Lactone, rhodamine

A simple example of an intramolecular Lewis acid/base reaction (thus avoiding the association step) is the xanthene dye rhodamine B, which exists in solution either in the red-coloured zwitterionic form (26a) or as the colourless lactonic form (26b) [175, 221, 222]. Solutions of rhodamine B in non-HBD solvents such as dimethyl sulfoxide,... 

Figure 3.82. TP photochemical rearrangement of o-nitrobenzaldehyde (181) into o-nitroso benzoic acid and consecutive dissociation into the corresponding anion and H+. H+ opens the lactone ring in 183a, resulting in rhodamine B (183b).

There is an equilibrium between the lactone and quinone forms of rhodamine B ... 

---