molecular photochromism 
 and high photostability are among the crucial 
 requirements for optically driven molecular memories , photoswitches , 
 and data processing materials . in this context special attention
is paid to aromatic schiff bases 
 whose photochemistry is governed by an ultrafast excited - state intramolecular 
 proton transfer ( esipt ) . in these molecules ,
the decay of the initially formed excited - state 
 tautomer to the ground state can either lead back to the original 
 species by proton transfer or trap a portion of the excited molecules 
 in a ground - state metastable photochromic form .
basic 
 prerequisites for an efficient molecular photoswitch are 
 the presence of at least two stable photochromic forms in the ground 
 electronic state , lack of overlap between their absorption bands in 
 the relevant excitation region , and radiationless decay routes from 
 the franck 
condon regions of both isomers leading to the other 
 switching form . in the literature
there are two main groups of chemical 
 compounds investigated in the context of molecular photoswitching , 
 which differ in their internal conversion mechanism : ring opening  ring 
 closure versus cis
esipt - exhibiting aromatic schiff bases introduce 
 a novel way to optically driven switching control : here , the cis  trans 
 photoisomerization is enabled ( at least from one side ) by the initial 
 excitation of an enol - form allowing for a chemical bond rearrangement 
 through proton transfer ( pt ) .
chemical structures relevant for the photoswitching 
 process and 
 general scheme of the photochemistry of salicylidene methylamine . in recent years
the photophysics 
 of aromatic schiff bases has been 
 intensively studied both experimentally and theoretically , and much 
 progress has been made toward an understanding of the underlying mechanisms . 

however , in the context of molecular photoswitching , there are still 
 important questions to be answered . in this paper
we investigate the 
 photochemistry and photoswitching properties of an isolated molecule 
 containing the minimal schiff base chromophore 
despite its simple structure , sma has been 
 studied less than other 
 model aromatic schiff bases , such as salicylidene aniline ( sa ) or 
 larger systems of similar type . from the experimental side ,
the enol - form of sma has been identified 
 as the most populated ground - state isomer , and the photochromic behavior 
 observed in solution has been attributed to the formation of the keto - form . 

the available experimental data ( absorption and transient absorption 
 spectra recorded in some nonprotic solvents ) indicate that no fluorescence signal is found upon uv  vis 
 excitation in the absorption band of the enol - form of sma .
moreover , 
 only a very weak emission is observed for lower - energy excitation 
 in the absorption band of the keto - form .
this suggests that sma undergoes 
 a barrierless esipt process followed by efficient nonradiative internal 
 conversion to the electronic ground state .
theoretical investigations 
 of sma photophysics performed so far include static energy landscape 
 calculations and wave packet photodynamics simulations of the esipt 
 process .
these studies 
 confirm barrierless pt in the first singlet excited state of sma and 
 point to the presence of a conical intersection arising from cc double 
 bond rotation as the main route for radiationless decay to the ground 
 state . in this work we present results of semiempirical on - the - fly 
 photodynamics 
 simulations for the two sma tautomers relevant to photoswitching : 
 the global minimum enol - form and
the photochromic keto - form ( denoted 
 later as  and  ; for chemical structures see figure 1 ) .
our main goal is to investigate the full switching 
 cycle and to determine the characteristic time scales and photoproduct 
 distributions of both the forward and back switching transformations .
the semiempirical calculations were performed using 
 the om2/mrci 
 method , as implemented in the mndo99 code . during dynamics simulations , all required energies , gradients 
 and nonadiabatic coupling elements
fock ( hf ) 
 formalism was applied in the self - consistent 
 field ( scf ) treatment ( i.e. , the orbitals were optimized for the leading 
 configuration of the s1 state with two singly occupied 
 orbitals ) . the active space in the multireference configuration interaction 
 ( mrci ) calculations included 10 electrons in 10 orbitals ( see the supporting information for plots of these orbitals ) . 
 in terms of the scf configuration , it comprised the four highest doubly 
 occupied orbitals , the two singly occupied orbitals , and the four 
 lowest unoccupied orbitals .
three configuration state functions were 
 chosen as references for the mrci treatment , namely the scf configuration 
 and the two closed - shell configurations derived therefrom ( i.e. , all 
 singlet configurations that can be generated from the homo and lumo 
 of the closed - shell ground state ) .
the mrci wave function was built 
 by allowing all single and double excitations from these three references . 

this semiempirical om2/mrci approach has been recently shown to perform 
 well in a comprehensive general evaluation of excited - state properties and in a number of recent excited - state studies . in trajectory
surface - hopping dynamics the initial conditions 
 are 
 usually generated by wigner sampling , which defines 
 initial coordinates and velocities based on classical or quantum harmonic 
 vibrational modes , or by employing canonical classical molecular dynamics 
 ( md ) simulations , in which the canonical distribution is satisfied 
 with the use of a thermostat . here
, we adopted the second approach 
 applying the nose - hoover chains algorithm ( with a chain length of 10 ) for thermostatting .
we used the default 
 characteristic time of 0.01 ps for the thermostat and a time step 
 of 1 fs for nuclear motion . for both target molecules (  
 and  isomers ) ,
4 ps of equilibration 
 dynamics were followed by a 100 ps production run , from which 600 
 initial atomic coordinates and velocities were randomly selected for 
 each molecule .
the starting points for the subsequent om2/mrci nonadiabatic 
 dynamics were chosen from this set on the basis of the computed s0 to s1 transition probabilities and the vertical 
 excitation energies .
a total of 321 surface - hopping trajectories for the  
 isomer and 333 for the  isomer were run up to 1 ps with all 
 relevant energies , gradients , and nonadiabatic coupling vectors being 
 computed on - the - fly as needed . for points with an s1  s0 energy 
 gap of less than 10 kcal / mol ,
the time step was chosen 
 to be 0.2 fs for nuclear motion and 0.0005 fs for electronic propagation . 

the unitary propagator evaluated at the middle point was used to propagate 
 electronic motion .
the empirical decoherence correction 
 ( 0.1 au ) proposed by granucci et al . was employed .
the final evaluations were done for the 259  and 
 306  trajectories that finished successfully and satisfied 
 our energy continuity criterion ( no changes greater than 30 kcal / mol 
 between any two consecutive md steps ) .
the scc - dftb sampling with 
 the nose - hoover chains technique was done with chemshell 3.4 and the nonadiabatic om2/mrci dynamics simulations were conducted 
 with mndo99 .
for specific validation of the om2/mrci method for sma , the relevant 
 isomers ( see figure 1 ) were optimized in the 
 ground and first excited states .
the first excited state is always of  * character in the om2/mrci calculations . for full geometrical structures 
 and corresponding ab initio results
the om2/mrci results are consistent 
 with the previously calculated 
 energy landscape . in the ground state
the  
 conformer is confirmed to be the most stable form and all other isomers , 
 except for  , are found to be planar  as indicated by 
 the given dihedral angles . in the first singlet
excited state the 
  isomer does not correspond to a minimum but transfers the 
 oh proton to the n atom upon geometry optimization , forming the  
 isomer in a barrierless esipt process .
in the course of the excited - state 
 optimization the  conformer relaxes to the ci2 conical intersection . 

the calculated vertical excitation energies to the s1 state 
 are reasonably close to the published values obtained from high - quality 
 ab initio methods ( cc2/aug - cc - pvtz on mp2/cc - pvdz geometries ) , which are documented in the supporting information for easy reference .
vertical energy profiles 
 for sma calculated at the om2/mrci and 
 cc2/aug - cc - pvtz levels ( see text ) . for further validation
, vertical 
 potential energy profiles were 
 calculated at the om2/mrci level and compared with corresponding ab 
 initio data .
the profiles were computed 
 for the variation of three geometrical parameters most relevant to 
 sma photochemistry : the c1c7nc8 dihedral 
 angle relating isomers  and  ( denoted as cn from now 
 on ) , the oh distance relating isomers  and  , and the 
 c2c1c7n dihedral angle relating isomers 
  and  ( denoted as cc from now on ) . for each point on 
 the profile , the corresponding parameter was constrained to a fixed 
 value and a full ground - state optimization was carried out for all 
 other degrees of freedom .
the energy of the first excited state was 
 then obtained from a single - point calculation .
the profiles obtained 
 from the semiempirical om2/mrci calculations and from single - point 
 ab initio cc2/aug - cc - pvtz calculations on mp2/cc - pvdz geometries are 
 very similar to each other and agree with regard to all important 
 features pertinent to sma photochemistry . in particular , they show 
 a barrierless proton transfer path in the first excited state and 
 exhibit only very small barriers for the excited - state deactivation 
 via ci1 and for the ground - state proton back transfer .
the small bumps 
 in the regions near the conical intersections are due to minor sudden 
 changes in the optimized geometries .
after vertical excitation to the 
 first singlet excited state , both the  enol - form and the  
 keto - form of sma undergo an ultrafast deactivation to the ground state . 
 in both cases
the average state populations during the  and  photodynamics 
 are plotted in figure 3 against the simulation 
 time .
the data are fitted by a sigmoid function ( see eq 1 ) which provides an approximate decay time .
the lifetime of 
 the excited state is defined as the point at which half of the trajectories 
 have decayed to the ground state .
the parameter k denotes the fraction of trajectories 
 that decay until the end of the simulation.1 the deactivation of the  isomer 
 may be realized through two different phototransformations  

( i ) the esipt process followed by cc bond twist or ( ii ) the photoisomerization 
 around the cn bond ( direct cis  trans transformation ) .
150 fs which 
 makes it slower by around 50 fs in comparison with the excited - state 
 decay of the  form ( see figure 3 ) . within 
 the total simulation time of 1 ps
, the internal conversion to the 
 ground state is very efficient for both sma forms  less than 
 6% ( 9% ) of the trajectories are left in the excited state for the 
  (  ) photodynamics , respectively
. the slightly higher 
 number of such trajectories in the  case may be attributed 
 to the very flat shape of the s1 potential energy surface 
 ( pes ) in the franck 
condon region and the lack of excess kinetic 
 energy from esipt ( compared with the  case ) .
average state populations 
 for the two sets of trajectories ( left : 
  isomer ; right :  isomer ) .
figure 4 and 
 figure 5 show the time evolution of key 
 parameters characterizing the photodynamical trajectories of the  
 and  isomers , respectively .
the plots for the two sample runs 
 include : the dihedral angle of cc and cn rotations , the oh and nh 
 distances , the s0 and s1 energies relative to 
 the global ground - state minimum of the  isomer , and the energy 
 gap between these two states .
blue vertical lines mark the hopping 
 events between the two energy surfaces , while the red lines indicate 
 an intramolecular proton transfer . to facilitate the analysis of the 
 graphs , the geometrical structures present at each stage of the dynamics 
 are indicated by a corresponding label ( i.e. ,  ,  ,  , 
 or  ) .
after photoexcitation occurs an 
 ultrafast proton transfer to the n atom ( finished within 30 fs ) accompanied 
 by a clear reduction of the s1/s0 energy gap . 

a subsequent rotational motion takes the system toward the ci1 conical 
 intersection , where it hops to the ground state at a simulation time 
 of ca .
700 fs ) ; cc rotation is not feasible in the ground 
 state , because during s1  s0 deactivation 
 the 
* orbital localized at the cc bond becomes unocuppied and 
 thus the energy barrier for cc rotation rises significantly .
the ground - state 
 proton transfer from the nitrogen back to the oxygen atom then occurs 
 at ca .
850 fs , regenerating the  isomer and restoring the interstate 
 energy gap to its initial value . at this point ,
top left : dihedrals , horizontal dashed lines 
 mark multiples of 180 ; top right : distances ; bottom left : state 
 energies ; bottom right : energy difference .
the trajectory starting from the  isomer in figure 5 shows a faster , single - step deactivation involving 
 only cc bond rotation ( there is no need for esipt to open the route 
 toward the ci ) .
70 fs and 
 thereafter exhibits a similar behavior as on the previously discussed 
  trajectory . a difference between the two  and  
 simulations can be seen in the oh and nh stretching amplitudes , which 
 should be associated with the ease of proton back transfer in the 
 ground state . after roaming on the s0 pes in the vicinity 
 of the  isomer for ca .
350 fs , the proton is transferred from 
 the n back to the o atom , and the system ends up in the  form . 

top left : dihedrals , horizontal dashed lines 
 mark multiples of 180 ; top right : distances ; bottom left : state 
 energies ; bottom right : energy difference . in line with a previously 
 proposed scheme
, we have analyzed the 
 sma isomers observed during the dynamics simulations in terms of photochemical 
 classes . depending on the values of the three key internal degrees 
 of freedom ( cn dihedral , cc dihedral , and oh distance ) we assign the 
 starting , hopping , and final structures from each trajectory to the 
  ,  ,  , or  sets ( for precise class definitions 
 see the supporting information ) . within 
 one particular photochemical class
all sma conformers are interconvertible 
 by rotations around single bonds that usually require only little 
 energy .
all class members have similar uv absorption spectra which 
 are , however , distinct from those of different - class representatives .
figure 6 shows results of such a structure 
 population analysis for the  photodynamics . at the starting 
 point
, the two key dihedral angles do not differ much from the optimum 
 values of 0 and 180 of the initial  isomer ( left panel ) . by 
 contrast , the hopping points ( middle panel ) have twisted structures , 
 with at least one of the two key dihedral angles differing strongly 
 from 0 and 180. these hopping points are concentrated in two 
 areas : cc  90 and cn  [ 120 160 ] 
 and cn  90 and cc  [ 0  30 ] . 

this reflects the competition between two internal conversion pathways 
 proceeding through the ci1 and ci2 conical intersections .
the first 
 one , involving esipt followed by cc bond rotation , happens much more 
 frequently than the second one , involving cn bond photoisomerization 
 ( ca .
condon point to ci1 is essentially barrierless and 
 initiated by the ultrafast esipt process , while at the same time there 
 is a small barrier on the route to the alternative ci2 conical intersection . 
 in the right panel with the final structures ( final points of the 
 simulation )
7:3:9:1 . evidently , the switching 
 forms   and   are strongly preferred 
 over the other two structures .
the small amount of the  form 
 represents the photoproducts of the alternative deactivation mechanism 
 via ci2 . on a somewhat longer time scale ,
the initially produced  
 isomers are expected to undergo a nearly barrierless ground - state 
 proton back transfer transforming them into the  photochemical 
 class .
the collected data also allow an estimate of the splitting 
 ratio at both conical intersections . comparing the overall number 
 of final structures classified as  and  that arise from 
 hopping around ci1 with the number of final  isomers , one gets 
 a 1:1 ratio .
an analogous analysis for the ci2 conical intersection 
 ( involving the  and  structures coming from the deactivation 
 at ci2 ) also gives a 1:1 ratio .
again , 
 the trajectories start in the vicinity 
 of the initial minimum ( left panel ) .
all the hopping points lie close 
 to the ci1 conical intersection ( middle panel ) .
condon region of the  isomer by cc bond rotation 
 ( note that ci2 can not be accessed from the  structure ) . at 
 the end of the simulation ( right panel )
most of the trajectories end 
 up as  or  isomers , with a photoproduct distribution 
 ::: of ca .
4:5:1:0 . thus , the overall 
 splitting ratio estimated for ci1 equals 9:1 for the  photodynamics .
dihedral 
 angle distribution of key structures in the trajectories 
 starting from the  isomer .
color code for assignment to isomers : 
 green   ; red   ; violet   ; 
 blue 
. dihedral angle distribution of key structures in the trajectories 
 starting from the  isomer .
color code for assignment to isomers : 
 green   ; red   ; violet   ; 
 blue  . the product distribution in the two photodynamics simulations 
 gives 
 valuable information about the expected photoswitching efficiency 
  the extent of the initial - to - final transformation at the 
 end of the simulation may be considered as one of its possible measures . 

comparison of the ::: ratio for the onward 
 (  photodynamics ) and backward (  photodynamics ) switching 
 shows better performance of the latter . the initial - to - final conversion 
 ( counting the  population as future  class ) amounts 
 to 50% for the    and to 90% for the  
   phototransformation .
these percentages almost perfectly 
 reflect the splitting ratio at ci1  the conical intersection 
 responsible for the dominant internal conversion mechanism in sma . 

therefore , it is worthwhile to focus attention on the structural bifurcation 
 at ci1 . the splitting ratio may be explained in terms of the interplay 
 between the effects of momentum conservation and coupling to the local density of states .
the first factor , 
 understood as favorable continuation of the rotational motion driving 
 the system to a conical intersection , always supports direct passing 
 through the ci and , thus , the photoswitching .
the second one , a purely 
 quantum effect , enters the dynamics through vibrational mode relaxation 
 that occurs after the hop to the ground state .
this vibrational cooling 
 is more efficient for the more pronounced ( deeper and/or wider ) of 
 the minima adjacent to the splitting ci point , which in our case is 
 the  isomer .
one should notice that in the  photodynamics 
 these two effects act against each other leading to a rather even 
 splitting distribution , while in the  case they act in the 
 same direction and thus favor formation of the  isomer .
figure 8 shows the time evolution of the 
 photochemical class populations for both photodynamics :   

after excitation of the 
  form , one observes an ultrafast  to  transformation 
 ( esipt ) occurring mainly in the first 30 fs of the simulation ; this 
 is in very good agreement with previous theoretical results and with experimental findings for similar systems .
50 fs after the excitation , 
 the population of the  form starts to grow which , after rising 
 to a 5% value , stays constant until the simulation ends .
this confirms , 
 as expected , that in the s1 state an intramolecular ballistic 
 transfer of a light proton occurs faster than a rotation around the 
 cn bond .
consequently , the internal conversion through ci1 plays a 
 predominant role in sma photodynamics .
further analysis of the left 
 panel of figure 8 ( 100 fs < t < 200 fs ) shows that the  population , after reaching its 
 maximum arising from the esipt process , decreases rapidly . at the 
 same time
70 fs after the pt ) the amount of  
 isomer starts to grow with a similar rate .
this reflects the internal 
 conversion process with a bifurcation between the  and  
 forms and is consistent with the previously estimated s1 excited - state lifetime .
after 200 fs the  population stabilizes 
 at its final value , and one observes an almost linear decrease ( increase ) 
 in the population of the  (  ) isomer , which continues 
 until the end of the 1 ps simulation . assuming this trend to continue , 
 the full switching process would be finished around 1.6 ps after the 
 initial photoexcitation of the  isomer , which is again in agreement 
 with experimental observations for analogous schiff base systems . from the right panel of figure 8 one
may 
 learn about the time characteristics of the back - switching process 
 starting with an excitation of the  form .
at the very beginning 
 the system stays close to the initial structure and then , again after 
 ca .
this happens 
 through internal conversion at ci1 with bifurcation between these 
 two forms , similarly as in the previously discussed  photodynamics . 

population of the  isomer initializes the subsequent  
 to  transformation , which again proceeds almost linearly with 
 time .
concomitantly , the  population reached after the s1 to s0 deactivation remains constant , and no traces 
 of the  form are observed . in analogy
to the analysis for the 
  case , the overall  switching time scale may be estimated 
 as ca . 2.0 ps .
in this paper we investigated 
 the full photoswitching cycle of 
 an isolated sma molecule by means of the semiempirical on - the - fly 
 photodynamics simulations .
for the two isomers that play a crucial 
 role in the switching (  and  ) , we find a very efficient 
 internal conversion ( 90% nonradiative decay of the excited 
 state ) that indicates excellent photostability and promising switching 
 efficiency of sma .
the computed time scales characterizing the crucial 
 phototransformations of this model schiff base system are all in the 
 ultrafast regime ( 1 ps and subpicosecond ) .
the simulations 
 predict that the switching process is reversible in sma and that the 
 competing photoisomerization around the cn bond plays only a minor 
 role in the photochemistry of this system .
the detailed analysis of 
 structural time evolution provides direct insight into the internal 
 conversion and switching mechanisms .
the strong dependence of the 
 splitting ratio at the ci1 conical intersection on the photoswitching 
 direction probably originates from the ground - state pes topology of 
 the  and  switching forms .
 ; cyan   +  ; violet   ; 
 blue . the esipt - based photoswitching mechanism in sma involves 
 pronounced 
 reorganization of the electron density accompanied by only minor changes 
 in its geometrical structure .
this should allow for switching selectivity 
 due to the strong photochromism and should also ensure structural 
 stability , which are both important factors in the design of molecular 
 electronics devices . in summary
, we believe that aromatic schiff bases 
 may have advantages over many previously proposed molecular photoswitching 
 systems because of their ultrafast photochemistry , high photostability , 
 and good excitation selectivity , and that they may thus be considered 
 as promising candidates for future photodriven molecular electronics 
 materials .
 photoswitching 
 of simple photochromic molecules attracts substantial 
 attention because of its possible role in future photon - driven molecular 
 electronics . here 
 we model the full photoswitching cycle of a minimal 
 photochromic schiff base  salicylidene methylamine ( sma ) . 
 we 
 perform semiempirical nonadiabatic on - the - fly photodynamics simulations 
 at the om2/mrci level and thoroughly analyze the structural time evolution 
 and switching efficiency of the system . 
 we also identify and examine 
 in detail the crucial steps in the sma photochemistry ruled by excited - state 
 intramolecular proton transfer . 
 the results place the investigated 
 model aromatic schiff base among the promising candidates for novel 
 photoswitching molecular materials . 
 our study also shows the potential 
 of the semiempirical multireference photodynamics simulations as a 
 tool for early stage molecular photodevice design . 