text: *The pi-pi charge-transfer interactions of bipyridinium with dioxyarenes or diaminoarenes have been used extensively for the construction of mechanically interlocked systems and in crystal engineering.
  *The use of crown ether binding with metal or ammonium cations is ubiquitous in supramolecular chemistry.
  *The formation of carboxylic acid dimers and other simple hydrogen bonding interactions.
  *The complexation of bipyridines or terpyridines with ruthenium, silver or other metal ions is of great utility in the construction of complex architectures of many individual molecules.
  *The complexation of porphyrins or phthalocyanines around metal ions gives access to catalytic, photochemical and electrochemical properties in addition to the complexation itself. These units are used a great deal by nature.