Steric Inhibition of Conjugation in Lowest Excited Singlet State of 9-Anthramide by Hydrogen Bond donor Solvents: Role of Solvent in Chemical Structure

9-Anthramide has electronic absorption and fluorescence spectra that, in water, are similar to those of anthracene. This result is attributed to steric hindrance of the 9-carboxamido group with the peri-hydrogen atoms in the 1- and 8-positions of the anthracene ring. However, in aprotic solvents, although the absorption spectrum of 9-anthramide is anthracene-like, its fluorescence spectrum is red shifted and structureless. This finding is attributed to excited-state rotation of the 9-carboxamido group into coplanarity with the anthracene ring and indicates that, in water, the hydrogen-bonded solvent cage affects the steric inhibition of conjugation in excited 9-anthramide. These findings suggest that studies of structure and reactivity of drugs in nonaqueous or solid matrixes are probably of only limited value, since in the strongly interacting aqueous media the aqueous solvent cage plays a substantial role in determining molecular structure and reactivity.