Absorption Spectra and Tautomerism of Aminonaphthazarins

2-n-Butylaminonaphthazarin (2) absorbs at 499 and 532 nm in ethanol but 539 and 577 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 absorption maxima 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 solutions