Porphyrin Appended With Ionophores: Design And Synthesis Of New Cation-Sensitive Fluorescent Cation Probes

The analysis and detection of the transition metal ions are currently of significant importance for chemistry because of its implications in many fields: chemistry, biology, medicine (clinical biochemistry), environment, etc. Fluorescent probes can transfer molecular recognition events into fluorescence signals, which make a bridge between the molecular analytes and the analyst as an observer. The advantages of fluorescenct probes include high sensitivity of detection down to the single molecule, nice selectivity,"on-off"switchability, subnanometer spatial resolution and submillisecond temporal resolution, observation in situ, remote sensing with the application of fiber optics, etc. In terms of sensitivity concerns, fluorescent probes exhibiting fluorescence enhancement on metal ion complexation are favored over those showing fluorescence quenching upon cation binding. The fluorescence intensities of common probes (both fluorescence quenching and fluorescence enhancement) are influenced by experimental factors such as excitation intensity, the micro-environment around the dye, and the concentration of the dye. The ratiometric probes can normalize the variation of these effects and provide more robust and precise measurement results.Porphyrins are attractive candidates of fluorescent probes owing to their good photophysical properties with strong fluorescence, large Stokes shifts and relatively long excitation (>400 nm) and emission (>600 nm) wavelengths that minimize the effects of the background fluorescence. Certain problems seriously restrict their wide applications such as the weak fluorescence emission of most of metalloporphyrins, the slow rate of formation of metalloporhyrins, and the poor selectivity, especially the interference from transition metal ions. Thus, in the present study porphyrins appended with the traditional ionophores were synthesized and applied for preparation of fluorescent probes. Along this line we synthesized a new porphyrin derivative (H2TPP-Cycalm) appended with cyclam (an ionophore of Hg²⁺). Following the method of synthesis of tetraphenylporphyrin, 5-(4-hydroxyphenyl)-10,15,20- triphenylporphyrin was prepared by controlling the ratio of benaldehyde and p-hydroxybenzaldehyde. 5-(4-Hydroxyphenyl)-10,15,20-triphenylporphyrin was connected with cyclam by 1,4-dibromobutane. H2TPP-Cyclam exhibits a linear response toward Hg²⁺ in the concentration range 1.0×10^-7-1.0×10^-4 M, with a working pH range from 5.0 to 8.0 and a high selectivity. It indicates that Hg²⁺ chelates to porphyrin ring with the help of cyclam as proved by the Uv-Vis spectra. The detection limit is 2.1×10^-8 M.Bipyridine is a good ionophore of Cu²⁺. Following the design of fluorescent probe, we synthesized a porphyrin derivative appended with bipyridine (H₂TPPBPy). Firstly we synthesized the ligand, 4-methyl-2,2′-bipyridine-4′-carboxylic acid. Selective SeO₂ oxidation of 4,4′-dimethyl-2,2′-bipyridine to the monoaldehyde was followed by Ag₂O oxidation of the latter to the monoacid (4-methyl-2,2′-bipyridine-4′-carboxylic acid). The monoacid was transformed to acetyl chloride and reacted with 5-(4-aminophenyl)-10,15,20-triphenylporphyrin. A Cu²⁺-sensitive optical chemical sensor was prepared by the entrapping H₂TPPBPy in a poly(vinyl chloride) membrane. It exhibits a linear response toward Cu²⁺ in the concentration range 2.0×10^-8-1.0×10^-5 M, with a working pH range from 6.0 to 8.0 and a high selectivity. The detection limit is 5×10^-9 M. Quenching of porphyrin fluorescence on irradiation is attributed to electron transfer from porphyrin to the acceptor (Cu²⁺), which is linked with the porphyrin by bipyridine.The selectivity of fluorescent probe must be viewed in terms of both selectivity of binding and selectivity of photophysical effects. On the base of H₂TPPBPy, we synthesized ZnTPPBPy. ZnTPPBPy exhibits a linear response toward Cu²⁺ in the concentration range 3.2×10^-7-1.0×10^-5 M, with a working pH range from 5.0 to 7.0 and a high selectivity. The detection limit is 5×10^-8 M. Quenching of zinc-porphyrin fluorescence on irradiation is attributed to electron transfer from porphyrin to the acceptor (Cu²⁺), which is linked with the porphyrin by bipyridine.Porphyrins as electron donors have been widely investigated in the photochemistry. Pyridine is a good electron acceptor. When porphyrin is connected with terpyridine (H2TPPTPy) through a linking bridge, it would make up a"donor-spacer-receptor"intramolecular PET transfer system. Firstly we synthesized the ligand (2,2′:6′,2′′-terpyridine-4′-carboxylic acid) in a simple procedure with high yield via one step oxidation. Finally, porphyrin-appended terpyridine (H2TPPTPy) was synthesized by the direct coupling of 2,2′:6′,2′′-terpyridine-4′-carboxylic acid and 5-(4-aminophenyl)-10,15,20-triphenylporphyrin with 2-chloro-1-methylpyridinium iodide (CMPI) as a coupling reagent and 4-dimethylaminopyridine (DMAP) as a base. In the probe, the fluorescence of porphyrin is quenched by way of transfer of the excited state electron from porphyrin to terpyridine. When the terpyridine moiety is bound to cadmium ion, the chelate would abrogate the PET process and the fluorescence of porphyrin is recovered. It shows a linear response toward Cd²⁺ in the concentration range of 3.2×10^-6 M to 3.2×10^-4 M with a limit of detection of 1.2×10^-6 M. The probe shows good selectivity for Cd²⁺ over a large number of cations such as alkali, alkali earth and transitional metal ions except Cu²⁺ and Zn²⁺.8-Hydroxyquinoline (8-HQ) is a more important chelating agent than pyridine. Its most interesting feature is that it has very low quantum yield in aqueous or organic solutions while the fluorescence enhancement arising from cation binding. 8-Hydroxyquinoline benzoates are highly sensitive probe for Hg²⁺. At the same time, tetraphenylporphyrin possesses four phenyls. Following the design of fluorescent probe, we synthesized a porphyrin (1) derivative appended with 8-HQ through a linking bridge (ester bond) and applied it for preparation of a ratiometric fluorescent probe for sensing of Hg²⁺ ions. Following the method of synthesis of tetraphenylporphyrin, 5-(4-carboxymethylphenyl)-10,15,20-triphenylporphyrin was prepared by controlling the ratio of benaldehyde and p-hydroxybenzaldehyde. And then 5-(4-carboxymethylphenyl)-10,15,20-triphenylporphyrin was hydrolyzed to 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin. Finally, the porphyrin derivative 1 was synthesized via a simple one-step reaction of 8-HQ with carboxylporphyrin using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC) as a coupling reagent and N,N'-dimethyl-4-aminopyridine (DMAP) as a base in good yield. It indicates that Hg²⁺ chelate only with the 8-hydroxyquinoline moiety as shown by the Uv-Vis spectra. The fluorescence intensity at 603 nm is enhanced and that at 646 nm is quenched. The proposed probe shows a selective ratiometric fluorescent response towards Hg²⁺ ions with a linear range covering from 3×10^-7 to 2×10^-5 M and a detection limit of 2.2×10-8 M.The quinazoline derivative contains an intramolecular hydrogen bond, which would undergo excited-state intramolecular proton transfer (ESIPT) at illumination. The process produces a highly Stokes shifted emission. The proton acceptor is commonly an amino group. Following the design of fluorescent probe, we modified the amino group with ionophore (e.g., DPA) and prepared a Co²⁺-sensitive fluorescent probe. Anthranilamide and o-nitrobenzaldehyde were reacted and transformed to the intermediate in the condition of acid. The intermediate was oxidate which was transformed to nitrophenylquinazoline under 2,3-dichloro-4,5-dicyano- 1,4-benzoquinone. Nitrophenylquinazoline was reduced to aminophenylquinazoline. Finally, aminophenylquinazoline was reacted with 2-chloromethylpyridine to form BpyPQ. The DPA moiety binds cobalt ion tightly with three nitrogen-donor atoms. So the tertiary amine would not accept proton anymore as its lone-pair electrons participated coordination with cobalt. In this way the cobalt inhibits the excited-state intramolecular proton transferring process, which results in the fluorescence quenching. It exhibits a linear response toward Co²⁺ in the concentration range 3.2×10^-8-1.4×10^-6 M, with a working pH range from 7.0 to 9.5 and high selectivity.The molecule with the lowest-excited nÏ€^* single state is weakly fluorescent. If complexation with ion induce transfer the lowest-excited single state from the nÏ€^* state to theππ^* state, it will emit fluorescence. Carbonyl oxygen lone pair in the ester derivatives of 8-HQ make these fluorophores weakly fluorescent as a result of a radiationless process via the nÏ€^* state. The radiationless decay is easily influenced by environment factors such as metal ions and H+ binding with the carbonyl oxygen. The lowest single state is transferred from the nÏ€^* state to theππ^* state. The fluorescence is recovered from aππ^* state. A new compound, di(quinolin-8-yl) isophthalate (DQIP), was synthesized and applied as a potential fluoroionophore for recognition of hydrogen ions. DQIP was synthesized via a simple reaction of 8-HQ with isophthaloyl chloride. Carbonyl oxygen lone pair will easily capture the H+ ions of environment with the help of the quinoline N atom. Thus, the radiationless decay is blocked and the emission originates from aππ^* state. It shows a linear response toward H+ in the pH range of 1-6.5 with moderate selectivity.