Bifunctional Behavior of a Porphyrin in Hydrogen-Bonded Donor–Acceptor Molecular Chains on a Gold Surface

Peculiar hydrogen-bonded molecular chainsare spontaneously created from the self-assembly on a goldsurface of a porphyrin functionalized with four aromatic aminemoieties. The molecular chains are formed by a sequence ofdyads, where the same molecule behaves alternately as ahydrogen-bond acceptor or donor as a whole at all its fouraromatic amino groups. This remarkable bifunctional behavioris due to the conformational flexibility of the functionalizingamino groups that switch from a planar, aniline-likeconformation in donors to a pyramidal, amine-like one inacceptors. Furthermore, we show that the acceptor porphyrinscan trap gold adatoms underneath their center. Combined scanning tunneling microscopy experiments and density functional theory calculations characterize the structural and electronic modifications suffered by such molecules to establish amino−amino interactions. Notably, scanning tunneling spectroscopy measurements show that the highest occupied molecular orbital−lowest unoccupied molecular orbital gaps of the acceptors and donors are, respectively, larger and smaller with respect to the isolated molecule according to the reduced extent of conjugation occurring in the acceptors. In summary, experimental and theoretical results reveal a remarkable hydrogen-bonded complex where the amino groups act as both hydrogen-bond donors and acceptors and suggest how hydrogen bonding can modify the geometrical and potentially also the electronic structures of highly conjugated molecules.