Lighting up solid states using a rubber

The study addresses the challenge of efficiently and reversibly regulating structures and properties of materials in the solid state. While many stimuli-responsive materials react to light, heat, magnetic fields, pressure, and mechanical forces, such responses are often slow and incomplete in solids. Chemical stimuli (pH, redox, coordination) are faster and more extensive but typically effective only in solution; in solids, both physical and chemical approaches are less efficient. Topochemical solid-state reactions can proceed with minimal atomic displacement but often require highly ordered systems and face issues of byproduct accumulation and irreversibility. The authors hypothesize that integrating tautomerization with topochemistry via multiple hydrogen-bonded donor–acceptor dimers can enable rapid, reversible, and efficient solid-state switching of optoelectronic properties under mild, green stimuli. Specifically, they propose that in ortho-pyridinyl phenol systems, double H-bonded dimers will undergo triboelectrically induced (negative charge) single-proton transfer, creating a bright tautomer stabilized in the solid state and thus turning on photoluminescence. They further suggest embedding the H-bonded couple within a π push–pull skeleton to tune properties.

Prior work established many stimuli-responsive materials across polymers, supramolecular assemblies, MOFs, and perovskites reacting to physical stimuli (light, pressure, temperature) but often with limited efficiency in solids. Topochemical reactions in solids rely on preorganization and minimal atomic motion but can be constrained by crystallinity requirements and irreversibility. Multiple hydrogen-bonded structures have been widely used in supramolecular and polymeric materials, remain intact in the solid state, and can mediate topochemical processes, energy transfer, and information transfer (chirality, sequence). These insights motivate using multiple H-bonded dimers to mediate proton transfer-driven tautomerism in the solid state as a route to fast, reversible property switching. The triboelectric effect is highlighted as a green means to generate surface charges; materials exhibit a triboelectric series with rubbers tending to acquire strong negative charge, which can potentially drive proton transfer and stabilize ionic tautomers in solids.

• Synthesis: A series of mono- and bis-ortho-pyridinyl phenols (compounds 1–7) were synthesized via Suzuki–Miyaura cross-coupling (Schemes S1–S9). Structures confirmed by 1H NMR, 13C NMR, and mass spectrometry (Figures S15–S71). Reference compounds 8–11 were prepared: 8 via deprotonation of 3 with KOH; 9 via full protonation of 3; 10 via methylation; 11 as a bis-para-pyridinyl phenol.
• Photophysical characterization: Solid-state absorption, excitation, emission spectra; fluorescence lifetimes (ns scale) to confirm fluorescence; quantum yields before/after rubbing; temperature-dependent luminescence.
• Rubbing protocol and triboelectric control: Solid powders were spread on frosted glass and rubbed with different materials (rubber, glass, paper, cotton, wool, PET, PVC, ceramic, agate) to probe triboelectric effects. Frictional force was controlled by varying mass load (200–760 g) and measured over time to correlate with luminescence intensity.
• In situ structural probes upon rubbing: FTIR tracked changes in O–H stretch (~3300–3500 cm−1) and emergence of N–H (pyridinium) bands at 2800–2900 cm−1. MALDI-TOF mass spectrometry in positive/negative ion modes identified protonated and deprotonated species formed after rubbing. Solid-state 13C NMR monitored changes in resonances associated with hydroxyl-bound vs oxygen-anion-linked carbons.
• Crystallography: Single crystals of compound 7 were grown (anthracene substituent aided crystallization). X-ray diffraction provided packing and dimer structures, including H-bond distances and π–π stacking metrics.
• Quantum-chemical calculations: DFT and TDDFT based on the crystal structure of 7 evaluated energy barriers for single- vs double-proton transfer, binding vs dissociation energetics, and oscillator strengths (S1–S0 transitions) in initial and proton-transferred states. Frontier molecular orbitals analyzed for neutral dimer and ionic tautomer.
• Control studies: Optical spectra and lifetimes of reference compounds 8–11 to test roles of anion formation, protonation, methylation (blocking H-bonds), and substituent geometry (para isomer). Doping 3 into PMMA films assessed effect of matrix on dimer formation and charge contact. Solution 1H NMR examined tautomerism in liquid.
• Reversibility tests: Rubbing-induced luminescence reset by wetting/wiping with water to disperse electrostatic fields; repeated rubbing after drying. Cycling measured emission intensity and quantum yield across several cycles; structural integrity checked by 1H NMR after multiple cycles. Powder XRD compared crystallinity before/after rubbing.
• Application demonstration: Information encryption using co-printed QR codes with 3 (non-emissive until rubbed) and a persulfurated arene emitter; QR readability tested pre- and post-rubbing under UV light.
• Biocompatibility: Cytotoxicity assay on HeLa cells, reporting 12 h survival at specified concentration.

• Rubbing-induced photoluminescence: All synthesized ortho-pyridinyl phenols show rubbing-activated emission; bis-ortho derivatives exhibit higher quantum yields than mono-ortho analogs (Table S1). Compound 3 transitions from non-emissive white powder to strongly green fluorescent yellow solid upon rubbing; new absorption at ~400 nm and emission at 528 nm appear; lifetimes on the ns scale confirm fluorescence.
• Evidence of proton transfer and tautomerism: FTIR shows weakening of O–H (~3300–3500 cm−1) and emergence of N–H (pyridinium) at 2800–2900 cm−1 during rubbing, indicating intermolecular proton transfer within H-bonded dimers. MALDI-TOF detects both protonated and deprotonated species after rubbing. Solid-state 13C NMR shows decreased hydroxyl-linked carbon resonance and increased oxygen-anion-linked carbon resonance after rubbing. Elevated temperature boosts rubbing-induced emission, supporting facilitation of proton transfer.
• Quantitative control via friction: Increasing load from 200 g to 760 g increases friction force from ~1.1 N to 4.5 N, linearly enhancing emission intensity of 3; emission intensity scales approximately linearly with frictional force, indicating controllable activation proportional to triboelectric energy.
• Solid-state dimer structure enabling topochemical tautomerism: Single-crystal XRD of 7 reveals double H-bonded dimers with unusually short H-bonds (1.842 Å) and π–π stacking distances of 3.444 Å; short H-bonds and applied pressure from rubbing favor proton transfer. Concentration-dependent 1H NMR (α-H of pyridine) indicates prevalent intermolecular H-bonding.
• Computation: The energy barrier for single-proton transfer in the double H-bonded dimer is low (~11 kcal/mol), whereas dimer dissociation is less favorable. TDDFT shows S1–S0 oscillator strength increases from f ≈ 0.01 (initial neutral dimer, non-emissive) to f ≈ 0.55 (proton-transferred state, 55-fold increase), consistent with bright emission from the anionic component and quenching tendency of pyridinium cations.
• Triboelectric origin and stabilization: Rubbing with materials across the triboelectric series shows rubber (−117 nC/J) yields the largest increase in quantum yield relative to other materials, indicating that negative charge induction is crucial. The extra static electric field in the solid state helps stabilize the bright ionic tautomer, hindering immediate reversion. Powder XRD shows crystallinity largely maintained despite packing changes.
• Controls validate mechanism: Deprotonated 8 (from 3 + KOH) reproduces the rubbed spectra and lifetimes, confirming the bright form is the oxygen anion species. Fully protonated 9 and methylated 10 (blocking H-bonding) show weak fluorescence (e.g., 9: Φ ≈ 2.5%, emission red-shift to 588 nm) and no rubbing response. Para isomer 11 (geometry disfavors double H-bonded dimers) is weakly absorbing and non-emissive before/after rubbing. Embedding 3 in PMMA suppresses rubbing response, consistent with reduced charge contact and dimer formation. In solution, 1H NMR shows no change, consistent with solid-state stabilization requirement.
• Tunability: Compounds 3–7 show rubbing-induced emission bands at different wavelengths, demonstrating that substituent modification tunes luminescence properties.
• Reversibility and durability: Wetting or soaking the rubbed powder disperses the electrostatic field and resets luminescence to off; after drying, rubbing reactivates emission. Multiple cycles maintain quantum yield with gradual intensity loss (sample loss). 1H NMR after ≥5 cycles matches fresh sample, indicating chemical stability. Crystallinity remains high per powder XRD.
• Application and safety: A co-printed QR code with 3 and a persulfurated arene is unreadable before rubbing (3 is dark) and becomes readable after rubbing, demonstrating information encryption. Cytotoxicity testing shows good biocompatibility (e.g., 96.2% HeLa cell survival at 2.84 μg/mL after 12 h).

The findings show that a deliberately designed double H-bonded dimer in ortho-pyridinyl phenols enables a topochemical tautomerism that can be triggered by triboelectric negative charges from rubbing. The single-proton transfer in the dimer converts a non-emissive neutral dimer into an ionic tautomer with a bright anionic species, drastically enhancing fluorescence in the solid state. The response is fast, efficient, and quantitatively controllable by friction force, addressing the limitations of conventional solid-state stimuli responses. The process is reversible upon removal of the stabilizing electrostatic field (e.g., by wetting), enabling repeated use. Structural modifications allow tuning of emission wavelength and quantum yield, broadening applicability. These results demonstrate a green approach to solid-state property switching, with implications for smart materials, optical data storage/encryption, and device integration.

The work introduces a green, efficient, and reversible solid-state luminescence switching strategy based on triboelectrically induced proton transfer within double H-bonded dimers of ortho-pyridinyl phenols. The approach achieves up to over 450-fold quantum yield enhancement, quantitative control via friction, structural robustness, and tunability through chemical modification. Demonstrations include reversible writing/erasing and encrypted QR codes, with preliminary biocompatibility. Future development may focus on expanding molecular designs to span broader emission colors, optimizing solid-state matrices and device architectures, and integrating triboelectric control in practical sensors and information storage systems.

• Effectiveness depends on solid-state dimer formation and proximity; embedding in inert polymer matrices (e.g., PMMA) suppresses the response due to reduced charge contact and dimerization.
• The bright ionic tautomer stabilization requires an electrostatic field; removing it (e.g., by wetting) resets the system, which is advantageous for reversibility but may limit retention without environmental control.
• The mechanism relies on materials capable of generating sufficient negative triboelectric charge; rubbing with less negative materials yields weaker activation.
• Single-crystal structures post-rubbing were not obtainable because rubbing pulverizes crystals, so post-activation crystallographic confirmation was not possible (mechanism supported by spectroscopy and computation instead).
• Luminescence intensity decreases over repeated mechanical cycles due to sample loss, though quantum yield remains stable.
• Tautomerization is not observed in solution, limiting applicability to solid-state systems.