Treatment of 1H-phenalen-1-one in 95% H2SO4 at 180 °C for 2 days yields a highly water soluble red dye. Although a previous literature report, under similar conditions, described this reaction as producing the 2-substituted sulfonato adduct, the 1H NMR spectrum we obtained for the product was inconsistent with the proposed structure. A detailed 1H NMR spectroscopic study, however, using 1H–1H spin-decoupling and steady-state NOE-difference experiments revealed the product to be 1H-phenalen-1-one-5-sulfonate. Despite the extended planar aromatic π-system typical linear Beer-Lambert behaviour, up to a concentration of ca 2mM, is observed for a solution of the dye in phosphate-buffered water.
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