Conformational Behavior of Chemisorbed Azobenzene Derivatives in

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J. Phys. Chem. C 2010, 114, 20556–20563

Conformational Behavior of Chemisorbed Azobenzene Derivatives in External Electric Fields: A Theoretical Study† Chris Chapman and Irina Paci* Department of Chemistry UniVersity of Victoria, P.O. Box 3065, Victoria, BC, V8W 3 V6, Canada ReceiVed: May 31, 2010; ReVised Manuscript ReceiVed: August 5, 2010

Azobenzene derivatives have been shown to act as molecular switches when exposed to an applied electric field or tunneling electrons. Many applications require the switching molecule to be adsorbed on a surface. However, stable conformations and the isomerization energetics of adsorbed azobenzenes can be very different from the analogous more thoroughly studied behaviors in the gas phase or liquid solution. In this study, we investigate the zero-density limit behavior of cis and trans N-(2-mercaptoethyl)-4-phenylazobenzamide chemisorbed on a Au(111) surface. For all calculations, we employ the Perdew-Burke-Erzenhof functional as implemented in the SIESTA package, with a double-ζ plus polarization basis set. A large number of starting geometries were equilibrated, and several stable configurations were identified for both the trans- and cis-adsorbed isomers. The most stable are those in which the azobenzene moiety is parallel to the surface. Applied external electric fields in the usual STM range of (1-3 V/nm produce minimal changes in these geometries. We find that the strength of the dispersive interactions between the extended conjugated system and the metallic surface is such that switching between parallel and upright geometries of single-molecules is unlikely to occur because of coupling to such a field. Although the presence of the surface slightly modifies the ground electronic state pathways for isomerization, this process is also not accessible through simple field-molecule coupling effects. 1. Introduction Molecular switches are molecules that can change reversibly between two or more stable states. The switching process can be triggered by a variety of factors, such as pH, temperature, electromagnetic radiation, chemical stimuli, electric fields, electronic tunneling, etc. Their existence has been long-known, pH indicators being a classic example. At the opposite end of the spectrum, the complex chains of reactions leading to a true/ false type of response, such as the genetic regulation of myelin production around axons in nerve cells,1 are another example of a molecular switch. In nanotechnology, it is expected that molecular switches can serve as circuit elements in molecular electronics. In cases when the switching behavior leads to a change in molecular geometry, the molecular switch can serve as a molecular motor. The archetypal example of a molecular switch in chemistry is the azobenzene molecule and its derivatives. The trans isomer of azobenzene can be reversibly converted into the less stable cis isomer by electromagnetic radiation.2,3 The process has been well studied, and three channels have been proposed:3-7 an inversion channel through bending of a CNN angle, a rotation channel around the CNNC torsional angle, in which the NN bond order becomes one in the excited state, and a concerted inversion channel in which the CNN angles change simultaneously. The reaction pathway realized depends on the excitation energy and the availability of free volume for rotation, but recent theoretical studies suggest that high barriers make the nonconcerted inversion pathway unlikely.6,7 A large number of applications for azobenzene switching have been implemented or proposed. They include optical data storage,8 dopants in nematic-cholesteric orientation changes in †

Part of the “Mark A. Ratner Festschrift”. * To whom correspondence should be addressed. E-mail: [email protected].

liquid crystals,9 polymer additives for photoactive materials,10 chemical sensors,11-14 and photobiochemical switches.14-16 For many of these applications, the photochromic material has to be immobilized on a solid surface. The presence of the surface adds significant complexity to the photoisomerization process because of a number of factors. A fundamental issue is that the chromophore can be either chemisorbed or physisorbed on the surface, which influences its behavior with respect to other important factors, such as monolayer density and surface quenching. Densely packed monolayers of azobenzene derivatives have generally shown poor photoisomerization yields.17 This is thought to be because of the significantly different geometry of the cis isomer, which requires a larger free volume than is often available in a transazobenzene monolayer.18 Exceptions to this were noted when collective isomerization of the monolayer occurred for rigid azobenzene derivatives.19,20 Solutions to the density problem have been found through the use of bulkier groups such as tertbutyl for chemisorbed azobenzene,21-23 the use of bulky surface binding groups,18 or, in coadsorption cases, the use of substituent chains that bring the photochromic group on top of the surrounding alkylthiol monolayer.24 These bulky substituents also serve to distance the azobenzene moiety from the metallic substrate, thus reducing surface quenching of the photoisomerization process. An alternate interpretation of the density-related suppression of trans-cis isomerization processes is based on lateral interactions. Needle-like trans-azobenzene molecules can interact (as well as pack) with surrounding molecules more effectively (through more atoms at optimal distances) than cis or laterally substituted azobenzenes. We have shown previously25 that, in medium- to high-coverage mixed monolayers, the trans-cis isomerization is fully inhibited by lateral interactions at densities

10.1021/jp104967e  2010 American Chemical Society Published on Web 08/31/2010

Chemisorbed Azobenzene Derivatives significantly lower than those where considerations of molecular footprint are relevant. Isomerization of adsorbed azobenzenes can also be caused by electron tunneling or the electric field of an STM tip.26,27 Methyl orange (4-dimethylaminoazobenzene-4-sulfonic acid) has been observed undergoing trans-cis isomerization on Au(111) surfaces, while the reverse process does not occur because its barrier is higher than that for dissociation.26,28 Alemani et al.29 observed field-induced trans-cis isomerization of 3,3′,5,5′-tetra-tert-butylazobenzene, with an STM tip held at a height of 5-6 Å from the surface. Also, unsubstituted azobenzene has been seen to undergo trans-cis isomerization at negative sample bias, with the reverse process occurring at positive bias.30 STM studies suggest that the isomer geometry in the adsorbed phase is significantly affected by substituents. In both the tertbutylazobenzene and unsubstituted azobenzene studies, the isomers seemed to largely preserve their gas-phase geometry when adsorbed. The trans isomer appeared in STM images to be parallel to the surface, while the cis isomer had only one ring parallel to the surface, and the other was tilted away from it. Morgenstern et al. performed STM studies of 4-amino-4′nitroazobenzene on Au(111) surfaces and observed both the trans and cis forms to be parallel to the surface.26,31 To minimize the steric repulsion that pushes the cis-azobenzene phenyl ring out of plane in the gas phase, the CNN bond angles increased to approximately 150° in this case. Yasuda et al.32 observed conductance switching in STM studies of mixed, chemisorbed, alkylthiol/azobenzene monolayers. A high-conductance form of azobenzene was stable in the close-packed alkylthiol monolayer. At boundary and pit sites, molecules exhibited a low-current form at low voltage, which underwent fast reversible switching to a high-current form at voltages above 0.25 V/nm. The authors interpret the motion as a trans-cis isomerization. Das and Abe33 offered an alternate interpretation, arguing that the high isomerization barrier makes the process unlikely at the reported voltages, and that the switching behavior may be due to rotation around one of the single bonds in the linker groups. A careful discussion of cis-trans isomerization versus sp3 rotation in adsorbed azobenzenes has been provided by Fuchsel et al.,34 although these authors did not consider the surface explicitly in their calculations. One important result of this quantum-chemical investigation was the observed large reduction in the barrier for the rotation pathway of the trans-cis isomerization in free azobenzene, when anionic species were considered. A similar reduction was not observed for the inversion pathway, but both pathways were enhanced when cationic species were examined. The authors also discussed the possible impact of applied electric fields on the isomerization process, but their calculations did not account for the effect of the field on the transition state energy. In this paper, we report on quantum chemical investigations of two possible field-induced switching processes in a chemisorbed azobenzene derivative, N-(2-mercaptoethyl)-4-phenylazobenzamide (AB): conformational changes due to physical desorption of the molecule from the surface and trans-cis isomerization in the ground electronic state. Desorption in this context refers to the motion of the molecule from a horizontal configuration in which it is held by dispersive interactions between delocalized electrons in the AB and the surface conduction-band electrons to an upright configuration where these interactions are absent. The AB molecules are, at all times, tethered to the surface through covalent S-Au bonds. Chemi-

J. Phys. Chem. C, Vol. 114, No. 48, 2010 20557 TABLE 1: Changes in the Atomization Energy, Eatom, with the Number of Gold Layers in the Slab layers

Eatom (eV)

1 2 3 4 6

7.06 6.63 6.67 6.66 6.58

sorption is often preferred in nanotechnological device applications over physisorption, as it confers stability to the selfassembled structure. We explored the zero-density limit, as Yasuda et al.32 showed experimentally that switching occurred only in pit sites in the otherwise highly compact mixed monolayers. Thus, we investigate here the possibility that pit-site switching is based on the conformational changes of single molecules, with minimal lateral interactions. The choice of quantum chemical methodology was limited by system size. Despite the known limitations of traditional density functional theoretical methods in describing dispersive interactions, we used the Perdew-Burke-Erzenhof35 (PBE) generalized gradient functional. This functional is known to generally underestimate dispersive effects in molecule-surface interactions,36 which is slightly preferable to other nonbonding37 or overbinding36 quantum methods applicable to our system size. 2. Computational Methods The SIESTA38,39 (Spanish Initiative for Electronic Simulations with Thousands of Atoms) code, version 2.02, was used for all calculations, with a PBE functional in combination with normconserving nonlocal Troullier-Martins40 pseudopotentials. For main group atoms, the valence electrons were treated explicitly. For gold, the 5d and 6s electrons were treated explicitly. The use of pseudopotentials contributes significantly to the high efficiency of SIESTA for large systems such as those considered here. A double-ζ plus polarization (DZP) basis set was used for all atoms. This basis set is standard in SIESTA and somewhat affordable for the size of the system considered here. The gold surface was modeled using a 128-atom regular (111) slab. The slab was minimized using PBE/DZP, with periodic boundary conditions, to remove spurious forces due to mismatch between the ideal fcc Au lattice and the minimum-energy lattice within the computational method. The atoms were then frozen for the remainder of the calculations. Unrealistic forces can also occur when a slab is used as opposed to what would ideally be a semi-infinite surface. To determine the minimum number of Au layers that are necessary to minimize this effect, we examined slab atomization energies by removing a single Au atom from the top surface layer. This atomization energy is given by

Eatom ) E1 + EN-1 - EN

(1)

where EN is the energy of a given N-atom slab, EN-1 is the energy of the slab with a single atom removed, and E1 is the energy of an isolated Au atom. The atomization energy measures the spurious reactivity of a given atom due to incomplete semisurfaces. In other words, we are calculating the potential acting on the atom to be removed due to its presence in the slab. This potential should be independent of the slab thickness considered, as soon as a sufficient number of layers have been included.

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Figure 1. Equilibrium configurations of trans (parallel (a) and upright (b)) and cis (parallel (c), semiparallel (d), and upright (e)) AB isomers chemisorbed on a Au(111) surface in the absence of an electric field. C, H, O, N, and S atoms are colored gray, white, red, blue, and yellow, respectively.

As is indicated in Table 1, two layers are sufficient to produce a value for Eatom that is close to the values obtained using three to six layers. This is consistent with the results of a study by Mavrikakis et al., where the effect of the number of Au layers on binding energy was investigated.41 Therefore, for the sake of computational efficiency, we used two layers with a surface area that fully supported an AB molecule regardless of conformation. Note that Table 1 suggests that a slab composed of more than six Au layers may be necessary for the forces on atoms at or near the surface to be well-converged. The use of such a thick slab, with the necessary surface area, would be impractical given our limited albeit substantial computational resources. Because we are interested in the behavior of an adsorbed molecule in the zero-coverage limit, the unit cells in the calculations that included AB were fixed and made large enough such that interactions between neighboring cells were negligible: cubic boxes with 50 Å edges were used, which provided sufficient vacuum space. Structural relaxations were performed with and without an external electric field. We found that the potential energy surfaces (PESs) were complicated for these systems and zerotemperature quantum chemical optimizations tended to become trapped in local minima. At least two starting conformations were used for both cis and trans isomers: one “parallel” to the surface and the other tilted nearly perpendicular to it. For

“parallel” structures, additional minima were located during the relaxation of nearby configurations along the isomerization paths. While “parallel” configurations are usually lower in energy and thus more stable, the optimization procedure within SIESTA almost never found them when starting from tilted initial configurations and using the default optimization displacement-step guard. Instead, local tilted minima were located. These are interesting, as they provide insight into π-d binding energies by comparison to the “parallel” configurations. Moreover, they are likely to be important in determining the molecular conformation at medium to high surface coverages, when the monolayers adopt tilted configurations.18,20 3. Results and Discussion 3.1. Zero-Field Equilibrium Structures of cis and trans Isomers. We investigated the zero-density limit, zero-field conformational space of Au(111)-chemisorbed cis and trans AB isomers. Figure 1 presents minima on the PES, found from starting points with tilted and parallel initial structures. Relative energies and heights for the five structures are presented in Table 2. Dipole moments, polarizabilities, and approximate field coupling energies are also included, and will be discussed in section 3.2. As noted experimentally for physisorbed azobenzene derivatives,21,30,31 the most energetically favorable structure is the trans isomer, in which the phenyl rings and azo group are oriented parallel to the surface (Figure 1a). This geometry is

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TABLE 2: Structural and Energetic Information for the Stable Configurations of Chemisorbed cis and trans ABa geometry

energy (eV)

µz0 (D)

parallel trans upright trans parallel cis semiparallel cis upright cis

0.00 2.77 1.71 2.85 3.34

0.71 2.87 2.97 4.60 0.86

Rzz height field coupling at (D · nm/V) (Å) (1 V/nm (×10-2 eV) 0.75 2.94 0.92 1.63

4.20 15.68 4.64 10.51 13.05

-6.0/4.5 -9.0/2.9 -7.1/5.2 -11.3/7.9

a Relative energies, components of the permanent dipole moment and of the polarizability along z (the direction of the surface normal), and molecular heights are reported for structures equilibrated in the absence of an external field. Heights are defined as the distances in the z direction between the surface and the atom furthest from it. Approximate field coupling energies are also presented (see section 3.2). Energies are reported relative to the parallel trans geometry.

stabilized by dispersive interactions between the extended π system of the AB molecule and the d electrons of the surface, as well as those involving the lone (n) pairs of electrons on the oxygen atom and the d gold electrons. The latter interaction causes the conjugation of the amide group to become distorted, and thus, the CO-NH group is no longer coplanar with the phenyl rings. In the less-favorable upright trans configuration (Figure 1b), the molecule is oriented approximately 35° from the surface normal. As π-d interactions are no longer present, related strains in the structure are removed, the amide group becomes coplanar with the azobenzene moiety, and the ethylthiol linker recovers its staggered configuration. The latter effect lifts the amide oxygen away from the surface. It remains pointing toward the surface due in part to the geometry of the amide group and in part to the interaction of its free electrons with the substrate. As shown in Table 2, the upright configuration is significantly less stable than the parallel configuration of the trans isobenzene isomer. The desorption energy of about 3 eV is largely due to dispersive interactions between the conjugated AB and the metal surface. In the gas phase, cis-AB is nonplanar because of steric repulsions between hydrogen atoms in the two phenyl rings. As a result, in principle, only one of the two benzene rings can be fully parallel to the surface when adsorbed. Our calculations suggest that, in practice, some of the repulsive energy is overcome by π-d interactions and the dihedral angle between the planes of the two rings decreases from 66° in the gas phase to 22° in its most stable adsorbed configuration (Figure 1c). The two phenyl rings are close to parallel to the surface, and again, the amide oxygen is twisted out of the conjugation plane and toward the substrate. This type of “parallel” cis structure has been previously reported by Henzl et al.31 for physisorbed push-pull azobenzene molecules. More often encountered or surmised in STM studies are semiparallel structures of the cis azobenzenes. In this geometry, one of the phenyl rings is roughly parallel to the surface, while the second is tilted away, due to steric repulsion. In the case of the chemisorbed azobenzene derivative considered here, the semiparallel cis geometry is a local minimum, less stable than the parallel configuration by 1.1 eV (see Figure 1c and d and Table 2). We found that, in this geometry, conjugation was maintained through the amide group and the neighboring phenyl ring. The oxygen was pointing toward the surface, which led to a slight tilt of the inner phenyl ring, away from the parallel configuration. In contrast, the parallel trans structure had the inner phenyl ring parallel to the surface and the amide group distorted. This discrepancy means that, within the PBE/DZP

approximation, the π-d interactions between one phenyl ring and the surface cannot overcome the energy cost of weakening conjugation through the amide group. Tilted initial structures optimize to an upright cis geometry (see Figure 1e). This geometry is higher in energy than the other two cis conformations described here, due to the lack of dispersive interactions between the molecule and the substrate. Physical desorption of the parallel cis structure leads to the upright cis configuration, with a somewhat lower energetic cost than that associated with the trans isomer. Despite deficiencies associated with PBE/DZP-based dispersion and conformational searches at 0 K, this conformational analysis provides an indication of expected behavior in the zerocoverage limit of AB-containing monolayers. Molecular heights are expected to correlate well with STM images, as the most visible features are generally phenyl rings, and thus, the contributions from molecular conductance to the brightness of the feature are relatively unchanged between images. The parallel trans form of AB would be seen as having the lowest height in an STM image. Although the height of the parallel cis structure is comparable to that of the parallel trans structure, the position of the two phenyl rings allows one to distinguish them.31 The semiparallel cis configuration has one phenyl ring protruding from the surface, and thus would be seen as two contrasting spots: a spot of low brightness associated with the lower phenyl ring and a brighter spot due to the upright phenyl ring. The upright trans and cis structures, which have the largest molecular heights, would be seen as the highest or brightest forms. 3.2. Effect of Applied Uniform Electric Field. In an STM experiment, the effect of the applied electric field is felt by the molecule because of coupling with the molecular permanent and induced dipole moments. This can result in geometrical changes, as this coupling acts as a torque on the molecule, tending to align it with the direction of the applied field. A free molecule will move to maximize the field-total dipole coupling. Adsorbed molecules may have to overcome significant binding energies in order for motion to occur. If one considers the z direction to coincide with the applied field, the z component of the permanent dipole moment µz0 and the zz projection of the molecular polarizability Rzz are particularly important. The values for µ0z associated with the four lowest-energy conformers shown in Figure 1 are presented in Table 2. The orientation of the amide group and the relative positions of the phenyl rings have the largest effect on µz0. Since the parallel trans and cis configurations are similar in this regard, their µz0 values are also. The semiparallel cis geometry has the largest µz0 value because its outer phenyl ring is aligned with the surface normal and its CO bond axis is closest to this normal. Total dipole moments µz reported by SIESTA include a large contribution from the redistribution of charge density within the gold slab. To properly account for this contribution, we calculated molecular dipole moments in two steps. First, an AB was optimized on a Au(111) surface in an electric field, applied in the z direction. Then, a single point calculation was performed on the optimized structure with the surface removed. In this calculation, the Au-S bridge bonds were replaced by a SdS bond, aligned with the z axis. This valency-capping method was used because the SdS group does not contribute significantly to µz. For consistency, the µz0 values reported in Table 2 were calculated using the same procedure. Note that the interaction between the surface and AB was not accounted for in the determination of µz.

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Figure 2. Effect of the electric field on µz. The slopes of the curves correspond to Rzz. Black circles and red squares represent parallel and upright trans configurations, respectively, and blue diamonds and green triangles represent semiparallel and parallel cis configurations, respectively.

Figure 3. Effect of the electric field on the energy of the system (AB and gold surface). Black circles and red squares represent parallel and upright trans configurations, respectively, and blue diamonds and green triangles represent semiparallel and parallel cis configurations, respectively. Values are reported relative to the zero-field energy of the parallel trans configuration.

The field dependence of µz for the four lowest-energy conformers is presented in Figure 2. Field strengths were chosen within the range reported for STM experiments for AB.32 We determined Rzz by linear regressions of data points in Figure 2. The resulting values are shown in Table 2. Because geometries were optimized for each field strength, there is a small geometric component of Rzz. The quality of the fits shown in Figure 2 suggests that this contribution was negligible except for the upright trans case. Generally, Rzz values are highest for structures that have a large conjugated structure aligned or nearly aligned with the z axis. This is the reason Rzz is highest in the upright trans structure, while the parallel trans configuration has the lowest value. Less efficient alignment with the field or with the substrate for the semiparallel and parallel cis configurations led to intermediate values of Rzz for these conformers. The effect of applied fields on the energies of structures is shown in Figure 3. The energies are those associated with geometries that were reoptimized in the fields. The parallel trans geometry is energetically favored at all field strengths, even though this geometry has the lowest Rzz (see Table 2). The

Chapman and Paci upright trans structure is stabilized at high fields due to a more favorable field-induced dipole coupling. However, this energy is not sufficient to compensate for a lack of stacking interactions in this configuration. We considered fields of up to 5 V/nm. At this field strength, the upright trans configuration is more stable by approximately 0.5 eV than the parallel configuration. However, it is unclear whether the molecules would be chemically stable in experiments which used such high fields. Geometric effects of the applied field were negligible in all of the configurations that exhibited π-d interactions with the substrate. Only minor twisting of the phenyl rings and very small changes in the distance between the oxygen atom and the surface were observed. The upright trans configuration exhibited changes in tilt angle; at high fields (both positive and negative), it had smaller tilt angles than at low fields. The results presented here suggest that the application of an external electric field does not induce a significant change in the equilibrium geometry through field alignment, in the density regimes where lateral interactions are negligible. An approximation of the field-coupling potential acting on each molecule, calculated as the dot product of the total dipole moment and the applied field,34 provides the values included in Table 2. Although the upright trans isomer can be stabilized at large fields by its high polarizability, it is clear from the table that the differences between field-coupling values for different isomers are only a few times the thermal energy at 300 K, and much smaller than the few eV values of the binding energies for the “parallel” structures. This calculation ignores a number of effects such as geometry changes with the field and higher-order polarizabilities, but our results suggest that these effects are small at applied fields within the standard STM regime. Thus, our calculations suggest that field-dipole coupling cannot overcome the binding energy in the parallel-trans AB, to induce the physical desorption necessary for configurational switching. 3.3. Conformational Stability of 4-Mercaptobutyl-4-phenylazobenzene. The behavior of the derivative 4-mercaptobutyl4-phenylazobenzene (AAB, alkyl azobenzene) was also studied, to determine the contribution of the n-d coupling terms to the binding energy in the various AB isomers and conformers. Possible effects of the replacement of the conjugated amide with an ethyl group are (1) a higher flexibility of the linking group, (2) decreased strains due to the competition between conjugation and oxygen-surface n-d coupling, (3) a loss of stabilization through this coupling, and (4) changes in the molecular dipole moment and polarizability and thus their coupling to the field. Minima on the zero-field PES of AAB are similar to those found for the amide derivative (see Figure 4). The most stable structure is that of the trans isomer parallel to the surface (see Figure 4a and Table 3). The molecule lies somewhat higher above the substrate than AB, because of the smaller attraction between the linker group and the surface. Upright trans and semiparallel and parallel cis structures were also found (see Figure 4b-d, respectively). However, their structures show notable differences when compared to the corresponding amide derivatives. The upright trans structure is tilted significantly closer to the substrate, with a tilt angle of 65° from the surface normal, and the azobenzene moiety in a T-configuration to the substrate. Also, the desorption energy is smaller by about 1.1 eV in this case than in the corresponding amide derivative. An analogous structure for the amide would experience steric repulsion between the linking group and the surface.

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Figure 4. Equilibrium configurations of trans (parallel (a) and upright (b)) and cis (semiparallel (c) and parallel (d)) AAB isomers chemisorbed on a Au(111) surface in the absence of an electric field. C, H, N, and S atoms are colored gray, white, blue, and yellow, respectively.

TABLE 3: Energies, Heights, and Permanent Dipole Moments of AAB Equilibrium Structures in the Absence of an Electric Fielda geometry

energy (eV)

height (Å)

µz0 (D)

parallel trans upright trans semiparallel cis parallel cis

0.00 1.71 1.10 1.21

5.01 10.15 6.94 5.22

1.77 1.39 2.97 2.00

a Energies are reported relative to the parallel trans geometry of AAB.

The 1.7 eV desorption energy of trans-AAB is in agreement with experimental data for small aromatic systems such as benzene, which has a desorption energy of about 0.65 eV.42,43 McNellis et al.44 find an experimental desorption energy of 1.7 eV for tetra-tert-butyl azobenzene. This agreement may be due to cancellations of errors45 arising from the inability of PBE to account correctly for dispersion interactions, basis set superposition errors for the small atomic-orbital basis set used here,43,46 and the fortuitous use of periodic boundary conditions. Note that our desorption energy values are significantly higher than those reported in ref 42 for plane-wave calculations on physisorbed azobenzene. The relative stability of the cis isomer configurations is different than that for AB. The semiparallel geometry is more energetically favorable than the parallel configuration (see Table 3). This is in part due to steric repulsion in the azo group region of the parallel cis structure. However, the main difference is that the inner phenyl ring of the semiparallel structure can be fully horizontal in AAB, because it lacks the intramolecular strains discussed above, which lead to a twist away from the horizontal plane for the analogous amide conformer. The equilibrium structures of AAB demonstrate that the carbonyl group of AB influences the adsorbed geometry and its associated energy. Coupling of the oxygen atom to the surface results in either the distortion of the inner phenyl ring from parallel or a loss in conjugation with the neighboring ring. When the amide unit is replaced with an ethyl group, the added flexibility and decreased surface coupling allow the molecule to adopt lower-energy metastable conformations. However, by removing the amide group, the coupling of the molecules to the field is significantly reduced. All things considered, fieldinduced switching of AAB due to desorption seems unlikely.

3.4. Ground State Inversion and Rotation Pathways of trans-cis Isomerization. We also conducted a preliminary investigation of the inversion and rotation pathways on the ground state PES for the trans-cis isomerization of AB in the presence of a gold surface. These pathways are relevant if a field-induced isomerization process is to exist for adsorbed AB in the zero-density limit. There are effects of the metallic substrate that may make these pathways different from the wellinvestigated gas-phase channels. We have shown in previous subsections how the presence of the surface led to parallel equilibrium geometries for the reactant (trans isomer) and product (cis isomer). The surface may also stabilize transition state geometries, such as parallel structures that would be energetically unfavorable in the gas phase. Furthermore, the surface restricts the adsorbed AB molecule from adopting certain conformations. We found that the surface has a profound influence on the various geometries along the two possible isomerization pathways. The relaxed rotation and inversion pathways of the parallel trans to parallel cis isomerization in the zero-density limit were calculated as follows. For the rotation pathway, the CNNC dihedral was changed and frozen from 180 to 15°. Parallel trans and cis geometries were the initial configurations for angles of 105-180 and 15-90°, respectively. For the inversion pathway, the CNN angle of the outer phenyl ring was varied from 120 to 240°, with the closest bound stationary state used as the reference geometry: parallel trans for angles of 120-175° and parallel cis for angles of 185-240°. Geometries, other than the frozen angular constraint, were relaxed for each point along the pathways. Because the inner ring of the AB molecule is attached to the surface, a concerted inversion pathway was not considered. Energy profiles for the inversion and rotation pathways are presented in Figure 5. Transition state structures are given in Figure 6. The inversion mechanism involves a succession of intermediate geometries that have both phenyl rings roughly parallel to the substrate. The azobenzene moiety was planar between the trans isomer and the transition state. As the CNN bond angle was increased further, the inner ring twisted away from the horizontal, as a result of steric repulsion. The activation energy was found to be approximately 2.3 eV for the trans-tocis pathway and 0.6 eV for cis-to-trans. The NNC angle (azo group nitrogen atoms and closest carbon atom of the inner ring) did not change toward a concerted pathway: it changed from 113° near the trans starting point to 137° at the cis geometry.

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Figure 5. Energy profile of trans-cis isomerization of the (a) inversion and (b) rotation pathway for the electronic ground state. In both figures, the left side corresponds to the trans configuration, while the right side corresponds to the cis isomer.

Figure 6. Inversion and rotation pathways of AB trans-cis isomerization on a Au(111) surface.

Rotation channels for gas phase isomerization generally have a higher barrier than inversion channels in the ground state, with this ordering reversed in excited states.4,47,48 Calculated rotational barriers range in height from 1.6 to 2.4 eV in the ground state, while inversion pathways have barriers of 1.6-2.2 eV.4,7,47,48 As an adsorbed molecule undergoes the rotational transformation, the isomerization mechanism requires a change in the CNNC dihedral, leading to a loss of planarity. The energy profile for this mechanism is illustrated in Figure 5b. The ground state energy barrier is approximately 0.4 eV lower than that for the inversion pathway. The transition state is reminiscent of the semiparallel cis structure (see the lower path in Figure 6 and Figure 1d). However, in this case, the inner phenyl ring and azo group are oriented nearly parallel to the surface. It is important to emphasize that all of the inversion and rotation calculations were performed in the electronic ground state. During the isomerization of AB, the NdN bond is partially broken in both inversion and rotation mechanisms. Our calculations allowed for the possibility of spin polarization, so they should be able to accurately account for this. However, the mechanisms described here do not apply to UV-light-induced isomerizations, which occur in excited states, or to tunnelinginduced transformations that involve charged states along the isomerization pathways. 4. Summary Substrate-supported azobenzene (AB) structures, whether in pure or mixed monolayers, show promise in a number of areas from switches for molecular electronics to biosensors. However, studies involving electric-field-induced changes in supported azobenzenes are still at an exploratory stage. From a theoretical perspective, these systems are challenging: (1) a large number

of atoms are necessary to describe the monolayer, (2) a multifaceted treatment of the interaction of the molecules with the applied field is required, (3) the dispersive interactions that contribute to structure formation and self-assembly are difficult to treat accurately, and (4) the inclusion of a many-electron metallic substrate is computationally costly. In this paper, we investigated the conformational and ground state isomerization behavior of two Au(111)-chemisorbed azobenzenes in the zero-density limit. We employ DFT calculations, which include a surface large enough to provide a substrate for the entire AB molecule, to investigate the influence of an applied electric field on (meta)stable AB geometries and their interconversion. Not unexpectedly, we found a rich potential-energy surface with multiple minima challenging for zero-temperature DFT minimization procedures. Two stationary state conformational motifs were identified for the trans isomer, and three for the cis isomer. Regardless of the applied field, within the usual range used in STM experiments, the equilibrium geometries in both isomers maximize π-d interactions with the gold substrate. Partial or full physical desorption costs a high energetic price, with energies in the 1-3 eV range. In contrast, the field-total dipole coupling energies are on the order of 0.1 eV. When present, the interactions between an amide group and the surface provide a large percentage of the binding energy. However, it also competes with the azobenzene moiety, through conjugation, in realizing the most effective geometry for binding to the surface. This is particularly important for the cis isomer, which was able to achieve more stable configurations when the amide group was replaced with an equal-length alkyl spacer. On the other hand, an amide group contributes significantly to the molecular permanent dipole moment, and so in addition to

Chemisorbed Azobenzene Derivatives the enthalpic cost associated with the switching process, the potential provided by the electric field to the molecule is also reduced in the alkyl derivative. We also investigated the two trans-cis isomerization pathways for chemisorbed molecules in their ground electronic state, with the two “parallel” equilibrium geometries as their endpoints. The inversion pathway was probed by increasing the CNN bond angle, whereas the rotation pathway was investigated by making changes to the CNNC dihedral. We found that, although the surface significantly affects the geometrical pathway of both mechanisms, the barriers for the two channels remain within the range calculated for analogous gas-phase isomerizations. The results of the present study suggest that the application of an electric field within the range usually used in STM experiments does not produce switching through either physical desorption or trans-cis isomerization changes in isolated Auchemisorbed AB molecules. However, there remains a distinct possibility that the zero-density approximation used here does not apply to molecules chemisorbed on the edges of monolayer pits. Lateral interactions between the matrix molecules found at these edges may stabilize upright AB structures. Thus, experimentally observed switching at pit sites could be the result of an upright process, be it cis-trans isomerization or some rotation in the linking group. We are in the process of examining these issues. The results will be the subject of a future publication. Acknowledgment. Funding was provided by NSERC, CFI, BCKDF, and the University of Victoria. This research was performed in part using the facilities of the WestGrid computing resources, which are funded in part by the Canada Foundation for Innovation, Alberta Innovation and Science, BC Advanced Education, and the participating research institutions. WestGrid equipment is provided by IBM, Hewlett-Packard, and SGI. References and Notes (1) Taveggia, C.; Zanazzi, G.; Petrylak, A.; Yano, H.; Rosenbluth, J.; Einheber, S.; Xu, X.; Esper, R. M.; Loeb, J. A.; Shrager, P.; et al. Neuron 2005, 47, 681. (2) Rau, H. In Photochromism: Molecules and Systems; Durr, H., Bouas-Laurent, H., Eds.; Elsevier Academic Press: Amsterdam, The Netherlands, 1990; p 1. (3) Tamai, N.; Miyasaka, H. Chem. ReV. 2000, 100, 1875. (4) Cattaneo, P.; Persico, M. Phys. Chem. Chem. Phys. 1999, 1, 4739. (5) Ishikawa, T.; Noro, T.; Shoda, T. J. Chem. Phys. 2001, 115, 7503. (6) Cembran, A.; Bernardi, F.; Garavelli, M.; Gagliardi, L.; Orlandi, G. J. Am. Chem. Soc. 2004, 126, 3234. (7) Diau, E. W.-G. J. Phys. Chem. A 2004, 108, 950. (8) Kawata, S.; Kawata, Y. Chem. ReV. 2000, 100, 1777. (9) Ichimura, K. Chem. ReV. 2000, 100, 1847. (10) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139. (11) Oosaki, S.; Hayasaki, H.; Sakurai, Y.; Yajima, S.; Kimura, K. Chem. Commun. 2005, 5226. (12) Horie, M.; Sakano, T.; Osakada, K.; Nakao, H. Organometallics 2004, 23, 18. (13) Liu, N.; Chen, Z.; Dunphy, D. R.; Jiang, Y.-B.; Assink, R. A.; Brinker, C. J. Angew. Chem. 2000, 115, 1847.

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