ABSTRACT

This work describes the synthesis of several photoactive molecular sensors functionalised with suitable receptors for the recognition of certain ionic species in solution and the study of the interaction of the chemosensors with the species of interest (determination of stability constants of the complexes, interferent studies, etc.).
The main chromophoric nucleus studied here is the 7-amino-3H-phenoxazin-3-one, whose structure consists basically of an amino group acting as electron donor and a carbonyl group connected through the p-electron system of the chromophore. The carbonyl acts as acceptor in two processes, the intramolecular charge transfer that takes place upon electronic excitation and the intermolecular hydrogen bonding formed in protic solvents. This chromophore possesses optimal spectroscopic characteristics such as high molar absorption coefficients, high fluorescence quantum yields and absorption and emission wavelengths centered above 500 nm. A series of model compounds have been synthetised for spectroscopic studies which helped to better understand the response mechanisms of the molecular sensors. The increase on electron donor strength of the substituent introduced through the amino group in the phenoxazinone (amido > urea > amino > dimethylamino > julolidino) results on an increase of fluorescence quantum yields and bathochromic shifts of the absorption and emission bands. Based on these results, a weak electron donor group (urea) as receptor is introduced in the phenoxazinone core for the detection of rich electron species (anions). This allowed to obtain a fluoroionophore able to detect the presence of acetate and dihydrogenphosphate in organic solvents via hydrogen bond interactions, with an increase on fluorescence emission and a colour change from orange to pink. In the case of the more basic anion fluoride, it is showed, via UV/Vis and 1H-NMR spectroscopic studies, that deprotonation of the chromophore occurs, instead of coordination via hydrogen bonds. Another interesting aspect of those chromophores is its hability to act as a ditopic receptor in certain situations. In the same way as for protonation, coordination of metal cations takes place through the carbonyl group of the dye. This coordination induces shifts to the red in the absorption and emission spectra and quenching of the fluorescence. By introducing different receptors through the nitrogen (macrocyclic ligands, ether crown type for alkaline-earth cations coordination and aza-thia-oxa type for heavy-metal cations coordination), it is possible to obtain selective hypsochromic shifts induced by the coordination of certain cations through the electron donor group of the molecule, whereas the rest of cations interact through the carbonyl group. Thus for example, a colorimetric and fluorogenic sensor for Hg2+ able to selectively detect this cation in aqueous solution in the ppb range has been developed. 
One of the most innovative aspects of this work has been the incorporation of some of these photoactive systems onto UVM-7, a mesoporous siliceous matrix MCM-41 type, with a hexagonal distribution of nanometric channels and a high specific surface. This made possible the development of heterogeneous sensors, either for organic vapours (grafting of a solvatochromic phenoxazinone derivative) or for anions in aqueous media (long-chain carboxylates, though the incorporation of a urea-phenoxazinone derivative onto a highly hydrophobic support). 
Another fluorescent system employed here has been an aminomethylanthracene derivative, grafted onto UVM-7 for the ATP anion detection in aqueous solution via fluorescence quenching of the anthracene at acidic pH. The inmobilization of the receptor on the solid support results on an increase of the interaction with the species of interest with respect to the free ligand in solution. This improvement of the interaction with the targeted anion has been assigned to an increase on the electrostatic interactions of the ammonium groups of the receptor with the ATP due to a surface concentration increase of the ligand. The synthesis of several materials with different fluorophore loads reveals that an increase on ligand concentration (thus of its space proximity) leads to the appearance of additional effects resides the fluorescence quenching induced by ATP coordination as the excimer appearance, what causes the dequalification of the response of the fluorescent probe.
Additionally, a model based on the Langmuir isotherm has been proposed for the evaluation of the interaction strength between the corresponding anions and the photoactive hybrid materials by means of the calculation of an adsorption constant.
Finally, the anchoring of chromo/fluorphores onto UVM-7 made possible to develop a method for the spectrophotometric determination of fluoride. The reaction of this anion with the silica support at acidic pH results in the dissolution of the silica and the liberation of the indicator molecule, with the consequent appearance of color/fluorescence emission in the solution (whose intensity is proportional to fluoride concentration).