Abstract The spectral properties of a series of new 11-acetyl- and 11-ethyl-carboxylate-10-methyl-7 H-benzo[ de]pyrazolo[5,1- a]isoquinolin-7-ones, their dimethylamino derivatives as well as their quinazolinium salts have been investigated. These compounds show bright fluorescence in solution, in poly methlmetacrylamide plastic matrices and in the solid state. The nature of the carbonyl substituent was found to affect the fluorescence quantum yields of the isoquinolinones, but apparently has no influence on the dimethylamino derivatives. These effects can be explained by the relative location of the S 1 ππ * and T 1 nπ * states. The presence of the dimethylamino group in the naphthalic moiety results in shifts of the absorption and emission maxima, and increases in quantum yields. These compounds are effective light shifters and have valuable properties for development of fluorescent markers for biomedical applications.