A synchrotron-radiation study of the lactone form of rhodamine B at 120 K

The structure of the lactone form of rhod­amine B, 3,6-bis­(diethyl­amino)-1′,3′-di­hydro­spiro­[xanthene-9,3′-isobenzo­furan]-1′-one, C28H30N2O3, has been determined at 120 K using synchrotron radiation at a wavelength of 0.496 A. The structure contains two independent rhod­amine B mol­ecules with virtually identical geometry. The xanthene main planes of the mol­ecules are inclined at an angle of 41.6 (2)° to one another. Molecule 2 has a statistically disordered ethyl group, with 71% in one orientation and 29% in a second orientation. The lactone C—O bonds are 1.497 (1) and 1.495 (1) A. There are no classical hydrogen bonds, but the structure is stabilized by two short C⋯O interactions. The crystals of the lactone form were produced by a novel hydro­thermal reaction.