The spectra of forty-five 2-pyrazolin-5-one dyes in the ultraviolet and visible regions are discussed on the basis of their hydrazone-keto structures. In particular, the effects of sterically-sensitive ortho-substitutions in the 1- and 4-positions on the pyrazolinone ring are investigated. The bathochromic effects on the visible absorption band of pyrazolinone dyes are caused by electron-attracting substituents in the 3-position, not by 1- and 4-substituents.
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H. Yasuda (1967) studied this question.
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