In our previous research, we demonstrated that the amine-type reduced CC3 cage can be easily transformed to its cationic ammonium analogue by acid treatment, which simultaneously altered the cage skeleton from hydrophobic to hydrophilic. This key insight prompted us to investigate whether liquid-like properties could be imparted to molecular cages through neutralization with organic acids bearing flexible, flowable chains. Conventional covalent modification strategies had failed, since the hydrophobic chain terminals readily threaded through the hydrophobic cage windows and occupied the internal cavities, thereby eliminating porosity. By contrast, our approach proved successful owing to the subtle transformation of the cage skeleton. The hydrophilic, positively charged ammonium windows effectively suppress the intrusion of hydrophobic chain ends, while the high charge density of the skeleton prevents pore distortion. Furthermore, the strong electrostatic complexation constrains the flexible chains around the cage molecules. This straightforward strategy is readily scalable, enabling one-pot synthesis of porous liquids in quantities exceeding one kilogram.
When we first shone a UV light (365 nm) onto the porous liquid, an unexpected scene unfolded: the pale yellow liquid gradually deepened into a dark brown hue, as if the liquid itself had come alive. This striking photochromic transformation hinted at something hidden beneath the surface—a charge-separated state, perhaps even the elusive generation of radicals. To verify our suspicion, we turned to electron spin resonance, which indeed revealed the unmistakable signature of radical species. But this raised deeper questions: how could such radicals form, and why did they persist for so long? Driven by curiosity, we embarked on a systematic exploration, combining spectroscopic characterizations, computational simulations, and carefully designed control experiments. We discovered that the radicals emerged through a photoinduced electron transfer process, and once born, found an unusual refuge in the extensive electron delocalization within the porous liquid. This delocalization acted like a protective cocoon, dramatically enhancing the stability of the radical state. To our surprise, the radicals endured not just for days or months, but over a year—an extraordinary timescale in radical chemistry. As far as we are aware, this makes our system the very first example of a radical porous liquid.
With the porous organic cage liquids in hand, we sought to explore applications that have been rarely investigated in the porous liquid field. The emergence of a dark-colored radical state with extended light absorption suggested to us that these liquids might serve as efficient near-infrared photothermal materials—a direction scarcely explored in the porous liquid field. Guided by this intuition, we tested their performance under 808 nm laser irradiation and were surprised to find a high photothermal conversion efficiency of 50.8%, with the temperature rapidly rising to 80 °C within just 10 minutes. Even more fascinating, this photothermal effect could directly regulate the thermal microenvironment surrounding the encapsulated Au clusters, thereby tuning their catalytic activity. By simply switching the heating mode from a conventional oil bath to local 808 nm laser irradiation, the catalytic rate of a model hydroamination reaction increased more than fivefold, highlighting the unique advantage of photothermal conversion in these cage liquids.
Collectively, these findings prove that the incompatibility between hydrophilicity and hydrophobicity could be an efficient principle for constructing porous liquids with task-specific applications (such as photothermal conversion and catalysis), which we anticipate to be pushed forward as competitive or even more attractive alternatives to traditional materials in various technologies and industries.
For more details, please read our article “Direct liquefying organic cages into porous liquid molecules for enhanced near-infrared photothermal conversion and catalysis” in Nature Communications (https://doi.org/10.1038/s41467-025-63126-6).