Two-dimensional photoactive frameworks:
towards dynamic covalent chemistry and exciton imaging

Combining structural tunability, chemical stability with a crystalline structure, covalent organic frameworks (COFs) are promising material platforms for a wide range of applications in (photo-) catalysis. Two-dimensional COFs, composed of a single atomic layer, exhibit peculiar opto-electronic properties, making them suitable platforms for light-induced up-conversion of chemicals. However, despite the growing interest in such 2D polymers, their synthesis is still a major challenge. A proposed approach relies on dynamic covalent chemistry, where the chemical equilibrium is tuned to enable reversible bond formation, allowing the polymer networks to overcome kinetic limitations and reach the crystalline configuration. Here, I will describe the application of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to elucidate the reversible formation of a two-dimensional boroxine framework. By controlling the water partial pressure and the sample temperature, the chemical equilibrium can be tuned, leading to the formation or dissolution of the framework. Light absorption in photoactive materials leads to the generation of excitons, the most fundamental light-induced excitations, composed of bound electron-hole pairs. Exciton dissociation into spatially-separated photocharges allows running photochemical reactions, as CO₂ reduction and water oxidation. However, a fundamental gap about the atomic-scale mechanisms remains to be filled, being of tremendous importance to improve the efficiency of photoenergy conversion in current devices. I will discuss the experimental challenge of observing excitons at the atomic scale. Novel microscopic approaches to resolve out-of-equilibrium states by atomic force microscopy (AFM) will be presented, opening to investigation of light-induced charge transfer in two-dimensional photoactive materials.