Turn-On Fluorescent Chemodosimeter For Cu²⁺ And CT Coupled PET In Anion Sensing

Cu²⁺ as the third abundant heavy metal ion in human body has attracted much attention due to its obvious biological importance and increasing environmental concerns as well.Detection of Cu²⁺ with high selectivity and sensitivity represents therefore a challenging subject.Fluorescence signaling is advantages in many respects such as high sensitivity and easy operation.Cu²⁺ as a paramagnetic species,however, shows an inherent fluorescence quenching nature.As a consequence,most of the reported detection of Cu²⁺ employing fluorescence signal operates under quenching mode.Fluorescence signaling showing an enhancement is for sensitivity reason prior to those exhibiting quenching,especially in aqueous solutions.Fluorescent chemosensors reported for detection of Cu²⁺ in aqueous solution with a fluorescence enhancement are indeed rare.Therefore,constructing fluoroionophores which could be fluorescently turned on in the presence of heavy metal ions becomes dominating in this regard.Because of the inherent difficulties of addressing anions characterized by larger sizes(larger than cations in general),a greater varied shapes and pH dependence,research into sensing of anions and design of anion receptors remains a challenging subject,too.In this dissertation,we reported a highly selective and sensitive turn-on catalytic chemososimeter for Cu²⁺ in aqueous solution,with sensing mechanism discussed.CT coupled PET sensing mechanism in anion sensing was also discussed,providing further hints for rational design of turn-on fluorescent anion sensors and thiourea-based organocatalyst.Finally,multiple hydrogen bond based receptors for amino acid was synthesized and examined.The dissertation consists of five chapters. Chapter 1 briefly reviews research progresses of photophysical and sensing mechanisms for molecular design of anion and cation receptors.Involved sensing mechanisms include(1)(n,Ï€^*)/(Ï€,Ï€^*) energy reversion,(2) fluorescent chemodosimeters,and(3) photo-induced electron transfer(PET).The objectives of this dissertation were proposed.Chapter 2 describes syntheses and characterization of 20 new compounds, including 2-aminobenzoic acid derivative(DiAPC),N-(9-methyleneanthracene) thioureido based derivatives,Cholic acid based derivatives.The equipments,materials and methods involved in this dissertation were also reported.Chapter 3 reports a catalytic chemodosimer for Cu²⁺ in aqueous solution. 2-Aminobenzoic acid(2-ABA) based derivative(DiAPC) was easily made available by a simply one-step reaction.Substitution of the amino group in 2-ABA led to DiAPC weakly fluorescent.In absorption and fluorescence spectral titrations,DiAPC showed a high selectivity for Cu²⁺ in aqueous solutions of biological pH as would have expected for a similar Cu²⁺-aminoacid binding motif.The excellent selectivity was further demonstrated in that the fluorescence enhancement by Cu²⁺ was not affected by the co-existence of other metal ions and counter anion of Cu²⁺.DiAPC was shown to operate under the mechanism of Cu²⁺-coordination promoted decomposition of the weakly fluorescent DiAPC to purple Stenhouse salt,allowing for a naked-eye detection of Cu²⁺,which underwent further hydrolysis to highly fluorescent 2-arninobenzoate.A detection limit of 1 ppb was achieved.DiAPC described herein,unlike the previously reported stoichiometric chemodosimeters,was shown to be a catalytic chemodosimeter,which accumulates and amplifies signal in response to Cu²⁺,leading DiAPC a potentially more sensitive chemodosimeter.Chapter 4 reports design and syntheses of a series of CT coupled PET anion receptors,following our previous N-amidothiourea design motif.The observation that the CT absorption in the presence of AcO^- was only made in X-BCTUA series provided a clear indication that the carbonyl group exhibits better electron delocalization efficiency than sulfonyl group.Due to the higher efficiency in communicating induction effect in X-BCTUA series,-NH protons of X-BCTUA series showed lower chemical shifts compared to those in X-BSTUA series bearing the same substituent.Also,X-BCTUA series showed lower binding constants with anions(CT processes in X-BCTUA series decreased the electron density of thiourea), and higher oxidation potentials of anthracene moiety than those of X-BSTUA series.The fluorescence quantum yields decreased with increasing Hammer constant of the substituent in both X-BCTUA and X-BSTUA series,indicating that a PET occurs from anthracene to thiourea moiety.From the reduction potential of the thiourea moiety in p-NO₂-BCTUA a negative free energy change was estimated from the Weller equation,supporting this PET with thiourea being electron acceptor. Combined with higher efficiency in communicating induction effect in X-BCTUA series,it is reasonable that X-BCTUA series show lower quantum yield than X-BSTUA series,and even in electron-donate substituted compound (p-CH-3-BCTUA) has almost the same value of quantum yield with electron-withdraw substituted compound(p-Br-BSTUA).Knowing that the thiourea group could bind metal ions,we observed that the presence of Pb²⁺ led to quenching of the fluorescence of the receptors(heavy atom effect was excluded),further supporting the PET direction from anthracene to thiourea.CT coupled PET theoretically supplied us with a possibility of designing turn-on neutral chemosensors for anions,and due to the higher efficiency in communicating induction effect in X-BCTUA series,the CT process in X-BCTUA series decreased the electron density of thiourea effectively when bind with anions,which theoretically provided us for selectively design of organocatalyst.Chapter 5 reports design and synthesis of a multiple hydrogen bonding based chemosensor ACCA for amino acids.This sensor despite not that well,however, provided instructive experiences for sensor design that coordinative binding and steric interaction should be considered.