Absorption spectra and tautomerism of aminonaphthazarins

2-n-Butylaminonaphthazarin (2) absorbs at 499 and 532 nm in ethanol but at 539 and 577 nm in alkali solution, while 2, 6-bis(n-butylamino)naphthazarin (3) absorbs at 461, 500 and 540 nm in ethanol but at 511 and 545 nm in alkali solution, with only a small bathochromic shift of 5 nm. The absorption curve of 3 was changed by addition of alkali and the λD_max at around 461 nm completely disappeared. In the case of 2-bromo-3, 6-bis(n-butylamino)naphthazarin (4), tautomerism between the 1, 4-quinone and 1, 5-quinone forms was observed and the isosbestic point was observed at 480 nm. Dye 2 easily deprotonated to give the 1, 4-dianion, but dyes 3 and 4 existed as tautomeric mixtures in solution and shifted to 1, 5-quinone forms in alkali solution.