UV-visible and 1H−15N^1H-^{15}N NMR spectroscopic studies of colorimetric thiosemicarbazide anion sensors

Four model thiosemicarbazide anion chemosensors containing three N – H bonds, substituted with phenyl and/or 4-nitrophenyl units, were synthesised and studied for their anion binding abilities with hydroxide, fl uoride, acetate, dihydrogen phosphate and chloride. The anion binding properties were studied in DMSO and 9 : 1 DMSO – H 2 O by UV-visible absorption and 1 H/ 13 C/ 15 N NMR spectroscopic techniques and corroborated with DFT studies. Signi fi cant changes were observed in the UV-visible absorption spectra with all anions, except for chloride, accompanied by dramatic colour changes visible to the naked eye. These changes were determined to be due to the deprotonation of the central N – H proton and not due to hydrogen bonding based on 1 H/ 15 N NMR titration studies with acetate in DMSO- d 6 – 0.5% water. Direct evidence for deprotonation was con fi rmed by the disappearance of the central thiourea proton and the formation of acetic acid. DFT and charge distribution calculations suggest that for all four compounds the central N – H proton is the most acidic. Hence, the anion chemosensors operate by a deprotonation mechanism of the central N – H proton rather than by hydrogen bonding as is often reported

Synthesis and physical evaluation of urea- and thiourea-based receptors and sensors for anions

THESIS 8241This thesis, entitled ?Synthesis and Physical Evaluation of Urea- and Thiourea-based Receptors and Sensors for Anions?, consists of five chapters. Chapter 1 briefly introduces the field of molecular sensing, followed by a general description of anion sensing, which includes the design principles of anion receptors and sensors as well as the advantages of their practical use. A review of various recent examples of urea- and thiourea-based fluorescent and colorimetric anion sensors is also presented, which is then followed by a summary of the anion sensing work carried out within the Gunnlaugsson research group. Finally, an outline of the project objectives is described. Chapter 2 discusses the synthesis of bis-phenyl urea and thiourea receptors, substituted at the 2- and 4-position of the aromatic rings by electron-donating (i.e. CH3 ) and electron-withdrawing (i.e. F, CF3 ) groups. .... In Chapter 3, the synthesis of polyaromatic thiourea-based fluorescent anion sensors is described and the physical evaluation of their anion binding abilities in DMSO or DMSO-d6 using various techniques (such as 1H NMR, fluorescence and CD spectroscopies) is discussed. ... Chapter 4 concentrates of the synthesis and anion binding studies of hydrazinebased naphthalimide colorimetric sensors, consisting of either a urea or thiourea moiety. ... Finally, Chapter 5 outlines the experimental procedures used in Chapter 2-4, and presents the characterisation of the compounds prepared

Fluorescent photoinduced electron transfer (PET) sensors for anions; from design to potential application

This mini review highlights the synthesis and photophysical evaluation of anion sensors, for nonaqueous solutions, that have been developed in our laboratories over the last few years. We have focused our research mainly on developing fluorescent photoinduced electron transfer (PET) sensors based on the fluorophore-spacer-anion receptor principle using several anthracene (emitting in the blue) and 1,8-naphthalimide (emitting in the green) fluorophores, with the aim of targeting biologically and industrially relevant anions such as acetates, phosphate and amino acids, as well as halides such as fluoride. The receptors and the fluorophore are separated by a short methyl or ethyl spacer, where the charge neutral anion receptors are either aliphatic or aromatic urea (or thiourea) moieties. For these, the anion recognition is through hydrogen bonding, yielding anion:receptor complexes. Such bonding gives rise to enhanced reduction potential in the receptor moieties which causes enhancement in the rate of PET quenching of the fluorophore excited state from the anion:receptor moiety. This design can be further elaborated on by incorporating either two fluorophores, or urea/thiourea receptors into the sensor structures, using anthracene as a fluorophore. For the latter design, the sensors were designed to achieve sensing of bis-anions, such as di-carboxylates or pyrophosphate, where the anion bridged the anthracene moiety. In the case of the naphthalimide based mono-receptor based PET sensors, it was discovered that in DMSO the sensors were also susceptible to deprotonation by anions such as F&minus; at high concentrations. This led to substantial changes in the absorption spectra of these sensors, where the solution changed colour from yellow/green to deep blue, which was clearly visible to the naked eye. Hence, some of the examples presented can act as dual fluorescent-colorimetric sensors for anions. Further investigations into this phenomenon led to the development of simple colorimetric sensors for fluorides, which upon exposure to air, were shown to fix carbon dioxide as bicarbonate.<br /