DUAL LUMINESCENCE, STRUCTURE AND EXCIMERS OF 9-ANTHRACENE CARBOXYLIC-ACID

9-Anthracene carboxylic acid (9ACA) shows two fluorescence spectra which have been variously attributed to twisted and planar molecular forms or to excimers and excited tetramers. The study of the concentration dependence shows that both emissions are of molecular origin. In alkaline solution the fluorescence is similar to that of anthracene, and this is explained by the wide energy gap between the antibonding orbitals of the aromatic ring and of the carbonyl group linked to the very strong donor O-.