Design, Synthesis And Photophysical Studies Of Coumarin And Anthracene Derivative Complexes

Molecular recognition involves the selective binding of a substrate,and ideally generates a specific function,which forms the basis for host-guest interaction and the assembly and disassembly processes.Fluorescent chemosensors are a class of molecules that can effectively express recognition events by fluorescence signaling,which is one of the important research areas in molecular recognition.In recent years,the molecular switch-based fluorescent sensors have become a hot area in sensor research.F(o丨¨)rster(or Fluorescence) resonance energy transfer(FRET) is an excited-state energy transfer process that occurs from the initially excited donor molecule to an acceptor molecule without involving an emission step from the donor molecule.FRET mechanism has been applied in a lot of systems reported in recent years for various applications,including the use of luminescent d-block transition metal complexes that show relatively long-lived excited states and low excitation energy used as the replacement for organic chromophores, long-lived lanthanide complexes of terbium and europium,semiconductor nanocrystals, conjugated polymer/CdTe NCs systems and so on.Since semiconductor nanocrystals (NCs) are promising for various applications,it is of interest to modify the surface of semiconductor NCs with functional organic groups as luminescent species for imaging, detection and biolabeling.So,in recent years,many composite systems involving the preparation of CdTe NCs linked to organic fluorophores have been reported.Efficient fluorescence resonance energy transfer has been reported to occur from CdTe quantum dots that served as donors to the organic molecules that served as the energy acceptor.On the contrary,there are only very few reports on the use of CdTe NCs as energy acceptors with emitting dyes as enery donors.The reasons for this which has been ascribed to the short fluorescence lifetime and the influence of interactions on the luminescent properties of the composites were almost not systematically investigated.In this thesis,the synthesis,characterization and photophysical properties of three different kinds of fluorescence sensors have been explored. Although f-block lanthanide-based FRET systems are known,FRET systems based on d⁶ transition metal polypyridine complexes,especially those of ruthenium(â…¡) and rhenium(â… ), which are well-known to show rich MLCT excited state properties,are less explored. Recently there has been a growing interest in the study of FRET systems involving metal polypyridine complexes,especially that of ruthenium(â…¡) and osmium(â…¡).As a continuation of the previous studies by the group of Yam on a series of rhenium(â… ) complexes containing a pyridine ligand with coumarin group,in Chapter 2,the synthesis, characterization and photophysical properties of several rhenium(â… ) complexes of 2,2'-bipyridine ligands with oligoether-tethered coumarin derivatives,[Re(CO)₃(L1)Br] (1),[Re(CO)₃(L2)Br](2),[Re(CO)₃(L3)Br](3),[Re(CO)₃(L4)Br](4) and[Re(CO)₃(L5)Br] (5) have been studied.Perturbation of the FRET process by metal ions was explored.The interaction of the complexes with metal ions was investigated by UV-vis,luminescence and NMR spectroscopic methods,and the binding constants of the complexes for various metal ions were determined.Excitation of the complexes atλ≤380 nm in acetonitrile solution at room temperature resulted in a strong yellow luminescence,attributed to the ³MLCT phosphorescence of the rhenium(â… ) diimine acceptor,and a weaker blue emission, assigned as the IL fluorescence of the coumarin donor.Upon addition of various alkali and alkaline-earth metal ions,their electronic absorption and emission spectra were found to vary.The complexation of the metal ions was also found to induce conformational changes that resulted in a variation of the FRET efficiency.¹H-NMR and ¹³C-NMR spectroscopy were also employed to probe the ion-binding properties of(3) with Mg²⁺ and Ba²⁺ ions.Downfield shifts of the ¹H-NMR and ¹³C-NMR signals were observed due to the weakening of the shielding effect on the neighboring protons resulting from the decrease of the electron density at the oxygen atoms.It is well know that,although a semiconductor feature can be downsized to nanometer scale,it will be difficult to approach the molecular size.In molecular computation, fluorescent molecules play pivotal roles because they are detectable as a single molecule and can simultaneously handle multiple inputs.Inspired by these works,in Chapter 3,we demonstrate the OR logic operation with AnCAD inβ-cyclodextrin-modified gold nanoparticle.The radius of aβ-cyclodextrin-modified gold nanoparticle has been estimated to be～5 nm using TEM.The fluorescence characteristics of AnCAD in aqueous solution under three different conditions,in which H⁺ and K⁺ concentrations are different, were studied.A strong fluorescence signal was observed only when H⁺ concentrations are high.The fluorescence signal was found to be very weak in the other cases.The fluorescence quantum yields of AnCAD in the presence of H⁺ and K⁺ were higher than that in the absence of H⁺ and K⁺.The lifetimes of AnCAD in the presence of H⁺ and K⁺ were also longer than that in their absence.Semiconductor nanocrystals(NCs) directly synthesized in aqueous solution have attracted much attention due to their unique chemical and physical properties.Incorporation of these NCs into organic fluorophore is a highly desirable approach to generate novel light-emitting materials.In Chapter 4,composite systems of a series of anthracene derivatives with blue fluorescence and functional NCs have been successfully designed and explored.Amongst the properties of NCs that are considered to be superior over traditional fluorescent labels based on organic fluorophores are high resistance to photobleaching,broad absorption spectra in combination with single,narrow and symmetric luminescence bands.The ability of CdTe NCs to act as energy acceptors in the ET process with anthracene serving as the donor has been investigated.The energy transfer can occur from anthracene to CdTe NCs.The energy transfer efficiency was found to vary with the size of CdTe.The order was found to be G-CdTe＞O-CdTe＞R-CdTe.At the same time,the emission characteristics of CdTe NCs were found to depend on pH. This work will contribute to the development of organic chromophore-quantum dot hybrids which are important for applications in medicine,biology,and material science.