Luminescence and charge transfer. Part 4. ‘On–off’ fluorescent PET (photoinduced electron transfer) sensors with pyridine receptors: 1,3-diaryl-5-pyridyl-4,5-dihydropyrazoles

Abstract

1,3-Diaryl-5-pyridyl-4,5-dihydropyrazoles, some of which have been previously synthesized for use as optical brighteners, satisfy the design criteria for fluorescent PET pH sensors by a proton-induced ‘switching off’ of fluorescence. In practice, they are the first family of ‘on–off’ fluorescent PET pH sensors whose signals cover most of the visible spectrum. They also show significant absorptiometric pH sensing action due to the coupling of the protonatable pyridine with the ICT (internal charge transfer) excited state of the 1,3-diaryl-4,5-dihydropyrazole. Experimentally identical pK_a values arise from the fluorimetric and absorptiometric pH dependences. The examination of 2-, 3- and 4-pyridyl regioisomers shows understandable differences in pK_a and maximum fluorescene quantum yield [φ_F(base)] values, but show no significant variations in fluorescence switching factors (FE).