Lighting up solid states using a rubber

Stimuli-responsive materials are crucial for advancements in material science. While various materials respond to stimuli like light, heat, or pressure, these responses are often slow and incomplete, resulting in minor property changes. Chemical reaction-driven responses, such as pH changes or redox reactions, are rapid and extensive in solution but less efficient in solid-state settings. This paper addresses the challenge of developing green and highly efficient strategies for regulating solid-state structures and properties. The researchers hypothesize that integrating tautomerization and topochemistry, utilizing multiple hydrogen-bonded structures, could provide a unique solution. Multiple hydrogen-bonded structures, common in supramolecular and polymeric materials, can be maintained in the solid state and facilitate topochemical reactions. These structures also support efficient energy and information transfer. The study proposes that tautomerism in such structures, triggered by mild stimuli like triboelectric effects, can enable material property switching. The research focuses on ortho-pyridinil phenol-based compounds, designed with a double H-bonded dimer structure to regulate optoelectronic properties through tautomerism.

The introduction extensively reviews existing literature on stimuli-responsive materials and their limitations. It highlights the challenges associated with achieving rapid and significant property changes in the solid state, contrasting solution-based approaches with the difficulties encountered in solid-state reactions. The literature review emphasizes the potential of topochemical reactions and the advantages of multiple hydrogen-bonded structures in facilitating such reactions and energy transfer. Prior work on stimuli-responsive materials, topochemical reactions, hydrogen-bonded structures, and their applications in supramolecular chemistry and materials science is referenced to establish the context and significance of the current study.

The researchers synthesized a series of mono- and bis-ortho-pyridinil phenols using Suzuki-Miyaura cross-coupling reactions. Structural confirmation was achieved through ¹H NMR, ¹³C NMR, and mass spectrometry. The study focused on bis-ortho-pyridinil phenols due to their higher quantum yields for rubbing-induced photoluminescence. The researchers investigated the rubbing-induced photoluminescence using various techniques including UV-Vis spectroscopy to analyze changes in absorption and emission spectra before and after rubbing. Fourier-transform infrared (FTIR) spectroscopy was employed to track in situ solid-state structural changes during rubbing, focusing on changes in characteristic stretching frequencies of the phenolic hydroxyl and N–H groups. MALDI-TOF mass spectrometry was used to identify the products resulting from proton transfer. Solid-state ¹³C NMR spectroscopy further investigated changes in resonance upon rubbing. The influence of frictional force, controlled by varying mass loads, on emission intensity was also studied. Single-crystal X-ray diffraction analysis of compound 7 provided structural information about hydrogen bonding within the dimers. Quantum-chemical calculations using density functional theory (DFT) and time-dependent DFT (TDDFT) were performed to determine the energy barrier for proton transfer and assess the intensity of S₁–S₀ transitions in both the initial and proton-transferred states. To further investigate the mechanism, additional reference compounds were synthesized and characterized, and the effect of doping compound 3 into a PMMA film was studied. Finally, the reversibility of the photoluminescence was examined by wetting the rubbed sample with water, and applications in information encryption were explored.

Rubbing the solid sample of bis-ortho-pyridinil phenols with a rubber resulted in a significant increase in photoluminescence, up to over 450-fold. FTIR spectroscopy revealed a rubbing-induced proton transfer between the hydroxyl and pyridinyl groups within the double H-bonded dimers. MALDI-TOF and solid-state ¹³C NMR confirmed this proton transfer, resulting in a tautomeric shift. The emission intensity was directly proportional to the applied frictional force. Single-crystal X-ray analysis showed short hydrogen bond lengths (1.842 Å) in the dimer, indicating a high probability for proton transfer. DFT calculations showed a low energy barrier (11 kcal/mol) for proton transfer. The proton-transferred state exhibited a significantly stronger S₁–S₀ transition intensity (f = 0.55) compared to the initial state (f = 0.01), explaining the observed enhanced luminescence. Control experiments using different materials for rubbing, including materials generating positive or neutral charges, confirmed the crucial role of the negative charge from the rubber in inducing the proton transfer. The luminescence was reversible; wetting the sample with water reversed the proton transfer and quenched the luminescence. This reversibility allowed for repeated cycles of switching the luminescence on and off, enabling potential applications in information encryption. Further structural modifications to the H-bonded dimers allowed tuning of emission wavelengths. The material also showed good biocompatibility.

The findings demonstrate a highly efficient and green strategy for inducing photoluminescence in the solid state by using only a rubber to induce a topochemical tautomerism. The unique double H-bonded dimeric structure is crucial for this effect. The quantitative controllability, reversibility, and tunability of the luminescence make this system highly promising for applications in smart materials and information encryption. The key innovation lies in using a simple, readily available stimulus (rubbing with a rubber) to achieve a significant and reversible change in material properties. This overcomes limitations of previous approaches and opens up opportunities for developing new types of stimuli-responsive materials.

This research successfully demonstrates a novel, green, and efficient strategy for achieving rubbing-induced photoluminescence in the solid state through topochemical tautomerism. The study's key contributions include the design of a highly effective double H-bonded dimeric structure, the use of readily available materials, the demonstration of reversible luminescence switching, and the potential for tuning material properties through structural modifications. Future research could explore further optimization of the material design for enhanced efficiency and broader applications in areas such as sensing, displays, and data security. The biocompatibility of the material also opens up avenues for biomedical applications.

The study primarily focuses on a specific class of compounds, and the generalizability of the findings to other materials needs further investigation. While the reversibility is demonstrated, the long-term stability and durability of the rubbing-induced photoluminescence under repeated cycles need further evaluation. The mechanism involves a complex interplay of factors such as triboelectric effects, proton transfer, and electrostatic field stabilization, which could be further explored through advanced characterization techniques.