Photoinduced electron-transfer reactions in small supramolecular systems, such as the 4-amwinonaphthalimide compounds, are interesting in that there are two alternative directions for the electron transfer to occur. Nevertheless, the electron transfer occurs only along one path, Lis deduced from pH-dependent fluorescence quenching studies of selected compounds. The role of the electronic coupling matrix element and the effect of the charges accompanying protonation arc considered so as to explain the directionality of the electron transfer and other results. A related mechanism is suggested for interpreting the behavior of similar molecules, which serve as fluorescent sensors of metal ions.