A nanoscale junction forces molecules into reactive configurations, allowing a small voltage to catalyze a chemical transformation that normally requires ultraviolet light.
(Nanowerk Spotlight) Chemical reactions follow rules. One of the most fundamental is that molecular orbital symmetry must be conserved when molecules transform from one form to another. This principle, established by Robert Woodward and Roald Hoffmann in the 1960s, revolutionized understanding of how atoms rearrange themselves during reactions. It explains why certain reactions happen easily while others are effectively impossible through conventional means.
When the symmetry of molecular orbitals in the reactants does not match that of the products, the reaction faces an enormous energy barrier, making it “forbidden” in practical terms. So fundamental is this rule that Woodward and Hoffmann once remarked that overcoming it would require a very powerful Maxwell demon, the hypothetical creature from thermodynamics thought experiments that can seemingly violate natural laws by sorting molecules based on their properties.
Nature, however, has already found workarounds. Enzymes, the protein-based catalysts that drive biochemistry, achieve notable feats by confining molecules within specialized active sites. This molecular constraint, combined with precisely positioned electric fields and careful stabilization of high-energy intermediate states, allows enzymes to accelerate reactions by factors that would otherwise seem impossible.
Scientists have sought to replicate these capabilities in artificial systems, creating so-called artificial enzymes that mimic biological strategies. One emerging approach uses nanoscale electrical junctions, devices where a single molecule bridges two electrodes separated by just a nanometer or so, to induce and control chemical reactions.
A study published in Advanced Materials (“Electrical Catalysis of Forbidden Transitions in Single‐Molecule Devices”) now demonstrates that this strategy can electrically drive a reaction that orbital symmetry rules nominally forbid. Researchers used a single-molecule break junction combined with single-molecule Raman spectroscopy to convert a norbornadiene derivative into its quadricyclane form at room temperature, simply by applying a modest voltage.
This particular transformation, a [2+2] cycloaddition where two pairs of atoms form new bonds simultaneously, ordinarily requires ultraviolet light because the molecular orbitals involved have incompatible symmetries for a direct thermal pathway. The energy barrier for the ground-state reaction exceeds 2.7 eV, far too high for thermal activation under normal conditions. The norbornadiene-quadricyclane system already attracts interest as a molecular solar thermal energy storage platform, making electrical control over this conversion particularly valuable.
NBD-QC Reaction Pathways and Associated Raman Spectra. a) Schematic of reaction pathways for NBD to QC conversion. Norbornadiene (NBD, blue molecule) is typically converted to quadricyclane (QC, red molecule) photochemically by exciting the NBD form (gray arrows) which relaxes to the QC form. Because of the orbital symmetry differences between the Highest
Occupied Molecular Orbital (HOMO) of NBD and QC there is a large barrier for the direct reaction between the two (dashed gray arrow). Alternatively, the symmetry is modified through steric control in a nanoconfined single-molecule junction (solid green arrow), thus enabling the reaction to be electrically driven at modest bias voltages. b) Schematic of the reaction pathway explored in this work. Using electrodes to modify the molecular configuration of the molecule to a point near the transition point decreases the resulting energy barrier enabling electrical control over the reaction. c) Examples of Raman spectra obtained for the system in each of the configurations explored. The upper panel shows experimental spectra, and lower panel are spectra calculated using DFT. Differences in the Raman spectra at each of the points enable tracking of the molecular configuration and identification of the emergence of configurations near the transition point. (Image: Reproduced with permission from Wiley-VCH Verlag) (click on image to enlarge)
The key insight lies in how the nanoscale junction affects the molecule before the voltage is even applied. When the norbornadiene derivative binds between two gold electrodes separated by approximately 1.0 nm, it cannot adopt its lowest-energy configuration. Instead, the physical crowding imposed by the electrodes forces the molecule into a distorted geometry that sits partway up the energy barrier toward the transition state.
This nanoconfinement effect mirrors what enzymes accomplish: placing a reactant in an energetically unfavorable but reactive configuration.
The researchers tracked this process in real time using single-molecule Raman spectroscopy, which detects the vibrational fingerprints of chemical bonds. By focusing a 785 nm laser on the nanocavity between electrodes, they captured spectral signatures that revealed precisely what configuration the molecule occupied at any moment.
When a molecule became caught between the electrodes, the electrical conductance suddenly jumped, a signal known as a blinking event that confirms a molecule has been captured. The Raman signal simultaneously changed character. These correlated signals confirmed that both measurements probed the same individual molecule.
To induce switching between the two molecular forms, the team applied a square-wave voltage alternating between 0.05 V and 0.55 V. At the lower voltage, conductance measured approximately 10⁻⁴·⁰ G₀, the conductance quantum being a fundamental unit of electrical conductance, which corresponds to approximately 7.75 nS and is characteristic of norbornadiene. At the higher voltage, conductance dropped to approximately 10⁻⁴·⁷ G₀ (about 1.54 nS), consistent with quadricyclane. This counterintuitive result, lower conductance at higher bias, clearly indicated molecular transformation.
The Raman spectra provided additional confirmation. A vibrational mode near 400 cm⁻¹, corresponding to a collective twisting motion within the molecular core, shifted reproducibly between the two forms. The researchers observed this shift reversibly tracking the conductance changes as the voltage cycled back and forth.
The Raman data also revealed why certain switching attempts succeeded while others failed. Density functional theory calculations predicted that molecules close to their ground-state energy would show minimal Raman activity between 150 and 550 cm⁻¹, aside from the persistent 400 cm⁻¹ peak. However, molecules pushed closer to the transition state by steric forces should exhibit numerous additional modes in this region.
Experimental observations matched this prediction strikingly well. In successful switching events, prominent Raman bands appeared at approximately 200 cm⁻¹, 270 cm⁻¹, and 515 cm⁻¹, all corresponding to scissoring motions of carbon atoms involved in bond formation. These modes appeared in approximately 80% of successful forward switches and 76% of successful reverse switches. When switching failed despite voltage cycling, these additional modes appeared in fewer than 10% of spectra.
The team analyzed nearly 2,000 Raman spectra from 30 successful switching events across three separate molecular junctions, alongside more than 1,000 spectra from 18 failed events. This statistical clarity strongly supports the proposed mechanism: successful electrical catalysis requires nanoconfinement to first position the molecule near the transition state, after which a modest applied voltage drives it over the remaining barrier.
The nanoconfinement serves three critical functions. It sterically distorts molecular orbital symmetry, relaxing the selection rules that would otherwise forbid the reaction. It elevates the molecule’s energy above its ground state, effectively pre-climbing part of the barrier. And it stabilizes these high-energy configurations long enough for the electrical stimulus to complete the transformation.
This work establishes a proof of concept for what the authors describe as biomimetic catalysis, combining the confinement and electric-field effects that enzymes use with the control and characterization capabilities of single-molecule electronics.
The ability to observe transition-state configurations through Raman spectroscopy while simultaneously controlling reactions electrically opens possibilities for studying reaction dynamics that are otherwise inaccessible. The demonstration that orbital symmetry rules can be relaxed through careful nanoscale engineering suggests new strategies for catalyzing high-barrier reactions without requiring the harsh conditions or rare-metal catalysts typically needed to overcome such obstacles.
For authors and communications departmentsclick to open
Lay summary
Prefilled posts
Nanowerk Newsletter
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
