Colour polymorphism of a bis(quinoxaline) compound

Synthesis of 2(1H)-quinoxalinone O-(2'-quinoxalinyl)oxime (3) results in tw o easily distinguishable crystal forms with different colours, providing a rare example of colour polymorphism. The crystal structures of the two form s have been determined and compared. Intermolecular interactions and light- absorption characteristics of the polymorphs have been analysed by a mixed quantum chemical/molecular mechanical method. In one of the polymorphic for ms the crystal field stabilises relatively high energy conformations by enh anced electrostatic interactions. The difference in the light absorption is found to be caused partly by a conformational difference of the molecules. The crystal field increases the difference in the absorption wavelengths a nd shift them towards higher values.