Synthesis Of Highly Discriminating Chromotropic Fluorescent Compounds And Their Sensing Applications

Solvatochromic probes have become powerful tools in environmental monitoring and biosensing because of their high sensitivity to the microenvironment,such as polarity,viscosity,temperature,pH and other specific binding effects.Slight changes in these factors usually cause emission spectra shifts of a solvatochromic probe,which are unequivocally and easily detectable.Information on the functions,activities and interactions of biomolecules within living systems,with spatial and temporal resolution,is available with solvatochromic probe techniques,and this approach finds use in environmental and materials applications as well.Owing to these advantages,a large number of solvatochromic probes have been developed during the past years.However,from the viewpoint of practical uses,there is still big room for research in this area.This is because most of these probes have some drawbacks in their photophysical properties.As an idea solvatochromic probe,the fluorophore needs to meet the flowing criterions:(1)absorption in the visible region(>400 nm),(2)high absorption coefficients(>30,000 M-1·cm-1),(3)great fluorescence quantum yields,(4)good photochemical stability,(5)strong solvatochromism,and(6)adaptive to solvents of very different properties.It is well known that a solvatochromic probe is structurally characterized by a conjugated ?-system functionalized with an electron-withdrawing and/or an electron-donating unit.Recently,professor Fang and his colleagues have focused on the development of 1,4-bis(phenylethynyl)benzene(BPEB)-based fluorescent sensing probes.Notably,it was found that these BPEB-based fluorophores demonstrated a number of favorable chemical and spectroscopic properties.These include 1)monomer emission,partially overlapped excimer emission and fully overlapped excimer emission,2)the presence of amino functionality connected to the BPEB backbone through a methylene unit,precluding PET,and protonation of the lone pair of electrons on the amino functionality which quenches fluorophore emission,and 3)the conformational versatility of the fluorophores,which can exist in different forms in solution and thin film states that are characterized by different emission profiles.In addition,the molecular structures of these probes are easily modified.However,as a class of ideal fluorescent probes,there are still many deficiencies.For example,their absorption bands occur at wavelengths shorter than 400 nm,their molar absorption coefficients are on the order of a few thousand cm-1·M-1,and these fluorophores exhibit relatively low fluorescence quantum yields in common organic solvents.To overcome these limitations,we have sought to identify solvatochromic species that have more favorable optical properties.Firstly,we will make detailed introduction on the excitation and emission processes of the solvatochromic probes,Lippert-Mataga model theory and other related photophysical parameters through the classic editions and literatures.Secondly,we will systematically summarized the different types of solvatochromic probes currently reported and their applications by regarding a large number of references.Based upon the conjugated structure of 1,4-bis(phenylethynyl)benzene(BPEB),a series of new solvatochromic probes were designed and synthesized by introduction of electron-donating units and electron-withdrawing units with novel structures.In order to enhance the ?-?conjugation effect,one of the terminal phenyl rings of BPEB is replaced with a naphthyl.At the same time,different functional groups were grafted onto the side positions of BPEN.It is revealed that these created solvatochromic probes demonstrate unprecedented discrimination abilities for the organic liquids with similar structures or properties.Part one:A new fluorescent 1,4-bis(phenylethynyl)benzene(BPEB)-containing fluorophore,PNBD,was designed and synthesized.To improve the solubility of the conjugate,two long alkyl chains were introduced as substituents of the central aromatic ring.Spectroscopic studies demonstrated that PNBD is a strongly solvatochromic probe which is characterized by a large molar absorption coefficient(>32 000 cm-1·M-1),long wavelength absorption(>410 nm),large solvatochromic emission range(470?650 nm),high photochemical stability,and good solubility in common organic solvents.The fluorescent quantum yield of PNBD is limited in some polar solvents due to dual emission,a phenomenon ascribed to radiative decay from a higher excited singlet state.To eliminate dual emission,a covalently bound dimer(BPNBD)of PNBD characterized by weak vibronic coupling,was designed and synthesized.As expected,BPNBD maintains almost all the strong points of the monomer,exhibits a substantial increase in fluorescence quantum yield,and eliminates dual emission by facilitating efficient internal conversion.Importantly,the use of PNBD and BPNBD in concert provides unprecedented discrimination among solvents of similar structures,such as(CH2Cl2,CHCl3,CCl4),(ethyl ether,THF,dioxane),or(methanol,ethanol,n-propanol,n-butanol,n-pentanol,n-hexanol,n-heptanol,n-octanol,n-decanol),allowing rapid and selective visual identification.Part two:Four highly fluorescent derivatives of bis(phenyl-ethynyl-)-2-naphthyl(BPEN)with push-pull structures were designed and synthesized,of which azetidine was adopted as an electron-donating unit.For the electron withdrawing moiety,it varies from hydrogen,to formyl,then to 2-ethoxyethyl derivative of dicyanovinyl and finally to dicyanovinyl itself,and the corresponding fluorophores are denoted as Al,A2,A3 and A4,respectively.To enhance the solubility of the compounds,two n-hexadecyl residues were grafted onto the side positions of BPEN.Interestingly,introduction of azetidine not only improves the fluorescence quantum yield and enlarges the Stoke's shift of the parent compound,but also endows them,in particular A2 and A4,exceptional capability to distinguish structurally relevant organic liquids,such as ethylbenzene and its isomers(o-xylenes,m-xylenes and p-xylenes),mono-alkyl-substituted benzene derivatives,gasolines of different grades and other organic liquids.Theoretical calculation and Lippert-Mataga equation-based tests revealed the intra-molecular charge transfer(ICT)nature of the solvatochromic properties of the compounds.Further quantitative analysis of the data obtained from studies of the probes/n-hexane-dioxane systems revealed the big differences in the dipole moments between the excited-and ground states of A1,A2,A3 and A4,which are close to 36 D,47 D,65 D and 63 D,respectively.Moreover,the four novel fluorophores possess exceptional photochemical stability as demonstrated by the fact that more than 2 hours UV light illumination did not result in detectable reduction in the fluorescence emission of the fluorophores.It is the long wavelength absorption(>380,?400,>410 and>430 nm),large molar absorption coefficient(>59 000,>52000,>39 000 and>34 000 cm-1·M-1),great color change(400?620 nm),and good solubility in common organic liquids that makes the as developed compounds,in particular A2 and A4,very competitive solvatochromic probes.Part three:Four new fluorescent dyes based on bis(phenyl-ethynyl-)-2-naphthyl(BPEN)were designed and synthesized.To improve the solvatochromic property and enhance the brightness of the fluorescent BPEN,an electron-donating unit of azetidine and/or an electron deficient group of-NO2 were introduced.The corresponding fluorophores are denoted as T1,T2,T3 and T4,respectively.Moreover,to facilitate derivatization of the probes,two ethoxy carbonyl residues were grafted onto the side positions of BPEN.Spectroscopic studies demonstrated that introduction of azetidine leads to superior solvatochromic properties and largely enhanced fluorescence quantum yields as evidenced by the fact that T3 shows more than 150 nm shift in its maximum emission when dissolved in solvents of very different polarities and displays high fluorescence quantum yields in the solvents studied.However,T2,which is the one bearing a-NO2 group,is non-fluorescent.Theoretical analysis and Lippert-Mataga modeling revealed the intra-molecular charge transfer(ICT)nature of the solvatochromic behavior of the compounds.Further test reveals that the fluorophores,in particular T3,are sensitive to the presence of trace water in less polar solvents,such as THF and 1,4-dioxane.Moreover,it is believed that the new fluorophores may serve as building blocks for creating environment-sensitive fluorescent sensors.Part four:Benzene is the simplest aromatic hydrocarbon with a six-membered ring.It is one of the most basic structural units for the construction of ? conjugated systems,which are widely used as solvatochromic probes and other luminescent materials for imaging applications and displays because of their enhanced spectroscopic signal.Presented herein is a series of water-soluble mini-fluorophores with single benzene.These compounds demonstrate high fluorescence quantum yields and large stokes shifts in some solvents with large polarity,for example,the stokes shifts of BAP in methanol and water are 200,230 nm,respectively.The compound of BAPA,the hydrolysate of BAP,exhibits relatively high fluorescence quantum yields(up to 0.20)in water.In addition,the emission wavelengths change depending on the polarity of solvent for some of these fluorophores.As for BAP,the maxima emission peak in the solvents studied extend from 550 nm in n-hexane to 630 nm in water.Furthermore,due to the presence of acid-base sensitive groups in the structure of fluorophores,BAPA can be used to detect pH in aqueous phase.