Non-Innocent π Linkers Affect Cooperativity in Hydrogen-Bonded Macrocycles

Rigid, linear π-conjugated acetylene linkers connecting a hydrogen-bond acceptor to a hydrogen-bond donor are established building blocks for self-assembled hydrogen-bonded macrocycles. Kohn-Sham molecular orbital and Voronoi deformation density analyses reveal that the acetylene linker plays an unprecedented, non-innocent role in the cooperativity of these hydrogen-bonded macrocycles. The acetylene linker can abstract electron density from the hydrogen-bond acceptor and donor, due to the linkers’ low-lying π-LUMO. As a result, the hydrogen-bond acceptor becomes less negatively charged, which both hampers the cooperativity, as well as the hydrogen bond strength, in the hydrogen-bonded macrocycles. This effect becomes more pronounced when the size of the acetylene linker increases. The findings presented in this work can act as design principles for the development of novel supramolecular macrocycles based on hydrogen bonds.