Fluorescent chemosensors for metal ions based on photoinduced electron transfer (PET) from sulfur

Mireille Vonlanthen
The use of fluorescence for the detection of metal ions is a very powerful technique since it allows the analysis of traces amounts of specific analytes by a simple optical method. However, metal ions are not fluorescent, and in order to detect non-emissive targets using fluorescence spectroscopy small organic compounds capable of signal transduction need to be developed. Such compounds are named chemosensors when they are of abiotic origin. They will have to present two different states in
more » ... r fluorescence emission indicating the occurrence of a supramolecular binding event via a transduction mechanism. A photophysical mechanism is required for the signal transduction, and the most commonly used is photoinduced electron transfer (PET) quenching. In the majority of the examples found in the literature, PET based chemosensors focus on the use of the lone pair of electron from a benzylic nitrogen atom as electron donor for the quenching of the fluorescence. Upon coordination with metal ions, or protonation, the energy of the lone pair of electron is lowered and electron transfer is no longer thermodynamically favored. This gives rise to an increase of the fluorescence emission giving a visual indication of the presence of the target. In this thesis, the use of a sulfur atom as electron donor for PET quenching was investigated. First, the case of a thioether functional group was studied with either a naphthalimide or a coumarin as fluorophore. The obtained compounds were not showing reduced quantum yields as expected in the case of an efficient PET quenching. This behavior could be explained by high redox potential of the thioether moiety leading to thermodynamically disfavored PET quenching process. The second case studied was the integration of a thiourea moiety as electron donor for PET quenching process with a naphthalimide fluorophore. In this case, low quantum yields in comparison with a non-quenched analog were observed indicating that PET quenching is occurring. A typical distance dependence of the efficiency of the process was observed and the study of the thermodynamics of the process indicates that PET is feasible. Subsequent addition of metal ions in methanol and in a water:methanol mixture was investigated and leads to recovery of the fluorescence. The addition of Hg2+ is showing the strongest affinity with an apparent binding constant of 1 µM and with a 10-to 20-fold increase of the fluorescence emission for all the compounds. The affinity of the thiourea based chemosensor to various metal ions can be modulated by variation of the appended binding domain. The introduction of a di-2-pycolylamine (DPA), which is a well-known ligand for Zn2+, as well as the introduction of other pyridine containing analogs is inducing interesting behavior. When DPA is placed very close in space to the thiourea, the chemosensor interacts with Zn2+ with only one of the pyridine, leading to 13 folds increase of the fluorescence but with lower affinity, as it would be expected for typical DPA-Zn2+ interaction. This hypothesis was confirmed by the crystal structure of a zinc complexe of an analog of our chemosensor bearing a phenyl group instead of the fluorophore. The addition of an ethyl bridge in the system gives more space between the DPA moiety and the sulfur atom leading to lower increase of the fluorescence enhancement but increased affinity for the interaction with Zn2+. Further modifications of the binding domain, for example by the addition of crown ethers containing oxygen or sulfur bridging atoms were investigated, and lead to interesting responses to various metal ion as for example Ag+ and Pb2+ in methanol and aqueous media. This work demonstrates that modulation of PET quenching by thioureas provides a basis for the development of metal-responsive fluorescent chemosensors in protic media. High affinity and intensity response to the addition of Hg2+ as well as very interesting variation of the response to the addition of Zn2+ depending on the additional binding domain was observed.
doi:10.5167/uzh-164261 fatcat:tawfwxznfvantg7qr2cchgg36u