Red Emissive BODIPY Dyes as Sensitizers for Near-Infrared Emission of Lanthanide Complexes & Structural & Spectroscopic Studies of Quinoline-Functionalized BODIPY Dyes

Bioimaging has become a significant tool for the visualization of biological events and the detection of numerous biomarkers. However, detection sensitivity is compromised by background signal from the biofluid which interferes with signals from the probes that usually emit in the visible region Alternative probes such as those that emit in the near-infrared (NIR) region are necessary to overcome these shortcomings. Lanthanide ions have demonstrated efficiency with sharp characteristic emission in the visible and (NIR) region. Emission however is mostly achieved by indirect sensitization process in which energy from the triplet state of a sensitizer is transferred to the excited states of lanthanide ion and thereby initiating emission in the NIR region when the lanthanide ion relaxes back to the ground state. Designing ligands that contain strongly absorbing chromophores that can conveniently be used to sensitize lanthanide ion in generating near-infrared emission at low energy has been a key requirement in synthesis of efficient of lanthanide complexes. All antenna ligands synthesized in this study were BODIPY dye derivatized ligands. BODIPY dye has been a choice of ligand due its excellent photophysical properties such as high extinction coefficient, narrow absorption and emission bands, high quantum yield and low photobleaching effect. Three major antenna ligands were synthesized for sensitization of the lanthanide ion. The first antenna ligand [iodinated BODIPY dye benzoic acid (AH9)] was employed in synthesis of ytterbium complex in the presence and absence of auxiliary ligand [1,10-phenanthroline]. The reaction mixture obtained from AH9 with auxiliary ligand was difficult to be post-treated by column chromatography purification due to insolubility of the reaction mixture in many nonpolar solvents. Reaction of 3 moles of AH9 with ytterbium triflate in absence of auxiliary ligand gave a reaction mixture that could be treated only in polar solvents. Fractions obtained from column chromatography had very weak NIR emission between 900-1100m. The insolubility of the reaction mixtures of the lanthanide complexes of AH9 in many non-polar solvents required further extensive study which will be considered a future work of this research. The second antenna ligand 2,9-Phenanthroline BODIPY dye was originally synthesized with the aim of employing the N-atoms of the phenanthroline ring in binding to the lanthanide ion center. The substitution of the 2,4-dimethylpyrrole moieties on C2 and C 9 of the phenanthroline ring was to design organic chromophore with not only high absorption coefficient but also an organic chromophore capable of eliminating influence of solvent molecule interaction with the metal ion center. However, 2,9-phenanthroline BODIPY dyes was observed to have a significant low fluorescence emission a phenomenon which contrasts with the class of 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene family (BODIPY dyes). To fully understand the underlying reasons causing the low fluorescence emission of 2,9-phenanthroline BODIPY dye, 2-quinoline BODIPY dye, 3-quinoline BODIPY dye and 4-quinoline BODIPY dye was synthesized and their photophysical properties alongside that of 2,9-phenanthroline BODIPY dye was reported. Quantum yield determination of the four quinoline BODIPY dyes showed that 2,9-phenanthroline BODIPY and 2-quinoline BODIPY dye had the lowest quantum yield. The distance of the boron on the BODIPY core to the nitrogen atom on the quinoline ring for all the four dyes established a relationship (B-N distance) that was found to be directly proportional to the fluorescence quantum yield result. It was further established based on the photo-induced electron transfer mechanism that the fluorescence quenching was because of electron transfer from the HOMO energy level of the quinoline ring to the HOMO energy level of the BODIPY core. The third antenna ligand synthesized was 4-amino benzene styryl di-iodinated BODIPY dye (AH11). The ligand had a broad absorption with maximum absorption at 645 nm, high molar extinction coefficient of 1.36 x 105 M-1 cm-1 and wavelength of maximum emission to be 681 nm. Future work on this research will focus on reaction of AH11 with 4-thiocyano-1,10-phenanthroline to make ligand AH12 which will be deployed for reactions with different lanthanide salts to make lanthanide complexes with excellent NIR emission yield