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

9-Anthracene carboxylic acid (9ACA) shows two fluorescence spectra whi ch have been variously attributed to twisted and planar molecular form s or to excimers and excited tetramers. The study of the concentration dependence shows that both emissions are of molecular origin. In alka line solution the fluorescence is similar to that of anthracene, and t his is explained by the wide energy gap between the antibonding orbita ls of the aromatic ring and of the carbonyl group linked to the very s trong donor O-.