A. Llanes-Pallas, C.-A. Palma, L. Piot, A. Belbakra, A. Listorti, M. Prato, P. Samori, N. Armaroli, D. Bonifazi,
J. Am. Chem. Soc. 2009, 131, 509-520
Discrete and multicomponent nanoscale noncovalent assemblies on surfaces featuring polygonal porous domains are presented. The molecular engineering concept involves multivalent molecular modules that are preprogrammed to undergo heteromolecular recognition by exploiting complementary multiple H bonds. Two types of molecular modules have been engineered: (i) a linear unit of twofold symmetry exposing two 2,6-di(acylamino)pyridyl [donor−acceptor−donor (DAD)] recognition sites at its extremities with a 180° orientation relative to each other and (ii) an angular unit constituted by a 1,3,6,8-tetraethynylpyrene core peripherally functionalized with four uracil groups [acceptor−donor−acceptor (ADA)] positioned at 60° and 120° relative to each other. These molecular modules self-assemble through H-bonds between the complementary recognition sites, forming supramolecular architectures. Their symmetry depends upon the type of each individual subunit and the stoichiometry as well as on the combination and distribution of the main symmetry axes. These so-formed two-dimensional (2D) supramolecular oligomers have been studied in solution by optical spectroscopy and on highly ordered pyrolitic graphite (HOPG) substrates by scanning tunneling microscopy (STM) at the solid−liquid interface. Steady-state UV/vis absorption and emission titration measurements suggest the reversible formation of multiple oligomeric species with slightly modulated fluorescence spectra. This likely reflects the presence of various aggregates between the two polytopic receptors, which exhibit somewhat different electronic delocalization as a function of the aggregate size. The presence of multiple species is further confirmed by time-resolved luminescence measurements: lifetime values are fitted as double/multiple exponentials and are always shorter than 6.5 ns. The formation of several oligomeric species is further supported by in situ STM measurements at the solid−liquid interface that provided evidence, with submolecular resolution, for the formation of multicomponent and discrete 2D polygon-like assemblies. We highlight the role of accurate control of the concentration required to image on the surface the 2D oligomeric species formed in solution, which allows us to bypass the determinant role of the substrate−molecule interactions in forming the thermodynamically stable monocomponent architectures at the solid−liquid interface.