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Switching the Conductance of a Single Molecule by Photoinduced Hydrogen Transfer ˘ ´ız˘ek,§ R. Ha¨rtle,†,| O. Rubio-Pons,†,| M. Thoss,*,†,| and C. Benesch,† M. F. Rode,‡ M. C ,‡ A. L. Sobolewski* Department Chemie, Technische UniVersita¨t Mu¨nchen, Lichtenbergstr. 4, D-85747 Garching, Germany, Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland, Faculty of Mathematics and Physics, Institute of Theoretical Physics, Charles UniVersity, Prague, Czech Republic, and Institut fu¨r Theoretische Physik und Interdisziplina¨res Zentrum fu¨r Molekulare Materialien, Friedrich-Alexander-UniVersita¨t, Erlangen-Nu¨rnberg, Staudtstr. 7/B2, D-91058 Erlangen, Germany ReceiVed: February 17, 2009; ReVised Manuscript ReceiVed: April 29, 2009
A new mechanism for optical switching of nanoscale single-molecule junctions between different conductance states is proposed. The mechanism is based on photoinduced excited state hydrogen transfer in the molecular bridge. Employing first principles electronic structure and transport calculations for prototype examples it is shown that the keto and enol tautomeric forms of the molecular bridge exhibit very different conductance properties, which realize the ‘on’ and ‘off’ states of the molecular switch. The feasibility and robustness of the mechanism is analyzed in some detail. Electronic devices that incorporate single molecules as active elements are considered to be promising alternatives to semiconductor-based electronics.1,2 Recent advances in experimental3-10 and theoretical11-14 methodology have allowed the study of the conductance properties of nanoscale molecular junctions, where single molecules are chemically bound to metal electrodes. It has been demonstrated that the current-voltage characteristics of single-molecule junctions may resemble those of basic electronic devices, such as rectifiers8 or transistors.4 An important element for the design of molecular memory or logic devices is a molecular switch. A molecular junction may be used as a nanoswitch, if the molecule can exist in two or more differently conducting states that are sufficiently stable and can be reversibly transferred into each other. A variety of different optical or current-induced mechanisms have been proposed to achieve reversible switching of molecular nanojunctions between different conductance states.15-22 Most mechanisms for optical switches considered so far are based on light-induced conformational changes, in particular isomerization reactions20,21 or ring-opening reactions17,18 of the molecular bridge. Nonoptical mechanisms include reversible redox reactions, for example, in catenane and rotaxane molecules triggered by voltage pulses19 as well as current induced hydrogen tautomerization using an STM.22 As an alternative mechanism for optical switching of a molecular junction between different conductance states, we propose in this paper to use photoinduced excited state hydrogen transfer. In contrast to most other mechanisms suggested previously to realize an optical molecular switch, in particular cis-trans isomerization reactions, hydrogen translocation within the molecular bridge has the advantage that the overall length * To whom correspondence should be addressed. † Technische Universita¨t Mu¨nchen. ‡ Polish Academy of Science. § Charles University. | Friedrich-Alexander-Universita¨t.
10.1021/jp901453b CCC: $40.75
SCHEME 1: Scheme of an Optically Induced Hydrogen Transfer Reaction between Two Tautomers Corresponding to the Oxo-amine (Keto) and Hydroxy-imine (Enol) Form, Respectivelya
a The side group R may be used as a molecular “crane” to facilitate photoinduced hydrogen transfer.
and thus the molecule-electrode binding geometry of the junction is not changed significantly. As a prototype example we consider the reaction depicted in Scheme 1. The molecule may exist in two different tautomers of planar structure, 3-hydroxy-2,4,6-heptatrien-butadienimine (in the following referred to as HB-enol), and 3-oxo-1,4,7-heptatrien-butadienamine (HB-keto) (for R ) H). Both tautomers are connected via the translocation of a hydrogen atom (i.e., a hydrogen transfer reaction). The side group (R) may be used as a molecular “crane” to facilitate photoinduced hydrogen transfer via the electronically excited state (see below). For similar systems, this reaction was theoretically predicted to be reversible23 and can be tuned in either direction by irradiating the sample with light of different wavelength. We first consider the simplest case, with R ) H. The right panel in Figure 1 depicts the keto and enol tautomers chemically bound via thiol groups to two gold clusters, which model the metal electrodes in a metal-molecule-metal junction. The theoretical methodology to describe conductance in molecular junctions has been described in detail previously.26 Briefly, we have used density functional theory (DFT) employing the B3LYP functional and an SV(P) basis set (including ECP-60-MWB on the gold atoms) as implemented in TURBOMOLE25 in combination with a partitioning procedure and 2009 American Chemical Society
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Figure 1. Current-voltage characteristics (left panel) of the two tautomeric forms of a molecular junction depicted in the right panel. The black line shows the current for the enol form (upper structure), whereas the red line depicts the result for the keto form (lower structure).
Green’s function techniques.26 Test calculations with larger basis sets reveal no qualitative changes of the results (see the Supporting Information). The geometry of the central molecular units bound to two gold atoms were optimized and mounted to gold clusters as shown in Figure 1. To describe the effect of infinite leads we have added the surface self-energy of a gold (111) surface to the atomic orbital energies of the gold atoms of the outer gold layer.26 The current for a given voltage was obtained from the Landauer formula27
I)
2e h
∫ T(E)[ fL(E) - fR(E)]
(1)
where T(E) denotes the transmission function of a single electron and fL(E) and fR(E) are the Fermi distributions of the electrons in the left and right lead, respectively. In the calculations, the Fermi energy of bulk gold has been used (EF ) -5.53 eV). Both the transmission function and the Fermi functions depend on the voltage. The transmission function is given by † T(E) ) trM{ΓL(E)GM (E)ΓR(E)GM(E)}
(2)
Here, GM(E) denotes the molecular Green’s function and the functions ΓL/R describe broadening of molecular states due to coupling with the metallic leads. The methodology to obtain these functions is described in ref 26. Figure 1 shows the calculated current-voltage characteristics of the two tautomers. Although the two tautomers only differ by the position of a single hydrogen atom, their conductance properties are drastically different. The keto form exhibits, in particular for positive voltages, a rather low current. The enol form, on the other hand, shows a significant current and an almost Ohmic current-voltage characteristics. Thus, for a given voltage, the two tautomers realize different conductance states of the molecular bridge corresponding to “on” (enol form) and “off” (keto form). The very different conductance properties of the two tautomeric forms can be rationalized in several ways. Considering the valence bond structures in Scheme 1, the enol tautomer corresponds to a fully conjugated system, whereas in the keto form, the conjugation is broken at two sites (oxo and amino moieties). Typically, conjugated systems have a delocalized π-electron density that facilitates conduction. This interpretation is corroborated by an analysis of the orbitals that contribute to the current. Figure 2 (right panel) depicts two representative examples. In the enol form these orbitals are delocalized over essentially the whole molecular bridge thus facilitating a large
Figure 2. Left: Transmission as a function of electron energy (relative to the Fermi energy) at zero bias voltage of the two tautomeric forms of the molecular junction: black, enol form and red, keto form. Right: Two representative molecular orbitals of the enol (top) and the keto (bottom) form.
current. In the keto form, on the other hand, the orbitals are more localized resulting in a smaller current. Further insight in the conductance mechanism can be obtained by considering the transmission function of a single electron, which upon integration via the Landauer formula (cf. methods section) gives the current. Figure 2 depicts the transmission function for zero bias voltage as a function of the electron energy (relative to the Fermi energy). The current at low voltage is mainly determined by the transmission function close to the Fermi energy. The results show that the enol form (black curve) has a much larger transmission than the keto form at the Fermi energy and throughout the whole energy range studied. The fact that the different conductance of the enol and the keto forms is based on a significantly different magnitude of the transmission function of the two tautomers over a broad energy range, and does not rely on the location of molecular levels with respect to the Fermi energy of the metal leads, demonstrates the robustness of the mechanism. The results presented above provide a proof of principle that tautomeric forms of a molecule, which are related to each other by the translocation of a hydrogen atom, may exhibit very different conductance behavior and thus realize different conductance states of the molecular bridge. A reversible change between the two tautomers would thus realize a molecular nanoswitch. The most promising mechanism to achieve efficient translocation of the hydrogen atom is photoinduced excited state hydrogen transfer.23,24 To this end, the molecule has to be functionalized by a hydrogen transferring unit (a molecular “crane”) at site R (cf. Scheme 1). As a prototypical example, we consider a molecular crane based on a pyridine ring realized in the molecular junctions depicted in Figure 3. The enol form is represented by a 2-pyridinyl-3-hydroxy-2,4,6-heptatrienbutadienimine molecule (in the following referred to as PHBenol) and the keto form by 2-pyridinyl-3-oxo-1,4,7-heptatrienbutadienamine (PHB-keto). We first consider the electronic structure and the possible hydrogen transfer reaction pathway for the isolated tautomers determined by electronic structure calculations. Specifically, the equilibrium geometries of the two tautomers of PHB and the minimum-energy reaction paths along the photophysically relevant reaction coordinates in the ground and in the lowest excited singlet states have been determined with the MP2 and the CC2 method, respectively.28,29 To allow efficient explorations of the high-dimensional potential-energy (PE) surfaces, the standard split-valence double-ζ basis set of TURBOMOLE with polarization functions on the heavy atoms (def-SV(P)) has been employed in these MP2 and CC2 geometry optimizations.
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Figure 3. Current-voltage characteristics (left panel) of the two tautomeric forms of a molecular junction depicted in the right panel. The molecular junction has been functionalized by a pyridine ring to facilitate efficient photoinduced hydrogen transfer. The black line shows the current for the enol form (upper structure), whereas the red line depicts the result for the keto form (lower structure).
Figure 4. PE profiles of the central molecular unit (PHB) of the junction depicted in Figure 3 in the ground (S0, circles) and the first excited ππ* (S1, squares) states. The different panels depict PE profiles along the minimum-energy path for hydrogen atom transfer in the keto form (a), pyridine ring torsion (b), and for hydrogen atom transfer in the enol form (c). The PE profile denoted with S0ππ* depicts the energy of the S0 state, calculated along the minimum-energy path of the ππ* state.
Single-point CC2/cc-pVDZ calculations were performed at the equilibrium geometries and along the minimum-energy paths on the S0 and S1 PE surfaces. The conical intersections between the S1 and S0 energy surfaces have been located at the CASSCF level, using the corresponding module of the Gaussian 03 program package.30 Details on the electronic structure methods for the characterization of the excited electronic states and the pathway for hydrogen transfer in the two tautomers of PHB are provided in the Supporting Information. Figure 4 shows the potential energy (PE) profiles of the ground (S0) and the first electronically excited (S1) state along the reaction path for hydrogen translocation. In addition, the “vertical” PE profile of the S0 state calculated at the optimized geometry of the S1 state is shown (dashed line). The left and right panels of Figure 4 correspond to in-plane hydrogen transfer from the amine and hydroxy group, respectively, toward the N atom of the pyridine moiety. These curves have been obtained by freezing only one, OH or NH, stretching coordinate, respectively, while the others were fully optimized (keeping the whole system planar, however). The central panel of Figure 4 describes the rotation of the pyridine moiety around the pyridinechain twist angle. At the highest level of theory employed in the electronic structure calculations, the keto tautomer is 0.16 eV more stable than the enol tautomer (cf. Table S1 of the
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10317 Supporting Information). Both tautomers are separated by a barrier of about 1 eV in the electronic ground state and thus cannot be thermally interconverted into each other. For both tautomers the first optically absorbing electronically excited state (S1) has ππ* orbital character. The vertical excitation energy to the S1 state of the enol tautomer is 3.97 eV and it is redshifted by 0.14 eV from the corresponding vertical excitation energy of the keto form. Optimization of the geometry of the S1 state with Cs symmetry constraint for both enol and keto tautomers results in stable hydrogen-transferred tautomers. Both hydrogen transfer processes are barrierless when starting from the ground-state equilibrium geometry (Franck-Condon geometry) of a given tautomer. Once the ππ* state is populated, the proton moves spontaneously from the hydroxy or amine group, respectively, toward the pyridine moiety. The reaction is fairly exoenergetic. For both tautomers, the hydrogen transfer reaction stabilizes the ππ* state by about 0.7 eV with respect to the vertical (Franck-Condon) energy. These hydrogen transfer structures represent, however, saddle-points on the ππ* PE surface, and are unstable with respect to the torsion around the central C-C bond that links the pyridine moiety with the central chain. The unconstrained geometry optimization of the ππ* state leads to a minimum energy-structure with a pyridine-chain twist angle C(O)-C-C-N of 109.3° and with a slight out-of-plane deformation of the central chain plane in the region of the keto group. The energy of the S1 global minimum is located 2.9 eV above the global minimum of the S0 PE surface but only 1.1 eV above the vertical energy of the S0 state calculated at this geometry. This result indicates that the S1 and S0 states may intersect each other in the vicinity of the perpendicular geometry of the pyrinine ring. Indeed, an optimization of the conical intersection between these states reveals a structure where the pyridine ring is oriented almost perfectly perpendicular to the polyene chain (see Figure S2a of the Supporting Information). The PE profiles shown in Figure 4 allow us to predict that optical excitation with a properly tuned wavelength can switch the system between the two tautomeric forms. This process involves hydrogen atom transfer from the central chain to the pyridine moiety. The subsequent twist of the pyridine ring around the central C-C bond leads to a global minimum on the PE surface of the S1 state with an almost perpendicular orientation of the two molecular moieties. The nearby conical intersection facilitates ultrafast internal conversion to the ground state where a bifurcation of the wave packet between the two tautomeric forms takes place. Since the overall photophysical process (hydrogen transfer and intramolecular twist in the S1 state, nonadiabatic S1 f S0 transition and hydrogen transfer in the S0 state) is expected to occur on the subpicosecond time scale, any optical pulse with duration longer then a picosecond and proper wavelength will allow to switch the system efficiently from one tautomeric form to the other. The efficiency can be further improved by employing pulses with optimized shape. The current-voltage characteristics of the molecular junction including the hydrogen transferring unit is depicted in the left panel of Figure 3. As in the case of the molecular junction without hydrogen transferring unit (cf. Figure 1), the enol form exhibits a larger current for any given voltage. Although the effect is not as pronounced as in the case of the molecule without hydrogen transferring unit, it can nevertheless be used as an optical switch. The “on/off” ratio of the current in the enol and keto tautomers can be further improved by optimization of the hydrogen transferring unit. This will be the subject of future work.
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In summary, we have proposed a new mechanism for optical switching of nanoscale single-molecule junctions between different conductance states based on hydrogen translocation in the molecular bridge. The mechanism relies on different conductance properties of the keto and enol form of the molecular bridge, which realize the “on” and “off” states of the switch. The results presented for prototypical systems demonstrate the feasability and the robustness of the mechanism. With the recent advances in techniques to address nanoscale molecular junction with laser pulses,10 an experimental realization of the optical switch proposed in this work appears to be promising. Acknowledgment. We thank Wolfgang Domcke for numerous helpful discussion. This work has been supported by the Ministry of Science and Education of Poland, the Deutsche Forschungsgemeinschaft (DFG) through the DFG-Cluster of Excellence Munich-Centre for Advanced Photonics and a research grant, a grant of the Interdisciplinary Center of Mathematical and Computer Modeling (ICM) of Warsaw University, and the German-Israel Science Foundation (GIF). Supporting Information Available: Electronic structure methods for the characterization of the excited electronic states and the pathway for hydrogen transfer in the two tautomers of PHB, absorption spectra, characterization of the conical intersection between the S1 and S0 state in PHB, and study of the effect of the basis set on the transport calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (2) Joachim, C.; Gimzewski, J.; Aviram, A. Nature (London) 2000, 408, 541. (3) Reed, M.; Zhou, C.; Muller, C.; Burgin, T.; Tour, J. Science 1997, 278, 252. (4) Park, H.; Park, J.; Lim, A.; Anderson, E.; Alivisatos, A.; McEuen, P. Nature (London) 2000, 407, 57. (5) Smit, R.; Noat, Y.; Untiedt, C.; Lang, N.; van Hemert, M.; van Ruitenbeek, J. Nature (London) 2002, 419, 906. (6) Reichert, J.; Ochs, R.; Beckmann, D.; Weber, H.; Mayor, M.; von Lohneysen, H. Phys. ReV. Lett. 2002, 88, 176804. (7) Qiu, X.; Nazin, G.; Ho, W. Phys. ReV. Lett. 2004, 92, 206102. (8) Elbing, M.; Ochs, R.; Koentopp, M.; Fischer, M.; von Ha¨nisch, C.; Weigend, F.; Evers, F.; Weber, H.; Mayor, M. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8815. (9) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173.
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