Expanding the Registry of Aromatic Amide Foldamers: Folding, Photochemistry and Assembly Using Diaza-anthracene Units

The synthesis of various 1,8-diaza-4,5-dialkoxy-2,7-anthracene dicarboxylic acid derivatives and their
incorporation into cyclic and helically folded aromatic oligoamides are reported. The ability of the diaza-anthracene monomers to undergo photoaddition or head-to-tail photodimerization was investigated in
the solid state and in solution. Quantitative conversion of a monomer diester to the corresponding head-to-tail photodimer could be achieved in the solid state without protection from oxygen. The formation of
an emissive excimer between two diaza-anthracene units appended at the end of a helically folded oligomer
was demonstrated. Intramolecular photodimerization was not observed in this compound, possibly due
to the low thermal stability of the head-to-head photoadduct. A cyclic oligoamide composed of two
diaza-anthracene and two pyridine units was shown to adopt a flat conformation and to form columnar
stacks in the solid state. Longer, noncyclic oligoamides composed of one or two diaza-anthracene units
were shown to adopt helical conformations that exist preferentially as double helical dimers.