Structural and spectroscopic investigation of 5-hydroxyflavone and its complex with aluminium

This study constitutes the third step in a program aimed to investigate the chelation of hydroxyflavones. After the 3-hydroxyflavone and 3′,4′-dihydroxyflavone, we now study the complexation of 5-hydroxyflavone (5HF) with Al(III). A stoichiometry of 1:1 was obtained from spectrophotometric methods for the complex in methanol. Important structural modifications of 5HF have been observed by density functional theory (DFT) calculation between free and complexed molecules. Whereas, 5HF adopts a staggered conformation, the complexed molecule is rather flat, involving participation of pyronium and cinnamoyl forms. Both γ-pyrone and B-rings are affected by complexation, and changes in the inter-ring linkage are observed. UV–vis and Raman spectroscopies have permitted to confirm the structural models of free and complexed 5HF as well as the modifications produced by complexation. The bathochromic shift of band I observed in the electronic spectrum of complex relative to the 5HF one, has been attributed to pronounced changes in the pattern of frontier orbitals involved in this transition.