ELECTRONIC SPECTRA OF MONO-SUBSTITUTED ANTHRAQUINONES AND SOLVENT EFFECTS

A systematic investigation of the near UV and visible spectra of mono-substituted anthraquinones and the solvent effects on the longest wavelength π → π* absorption band of these compounds reveals that the longest wavelength π → π* band of 1- and 2-substituted anthraquinones with electron donating groups can be assigned to the intramolecular electron transfer transition from the substituents to the anthraquinone nucleus. The effect of the intramolecular hydrogen-bonding between the carbonyl group of the quinone and the adjacent substituents on the π → π* absorption band has also been investigated. The absorption frequency-shifts in various solvents indicate that the value of the frequency-shift mainly depends on the strength of the hydrogen bond between the substituents and the solvent molecules. The largest shifts are caused by the interaction between active hydrogen of the substituents and the proton accepting solvents. In the case of 1-substituted anthraquinones, however, the intramolecular hydrogen-bonding is favoured and the solvent shifts are small. In the absence of the intermolecular hydrogen bonds, the solvent shifts are well interpreted by McRae's equation. © 1968.