Chlorine-Functionalized Covalent Organic Frameworks Harvest Sunlight to Supercharge Solar Water Evaporation

Fresh water scarcity is one of the defining challenges of the twenty-first century, and for billions of people living near coastlines or in arid regions, the ocean remains an untapped reservoir separated from human use only by the energy cost of desalination. Solar-driven interfacial evaporation, in which a floating photothermal material converts sunlight directly into heat at the water surface to generate clean vapor, has emerged as one of the most promising low-carbon routes to decentralized purification. Yet the technology has long been constrained by a stubborn materials problem: most photothermal absorbers capture visible light efficiently but let much of the solar spectrum, particularly the infrared and ultraviolet portions that carry a substantial fraction of the sun’s energy, slip away unused. A new study published in Nature Water reports a class of chlorine-functionalized covalent organic frameworks that appears to have cracked this problem, achieving ultra-broadband light absorption and record-setting evaporation performance in a device stable enough to run outdoors for two months.

Covalent organic frameworks, or COFs, are crystalline porous polymers built from organic molecular building blocks linked into periodic two- or three-dimensional networks. Their appeal lies in their atomic precision: by choosing the aldehyde and amine monomers, chemists can dictate pore size, topology, and electronic structure with a degree of control that inorganic materials rarely allow. COFs have already proven themselves in gas separation, catalysis, and energy storage, and their low density, high surface area, and tunable conjugation make them natural candidates for photothermal applications. The catch, as the research team led by Yachao Xu, Zhong Zhou, and Bojing Sun, working under the direction of corresponding authors Youxing Liu, Hongbo Li, and Shaojun Guo, points out, is that reported COFs have intrinsically lacked the capacity to harvest infrared light. Because roughly half of the solar energy reaching Earth arrives at wavelengths beyond the visible range, that blind spot translated directly into inferior photothermal conversion efficiency and, consequently, sluggish water evaporation rates.

The solution reported in the new paper is deceptively simple in concept: decorate the COF backbone with chlorine atoms. The researchers synthesized a family of Cl-functionalized COFs by condensing 2,4,6-trichloro-1,3,5-trialdehyde benzene, abbreviated TTB, with p-phenylenediamine, abbreviated pPA, yielding the flagship material TTB-pPA COF. The chlorine substituents proved to be far more than passive appendages. According to the team’s theoretical calculations, they reshape the electronic structure of the framework in ways that extend absorption across an extraordinary 200 to 1,500 nanometer window, spanning the ultraviolet, the entire visible spectrum, and a deep swath of the near-infrared. Equally important, the Cl-functionalized framework exhibits strong non-radiative relaxation capacity, meaning that excited electrons shed their absorbed energy as lattice vibrations, that is, as heat, rather than re-emitting it as light that escapes back into the environment.

The photothermal consequences are dramatic. Under illumination at one sun, the standard intensity of noonday sunlight, the TTB-pPA COF reached a surface temperature of 125.3 degrees Celsius within just 60 seconds. A structurally identical COF lacking the chlorine atoms climbed to only 60.7 degrees Celsius under the same conditions, a difference of nearly 65 degrees that isolates the halogen functionality as the decisive variable. That kind of instantaneous, high-amplitude heating is exactly what an interfacial evaporator needs, because vapor generation is governed by the local temperature at the water-air boundary rather than by the bulk temperature of the water body. The faster and hotter the photothermal layer becomes, the more vigorously water molecules are driven off the surface and into the vapor phase.

To translate material performance into device performance, the team constructed a solar-driven water evaporation device using the TTB-pPA COF as the photothermal conversion layer. In laboratory testing under one-sun illumination, the device achieved a water evaporation rate of 7.62 kilograms per square meter per hour, a figure the authors report as significantly outperforming previously reported water evaporation devices. For context, the theoretical evaporation limit under one sun for a conventional dark absorber sits well below this value, and rates approaching or exceeding this level typically require optical concentration, elaborate thermal insulation, or multi-stage architectures. The chlorine-engineered COF reaches its performance through a combination of broadband absorption, rapid non-radiative heat release, and the porous framework’s ability to draw water to the heated surface through capillary action.

Laboratory metrics, however, have a habit of fading under real skies, where cloud cover, wind, changing solar angles, and salt fouling conspire against even the best evaporators. The team therefore developed a custom water evaporation device and subjected it to long-term outdoor testing under natural light conditions. The device operated continuously for 60 days, demonstrating a level of durability that few photothermal materials can claim, particularly organic polymers, which are often vulnerable to photochemical degradation and hydrolysis over extended deployment. Over that period, the system produced purified water with an impurity content below 0.1 percent at a rate of 30 to 45 liters per square meter per day. The authors calculate that a single square meter of the device can meet the drinking water needs of 12 to 18 people, a strikingly concrete benchmark for a technology aimed at households and small communities rather than industrial desalination plants.

The mechanistic story behind these numbers is as interesting as the performance itself. When photons strike the Cl-functionalized framework, electrons are promoted into excited states across an unusually wide range of excitation energies. In a typical fluorescent or reflective material, much of that energy would be lost through radiative decay or through charge transport that carries energy away from the absorption site. In the TTB-pPA COF, the chlorine substituents and the resulting electronic asymmetry channel the excitation energy into vibrational modes of the molecular lattice instead. First-principles calculations of the photothermal conversion mechanism, supported by molecular dynamics simulations of heat dissipation, indicate that this non-radiative decay pathway is both fast and efficient, allowing the framework to act as a molecular-scale solar thermal converter. The result is a material that behaves less like a semiconductor absorbing light and more like a blackbody engineered at the level of individual chemical bonds.

The broader significance of the work lies in its design principle rather than in any single number. The authors emphasize that the key to next-generation solar-powered seawater desalination lies in simultaneously engineering two properties that have usually been pursued separately: a broad light absorption range and strong non-radiative electron decay capability. Halogen functionalization, they show, offers a synthetically accessible route to both. Chlorine is cheap, abundant, and compatible with the condensation chemistry used to build COFs, which means the strategy could in principle be extended across many framework chemistries and device formats. If the approach generalizes, it could reshape how photothermal materials for water treatment are designed, shifting the field away from complex plasmonic or ceramic absorbers toward rationally functionalized organic frameworks that are lightweight, scalable, and inexpensive.

Challenges remain before chlorine-functionalized COFs reach the field at scale. Large-area synthesis of crystalline frameworks with consistent quality, mechanical integration into floating evaporator architectures, and cost accounting at production volumes all require further engineering. But the demonstration of a 7.62 kilogram per square meter per hour evaporation rate, a 60-day outdoor operational lifetime, and drinking-quality output sufficient for more than a dozen people per square meter marks a genuine advance in the solar steam literature. As climate change intensifies droughts and groundwater depletion accelerates worldwide, materials that turn ordinary sunlight into safe drinking water with no moving parts and no external energy input are moving from laboratory curiosity to practical necessity. With this work, covalent organic frameworks, long celebrated as precision platforms for molecular chemistry, have now claimed a place at the front line of the global water challenge.

Subject of Research: Chlorine-functionalized covalent organic frameworks with ultra-broadband light absorption for efficient solar-driven water evaporation and desalination

Article Title: Covalent organic frameworks with ultra-broadband absorption for efficient solar-driven water evaporation

Article References: Xu, Y., Zhou, Z., Sun, B., Li, Q., Wang, Y., Zhao, R., Lin, Z., Sun, Z., Liu, Y., Li, H., & Guo, S. (2026). Covalent organic frameworks with ultra-broadband absorption for efficient solar-driven water evaporation. Nature Water, 4(9), 1157-1165. https://doi.org/10.1038/s44221-026-00687-w

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00687-w

Keywords: covalent organic frameworks, solar-driven water evaporation, photothermal conversion, seawater desalination, infrared absorption, non-radiative relaxation, chlorine functionalization, Nature Water, clean water, photothermal materials, interfacial evaporation, water purification