Molecular photoswitches are crucial tools in various scientific disciplines, enabling responsiveness at the nanoscale. Recent advancements have expanded their applications to remote control of drug bioactivity, material property modification, catalytic selectivity steering, influence on supramolecular assemblies, and manipulation of molecular machines. While azobenzenes, hydrazones, Stenhouse dyes, and indigoid photoswitches have shown promise, the limited functionalization possibilities of hemiindigo (HI) derivatives have hampered their widespread applicability. This research addresses this limitation by presenting a novel synthetic route for diaryl-HIs, which introduces an additional aromatic residue at the central double bond. This modification is anticipated to unlock enhanced properties and broader applicability for HI photoswitches, making them suitable for advanced applications in research on molecular machines and switches, photoisomerization mechanisms, and the creation of smart, addressable materials. The unique combination of red-light responsiveness, high thermal bistability, and strong isomer accumulations in both switching directions, along with tunable acid responsiveness and acid gating, makes these diaryl-HIs particularly attractive. The study aims to comprehensively investigate the photophysical and photochemical properties of these novel photoswitches and demonstrate their potential in multi-state switching and functional materials applications, specifically showcasing their use in creating photochromic transparent polymers.

The field of photoswitch research has witnessed rapid progress, with several classes of photoswitches being actively explored. Modified azobenzenes, hydrazones, Stenhouse dyes, and imidazolyl-radical photoswitches, each with specific advantages, have been reported. The authors' group has extensively studied indigoid chromophores, transforming many known dyes into capable photoswitching motifs. The advantage of indigoid core-chromophores lies in their visible light responsiveness, avoiding damaging UV irradiation. Their typically rigid geometries limit degrees of freedom, enabling precise control over the shapes and spatial relations of functional groups. Hemithioindigo (HTI), a prominent representative, has seen significant advancements in applicability and photoswitching performance with the introduction of a fourth substituent at the double bond. This substitution pattern facilitated the direct evidence of the hula twist photoreaction, the discovery of a new photoreaction (dual single bond rotation, DSBR), the development of diverse molecular motors, advanced multi-state photoswitches, and the first light-energy powered molecular gearing systems. The related HI structure, featuring an indigo fragment instead of the thioindigo fragment, shows similar potential. Monoaryl-HIs with strong electron donors provide nearly perfect photoswitching characteristics, exhibiting quantitative isomer conversions, red-light responsiveness, high extinction, strong photochromism, and high thermal bistability. Nitrogen substitution at the indigo fragment allows facile functionalization, leading to applications as chiroptical switches and ionic chromophores. Applications in biological contexts have also been demonstrated. Despite this promise, the full potential of HIs remains unexplored. Current research focuses on modifying monoaryl-HIs to enhance their properties. The introduction of diaryl-substitution, inspired by advancements in HTIs, is identified as a crucial step to significantly improve the capacity of HI photoswitches.

The synthesis of diaryl-HIs presented several challenges, as methods successful for diaryl-HTIs proved ineffective for diaryl-HIs. Attempts using Dieckmann-type condensations and condensations of indoxyl derivatives with ketones failed to yield the desired products. The authors developed a three-to-four-step modular synthesis. The synthesis began with the high-yield condensation of commercially available aldehydes and indoxyl acetate to produce parent HIs. Chlorination of the double bond was then achieved using N-chlorosuccinimide (NCS), allowing subsequent substitution reactions with various nucleophiles. This chlorination step is novel and provided a versatile intermediate for diversification. A final N-alkylation of the indoxyl nitrogen introduced functionalities for later-stage covalent attachment. The specific R-groups at the diaryl moiety were selected to optimize photoswitching properties. A p-aniline (p-NMe₂) residue, providing electron richness, was combined with a neutral or electron-poor residue. The synthesis involved condensation, chlorination, cross-coupling (Suzuki-Miyaura), and NH-substitution. Crystal structures obtained for compounds 1a and 3b confirmed the twisted nature of the diaryl moieties. The thermal behavior, characterized by slow isomerization at elevated temperatures, was measured to determine the relative free enthalpy differences (ΔG) and Gibbs energies of activation (ΔG‡). Photoswitching properties were analyzed through UV/Vis and ¹H NMR spectroscopy. Irradiation with different wavelengths of light induced isomerization and the quantum yields (QYs) of these photoisomerization reactions were determined. The effects of solvent polarity on thermal stability and photoswitching were also investigated. Finally, the diaryl-HIs were incorporated into polystyrene (PS) polymers, and the resulting photochromic behavior was evaluated. Acid-base induced switching was studied using UV/Vis and ¹H NMR spectroscopy, investigating the effects of different acid concentrations and the electronic properties of the diaryl-HIs. Mass spectroscopy was used to confirm the protonation state.

The developed synthesis yielded diaryl-HIs with high yields. The diaryl-HIs demonstrated excellent thermal bistability, with half-lives ranging from hours to over 100 years at 25 °C, depending on the substituents. Significant photochromism was observed, with color changes ranging from red to deep purple. Irradiation with appropriate wavelengths led to high isomer enrichments. Most notably, the diaryl-HIs exhibited multi-stimuli responsiveness, with green and red light, temperature, and acid/base stimuli all affecting their isomerization. The addition of small amounts of acid induced isomerization; while the addition of a large excess of acid yielded distinct, protonated forms. The addition of base neutralized the acid and reverted the system to its neutral state. This acid-base induced switching mechanism demonstrated reversible four-state switching and a chemical fueling process, enabling enrichment of the thermodynamically less stable isomer. Incorporation into polystyrene polymers yielded transparent, photochromic materials, where information could be written with light and erased by another wavelength of light, by addition of acid, or by exposure to sunlight. The QYs for photoisomerization from the hypsochromic to bathochromic species were typically low; however, QYs for photoisomerization from the bathochromic to hypsochromic species were higher, ranging from 4% to 6% for selected compounds. The UV/Vis absorption maxima of the hypsochromic isomers showed a bathochromic shift in polar solvents, while the bathochromic species exhibited hypsochromic or bathochromic shifts, indicating solvent dependence. The study also noted that stronger donor/acceptor systems on the stilbene moiety resulted in more bathochromically shifted UV/Vis absorption bands and better isomer enrichment. Functionalization at position R³ improved photochemical behavior.

The findings demonstrate that diaryl-HIs represent a significant advancement in photoswitch technology. The ease of synthesis and the modular nature of the synthetic route allow for extensive structural diversification, enabling fine-tuning of the photoswitch properties for specific applications. The remarkable combination of visible light responsiveness, high thermal bistability, strong photochromism, and multi-stimuli responsiveness positions diaryl-HIs as superior photoswitches compared to previous generations. The ability to achieve four distinct states using light, heat, and pH manipulation expands the potential for complex switching applications in molecular machines and functional materials. The successful demonstration of reversible inscription into transparent polymers highlights the practical utility of these photoswitches. The acid-gating of photoresponsiveness introduces another layer of control and complexity. The ability to chemically fuel the system to favor the thermodynamically less stable isomers opens up new avenues for energy-efficient molecular machines and devices. The detailed photophysical and photochemical characterization provides a strong foundation for future design and optimization of diaryl-HIs.

This work presents a novel synthetic route and a thorough characterization of a new class of photoswitches, diaryl-HIs, demonstrating their superior properties compared to existing photoswitches. The remarkable multi-stimuli responsiveness, four-state switching ability, and successful implementation in photochromic polymers highlight their broad applicability. Future research could focus on exploring the full potential of this structural space, further optimizing properties, and investigating applications in diverse fields such as drug delivery, catalysis, and data storage.

While the study demonstrates the potential of diaryl-HIs, some limitations exist. The quantum yields for photoisomerization from the bathochromic to hypsochromic species were relatively low. Further optimization of the molecular structure might enhance these yields. The long-term stability of the photochromic polymers under intense sunlight exposure requires further investigation. The study predominantly used toluene as a solvent; a broader investigation of solvent effects on performance is warranted.