Controlling the Two-Dimensional Self-Assembly of Functionalized Porphyrins via Adenine-Thymine Quartet Formation

The development of supramolecular synthons capable of driving hierarchical two-dimensional (2D) self-assembly is an important step toward the growth of complex and functional molecular surfaces. In this work, the formation of nucleobase quartets consisting of adenine and thymine groups was used to control the 2D self-assembly of porphyrins. Tetra-(phenylthymine) zinc porphyrin (Zn-tetra-TP) and tetra-(phenyladenine) porphyrin (tetra-AP) were synthesized, and scanning tunneling microscopy (STM) experiments were performed to visualize their self-assembly at the liquid-solid interface between an organic solvent and a graphite surface. Monocomponent solutions of both Zn-tetra-TP and tetra-AP form stable 2D structures with either thymine-thymine or adenine-adenine hydrogen bonding. Structural models based on STM data were validated using molecular mechanics (MM) simulations. In contrast, bicomponent mixtures showed the formation of a structure with p4 symmetry consisting of alternating Zn-tetra-TP and tetra-AP molecules in a chessboard type pattern. The relative positions of the porphyrin components were identified from STM images via contrast changes associated with the zinc atom present in Zn-tetra-TP. MM simulations suggest that hydrogen bonding interactions within these structures are based on the formation of adenine-thymine (ATAT) quartets with Watson-Crick base pairing between adenine and thymine groups.