ROLE OF NONFLUORESCENT TWISTED INTRAMOLECULAR CHARGE-TRANSFER STATE ON THE PHOTOPHYSICAL BEHAVIOR OF AMINOPHTHALIMIDE DYES

Photophysical behavior of 4-aminophthalimide and its N,N-dimethyl deri vative is studied in a series of solvents. A comparison of the fluores cence properties of the two compounds reveals that they behave quite d ifferently despite their structural similarity. The efficient nonradia tive decay of the dimethyl derivative in polar media is interpreted in terms of a nonemitting twisted state. Semiempirical calculations base d on the AM1 method have been performed, and the results support the t wisting mechanism. Calculations of the ground and excited state profil es as a function of torsion angle provide a clear picture of whether t he twisting can influence the photophysical behavior of this class of dyes. The results provide insight into the design of efficient dye sys tems.