Electronic Structure Modulation via Composition-Preserving Phase Transformations in Metal-Organic Assemblies on the Surface.

Supramolecular self-assemblies based on metal-organic coordination provide a tunable platform for constructing functional nanostructures on surfaces, with potential applications in catalysis, magnetism, and optoelectronics. Rational design of assemblies requires understanding how their electronic structures can be tuned by various factors, such as precursor design and substrate choice. While phase transformations can significantly alter both structural and electronic properties, most reported cases involve changes in chemical composition or bonding configurations. In contrast, systematic studies of geometric relaxation, which is modulated without altering chemical identity, remain limited, as such transformations often demand high temperatures that risk side reactions. In this study, we demonstrate a room-temperature phase transformation in a supramolecular self-assembly of Ag-carboxylate complexes derived from 3,5-dinitrobenzoic acid on Ag(111), proceeding without any changes in chemical composition. Combining scanning tunneling microscopy and spectroscopy with density functional theory calculations, we track a stepwise transformation among three distinct hexagonal lattices. The transformation induces subtle geometric relaxation that strengthens metal-molecule interactions, thereby modulating the collective electronic structure. Unlike previous studies on polymorphic organic assemblies, this work reveals a composition-preserving, phase-transformation-driven route to modulate electronic structures in metal-organic coordination assemblies, enabling tuning of physicochemical functionalities in surface-confined molecular architectures.