1,3,5-Triarylpyrazolines - pH-driven off-on-off molecular logic devices based on a receptor1-fluorophore-spacer-receptor2 format with internal charge transfer (ICT) and photoinduced electron transfer (PET) mechanisms

The excited state photophysical properties of the 1,3,5-triarylpyrazolines 1-4 were studied in methanol and 1: 1 (v/v) methanol-water, as well as 1: 4 (v/v) methanol-water and water by fluorescence spectroscopy. The molecules 2-4 incorporate a receptor(1)-fluorophore-spacer-receptor(2) format while 1 is a reference compound based on a fluorophore-receptor(1) design. The molecular probes operate according to photoinduced electron transfer (PET) and internal charge transfer (ICT) processes. At basic and neutral pHs, 2-4 are essentially nonfluorescent due to PET from the electron-donating dimethylamino moiety appended on the 5-phenyl ring to the excited state of the 1,3,5-triarylpyrazoline fluorophore. At proton concentrations of 10-3 mol/L, the dimethylamino unit is protonated resulting in a strong blue fluorescence about 460 nm with significant quantum yields up to 0.54. At acid concentrations above 10(-2) mol/L, fluorescence quenching is observed by an ICT mechanism due to protonation of the pyrazoline chromophore. Symmetrical off-on-off fluorescence-pH profiles are observed, spanning six log units with a narrow on window within three pH units. Hence, 2-4 are novel examples of ternary photonic pH sensing molecular devices.