LUMINESCENCE AND CHARGE-TRANSFER .4. ON-OFF FLUORESCENT PET (PHOTOINDUCED ELECTRON-TRANSFER) SENSORS WITH PYRIDINE RECEPTORS - 1,3-DIARYL-5-PYRIDYL-4,5-DIHYDROPYRAZOLES

1,3-Diaryl-5-pyridyl-4,5-dihydropyrazoles some of which have been prev iously 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-o ff' fluorescent PET pH sensors whose signals cover most of the visible spectrum. They also show significant absorptiometric pH sensing actio n due to the coupling of the protonatable pyridine with the ICT (inter nal charge transfer) excited state of the 1,3-diaryl-4,5-dihydropyrazo le. Experimentally identical pK(a) values arise from the fluorimetric and absorptiometric pH dependences. The examination of 2-, 3- and 4-py ridyl regioisomers shows understandable differences in pK(a) and maxim um fluorescene quantum yield [phi(F)(base)] values, but show no signif icant variations in fluorescence switching factors (FE).