Studies On Synthesis And Fluorescence Properties Of Xanthene Corannulene Derivatives

Fluorescence is the light emitted by a molecule when it returns from its excited singlet state to its ground state.However,most of the molecules under test do not have fluorescence properties themselves,and rarely emit fluorescence in chemical reactions.Changes in fluorescence can be achieved through physical or chemical changes in compounds.The fluorescent probe is generally composed of three parts:the fluorescent group,the ligand,and the molecular recognition part that can generate a response with the detected target.Traditionally,three classical types of fluorescence have been developed to produce fluorescence response mechanisms,namely photoelectron transfer（PET）,fluorescence resonance energy transfer（FRET）,and intramolecular charge transfer（ICT）.The fluorescent probe is a new technology based on an organic combination of molecular recognition and fluorescence analysis,which can specifically identify and label substrates through chemical reactions,and realize in situ and real-time detection of living cells and living organisms.Fluorescent probes have become a powerful biological tool to promote cell research with fluorescence,and have promising application prospects in the discovery of new drugs,detection of environmental pollutants,and further detection of cancer.Fluorescent groups mainly include fluorescein and rhodamine,fluoroboron fluorescent dye,coumarin and naphthalimide.This article focuses on the first two.In xanthene-based fluorescent dyes,the emission wavelength of fluorescein is 510 nm,and the quantum yield is 0.98;the emission wavelength of rhodamine B is 554 nm,the quantum yield is 0.90.The fluorescein and rhodamine dyes are usually emitted at about 510 nm because the xanthene skeleton is used as the fluorescent group,and the solubility is good.Fluorescein and its derivatives are an important class of fluorescent materials.There are two resonance structures of the lactone type and quinone type in fluorescein molecule,which have the characteristics of"on-off"ring:lactone structure is not conducive to the production of fluorescence;however,when some reactions occur,lactone structure changes to quinone type,and the fluorescence can be turned on.The structure of rhodamine B is similar to that of fluorescein,which belongs to oxaxanthene dyes.The oxaxanthene ring is connected by an oxygen bridge.It’s a good rigid coplanar structure,with a high molar absorption coefficient and quantum yield.Rhodamine B is also an ideal precursor for the construction of"on-off"fluorescent probes.The emission wavelength of BODIPY is 512 nm,quantum yield is0.7,green fluorescence.BODIPY is a high fluorescence compound.Because of its small Stokes Shifts,high quantum yield and strong excitation and emission,which make them useful as fluorescent markers and laser dyes for biological systems.Corannulene(C₂₀H₁₀)is a kind of polycyclic aromatic hydrocarbons（PAHs）,which is composed with sp²carbon atoms（Figure 2）.The synthetic route of corannulene was first reported in 1966,but the synthetic route is complex,a low yield which hindered the further research and development of the corannulene.In 2012Siegel group has developed a new method for synthesis of corannulene of kilogram grade,which means that the corannulene can be used for commercial synthetic rather than purely academic research,makes the study of corannulene get further development.Since then,bromocorannulene derivatives have become the research objects of many researchers.Bromocorannulene is the key intermediate of the mono-functional group of bromocorannulene derivatives as well as the key to the connection between the mono-functional group and other molecules.This research is also based on bromocorannulene.Compared with six electrons of Buckminster fullerene,corannulene can accept up to four electrons,and each reduction reaction will cause a significant change in the absorption spectrum,however,it possesses many advantages over C₆₀.Firstly,corannulene can be synthesized at a kilogram-scale cost-effectively;Secondly,it has a high solubility in a variety of common solvents used for device processing;Thirdly,corannulene can be substituted/functionalized in a precise and controlled fashion without disrupting the electronic communication of the fundamental carbon scaffold.In light of this,one would expect extensive applications of corannulene and its derivatives in the arena of organic electronics.Different from the traditionalπ-conjugated systems with planar structure,such as naphthalene,phenanthrene,anthracene,fluoranthene,pyrene,flexor and so on,theπ-conjugated system with curved surface leads to the decrease of the overlap rate of each p-orbital in the SP²hybrid carbon atom in the corannulene structure,the increase of the s-orbital characteristics in the carbon atom,the decrease of LUMO level in the corannulene,and the increase of the electronic receiving performance of the corannulene.Corannulene has two very desirable properties:1.It has an electron-deficientπ-system.This makes it highly resistant to“photobleaching”-oxidative degradation in the singlet or triplet excited state.2.The synthetic chemistry of corannulene is now highly and advanced and modular.This will allow ready access to numerous potential fluorescent probes.However,a central limitation is that corannulene is not very fluorescent,and insoluble in water.Although the corannulene themselves exhibit poor fluorescence properties,corannulene can accept electrons and their molecular five-fold symmetry,with uneven distribution of electrons on concave and convex surfaces,so we can change their electrical properties by forming various substituents with different compositions,lengths and numbers.We are working on preparing corannulene derivatives,to try to“teach”corannulene to fluoresce brightly.The fluorescence characteristics of polyaromatic hydrocarbons were studied by using this unique curved polyaromatic structure.So far,compounds with fluorescent properties are roughly divided into two categories based on structural characteristics:one is planarπ-conjugated chromophore,and the other is twistedπ-conjugated chromophore.At present,many studies have been published relating to the corannulene derivatives and their applications,but there is still a large blank in the systematic study of corannulene-fluorophore.So in this paper,we mainly connected the corannulene with the known fluorescence group to expand theπsystem to enhance its fluorescence.The fluorescence properties of corannulene derivatives with different fluorescent groups（fluorescein,rhodamine）as substituents were studied.In this paper,we mainly study the influence of unilateral substituents on the fluorescence properties of corannulene.By extendingπconjugation,we can change the fluorescence properties of corannulene and make it emit strong fluorescence.The molar absorption coefficient of the new compounds in this paper is the absorbance（A）under the maximum absorption wavelength measured by the standard configuration of different solution concentrations,and the linear relationship between concentration and absorbance is obtained（R²≥0.999）.The slope is the molar absorption coefficient.As we all known,the excitation wavelength of corannulene is254 nm,the emission wavelength of corannulene is 421 nm,quantum yield is 0.02,with weak blue fluorescence.Many traditional fluorescent molecules have solvent effect.In order to better understand the effect of solvent on the fluorescence properties of the corannulene derivatives,the UV and fluorescence data of compounds3,4 in acetonitrile and methanol were obtained.Corannulene is connected to the fluorescein（Figure 3）,compound 3 the molar absorption coefficient is 1360.2L·mol^-1cm^-1,with yellow color.In methanol,the quantum yield is 0.25,the excitation wavelength is 503 nm,the red shift is 249 nm,the emission wavelength is 537 nm,the red shift is 116 nm,the stokes shift is 34 nm;in acetonitrile,the quantum yield is 0.25,the excitation wavelength is 502 nm,the red shift is 248 nm,the emission wavelength is 557 nm,the red shift is 136 nm,the stokes shift is 55 nm.compound 4 the molar absorption coefficient is 14422 L·mol^-1cm^-1,with pink color.In methanol,the quantum yield is 0.13,the excitation wavelength is 568 nm,the red shift is 314 nm,the emission wavelength is 597 nm,the red shift is 176 nm,the stokes shift is 29 nm;in acetonitrile,the quantum yield is 0.13,the excitation wavelength is 569 nm,the red shift is 315 nm,the emission wavelength is 601 nm,the red shift is 180 nm,the stokes shift is 32 nm.The absorption and emission spectra of the corannulene derivatives show a red shift to some extent,and the quantum yield is improved,which indicates that the energy band gap between HOMO and LUMO is reduced by the fluorescent group modified corannulene.At the same time,we also characterized the HR-MS,NMR,UV,FT-IR properties and melting point of the new compounds.The absorption and emission of compound 4 in the near-infrared region provide the possibility for the development of near-infrared fluorescent probes.Because the corannulene is insoluble in water,the research on the fluorescent corannulene derivatives with water solubility is also very lacking.In this paper,the water solubility of compounds 3 and 4 is simply explored.It is found that compound 3 has good water solubility.The compound 3 in water,the excitation wavelength is 367 nm,the emission wavelength is 430 nm,the quantum yield is 0.35 and the stokes shift is63 nm;in the buffer solution（p H=7.4）,the excitation wavelength is 359 nm,the emission wavelength is 435 nm,the quantum yield is 0.03 and the stokes shift is 76nm.It is preliminarily speculated that the fluorescence intensity of 3 is related to the p H value of the solution,which shows that 3 can lay a foundation for further construction of fluorescence probe for p H detection.To sum up,an attached fluorescent group can start to"teach"the bright fluorescence of the corannulene.At present,the new corannulene derivatives have good optical properties in practical applications.The synthesis of the new corannulene-fluorescein and corannulene-rhodamine provide a new way to study the fluorescence properties of the corannulene derivatives,as well as a new way to study the water-soluble corannulene fluorescence probes,and expands the application scope of the corannulene derivatives.