High performance mechano-optoelectronic molecular switch

The study addresses the need for simplified, energy-efficient electronic devices that respond to multiple external stimuli (voltage, light, mechanical stress) for applications such as soft robotics, neuromorphic computing, and IoT. Prior research has largely focused on single-stimulus switching. In-situ molecular photoswitches have struggled with quenching and spontaneous back-switching, while molecular mechanical switches are rarely reported. The authors propose aligning materials design with device architecture by mechanically controlling photoswitching via aggregation-induced emission (AIE) from tetraphenylethylene (TPE) headgroups in SAMs. The hypothesis is that mechanical bending of the supporting electrode will direct supramolecular aggregation, suppress non-radiative relaxation, and enable fast, reversible, high on/off ratio light-controlled conductance switching without light-induced conformational changes, thereby creating a dual-gated mechano-optoelectronic molecular switch.

Conventional high on/off ratio (>100) molecular photoswitches typically rely on optically triggered conformational changes (photoisomerization or photocyclization) and require long illumination times (>30 min), delivering non-volatile behavior suitable for memory but not fast in-situ switching. Volatile photo-assisted conduction approaches avoiding conformational change achieved sub-second speeds but low on/off ratios (~1.4) and limited stability. Landmark single-molecule graphene–diarylethene–graphene junctions achieved >10^2 on/off ratios with <100 ms switching. Large-area SAMs of hemicyanine dyes produced ~100 on/off ratios with <60 s switching. The challenge is simultaneous attainment of large on/off ratios, fast, stable, reversible in-situ switching absent photo-quenching or thermal relaxation. AIE-active systems offer a route to overcome quenching by promoting emission and exciton formation upon aggregation. The authors bridge mechanical and optical stimuli by integrating AIE-active TPE into flexible tunnel junctions to realize mechano-optoelectronic switching.

• Device architecture: SAMs of HS(CH2)10-O-tetraphenylethylene (HSC10-O-TPE) assembled on ultra-flat 30 nm Au films supported on transparent flexible PET. Top contact formed by Ga2O3/EGaIn cone-shaped electrodes to yield PET/Au–SC10–O–TPE//Ga2O3/EGaIn junctions.
• Mechanical actuation: Substrates mechanically bent to defined radii of curvature R (convex negative, concave positive). Radii extracted from images via curve fitting (quartic fit, differential at x=0) with multiple repeats to determine mean and error. Operational range set to convex R = −18.3 mm to concave R = 17.0 mm to avoid damage observed at extreme |R| = 9.4 mm.
• Optical stimulus: 365 nm UV illumination from beneath the junctions for in-situ light-controlled switching.
• Electrical measurements: J–V collected with EGaIn biased, bottom Au grounded. For each junction type, 20–24 J(V) traces from >20 junctions over sequence 0 V → 1.0 V → 0 V → −1.0 V → 0 V, 100 mV step, 0.2 s delay. Current integration time 40 ms, time resolution 100 ms. Data analyzed using Gaussian log-mean of |J|(V), with error bars from FWHM of Gaussian fits.
• Stability and dynamics: Real-time UV on/off cycling to quantify on/off ratios and switching times; >1600 cycles recorded at concave R = 17.0 mm. Switching time statistics from 1115 cycles.
• Controls: Photo-inactive SAMs (Au–SC10 and Au–SC18) tested under same mechanical and optical conditions. Temperature dependence assessed at 25–80 °C to probe transport mechanism.
• Surface characterization: AFM (1 × 1 µm scans; tapping mode) of Au surfaces post-bending across R values to evaluate RMS roughness and detect damage.
• Spectroscopy for aggregation and electronic structure: Fluorescence spectroscopy (excitation 365 nm) to monitor AIE intensity and peak shifts vs R. Angular-dependent C K-edge NEXAFS to determine TPE tilt angle α vs R. UPS to determine HOMO energy vs R relative to EGaIn Fermi level; transition voltage analysis performed.
• Molecular modeling: Classical MD simulations of SAMs on Au(111) slabs (area ~5.19 × 5.5 nm^2; 100 molecules; CGenFF) to generate flat/concave/convex configurations; production runs 1 µs, then curved-substrate models in larger cells with an additional 1 µs. Molecule–molecule packing energies computed from Coulomb and van der Waals interactions; distributions of π–π distances analyzed. TD-DFT (Gaussian16, B97D/cc-pVTZ; tether replaced by methyl) to compute UV–vis spectra of monomer and dimers representative of crystal, concave-SAM, and convex-SAM packings. DFTB+ transport for convex and concave dimers between Au electrodes at 0.5 V to evaluate DOS and transmission changes.
• Safety margins: Maximum working bends chosen to prevent Au surface ruptures or humps that increase roughness and leakage.

• High-performance in-situ photoswitching achieved only under concave bending that induces TPE aggregation: at R = 17.0 mm, average on/off ratio J_UV-on/J_UV-off at 1.0 V is (3.8 ± 0.1) × 10^3 across >1600 reversible cycles; average switching time 140 ± 10 ms (from 1115 switch events); device is 10–100× faster than many prior approaches.
• Mechanical control of switching: On/off ratio depends strongly on curvature R. Convex (R = −18.3 mm): ratio ≈ 1 (no switching). Flat: ratio ≈ 7. Concave (R = 17.0 mm): ratio ≈ 3.8 × 10^3. Demonstrates dual gating by mechanical bending and light.
• Control SAMs (Au–SC10, Au–SC18) show no light-induced change, ruling out thermal or plasmonic artifacts; temperature-dependent measurements (40–80 °C) show indistinguishable J–V vs 25 °C, confirming temperature-independent coherent tunneling dominates in both UV-on and UV-off states.
• Mechanism: Concave bending increases AIE via aggregation that suppresses intramolecular rotations/vibrations, enhances photoexcitation and electron–hole (exciton) formation. This reduces the effective tunneling barrier (ΔE_ME lowered from ~1.6 eV off-resonance), likely through creation of an in-gap SOMO of the photo-excited cation; DOS calculations show LUMO moving closer to the Fermi level in concave SAMs.
• Structural and spectroscopic corroboration: AFM shows minimal change in RMS roughness across working R (≈0.219–0.312 nm), with surface damage only at extreme |R| = 9.4 mm. Fluorescence intensity increases and exhibits a small red shift with increasing concavity, consistent with stronger aggregation. NEXAFS indicates increased tilt angle α of TPE in concave SAMs (e.g., from ~56–60° convex/flat to ~69 ± 5° at R = 17.0 mm). UPS shows HOMO levels remain far below EGaIn Fermi level (−4.2 eV) and shift slightly upward with concavity, consistent with Fermi level pinning and stronger π–π interactions.
• Modeling: MD reveals densified sub-0.5 nm π–π networks and most stabilizing packing energies in concave SAMs; TD-DFT spectra of concave-SAM dimers resemble crystal dimers (strong aggregation), while convex SAM dimers resemble monomers (weak aggregation). DFTB+ indicates enhanced electronic coupling/DOS near EF in concave aggregates.
• Molecular engineering: Increasing the number of TPE units per molecule at R = 17.0 mm boosts on/off ratio: mono-TPE (3.8 ± 0.1) × 10^3; di-TPE (2.5 ± 0.2) × 10^4; tri-TPE (3.3 ± 0.2) × 10^4; tetra-TPE (4.8 ± 0.1) × 10^5. Fast switching (~100–140 ms) maintained. Performance scales approximately exponentially with TPE count up to tri-TPE, with further gain for tetra-TPE due to maximized phenyl density.

The findings validate the hypothesis that mechanical control over supramolecular packing can enable robust, fast, and large in-situ optical conductance switching in molecular junctions. Concave bending drives aggregation of AIE-active TPE headgroups, suppressing non-radiative intramolecular motions and stabilizing photo-excited electron–hole pairs. This aggregation modulates the interfacial electronic structure and lowers the effective tunneling barrier without requiring photoisomerization, thereby overcoming quenching and back-switching issues that limit conventional photoswitches. The strong dependence of on/off ratio on curvature demonstrates a dual-gated mechanism and suggests applications as mechanosensitive optoelectronic switches. Spectroscopy (fluorescence, NEXAFS, UPS) and modeling (MD, TD-DFT, DFTB+) together support a mechanism of mechanically enhanced π–π stacking that produces crystal-like local ordering, reduces conformational entropy, and increases conductance under illumination. Molecular engineering further shows that device performance is tunable, with multiple TPE termini enabling ultra-high on/off ratios while retaining fast response, highlighting the generalizability of the mechano-optoelectronic design paradigm.

This work introduces a mechano-optoelectronic molecular switch based on AIE-active TPE SAMs on flexible Au/PET electrodes, achieving fast (≈140 ms), reversible, and stable in-situ optical conductance switching with large on/off ratios (≈3.8 × 10^3 over >1600 cycles), maximized under concave bending. Structural, spectroscopic, and theoretical analyses reveal that mechanically induced aggregation strengthens π–π interactions and lowers tunneling barriers, enabling high-performance volatile photoswitching without conformational changes. Molecular engineering with multi-TPE units raises the on/off ratio to (4.8 ± 0.1) × 10^5 while maintaining rapid switching, representing best-in-class performance. Future directions include exploring other AIE-active headgroups and linker chemistries, optimizing electrode curvature and surface morphology, integrating devices for flexible mechanosensing and optomechatronic applications, and refining models to include substrate restructuring under mechanical/optical perturbations.

• Extreme bending (|R| = 9.4 mm) damages Au surfaces (ruptures or humps) and increases roughness, so operational bending was limited to R = −18.3 mm (convex) to R = 17.0 mm (concave).
• Transport and spectroscopy were performed under 365 nm UV illumination; although thermal/plasmonic artifacts were ruled out by controls, operation currently requires UV light.
• Computational models constrained Au substrate atoms and did not include local substrate restructuring under mechanical/optical perturbations, which may influence SAM conformation and electronic properties.
• HOMO remains far below the electrode Fermi level with Fermi-level pinning; thus conductance modulation relies on photo-excited states and aggregation effects rather than ground-state level alignment, which may limit behavior under different electrode materials or wavelengths.