Dual responsive chemosensors for anions: the combination of fluorescent PET (photoinduced electron transfer) and colorimetric chemosensors in a single molecule

The design and synthesis of two novel fluorescent PET anion sensors is described, based on the principle of 'fluorophore-spacer-(anion)receptor'. The sensors 1 and 2 employ simple diaromatic thioureas as anion receptors, and the fluorophore is a naphthalimide moiety that absorbs in the visible part of the spectrum and emits in the green. Upon recognition of anions such as F- and AcO- in DMSO, the fluorescence emission of I and 2 was 'switched off', with no significant changes in the UV-vis spectra. This recognition shows a 1:1 binding between the receptor and the anions. In the case of F-, further additions of the anion, gave rise to large changes in the UV-vis spectra, where the lambda(max) at 455 nm was shifted to 550 nm. These changes are thought to be due to the deprotonation of the 4-amino moiety of the naphthalimide fluorophore. This was in fact found to be the case, using simple naphthalimide derivatives such as 6. Sensors 1 and 2 can thus display dual sensing action; where at low concentrations, the fluorescence emission is quenched, and at higher concentrations the absorption spectra are modulated. (C) 2003 Elsevier Ltd. All rights reserved.