A sharp J-band-like electronic absorption peak observed for the chromonic liquid crystal of bisazo dye Ponceau SS in water

The bisazo dye Ponceau SS forms an acute oblique (V-type) dimer in dilute aqueous solution. In concentrated aqueous solution, these V-type dimers are considered to stack to form V-type blocks of chromonic liquid-crystalline (LC) columns. The weak interactions among these LC columnar V-type blocks have been spectroscopically investigated. On increasing the concentration of Ponceau SS at 25 °C, the V-type LC columnar blocks form a rhombus-type LC columnar block at 2 wt% concentration. These rhombus-type LC columnar blocks are further aligned at an acute angle at 5 and 10 wt% concentrations. The rapidly cooled 10 wt% LC sample shows an unusually sharp J-band-like peak in the electronic absorption spectrum. The emergence of this J-band-like peak has been analyzed from the viewpoint of an exciton model, suggesting that many nearest neighbor unique molecules belonging to many different rhombus-type LC columnar blocks are aligned in a head-to-tail manner to give a giant quasi-linear head-to-tail-type exciton. Graphical abstract