a single molecule that 
 functions as a molecular conductance switch 
 under external stimuli can be useful for molecular memory and logic 
 devices .
a broad range of molecular switches have 
 been reported in the literature , activated with different kinds 
 of external stimuli .
light is a very attractive stimulus due to its 
 fast response time and compatibility with already existing experimental 
 setups .
the existing switches can roughly 
 be divided into two categories : redox / polarization switches , where the molecule ( or part of it ) takes up or loses an electron , 
 or isomerization switches , where the 3d - structure ( geometry and/or connectivity ) of the molecule 
 is changed .
the basic requirements for a molecular reaction to be 
 suitable for switching applications are : ( i ) it must have high difference 
 in the magnitude of the conductance for the on and off states , i.e. , 
 high switching ratio ( sr ) , ( ii ) it must be stable , and ( iii ) it needs 
 a sufficiently high barrier between the on and the off states to hinder 
 accidental ( de)activation .
isomerization 
 switches should ideally have the on state represented 
 by a planar compound with a linearly conjugated path with maximal 
 p-orbital overlap .
the compound representing the off state , 
 on the other hand , should have a disruption in its p-conjugation , 
 and such a disruption could be accomplished by a saturated molecular 
 segment ( structural isomerization ) or by a large 
 twist in the conjugated path ( stereoisomerization ) . at the same time , for a possible use in a solid state device the 
 length of the molecule should stay essentially the same which has 
 been a major drawback for some of the previously studied molecular 
 photoswitches , i.e. , azobenzenes .
further , it should be 
 possible to switch the molecule between the two states with an external 
 stimulus , and most interesting is the possibility to switch in one 
 direction with one stimulus and in the other direction with another 
 stimulus . the -silyl shift for formation of a si = c 
 double bonded 
 compound ,
a so - called silene , from an acyloligosilane is a structural isomerization that potentially 
 could be used for a novel type of molecular switch .
this reaction 
 was used by brook and co - workers to form the first isolable silene 
 in 1981 ( scheme 1 ) .
importantly , the acyloligosilanes such as 1a - off and 1b - off were turned into the brook - silenes photolytically through 
 irradiation by mercury spot lamps .
however , it was also observed that 
 the silenes rearranged back to the acyloligosilanes ( e.g. , silene 1b - on has a half - life of 15.8 h at 25 c in ether solution ) .
silene 1a - on , on the other hand , 
 could be observed spectroscopically ( ir , uv , nmr ) over a period of 
 two weeks at room temperature , during which it gradually rearranged 
 back to 1a - off . with regard to 1a - off/1a - on
, we report herein ( vide infra ) the calculated reaction energy for 1a - on formation to be 51.2 kj / mol , and the activation energy 
 for the thermal back - rearrangement from the silene to silane to be 
 82.1 kj / mol ( scheme 1 ) . in a recent computational 
 study ,
eklf , guliashvili , and ottosson found silene 1c - on to be 20.1 kj / mol higher in energy then 1c - off , and an activation barrier of 123.4 kj / mol separated the silene 
 from the acyloligosilane .
this property 
 of the acyloligosilane / brook - silene system should be possible to elaborate 
 further and potentially exploit in a molecular conductance switch . 

the described activation energies for thermal back - rearrangements 
 from the silene to the silane to progress at moderately elevated temperatures 
 should be in the range 90120 kj / mol , and the reaction energies 
 for this process should be moderately exothermic .
reaction and activation free 
 energies for the thermal processes of 1a computed at 
 the b3lyp/6 - 31g(d ) level as reported herein ( vide infra ) . transition 
 state abbreviated as ts .
our earlier computational 
 study showed a distinct difference in 
 the relative energies between the brook - type 1,1-dimethylsilenes as 
 compared to the analogous brook - type 1,1-bis(trimethylsilyl)silenes 
 where the former have higher relative energies by 4060 kj / mol 
 compared to the latter species .
this 
 feature is interesting as an acyloligosilane / brook - silene switch should 
 have the silene ( the on state ) slightly higher in energy than the 
 acyloligosilane ( the off state ) so that a driving force for the thermal 
 back - rearrangement is established .
consequently , the relative energy 
 of silene versus acyloligosilane can be tuned by a proper choice of 
 substituents at the central si .
moreover , a change from a migrating 
 silyl group to a migrating germyl ( stannyl ) group weakens the o  e 
 bond strength ( eo  e = si > ge > sn ) , raising the energy of the silene ( on ) side . 

increased 
 steric bulk of the migrating group should also disfavor the acylsilane 
 ( off ) side as recently found by bravo - zhivotovskii et al .
finally , one can expect that increased conjugation , 
 as found in the silenes studied herein , will lower the relative energies 
 of these species .
our initial model species for the acylsilane / brook - silene 
 switches , together with their carbon analogues , are displayed in scheme 2 .
these model silenes have a linearly conjugated 
 path stretching between the two ends of the molecule , whereas the 
 acyloligosilanes have an interruption .
we first examine the acyloligosilane / brook - silenes 
 in their trans - conformers with thiol end groups since 
 those ( still ) are ubiquitous anchors to gold electrodes , even though 
 a range of other groups with improved anchoring properties recently 
 have been identified .
thus , 
 we report on a new class of constitutional molecular switches 
 based on the -silyl shift going from a silane to a silene .
we 
 start by the linear acyloligosilane / brook - silene molecules where we 
 calculate the reaction ( rxn ) and activation energies as well as the 
 transport properties using density functional theory ( dft ) and nonequilibrium 
 green s functions ( negf ) .
however , based on our work on these 
 linear switches , we found that it is important to constrain the motion 
 when the system progresses along the reaction coordinate over the 
 transition state ( ts ) . consequently , these first candidates needed 
 to be further modified to perform optimally as molecular conductance 
 switches in solid state devices .
the second half of the study is therefore 
 devoted to computational design of a set of more realistic molecular 
 switch candidates .
reaction and activation energies were computed 
 for the new cyclic silane / silene switches , where we also find constrained 
 motions .
furthermore , transport investigations reveal high switching 
 ratios . using a single - level transport model , we estimate the location 
 of the most dominating level for electron transport ( e0 ) and the broadening of the level (  ) and their 
 influence on the switching ratio .
full geometry optimizations without symmetry constraints were carried 
 out with the gaussian09 program package .
all geometry optimizations reported were performed with the b3lyp 
 hybrid density functional theory ( dft ) method in combination with 
 the 6 - 31g(d ) valence double- basis set .
frequencies 
 were computed to confirm that the calculated structures correspond 
 to local minima or transition states on the potential energy surface .
the b3lyp hybrid dft method was chosen as we earlier have found 
 that the deviations in the geometries of a set of 15 small substituted 
 silenes in a total of 27 different conformers , including silenes with 
 natural ( si = c ) 
 as well as reverse ( si = c ) bond polarization , when compared against ccsd / cc - pvtz geometries , 
 are smallest for hybrid dft methods .
neither 
 mp2 nor generalized gradient approximation ( gga ) dft methods , as computationally 
 inexpensive methods , lead to better agreements .
the m06 - 2x meta - hybrid 
 gga dft method was found to give a slightly 
 better mean absolute deviation but , on the other hand , gave spurious 
 geometry deviations for a few of the test silenes when compared against 
 the ccsd geometries .
the effect of the basis set on the calculated geometries 
 was also examined on the test set of small silenes mentioned above , 
 and it was revealed that the mean absolute deviation , as compared 
 to ccsd / cc - pvtz , was 0.017 and 0.019  with regard to the si = c 
 double bond lengths calculated at b3lyp / cc - pvtz and b3lyp/6 - 31g(d ) , 
 respectively . for the 15-off/15-on and 17-off/17-on silane / silene switches , we also 
 examined the quality of the b3lyp/6 - 31g(d ) calculations with regard 
 to basis set and with regard to treatment of sterically congested 
 situations ( dispersion ) .
b3lyp calculations with the 6 - 31g(d ) valence 
 double- basis set were compared against b3lyp calculations 
 with the 6 - 311+g(d , p ) valence triple- basis set , and both reaction and activation energies when 
 calculated at the b3lyp/6 - 311+g(d , p)//b3lyp/6 - 31g(d ) level were found 
 to agree well ( see supporting information ) . when the b3lyp/6 - 31g(d ) energies are compared against calculations 
 with the m06 - 2x hybrid meta gga functional , i.e. , a dispersion corrected 
 functional which is better able to describe sterically congested situations , 
 it was also found that the deviations in energies were modest ( see supporting information ) .
time - dependent 
 dft ( td - dft ) calculation were carried out with b3lyp 
 in combination with a 6 - 311+g(2d , p ) basis set , considering ten transitions 
 each ( singlets and triplets ) .
transport calculations were carried 
 out from first principles with 
 a method based on nonequilibrium green s functions ( negf ) combined 
 with dft as implemented in the transiesta package .
the relaxed molecular structures were inserted as their 
 dithiolates between two au surfaces and relaxed once more to 
 optimize the au 
the device consists of three parts : 
 left electrode , molecule , and right electrode .
the electrodes are 
 modeled by six layers of gold atoms where the three outer layers are 
 relaxed , while the others are kept at the experimental bulk positions . 
 in the lateral dimension
a 6  6 supercell ( 17.5   
 17.5  ) was used , large enough to remove interactions between 
 periodic images .
all relaxations are performed at the dft level with 
 the siesta package , and core electrons are modeled 
 using troullier - martins soft norm - conserving 
 pseudopotentials .
the valence electrons are expanded in a basis set 
 of local orbitals using a double- plus polarization orbital 
 ( dzp ) set for electrons in the molecule and a single- plus 
 polarization orbital ( szp ) for electrons in the gold electrode .
the off states of switches 2a , 2b , 3a , and 3b were optimized 
 in the anti - conformers , and the on states were optimized 
 as e - isomers to match the expected structures of 
 a rigidly confined acyloligosilane / brook - silene 
 switch . for these species , it becomes clear that for the two acyloligosilane / brook - silene 
 pairs the acyloligosilane and the silene side are nearly isoenergetic 
 ( see table 1 ) , while for the all - carbon -silyl 
 ketone / siloxyalkene pair the alkene is much lower in energy .
the very 
 large difference in relative energy for the two sides of the all - carbon 
 pairs can not be changed significantly through substitution , and our 
 calculations thereby reveal why the -silyl shift to form an alkene , 
 and the reverse reaction , are not suitable to function as a reversible 
 molecular switch .
the -shift of a silyl group from the -carbon 
 of a ketone to the carbonyl oxygen is thus a purely hypothetical molecular 
 switching reaction .
however , with a suitable tailoring of the acyloligosilane / brook - silene 
 switch pair , it should be possible to design a system with the silene 
 slightly above the acyloligosilane in energy and which has an activation 
 energy for the thermal back - reaction which is reasonably high ( 90120 
 kj / mol ) so that moderately elevated temperatures can be used to turn 
 off the switch .
the activation energy for the -silyl shift from 1a - off to 1a - on is calculated to be 133.3 kj / mol 
 at the b3lyp/6 - 31g(d ) level , but the activation energy for the back - reaction 
 taken relative to 1a - on is only 82.1 kj / mol . as this 
 silene gradually rearranged back to the silane during a period of 
 two weeks , it is obvious that the barrier for a silane / silene switch 
 needs to be slightly higher .
free energy 
 of activation relative 
 to the off - state ( normal print ) [ kj / mol ] .
free energy of activation relative 
 to the on - state ( in italics ) [ kj / mol ] . restricted ss 
 distance .
importantly , the 
 ss distances within each of 
 the four pairs stay essentially constant when going from the off structure 
 to the on structure ; however , investigations of the transition states 
 revealed that the migration of the silyl group requires a significant 
 rotation around the central si 
this rotation induces 
 a large motion of the two thiol anchors during the ( thermal ) rearrangement 
 and consequently a significant shortening of the ss 
 distance to 9.57  for 2b - ts , i.e. , a reduction 
 by approximately 4  .
attempts to mimic the rather fixed distance 
 situation , as would be seen in a solid - state setup , by virtually fixing 
 the ss distance to 13.54  resulted in a large 
 increase in the activation energy from 115.7 to 190.0 kj / mol of 2b - ts , a prohibitively high activation barrier for thermal 
 back - rearrangement at reasonably elevated temperatures .
brook 
 and co - workers used acyloligosilane 1a - off ( scheme 1 ) to form silene 1a - on , which has uv / vis 
 absorption maxima at 348 nm (  = 100 ) and 339 nm (  = 
 5200 ) , respectively , showing good agreement with td - dft calculated 
 values of 362 nm ( oscillator strength f = 0.0118 ) 
 for 1a - off and 351 nm ( f = 0.2165 ) for 1a - on .
the presently investigated acyloligosilanes 2a - off and 2b - off have their computed lowest uv / vis absorptions 
 with significant calculated oscillator strengths at 395.7 nm ( f = 0.1194 ) and 381.3 nm ( f = 0.0352 ) , 
 respectively ( see supporting information ) .
from this we can conclude that the present silane / silene pairs 
 could form the basis for a light - driven molecular switch under the 
 condition that the motion along the reaction path is constrained . to estimate the switching properties of the acylsilane to silene 
 transition
, we performed first - principles dft calculations for the 
 acetylenic compounds ( 2a/3a ) shown in scheme 2 .
compounds ( 2b/3b ) showed a different 
 behavior in the electrode setup which is briefly discussed in the supporting information .
the current response 
 for bias voltages of 0.1 , 0.2 , 0.5 , and 1.0 
 v are shown in figure 1 .
looking at trends 
 in the current response , both compounds 2/3a - off and 2/3a - on show a similar behavior with a linear increase of 
 the current for low voltages and more rapidly increasing current at 
 higher voltages . to investigate their performance as switches , we 
 calculate the on / off switching ratio of the current for both the zero - bias 
 limit as well as for a bias of + 1 v. due to the unsymmetric nature 
 of these molecules , we also investigate if they have a rectifying 
 property on the current , hence if they could function as diodes .
the on / off ratio is higher for compound 2a compared 
 to the all - carbon switch 3a . from figure 1
it can be seen that this is due to a more conducting off 
 state for 3a , while the current response is very similar 
 for the two on states .
when going to higher biases , the switching 
 ratio decreases due to the stronger current response from the low - conducting 
 acyloligosilanes and ketones when the bias is increased .
calculated current  voltage ( i  v ) characteristics for off and on state structures of compounds 2a and 3a , respectively .
note the different scales 
 for the calculated currents in the on and off states .
rectifying ratio , calculated as 
 the ratio between the currents at bias voltages of + 1 and 1 
 v. to explore the physical 
 mechanisms behind the switching , we start 
 by calculating the partial density of states ( pdos ) for the junctions 
 with 2a under equilibrium conditions . the density of 
 states spatial distribution can be calculated 
 by summing up the states in the transverse direction and normalizing 
 to the atomic density .
this gives the number of states/(ev  
   atom ) along the transport direction which is shown in 
 figure 2 , where the x - axis 
 is the coordinate along the transport direction and the y - axis shows the energy ( e  ef ) for the state , where the magnitude of the dos is indicated 
 by the color , from blue ( low ) to red ( high ) .
space resolved normalized 
 density of states at zero bias for the 
 junctions with compound 2a .
the off state is shown in 
 the top panel and the on state in the bottom panel .
the space resolved 
 density of states have the transport coordinate on the x - axis and the energy of the state on the y - axis . 

the color scale goes from blue ( low density of states ) to red ( high 
 density of states ) . for 2a - off the homo of the molecule
is located about 
 0.5 ev below the fermi level ( ef ) of the 
 electrodes and is not so well coupled to the electrodes .
the electronic 
 states in the molecule that are induced by the interaction with the 
 metallic electrodes are often referred to as metal - induced gap states 
 ( migs ) .
migs can be seen in the spatial 
 dos plot as light - blue protrusions penetrating into the molecule from 
 both electrodes but rapidly decaying toward the center of the switch .
the -silyl shift leads to the formation of 2a - on ( 3a - on ) having a si = c ( c = c ) double bond . 

the dos spatial distribution for 2a - on ( bottom panel ) 
 shows that the homo of the molecule is shifted closer to ef , and at the same time it is delocalized over the molecule 
 and coupled to both electrodes .
coupling of the homo to the electron states in 
 the electrodes causes broadening of the dos peak .
we see two light 
 blue  satellites  of the homo slightly above and below 
 in energy in the dos spatial distribution plot created by the extended 
 migs overlapping with the broad homo peak .
the broadening and delocalization 
 of the homo and surrounding migs create a relatively smooth dos close 
 to the fermi level .
this density is responsible for the absence of 
 current onset around the position of the homo ( 0.2 or 0.4 
 v bias ) , when increasing the bias voltage for 2a - on ( 3a - on ) .
furthermore , we show the wave function of the 
 most conducting scattering 
 state at zero bias in figure 3 .
the isosurface of the scattering state has different 
 colors depending on the phase ( + 1 , 1 , + i , 
 i ) as explained in the figure caption .
the 
 magnitudes of the isosurface lobes indicate where the electrons incoming 
 from the left electrode are traveling since the absolute square of 
 the wave function corresponds to the density of traversing electrons . 
 for a wave , incoming from the left and which is mostly reflected
, 
 the real part will show an exponential decay into the junction from 
 the left , while the imaginary part will be very small and in most 
 cases not visible ( if the same isovalue is used when plotting ) .
as 
 in the case of dos spatial distribution for 2a - off , the 
 scattering state ( top panel ) shows a very weak coupling through the 
 central si  c bond , where the real part of the state decays 
 rapidly from the left into the junction indicating that in this case 
 we have a poor conductor acting as a tunneling barrier .
the imaginary 
 part of the wave function is not visible due to the low transmission 
 of the channel and that the wave function is chosen to be real at 
 the incoming side .
visualization of the most transmitting eigenchannel wave 
 function 
 ( incoming from the left electrode ) at the fermi energy for the junctions 
 with compound 2a . off state shown in the top panel and 
 on state in the bottom panel .
the isosurfaces of the eigenchannel 
 wave function are colored according to phase and sign , the positive / negative 
 real part being colored in deep - purple / light - blue ( the imaginary part 
 is below the cutoff value and not visible ) , and both junctions plotted 
 with the same isovalue to be comparable .
from the scattering state of 2a - on ( bottom panel ) 
 the si = c double bond formed from the -silyl shift shows 
 a good connection through the central bridge providing a higher conductance 
 through the migs at the fermi level .
it is still almost only the real 
 part of the wave function that is visible , but it does not decay as 
 fast into the junction . from the shape of the lobes we can see that 
 the main transport goes through the p-orbital system of the 
 molecule .
similar conclusions can be drawn for 3a ; i.e. , 
 for the on state we have a very good linear conjugation through the 
 complete molecule , while for the off state the conjugation is broken 
 in the central unit due to the si
 c ( c  c ) single bond ; 
 hence , the transmission drops significantly . even though 
 the studies of reaction thermodynamics as well as electron transport 
 of 2a , b reveal that these compounds exhibit 
 the desired properties , the large structural change along the reaction 
 pathway led us to a redesign .
in particular , the twisting about the 
 central sic bond that takes place during the -silyl shift , leading 
 to drastic variations in the ss distance during 
 the reaction , needs to be reduced , and a silane / silene unit with less 
 conformational flexibility is required .
this can be accomplished by 
 introduction of a fused cyclic system that would prevent large motions 
 leading to / from the ts . however , this restricted system must not have 
 a raised energy of the ts state as this would hamper a thermal back - reaction . 
 to provide a cyclic system with an overall p-conjugated pathway 
 in the on state
, we designed a five - membered ring with a central sic 
 unit having a si = c double bond and two neighboring exocyclic 
 c = c double bonds . to accommodate for steric congestion , either 
 a benzene or a phenylacetylene group
is attached to the double bonds 
 in an all - trans arrangement . to provide suitable 
 anchors
, we kept the thiol groups in para - positions 
 of the phenyl moieties .
our initial studies on the linear silane / silene 
 systems indicate approximately the desired thermodynamic and kinetic 
 properties for a light - triggered on switch with a back - reaction which 
 is thermally feasible .
one advantage of this rearrangement is the 
 possibility of tuning both thermodynamics and kinetics by slight variations . 
 for our further investigations on the cyclic systems
, we focused on 
 three parameters : the nature of the migrating group ( silyl or germyl ) , 
 the steric bulk of the migrating group ( eme3 or ebu2me , where e = si or ge ) , and the acceptor group ( a 
 = o or = nme ) ( figure 4 ) . suggested cyclic molecular 
 conductance switches with different 
 acceptor moieties and migrating groups .
a = nme or o , and r = sime3 , sibu2me , geme3 , or gebu2me .
blue bars denote n acceptor groups ( a = nme ) , while red 
 bars denote the oxygen acceptors ( a = o ) .
simple variation of these three parameters gives a tremendous 
 impact 
 on the reaction thermodynamics ( table 3 ) .
while 
 the silyl migrating groups in combination with an oxygen acceptor 
 favor the on state ( from 1.1 to 52.5 kj / mol ) , migration to the nitrogen - based 
 acceptor ( an imine in the off state ) is strongly dependent on the 
 steric bulk of the silyl group ( figure 5 ) . 

changing the 
 central atom of the migrating group from silicon to germanium favors 
 the off state by up to 76 kj / mol , depending on the steric demand of 
 the alkyl substituents and the acceptor group .
however , a general 
 trend for the two different acceptor groups was not found .
we further 
 investigated the transition states for the thermal rearrangement of 
 the cyclic silane / silene combinations with the most promising thermodynamic 
 properties .
introduction of the five - membered ring results in a reduced 
 ts motion and , moreover , provides suitable kinetic parameters .
the 
 activation energies relative to the off states are 126160 
 kj / mol .
the on states are in most cases higher in relative energy , 
 which will allow for a thermal back - rearrangement at ( slightly ) elevated 
 temperatures as the activation free energies for all except one system 
 are gathered in the range 108144 kj / mol .
the drawbacks of 
 the linear systems , i.e. , the large motion leading to / from the ts , 
 were also successfully circumvented by the cyclic silane / silene switches . 

the largest absolute displacements were calculated to be in the range 
 0.22.0  , which corresponds to relative motions of less 
 than 10% of the absolute ss distance .
we assume 
 that these small changes can easily be balanced by lateral movement 
 on the gold substrates , without significant changes in reaction energies .
free energy 
 of activation relative 
 to the off state ( normal print ) [ kj / mol ] .
free energy of activation relative 
 to the on state ( in italics ) [ kj / mol ] . to prove the viability of photochemically 
 induced silene formation 
 we also performed td - dft calculations for the extended cyclic systems 16 and 17 in their on and off configurations . 
 for compound 16-off ,
the first allowed uv / vis transition 
 should occur at around 416 nm ( f = 0.0424 ) with a 
 stronger absorption at 404 nm ( f = 0.2193 ) . similarly , 
 but slightly red - shifted , we found for 17-off a weaker 
 ( 417 nm , f = 0.0342 ) and a stronger ( 407 nm , f = 0.1301 ) transition .
the extension of the p-system 
 in the corresponding silenes , 16-on and 17-on , is also clearly reflected in the calculated uv / vis spectra of compounds 16-on ( 556 nm , f = 1.1302 ) and 17-on ( 544 nm , f = 0.9959 ) as shown by their longest 
 wavelength absorptions .
these long - wavelength absorptions for the 
 on states potentially allow the suggested switches to operate in a 
 reversed manner , in which the on states might rearrange to the corresponding 
 silanes by irradiation with visible light .
however , 16-on and 17-on also absorb at the shorter wavelengths , and 
 this feature may potentially complicate the formation of the silenes , 
 which when formed can absorb the irradiation used for their generation 
 and photorearrange back to the silanes . yet , detailed computational 
 and experimental studies and optimization of the photochemistry of 
 the cyclic silane / silene switches are outside the scope of the present 
 investigation .
for example , for an optimal silane / silene switch the 
 position of the photostationary state needs to be optimized so that 
 predominantly the silene is formed upon irradiation at the chosen 
 wavelength .
the current  voltage characteristics 
 have been examined by 
 dft - negf calculations , and due to computational cost we have chosen 
 to study the compounds 4 , 12 ( o - acceptor 
 and trimethylsilyl migrating group ) , 8 , 16 ( nme - acceptor and trimethylsilyl migrating group ) , and 17 ( nme - acceptor and simebu2 migrating group ) 
 here .
the current  voltage results and their switching ratios 
 are shown in figure 6 and table 4 , respectively .
compared to the previous linear compound , 
 we find larger switching ratios at zero bias . by introducing the cyclic 
 unit interference effects , similar to those observed by solomon et 
 al . in phenyl rings ,
the interference effects can clearly be seen in the transmission 
 spectra where there are several pronounced dips for the off compounds 
 with the phenylacetylene group ( cc  ph ) shown in the 
 top panel of figure 7 . for
the shorter compounds 
 ( 4 , 8) transmission in the -system 
 of the molecules can now start to play an important role , and this 
 can hide interference effects in the p-system . in the top ( bottom ) 

panels of figure 8 the local currents in compound 16 are shown for the off ( on ) state .
for 
 the off state we find that the interference arises due to ring currents 
 in the ring segment with three saturated carbon atoms . on the other 
 hand , for the on state we observe an undisturbed current path which 
 follows the p-conjugated path through the molecule .
conductance on / off switching ratio . rectifying ratio , calculated as 
 the ratio between the current at bias voltage + 1 and 1 v. distance between the most conducting 
 level and ef ( v = 0 ) .
proportionally factor for movement 
 of the conducting level under bias ( 0 = e0  ( v/ ) for  v , + = r/l ) . a measure of 
 the unsymmetry of the 
 effective coupling to the electrodes .
markers denote values obtained from the dft - negf calculations , while 
 the full lines are obtained from the model calculations .
note the 
 different scales for the calculated currents in the on and off states .
transmission spectra , at zero bias ( v = 0 ) , for 
 the off compounds ( top panel ) and the on compounds ( bottom panel ) .
local currents in compound 16-off ( top panel ) and 16-on ( bottom panel ) , where the cross - sectional 
 area of the 
 cylinder is proportional to the current density .
red currents represent 
 positive transport direction , and blue currents represent negative 
 direction .
the currents are calculated at the fermi energy ( ( l + r)/2 ) , but a small positive bias voltage 
 is assumed for calculation purposes . for a full derivation see okabayashi 
 et al . and paulsson and brandbyge .
at higher bias , the switching ratio drops to moderate levels 
 for 
 the cyclic compounds similar to the linear analogues . as in the case 
 of the reaction energies we see that the nme acceptor groups ( compounds 8 , 16 , 17 ) are the most promising 
 candidates with switching ratios on par or higher compared to other 
 compounds studied here and in previous investigations .
introduction of the bulkier migration group ( simebu2 , compound 17 ) not only will provide higher steric 
 protection and hence higher kinetic stability but also enhances the 
 switching ratio .
the on compounds have rectification ratios ( rr ) close 
 to unity , which is beneficial for practical applications since there 
 will be no directional dependence when running a current through the 
 switch . on the other hand ,
some of the off compounds have large rr 
 ( especially compounds with o - acceptor , 4 and 12 in table 4 ) , opening the possibility to use 
 them as diodes . for switch applications , the rr of the off compound 
 is not important since there should be no ( or only a negligible ) current 
 running through them , but it will give a difference in sr between 
 + 1 v and 1 v ( here , for rr > 1 , the sr at 1 v will 
 be larger than the sr at 1 v ) .
we apply a simple single - level 
 transport model to investigate the influence of the effective coupling 
 ( l,r ) to the left and the right 
 electrode , and the distance between the closest molecular orbital 
 ( e0 ) and ef of the electrodes on the current . from the landauer formula
the transmission is obtained as1 we started 
 by considering a static level and symmetric coupling 
 to the electrodes as the earlier literature suggested .
, we had to expand the model with 
 a level that could move with the potential level in the electrodes 
 under bias ( 0 = e0  ( v/ ) 
 for  v , see table 4 for 
 values of  ) and nonsymmetric coupling to the electrodes .
the 
 parameters obtained from the model are given in table 4 , showing that the unsymmetry of the coupling ( l/r ) we find from our model agree reasonable 
 well with the rr of the molecules .
the results from the model are 
 shown in figure 6 ( full lines ) , together with 
 the dft - negf results ( markers ) .
as can be seen from the figure , this 
 extended model can explain our results reasonably well . from this 
 model we can locate the most conducting mo at about 0.5 ev below ef for the off states and almost resonant ( 0.050.15 
 ev below ef ) for the on states .
this can 
 be compared with the transmission spectra at zero bias ( shown in figure 7 ) from the dft - negf calculations , and we find that 
 the peaks in the spectra are located at similar values as obtained 
 from the model ( a comparison between transmission spectra at different 
 biases from dft - negf and our model is shown in the supporting information ) . from the model results we are able 
 to deduce the important parameters affecting the sr . in figure 9
the magnitude of the sr ( symbolized by the size 
 of the filled circles ) is shown as a function of the change in coupling 
 to the electrodes (  = ( lr ) ) and the change in distance between the closest mo 
 and ef . the switching occurs due to either 
 a change of the distance between the closest mo and ef , a change in the coupling between the mo and the electrodes , 
 or a combination of both . to have a large sr
ideally the molecule 
 should be in the top right corner of figure 9 , hence it is favorable to have both a large change in distance between ef and the closest mo and a large change in the 
 coupling between the electrodes and the mo .
magnitudes of the switching 
 ratio ( sr ) as a function of the change 
 of the closest mo ( e0 ) and the 
 coupling between the mo and the electrodes (  ) .
the behavior of the compounds 
 studied here is different compared 
 to the photoswitches studied previously by kim et al . for which they 
 obtained switching due to the change either in coupling to the electrodes or in the relative position of the mo . by the possibility to change both the coupling 
 and the relative position of the mo , as in the case for compounds 4 and 17 , we can obtain a higher switching ratio 
 compared to previous studied molecular photoswitches . at the same 
 time
thus , they should represent good candidates of a 
 new set of molecular conductance switches . as pointed out by our criteria 
 to a molecular switch
, stability plays an important role , which is 
 especially relevant to the suggested silane / silene based systems . 

it is known that silanes ( off states ) are persistent species at ambient 
 conditions and could easily be inserted between two gold electrodes 
 prior to switching applications
. however , the silenes ( on states ) 
 are reactive , moisture sensitive , and have a high dimerization aptitude .
the situation can be significantly changed when 
 the silanes are immobilized between the gold electrodes , and provided 
 the concentration of silanes on the surface is low , the resulting 
 spatial separation between the silenes when formed during switching 
 will inhibit dimerization as a possible decomposition pathway
. moreover , 
 the environment on the gold surface around the silane / silene switch 
 can be covered by unreactive alkanethiols or alkanedithiols , e.g. , 
 as recently developed by us in a au nanoparticle / nanoelectrode setup .
the dimerization 
 aptitude of the silenes may also be reduced through a range of bulky 
 substituents .
the moisture sensitivity , on the other hand , is delicate , 
 and the acylsilane / brook - silene must likely operate under moisture - free 
 conditions .
however , reverse polarization , i.e. , as observed for oxygen- 
 or amino - substituted systems , reduces 
 moisture sensitivity of these silenes and might significantly increase 
 the stability of the suggested molecular switches .
for example , we 
 found recently that 2-amino-2-siloxysilenes , which are transient silenes 
 extensively influenced by reverse si = c bond polarity ( si = c ) , are unreactive 
 to alcohols .
it has also been observed 
 that 1-silaallenes , i.e. , si = c double bonded compounds with 
 an inherent si = c polarization influence , become resistant to water when they bear 
 substituents with large steric bulk .
in conclusion , we have introduced 
 a new class of synthetically 
 realistic compounds that can perform as molecular conductance switches 
 which could be triggered by light and/or thermal energy .
moreover , 
 we were able to confirm that these switches fulfill the criteria for 
 a useful reversible molecular switch , as given in the introduction , i.e. , a high on / off conductance ratio , high 
 enough barrier separating the two states , and a possibility to realize 
 the switch in a solid state device .
the reaction investigated is the 
 -silyl ( germyl ) shift from a low - conducting silane to a conducting 
 silene , and the charge transport characteristics are investigated 
 using dft - negf calculations and a simple landauer model .
we started 
 with a set of linear molecules and found by dft - calculations that 
 they can be switched photochemically in one way and switched back 
 thermally .
the single - molecule conductance showed good switching ratios 
 at low bias ; however , it was also found that large length variations 
 when progressing along the reaction coordinate , especially at the 
 ts , hamper their potential use as a solid state device . to overcome 
 this last issue
we propose a cyclic central unit of 
 the molecule reducing the flexibility while at the same time keeping 
 the switching properties of the linear molecules .
calculations further 
 show that we can tailor the reaction dynamics of the compounds , in 
 terms of both reaction barrier ( kinetics ) and energy ( thermodynamics ) 
 between the two states by changing the acceptor unit and/or the migration 
 group .
we find that the switching ratio is increased going from the 
 linear molecules to the cyclic , and it is 200 or greater for all considered 
 molecules at zero - bias voltage .
we have further analyzed the 
 transport mechanism using the most 
 conducting scattering state , the density of states in the junction , 
 and a single - level landauer model .
we find that for the off states 
 we have a mo localized in the central part of the molecule about 0.5 
 ev below ef .
we also see interference 
 effects in some of the off states decreasing the transmission further . 
 for the on states , on the other hand , we have a delocalized -electron 
 system spanning the whole molecule and metal - induced gap states at ef increasing the effective coupling to the electrodes . 

the most conducting mo is now ( almost ) resonant , enhancing the conductance 
 significantly .
these results should be of great interest for 
 the development of 
 a solid state switch or memory device with a high on / off conductance 
 ratio and the possibility to be switched by two different stimuli 
 ( photochemically and thermally ) . with this study
we propose a rearrangement 
 and a class of compounds which are completely new to the field of 
 molecular electronics , thus expanding the pool of compounds used in 
 this research area .
 on 
 the basis of first - principles density functional theory calculations , 
 we propose a new molecular photoswitch which exploits a photochemical 
 [ 1,3]-silyl(germyl ) shift leading from a silane to a silene ( a si = c 
 double bonded compound ) . 
 the silanes investigated herein act as the 
 off state , with tetrahedral saturated silicon atoms disrupting the 
 conjugation through the molecules . 
 the silenes , on the other hand , 
 have conjugated paths spanning over the complete molecules and thus 
 act as the on state . 
 we calculate on / off conductance ratios in the 
 range of 1050 at a voltage of + 1 v. in the low bias regime , 
 the on / off ratio increases to a range of 2001150 . 
 the reverse 
 reaction could be triggered thermally or photolytically , with the 
 silene being slightly higher in relative energy than the silane . 
 the 
 calculated activation barriers for the thermal back - rearrangement 
 of the migrating group can be tuned and are in the range 108171 
 kj / mol for the switches examined herein . 
 the first - principles calculations 
 together with a simple one - level model show that the high on / off ratio 
 in the molecule assembled in a solid state device is due to changes 
 in the energy position of the frontier molecular orbitals compared 
 to the fermi energy of the electrodes , in combination with an increased 
 effective coupling between the molecule and the electrodes for the 
 on state . 
Introduction
Computational Methods
Results and Discussion
Conclusions and Outlook