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Substituent-Induced Aggregated State Electrochemiluminescence of Tetraphenylethene Derivatives Zhengang Han, Yinpan Zhang, Yanxia Wu, Zhimin Li, Lei Bai, Shuhui Huo, and Xiaoquan Lu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b02357 • Publication Date (Web): 02 Jun 2019 Downloaded from http://pubs.acs.org on June 3, 2019

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Analytical Chemistry

Substituent-Induced Aggregated State Electrochemiluminescence of Tetraphenylethene Derivatives Zhengang Han,†,a Yinpan Zhang,†,a Yanxia Wu,a Zhimin Li,a Lei Bai,a Shuhui Huo,a and Xiaoquan Lu*a,b aKey

Laboratory of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, People’s Republic of China bTianjin Key Laboratory of Molecular Optoelectronic, Department of Chemistry, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: The development of highly active, eco-friendly and structurely fine-tunable organic luminophores is currently desirable for electrochemiluminescence (ECL). Tetraphenylethene (TPE) derivatives are the most representative aggregation-induced emission characteristic (AIEgens). In contrast, their aggregation-induced ECLs have not been detailed studied. Herein, we report the bright cathodic aggregated state electrochemiluminescence (ECL) of TPE derivatives by a co-reactant approach. In this system, the substituents profoundly affect ECL emissions by changing the relative intensities of R and B band intensity ratios in their UV-Vis spectra as well as the HOMO and LUMO energies. It was discovered that electron-withdrawing nitro-substituted TPE-(NO2)4 with a smaller LUMO/HOMO band gap and stronger R band featured the strongest ECL emissions, and became as the best luminophore for the highly efficient detection of iodide (I−) in aqueous phase. This work not only reveals the influence of R and B bands in TPE derivative’s UV-Vis spectra on their optical properties but also constructed a novel aggregation-induced ECL sensing.

Since the pioneer work by Tang and coworkers in 2001, an intriguing light emission phenomenon, in which luminophores abnormally show weakly fluorescence or non-fluorescence in the solution state, but can fluoresce strongly in the aggregated state, namely aggregationinduced emission (AIE)[1−4], has indeed contributed to solve the problems lying in organic light emitting materials caused by aggregation-caused quenching (ACQ), and been applied in numerous fields, such as chemical detections, biosensors, and imagines.[5−9] Among hundreds of AIE-

ECL has been widely used in immune and nucleic acid analysis, food and water quality monitoring, biological warfare monitoring and other fields.[20−23] Among numerous ECL systems, organic luminophoresbased ECLs are currently of great interest due to their nonmetallic containing, environmental friendly, nontoxicity, high activity, water-insolubility, and inherently convenient adjustion of emission wavelengths through structural modification and functionalization. As for organic-based TPE derivatives, although the mechanism between fluorescence and ECL is similar to a large extent, only a few studies have involved in their ECLs, and very far less than their fluorescence studies. Very recently, Lu and coworkers reported the TPE aggregates on the surface of gold electrode could significantly enhance the ECL signal of luminol in neutral aqueous solution due to its porous noncoplanar structure.[24] Ju and coworkers synthesized donor-acceptor conjugated polymer dots for tunable ECL activated by AIEactive TPE moieties.[25] Zhuo and coworkers reported the anodic ECL behavior of TPE microcrystals.[26] The research above reveals TPE derivatives could effectively improve the ECL performance for some luminophores, and show potential for ECL influenced by their different aggregated states.[27] Still, ECL of TPE derivatives are not well detailed investigated. In this work, to verify the superiority and extensibility of AIE materials, we report the bright aggregated state cathodic ECLs of TPE-based derivatives by a coreactant approach for the first time. The novel ECL system is simple design, convenient to use, low interference to analytes, and

characteristic compounds (AIEgens), tetraphenylethene (TPE)-based derivatives have been paid on much efforts, and become the most attractive AIEgens due to their excellent properties, facile synthesis, and easy functionalization.[10−14] However, to the best of our knowledge, the research of TPE derivatives in other numerous photoelectric fields besides fluorescence is surprisingly scarce. Hence, the development of their more extensive applications according to the AIE strategy are still highly desirable for both AIE research and material chemistry. Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a phenomenon in which luminophores on the electrode surface undergo electron transfer by electrochemical methods to form excited states that then emit light.[15−19] It combines chemiluminescence and electrochemistry characteristics to become a new analytical technology, and features the advantages of strong selectivity, high sensitivity and low destructive. Therefore,

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achieves the highly efficient detection of iodide (I−) with a lower detection limit of 0.23 nM in aqueous phase.

KH2PO4) as the electrolyte, the potential range ( 1.6 ~ 0 V), and the scanning rate (100 mV/s) under the N2-saturated atmosphere.

EXPERIMENTAL SECTION

Calculation All calculations were carried out in the programs of Gaussian 03 (revision E.01). The geometry structures, the optimized 3D chemical structures and molecular orbital plots of TPE derivatives were all optimized by the density functional theory (DFT) method using 6-31+G(d) and UB3LYP basis set. The LUMO and HOMO represented the lowest occupied molecular orbital and highest unoccupied molecular orbital, respectively.

Chemicals and Materials All the reagents in this experiment were analytically grade. Potassium peroxydisulfate (KPS), KCl, K2HPO4, KH2PO4 and HNO3, NH2NH2•H2O, H2SO4 and Pd/C were purchased from Sigma-Aldrich Co. Ltd (St. Louis, MO, U.S.A.). TPE and diphenylacetylene (Tolane) were purchased from SinopharmGroup Co. Ltd (Shanghai, China). All the electrochemical measurement used phosphate buffered (PBS, pH = 7.5) containing mixture solutions of 0.1 M KCl, 0.1 M (K2HPO4 and KH2PO4) as the electrolyte. The detailed preparations of TPE derivatives were seen in Supporting Information. All organic compounds were characterized by mass spectrum (EI) and nuclear magnetic resonance (NMR). The 1H NMR spectroscopic data of compounds were determined by Bruke AV 400M NMR spectrometers. The chemical shifts of 1H NMR used deuterated Me4Si as a reference. Mass spectra (EI) were texted on a Shimadzu LCMS-2010EV mass spectrometer.

Detection The accurate amount of potassium iodide (KI) was dissolved in PBS buffer solution, and then diluted into a series of standard solutions of different concentrations for reserve (seeing details in Supporting Information Page S13).

RESULTS AND DISCUSSION ECL Signals of the TPE Derivatives ECL can mainly be divided into two types of annihilation and co-reaction[15−19]. As mentioned earlier, some ECL luminophore derivatives exhibit different luminescent properties in the solution state and solid state due to their unique structures.[17,28−30] Based on this, firstly, we investigated the annihilation ECL of TPE both in homogeneous solution (CH2Cl2) and the aggregated state modified at the surface of the glassy carbon electrode under electrochemical conditions (0.1 M PBS, pH = 7.5, a potential range: 2 V ~ 2 V (vs Ag/Ag+), and 100 mV/s scanning rate), respectively. Unfortunately, not any ECL signals could be observed in either case. It was reported that co-reactants could produce intermediates that then react with ECL luminophores to produce excited state and significantly generate ECL.[15−19] To explore the most suitable coreactants, three kinds of coreactants, oxalate ion (C2O42−), tri-npropylamine (TprA) and potassium persulfate (K2S2O8), were examined respectively, and only K2S2O8 could obviously enhance the ECL emission of TPE modified at the surface of GCE. It could be seen that the ECL intensity of TPE/K2S2O8 was amplified approximately 6-fold higher than that K2S2O8 existed alone (Figure 1A). In contrast, the ECL of TPE in the homogenous organic solution as a monomer state was also investigated with K2S2O8 coreactant, but the signal lowered to approximately one fifth (from 1000 to 200 a.u.) of the aggregated state ECL (Figures 1B). This might be due to active molecular motions in the solution state served as a relaxation channel for the excited state to decay non-radiatively, whereas in the aggregated state, the motions could be suppressed greatly by its propeller-like noncoplanar structure (Figure S3), which blocked the nonradiative decay channel and promoted the radiative decay of the excited state to the ECL emission. The phenomena also indicated that TPE derivatives featured higher ECL signals only in the aggregated state and similar characteristics like their fluorescences, namely aggregation-induced ECL emission (AIEECL). Additionally, this heterogeneous ECL[31,32] originating

Preparation of TPE-Based Aggregates The reserved THF solutions of TPE, TPE-(NO2)4, TPE-(NH2)4, and tolane with the concentration of 10-3 M were prepared, respectively. After adding an appropriate amount of THF, a certain amount of deionized water was added dropwise under intense agitation and compounds slowly aggregated in solutions. Finally, aggregates (10-5 M) were obtained for subsequent applications. Fabrication of TPE-Modified Carbon Electrodes First, glassy carbon electrode (GCE, 3 mm in diameter) was polished to a mirror using 0.05 μm alumina slurry (Gaoss Union, Wuhan), followed by sonication in water, ethanol, and water, respectively. Finally, the electrode was rinsed thoroughly with ultrapure water and dried in the N2 flow. After tested by cyclic voltammogram under the CV potential (2.0 ~ 2.0 V) at the scanning rate of 100 mV/s until observing a stable redox wave of K3[Fe(CN)6]. The modified GCE was prepared by casting 5 μL aggregates of 12.0 mmol/L TPE or TPE-based derivatives in THF/H2O mixture solution on a clean GCE surface and dried at room temperature (Figure S4). Electrochemical and ECL Experiments A three-electrode system containing a Pt wire as counter electrode, a glassy carbon electrode (GCE with a diameter of 3 mm) as a working electrode, and Ag/AgCl (saturation KCl solution) as a reference electrode was used to measure electrochemical experiments. ECL and cyclic voltammogram (CV) were measured on a MPI-A ECL detection system (Remex Electronic Instrument Lt. Co., Xi'an, China) equipped with a homemade ECL electrolytic tank. The voltage of the photomultiplier tube (PMT) was set as 800 V during the whole experiment process. The ECL spectrum was acquired using a CHI 650D electrochemical workstation in conjunction with a fluorescence spectrophotometer (F97XP, Shanghai Lengguang Technology Co., Ltd.) in the closed state of the xenon lamp. The ECL reactions were measured in 0.1 M PBS (pH = 7.5) containing mixture solutions of 0.1 M KCl, 0.1 M (K2HPO4 and

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intensity of TPE-(NO2)4/K2S2O8 increased greatly and was amplified approximately 12-fold higher than that K2S2O8 existed alone, which indicated that electron-withdrawing nitro group (−NO2) could help to enhance its ECL performance. At the same time, the aggregated state ECL of TPE-(NH2)4 containing electron-donating groups (−NH2) was weaker and only half ECL intensity of TPE. As a comparison, non AIEactive coplanar structure diphenylacetylene (tolane) (Figure S3) showed almost little ECL with K2S2O8 under the same experimental conditions (Figure 1A). The results revealed such AIE characteristics caused by unique propeller-like noncoplanar structures made great contribution to generate the aggregated state ECLs of TPE and its derivatives, and the substituents had also profoundly affected their ECL emissions. Finally, TPE-(NO2)4/K2S2O8 became the best ECL luminophore because of its strongest ECL characteristics. Additionally, some important influencing factors (scanning rate, pH value, the concentrations of TPE-(NO2)4 and K2S2O8) for TPE-(NO2)4/K2S2O8 system were also optimized to obtain the best experimental conditions, which was a potential range (0.1 M PBS containing 0.1 M KCl and 0.1 M K2S2O8, pH = 7.5, 12.0 mM concentration of TPE-(NO2)4, 1.6 V ~ 0 V (vs Ag/Ag+)), and 100 mV/s scanning rate) for further study (Figure S7).

from the modification of AIEgens at the electrode surface could effectively solve the aggregation-caused quenching (ACQ)[33−36] lying in the organic-based ECL system due to its design and high efficiency. TPE

TPE-(NO2)4 O 2N

(1)

O 2N

NO2

(2)

AIE

NO2

TPE-(NH2)4

H 2N

Tolane

NH2

H 2N

(3)

NH2

(4)

non-AIE

Scheme. 1 Chemical structures of TPE derivatives and tolane

Substitutes play an important role on the luminescent properties of organic compounds. To further investigate the substituent effects, the ECL properties of some similar TPEbased derivatives, 1,1,2,2-tetrakis(4-nitrophenyl)ethene (TPE(NO2)4), and 4,4',4'',4'''-(ethene-1,1,2,2-tetrayl) tetraaniline (TPE-(NH2)4), were also measured under the same electrochemical conditions. As shown in Figure 1A, the ECL

Figure. 1 ECL intensities vs. different TPE derivatives/K2S2O8 systems: A. In the aggregated state, the modificaiton of aggregated TPE derivatives at the surface of GCE. The experimental conditions (0.1 M PBS containing 0.1 M KCl and 0.1 M K2S2O8, pH = 7.5, a potential range from 1.6 V to 0 V (vs Ag/Ag+), and 100 mV/s scanning rate); B. In the homogenous solution at the same conditions like A, 12 mM TPE derivatives in the mixted solvent containing 75% dimethyl sulfoxide and 25% H2O.

On the other side, an interesting phenomenon observed from the UV-Vis spectra of three TPE derivatives illustrated that the ECL intensities were very in direct proportion to their R/B absorbance band intensity ratios, and the larger R/B ratios represented the stronger ECL emissions (Figure 2), where the B bands represented the characteristic absorption bands of aromatic groups,

and the R bands represented the characteristic absorption bands of substituents (−NO2, −NH2) (Figure 2B). It could be seen clearly TPE-(NO2)4 with the largest R/B intensity ratio showed the strongest ECL signal. The new finding was of great significance to reveal the origination of ECL and design TPE-based luminophores featuring better luminescent properties.

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Figure. 2 A. Synchronous ECL intensity vs. potential: a. TPE-(NO2)4 + K2S2O8, b. TPE + K2S2O8, c. TPE-(NH2)4 + K2S2O8, d. Tolane + K2S2O8, e. K2S2O8. The experimental conditions: 0.1 M PBS solution containing 0.1 M K2S2O8 and 0.1 M KCl, a potential range from 1.6 V to 0 V (vs Ag/AgCl), 100 mV/s scanning rate, and pH = 7.5; B. The UV-Vis absorption spectra of TPE, TPE-(NO2)4, TPE-(NH2)4, and Tolane in aggregated state. (The B bands represent the characteristic absorption bands of aromatic groups, and the R bands represent the characteristic absorption bands of substituents (−NO2, −NH2), respectively)

DFT Calculations To fully understand the effect of electronic structure on such ECL emission, the geometries and harmonic vibration frequencies of TPE derivatives and tolane were optimized using the DFT method with UB3LYP and 6−31+G(d) basis set. The calculated optimized 3D chemical structures and molecular orbital plots for TPEs were given in Figure 3, respectively. The HOMOs of three TPE derivatives displayed a similar surface and mainly arised from the C=C double bond of the vinyl group. The LUMOs of TPE, TPE-(NO2)4, and TPE-(NH2)4 were mainly dominated by the π orbitals originating from the intense interaction between C=C double bond and four aromatic groups. Unlike TPE and TPE-(NH2)4, the LUMO energy level of TPE(NO2)4 was very low at −4.25 eV, which leaded to a high electron affinity, a smaller LUMO-HOMO band energy gap, and contributed to the easy occurrence of electron transfer reactions to generate the strongest ECL emission. This might be due to the strong electron-withdrawing nitro group (−NO2) that contributed its LUMO hybridization (Figure 3). The calculations also revealed that the HOMO/LUMO energy levels played an important influence on their ECL emissions because of the substituent effects, especially the low-lying LUMO levels could help to enhance the ECL signals, which provided another way to understand the emissive mechanism and design the more efficient TPE-based luminophores.

Figure 3. Molecular orbital amplitude plots of the HOMOs and LUMOs and calculated orbital energy levels for the TPE, TPE(NO2)4, and TPE-(NH2)4 and Tolane molecules.

Stability Experiment ECL stability determined its further application in the sensor field. Figure 4. presented the ECL stability of TPE(NO2)4/K2S2O8 system under consecutive cyclic potential scans for 22 cycles in the buffer under experimental conditions (0.1 M PBS solution containing 0.1 M K2S2O8 and 0.1 M KCl, pH = 7.5, a potential range: 1.6 V ~ 0 V (vs Ag/AgCl), 100 mV/s scanning rate, and 12.0 mM concentration of TPE-(NO2)4). It could be seen that the stability of the ECL system was very good and the

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Analytical Chemistry negative potential region. Then the TPE-(NO2)4 radical anions could quickly react with S2O82− to generate SO4•− that featured the strong oxidizability. In addition, an irreversible reduction peak observed at about 1.37 V (Figure 6) belonged to the electrochemical reaction between TPE-(NO2)4 anions and S2O82−.

calculated relative standard deviation was 1.06%, which made it as a potential ECL sensor.

Figure 4. ECL intensity vs. time curve for TPE-(NO2)4/K2S2O8 system under optimal conditions for continuous 22 cycles.

The Possible Mechanism of ECL

Figure 6. CV waves for the K2S2O8 (blue line), TPE-(NO2)4 aggregates (black line) and the mixture of TPE-(NO2)4 + K2S2O8 (red line)

The maximum ECL spectrum for TPE-(NO2)4/K2S2O8 system was at 537 nm (Figure 5), which was very approximate with the photoluminescence (PL) at 525 nm (Figure S6, Table S1), indicating ECL emissions went through a similar mechanism like PL. The calculated ECL efficiency was 41.2% for TPE-(NO2)4/K2S2O8 compared with the elegant Ru(bpy)32+/TPrA system (Table S2), and it belonged to the relatively high value reported at present.[15−23].

The strong oxidant species SO4•− could quickly oxidize the TPE-(NO2)4 anions through electron transfer reaction to generate the excited TPE-(NO2)4*, which would immediately go back to the ground state and then emit ECL light (Figure 7). At the same time, because of the highly spatial constraint in the aggregated state, intramolecular motions of TPE-(NO2)4 was blocked, which could avoid the nonradiative decay[4] and be benefit for ECL enhancement. 1)

TPE-(NO2)4 + e-

2)

TPE (NO2)4 + S2O82-

TPE (NO2)4 TPE (NO2)4 + SO42- + SO4 SO42- + TPE (NO2)4*

3) TPE (NO2)4 + SO4 4) TPE (NO2)4*

TPE (NO2)4 + hv (ECL)

Figure 7. Proposed ECL mechanism of TPE-(NO2)4/K2S2O8 system.

Detection of Iodide (I−)

Figure. 5 (a) Normalized photoluminescence (PL) spectrum of TPE-(NO2)4 in the aggregated state (black line); (b) Normalized electrochemiluminescence (ECL) spectrum of TPE-(NO2)4/K2S2O8 system in the aggregated state (red line).

Iodide (I−) plays a vital role not only for human growth and metabolism, but also for chemical industry, medicine, food, and so on.[39,40] Although many methods have been developed for the detection of iodide, they all have some shortcomings because of the volatilization, instability, and low content of iodide.[41−46] So it is currently of great demand to explore a new method with high sensitivity, simple instrumentation, fast analysis, low background interference, and easy control for the detection of iodide. ECL analysis could just make up for the analytical shortcomings above because of its high selectivity and sensitivity.[16,17]

To verify the mechanism, cyclic voltammograms had been also investigated (Figure 6). The reduction CV peak of TPE-(NO2)4 in the aggregated was at 1.28 V and close to their as monomers in CH2Cl2 solution (1.22 V see in Figure S9). After mixing TPE-(NO2)4 and K2S2O8, the reduction peak of cyclic voltammogram was observed at 1.37 V. So the possible proposed mechanism was that TPE-(NO2)4 could obtain an electron to form corresponding TPE-(NO2)4 radical anion[37,38] when the potential was scanned in the

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Selected Experiments

It noted that iodide was prone to redox reaction with peroxydisulfate. In order to verify the application of the TPE-(NO2)4/K2S2O8 ECL system, the detection of iodide was performed in this study. As shown in Figure 8, the ECL intensity quenched linearly when the iodide (from 0~2000 nM) was added to the TPE-(NO2)4/K2S2O8 ECL system, which illustrated a potential application to the sensing of iodide. To the best of our knowledge, there had been no documented example of ECL analysis for sensing of iodide (I−).

In order to verify the specificity of the ECL detection, , we had also investigated the effect of other various interference ions (Na+, K+, Ca2+, Zn2+, Cu2+, Fe3+, Mg2+, Br−, Cl−, NO3−, NO2−, SO42−, S2O52−, ClO4−, IO4−, and OH−.) both exiting alone and in the mixture of iodide (I−). As shown in Figure 10 and S13, only the iodide (I−) responsed to the ECL system, while others almost exhibited very slight ECL intensity changes. This might be due to such redox reaction was very specific to the iodide (Figure S12).

Figure 8. ECL intensity of TPE-(NO2)4/K2S2O8 system under optimized conditions in the presence of various concentration of iodide (I−).

Figure 10. Comparison of the quenching effect of various interferences for the TPE-(NO2)4/K2S2O8 system.

Figure 9 showed the linear correlation between the iodide concentration and the ECL intensity of TPE-(NO2)4 in the range of plotting ln(Io/I) (here I0 and I stood for the ECL intensities in the absence and presence of iodide, respectively) against the iodide concentration, which showed a good correlation coefficient with R2 = 0.9981 (n = 3). The linear range indicated a detectable range of iodide (I-) was from 5 to 2000 nM (Figure 8). In addition, the new ECL analysis for the detection of iodide (I−) showed a relative lower limit of detection (0.23 nM, signal/noise ratio = 3). Beyond that, the determination of iodide (I−) in real samples (river water) by the new ECL method have been also measured, it was found to be 31.25 nM from the wellestablished method (gas chromatography). As shown in Table S4 and S5, the proposed ECL method shows more accurate for the quantification of iodide (I−) in the actual sample determination.

All of these above suggested the new ECL method exhibited highly specific recognition to the iodide (I−) in aqueous phase compared with the present methods (Table S3).[41−46]

CONCLUSIONS In summary, we have successfully established a novel substituent-induced aggregated state ECL based on AIEactive TPE derivatives. The new ECL system exhibits selective recognition, and highly sensitive detection of iodide (I−) in water phase for water-insoluble organic-based ECL. Our work also indicates the relative intensities of R and B bands in UV-Vis could reflect the HOMO and LUMO energies of TPE derivative’s, and then directly influence their luminescent properties. This finding expands the applications of AIE-active TPE derivatives and provides a useful guidance for designing better TPEbased luminophores.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Figure 9. Linear calibration plot for iodide (NO2)4/K2S2O8 system.

(I−)

*[email protected] *[email protected]

detection by TPE-

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Analytical Chemistry Author Contributions †Zhengang

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Han and Yinpan Zhang contributed equally to this

work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the Natural Science Foundation of China (grant Nos. 21575115 and 21705117) and Postdoctoral Science Foundation of China (grant No. 2019M653898XB) for financial support.

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