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Bianthrone at a metal surface: Conductance switching with a bistable molecule
made feasible by image charge effects
Victor Geskin, Samuel Lara-Avila, Andrey Danilov, Sergey Kubatkin, Saïd Bouzakraoui, Jérôme Cornil, and
Thomas Bjo/rnholm
Citation: AIP Conference Proceedings 1642, 469 (2015);
View online: https://doi.org/10.1063/1.4906722
View Table of Contents: http://aip.scitation.org/toc/apc/1642/1
Published by the American Institute of Physics
Bianthrone at a Metal Surface: Conductance Switching with
a Bistable Molecule Made Feasible by Image Charge Effects
Victor Geskin1, Samuel Lara-Avila2, Andrey Danilov2, Sergey Kubatkin2, Saïd
Bouzakraoui1, Jérôme Cornil1, Thomas Bjørnholm3
1
Université de Mons, Service de Chimie des Matériaux Nouveaux, Place du Parc 20, B-7000 Mons, Belgium
Department of Microtechnology and Nanoscience, Chalmers University of Technology, Kemivägen 9, S-41296
Göteborg, Sweden
3
Nano-Science Center & Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100
Copenhagen, Denmark
2
Abstract. Bianthrone is a sterically hindered compound that exists in the form of two non-planar isomers. Our
experimental study of single-molecule junctions with bianthrone reveals persistent switching of electric conductance at
low temperatures, which can be reasonably associated to molecular isomerization events. Temperature dependence of the
switching rate allows for an estimate of the activation energy of the process, on the order of 35-90 meV. Quantumchemical calculations of the potential surface of neutral bianthrone and its anion, including identification of transition
states, yields the isolated molecule isomerization barriers too high vs. the previous estimate, though in perfect agreement
with previous experimental studies in solution. Nevertheless, we show that the attraction of the anion in the vicinity of the
metal surface by its image charge can significantly alter the energetic landscape, in particular, by reducing the barrier to
the values compatible with the observed switching behavior.
Keywords: single-molecule junction, conformational switch, bianthrone, image charge effect, hybrid DFT calculation.
INTRODUCTION
Molecular electronics is a fast developing field which has a potential to dramatically extend the miniaturization
limits of the integrated circuit technology. The ultimate ambition of molecular electronics is to implement the
desired electronic functionality at a single-molecule level by a proper molecular design [1]. In view of possible
device applications, molecular switches appear to be the most promising candidates as CMOS successors. Among
these, conformational switches have a potential to be less prone to environmental uncertainties.
We present a conformational switch based on the bianthrone (BA), an overcrowded bistricyclic aromatic ene
(BAE) [2], which has two well-defined conformational isomers [3] as illustrated in Fig. 1. A coplanar conformation
of the molecule, though favorable for conjugation, is sterically impossible because of overcrowding in the vicinity of
the double bond in the middle (in the “fjord” region): the pairs of adjacent hydrogen atoms (a and a´, b and b´in Fig.
1a) can not share the same space: either a or a´ (b or b´) should be on top of the other. Two distinct conformations –
A and B - arise from two ways for the molecule to reduce the strain by out-of-plane deformation: (i) by sacrificing
the planarity of the anthrones but conserving the coplanarity around the double bond in the folded conformation A
(Figure 1b), where two protons linked to the same anthrone group are both on top, (ii) by conserving the former and
sacrificing the latter in the twisted conformation B (Figure 1c), where one proton is on top and the other underneath.
Pronounced difference in the optical properties of the isomers readily attributed the thermochromism of bianthrone
discovered a century ago to statistical A  B (stable to metastable isomer) multimolecular switching as more
molecules overcome the barrier with increasing temperature [3].
More controlable and reversible switching of BA between its isomers can be realized under electrochemical
stimulus in solution, due to reversed relative stability of the forms upon reduction in favor of the twisted B-form,
more stable in anions and dianions [4]. Electrochemical reduction of the A-form in solution is thus associated with
isomerization to B-form, while the oxidation leads to the isomerization back to A-form as the molecule returns to the
neutral state. The structural constraint which ensures the existence of two forms can not be overruled by any
environmental perturbations as long as the molecule retains its chemical identity. We thus expected that electric
switching behavior would be a robust feature of bianthrone single molecule junctions when we undertook this study.
Proceedings of the International Conference of Computational Methods in Sciences and Engineering 2010 (ICCMSE-2010)
AIP Conf. Proc. 1642, 469-472 (2015); doi: 10.1063/1.4906722
© 2015 AIP Publishing LLC 978-0-7354-1282-8/$30.00
469
(a)
(b)
(c)
(d)
FIGURE 1. The valence formula of bianthrone highlighting the sterically strained hydrogen atoms (a) and the structures of its
form A (b), B (c), and transition state (d) as obtained by B3LYP/6-31g** optimization.
EXPERIMENTAL OBSERVATIONS
We prepared single-molecule BA junctions following a fabrication procedure described elsewhere [5]. All the
samples we measured demonstrated hysteretic conductance switching between two states [6]. For the bias sweeps,
the sample is initially in the “Hi” state and will eventually switch to the “Lo” state (called so according to the high
and low value of the current through the molecular junction). For different scans, the switching occurs at slightly
different biases, though all switching events are obviously clustered around some characteristic bias. Notably, the
Hi  Lo switching is temperature dependent in the range 4-19 K; therefore, the barrier U between the two
conformations should be rather low. Indeed, as explained in details in [7], if the switching rate is temperature
dependent than it can be roughly estimated as  = 0 exp(-U / kBT ), where an attempt frequency 0 is in the THz
range for all molecular vibrations. Therefore, the experimentally observed switching rate  of a few Hz implies that
the barrier for this transition does not exceed the value of 35-90 meV [6].
As persistent switching in a planar device can be reasonably associated to molecular isomerization events, the
barrier which separates the two conformations is dramatically reduced as compared to the barrier for neutral BA
isomerization in solution, as discussed in the next section. This apparent discrepancy motivated us to study
theoretically the rearrangement of bianthrone, first, as isolated molecule and then by modelling the presence of the
surface of a metal leading to the appearance of image charge effects and structural constraints.
QUANTUM-CHEMICAL MODELING
We studied various forms of BA at the hybrid DFT B3LYP/6-31g** level. Both isomers of neutral BA
correspond well to a quinoid valence structure, with a double bond in the middle and sufficient conjugation between
the anthracenes, though the immediate surrounding of the double bond is practically coplanar in A but rather twisted
in B. The calculated relative stability (Figure 2) of the isomers predicts A (shown in Figure 1b) to be more stable
than B (shown in Figure 1c) by 0.12 eV, in excellent agreement with the experimental value of 3 kcal/mol (0.13 eV)
[8]. The relative stability of the anionic forms is predicted to be inversed vs. neutral BA: B is calculated to be more
stable than A by 0.88 eV, again in excellent agreement with the experimental value of 18 kcal/mol (0.78 eV) [8].
The most important factor responsible for the reversal of the relative stability of the forms in the anionic BA is
470
1.0
BA neutral
0.5
Relative energy (eV)
0.80 0.89
0.0
BA-Ag
0.12
0.14
0.27
0.53
-0.5
1.39
2.38
-1.0
0.31
BA negative
-1.5
0.88
-2.0
-2.5
form A
TS
form B
FIGURE 2. Calculated relative energies for the essential structures of free bianthrone and its complex with Ag.
certainly the difference in the vertical electron affinities of the forms, which is increasing with the twist angle
between the antraquinones in BA, in perfect correlation with the decreasing LUMO energy.
The neutral transition state (TS, shown in Figure 1d) is 0.89 eV higher than the stable A form, in excellent
agreement with the experimental barrier height of 0.87 eV [8]. This barrier, low compared to those typical for
rotations around double-bond, due to the ground state destabilization, is still much higher than estimated for lowtemperature switching in the junction. For the radical anion, the A to B isomerization barrier is further decreased
with respect to the neutral molecule; the calculated value is only 0.31 eV, to be compared to the experimental value
of 10.8 kcal/mol (0.47 eV) [4]. This lowering of the barrier is again due to increased electron affinity of BA with tilt
angle, as well as the reversed stability of the isomers discussed above.
At this stage, it is reasonable to check whether negative charging can be induced by the interaction of BA with
metal (silver in particular, as this is the contact material) when the molecule is captured in the junction in our
experiments. To get an idea, we obtained fully optimized structures at the B3LYP/LANLDZP level for the
complexes of one Ag with the A and B forms, as well as the transition state for the rearrangement between A and B
forms when bound to Ag. Silver is found to donate only 0.04|e| to the A form, but with the B form, 0.35|e| are
donated, leading to more pronounced changes in the geometry of the organic part.
As BA can be charged, it is relevant to consider the possible role of the image charge effects. Note that the
classical image charge model was shown to be fully compatible with DFT [9], therefore B3LYP/6-31g** level is
appropriate. We model the image interaction of the BA forms bearing the negative charge of full -|e|. While the
geometry of the B form is not changed appreciably, one of the anthracene units in the A form undergoes notable
planarization, which, in particular, permits its oxygen to decrease the distance to its image. As a result, image
interaction with the optimized structures is more favorable for A, the net relative stabilization of A is thus 0.30 eV.
This behavior is robust when tuning the distance from the image plane. This relative stabilization is even more
pronounced for the TS taken in its initial geometry and optimized in the presence of image charges, under the same
constraints as above. Similarly to the A form at the surface, TS shows the closest anthracene unit getting planarized.
This allows the oxygen of this ring, as well the other oxygen to get closer to their image charges. This leads to
effective decrease of the barrier for the A anion reorganization to 0.10 eV. Though the barrier height in the presence
of the image charges calculated as described here has rather an indicative value, this is of the order of magnitude
suggested by our switching experiments.
471
(a)
(b)
FIGURE 3. Essential structures of bianthrone radical anion as obtained by constrained optimization (atoms no closer than 1Å to
the xOy image plane) with B3LYP/6-31g** in the presence of its image charges (point charges opposite in sign to the atomic
Mulliken charges placed at the molecular mirror image position vs. the xOy image plane): a) form A, b) transition state.
CONCLUSION
Conductance measurements on bianthrone in single-molecule junctions revealed conductance bi-stability and
hysteretic switching between high and low conductance states for all samples prepared with the barrier height in the
range 35-90 meV. However, the experimentally established isomerization barrier for neutral bianthrone in solution
(ca. 0.9 eV) is an order of magnitude higher than the values suggested by our analysis for a molecular switch.
For the neutral and negatively charged single molecule, our computational results are fully consistent with the
experimentally established numbers. When placed in contact with silver, one of the bianthrone forms acquires
negative charge that shifts molecular geometry towards the one of the anion, thus lowering the transition barrier.
More pronounced barrier suppression arises if one takes into account the interaction between the charge on the
molecule and the image charges on the substrate. It appears that the transition state is less rigid than any of the
conformations, which allows the atomic charges in the anion to approach closer to their images, so that the
electrostatic interaction compensates the distortion energy, thus lowering the transition barrier for the anion at a
metal surface down to ca. 0.1eV, in good agreement with our experimental findings. This result shows that the
environment plays a very important role in the performance of single molecules devices; highly polarizable
substrates and metallic electrodes can dramatically affect the energy landscape of the molecular kernel.
ACKNOWLEDGMENTS
The research leading to these results has received funding from the European Community's Seventh Framework
Programme (FP7/2007-2013) under the grant agreement SINGLE no. 213609 and from the Interuniversity
Attraction Pole IAP 6/27 Program of the Belgian Federal Government.
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