Synthesis, Characterization And Photophysical Properties Of Cyclometalated Platinum (Ⅱ) Complexes With Phosphonate And Phosphonic Acid Groups

The chemistry of square-planar platinum (II) complexes has attracted considerable attention in recent years due to their rich spectroscopic behaviors, useful photophysical and biological properties. In this thesis, a series of platinum (II) complexes of 6-phenyl-2, 2'-bipyridine with phosphonate and phosphonic acid groups, have been synthesized and characterized. Their photophysical properties,solvent effects and applications in the sensing aspects have been studied. Many interesting results were obtained:1. Synthesis and spesctroscopic study of cyclometalated Platinum (II) bipyridyl complexes with phosphonate group:Four cyclometalated Pt (II) polypyridyl complexes Pt-1, Pt-2, Pt-3 and Pt-4 with phosphonate group were studied. All complexes have strong absorption and emission properties. According to the order of complexes Pt-3, Pt-2, Pt-1 and Pt-4, their absorption and emission bands have displayed a red shift. By introducing conjugated phenyl into main ligand, the conjugated systems of complexes have been enlarged to reduce energy of the metal-to-ligand charge transfer (³MLCT), leading to the red shift of emission bands. However, due to introducing -COOMe group, the strong attracting electron has dramatically decreased the lowest unoccupied molecular orbital (LUMO) energy, which increases the energy gap of the metal-to-ligand charge transfer(MLCT)and d-d states to exhibit a red shift. Meantime, all complexes have prominence solvatochromic response. With increasing solvent polarity, the low-energy absorption bands exhibit a blue shift, but the absorption bands under 350 nm almost have no effect. This is in agreement with the attribution of those bands.2. Synthesis and spesctroscopic study of cyclometalated Platinum (II) bipyridyl complexes with different auxiliary ligand:By varying the auxiliary ligand from Cl^-,Br^- to SCN^-, we can adjust their emission spectra. The complexes Pt-2, Pt-2a and Pt-2b were synthesized and characterized. The result of spesctroscopic study showed that the emission spectra were red shifted with the increase of donating electron ability, and all complexes have no intermolecular Pt-Pt interaction. The complex Pt-2b has two emission bands at 564 nm and 594 nm, this is due to adopting different coordination modes in solution to form respectively Pt-SCN and Pt-NCS coordination complexes.3. Synthesis and spesctroscopic study of binuclear cyclometalated Platinum (II) bipyridyl complexes with phosphonate group:By using complex Pt-2 as structural unit, the two Pt atoms are bridged by dppm and dppe ligands, obtaining two new complexes Pt-9 and Pt-10. The spesctroscopic result indicated that the lowest excited state originates from ³MLCT. According to different chain length of dppm and dppe, the strong spesctroscopic spectra of Pt-9 ascribe to ³MMLCT because of metal-metal and ligand-ligand interaction, but those of Pt-10 contribute to ³MLCT due to no metal-metal and ligand-ligand interaction.4. Synthesis, spesctroscopic study and solvatochromic response of cyclometalated Platinum (II) bipyridyl complexes with phosphonic acid group:The spesctroscopic properties of complexes Pt-5～Pt-8 in DMSO and solid state have been discussed in detail. The results showed that complexes Pt-7 and Pt-8 only have prominent solvatochromic response in the present of DMSO molecule, but no effect is detected with other solvents. Complex Pt-7 exhibits a solvatochromic shift to 704 nm in emission maximum from emerald powder to red one, the emission band at 556 nm was red shifted to 683nm from yellow powder to red one for complex Pt-8. Nevertheless, no solvatochromic response is found for complexes Pt-5 and Pt-6 in the present of DMSO and methanol molecules. We think that the strong hydrogen bond formed between the neighbour molecules have played an important role in the emergence of solvatochromic response for complexes Pt-7 and Pt-8, resulting in decreasing the distance between two neighbour molecules to produce the intermolecular interaction. In addition, no solvatochromic response has been related to their large substituted groups of Pt-5 and Pt-6, badly holding back the closing of neighbour molecules to weaken the intermolecular interaction.5. Synthesis of cyclometalated Platinum (II) bipyridyl complexes with phosphonic acid group and Effect of pH on their electronic spectra:The absorption and emission spectra of HL-4 and Pt-11 were studied at different pH. The dissociation constants with pK_a values at 6.2 and～2.0 were measured by the potentiometric titration method. The mechanism of sensitivity to pH value has also been discussed in detail. With increasing pH value, the—PO₃H₂ group was dissociated into -PO₃H^- and -PO₃^2- groups, resulting in the enhancement of ability to supply electron. Electronic spectral studies showed that the absorption bands at 322 nm, 270 nm and 241 nm were blue shifted to 292 nm, 262 nm and 234 nm respectively, the. emission band at 428 nm was blue shifted to 353 nm with increasing the pH value. For binuclear complex Pt-11, the emission band at 625nm has a blue shift to 567nm with increasing the concentration of triethylamine. In addition to the enhancement of ability to supply electron, we think that due to its strong ability to combine proton, triethylamine has played a major part in blue shift because of breaking the intermolecular hydrogen bond formed between neighbour phosphonic acid in Pt-11. The breaking of hydrogen bond is leading to the increase of distance between two structural units of Pt-11 to have no interaction. In consequence, owing to the active phosphonic acid in ligand HL-4 and complex Pt-11, they show good sensitive function to pH values, reaching the protonic detection.