The structure of dimers and the nature of azulene chromaticity

The structure of supramolecular azulene dimers responsible for the blue coloration of its crystals and solutions has been discussed on the basis of result of optical spectroscopy and literature data. It is established that two types of these dimers (I and II) absorb the light in the red region of the visible (VIS) spectrum and differ by the mutual orientation of the molecules. Dimers I have a VIS band with a vibronic structure; the azulene molecules in dimers I match their own seven-membered rings in type (stacking structure), and five-membered rings of the molecules are separated so that the molecular C _2v axes form an obtuse angle. The spectra of dimers II do not have a vibronic structure in the VIS band. The dipole moments of the molecules in these dimers are oriented antiparallel (five-membered rings are located over (or under) seven-membered rings, forming a structure with a center of inversion). It is concluded that due to their structure, dimers I should have a certain dipole moment, while dimers II have no dipole moment.