We report a combined computational and experimental study to rationalize the behavior of a well-known singlet oxygen (O-1(2)) probe, that is, the chromophore of the Singlet Oxygen Sensor Green: a fluoresceine-based sensor. In particular, we evidence that the presence of an intramoleculer charge transfer state that is no more present upon reaction with O-1(2) explains the fluorescence enhancement observed in the presence of reactive oxygen species. Furthermore, we also unequivocally show the photophysical pathways leading to the fluorescence enhancement of fluoresceine upon irradiation with UVA lights and also in the absence of any oxygen activator. More specifically, we evidence that the presence of a possible intersystem crossing upon population of higher energy singlet electronic excited states will lead to the population of the fluoresceine triplet manifold and hence to the self-production of O-1(2).