一种具有化学发光性能的锆基金属有机四面体

杜顺福，王文经，EL-SAYED El-Sayed M.，3，苏孔钊，袁大强，洪茂椿

（1.福建物质结构研究所结构化学国家重点实验室,福州 350002；2.中国科学院大学,北京100049；3.埃及石油研究所炼油部化学精炼实验室,开罗 11435）

1 Introduction

Chemiluminescence（CL）is the emission of cold light caused by radiative deactivation of electronically excited species generated by chemical conversion［1—4］.Several CL reactions of luminol and analogs，cypridina luciferin analogs，dioxetanes，and peroxyoxalates have been reported during the last decades［5—8］.Among various CL kinds，peroxyoxalate chemiluminescence（POCL）system garners much attention due to its high luminescent intensity，high quantum yields，and prolonged lifetime.As a cold light source，POCL reactions have widespread implementations in analytical detection，emergency lighting，and fluorescent sticks［9—12］.Although research on POCL has made significant progress，developing an efficient POCL system to fulfill the growing application needs remains a challenge.

Recently，the functional microreactor has been proven to be an elegant strategy for constructing the POCL system［13—16］.Based on this concept，various efficient platforms for POCL have been developed，such as metal-organic frameworks（MOFs）and integration of catalytic and chromophoric centers in hydrogels and tablets［17，18］.However，using metal-organic cages（MOCs）as POCL materials is rarely explored.MOCs，formed by the self-assembly of metal ions or clusters and organic ligands，have been extensively investigated due to their potential properties and applications，such as host-guest chemistry，gas sorption，catalysis，nanoreactors，and fluorescence［19—22］.Nevertheless，most reported MOCs are unstable in aqueous conditions，limiting their potential applicability.In 2013，we first reported zirconium-based metal-organic cages（Zr-MOCs）through the assembly of trinuclear zirconium clusters and carboxylate ligands［23］.After that，a series of related works has been reported by our group and others.Importantly，Zr-MOCs demonstrate high stability in aqueous solutions with wide pH ranges（2—10）due to their strong Zr—O bonds（Zr—O bond energy：776 kJ/mol）［24—27］.Therefore，Zr-MOCs seem to be a good candidate for relative applications of POCL.

In this work，we constructed a zirconium coordination tetrahedron（ZrT-6），in which a fluorescent molecular，9，10-diphenylanthracene，is fixed as edge.The chemiluminescence tests reveal that ZrT-6 exhibits good CL properties，indicating that using MOCs to improve the efficiency of the POCL system is an effective strategy.Finally，as demonstrated by mass spectrometry，the structure of ZrT-6 can be maintained after chemiluminescence，implying its recyclability.

2 Experimental

2.1 Reagents and Instruments

Zirconocene dichloride，N，N-dimethylacetamide（DMA），Bis（2，4，5-trichloro-6-carbopentoxyphenyl）oxalate（CPPO），dibutyl phthalate，H2O2（30%aqueous solution），carbon tetrachloride（CCl4），dimethyl phthalate，andtert-butanol were commercially available and used without further purification.4，4′-（anthracene-9，10-diyl）dibenzoic acid（PAP）were purchased from Beijing Hwrkchemical Company Limited.

Elemental analyses for C，H，and N were performed on an Elementar Vario MICRO elemental analyzer.Fourier transform infrared（FTIR）spectra of the samples were recorded on KBr pellets in the 4000—400 cm-1range using a PerkinElmer Spectrum One FTIR spectrometer.Electrospray ionization mass spectrometry（ESIMS）data were obtained on a Bruker Impact II Q-TOF spectrometer.Fluorescence spectral measurements were carried out on an FLS920 spectrometer.Single-crystal X-ray data were collected at 293（3）K on a SuperNova diffractometer equipped with a copper micro-focus X-ray source（λ=0.15406 nm）and an Atlas CCD detector.

2.2 Synthesis of Zr T-6

Although this compound has been reported，the defined crystal structure can helps us to better understand the structure-activity relationship，so we developed a new synthesis method［28］.Excess zirconocene dichloride（0.015 g，0.05 mmol）and PAP（0.005 g，0.01 mmol）were dissolved in a ternary solvent system of DMF（1 mL），CCl4（1 mL）and distilled water（4 drops，about 0.2 mL）.Following that，the mixture was heated at 55℃for 10 h，affording colorless bulk crystals of［Cp3Zr3μ3-O（μ2-OH）3］4（PAP）6·Cl4·nS（ZrT-6·Cl4·nS）with a yield of 69%.IR（KBr pellet），ν͂/cm-1：3350，1606，1581，1529，1421，1178，1101，1018，943，815，765，711，669，609，515，466.Elemental analysis calcd.（%）for（ZrT-6·2DMA·5CCl4·13H2O）：C 49.85，H 3.65，N 0.48；found（%）：C 49.19，H 3.98，N 0.48.

2.3 Chemiluminescence Process

A common procedure of CL study is as following：CPPO（150 mg），dimethyl phthalate（0.205 mL），H2O2（30%aqueous solution，0.480 mL），andtert-butanol（0.815 mL）were added in 1.50 mL dibutyl phthalate solution.After stirring for 2 min，fluorescent agents were added to the prepared mixture.Then the mixed solution was detected by FLS920 fluorescence spectrophotometer at different times.

3 Results and Discussion

Single crystal X-ray analysis reveals that ZrT-6 crystallizes inF4132 space group（Table S1，see the Supporting Information of this paper），and four Cp3Zr3O（OH）3SBUs located in the vertexes are linked by six ligands and result in coordination tetrahedron with V4E6（V=vertex and E=edge）topology（Fig.1）.ZrT-6 is a cationic MOC balanced by chlorine ions，with intrinsic void diameter and volume of 0.624 nm and 0.12734 nm³，respectively.

Fig.1 Structures of Cp 3Zr 3O(OH)3 SBU(A),4,4′⁃(anthracene⁃9,10⁃diyl)dibenzoic acid(B)and cationic Zr T⁃6(C)

To better understand the stability of MOC，ESI-MS analysis was employed on ZrT-6 dissolved in DMSO.ESI-MS reveals that［M-4Cl-］4+ions（M represents the intact ZrT-6）locate atm/z1160.7351，consistent with a whole tetrahedron including four Cl-.Meanwhile，the peak attributable to［M-4Cl--H+］3+is observed atm/z1547.3104.The obtained MS datum of each peak is in good agreement with calculated values atm/z1160.8618 andm/z1547.4801，respectively（Fig.S1，see the Supporting Information of this paper）.

The stable framework of ZrT-6 incorporating open fluorescent molecules was used for CL testing.The POCL reaction was performed following a common procedure using H2O2and CPPO in a mixture of dimethyl phthalate，dibutyl phthalate，andtert-butanol.In POCL system，ZrT-6 displayed a sustained emission lasting about 250 min.To accomplish a detailed study of CL，CL properties under different conditions have been demonstrated by spectrum，and each condition is listed in Table S2（see the Supporting Information of this paper）.In entry 1，no light emission was detected when no fluorescent molecule was added，excluding the possibility of other substances in the system emitting light（Fig.S2，see the Supporting Information of this paper）.Under the same test conditions，entries 2 and 3 displayed that after integrating the same amount of fluorescent agent into ZrT-6，the intensity of CL is twice that of POCL system in which the ligand acts as the fluorescent agent.In addition，it is observed that the light-emitting intensity of entry 2 reached its maximum at 50th min，whereas the light-emitting intensity of entry 3 reached its maximum at 30th min（Figs.S3 and S4，see the Supporting Information of this paper）.This interesting phenomenon encourages us to further explore the influence of ZrT-6 on POCL system.As demonstrated in entries 3—5，Table S2，by increasing ZrT-6 consumption from 10 to 30 mg，the maximum emission intensity continues to increase，followed by rapid decay（Figs.S5 and S6，see the Supporting Information of this paper）.

Obviously，assembling fluorescent agent molecules in ZrT-6 can improve the efficiency of CL［Fig.2（A）］.Several research groups［13—16］have reported similar phenomena when fluorescent molecules were used in the skeleton of MOFs.Normally，4，4′-（anthracene-9，10-diyl）dibenzoic acid tends to aggregate in solution due to the presence ofπ-πstacking interactions，which not only consumes the energy of excited fluorescent agent in the form of non-radiative transitions，but also prevents energy transfer between reactive species and conjugated fluorescent agents（the close packing between fluorescent agents）.However，when the fluorescent molecules are assembled on the edges of a tetrahedron，the framework effectively prevents them from accumulating and provides more contact opportunities between reactive species and fluorescent agents.This interesting phenomenon encourages us to further explore the influence of ZrT-6 on POCL system.As displayed in entries 3—5，Table S2，by increasing ZrT-6 consumption from 10 to 30 mg，the maximum emission intensity continues to increase，followed by rapid decay［Fig.2（B）］.Because there is a shift in chemical equilibrium，at the beginning of the reaction between CPPO and H2O2，a large amount of 1，2-dioxetanedione is produced，which requires more ZrT-6 to obtain electrons to complete energy transfer.Therefore，the larger the number of ZrT-6 cations is，the more intense blue light emission that the concurrent reaction with more 1，2-dioxetanedione can exhibit.Therefore，it also accelerates the forward progress of the chemical reaction，making the system exhibits a shorter lifetime.

Fig.2 CL intensity of ligand and MOC as a function of reaction time(A),CL intensity of different amounts of MOC as a function of reaction time(B)and changes in brightness of CL of MOC from 0 to 240 min(entry 3)(C)

Compared with the common homogeneous CL process，this process is a heterogeneous reaction，which can effectively avoid pollution and waste because ZrT-6 can be recovered through filtration and centrifugation.The stability of the structure is an essential consideration in chemiluminescence applications.The obvious peaks for ZrT-6 can be observed by high-resolution mass spectrometry after CL process（Fig.3）.

Fig.3 ESI⁃MS spectra of Zr T⁃6 after CL process

Chemically initiated electron exchange luminescence（CIEEL）can explain POCL system.First，H2O2reacts with oxalate ester under nucleophilic activation of SBUs to produce active intermediate 1，2-dioxetanedione.Second，the fluorescer fixed on the edge of the ZrT-6 tetrahedron and reactive species have single electron transfer，making ZrT-6 into the excited state，while reactive species are decomposed into carbon dioxide molecules，demonstrated by a large number of bubbles formed.Third，ZrT-6 returns from the excited state to the ground state and emits a steady stream of light（Fig.S7，see the Supporting Information of this paper）.

4 Conclusions

In summary，we designed and synthesized the first metal-organic polyhedra for CL，demonstrating high performance and prolonged lifetime.ZrT-6 provides not only catalytic sites but also efficiently assembled fluorescent agents.More importantly，its structure withstood harsh POCL conditions.This work provides a good method for the rational assembly of fluorescent molecules in POCL systems.

The supporting information of this paper see http://www.cjcu.jlu.edu.cn/CN/10.7503/cjcu20210628.