Fluorescent PET chemosensors for lithium

The synthesis of the chiral diaza-9-crown-3 derivatives 1 (S,S) and 2 (R,R) is described. These sensors were designed as luminescent chemosensors for lithium where the fluorescence emission from the naphthalene moieties was ‘switched on’ upon Li⁺ recognition by the crown ether moiety in organic solvents, showing excellent selectivity over other group I and group II cations. Even though the recognition of Li⁺ was not achieved in water (pH 7.4) or aqueous alcohol solution, the fluorescence (which was switched on at pH 7.4) was substantially modulated by spherical anions, where the fluorescence emission was quenched in the presence of Br⁻ and I⁻ but less by Cl⁻ and not by acetate. This indicates that the emission was quenched by heavy-atom affect. The recognition of Li⁺ was also investigated by ¹H NMR in CD₃CN and by observing the changes in the circular dicromism spectra. For the former, the resonances for the crown ether moiety and α-methyl protons adjacent to the ring were sifted upfield and broadened, whereas for 1 the intensity of the CD signal for the π-π transition was substantially modulated upon Li⁺ recognition.     

4-Amino-1,8-naphthalimide-based anion receptors: employing the naphthalimide N-H moiety in the cooperative binding of dihydrogenphosphate

The 4-amino-1,8-naphthalimide-based anion receptor 3 binds dihydrogenphosphate with 1:1 stoichiometry through cooperative hydrogen bonding to a naphthalimide N-H and thiourea N-H groups. This was clearly established from ¹H NMR titration experiments in DMSO-d₆ where a substantial shift in the resonance for the naphthalimide N-H was observed concomitant with the expected thiourea N-H chemical shift migration upon successive additions of H₂PO₄⁻. However, whilst ¹H NMR titration experiments indicate that 3 was capable of binding other anions such as acetate, the naphthalimide N-H does not participate and the N-H resonance was essentially invariant during the titration. The lack of cooperative binding in this instance was justifiable on steric grounds.     

Synthesis and photophysical evaluation of charge neutral thiourea or urea based fluorescent PET sensors for bis-carboxylates and pyrophosphate

The synthesis of the fluorescent photoinduced electron transfer (PET) chemosensors 1-3 for bis-anions such as bis-carboxylates and pyrophosphate in organic solvents is described herein. These sensors are based on the receptor-spacer-fluorophore-spacer-receptor motif where the receptors are charge neutral aromatic thiourea or urea receptors and the fluorophore is anthracene. The anion recognition was evaluated using 1H NMR as well as absorption and fluorescence spectroscopy in DMSO. For simple anions such as acetate or fluoride, the recognition was shown to be through hydrogen bonding of the corresponding anion to the receptors. This gave rise to only minor changes in the absorption spectra, but significant changes in the fluorescence emission spectra, which was substantially (70-95%) quenched. Analysis of these recognition events implied a 1 : 2 (sensor : anion) binding and ideal PET behaviour for ions such as AcO- and H2PO4-. For F-, the luminescent quenching indicated a 1 : 1 binding, but we deduced that this was due more to complete quenching of the excited state after the addition of one equivalent of the anion. For all of the anions, the quenching contributed to enhanced efficiency of PET from the receptors to the excited state of the fluorophore. In the case of the bis-anions (ambient), such as di-carboxylates, similar fluorescence quenching was observed. However, here either a 1 : 1 or a 1 : 2 binding was observed depending on the length of the spacer separating the two carboxylate moieties and the nature of the receptor. Whereas both pyrophosphate and malonate gave rise to a 1 : 1 binding, glutarate gave rise to approximately 1 : 2 binding for the thiourea sensors 1 and 2. However, for the urea based sensor 3, the binding was found to be 1 : 1 for all the bis-anions. For such a 1 : 1 binding we propose that the anion most likely bridges the fluorophore moiety. This was also evident from the 1H NMR (DMSO-d6) spectrum where the anthracene resonances were significantly affected. By simply modifying the electronic structure of the receptor, the sensitivity of the recognition process could also be modified; e.g. compound 1, bearing the trifluoromethyl substituent, showed stronger binding to the bis-anions than 2, which possessed a simple phenyl moiety.     

Synthesis and characterisation of novel 3′-O- and 5′-O- modified azobenzene-thymidine phosphoramidites and their oligonucleotide conjugates as colorimeter DNA probes and FRET quenchers

The synthesis of two new ‘first generation’ azobenzene based thymidine phosphoramidites 1 and 2 having the chromophore (DABCYL) covalently incorporated as an ester in the 3′-O- and 5′-O-positions of the deoxyribose, and the incorporation of these molecules into 16-mer Chronic Myeloid Leukaemia (CML) antisense oligonucleotides, giving 7 and 8, respectively, is described. These compounds were designed as highly coloured probes, and to participate in a Fluorescence Resonance Energy Transfer (FRET) mechanism in the design of novel molecular beacons.     

Sensitivity of <Superscript>57</Superscript>Fe emission Mössbauer spectroscopy to Ar and C induced defects in ZnO

Emission Mössbauer Spectroscopy (eMS) measurements, following low fluence (<10¹² cm^?2) implantation of ⁵⁷Mn (t _1/2 = 1.5 min.) into ZnO single crystals pre-implanted with Ar and C ions, has been utilized to test the sensitivity of the ⁵⁷Fe eMS technique to the different types of defects generated by the different ion species. The dominant feature of the Mössbauer spectrum of the Ar implanted ZnO sample was a magnetic hyperfine field distribution component, attributed to paramagnetic Fe³⁺, while that of the C implanted sample was a doublet attributed to substitutional Fe²⁺ forming a complex with the C dopant ions in the 2^? state at O vacancies. Magnetization measurements on the two samples, on the other hand, yield practically identical m(H) curves. The distinctly different eMS spectra of the two samples display the sensitivity of the probe nucleus to the defects produced by the different ion species.     

Spreadsheet based analysis of Mössbauer spectra

Using spreadsheet programs to analyse spectral data opens up new possibilities in data analysis. The spreadsheet program contains all the functionality needed for graphical support, fitting and post processing of the results. Unconventional restrictions between fitting parameters can be set up freely, and simultaneous analysis i.e. analysis of many spectra simultaneously in terms of model parameters is straightforward. The free program package Vinda – used for analysing Mössbauer spectra – is described. The package contains support for reading data, calibration, and common functions of particular importance for Mössbauer spectroscopy (f-factors, second order Doppler shift etc.). Methods to create spectral series and support for error analysis is included. Different types of fitting models are included, ranging from simple Lorentzian models to complex distribution models.     

Structural studies in aqueous solution of new binuclear lanthanide luminescent peptide conjugates

The synthesis, NMR and luminescent analysis of a novel polypeptide possessing a sensitising naphthalimide antenna at the amino terminus, and a model compound with a Trp moiety, and their Eu(iii) and Tb(iii) complexes are described.     

Solution studies of trimetallic lanthanide luminescent anion sensors: towards ratiometric sensing using an internal reference channel

Luminescent europium and terbium complexes and a mixed Eu(III)-Tb(III) complex were prepared, each with three macrocycles coordinating to a single lanthanide ion to form a trimetallic system, and can be used for the ratiometric sensing of anions in the case of the mixed complex.     

The recognition and sensing of anions through "positive allosteric effects" using simple urea-amide receptors

The design, synthesis, and X-ray crystallographic analysis of three simple diaryl-urea based anion receptors possessing an amide moiety on one of the aryl groups, and an electron withdrawing CF(3) group on the other, is described. The three receptors differ only in the position of the amide functionality relative to the hydrogen bonding urea moiety (being para, meta, and ortho for 1, 2, and 3, respectively). This simple modification was shown to have a significant effect on the anion recognition ability, the strength of the recognition process, and the stoichiometry (host/guest) for these sensors. We demonstrate, by using both UV-vis absorption and (1)H NMR spectroscopy, that these factors are caused by the ability of the amide moiety to both modulate the anion binding selectivity and the sensitivity of the urea moiety. We also demonstrate that, in the case of 1 and 2, this anion recognition at the urea moiety leads to concomitant activation (through enhanced inductive effect) in the amide functionality toward anions, which leads to the formation of an overall 1:2 (sensor/anion) binding stoichiometry for these receptors. This "activation" we describe as being an example of a "positive allosteric activation" by the urea site, caused directly by the first anion binding interaction, which to the best of our knowledge, has not been previously demonstrated for anion recognition and sensing.