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A photoinduced electron transfer (PET)-based chemosensor possessing dual PET processes by simultaneously introducing both nitrogen and sulfur donors was achieved. The fluorescence signal of the free chemosensor is in normal-off state due to the sulfur donor being insensitive to environmental pH stimuli. As a result, the device can be used over a wide pH span of 3-11. Upon binding Al³⁺, a significant fluorescence enhancement with a turn on ratio over 110 fold was triggered by the inhibition of PET processes from both the sulfur and the nitrogen donors to the fluorophore.