Light- and Electric-Field-Induced Switching of Thiolated Azobenzene

Subscriber access provided by LAURENTIAN UNIV

Article

Light and Electric Field Induced Switching of Thiolated Azobenzene Self-Assembled Monolayer Jin Wen, Ziqi Tian, and Jing Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp404434r • Publication Date (Web): 03 Sep 2013 Downloaded from http://pubs.acs.org on September 16, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Light and Electric Field Induced Switching of Thiolated Azobenzene Self-Assembled Monolayer Jin Wen, Ziqi Tian, and Jing Ma∗ Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China E-mail: [email protected]

Phone: +86 025-83597408. Fax: +86 025-83596131

∗ To

whom correspondence should be addressed

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The reversible isomerization of the azobenzene (AZO) based self-assembled monolayers (SAMs) under external stimuli is the key to their application as molecular switches. To establish the relationship between electronic structure and switching function, AZO and its derivatives with electron-donating (NH2 ) and withdrawing (NO2 ) terminal groups, respectively, are investigated in the light and electric field triggered configuration changes by using density functional theory (DFT) and molecular dynamics (MD) simulation. Using the modified force field, whose parameters are taken from DFT calculations on the ground and the first excited states, the non-equilibrium molecular dynamics simulations show the collective structural transitions in Au(111) surface supported AZO SAMs under ultraviolet-visible light and external electric field stimulation. Along MD trajectories, an index function, S, is then defined to depict the SAM switching dynamics between “on” (S = 1) and “off” (S = 0) states. The charge transfer between SAM and surface and dipole interactions under the external electric field are revealed. The joint configuration changes of the AZO molecules in the SAM are also displayed to be able to lift the alkythiol coated mercury droplet in Au(111)–SAMAZO //SAMC12 –Hg junction model, in agreement with experimental observations on the photo switching of the current in molecular junction. In addition to the manipulation of switching by light irradiation, it is predicted that the AZO SAMs, with or without substitutions, may also work as a molecular cargo lifter under the electric field.

Keywords photo-isomerization; charge transfer; density functional theory; molecular dynamics simulations; electric field; cargo lift 2 ACS Paragon Plus Environment

Page 2 of 38

Page 3 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction The topic of molecular switches stimulated by external force has attracted extensive attention recently since these nanomaterials are tunable in the single molecular level. 1,2 The trans-cis reversible molecules, such as azobenzene (AZO) and its derivatives, are irradiated by ultravioletvisible (UV-Vis) light with configuration rearrangement. 3,4 Many applications based on the AZOs, such as optimal data-storage devices, 5 tunable metal-organic junctions, 6,7 and light-powered molecular machines 8,9 have been proposed. One of the problems in AZO devices is that the lifetime in their cis-configuration is short in solution, making electric signal hard to be detected. 10–12 Deposition of AZO from solution onto surface is an essential process, which makes the molecular switch more feasible. The self-assembled monolayers (SAMs) formed by the conformationally flexible alkythiols exposing azobenzene head-group, have been reported to be either non or poorly responsive to light excitation. 10 To improve the switching behavior, a high trans-cis conversion azobenzene with aromatic group rather than the alkythiol terminal group has been reported with long-lifetime cis-isomer on the Au(111) surface. 11,12 It has been found that a functional fieldeffect transistor based on an azobenzene SAM is applied with Au source and drain electrons, which can modify charge injection reversibility. 6 Furthermore, a high on/off ratio of up to 7×103 in azobenzene-thiophene derivative has been reported with cis-isomer in high conductance state. 13 Inspired by applications in the molecular devices, a stable photo-induced current from the lifting and lowering the Hg electrode in a AZO based junction has been obtained, 14 which fabricates the potential usage of AZO in mechanical work as a “cargo” lifter. In addition, the reversible switching of AZOs is not only stimulated by light, but also by the external electric field. It has been demonstrated that the electric field caused by the bias voltage applied between scanning tunneling micro-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

scope (STM) tip and the sample surface can induce the reversible trans-cis isomerization. 15–19 In the search for novel photo or electric field responsive molecules suitable for molecular electronics, an in-depth study is urgently called for understanding the stimuli triggered collective isomerization process in the surface amounted AZO SAMs. The most popular explanation of AZO isomerization was originated by Rau’s observation 20 and was later demonstrated by Monti et al.’s configuration-interaction calculations along potential energy surfaces. 21 Then extensive studies with theoretical calculations have been carried out to trace the isomerization process. 20,22–26 A conical intersection (CI) has been found in a rotation mechanism with the optimized geometry of excited states, and an inversion mechanism dominates in the higher excited states. 22–24,26–28 Via the conical intersection, the photo-isomerization has been explored by the non-adiabatic dynamics simulations recently. 29,30 Moreover, the switching behavior of AZOs in gas phase is rationalized by the isomerization pathway and the barrier at CI depending on the substituted groups. 31–33 Although the ab initio or semiclassical dynamics simulations have been successfully applied in the study of isomerization process for single AZO-based functional molecule, 20–29,34 they are still too time-consuming for the complex AZO SAM/Au(111) system. There is an alternative way to simulate the switching processes within the force field framework by using the non-equilibrium molecular dynamics (MD) simulations. 34–38 Through changing the form and parameters of the N=N rotation potential, this simple model has been successfully used to simulate the optical properties of azobenzene based chromophore in solution, 34 and the photo-induced switchable peptides, 35 a phase transition in liquid crystal azobenzene based systems, 37 and isomerization by laser excitation in the middle layer of layered silicates. 38 Since the quantum yield of those isomerization conversion of azobenzene derivatives in solution is quite low, 25,25,39,40 some recent theoretical 4 ACS Paragon Plus Environment

Page 4 of 38

Page 5 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


studies concentrated on the switching behavior of the AZOs, which are connected to Au cluster, 41 nanotubes, 42 and Si(111) surface 43 using Green function technique. 44 A significant conductance change has been reported when the molecule is sandwiched between metal electrodes. 10,15,41 Recently, we also performed a reactive molecular dynamics simulation on the configurational changes of the azobenzene-based monolayer on Au surface. 36 Although a good agreement between the simulated film thickness and the experimental observation was achieved, there are still some other open questions about the substituent effects on the switching ability and the nature of electric field driven isomerization. For efficient molecular switches design, we will investigate the substituent effects, using the electron donating (NH2 ) and withdrawing (NO2 ) groups, respectively, on the configuration changes in AZO SAMs, which are driven by both light and electric field on the Au(111) surface, as illustrated in Scheme 1. These SAMs are found to be tunable with the variation in the wave length of UV-Vis light and direction of electric field. In this article, we focus on the understanding of the switching processes of three kinds of AZO derivatives, AZO, AZO(NH2 ), and AZO(NO2 ) (Scheme 1) from both DFT calculations and MD simulations. The modified force field parameters are taken from the isomerization potential curves, which are calculated by DFT and complete active space with second-order perturbation theory (CASPT2). We further study the relation between dipole interaction and AZO polarizability under the external electric field. Molecular dynamics simulations are carried out to study morphology changes of AZO SAMs deposited on the Au(111) surface. The trans-cis reversibility of Au(111)/SAMAZO combined with dodecanethiol SAM–coated Hg droplet is simulated by our modified force field. This AZO based junction could work as a cargo lift by both light irradiation and electric field.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Theoretical Models and Computational Details In the study of isomerization mechanism of AZOs, different functionals in DFT are employed to optimize the ground state (S0 ) geometry and to calculate the adsorption spectra of 4’-(biphenyl4-ylazo)-biphenyl-4-thiol in chloroform solution with polarizable continuum model (PCM) model (Figure S1, Supporting Information). It shows that CAM-B3LYP functional could reproduce well the experimental maximum adsorption wave lengths, λmax , (trans: 360 nm and cis: 450 nm). Moreover, the 6-31G(d) basis set gives the same excitation energy of AZO with the 6-31+G(d) basis set. Thus the 6-31G(d) basis set is adopted to save computational time in the following calculations. The LANL2DZ effective core potentials for Au atoms are employed in DFT calculations. As shown in the flowchart of Scheme 2, the ground state S0 potential curves of AZOs are scanned with the rotational or inversional angles fixed in geometry optimization by DFT method. In fact, the other quantum mechanical (QM) models, such as complete active space self-consistent field (CASSCF) and CASPT2, can be applied to get the S0 potential curves, if their computational costs are affordable for the studied system. In the present work, only the gas phase potential curves of the unsubstituted AZO are validated by using the CASPT2 method, while the other substituted AZOs absorbed on top of Au(111) are treated by DFT. To model the binding of AZOs on Au(111) surface, the extended molecular models are built by coupling the AZOs to the Au13 , Au31 , and Au37 cluster models, with the increasing cluster size. As shown in Figure S2, adsorptions of AZOs at top, bridge, hcp, and fcc hollow sites, respectively, are considered with the fixed bulk Au· · · Au distance of 2.88 Å. It shows that the binding interactions on hcp and fcc hollow sites are similar, which are stronger than the other sites. So the hcp hollow site in the Au31 cluster model is selected to investigate the Au· · · thiol interaction under electric field.


Page 6 of 38

Page 7 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Then the vertical excitation potential for the first singlet excited state (S1 ) is calculated by timedependent density functional theory (TDDFT) at the optimized S0 geometry. To test the accuracy of the TDDFT method, we also performed a CASPT2(12/10) calculation, complete active space with second-order perturbation theory with 12 electrons in 10 orbitals in active space, on the AZO isomerization in gas phase. The active orbitals are selected as 8 π valence and two doubly occupied nitrogen lone pairs. All these quantum chemical calculations are performed by using Gaussian 09 45 and Molpro 46,47 packages. Go beyond the oversimplified model of the isolated AZOs in gas phase, molecular dynamics simulations of collective isomerization processes induced by both light and electric field are then carried out in the slab models, which are in the size of 34.61 × 29.97 × 54.71 Å3 with 36 AZOs on the Au(111) surface. The canonical NVT ensemble is used to simulate the packing conformations of AZOs with all Au atoms fixed during molecular dynamics simulations. In this NVT simulation, the temperature is set to 298 K under an Andersen thermostat with a 1 fs time step. The trajectories are collected every 50 fs during the 1 ns simulations. For these MD simulations on the equilibrium S0 state, the consistent valence force field (CVFF) is employed with a slight modification on the intermolecular interaction potential. As shown in Table 1, the Lennard-Jones potential (Evdw = Ai j /r12 − Bi j /r6 ) is employed to describe the non-bonded intermolecular interaction between the thiol anchoring groups. Furthermore, the Morse potential (EAu···S = D · {1 − exp[−k · (r − r0 )]}2 ) is introduced to simulate interfacial interaction between thiol and Au(111) surface. 48 Such modified parameters for the interfacial Au· · · S interaction were satisfactorily applied in the simulations of SAM on the porous-network modified Au(111) surface. 48 To further model the cis/trans isomerization, several kinds of modifications on the N=N bond torsion potential, Etorsion = Kαi × [1 + cos(n × αi − α0 )], were suggested. 34–38 In the early version, 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the rotation force constant, Kα , was determined from the S0 → S1 excitation energy. 34 Taking the cis → trans isomerization process for example, when the photo excitation takes place at the cis conformation with αi = α0 = 0, the value of [1 + cos(n × αi − α0 )] term is 2 (n=1), and the torsion potential, Etorsion , equals to the S0 → S1 excitation energy (80 kcal/mol). Therefore, the force constant, Kα , is set to 40.0 kcal/mol, which is half of the vertical excitation energy at the cis configuration. However, the parameters for the electric field induced isomerization have not been explored yet. Here, we use the similar strategy to describe the electric field induced transition process. When the electric field lies parallel to the AZO, the energy difference between trans and cis configuration is predicted to be 120 kcal/mol at CAM-B3LYP calculation level, so Kα is taken as 60 for the cis → trans isomerization under electric field. For those non-equilibrium switching processes of the AZO SAMs on the Au(111) surface, the MD simulations using the modified N=N torsion potential are carried out in NVE ensemble. Since we lay emphasis on the influence of substituent groups and external electric field on the switching behavior, the random backward reaction events 36 are neglected in the present work. AZO SAMs finish the isomerization within 1ps, so the trajectories are recorded every 1 fs in 5 ps NVE simulations. On top of Au(111) supported SAMAZO model, a cargo lift model is also built with 100 dodecanethiol (C12SH) chains adsorbed on mercury electrode (using the liquid density of mercury, 13.53 g/cc). The size of the Au(111)–SAMAZO //SAMC12 –Hg junction model is 46.25 × 39.78 × 81.71 Å3 . Both NPT and NVT ensembles are employed in 500 ps simulations for this junction model. MD simulations are performed by using Material Studio software package. 49


Page 8 of 38

Page 9 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Results and Discussion A general picture of isomerization The adsorption spectra of AZO in Figure S3 are obtained by TDDFT calculation. As addressed by many other works, 22,24 theoretical calculations can reproduce the experimental spectra for both trans and cis configurations. As shown in Figure S3, the excitation transition is mainly n → π ∗ types from the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) of azobenzene. Previous reports have demonstrated that the isomerization is mainly controlled by the rotational mechanism, while in inversion path it requires a larger activation energy. 29–34 Here, the isomerization pathways (both rotation and inversion) are also calculated by CAM-B3LYP with 6-31G(d) basis set. As shown in Figure S4a, the rotational barrier with unrestricted wavefunction is found to be around 32 kcal/mol, which is close to the CASPT2 calculation results of 37–38 kcal/mol with 14 electrons in 12 orbitals in active space, 22,30 while the rotational barrier with restricted wavefunction is overestimated by 15 kcal/mol. To compare these two isomerization mechanisms, the scanned potential surfaces as functions of the C–N=N–C torsion angle, α, and the C–N–N bend angle, σ , are shown in Figure S5, where the TDDFT calculation reveals that activation energy along the rotational path is much lower than that along the inversion path. It also agrees with the previously reported CASPT2 calculation result, 22 which shows that the inversion mechanism plays a role only with an activation energy of more than 25 kcal/mol, while the rotational pathway only needs 2 kcal/mol activation energy. To evaluate the TDDFT accuracy, the rotational potential profile is also calculated by CASPT2 method (displayed in the inset of Figure 1) as a comparison. Based on the DFT optimized geometries, the CASPT2(12,10)/6-31G* calculation gives an activation energy of about 14 kcal/mol. This activation energy would be lower with 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CASPT2 optimized geometries, but CASPT2 optimization costs much more computational time. Comparing to previous reports on the activation energy of 2 kcal/mol with the CASPT2(14,12) optimized geometries, 22 however, our TDDFT calculation results give a reasonably close activation energy of about 8 kcal/mol. Furthermore, calculations are done with other two kinds of AZO derivatives with NH2 and NO2 terminal groups, that have electron donating and withdrawing abilities, respectively. We find the substitute groups do not change the energy gap in this cross section. The isomerization of the substituted AZOs are also dominated by the rotation of C–N=N–C bond. For 4,4’-diaminoazobenzene and 4,4’-dinitroazobenezne, it was reported that the difference in the excitation energies (as well as cis-trans rotational barriers) between substituted azobenzene and unsubstituted azobenzene is quite small (within 4.0 kcal/mol). 33 In our series of 4’-(biphenyl-4-ylazo)-biphenyl-4-thiol molecules, the excitation energy is similar between AZO and AZO(NH2 ), but it is 10 kcal/mol higher in AZO(NO2 ) than that in AZO. We also investigate the excitation energies of AZO on the Au(111) surface in Figure 1b. According to the potential curves on Au31 cluster model at ground (S0 ) and the first excited states (S1 ), the surface-supported photo-isomerization also favors the path of bond rotation.

Isomerization of AZOs induced by the external electric field The presence of an electric field leads to the possible isomerization beyond its threshold voltage. The 3,3’,5,5’-tetra-tert-butylazobenzene molecule has been polarized under the electric field in the STM junction, which makes the isomerization barrier lower. 16 To study the switching behavior of AZO under electric field, we further calculate the potential curves of AZOs on the Au cluster


Page 10 of 38

Page 11 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


model by DFT/CAM-B3LYP calculations. The rotational potentials of three kinds of AZOs are calculated under two electric fields, denoted as E+ and E− , which are parallel and anti-parallel to the AZO axes, respectively. As shown in Figure S4b, the difference in rotational barrier obtained by restricted and unrestricted wavefunction is negligible for the AZO/Au31 model, thus restricted wavefunction is used in the following calculations. Furthermore, when the bias-voltage is 10.0V with the electric field applied on AZO(NO2 ) (Figure 1d), AZO (Figure 2a), and AZO(NH2 ) (Figure 2b) there is no rotational barrier in the cis-to-trans isomerization. In contrast with the energy barrier of 50 kcal/mol in gas phase (Figure 1c), the absence of rotational barrier here shows that metal surface plays an important role in the molecular switch. To confirm the AZOs are still adsorbed to the Au(111) surface under the electric field, we calculate the binding energy between the AZOs and surfaces, which turns out to be still negative at 10.0 V, as shown in Figure 3. It should be mentioned that the strength of external electric field in our theoretical model is not strictly related to the applied biased voltage in the experimental STM manipulation. 16,50 In the STM tip-pulsing experiment, the current between monolayer and surface is observed, 10 while no current is adopted in our theoretical calculations. Although we can give a qualitative prediction of the switching behavior of the AZO molecules in this Au cluster model, it should be noted that the cluster model is not sufficient to model a practical photo-switching AZO device process with a collective nature. The UV field is not necessarily localized on the extended molecular model with one AZO molecule and a few Au layers. In addition, the penetration length of electric field is more than 10 nm for Au surface and photo-generated electron in the surface/bulk metal can be injected to molecules when the coverage rate is low. When coverage rate is very high and molecular SAM is thick, the above effect may be reduced. But the simulation of such a close-packed AZO SAM requires at least twice the cluster size in our present model, which is impractical for the QM treatments, even using 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the economical DFT calculations. Therefore, in the next subsection, we will embed the excitation induced isomerization picture into the force field framework and employ the MD simulation to trace the joint structural transitions in AZO SAMs. To better understand the switching behavior of these molecules under electric field, we further study the charger distribution and dipole moment of the AZO/Au31 models. As shown in Figure 4, charge transfer between Au(111) surface and AZOs is negligible in the absence of electric field. When electric field is applied, however, the direction of electron transfer in AZOs/Au31 models is sensitive to the direction of the field. The electrons transfer from Au(111) surface to AZOs under E+ , and the transfer direction reveres under E− for both trans and cis-configurations. To further study the polarization of AZO derivatives, we show the electron density difference induced by the electric field. The blue color indicates an increase in the electron density and yellow indicates a decrease in the electron density. In other words, electron density on the Au(111) surface is increased by applying E− , while it is accumulated in AZOs molecules by applying E+ . The dipole moments of the AZOs/Au31 model, on the other hand, dramatically increased, with the direction anti-parallel to that of electric field. The dipole moment in the case of transconfiguration is larger than that in the case of cis-configuration. Moreover, the charge redistribution and change in dipole moment of the AZO(NO2 ) molecule (Figure S6) is similar to that of the AZO molecule (Figure 4). In all, direction of charge transfer between Au(111) surface and AZOs is not sensitive to the substituent when the electric field is applied. The energy gap between frontier orbitals is similar in all cis-AZOs, but it changes lightly in the trans-AZOs with different substituents. The HOMO–LUMO gap of AZOs/Au31 changes with electric field strength as shown in Figure 4 and S6. The energy gap is reduced by the presence of the electric field. In this sense, the strength 12 ACS Paragon Plus Environment

Page 12 of 38

Page 13 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of electric field can be used to tune the switching behavior of AZOs on the Au(111) surface. The electric field can induce a charge transfer between the metal substrate and AZO molecules, which, in turn, renders the transition between cis- and trans-configurations.

Collective switching processes in SAM In the above discussions, it has been clarified that the isomerization mechanism of AZO with relative low activation energy (less than 10 kcal/mol) is mainly rotation of C–N=N–C bond due to the presence of external forces. It is necessary to extend the simulation of switching event from the single AZO molecule model to the AZO SAM on the Au(111) surface. In the following part, we will use the modified force field to simulate the collective isomerization process of AZO SAM on the Au(111) surface. In previous reports, the non-equilibrium MD simulations have been successfully applied in the photo-isomerization process successfully by modifying the parameters for the C–N=N–C torsion angle in force fields. 34–38 To simulate the transition process between trans and cis AZOs in SAM, we also changed some force field parameters within the CVFF, with modified parameters listed in Table 1. The potential curves of AZO during the isomerization process are shown in Figure 5(a-b). The blue curves are obtained based on the S0 and S1 potential surfaces from the DFT and TDDFT calculation (Figure 1), respectively. The modified CVFF potential curves (in red) reproduce these results along the rotational path. For the non-bonded interaction, it has been demonstrated that the default parameters in CVFF overestimate the interaction between aromatic thiols. 48 Therefore, van der Waals and Morse potential modified CVFF parameters are also adopted in the thiol· · · thiol and thiol· · · Au interactions, respectively. It was also shown in Figure 5 and S7 that the modified



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 38

parameters reproduce well the MP2 potential curves. To simulate the isomerization process, we use different parameters in the torsion energy expression as discussed above. So the trans → cis (UV) parameter in Table 1 is used to simulate the on → off switching process when the wavelength of the UV light is 370 nm, while the parameter of cis → trans (UV) is used for the 450 nm visible light. 11,12 Contrasting to the reversible trans-cis isomerization, only cis→ trans process appears when the electric field is involved as it has been discussed above in our Au cluster model. In the simulation of the AZO SAM switching behavior between trans- and cis-configurations, the collective isomerization dynamics can be characterized by an index function, S, which is defined as S = 1−

1 N ∑ (1 + cos αi)/2, N i=1

(1)

where αi is the torsion angle of ith chain in the monolayer. Thus S = 0 presents the cis-monolayer and S = 1 corresponds to the trans-monolayer. In our simulations, the switching index, S, changes with time evolution for AZO (Figure 6a) and AZO(NO2 ) (Figure 6b) monolayers, respectively. It shows that the monolayer switches between trans- and cis-configurations within 5.0 ps. We just exemplify the process by the AZO(NO2 ) monolayer when the bias voltage of -10.0 V is applied. The snapshots in different time are also presented in the inset of Figure 6. To give a closer look at the configuration of each individual AZO molecule, we find the transition state is mainly distributed at 0.5 ps with a torsion angle about 87◦ . The index function switches from 1 to 0 within 1 ps. We may conclude that the torsion angle switches between 0◦ and 180◦ within 1ps, while the following 4 ps is needed for the SAMAZO to reach an equilibrium and orderly-packed state with the AZO chains almost parallel to each other.


Page 15 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We can also estimate the SAM thickness from the switching index. The thickness of AZO SAMs increase when S changes from 0 to 1 as illustrated in Figure 7 at different coverage. The vertical heights are reduced by about 1.5 and 2.0 Å in the trans-to-cis isomerization at the higher coverage (3.5×1018 molecules/m2 ) and lower coverage ( 2.3×1018 molecules/m2 ), respectively. Unit cell of the SAMAZO is shown in the top view with parameters, a = 5.8 Å, b = 10.8Å, and γ = 92.6◦ . ( Figure 7) Due to the steric effect of AZO monolayer, the thickness of AZO SAM changes with the coverage. To find the coverage influence on the switching behavior, we further analyze the changes in tilt angle, ω. It is defined as the angle between the long axes of AZO chains to the [111] axis on the Au(111) surface. From the tilt angle distributions (Figure S8), we find the trans-AZO molecules tend to stand up with the increase of their coverage on the surface. The tilt angle of the AZO chain has a wider distribution at a low coverage (1.5×1018 molecules/m2 ) than that at a higher coverage (3.5×1018 molecules/m2 ). When the coverage is lower than 2.3×1018 molecules/m2 , the AZO SAM tends to lie flat on the Au(111) surface. The high coverage used here is close to the experimental coverage, 2.3×1018 molecules/m2 . 11,12

Configuration transition in cargo lift Taking advantage of the switching behavior of AZO, the AZO-based SAM can be used in a junction of a cargo lift together with mercury droplet. 14 Interaction between mercury atoms is weak and the droplet of mercury can be lifted by external force. The average force has been calculated as being 2.6 × 10−14 N for a close-packed AZO SAM. 14 Furthermore, a stable current in this type of Au(111)–SAMAZO //SAMC12 –Hg junction has also been observed. 14 Inspired by this experiment, we build a very complicated junction model, which is interpreted in Figure 8 for AZO(NO2 ) SAM



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and Figure S9 for AZO SAM, respectively. In this type of junction, the SAMC12 is adsorbed in the Hg droplet, which is contacted with the SAMAZO on the Au(111) surface. It is interesting to use the external electric field to induce the conversion of the SAM in this junction, although it was reported to switch with photo excitation. 14 The bias voltage of -10.0 V is also used in the electric filed application. With the changes in AZO SAM thickness during the isomerization process, the contacting Hg electrode is switching between lifting and lowering. From the snapshot of our simulations of close-packed SAM junction, the Hg droplet height is changed by around 1.2–1.4 Å, smaller than that (7 Å) predicted from a single AZO molecule length. 14 Local chain distortions and disorder packings in AZO SAM may depress the upright thickness. We predict this cargo lift also works under external electric field, by which the mechanical work can be fulfilled. As we find thickness difference is about 2 Å at lower SAMAZO coverage (Figure 7), the lifting height could also be tunable by different coverage.

Conclusions The isomerization mechanism driven by the UV-Vis light irradiation and electric field in AZO and its derivatives AZO(NH2 ) and AZO(NO2 ) is studied by DFT calculations. The activation energy between ground state and the first excited state is lower in the rotational path than that in the inversional path. With the presence of electric field, charge transfer occurs between the AZOs and the Au(111) surface. The threshold bias-voltages are +10.0 and -10.0 V in E+ and E− respectively, obtained by our Au31 cluster model, which gives a qualitative prediction for the cisto-trans isomerization in AZOs with electric field. In the framework of our modified force field, a switching function describes the collective isomerization dynamics process. In the application


Page 16 of 38

Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of this switchable monolayer, we also simulate the Au(111)–SAMAZO //SAMC12 –Hg junction and find that the height of Hg droplets is reduced by 1.2–1.4 Å when the AZO SAM switches in the trans-to-cis isomerization. This junction model can also be used as a cargo lift when the electric field in involved. Our simulation results open a new perspective for designing the switchable AZO SAM in a tunable fashion.

Acknowledgment This work is supported by the National Basic Research Program (Grant No. 2011CB808600), National Natural Science Foundation of China (Grant No. 21290190 and 21273102). The authors gratefully thank the High Performance Computing Center of Nanjing University for providing the IBM Blade cluster system. Jin Wen thanks Dr. Yingjin Ma for the helpful discussion in the CASPT2 calculations.

Supporting Information Selection in functionals in DFT calculations, electronic structures of AZO/Au31 cluster model, validation of modified force field, and packing conformations of AZOs on the Au(111) surface are discussed here. This information is available free of charge via the Internet at http://pubs.acs.org.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

References 1. Kudernac, T.; Katsonis, N.; Browne, W. R.; Feringa, B. L. Nano-Electronic Switches: LightInduced Switching of the Conductance of Molecular Systems. J. Mater. Chem. 2009, 19, 7168–7177. 2. Browne, W. R.; Feringa, B. L. Light Switching of Molecules on Surfaces. Annu. Rev. Phys. Chem. 2009, 60, 407–428. 3. Hu, J.; Yu, H.; Gan, L. H.; Hu, X. Photo-Driven Pulsating Vesicles from Self-Assembled Lipid-Like Azopolymers. Soft Matter 2011, 7, 11345–11350. 4. Sortino, S. Photoactivated Nanomaterials for Biomedical Release Applications. J. Mater. Chem. 2012, 22, 301–318. 5. Liu, Z. F.; Hashimoto, K.; Fujishima, A. Photoelectrochemical Information Storage Using an Azobenzene Derivative. Nature 1990, 347, 658–660. 6. Crivillers, N.; Orgiu, E.; Reinders, F.; Mayor, M.; Samorì, P. Optical Modulation of the Charge Injection in an Organic Field-Effect Transistor Based on Photochromic Self-AssembledMonolayer-Functionalized Electrodes. Adv. Mater. 2011, 23, 1447–1452. 7. Wang, Y.; Cheng, H.-P. Electronic and Transport Properties of Azobenzene Monolayer Junctions as Molecular Switches. Phys. Rev. B 2012, 86, 035444. 8. Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.; Seitz, M.; Gaub, H. E. Single-Molecule Optomechanical Cycle. Science 2002, 296, 1103–1106.


Page 18 of 38

Page 19 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9. Yu, Y.; Nakano, M.; Ikeda, T. Photomechanics: Directed Bending of a Polymer Film by Light. Nature 2003, 425, 145–145. 10. Yasuda, S.; Nakamura, T.; Matsumoto, M.; Shigekawa, H. Phase Switching of a Single Isomeric Molecule and Associated Characteristic Rectification. J. Am. Chem. Soc. 2003, 125, 16430–16433. 11. Pace, G.; Ferri, V.; Grave, C.; Elbing, M.; von Hänisch, C.; Zharnikov, M.; Mayor, M.; Rampi, M. A.; Samorì, P. Cooperative Light-Induced Molecular Movements of Highly Ordered Azobenzene Self-Assembled Monolayers. Proc. Natl. Acad. Sci. 2007, 104, 9937–9942. 12. Elbing, M.; Błaszczyk, A.; von Hänisch, C.; Mayor, M.; Ferri, V.; Grave, C.; Rampi, M. A.; Pace, G.; Samorì, P.; Shaporenko, A. et al. Single Component Self-Assembled Monolayers of Aromatic Azo-Biphenyl: Influence of the Packing Tightness on the SAM Structure and Light-Induced Molecular Movements. Adv. Funct. Mater. 2008, 18, 2972–2983. 13. Smaali, K.; Lenfant, S.; Karpe, S.; Oçafrain, M.; Blanchard, P.; Deresmes, D.; Godey, S.; Rochefort, A.; Roncali, J.; Vuillaume, D. High On-Off Conductance Switching Ratio in Optically-Driven Self-Assembled Conjugated Molecular Systems. ACS Nano 2010, 4, 2411– 2421. 14. Ferri, V.; Elbing, M.; Pace, G.; Dickey, M.; Zharnikov, M.; Samorì, P.; Mayor, M.; Rampi, M. Light-Powered Electrical Switch Based on Cargo-Lifting Azobenzene Monolayers. Angew. Chem. Int. Ed. 2008, 47, 3407–3409. 15. Choi, B.-Y.; Kahng, S.-J.; Kim, S.; Kim, H.; Kim, H. W.; Song, Y. J.; Ihm, J.; Kuk, Y. Confor-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

mational Molecular Switch of the Azobenzene Molecule: A Scanning Tunneling Microscopy Study. Phys. Rev. Lett. 2006, 96, 156106. 16. Alemani, M.; Peters, M. V.; Hecht, S.; Rieder, K.-H.; Moresco, F.; Grill, L. Electric FieldInduced Isomerization of Azobenzene by STM. J. Am. Chem. Soc. 2006, 128, 14446–14447. 17. Kirakosian, A.; Comstock, M. J.; Cho, J.; Crommie, M. F. Molecular Commensurability with a Surface Reconstruction: STM Study of Azobenzene on Au(111). Phys. Rev. B 2005, 71, 113409. 18. Henzl, J.; Mehlhorn, M.; Gawronski, H.; Rieder, K.-H.; Morgenstern, K. Reversible cis-trans Isomerization of a Single Azobenzene Molecule. Angew. Chem. Int. Ed. 2006, 45, 603–606. 19. Nakamura, H.; Asai, Y.; Hihath, J.; Bruot, C.; Tao, N. Switch of Conducting Orbital by Bias-Induced Electronic Contact Asymmetry in a Bipyrimidinyl-biphenyl Diblock Molecule: Mechanism to Achieve a pn Directional Molecular Diode. J. Phys. Chem. C 2011, 115, 19931– 19938. 20. Rau, H.; Lueddecke, E. On the Rotation-Inversion Controversy on Photoisomerization of Azobenzenes. Experimental Proof of Inversion. J. Am. Chem. Soc. 1982, 104, 1616–1620. 21. Monti, S.; Orlandi, G.; Palmieri, P. Features of the Photochemically Active State Surfaces of Azobenzene. Chem. Phys. 1982, 71, 87–99. 22. Cembran, A.; Bernardi, F.; Garavelli, M.; Gagliardi, L.; Orlandi, G. On the Mechanism of the cis-trans Isomerization in the Lowest Electronic States of Azobenzene: S0, S1, and T1. J. Am. Chem. Soc. 2004, 126, 3234–3243.


Page 20 of 38

Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Ciminelli, C.; Granucci, G.; Persico, M. The Photoisomerization Mechanism of Azobenzene: A Semiclassical Simulation of Nonadiabatic Dynamics. Chem. Eur. J. 2004, 10, 2327–2341. 24. Ishikawa, T.; Noro, T.; Shoda, T. Theoretical Study on the Photoisomerization of Azobenzene. J. Chem. Phys. 2001, 115, 7503. 25. Ootani, Y.; Satoh, K.; Nakayama, A.; Noro, T.; Taketsugu, T. Ab initio Molecular Dynamics Simulation of Photoisomerization in Azobenzene in the nπ∗ State. J. Chem. Phys. 2009, 131, 194306. 26. Cusati, T.; Granucci, G.; Persico, M. Photodynamics and Time-Resolved Fluorescence of Azobenzene in Solution: A Mixed Quantum-Classical Simulation. J. Am. Chem. Soc. 2011, 133, 5109–5123. 27. Shao, J.; Lei, Y.; Wen, Z.; Dou, Y.; Wang, Z. Nonadiabatic Simulation Study of Photoisomerization of Azobenzene: Detailed Mechanism and Load-Resisting Capacity. J. Chem. Phys. 2008, 129, 164111. 28. Sauer, P.; Allen, R. E. Multiple Steps and Multiple Excitations in Photoisomerization of Azobenzene. Chem. Phys. Lett. 2008, 450, 192–195. 29. Cao, J.; Liu, L.-H.; Fang, W.-H.; Xie, Z.-Z.; Zhang, Y. Photo-Induced Isomerization of Ethylene-Bridged Azobenzene Explored by Ab Initio Based Non-Adiabatic Dynamics Simulation: A Comparative Investigation of the Isomerization in the Gas and Solution Phases. J. Chem. Phys. 2013, 138, 134306. 30. Liu, L.; Yuan, S.; Fang, W.-H.; Zhang, Y. Probing Highly Efficient Photoisomerization of a



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Bridged Azobenzene by a Combination of CASPT2//CASSCF Calculation with Semiclassical Dynamics Simulation. J. Phys. Chem. A 2011, 115, 10027–10034. 31. Wang, L.; Wang, X. Ab initio Study of Photoisomerization Mechanisms of push-pull p,p’Disubstituted Azobenzene Derivatives on S1 Excited State. J. Mol. Struct.: THEOCHEM 2007, 847, 1–9. 32. Crecca, C. R.; Roitberg, A. E. Theoretical Study of the Isomerization Mechanism of Azobenzene and Disubstituted Azobenzene Derivatives. J. Phys. Chem. A 2006, 110, 8188–8203. 33. Füchsel, G.; Klamroth, T.; Doki´c, J.; Saalfrank, P. On the Electronic Structure of Neutral and Ionic Azobenzenes and Their Possible Role as Surface Mounted Molecular Switches. J. Phys. Chem. B 2006, 110, 16337–16345. 34. Liu, Z.; Ma, J. Effects of External Electric Field and Self-Aggregations on Conformational Transition and Optical Properties of Azobenzene-Based D-π-A Type Chromophore in THF Solution. J. Phys. Chem. A 2011, 115, 10136–10145. 35. Nguyen, P. H.; Stock, G. Nonequilibrium Molecular Dynamics Simulation of a Photoswitchable Peptide. Chem. Phys. 2006, 323, 36–44. 36. Tian, Z.; Wen, J.; Ma, J. Reactive Molecular Dynamics Simulations of Switching Processes of Azobenzene-Based Monolayer on Surface. J. Chem. Phys. 2013, 139, 014706. 37. Xue, X.-G.; Zhao, L.; Lu, Z.-Y.; Li, M.-H.; Li, Z.-S. Molecular Dynamics Simulation Study on the Isomerization and Molecular Orientation of Liquid Crystals formed by Azobenzene and (1-cyclohexenyl)Phenyldiazene. Phys. Chem. Chem. Phys. 2011, 13, 11951–11957.


Page 22 of 38

Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


38. Heinz, H.; Vaia, R. A.; Koerner, H.; Farmer, B. L. Photoisomerization of Azobenzene Grafted to Layered Silicates: Simulation and Experimental Challenges. Chem. Mater. 2008, 20, 6444– 6456. 39. Baranov, V. I.; Gribov, L. A.; Dridger, V. E.; Iskhakov, M. K.; Mikhailov, I. V. Simulation of Photochemical Processes and Calculation of the Quantum Yields of Isomerization Reactions of Substituted Dienes. High Energ. Chem. 2009, 43, 489–495. 40. Tiberio, G.; Muccioli, L.; Berardi, R.; Zannoni, C. How Does the Trans-Cis Photoisomerization of Azobenzene Take Place in Organic Solvents? ChemPhysChem 2010, 11, 1018–1028. 41. Zhang, C.; Du, M.-H.; Cheng, H.-P.; Zhang, X.-G.; Roitberg, A. E.; Krause, J. L. Coherent Electron Transport through an Azobenzene Molecule: A Light-Driven Molecular Switch. Phys. Rev. Lett. 2004, 92, 158301. 42. del Valle, M.; Gutierrez, R.; Tejedor, C.; Cuniberti, G. Tuning the Conductance of a Molecular Switch. Nat Nano 2007, 2, 176–179. 43. Nozaki, D.; Cuniberti, G. Silicon-Based Molecular Switch Junctions. Nano Research 2009, 2, 648–659. 44. Pecchia, A.; Carlo, A. D. Atomistic Theory of Transport in Organic and Inorganic Nanostructures. Rep. Prog. Phys. 2004, 67, 1497–1561. 45. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09 Revision A.02. Gaussian Inc. Wallingford CT 2009.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

46. Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; Celani, P.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G. et al. MOLPRO, version 2010.1, a package of ab initio programs, see http://www.molpro.net. 47. Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M. Molpro: A GeneralPurpose Quantum Chemistry Program Package. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 242–253. 48. Wen, J.; Ma, J. Modulating Morphology of Thiol-Based Monolayers in Honeycomb Hydrogen-Bonded Nanoporous Templates on the Au(111) Surface: Simulations with the Modified Force Field. J. Phys. Chem. C 2012, 116, 8523–8534. 49. Materials Studio, version 4.0, Accelrys Inc., San Diego, 2006. 50. Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Fréchet, J. M. J.; Trauner, D.; Louie, S. G. et al. Reversible Photomechanical Switching of Individual Engineered Molecules at a Metallic Surface. Phys. Rev. Lett. 2007, 99, 038301.


Page 24 of 38

Page 25 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table and Figure Captions Table 1. Modified Force Field Parameters Scheme 1. Schematical representation of the AZO SAMs with different terminal groups. Scheme 2. The flowchart of potential curve calculation and MD simulation of SAM morphology. Figure 1. Rotation pathways along the C–N=N–C torsion angle, α of (a) ground (S0 ) and the excited (S1 ) state of AZO calculated by TD-CAM-B3LYP at 6-31G(d) level for gas phase and (b) on Au31 cluster model. The potential curves under different external fields for (c) AZO(NO2 ) gas phase, and (d) AZO(NO2 )/Au31 calculated by CAM-B3LYP, respectively. Figure 2. Relative energies of (a)AZO and (b) AZO(NH2 ) molecules on the the Au31 cluster models, as a function of C–N=N–C torsion angles, α, under external electric fields with the bias voltage 10V along different directions. Figure 3. Binding energies of (a) AZO, (b) AZO(NH2 ), and (c) AZO(NO2 ) molecules on the Au31 cluster models, as a function of C–N=N–C torsion angles, α, under external electric fields. Figure 4. Frontier orbital energies of (a) trans-AZO, (b) cis-AZO, (c) trans-AZO(NH2 ), and (d) cis-AZO(NH2 ) molecules change with electric field direction on Au31 cluster model. The dipole moment, µ, (in units of Debye) and the direction of their electron transfer is shown as pink and green arrows, respectively. Figure 5. Potential energies of the irradiation of (a)-(b) AZO by UV-Vis light and (c)-(d) AZO(NO2 ) molecules by electric field calculated using CAM-B3LYP (blue), default-CVFF (black), and modifiedCVFF (red) methods, respectively. Figure 6. “On” and “off” switching behavior of (a) AZO and (b) AZO(NO2 ) SAMs on the Au(111) surface in 10 ps under UV-Vis light (black) and electric field (red).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

Figure 7. Snapshots of AZO SAMs with different coverages (a) 3.5×1018 molecules/m2 and top view of the unit cell parameters, (b) 2.3×1018 molecules/m2 . The tilt angle, ω, is defined as the angle between the long axes of AZO chains to the [111] axis. Figure 8. The cargo lift models of (a) trans and (b) cis-SAMAZO(NO2) in Au(111)-SAMAZO(NO2) //SAMC12 Hg junctions.


Page 27 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table and Figures Table 1: Modified Force Field Parameters (1) Evdw = Ai j /r12 − Bi j /r6 (Ai j = atom type Ai a c’ 1668753.3590 b s 365906.4000

p p Ai · A j , Bi j = Bi · B j ) Bi 825.70810 7050.80000

bond typed Au-sh

(2)EAu···S = D · {1 − exp[−k · (r − r0 )]}2 r0 D 1.8000 80.2580

k 1.0530

transition trans → cis (UV) cis → trans (UV) cis → trans (E+ ) cis → trans (E− )

(3)Etorsion = Kαi × [1 + cos(n × αi − α0 )] Kαi n 25.0 1 40.0 1 60.0 1 46.0 1

α0 180.0 0.0 0.0 0.0

a b

C atom in phenyl group of AZOs. S atom in thiols.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1: Schematical representation of the AZO SAMs with different terminal groups.


Page 28 of 38

Page 29 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2: The flowchart of potential curve calculation and MD simulation of SAM morphology.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1: Rotation pathways along the C–N=N–C torsion angle, α of (a) ground (S0 ) and the excited (S1 ) state of AZO calculated by TD-CAM-B3LYP at 6-31G(d) level for gas phase and (b) on Au31 cluster model. The potential curves under different external fields for (c) AZO(NO2 ) gas phase, and (d) AZO(NO2 )/Au31 calculated by CAM-B3LYP, respectively.


Page 30 of 38

Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: Relative energies of (a)AZO and (b) AZO(NH2 ) molecules on the the Au31 cluster models, as a function of C–N=N–C torsion angles, α, under external electric fields with the bias voltage 10V along different directions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3: Binding energies of (a) AZO, (b) AZO(NH2 ), and (c) AZO(NO2 ) molecules on the Au31 cluster models, as a function of C–N=N–C torsion angles, α, under external electric fields.


Page 32 of 38

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: Frontier orbital energies of (a) trans-AZO, (b) cis-AZO, (c) trans-AZO(NH2 ), and (d) cis-AZO(NH2 ) molecules change with electric field direction on Au31 cluster model. The dipole moment, µ, (in units of Debye) and the direction of their electron transfer is shown as pink and green arrows, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5: Potential energies of the irradiation of (a)-(b) AZO by UV-Vis light and (c)-(d) AZO(NO2 ) molecules by electric field calculated using CAM-B3LYP (blue), default-CVFF (black), and modified-CVFF (red) methods, respectively.


Page 34 of 38

Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: “On” and “off” switching behavior of (a) AZO and (b) AZO(NO2 ) SAMs on the Au(111) surface in 10 ps under UV-Vis light (black) and electric field (red).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7: Snapshots of AZO SAMs with different coverages (a) 3.5×1018 molecules/m2 and top view of the unit cell parameters, (b) 2.3×1018 molecules/m2 . The tilt angle, ω, is defined as the angle between the long axes of AZO chains to the [111] axis.


Page 36 of 38

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8: The cargo lift models of (a) trans and (b) cis-SAMAZO(NO2) in Au(111)– SAMAZO(NO2) //SAMC12 –Hg junctions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC Graphic


Page 38 of 38

Light- and Electric-Field-Induced Switching of Thiolated Azobenzene

Recommend Documents