Fluorescent Heteroditopic Ligands Targeting Zinc (II) Ion

The photophysical characteristics of 5-arylvinyl-5Oe-methyl-2,2Oe-bipyridyls (AVMB) and their zinc complexes were investigated in order to recognize the factors that influence the emission band shift and fluorescence quantum yield change of the title fluorescent heteroditopic ligand system upon zinc binding. The results of this study will advance the fundamental understanding of the coordination-driven photophysical processes embodied in the AVMB platform in addition to facilitating the rational design of fluorescent probes for metal ions, mainly zinc. AVMBs containing electron-donating aryl groups exhibit absorption and emission in the visible region attributed to charge-transfer transitions as supported by solvent-dependency and computational studies. The binding events between AVMB ligands and zinc ion in acetonitrile were studied implementing isothermal titration calorimetry (ITC). A multicomponent equilibrium model is proposed that interprets the multiple transitions demonstrated in fluorescence titration isotherms. A bathochromic shifts in both absorption and emission occurs as a result of stabilization of the charge-transfer excited state of an AVMB ligand with an electron-donating aryl substituent. However, in contrast to the emission band shift, the fluorescence quantum yield change upon zinc complex formation does not correlate with the electronic character of the aryl substituent. Nonradiative and radiative decay rate constants were calculated from lifetime measurements determined by Time-Correlated Single Photon Counting method. Both rates of an AVMB ligand are reduced upon zinc coordination. Overall, the change in fluorescence quantum yield lack the connection with the electronic features of the aryl group. The special features of the arylvinyl-bipy ligands promoted its application in aqueous media as well as real cell imaging as will be evident in chapter seven. The response of the presented ligands to the applied conditions was quite impressive and allows us to present biocompatible candidates for ratiometric imaging in biological systems. Following that study, detailed synthesis and characterization of a series of charged Iridium complexes will be presented in chapter eight. On the other hand, the preparation and investigation of two new fluorescent heteroditopic ligands for zinc ion is reported. The extent of emission bathochromic shift and enhancement is determined by two photophysical processes, intramolecular charge transfer (ICT) and photoinduced electron transfer (PET). The high degree of ICT in the excited state in the electron-rich ligand overrides PET where no enhancement in quantum yield accompanies the large bathochromic shift of emission upon Zn2+ coordination. However, the electron-poor ligand displays enhanced emission without significant spectral shift when Zn2+ binds. Experimental and computational approaches shed light into the photophysical balance encountered in the heteroditopic design and stress the prevailing pathways involved (e.g. ICT and PET) when the fluorophore is electron-rich or poor. In an attempt to improve the resolution of detection, the heteroditopic ligand platform will be extended to a two fluorophores system in which sequential fluorescence enhancement and fluorescence resonance energy transfer will take place over a zinc ion gradient addition. The applied strategy will involve the modulation of metal-coordination modulated PET, internal ICT, and fluorescence resonance energy transfer (FRET) in one synthetic fluoroionophore. An enhanced resolution (~100 nm) of two emission channels is obtained providing an advantage over the single-fluorophore heteroditopic ligands previously reported. The new strategy will allow creating new dual-emission fluorescent indicators where low- and high-target concentration regimes could be analyzed using independent emission filter sets. Chapter five will open the seals for a new system of coumarin-bipyridine analogs exhibiting dual fluorescence characteristics. The developed model reveals interesting properties showing the emission from two independent excited states. The difference in the electronic distribution between the donor and the acceptors play a major role in the relative population of the two excited states. This postulation was examined when the equilibrium of duality appears to shift in favor of the Zn2+-coordinated site. The effect of metal coordination of 2,6-bis(1-(9-anthryl)methyl-1,2,3-triazol-4-yl)pyridine and 2-(1-(9-anthryl)methyl-1,2,3-triazol-4-yl)-6-(1-octyl-1,2,3-triazol-4-yl)pyridine on the emission of the anthryl group was investigated in CH3CN. Titrating Zn2+ or Pb2+ into the solution of 2,6-bis(1,2,3-triazol-4-yl)pyridyl (gclickateh)-containing ligand results in a quenching process. The Zn2+-coordination chemistry of the clickate was probed by 1H NMR titration, X-ray crystallography, and isothermal titration calorimetry (ITC). Regarding the fluorescence modulation of the anthryl group upon binding Zn2+, computational analysis and cyclic voltammetry supports the occurrence of photoinduced electron transfer (PET) from the excited state of the anthryl group to the Zn2+-bound clickate moiety at 1:1 binding stoichiometry.