Selectivity in single-molecule reactions by tip-induced redox chemistry
Article source: Florian Albrecht, Shadi Fatayer, Iago Pozo, Ivano Tavernelli, Jascha Repp, Diego Peña, Leo Gross. Selectivity in single-molecule reactions by tip-induced redox chemistry . Science 2022, 377, 298-301.
Abstract: The selectivity of -controlled reactions is a subject of constant exploration in chemistry. In this work, the authors demonstrate that reversible and selective bond formation and dissociation on the surface promote the tip-induced redox reaction of to . Molecular rearrangements leading to different structural isomers are selected by polarity and size of applied voltage pulses through scanning tunneling and atomic force microscopy tips. Characterizing the voltage dependence of the reaction and determining the reaction rate demonstrate the selectivity of the constitutive isomerization reaction and provide insight into the underlying mechanism. Supported by density functional theory calculations, the authors found that the energy distribution of isomer in different charge states is important for reasonable selectivity. Tip-induced selective single-molecule reactions increase the authors' understanding of redox chemistry and may lead to new molecular machines.
Controlling the selectivity of reactions is a central goal of chemistry. In solution, this control can be achieved by manipulating valence electrons by adjusting pH or electrochemical potential. However, by these means, reaction conditions are altered to such an extent that the underlying mechanisms controlling selectivity often remain elusive. The exploration of how external electrostatic fields and charge state manipulation affect the chemical bonds of and is still in its infancy.
Studying surface chemistry through scanning tunneling microscopy (STM) and atomic force microscopy (AFM) provides the possibility to study basic chemical mechanisms under atomically well-defined conditions. The reaction itself can even be triggered directly at will with the tip of a scanning tunneling microscope. With advances in the molecular characterization of STM and AFM, new cutting-edge -induced responses to and reaction mechanisms have been discovered. Typically, in tip-induced chemistry, precursors are designed so that specific bonds can be broken, allowing designated masking groups to detach. This unmasking can in turn cause other reactions, such as intermolecular bond formation, intramolecular bond formation, or skeletal rearrangements.
can control configuration conversion, bond formation and dissociation, and hydrogen tautomerization reactions through charge attachment and charge state manipulation. Through the electric field, the configurational isomers can be controlled and the yield of the Diels-Alder reaction can be controlled. Furthermore, selectivity between molecular translation and desorption, controlled by inelastic tunneling electrons, and selective bond dissociation resulting from adsorption-substrate bond alignment is achieved. Even cutting-edge controlled artificial molecular machines have been demonstrated: for example, by inducing alternating reactions of conformation and configurational isomerization to drive molecular motors to translate molecules.
Here, the authors demonstrate the potential of tip-induced electrochemistry to obtain chemical selectivity in single-molecule reactions, that is, the authors demonstrate that multiple constitutive isomerized reactions can be controlled and selected by voltage pulses from the tip. By selecting specific voltage pulses, different cross-ring covalent bonds are formed.
As a molecular precursor, the author synthesized 5,6,11,12-tetrachlorotetraene [compound 1 (C18H8Cl4), see Figure 1). The authors prepared compound 1 by thermal sublimation on the Cu(111) substrate at a temperature of T ≈ 10 K. Unless otherwise stated, experiments were performed on Cu(111) in 2-ML NaCl at T = 5 K. All reported images were obtained with CO-functionalized probes. All AFM images were recorded in constant height mode with a sampling voltage of 0 V.
Figure 1
Figure 2A is the AFM image of compound 1. The difference in brightness of Cl atoms is due to the different adsorption heights caused by steric hindrance between adjacent Cl atoms. Using voltage pulses from a probe tip positioned above the molecule, the authors separated Cl atoms. The authors observed a threshold voltage of approximately +3.5 V and a tunneling current of I = 1 pA for the dissociation of the first two Cl atoms from compound 1. Typically, when a voltage pulse is applied, molecules also move a few nanometers across the surface.
Figure 2B shows the partially dechlorinated intermediate compound 2 (C18H8Cl2), in which the two Cl atoms of compound 1 are dissociated. The AFM image of compound 2 shows a 10-membered ring on the dechlorinated side, showing characteristic and bright features above the triple bond. This indicates that a retro-Bergman cyclization reaction occurred with intermediate compound 1a as the transition structure. Under voltage pulses from +4 to +4.5 V, the authors dissociated the remaining two Cl atoms, creating a different structure with the chemical formula C18H8. The authors observed structural isomers of compounds 3, 4, and 5 (Figures 2C-2E) and, in rare cases, other isomers.
These molecular structures are highly taut and potentially very active, and none of them have been reported before. Due to the inertness of the NaCl surface and the lower temperature, they are stable under the authors' experimental conditions (|V| 0.7 V). The central parts of the isomers are different. They either appear as a 10-membered ring (such as compound 3), a four-membered ring fused to an eight-membered ring (such as compound 4), or two six-membered rings fused. , generating a carbon backbone similar to chrysene (such as compound 5). In most cases (62%), the authors found that compound 3 was obtained directly after dissociation of all Cl atoms.
Figure 1 shows possible synthetic routes to the formation of these taut isomeric hydrocarbons . First, the cleavage of two Cl atoms of intermediate compound 2 may produce sigma diradical compound 2a, which can produce compound 3 through Cope-type rearrangement. This reaction may occur under transient charging conditions. This would be consistent with a previously proposed mechanism for enediyne cyclization, which is promoted by the formation of the radical -anionic species. Although compound 3 is a plausible resonance structure with two cumulative alkene groups, it is also possible to consider combining an alkene and a cumulative alkene group (compound 3') or two alkenes (compound 3'') within the central 10-membered ring alternative structure. Cross-ring C-C bond formation between radicals in compound 3′’ would lead to the formation of diyne compound 4, while C-C bond formation between radicals in compound 3′ would provide chrysen-based diyne compound 5.
Figure 2
First, the author characterized products 3, 4, and 5 on the NaCl surface. The STM measurement diagram of the electron resonance is shown in Figure 3, accompanied by density functional theory (DFT) calculations. The authors found that compounds 4 and 5 are neutrally charged, and their boundary orbital density image shapes are consistent with DFT calculations (Figure 3A-3K), which are the highest occupied and unoccupied molecular orbitals (HOMO and LUMO) respectively. For compound 4, the authors did not observe the positive ion resonance (HOMO) with voltages up to -2 V. For compound 3, experiments indicate that the molecule is in the anionic charge state. This is consistently shown by the STM images of the interface state electrons, the Kelvin probe force spectrum and the electron resonance (Figures 3B and 3C) and their comparison with theory. Compared to closed-shell compounds 4 and 5, the identification of compound 3 as a radical anion is consistent with the expected trend of sigma radicals being reduced.
Furthermore, the authors' measurements showed that the anion of compound 3 undergoes a Jahn-Teller distortion, which was also found by b3lyp-based DFT (Figure 3P) and explains the observed distortion in the two electron resonances with respect to long molecular axes. The symmetry is broken (Figures 3B and 3C). Structurally, the Jahn-Teller distortion of anion compound 3-1 is characterized by an inward bending on one long side of the 10-membered ring (Figure 3P), and can be observed in the high-resolution AFM image (Figure 3M) . The first electron resonance under negative bias shows two lobes of increased orbital density on the inward-curved side (Figures 3B and 3H). However, the first electron resonance at positive bias shows an increase in density on the side opposite the inward bend (Figures 3C and 3I), in excellent agreement with experiment and theory.
Next, the authors studied tip-induced reactions between compounds 3, 4, and 5. The authors were able to convert the molecule between all three structures when they applied relatively large bias voltage pulses (+2.5 V for V and on the order of 10 pA for I), however, they had limited control over the results. Rearrangements after a bias pulse V = +2.5 V resulted mainly in compound 5 (about 50% of attempts) and less frequently in compounds 4 or 3 (25% each). |V| Compound 5 is stable at 2 V.However, |V| a voltage pulse of 2 V, when applied to compounds 3 and 4, produced different responses depending on the applied voltage.
Figure 3
Figures 4A and 4B show histograms of the results of voltage pulses applying a 0.5 pA current as the initial structure over compounds 3 and 4, respectively. The author's research results are as follows: (i) |V| Compounds 3 and 4 are stable at 0.7 V. (ii) -1.7 V V At -0.9 V, bidirectional switching occurs between compounds 3-4. That is, compound 3 can be transformed into compound 4 (the transition is labeled α) and vice versa (the transition is labeled β), and only with a small probability (10%) will compound 5 be formed. (iii) At +0.9 V V +1.7 V, unidirectional switching of compounds 4 to 3 occurs. That is, compound 4 is converted to compound 3 (transition γ), but compound 3 is stable at these voltages. (iv) When V ≈ 1.9 V, compound 3 is converted into compound 5 with a higher yield, and compound 4 is converted into compound 3 or 5.
Switches between structures with similar thresholds on the compounds 1-, 2- and 3-ML NaCl islands. The data shown in Figure 4 were measured at 2-ML NaCl. The observed voltage dependence of on allowed the authors to select the consequences of tip-induced rearrangements. The authors could choose to form a cross-ring C-C bond within the 10-membered ring of compound 3. In the pulse range of -1.1 ~ -1.7 V, the authors mainly generated compound 4 and compound 5, V = -1.9 V, showing selectivity in single-molecule constitutive isomerization reactions.
For the transition between compounds 3 and 4, the authors studied the respective reaction rates (Figure 4C). Transitions of α (from compound 3 to 4 at negative V) and β (from compound 4 to 3 at negative V) at V = -1.3 V, and γ (from compound 4 to 3 at positive V) at V = +1.1 V 3) as a function of current using different tip heights. The slope of the linear fit in the log-log plot is 1.86 ± 0.18 for α, 1.90 ± 0.26 for β, and 1.61 ± 0.76 for γ. This indicates that transitions α and β are two-electron processes. For γ, the error is too large to distinguish between one-electron and two-electron processes.
Figure 4D visualizes the transition between compounds 3 and 4. Transitions α and γ begin to coincide with the ion resonances of the initial structure detected by STM (Figures 3B and 3A, respectively), indicating that these transitions involve charging (discharging) the initial structure. Note that at electron resonance, the charge state changes instantaneously through the transfer of charge between the tip and the molecule. After a typical lifetime of a few picoseconds on 2-ML NaCl, charge transfer between the molecule and the metal substrate restores the molecule's charge ground state. Structural relaxation after charge transitions can oscillate for tens of picoseconds.
For Figure 4D, the authors calculated the ground state energy and the associated relaxation energy Δ of compounds 3 and 4 in different charge states on the NaCl surface. The relatively large energy of the intermediate compound 30 transitioning from compound 3-1 results from the Jahn-Teller distortion of compound 3-1. When assuming similar energies for compounds 40 and 3-1, the calculated charge transition energies agree well with the STM measured resonances. This junction geometry allows for a local voltage drop of approximately 20% across NaCl. The two-electron process of α observed by
indicates that in addition to the charge transition, a second carrier is required to provide additional energy during the inelastic electron tunneling (IET) process. In contrast, transition β does not involve charging of the initial compound 40, but the transition to compound 30 may occur only through the IET process, followed by charging from the substrate to compound 3-1, the charge ground state of compound 3. In the transition γ, as shown in Figure 3G, the LUMO of compound 4 is briefly occupied. This orbital presents a node plane on the long axis of the molecule and is therefore an antibonding state relative to the central bond compound 4, promoting its cleavage and thus transition to compound 3. Note that at a small current of I ≈ 0.5 pA, the reaction rate is on the order of minutes, so orbital density images can be obtained (Figure 3A-3C). The authors' results show that these reactions are triggered by electron attachment rather than by electric fields alone. The smaller effect of the latter can be explained by the reaction coordinates, i.e. the motion of the molecular plane and the atoms are parallel to the substrate, whereas the electric field applied by the tip is orthogonal to the substrate and reaction coordinates.
surface calculations show that compound 5 is less energetic than compounds 3 and 4 in both the neutral and negatively charged states, explaining the dominant switching to compound 5 observed at larger deviations and at |V| 2.0 V stability. Gas-phase calculations show that the reaction barrier (including neutral and negative charge states) to compound 5 is higher than that between compounds 3 to 4, which explains the higher voltage required to switch to compound 5 than to switch to compound 3 to 4. voltage between. The high potential barrier between compounds 4 and 5 in the neutral and negatively charged states suggests that the transition from compound 4 to 5 observed at V = -1.9 V (Figure 4B) proceeds through compound 3 as an intermediate process.
Figure 4
Switching control at higher bias voltages |V| 2.0 V and currents on the order of 10 pA is less controlled and more difficult to understand, and its detailed description is beyond the scope of this work. Due to the large bias voltage, several consecutive and branched charge transitions and structural rearrangements must be considered. Current-induced barrier and catalytic reduction may also play a role. Furthermore, switching between all three structures may include higher excited states, e.g., occupancy of LUMO+1 for compound 5, which is accessed at V = 2.5 V. It can proceed through the dianion charge state, the ground state energies of the three structures are similar, and the transient occupation of the dianion by compound 3 is observed through resonance tunneling at V = 1.1 V in Figure 3C. The
authors' experiments show that for a molecule on a surface, several chemical transitions between multiple structural isomers can be controlled by tip-induced redox chemistry. At different voltages and polarities, the authors selectively activated one, two or all three transitions between three different isomers (compounds 3, 4, 5). The authors demonstrate directional and reversible switching between two non-ground-state isomers (compounds 3 and 4) and selective formation of anticyclic covalent bonds, intentionally converting compound 3 into compound 4 or 5. The authors learned that reaction selectivity is accessed by applying a bias voltage, promoting a changing energy landscape as a function of transient charge. Generally, the charge ground state can be tuned by the work function of the substrate. The study of redox reactions by
tip-induced electrochemistry will help to better understand important redox reactions in organic synthesis and in nature. For future artificial molecular machines, as shown in the authors' work, controllable, reversible, and selective switching between more than two different structural isomers can enable new functionalities. In addition, at higher temperatures, the high-energy barrier of the 1 eV energy level can facilitate the workload and operation of constitutive isomerization reactions.
