Knowledge Bank

From vibrationally well-resolved fluorescence spectra of amino and hydroxy-anthra-quinones, we have been studying the intramolecular hydrogen bonds these molecules are known to contain. The measurements have been made in n-alkane Shpol ‘skil matrices with site selection afforded by tunable dye laser excitation. Vibronic bandwidths are typically on the order of $5cm-1$. Among the molecules studied are 1,4-, 1,5-, and 1,8-dihydroxyanthraquinone, 1-aminoanthraquinone, and the hydroxy-deuterated isomers of 1,4-dihydroxyanthraquinone (quinizarin). There is strong evidence for excited state proton transfer in 1,5 -, and 1,8-dihydroxyanthraquinone, but not in quinizarin. The vibronic pattern of the 1-aminoanthraquinone fluorescence is highly unusual; almost all of the vibronic activity is contained in four fundamentals with frequencies between 300 and $500cm-1$. There is no discernible activity of either carbonyl stretching vibrations or aromatic ring stretching vibrations. The deuteration experiments on quinizarin indicate strong coupling between a mode involving the hydrogen atom of the hydrogen bond and various ring stretching vibrations. In this talk we shall present the relevant spectra, vibrational assignments, and our current understanding of the nature of the intramolecular hydrogen bonds in these molecules.