Photoswitching Azobenzene Derivatives in Single Molecule Junctions

Article pubs.acs.org/JPCC

Photoswitching Azobenzene Derivatives in Single Molecule Junctions: A Theoretical Insight into the I/V Characteristics Silvio Osella,† Paolo Samorì,‡ and Jérôme Cornil*,† †

Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium ISIS & icFRC, Université de Strasbourg & CNRS, 8 allée Gaspard Monge, 67000 Strasbourg, France

‡

S Supporting Information *

ABSTRACT: The I/V characteristics of several photoswitching azobenzene derivatives connected to two gold electrodes to form single-molecule junctions are investigated within the nonequilibrium Green’s function formalism coupled to density functional theory. We focus here on the changes in the I/V characteristics as a function of the length and degree of fluorination of the conjugated backbones as well as different coupling strength at the electrodes (chemisorption versus physisorption) upon trans/cis isomerization. The calculations illustrate that the conductance is larger for the trans isomer when the molecule is chemisorbed at both electrodes. However, a larger conduction for the cis isomer is found in the presence of a physisorbed contact at one electrode for specific geometries of the isomer in the junction, in full consistency with the apparent discrepancies observed among experimental measurements. The I/V curves are fully rationalized by analyzing the evolution under bias of the shape of the transmitting molecular orbitals.

1. INTRODUCTION In the past two decades, the study of the I/V characteristics of molecular devices has triggered a growing interest in view of the possibilities to scale electrical components down to the nanoscopic level.1−12 Introducing functionalities by switching the device characteristics via an external stimulus (such as light, redox chemistry, or a gate bias) opens the way to potential applications of such switches in molecular electronics,13−18 molecular machines,19 and biosensors.20 Among the different compounds studied, azobenzene derivatives are highly suitable candidates for photoresponsive molecular switches based on their reversible conformational changes from a trans to cis configuration that lead to two different states21,22 (high versus low conductance); see Figure 1a. Several theoretical and experimental studies have pointed to higher conductance values for the trans isomer,23,24 as intuitively expected from the lower gap of the closed form that should translate in lower charge injection barriers; however, the opposite trend has also been observed.14,19 For instance, Cuniberti et al.23 showed by first-principles calculations that the transmission of the trans isomer is larger than that of the cis isomer when chemisorbed between two carbon nanotubes, while the opposite scenario prevails in the presence of silicon electrodes. Cheng and co-workers24,25 showed by first-principles calculations that the cis isomer has a lower transmission than the trans form when the molecules are chemisorbed on gold electrodes. Nichols et al.26 reported experimentally with break junction measurements a strong increase in conductance for the cis isomer when the chemisorbed (via carboxylate linkers) tilted phenyl group gets in close proximity to the gold electrode. These different evolutions are attributed to changes in the nature of the © 2014 American Chemical Society

Figure 1. (a) Representation of the azobenzene photoswitching mechanism from the trans to cis isomer. (b) Chemical structures of the azobenzene derivatives under study.

anchoring group, in the geometric orientation of the molecules in the junction (associated for instance to a different degree of coverage), or in the molecular conformation;27 in turn, this impacts the nature of the orbital hybridization between the molecule and the metallic electrodes and/or the shape of the Received: May 9, 2014 Revised: July 23, 2014 Published: July 26, 2014 18721

dx.doi.org/10.1021/jp504582a | J. Phys. Chem. C 2014, 118, 18721−18729

The Journal of Physical Chemistry C

Article

orbital over the molecular backbone28−30 and hence the transmission coefficient.31 The discrepancy found for azobenzene molecules chemisorbed on different metal electrodes is also evidenced in the presence of physisorbed contacts, at both the experimental32 and theoretical33 levels. In two separate studies both Samori ̀ et al.14 and Vuillaume et al.34 showed by conducting AFM measurements that a larger conductance is found for the cis isomer when the molecules are physisorbed on one gold electrode, in contrast to the situation prevailing with chemisorbed contacts. Cheng et al.33 showed by first-principles calculations on azobenzene linked to gold electrodes by methylenethiol groups that different degrees of coverage and orientations of the molecule in the junction affect the transmission properties of both isomers with a possible reversal in the ratio of conductance. Note, however, that the conductance can drop by several orders of magnitude when going from a chemisorbed to physisorbed contact.9,35 The previous considerations motivate the use of theoretical approaches to predict and rationalize the conductance ratio between the two forms for various derivatives in well-defined architectures. In this context, we report here a first-principles study of the electronic structure and transport properties of molecular junctions including azobenzene derivatives varying by their length and degree of fluorination of the conjugated backbone (see Figure 1b). These compounds have been recently synthesized and chemisorbed on metallic electrodes to vary their work function by switching the structure from the trans to cis isomers, thus opening the way to a dynamical switching of charge injection barriers in optoelectronic devices.36,37 In order to isolate different contributions affecting the transmission, we consider three different azobenzene derivatives, namely AZO1, AZO2, and AZO3 (see Figure 2). The derivative AZO1 is made of a

where a benzene ring of the lower branch has been removed. Interestingly, our calculations indicate that the trans isomer yields the largest current when the molecule is chemisorbed on the two electrodes; in contrast, the cis isomer yields the largest current only in the presence of physisorption at the interface with one electrode for specific geometries of the molecule in the nanogap.

2. METHODOLOGY The geometric structures of all azobenzene derivatives have first been optimized in gas phase at the (density functional theory) DFT level, with the B3LYP functional38 and a 6-31G(d,p)39 basis set, with the Gaussian09 software.40 When the molecules are chemisorbed via metal−sulfur bond, the hydrogen atom of the thiol groups was removed and the molecules connected to two semi-infinite parallel gold (111) surface electrodes. These electrodes sandwiching the molecules are described by 3 layers (left side) and 4 layers (right side), with each layer made of 25 (5 × 5) gold atoms; periodic boundary conditions are applied in the plane perpendicular to the transmission direction. The unit cell has been chosen to match the size of the anchored molecules and to ensure weak interactions with molecules in the image cells (with interdistances larger than 8 Å). In all junctions, the chemisorbed sulfur atoms are anchored on a top site of the Au (111) surface, at a fixed distance of 2.42 Å suggested by experiments and theoretical calculations.41−43 In presence of a physisorbed contact, the shortest distance between a fluorine atom of the molecule and the gold atom of the surface has been fixed at 2.5 Å (lower than the sum of the van der Waals radii of fluorine (1.47 Å) and gold (1.66 Å) atoms); note that reducing (increasing) this distance down to 1.5 Å (up to 3.5 Å) yields no major differences in the overall shape of the transmission spectra though the intensity of the transmission peaks is reduced with an increasing interatomic distance. The unit cell is next replicated along the x and y axes perpendicular to the charge transport direction in order to generate infinite two-dimensional slabs. In a last stage, the two slabs are connected to semi-infinite electrodes to compute the I/V characteristics of the individual junctions. The transmission spectra of the gold−molecule−gold junctions have been calculated using the widely used nonequilibrium Green’s function (NEGF) formalism coupled to a DFT method, as implemented in the ATK2008.10 package.44,45 The exchangecorrelation GGA.revPBE functional46 has been used, with a SZP (DZP) basis set for valence gold electrons (the valence molecular orbitals), a (5 × 5 × 50) k-point sampling, a mesh cutoff of 180 Ry, and a temperature of 300 K. The parameters of the DZP basis set have been adapted in the same way for all atoms so that the work function of the clean gold (111) surface of 5.26 eV matches the experimental value of 5.2 eV.47 The core electrons are frozen and included in norm-conserving Troullier−Martins pseudopotentials.48 The I/V characteristics have been calculated on the basis of Landauer formula49 in a bias window between −1.2 and +1.2 V. The GGA functional used in the present study is known to underestimate the HOMO−LUMO gap of conjugated molecules due to the lack of derivative discontinuity. However, this limitation proves useful in the present case since it accounts implicitly for the reduction of the electronic gap of molecules inserted into molecular junctions induced by image effects. Over the recent years, this DFT/NEGFT approach has been widely exploited as a useful and robust tool to identify transmission pathways,50 estimate the magnitude of the current

Figure 2. Structures of the three azobenzene derivatives inserted within the junction for both the trans and cis isomers. cis-I and cis-II refer to a tilted and normal orientation of the lower branch of AZO2 with respect to the bottom electrode.

central NN bond connected to two biphenyl units. We first consider the chemisorption of AZO1 (via covalent Au−S bonds to gold (111) surfaces, AZO1−Au) and analyze the impact of the fluorination of the top phenyl ring (AZO2−Au). In a second stage, we address the impact of introducing weak contact (physisorption) by removing the thiol group on the top fluorinated ring (AZO2|Au). Finally, we look at the influence of channel length on transmission properties while keeping a physisorbed contact with the AZO3 compound (AZO3|Au), 18722

dx.doi.org/10.1021/jp504582a | J. Phys. Chem. C 2014, 118, 18721−18729

The Journal of Physical Chemistry C

Article

Figure 3. Semilog transmission spectra at zero bias for all structures studied in their trans (a) and cis (b) isomers. The vertical dashed line refers to the Fermi energy set to zero.

on a relative basis for different molecules, and investigate new concepts and trends for charge transport in molecular junctions. It is worth stressing that the shapes of the frontier orbitals obtained with the PBE functional are very similar to those provided by other functionals, such as the widely used hybrid B3LYP approach.51 The transmission coefficient characteristic of vacuum tunneling across the junction (estimated from the shortest AZO3|Au junction without the azobenzene molecule) exhibits a very small value at the Fermi level, on the order of 10−9 (to be compared to 10−2 for the junction with the molecule), confirming that the trends described hereafter are not governed by vacuum tunneling The electronic structure of the molecules within the junction has been further characterized by a molecular projected selfconsistent Hamiltonian (MPSH) analysis,45 which consists in the diagonalization of the Hamiltonian of the central scattering region within an LCAO basis set restricted to a chosen set of atoms of the central region. Such analysis provides information about the alignment of the molecular levels with respect to the Fermi level of the electrodes and thus helps in rationalizing the nature and intensity of the peaks in the transmission spectra on the basis of the shapes of the associated orbitals.52

Figure 4. Shape of the occupied orbitals responsible for transmission, within the junction (MPSH analysis) and in gas phase (MO analysis) for the cis (a) and trans (b) isomers of the AZO1 structure. The MPSH level energies are given with respect to the Fermi level energy set to 0 eV.

3. RESULTS AND DISCUSSION 3.1. Zero-Bias Transmission. In the case of the AZO1−Au derivative, the transmission through the single molecule is large at the Fermi level of the electrodes (5.9 × 10−2 for the trans form and 1.4 × 10−2 for the cis form) due to the strong coupling between the molecule and the two gold electrodes, as evidenced by the large broadening of the transmission peak lying close to the Fermi level (Figure 3a,b, black lines). For the cis isomer, the MPSH analysis reveals that the level associated with the transmission peak near the Fermi energy (referred to as the MPSH HOMO hereafter, Figure 4a) is very similar to the HOMO level of the isolated molecule, while a different scenario arises for the trans isomer. As a matter of fact, the HOMO level of the isolated molecule is localized around the NN central bond while, within the junction, the HOMO MPSH is delocalized over the molecular backbone and strongly resembles the HOMO−1 level of the isolated molecule, as depicted in Figure 4b. Actually, the MPSH HOMO and HOMO−2 both dominate the transmission peak lying close to Fermi level while the MPSH HOMO−1 has a smaller contribution due to the poor delocalization of the orbital over the backbone.

In the gas phase, the highest delocalized level is destabilized going from the trans to the cis isomer (from −4.86 eV for the HOMO−1 in the trans form to −4.52 eV for the HOMO in the cis form). In contrast, the ordering of the MPSH orbitals is reversed in the junction since the MPSH HOMO of the trans form lies closer to the Fermi energy than that of the cis form (−0.13 eV versus −0.17 eV, while the MPSH HOMO−2 of the trans isomer lies at −0.24 eV). As a result, the lower transmission coefficient at the Fermi level for the cis form originates from a downward energy shift of the broadened transmission peak associated with the highest delocalized level. The highest delocalized level of AZO1 trans isomer (HOMO−1) gets slightly stabilized by 0.11 eV (from −4.86 to −4.97 eV) in the gas phase upon fluorination of the top benzene ring (AZO2−Au); this shift results from the electron accepting character of the fluorine atoms and the increase in the torsion angle between the benzene ring and the fluorinated ring (from 35.5° in AZO1 to 41.5° in AZO2) that further stabilizes the electronic level. A similar scenario is found for the HOMO−2 level, which gets stabilized by 0.21 eV (from −5.08 to −5.29 eV) when fluorination is present. In contrast, the MPSH analysis yields the same energy for the MPSH HOMO 18723

dx.doi.org/10.1021/jp504582a | J. Phys. Chem. C 2014, 118, 18721−18729

The Journal of Physical Chemistry C

Article

to the cis isomer (7.8 × 10−5) (see Figure 3a,b). The trend is thus reversed compared to the junction with two chemisorbed contacts. In the trans isomer, the loss of the covalent Au−S bond on the right side of the junction induces a localization of the MPSH HOMO level over the unsubstituted branch strongly coupled to the gold electrode (see Figure 6a). The

and MPSH HOMO−2 levels when comparing AZO1−Au and AZO2−Au (−0.14 eV for MPSH HOMO and −0.24 eV for MPSH HOMO−2 in both cases). This points to the occurrence of a Fermi level pinning effect which makes the position of the MPSH HOMO and MPSH HOMO−2 levels quite insensitive to the substitution of the conjugated backbone (see Figure 5a), as evidenced and rationalized in a recent study

Figure 6. HOMO (HOMO−1) level within the junction (MPSH analysis) and in the gas phase (MO analysis) for the trans (a) and cis (b) isomers of the AZO2 physisorbed at one contact. The gap state responsible for the sharp peak at the Fermi energy is also shown for each isomer (c). The MPSH level energies are given with respect to the Fermi level energy set to 0 eV. Figure 5. Shape of the occupied orbitals responsible for the transmission, within the junction (MPSH analysis) and in gas phase (MO analysis) for the trans (a) and cis (b) isomers of the AZO2 structure. The MPSH level energies are given with respect to the Fermi level energy set to 0 eV.

absence of a significant electronic density in the right part of the molecule, intimately related to the presence of a weak contact, is thus responsible for the huge drop in transmission when introducing a physisorbed contact (from 4.4 × 10−2 for AZO2− Au to 5.5 × 10−5 for AZO2|Au). Note that a similarly large drop is observed for the maximum value of the transmission coefficient for the peak associated with the MPSH HOMO level (from 5.9 × 10−1 to 1.3 × 10−4). Interestingly, the MPSH HOMO−2 gets strongly stabilized by 1 eV (from −0.24 to −1.24 eV) and hence does not contribute to the transmission peak at the Fermi energy. The drop is less pronounced (from 5.7 × 10−4 to 7.8 × 10−5) for the cis-I isomer since the transmission coefficient is already pretty low with a covalent bonding on both sides. The MPSH analysis also indicates that the alignment with respect to the Fermi level of the MPSH HOMO of the two isomers of AZO2 physisorbed at one contact is significantly different from the corresponding values found for the same derivatives anchored on both sides (−0.24 and −0.34 eV for the MPSH HOMO of the trans and cis-I isomers for AZO2|Au versus −0.14 and −0.19 eV for AZO2− Au). We have also considered another orientation of the cis AZO2 derivative in the junction with the lower branch standing perpendicular to the gold surface and the second branch physisorbed parallel to the other gold electrode (AZO2|Au cisII); see Figure 6b. The two different geometries might reflect the influence of the degree of coverage in the experiments since the latter geometry is not likely to be accommodated at high packing density. Here, the transmission coefficient increases by 2 orders of magnitude, from 7.8 × 10−5 for the initial orientation of the cis-I isomer to 2.7 × 10−3 for the new cis-II geometry (see Figure 3b). Based on the exponential dependence of the transmission as a function of chain length, this evolution can be rationalized by the changes in the effective length of the junction between the two cases. While the electrons have to flow across the full backbone in the cis-I

by a compensation effect induced by the formation of interface dipoles.51 Moreover, this rationalizes that the transmission spectrum of the AZO1−Au trans isomer is hardly affected by the fluorination (coefficient of 4.4 × 10−2 for AZO2−Au and 5.9 × 10−2 for AZO1−Au at the Fermi level; see Figure 3a, blue lines). A different situation arises when considering the cis isomer for which the transmission coefficient decreases by 2 orders of magnitude, from 1.4 × 10−2 to 5.7 × 10−4 going from AZO1− Au to AZO2−Au. The MPSH analysis shows that the HOMO of AZO2−Au is pinned at −0.19 eV with respect to the Fermi level, to be compared to −0.17 eV for AZO1−Au, thus indicating that a change in the energy level alignment is not responsible for the drop in transmission. The latter is actually rationalized by the fact that the introduction of the fluorine atoms asymmetrizes the molecule and the coupling between the molecule and the electrodes, leading to a stronger localization of the HOMO over one branch of the molecule (see Figure 5b). Interestingly, the AZO2 HOMO−1 is more localized over the donor (unsubstituted) part in the gas phase as expected, whereas a larger localization on the fluorinated branch and larger coupling to the right electrode are observed in the junction. The transmission peak at 0.2 eV calculated for AZO2−Au in the cis form in Figure 3b originates from a gap state essentially localized on the gold substrate and the sulfur linkers. In the next step, we have removed one thiol group from the AZO2 derivative to promote a physisorption process at the top electrode while maintaining the orientation of the molecule in the junction (AZO2|Au). This drastically decreases the transmission coefficient for both isomers, with values smaller at the Fermi energy for the trans isomer (5.5 × 10−5) compared 18724

dx.doi.org/10.1021/jp504582a | J. Phys. Chem. C 2014, 118, 18721−18729

The Journal of Physical Chemistry C

Article

geometry, they have only to travel along the lower branch in the cis-II geometry. In the presence of a physisorbed contact, the transmission spectra of both isomers further display a sharp peak at the Fermi level energy due to a gap state essentially localized on the left gold substrate and the sulfur linkers for the trans and cis-II isomers, while it is localized on the right gold substrate and the adjacent fluorine atoms for the cis-I isomer (see Figure 6c). In a last step, we have analyzed the impact of removing in AZO2 a phenylene ring in the lower branch on the I/V characteristics of the junction while keeping a physisorbed contact at the top electrode (AZO3|Au); we have only considered the geometry with the shorter channel length for the cis isomer. Doing so, the channel length is reduced from 20.9 down to 16.5 Å for the trans isomer and from 10.3 to 6.9 Å for the cis isomer when going from AZO2|Au to AZO3|Au. The highest delocalized level of AZO2 (HOMO−1) trans isomer gets stabilized by 0.12 eV in AZO3 (HOMO−1) (from −5.02 to −5.14 eV) in the gas phase upon decreasing the chain length; the same trend prevails for the HOMO−2 level (from −5.63 to −5.87 eV). In contrast, the same energy is kept for the cis isomer (from −4.65 to −4.61 eV). Similarly, the removal of one phenylene ring slightly perturbed the level alignment in the junction with the MPSH HOMO (MPSH HOMO−2) level localized −0.31 eV (−1.55 eV) below the Fermi level for the trans isomer and −0.34 eV below for the MPSH HOMO of the cis isomer (to be compared to −0.24 and −0.32 eV for the MPSH HOMO of the trans and cis isomers of AZO2|Au, respectively). As it is the case for the AZO2|Au physisorbed trans isomer, the AZO3|Au MPSH HOMO−2 is strongly stabilized and does not contribute to the high-energy transmission peak. Because of the exponential decay of transmission with channel length, higher transmission coefficients are obtained at the Fermi level for AZO3|Au: 1.0 × 10−2 for the cis configuration and 8.4 × 10−4 for the trans isomer (to be compared to 2.7 × 10−3 and 5.5 × 10−5 for AZO2|Au). The impact of reducing the chain length on the transmission coefficient is more pronounced for the trans isomer (drop by 1 order of magnitude) compared to the cis isomer (drop by a factor of 3). The higher transmission coefficient calculated for the cis form is rationalized by the MPSH analysis showing that the external ring of the trans isomer lacks a significant electron density in the MPSH HOMO level whereas significant LCAO coefficients are found on all atoms of the lower branch connecting the two electrodes in the cis geometry, as depicted in Figure 7a,b. Once again, a sharp peak related to a gap state at the Fermi energy, essentially localized on the left gold substrate and the sulfur linker, is found in the transmission spectra for both isomers (Figure 7c). In the coherent regime, the transmission coefficient through the molecule typically follows an exponential decay quantified by a β-decay factor: T = T0e−βl

Figure 7. HOMO (HOMO−1) level within the junction (MPSH analysis) and in gas phase (MO analysis) for the trans (a) and cis (b) isomers of the AZO3 derivative. The gap state responsible for the sharp peak at the Fermi energy is also shown for each isomer (c). The MPSH level energies are given with respect to the Fermi level energy set to 0 eV.

of T when l = 0. The decay factor is here estimated to be 0.68 and 0.69 Å−1 for the trans and cis-II isomers, respectively, on the basis of the transmission intensities calculated for AZO2|Au and AZO3|Au varying by one phenylene ring; note that T0 is the same for the two chain lengths since the contact geometry is systematically preserved. These values are consistent with those reported in the literature for both chemisorption and physisorption for small molecules (0.4−0.6 Å−1) in the coherent transport regime.53 Note that the hopping regime typically appears for wires longer than 4−5 nm and is recognized by a characteristic reduction in the decay factor value.12 This further validates the use of a coherent transport regime throughout our study. Table 1 summarizes the transmission coefficients calculated at the Fermi energy for all derivatives under study. To be of Table 1. Transmission Coefficient of the Trans and Cis Isomers and Transmission Ratio (Trans/Cis for Chemisorbed Contacts and Cis/Trans in the Presence of a Physisorbed Contact)a AZO1−Au

AZO2−Au

transm trans transm cis

5.9 × 10−2 1.4 × 10−2

4.4 × 10−2 5.7 × 10−4

TR (Ttrans/ Tcis)

4.26

77

AZO2|Au 5.5 × 10−5 7.8 × 10−5 (2.7 × 10−3) 1.42 (49)

AZO3|Au 8.4 × 10−4 1.0 × 10−2 12

a

The numbers between parentheses for AZO2|Au correspond to the cis isomer in the cis-II geometry.

practical interest for switching molecular devices, the molecules should display a high transmission ratio (TR) between the two isomers acting as ON/OFF states. Interestingly, the trans isomer promotes the larger transmission when the molecules are contacted covalently to both electrodes, whereas the cis form dominates only in the presence of a physisorbed contact with a short junction geometry. The largest TR value of 77 is obtained for AZO2−Au chemisorbed at both contacts due to the significant drop of the transmission in the cis isomer (vide supra). In the case of physisorption, the TR value is much larger for the cis geometry in the short channel configuration (cis-II) of AZO2|Au compared to the cis-I configuration (49 versus 1.42).

(1)

where the transmission T exponentially decreases with channel length l with a drop dictated by the β value.53,54 This parameter primarily depends on the position of the transmitting molecular levels relative to the Fermi level. For low band gap systems such as alkyne-bridged metal complexes and oligo-zinc porphyrins, the value of the decay factor is very low (