repeated multiple times 
 ( see figure s23 in the supporting information ) .
hence , these combined h nmr and uv  vis experiments 
 reveal that dynamic regulation of the rotation rate is feasible via 
 addition of chemical stimuli .
uv  vis spectrum of l1 ( 2 
  10 m in ch2cl2/ch3oh , 40:1 , solid 
 line ) upon addition of either ( a ) zncl2 or ( b ) na2[pdcl4 ] ( dashed line ) followed by the addition of pyridine 
 ( dotted line ) .
in 
 conclusion , we have presented the synthesis and characterization 
 of zinc , palladium , and platinum dichloride complexes of molecular 
 motor l1 .
the photochemical and thermal isomerization 
 behavior of these metal  motor complexes has been studied in 
 detail .
interestingly , the speed of rotation of molecular motor l1 increases through metal complexation in the order zn(cl)2(l1 ) < pd(cl)2(l1 ) 
 < pt(cl)2(l1 ) , which is in agreement with 
 predictions based on dft geometry optimization .
binding of metal ions 
 to l1 leads to a contraction of the 4,5-diazafluorenyl 
 lower half and the simultaneous reduction of the steric crowding in 
 the  fjord  region .
furthermore , the metal  motor complexes 
 can be generated in situ , and the addition of a competitive 
 ligand causes full metal decomplexation .
consequently , the rotational 
 behavior of these motors can be tuned reversibly in a highly practical 
 and versatile manner with a level of control that is unprecedented . 

therefore , this work represents a critical step forward to integrated 
 chemical regulation in molecular machines that are operated by light . 

other applications are foreseen in photoswitchable transition metal 
 catalysis , surface anchoring , and molecular sensing .
 the 
 rotational speed of an overcrowded alkene - based molecular rotary 
 motor , having an integrated 4,5-diazafluorenyl coordination motif , 
 can be regulated allosterically via the binding of metal ions . 
 dft 
 calculations have been used to predict the relative speed of rotation 
 of three different ( i.e. , zinc , palladium , and platinum ) metal dichloride 
 complexes . 
 the photochemical and thermal isomerization behavior of 
 these complexes has been studied in detail using uv  vis and 1h nmr spectroscopy . 
 our results confirm that metal coordination 
 induces a contraction of the diazafluorenyl lower half , resulting 
 in a reduction of the steric hindrance in the  fjord  
 
 region of the molecule , which causes an increase of the rotational 
 speed . 
 importantly , metal complexation can be accomplished in situ and is found to be reversible upon the addition 
 of a competing ligand . 
 consequently , the rotational behavior of these 
 molecular motors can be dynamically controlled with chemical additives . 
Introduction
Results and Discussion
Conclusions