Synthesis And Applications Of Fluorescent Probes Based On Intramolecular Charge Transfer

In recent years, biological chemistry as an interdisciplinary subject develop rapidly, much attention and interest in fluorescent probes have been earned aiming to detect intracellular functional molecules, especially those biomedically related small molecules. In this thesis, several chemical sensors derived from different dyes were designed based on the intramolecular charge transfer mechanism. Their design, synthesis and application on vivo imaging of small molecules were discussed.In the chapter 2, two ratiometric fluorescent probes, probe 1 and probe 2, were designed by using 1,8-naphthalimide as the fluorophore and 4-azidobenzyl carbamate as the H2S response site. The probe 1 shows high selectivity towards H2S over other biothiols and reactive species. Moreover, probes are proved to have low detection limit with 0.15 uM. Most importantly, probe 2 gives excellent two-photon fluorescence signal changes towards mitochondrial H2S in living cells.In the chapter 3, we will discuss a novel design of fluorescent probe for simultaneous detection of H2S and thiol. The QME-N3 was derived from quinonline as fluorephore; azide group and α, β-unsaturated bond were chosen as recognition site for H2S and thiol, respectively. The purpose of the design is to study the possible effect of H2S on biothiols detection in living cell in order to achive simultaneous detection. Our results revealed that QME-N3 can simultaneously detect H2S and thiol, exhibiting distinct fluorescence change. By control experiment, it was demonstrated that H2S reacted with a, (3-unsaturated bond with a very slow reaction rate.Chapter 4 is mainly about development of water soluble fluorescent probes for thiophenols,6HQM-DNP and 7HQM-DNP. These two probes contain 6-or 7-hydroxy quinonlinium as fluorophore and 2,4-dinitrophenoxy (DNP) as nucleophilic recognition unit. Strongly fluorescent quinolinium could be released through aromatic nucleophilic substitution (SNAr) by thiolate anions from thiophenols, (much larger fluorescence enhancement was found for 7HQM-DNP). The sensing reaction is highly sensitive (detection limit of 8 nm for 7HQM-DNP) and highly selective to thiophenols over aliphatic thiols and other nucleophiles under neutral conditions (pH 7.3). The probes respond rapidly to thiophenols, with second-order rate constants k=45 M-1s-1 for 7HQM-DNP and 24 M-1s-1 for 6HQM-DNP. Furthermore, the selective detection of thiophenols in living cells by 7HQM-DNP was demonstrated by confocal fluorescence imaging. Importanly, these quinolinium salts show excellent chemical and thermal stability. Overall, such probes may be found use in the detection of thiophenols in environmental samples and biological systems.Chapter 5 further illustrates the rational design and development of novel ratiometric fluorescent probe based on quinolinium fluorophore. We have designed and synthesized a hydrogen peroxide-responsive fluorescent probe 7HQM-E. It contains aryl boromate as hydrogen peroxide recognition site, releasing fluorophore 7HQM by the hydrogen peroxide-induced nucleophilic rearrangement reaction, resulting in ratiometric fluorescence response from blue emission to strong green emission. The fluorescent probe displays high selectivity and good solubility with 70 μM, laying foundation for potential application in living cell.The influence of structural variations (including steric and electronic effects) of triarylboranes on their anion-sensing properties is not entirely understood. So in chapter 6, with systematic structural variations, the fluoride-sensing properties of tridurylboranes including the fluorescence mode, fluoride-binding constants, and the selectivity were discussed and compared with their cyanide-sensing behavior. The fluorescent response mode of a tridurylborane to fluoride depends on its properties of ICT state and the local excited (LE) state. The ICT-state property is determined by thermodynamics of the ICT process. The fluoride-binding constants depend on the Lewis acidity of the centre boron of tridurylboranes, which is determined by electronic effects of three aryl groups. The selectivity of tridurylboranes to fluoride over cyanide ions was observed in aprotic solvents for the first time, and originates from the favorable steric effect of duryl groups for smaller fluoride ions.The strong Lewis acidity resulting from strong electron-withdrawing aryls would decrease the selectivity to fluoride, for instant, tri(p-cyanoduryl)borane (TCDB) can bind both fluoride and cynaide anion. Moreover, solvent effects on the sensing reaction of TCDB and theoretic calculation reveal that the sensing property of tridurylboranes is dependent of solvent.