Robert Turanský

Switching of functionalized azobenzene suspended between gold tips by mechanochemical, photochemical, and opto-mechanical means

Optical, purely mechanical, and combined opto-mechanical switching cycles of a molecular switch embedded in a metal junction are investigated using density functional theory and (excited state) ab initio molecular dynamics. The nanomechanical simulations are done on realistic models of gold electrode tips bridged by a single dithioazobenzene molecule. Comparison of different tip models shows that the nature of the tips affects switching processes both qualitatively and quantitatively. The study predicts that purely photochemical cis⇌trans switching cycles of suspended azobenzene bridges are mechanically hindered; combined opto-mechanical as well as purely mechanochemical forward and backward switching is, however, feasible.

Optical, Mechanical, and Opto-Mechanical Switching of Anchored Dithioazobenzene Bridges

Constructing nanoscale devices and nanomechanical machines based on “active” molecular building blocks as functional cores is a rapidly expanding field due to the broad range of promising applications. Most crucial to any such endeavor is the possibility to manipulate, in a reversible manner, properties of the functional molecular unit, and thereby the nanostructure it is part of, via external control at the macroscopic level. This is achieved by inducing conformational changes, that is, isomerizations, which lead to different molecular properties. In addition to inducing such changes by electric currents in STM setups, using light in conjunction with optically isomerizable molecules 9] is a widespread and extremely flexible technique exploited to realize such molecular triggers. The powerful photoswitching capabilities of this kind of molecules have led to a wealth of exciting prospects, such as their use in optomechanical molecular engines or in opto-electronic devices 15] as well as in sensors and data storage media exploiting their isomer-dependent structural and electronic properties. The most prominent archetypal photoswitchable unit used is azobenzene (AB), C12H10N2, which undergoes reversible lightinduced changes between its cis (CAB) and trans (TAB) conformations, using laser light of appropriate wavelength, see Figure 1. Isomerization via the first excited singlet state (S1) proceeds by rotation about the N=N bond and can be monitored by the dihedral angle f=aCNNC. In the ground state, isomerization can take place either via rotation or inversion, the main reaction coordinate of the latter being the bond angle q=aNNC. 19] Most importantly, the CAB!TAB isomerization is accompanied by both a significant increase in the molecule’s length (by 2.4 ) and a dramatically increased conductance of TAB. One challenge here is to interface the nanometer-sized molecular units with the outside world. This is mostly done by functionalizing AB with thiolated derivatives leading to 4,4’-dithioazobenzene (DAB, C12H10N2S2), which readily form covalent bonds, that is, sulfur bridges, to gold surfaces, tips, or mechanically controllable break-junctions (MCBJ), see Figure 2. The harder challenge, pursued herein, is the ability to switch DAB embedded into junctions in a controlled manner. It is often assumed in theoretical considerations that such anchored molecular switches will easily be two-way light-switchable, but experiments do not provide a clear answer. On the one hand, experiments on such AB chromophores and on related molecules indicate that two-way switching of these molecules may not be straightforward. On the other hand, light-driven switching of an AB polymeric chain was reported with only a tiny part of AB units being actually switched. Using ab initio modeling we address, for the first time, the fundamental question concerning the possibility of a complete opto, mechano, and combined opto-mechano switching cycle of a single DAB chromophore suspended between two gold tips. The pure opto-controlled switching is mechanically hindered. On the other hand, both pure mechanoand combined opto-mechano-driven switching are feasible. Mechanical switching calculations are performed using the Perdew–Burke–Ernzerhof (PBE) density functional, a 19-electron (semicore) Au pseudopotential, and an all-electron representation for the other species using DMol3 with a double numerical-plus-polarization basis set at the G-point and a 0.005 Ha electronic smearing. In order to reflect the statistical character of the MCBJ, various gold tips, differing by size/symmetry/sharpness, each composed of 40–60 Au atoms were prepared using an empirical potential together with Langevin dynamics by pulling apart a gold rod coupled to two gold [a] Dr. R. Turanský, Prof. Dr. I. Štich Institute of Physics Slovak Academy of Sciences 84511 Bratislava (Slovakia) Fax: (+ 421) 2 5477 6085 E-mail : ivan.stich@savba.sk [b] Dr. M. Kon pka Department of Physics Slovak University of Technology (FEI STU) Ilkovičova 3, 81219 Bratislava (Slovakia) [c] Dr. N. L. Doltsinis Department of Physics King’s College London London WC2R 2LS (United Kingdom) [d] Prof. Dr. D. Marx Lehrstuhl f r Theoretische Chemie Ruhr-Universit t Bochum 44780 Bochum (Germany) Figure 1. Schematic ground-state (S0) and excited-state (S1) energy profiles along the S0 rotation path, including structures at the respective minima. The top inset defines the main coordinates describing the rotation (via f) and inversion (via q) isomerization mechanisms of CABÐTAB.

Understanding frictional duality and bi-duality: Sb-nanoparticles on HOPG

Antimony nanoparticles deposited under UHV conditions on HOPG are found to exhibit an intriguing frictional behavior characterized by a distinct clearly separated double dual behavior of dependence of the frictional force on contact area. We present the first realistic simulations, density functional modeling adapted to accommodate van der Waals interactions, of the (double) dual frictional behavior. The simulations provide insights into the physics/chemistry of all the frictional branches in terms of incommensurable interfaces, mobile spacer molecules as well as a novel concept of mobile oxidized multi-nanoasperities.

Organometallic Nanojunctions Probed by Different Chemistries: Thermo-, Photo-, and Mechano-Chemistry

Based on ab-initio simulations, three different types of chemistry, namely thermo-, photo-, and mechano-chemistry are compared for organometallic nanojunctions. In the first part we provide the first direct comparison of mechanical versus thermal activation of bond breaking. Study of thiolate/copper interfacesthiolate/copper interfaces provides evidence for vastly different reaction pathways and product classes. This is understood in terms of directional mechanical manipulation of coordination numbers and system fluctuations in the process of mechanical activation. In the second part mechanically and opto-mechanically controlled azobenzene (AB) switch based on AB-gold break-junction have been studied. It was found that both cis→trans and trans→cis mechanically driven switchings in the lowest singlet state are possible. Bidirectional optical switching of mechanically strained AB through first excited singlet state was also predicted, provided that the length of the molecule is adjusted towards the target isomer equilibrium length. The simulations reveal the paramount importance played by mechanical activation for this class of systems.

Azobenzene–Metal Junction as a Mechanically and Opto–Mechanically Driven Switch

Mechanically and opto–mechanically controlled azobenzene (AB) switch based on AB–metal break–junction have been studied using ab–initio simulations. It was found that both cis→trans and trans→cis mechanically driven switchings in the lowest singlet state are possible. Bidirectional optical switching of mechanically strained AB through first excited singlet state was also predicted provided that the length of the molecule is adjusted towards the target isomer equilibrium length.