PHOTOPHYSICS OF AMINOPHTHALIMIDES IN SOLUTION .1. STEADY-STATE SPECTROSCOPY

In non-protic solvents the dipole moment of the first excited singlet state of 4-aminophthalimide (4AP) increases to 7 D from the ground state dipole moment of 3.5 D. In protic solvents such as alcohols and water, hydrogen bonding results in a further increase in dipole moment to about 18 D, corresponding to nearly total charge transfer (CT) from the aromatic ring to the acceptor imide system. Quenching of the fluorescence in the CT state takes place by protonation in protic solvents and acid solutions. In mixtures of non-polar/protic solvents, the processes of relaxation and of preferential solvation are complicated by the two sequential increases in the dipole moments as well as by the quenching of hydrogen bonded CT state emitters. The 3AP isomer behaves differently, and does not show such large excited state dipole moment and quenching effects. This may be related to the presence of an intramolecular hydrogen bond which involves one of the carbonyl groups.