Tunable Interlayer Interactions in 2D van der Waals Frameworks

Two-dimensional materials can be isolated as monolayer sheets when interlayer interactions involve weak van der Waals forces. These rigorously atomically thin structures enable novel topological physics and open chemical questions of how to tune the structure and properties of the sheets while maintaining the sheets as isolated monolayers. Interactions between sheets and the properties they generate remain relatively neglected, as a consequence of this focus on their properties as monolayers. Here, we investigate two-dimensional porous sheets that exfoliate into isolated monolayers, but aggregate upon oxidation, giving rise to tunable interlayer charge transfer absorption and Stokes-shifted photoluminescence. This optical behavior resembles interlayer excitons, now intensely studied due to their long-lived emission, but which remain difficult to tune through synthetic chemistry. Instead, the interlayer excitons of these framework sheets can be modulated through control of solvent, electrolyte, oxidation state, and the composition of the framework building blocks. In comparison to other two-dimensional materials, these framework sheets display the largest known interlayer binding strengths, attributable to interactions between specific components within the sheets. Taken together, these results provide a microscopic basis for manipulating long-range opto-electronic behavior in van der Waals materials through molecular synthetic chemistry.