SOLVATOCHROMISM OF ANTHRAQUINONE AND SYMMETRICAL DIHYDROXY DERIVATIVES - LOCAL INTERACTIONS

The solvent effects on the electronic absorption spectra of 9,10-anthr aquinone (AQ) and its symmetric dihydroxy derivatives namely 1,5-dihyd roxyanthraquinone (1,5-DHAQ) and 2,6-dihydroxyanthraquinone (2,6-DHAQ) have been studied in pure solvents and some binary solvent mixtures. The frequencies of the absorption for AQ and 2,6-DHAQ are quite solven t sensitive while those for 1,5-DHAQ are not. Due to the intramolecula r hydrogen bond between the C=O and OH groups, no influence of solvent hydrogen bond acceptors is observed in 1,5-DHAQ. This hydrogen bond g ives a stable six member cycle which is not broken even by the stronge st hydrogen bond acceptor solvents used in this work, such as DMSO and DMF. The Taft and Kamlet's solvatochromic comparison method was appli ed for AQ and 2,6-DHAQ. Aromatic solvents and aliphatic amines were no t included in the correlations since they strongly deviate suggesting another type of interactions. All the pi-->pi bands of AQ and 2,6-DHA Q show strong influence of pi despite the fact that their dipole mome nt is zero. Although it would be reasonable to expect that in the abse nce of a solute dipole moment there is not significant orientation of solvent molecules around the solute molecules, in this case dipolar in teractions between solute and solvent due to local effects might be ex pected. AQ may be considered as formed by two carbonyl groups weakly i nteracting with the benzene rings; that means that the carbonyl group can behave as an isolated dipole and independently of the other. To de tect possible specific interactions between the AQ and aliphatic amine s and aromatic hydrocarbons, preferential solvation in mixed solvent w as investigated. It is concluded that EDA interactions are important i n the solvation of AQ with these compounds as solvents.