Electrochemiluminescence Behavior Of Fully Conjugated Donor-Acceptor Covalent Organic Framework And Organic Cocrystal

Electrochemiluminescence（ECL）represents the combination of electrochemical and spectral methods.In the 100 years since the phenomenon of ECL was first discovered,the discipline of ECL has rapidly developed to a powerful electrical analysis technology,playing a significant role in global environmental pollution,biology,medicine,and other fields.At present,the widely used inorganic phosphors Ru（bpy）₃²⁺and Ir（ppy）₃ are difficult to improve the efficiencies because the structures are difficult to be further modified.However,the ECL of easily functionalized organic phosphors in aqueous media is often significantly quenched by water and oxygen.In addition,the application of these emitters requires the addition of poisonous co-reactants（such as tripropylamine）to amplify the signal.Therefore,it is quite urgent to develop the non-toxic and efficient ECL systems in aqueous media.In addition,in the field of basic research,due to the lack of general design methods,the difficulty of finding new efficient ECL light emitters is tantamount to"blind people touching the elephant".Most studies only focused on the structural modification or functionalization of reported emitters to improve ECL properties.For example,the harmfulπ-πaccumulation of organic luminescence can be eliminated by modification with molecules with aggregation-induced effect（such as tetrastyrene）,amphiphilic substances（such as cyclodextrin）or coordination regulators,but this modification method is not universal for all organic molecules.More generally,when the construction method is not clear,it is challenging to regulate the ECL performance from the molecular or even atomic level and to clarify the structure-activity relationship between the atomized structure and ECL systems.This paper focuses on the construction and application of new ECL systems,the main contents are as follows:1.Boron carbide as a novel tris（2,2’-bipyridine）ruthenium（ii）ECL coreactant for ultratrace detection of mercury（II）For the first time,boron carbide nanosheets（B₄C NSs）were discovered as anodic coreactants of Ru（bpy）₃²⁺to enhance the anode ECL phenomenon of Ru（bpy）₃²⁺through surface boron radicals and hydroxyl radicals.When the mercury（II）(Hg²⁺)exists in B₄C NSs/GCE-Ru（bpy）₃²⁺system,Hg²⁺competes with Ru（bpy）₃²⁺for the interaction site with B₄C NSs,which makes the anodic ECL signal of Ru（bpy）₃²⁺decrease,and its intensity has a good linear relationship with the logarithm of Hg²⁺concentration.When applied to environmental water samples,it represents high recovery rate,good selectivity,and low detection limit（LOD=0.3 p M）for Hg²⁺detection,without the need for time-consuming sensor construction and assembly processes,showing important environmental significance and broad application prospects.2.A general design approach toward covalent organic frameworks for highly efficient electrochemiluminescence.A general construction approach of covalent organic frameworks（COFs）with ECL properties is proposed,and the structure-related ECL properties and mechanisms are systematically studied,providing innovative ideas for the structural design and performance improvement of ECL emitters.Employing the linear/triangular aldehyde monomers as donor sub-units,16 kinds of fully conjugated COFs were synthesized by introducing the highest occupied molecular orbital（HOMO）position of acceptor units with different symmetries.By limiting both the electron donor（D）and the acceptor（A）to the COFs’tight electron configurations and constructing high-speed charge transport networks through olefin bonds,the electrons and holes of fully conjugated D-A COFs are effectively separated after excitation,the overlap of molecular orbitals is reduced,and the intramolecular charge transfer from the donor to acceptor occurs in the whole molecule,thus activating the ECL of COFs.It enables COFs to achieve efficient ECL in aqueous media with internal dissolved oxygen as coreactant.The unique structurally tunable properties of COFs endow them with broad potential in further improving ECL.We propose two enhancement strategies for ECL performance to inspire researchers to obtain more efficient ECL emitters suitable for practical applications.（1）Increasing the chain length:the addition of a p-phenyl molecule in each direction of linear/triangular aldehyde monomers can greatly enhance the ECL by increasing the distorted conformation and increasing theπ-extension main chain.The fine adjustment of COFs molecular orbitals and ECL properties can be achieved by simply changing the composition of polyaromatics in the COFs skeleton.（2）Increasing the conjugation:it is found that the novel COFs constructed by acetylene functional aldehyde monomers can enhance COFs’coplanar conformation and conjugation degree,the unique ability of O₂ adsorbing of carbon-carbon triple bond can also shorten the distance between O₂ and COFs in space,promoting the oxidation reactions on electrode surface,and significantly improving the ECL.Based on above experimental and theoretical results,it is expected that high-throughput virtual screening can be used to design efficient ECL-COF emitters with different emission properties and open up more extensive applications of COF materials and ECL technology.3.Advanced uranyl electrochemiluminescence monitoring platforms based on covalent organic frameworksBased on the analysis of radioactive contamination,a design of uranyl ion UO₂²⁺monitoring system with fast response,low detection limit and high selectivity is proposed.The electron pulling molecule,2,4,6-trimethylbenzene-1,3,5-tricarbonitrile（TBTN）,was assembled with the strong electron donor molecules 4-[4-[4-（4-formylphenyl）-N-[4-（4-formylphenyl）phenyl]anilino]phenyl]benzaldehyde（BCBA）and 4-[4-[3,5-bis[4-（4-formylphenyl）-phenyl]phenyl]phenyl]benzoic acid（DAFB）into the olefin linked COFs molecular crystal skeleton to form a highly stable and ordered D-π-A structure,which promotes the intramolecular charge transfer in the skeleton.The C=C bond as a joint allows sp² carbon to form a fully conjugated system,providingπ-electron communication along skeleton.In COFs’extended charge transport network,anion and cation radicals collide with each other,resulting in strong cathodic ECL.The newly developed COFs can produce stable ECL in aqueous media without additional toxic coreactants,avoiding the unpredictable interference caused by the introduction of exogenous coreactants to accurate detection of trace targets.The strong absorptive molecular unit TBTN in COFs not only draws the flow of electrons,but also provides a variety of post-modified functional chelating sites for practical applications.Through a simple one-step hydrothermal reaction,we modified the amidoxime（AO）group on hierarchical open pores of COFs,which can quickly and firmly capture UO₂²⁺.UO₂²⁺transfers electrons between dissolved oxygen and COFs,recovers the ECL signal of COF-AO which was quenched due to the decrease of charge separation and increase of bandgap.Thus a"turn-on"UO₂²⁺sensing platform was constructed,which is more sensitive than all reported methods（with low detection limit of 0.35 p M）.The results obtained when applied to actual water samples are consistent with inductively coupled plasma mass spectrometry,indicating the practical prospect of proposed UO₂²⁺sensing platform in environmental field.4.Electrochemiluminescence beyond chemical bond from organic cocrystal engineering.A new family of ECL emitters was developed based on organic cocrystal engineering,the ECL behavior from non-covalent bond was discovered for the first time,and the internal relationship between atomic composition,molecular structure,and ECL properties was explored.It is the first time that the ECL-emitting cocrystals can be quickly obtained from the non-emitting moleculars by simple solution volatilization at room temperature.Taking the donor-acceptor non-covalent interaction andπ-πinteraction as main driving forces,this design avoids the harsh experimental conditions commonly used in chemical bond synthesis.Through molecular solid phase hybridization and crystallization,donors and acceptors are arranged and stacked into arrays to obtain accelerated electron transfer,reduced optical bandgaps and overlap of excited electrons and holes.As a result,the obstacle that the traditional organic emitters can only emit in organic phase and quenching in aqueous media is overcome.At the same time,the cathodic ECL can be obtained from the collision of the inherent sufficient cationic free radicals and anion free radicals produced by cathode reduction,which really does not need for any coreactants and has excellent stability.For the first time,we find that the combination of different kinds and numbers of heteroatoms（N,S,O）in cocrystals can regulate the ECL performance at atomic precision by affecting the molecular orbitals and the number of cation radicals.Further extending the research to the molecular level,it was found that increasing the degree of conjugation of either donor or acceptor unit in cocrystals can further improve the charge transport performance and ECL efficiency by extending the range of non-covalent interactions between cocrystal monomers,indicating that rational design and selection of atomic/molecular interaction modes on ECL-cocrystal platform is the most concise,efficient,and potential way to develop ECL luminophores,modulate ECL properties,and in-depth understanding ECL mechanisms.