Scientists create artificial leaves from supramolecular hydrogels that efficiently convert light into chemical energy for green, recyclable photocatalysis.
(Nanowerk Spotlight) The conversion of sunlight into usable energy lies at the center of modern strategies for renewable energy development. Among natural processes, photosynthesis remains the most elegant and efficient example of how light energy can be transformed into chemical energy. In plants, this process is carried out by chloroplasts, which contain light-harvesting complexes that capture sunlight and direct that energy to drive chemical transformations essential for growth.
Scientists have tried to build artificial systems that mimic this behavior, not just to study nature, but to apply such mechanisms to practical tasks like sustainable chemical production. Despite numerous efforts, progress has been slowed by challenges related to material stability, reaction scope, recyclability, and the difficulty of integrating light capture with catalytic function in a single, practical system.
One persistent limitation has been the inability of existing artificial light-harvesting systems to effectively utilize the captured energy beyond simple transfer. Many such systems are based on supramolecular chemistry—a field that uses non-covalent interactions to assemble complex structures—but these often remain confined to homogeneous environments where the catalysts are dissolved in solution. This creates barriers to recovery and reuse. Additionally, earlier designs were typically restricted in the types of reactions they could catalyze, limiting their practical use.
Recent advances in nanomaterials and supramolecular engineering have allowed researchers to design better structured, more versatile systems. A critical development is the use of aggregation-induced emission (AIE) materials, which become luminescent when aggregated, offering new pathways for efficient light utilization. When combined with hydrogels—soft, water-rich materials that can be engineered for mechanical stability and chemical functionality—these elements lay the foundation for more realistic biomimetic models.
In a study published in Advanced Science (“Bionic Artificial Leaves Based on AIE-Active Supramolecular Hydrogel for Efficient Photocatalysis”), researchers at Nanjing University of Aeronautics and Astronautics and collaborating institutions report the construction of a new type of artificial leaf. This system integrates supramolecular chemistry, light-harvesting capability, and photocatalytic function into a hydrogel matrix that operates efficiently in water and is fully recyclable.
The key innovation lies in embedding a synthetic host-guest complex into a gelatin-based hydrogel and combining it with a dye that absorbs light, creating a soft, stable material that mimics photosynthesis and catalyzes specific chemical reactions using only light and water.
Schematic illustration of artificial leaves and their photocatalytic processes. (Image: Reprinted from DOI:10.1002/advs.202504993, CC BY) (click on image to enlarge)
The artificial leaf is built around a compound known as m-TPEWP5, a water-soluble derivative of a molecule called pillar[5]arene. This structure is modified with tetraphenylethylene units that display aggregation-induced emission, meaning they fluoresce more strongly when clustered. These host molecules are combined with a specially designed guest molecule containing a sulfonate group for solubility and an acrylate group for later polymerization. Together, they form a host-guest supramolecular complex (HGSM). When this complex is mixed with gelatin methacryloyl (GelMA), a biologically derived polymer, and exposed to UV light in the presence of a photoinitiator, it forms a crosslinked hydrogel known as HGGelMA.
The resulting hydrogel is both mechanically stable and optically active. Importantly, it serves as a platform to incorporate a dye molecule called eosin Y, which functions as an energy acceptor. The donor-acceptor pairing between the m-TPEWP5-based complex and eosin Y enables a process known as Förster resonance energy transfer (FRET), where energy absorbed by the donor is efficiently transferred to the acceptor. This configuration closely mimics the energy flow in natural light-harvesting systems and allows the material to be activated by light to trigger chemical reactions.
To evaluate the photocatalytic potential of the artificial leaf, the researchers tested two types of reactions. First, they studied the dehalogenation of bromoacetophenone derivatives—a common reaction in organic chemistry used to remove halogen atoms and generate simpler molecules. Under UV light in water, the HGGelMA⊃ESY hydrogel (where “⊃” indicates encapsulation of eosin Y) achieved over 99% yield in converting bromoacetophenone into acetophenone, significantly outperforming controls where the donor or acceptor was absent. Even when different chemical substitutions were introduced on the starting materials, the system maintained near-quantitative efficiency, demonstrating broad applicability.
Second, the artificial leaf was applied to the oxidative coupling of benzylamine, a reaction that forms imines—important intermediates in pharmaceuticals and fine chemicals. Here, the hydrogel acts as a type-II photosensitizer, generating singlet oxygen (1O2), a reactive form of oxygen that facilitates the oxidation step. Using small amounts of the catalyst under UV light and oxygen-rich conditions, the system yielded the desired product in up to 69% yield. Importantly, this reaction was also effective for derivatives of benzylamine carrying both electron-donating and electron-withdrawing groups, showing flexibility in substrate compatibility.
Beyond their catalytic efficiency, these artificial leaves show a key practical advantage: reusability. After each reaction, the hydrogel can be recovered through simple washing and reused without significant loss of activity. This contrasts with many solution-phase photocatalysts, which are difficult to separate from reaction mixtures and often degrade over time. The soft, porous nature of the hydrogel provides mechanical integrity while enabling efficient diffusion of reactants and products, which is essential for scalability.
At the mechanistic level, the artificial leaf operates via a multi-step energy relay. Upon UV irradiation, the m-TPEWP5-based donor absorbs light and transfers energy to eosin Y. This energy input activates eosin Y to an excited state, which can then participate in electron transfer or generate singlet oxygen, depending on the reaction. In the case of dehalogenation, eosin Y in its excited state receives an electron from a compound called Hantzsch ester and passes it on to the brominated substrate, initiating the removal of the halogen atom. For the oxidative coupling reaction, the excited dye transfers energy to molecular oxygen, generating reactive oxygen species that mediate the transformation of benzylamine.
The design also highlights the modularity of supramolecular chemistry. By carefully selecting the host and guest molecules and controlling their interactions, the researchers tuned the structural and electronic properties of the resulting hydrogel. The aggregation-induced emission characteristic of m-TPEWP5 ensures that the material remains optically active in its solid or gelled form, avoiding one of the common problems in solid-state light-harvesting systems, where fluorescence tends to be quenched.
This work introduces a hydrogel-based system in the development of heterogeneous light-harvesting systems that integrate energy collection with photocatalysis in a reusable and scalable format. By drawing on lessons from nature and implementing them through synthetic chemistry, the team has demonstrated how molecular design and materials engineering can converge to solve practical problems. The artificial leaf demonstrates functionality beyond preliminary designs and a functional platform that could be adapted for broader applications in green chemistry, environmental remediation, and solar-driven synthesis.
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