Donor-Acceptor Conjugated Polymer Dots for Tunable

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Donor-Acceptor Conjugated Polymer Dots for Tunable Electrochemiluminescence Activated by AIE-Active Moieties Ziyu Wang, Yaqiang Feng, Ningning Wang, Yixiang Cheng, Yiwu Quan, and Huangxian Ju J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02087 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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The Journal of Physical Chemistry Letters

Donor-Acceptor Conjugated Polymer Dots for Tunable Electrochemiluminescence Activated by AIE-Active Moieties Ziyu Wang,† Yaqiang Feng,‡ Ningning Wang,‡ Yixiang Cheng,*,† Yiwu Quan,*,† and Huangxian Ju*,‡ †

MOE Key Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing 210023, P. R. China ‡

State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and

Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China

KEYWORDS: Low electrochemiluminescence potential, Enhanced electrochemiluminescence emission, Aggregation-induced emission, Donor-acceptor structure

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The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Low-potential electrochemiluminescence (ECL) luminophores with excellent ECL behavior has attracted considerable interest in biological analysis. Herein we synthesized two aggregation-induced emission (AIE)-active conjugated polymers with donor-acceptor (D-A) system via Suzuki coupling polymerization reaction. The intramolecular D-A pairs enhanced their luminescence intensity in aggregate states, which was beneficial for preparation of conjugated polymer dots (Pdots) as ECL materials. The ECL emission of the P-2 Pdots showed obvious red shift from 578 nm to 598 nm after replacing the fluorene moiety (P-1) with stronger electrondonating carbazole moiety. The ECL property could be regulated with intramolecular charge transfer of the D-A moieties, which led to a sharp decline (553 mV) of ECL anodic potential. Furthermore, the ECL intensity significantly increased for about 6 times due to the low LUMO level, which facilitated the electron injection into the conjugated polymer backbone. This work provides an effective strategy for developing AIE-active ECL materials with low potential and high ECL emission intensity via adjusting D-A electronic structure.

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Aggregation-induced emission (AIE) effect can cause fluorescent emission enhancement of chromophore in the aggregate state1 and has been successfully applied in various fields, such as OLEDs,2 bioprobes,3 enantioselective recognition,4 photodynamic cancer therapy5 and circularlypolarized luminescence.6 Among the known AIE-active luminogens, tetraphenylethene (TPE)based luminophores have been regarded as one of the most important families due to their excellent properties including simple synthesis procedure and high photoquantum yields.7,8 Recently, two AIE-active TPE bridged tetraimidazolium salts have been reported to display “turnon” fluorescence signal after complexing with AgI and AuI due to the inhibition to the intramolecular rotation of phenyl rings in the chelate complex,9 and AIE-active Pt (II) complexes have been designed to produce the enhanced electrochemiluminescence (ECL) signals.10 The donor-acceptor (D-A) type conjugated polymers generally undergo the intramolecular charge transfer (ICT) from electron donor to acceptor,11 which forms an inner complex donoracceptor pair since the band gap depends on the overlap of D/A wave functions.12 These polymers have been used as organic photovoltaic materials,13 electrochromic devices,14 memory devices,15 and OLEDs.16 The HOMO and LUMO energy levels of D-A type conjugated polymers can be easily tuned from the donor-acceptor electron ability.17,18 Owing to the strong electron accepting ability, boron-dipyrromethene (BODIPY) derivatives have become one of the most excellent acceptors for design of D-A type organic materials.19,20 Furthermore, the excellent electrochemical properties have also led to their ECL application.21 The ECL process involves the electron transfer of luminophores at electrode surface to form excited states and emit photons.22-25 Thus, adjusting the D-A electronic structure of the conjugated Pdots can efficiently tune the ECL behaviors of the D-A type luminophores.17,26 To achieve small band gap and low ECL potential, which benefits high selectivity and sensitivity of ECL biosensing



and avoids some backgrounds,27,28 this work designed and synthesized two kinds of threecomponent AIE-active conjugated polymers P-1 and P-2 with D-A type structure (Scheme 1). While the fluorene linker of P-1 was replaced by the strong electron-donating carbazole moiety (P-2), the ECL anodic potential sharply decreased for 553 mV and the ECL emission intensity increased obviously for about 6 times. Two intramolecular FRET pairs in the D-A type Pdots underwent from fluorene or carbazole linker to AIE-active TPE moiety and then from TPE moiety to BODIPY chromophore, respectively, which greatly enhanced the luminescence emission intensity and activated ECL behaviour in aggregate states. This work provides a new strategy on developing excellent ECL materials with low ECL potential and high ECL signal promoted from D-A electronic structure and intramolecular FRET mechanism.

Scheme 1. Synthetic Routes of P-1 and P-2 The detailed synthesis procedures and characterization data of P-1 and P-2 and model polymers 1 to 4 were outlined in Scheme S1 and Table S1 (Supporting Information). Here the groups in P-1 and P-2 acted as different functions: i) the fluorene/carbazole-BODIPY components as the D-A electronic system, fluorene or carbazole unit as the electron donor and the BODIPY component as the electron acceptor; ii) the fluorene/carbazole-TPE components as the first intramolecular FRET


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pair, fluorene or carbazole unit as the FRET donor and TPE component as the FRET acceptor; iii) the TPE-BODIPY components as the second intramolecular FRET pair, TPE component as the FRET donor and BODIPY component as the FRET acceptor. Both P-1 and P-2 contained one electron D-A pair and two intramolecular FRET pairs. The P-1 and P-2 Pdots could be prepared by nanoprecipitation method utilizing P-1 and P-2 as luminescent precursor and poly(styrene-co-maleic anhydride) (PSMA) as functional reagent (Supporting Information). The latter could produce the carboxyl groups on the Pdots surface by hydrolysis of the maleic anhydride.26 The AFM images of the P-1 and P-2 Pdots demonstrated an essential spherical morphology and monodisperse feature (Figure 1A and 1D) and their corresponding height profiles indicated the average particle size of 8.9 nm for P-1 Pdots (Figure 1B) and 13.4 nm for P-2 Pdots (Figure 1E). DLS data gave an average hydration diameter of 10.4 nm for P-1 Pdots (Figure 1C) and 15.1 nm for P-2 Pdots (Figure 1F), which were consistent with the AFM analysis.

Figure 1. AFM images of P-1 Pdots (A) and P-2 Pdots (D); and corresponding height profiles along the lines for P-1 Pdots (B) and P-2 Pdots (E). DLS of P-1 Pdots (C) and P-2 Pdots (F). Vertical color bar: 20 nm.



P-1 and P-2 in THF showed strong UV-vis absorption at 346 nm and 300 nm with a shoulder peak (343 nm), respectively (Figure 2), which could be assigned to the extended π-conjugation polymer backbone. Compared with P-1, P-2 showed about 46-nm blue shift and another shoulder peak owing to the difference of electron donating ability between fluorene and carbazole moieties30,31 and the cross-shaped conjugation structure of P-2.32-34 Both P-1 and P-2 showed weak absorbance peaks at around 538 nm, which could be attributed to the ICT between electronrich donor and electron-deficient acceptor along the conjugated polymer chain backbone.30

Figure 2. UV-vis absorbance of P-1 and P-2 in THF at 1.0×10-5 M corresponding to fluorene or carbazole moiety. The fluorescence spectrum of P-1 in THF-H2O mixture with the excitation wavelength of 370 nm gave two main emission peaks at 411 and 575 nm (Figure 3A). The weaker emission at 411 nm was from the fluorene moiety, while the strong peak at 575 nm could be regarded as the emission of the BODIPY chromophore. With the increasing water fraction (fw) from 0% to 40%, the emission peak at 411 nm did not obviously change (Figure S1), but it decreased and completely disappeared at fw > 60%. In addition, the emission peak at 575 nm appeared gradual increase at fw > 40% and reached the maximum intenisty at fw = 80% with a 5 nm red shift (photoquantum yield: 16% using 3.33 µg mL-1 quinine sulfate in 0.1 M H2SO4 solution as a standard), which could be attributed to the intramolecular FRET from TPE moiety to BODIPY


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chromophore.35,36 Due to the non-emission character of TPE moiety, the intramolecular FRET process could not occur in solvent THF. Therefore, P-1 exhibited obvious fluorene emission signal at fw ˂ 40%. With the increase of water fraction (fw), the polymer gradually formed the aggregate, which restricted molecular vibration of phenyl rings in the TPE moiety and led to aggregation-induced fluorescence enhancement. At fw > 50%, AIE-active P-1 emitted strong orange-colored fluorescence from BODIPY chromophore and enhanced gradually via

Figure 3. (A) P-1 and (B) P-2 in THF-H2O mixtures. λex = 370 nm. (C) UV-vis absorbance of P-1 in THF (a), PL spectra of model polymer 1 in THF (b) and model polymer 2 in THF-H2O mixture (fw = 99%) (c); λex = 370 nm. (D) UV-vis absorbance of P-2 in THF (a), PL spectra of model polymer 3 in THF (b) and model polymer 4 in THF-H2O mixture (fw = 99%) (c); λex = 370 nm. (E) P-1 and (F) P-2 in THF-H2O mixtures; λex = 520 nm. Polymer concentration: 1×10-5 M corresponding to fluorene or carbazole moiety.



intramolecular FRET mechanism. P-2 exhibited similar AIE behavior (photoquantum yield: 10%) but much weaker emission intensity than P-1 (Figure 3B). Two emission peaks situated at 426 and 595 nm could be assigned to carbazole moiety and BODIPY moiety, respectively. The emission peak at 426 nm disappeared at fw > 10%, while the emission of BODIPY chromophore gradually enhanced at fw > 40% and reached the maximum intensity at fw = 80% with a 5 nm red shift (Figure S2). Compared with the weaker electron donating fluorene group in P-1, P-2 showed 20 nm red shift due to the stronger intramolecular donor-acceptor interaction between carbazole and BODIPY moieties.37 To explain the mechanism of two intramolecular FRET pairs in P-1 and P-2, we synthesized four model polymers 1 to 4. The fluorescence emission peak of model polymer 1 with only fluorene moiety centered at 416 nm (Figure 3C, curve b), which overlapped with the UV-vis absorption of P-1 (Figure 3C, curve a) in the region from 380 to 430 nm, demonstrating the first FRET pair from fluorene moiety to TPE moiety.38 As an evidence for the second intramolecular FRET process, the fluorescence spectrum of model polymer 2 in aggregate state (Figure 3C, curve) overlapped with the UV-vis absorption of P-1 (Figure 3C, a) in the region of 490 to 580 nm, indicating the effective energy transfer from TPE moiety to BODIPY moiety. Similarly, the mechanism of two intramolecular FRET pairs in P-2 could be confirmed in Figure 3D. To further explore the second intramolecular FRET of these two polymers, we recorded the fluorescence emission spectra at λex of 520 nm (Figure 3E and 3F). Only the emission of BODIPY could be observed at 578 nm (P-1) and 597 nm (P-2), which indicated that no FRET process from TPE to BODIPY occurred due to the non-emission transition of fluorene and TPE moieties. Interestingly, both P-1 and P-2 in the aggregate state showed obvious aggregation-caused quenching (ACQ) behavior, which could further demonstrate the second intramolecular FRET pair from TPE to


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BODIPY chromophore. Meanwhile, the weighted average lifetimes of P-1 (2.17 ns) and P-2 (1.59 ns) were much longer than those of model polymer 2 (1.10 ns) and model polymer 4 (0.88 ns), respectively (Figure S3), which confirmed the second FRET pocess. The electrochemical studies of P-1 and P-2 were carried out in CH2Cl2 solution containing 0.1 M Bu4NBF4 as the supporting electrolyte. The energy levels were evaluated from the cyclic voltammograms (CVs) and summarized in Table 1. The CVs of P-1 showed two quasi-reversible oxidation peaks at +0.982 and +1.385 V with an onset oxidation potential of +0.788 V, while the oxidation peak potentials of P-2 appeared at +0.911 and +1.257 V, respectively, with an onset oxidation potential of +0.778 V (Figure 4A and 4B). These oxidation peaks were mainly contributed by the oxidation of TPE, which gave two consecutive one-electron oxidation peaks at +1.246 V and +1.549 V (Figure 4C).39 The D-A electronic structure could be extended to the whole conjugated system, which led to more prone to oxidation in polymer than that in TPE monomer. Moreover, the acceptor BODIPY monomer showed an Nernstian one-electron oxidation and reduction at half-wave potential of +1.009 and −1.434 V, respectively (Figure 4D).40 More carefully, P-2 could be oxidized more easily than P-1 (Figure 4B), which could be attributed to the stronger electron-donating ability of the carbazole moiety in P-2.41 Moreover, P-2 Table 1. Electrochemical data and calculated energy levels of P-1 and P-2.

a

Polymer

Ox Eonset (Va)

Red Eonset (Va)

HOMO (eVb)

LUMO (eVb)

Eg (eVb)

P-1

+0.788

−1.396

-5.468

-3.284

2.184

P-2

+0.778

−1.123

-5.458

-3.557

1.901

Potential was versus Ag/Ag+. b Ferrocene couple (Fc/Fc+) was used as the internal reference. The

energy levels were calculated using the following equations: EHOMO = − (EOx onset −EFc/Fc+ +4.8) eV, + ELUMO = − (ERed onset −EFc/Fc+ +4.8) eV, EFc+/Fc = 0.12 V vs. Ag/Ag , Eg = ELUMO − EHOMO.



Figure 4. CVs of P-1 and P-2 (A), TPE (C), and BODIPY (D) in 0.1 M Bu4NBF4 CH2Cl2 solution. (B) magnified CVs from black rectangle in (A). Concentration: 1.0 mM. Scan rate: 100 mV s−1. showed better reversibility than P-1, indicating the radical cations of P-2 was more stable than P1, which was helpful for improving the efficiency in the annihilation routes or coreactant routes. Both P-1 and P-2 did not show the reduction wave until −1.396 and −1.123 V, respectively (Figure 4A), where electrons began to inject into the polymer to produce corresponding radical anions. The band gaps of P-1 and P-2 could be calculated by the separation between the onset oxidation potential and onset reduction potential (Table 1) and gave the value of 2.184 and 1.901 eV for P-1 and P-2, respectively. Their optical band gaps in THF could also be calculated as 2.156 eV for P-1 (emission wavelength: 575 nm, Figure 3A) and 2.084 eV for P-2 (emission wavelength: 595 nm, Figure 3B) through the formula: Eg = 1239.8/λ (eV). The electrochemical gaps were very close to their optical energy gaps, indicating that the electrochemical measurement of the LUMO and HOMO energy levels were reliable. Moreover, the band gap of P-2 showed an


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obvious decline compared with P-1, which could be mainly assigned to its stronger ICT interaction. In nitrogen-saturated 0.1 M pH 7.4 PBS the P-1 Pdots modified electrode showed an oxidation peak at +0.984 V (Figure 5, curve a), which was lower than +1.422 V of P-2 Pdots (Figure 5, curve b), indicating that both Pdots could be electrochemically oxidized to radical cation and the oxidization of P-2 Pdots was more difficult in the aqueous medium. Both P-1 and P-2 Pdots exhibited the ECL emission in the anodic potential window (Figure 5, curves a’ and b’), indicating that the radical anions of Pdots were more stable than their radical cations. The P-1 Pdots modified electrode showed an anodic ECL emission peak at +1.476 V with an onset potential of +1.174 V, while the P-2 Pdots modified electrode showed an anodic ECL emission peak at + 0.923 V with an onset potential of +0.722 V. Interestingly, compared with P-1 Pdots, P-2 Pdots modified electrode not only gave an obvious decline of ECL peak potential by 553 mV, but also exhibited a significantly enhanced ECL emission. This phenomenon could be attributed to the stronger D-A electronic structure in P-2 than P-1, which led to a lower LUMO energy level and facilitated the electron injection into polymer. The mechanism for this annihilation ECL could be proposed in eq. (1)-(4):



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Figure 5. CV (a, b) and ECL (a’, b’) of P-1 Pdots (a, a’) and P-2 Pdots (b, b’) modified GCE in 0.1 M pH 7.4 nitrogen-saturated PBS. Scan rate: 100 mV s−1 for CV and 500 mV s−1 for ECL. PMT: 850 V. Pdots + e– → Pdots•–

(1)

Pdots – e– → Pdots•+

(2)

Pdots•+ + Pdots•– →Pdots*

(3)

Pdots* → Pdots + hv

(4)

To explore the most suitable coreactant, four kinds of coreactants were examined, and only TPrA could obviously enhance the ECL emission of these Pdots (Figure 6, Figures S4 and S5). After adding coreactant TPrA into the PBS, the Pdots modified electrodes displayed a strong ECL emission at +1.515 V for P-1 Pdots (Figure 6A, curve a) and +1.028 V for P-2 Pdots (Figure 6A, curve b), respectively. The obviously enhanced ECL emission of P-2 Pdots could also be observed, which gave about 6 times enhancement comparing with P-1 Pdots. Meanwhile, the oxidation of TPrA appeared between +0.588 and +1.305 V (Figure 6B, curve c), which was between the oxidation peaks of P-1 Pdots and P-2 Pdots modified electrodes (Figure 6B, curves a and b), indicating that the direct charge transfer for oxidizing TPrA was apparently blocked.42 The large potential window up to +1.8 V of PBS solutions provided a suitable medium for ECL study.


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Figure 6. (A) Coreactant ECL and (B) corresponding CV of P-1 Pdots (a), P-2 Pdots (b) modified GCE, and bare GCE (c) in 0.1 M pH 7.4 PBS in the presence of 0.1 M TPrA as anodic coreactant. Scan rate : 100 mV s−1. PMT: 700 V. In the presence of TPrA in PBS, the ECL spectrum of P-1 Pdots modified carbon electrode showed an ECL emission peak at 578 nm with a full width at half-maximum (fwhm) of 123.2 nm (Figure 7A), which was consistent with the fluorescence emission peak of P-1 in 80% H2O/THF mixtures (580 nm), indicating the same excited species. Similarly, the ECL spectrum of P-2 Pdots modified electrode displayed a more symmetric and sharp ECL emission peak at 598 nm with a fwhm of 58.6 nm (Figure 7B), which was consistent with the fluorescence emission peak of P-2 in 80% H2O/THF mixtures (600 nm). Thus both ECL emissions originated from the band gap emission of corresponding Pdots.29 The detailed ECL mechanism of Pdots/TPrA system was described as follows: TPrA – e– → TPrAH•+

(5)

TPrAH•+ – H+ → TPrA•

(6)

Pdots – e– → Pdots•+

(7)

Pdots•+ + TPrA• → Pdots* + Pr2N+HC=CH2CH3

(8)

Pdots* → Pdots + hv

(9)



Figure 7. ECL spectra of P-1 Pdots (A) and P-2 Pdots (B) in 0.1 M pH 7.4 PBS containing 30 mM TPrA as coreactant.

During the anodic scanning, the coreactant TPrA was firstly into TPrAH+• between +0.60 and +1.20 V (eqs 5) and then underwent deprotonation to produce TPrA• as a strong reducing agent (eqs. 6). At higher potentials (> 1.20 V), the Pdots were oxidized to form Pdots radical cation (eqs. 7). Then the TPrA• injected electrons into LUMO energy level of the Pdots radical cation to produce an excited state (eqs. 8). Finally, the intense ECL was obtained to give P-1 Pdots centered at 578 nm and P-2 Pdots centered at 598 nm, while the excited state decayed back to the ground state (eqs. 9). The ECL efficiencies of P-1 and P-2 Pdots relative to the Ru(bpy)32+/TPrA system were calculated to be 5.8% and 11.8%, respectively (See Experimental Section and Table S2 in Supporting Information). It should be pointed out that the ECL signals of both P-1 and P-2 Pdots modified GCEs in 0.1 M PBS (pH 7.4) containing 0.1 M KNO3 and 0.1 M TPrA constantly decreased with the continuously cyclic voltammetric scanning. This was ascribed to the consumption of the immobilized Pdots (Figures S6). In summary, two AIE-active D-A type conjugated polymers P-1 and P-2 containing fluorene, carbazole, TPE and BODIPY moieties were synthesized via Suzuki reaction. Two intramolecular FRET pairs were introduced in the polymers with TPE moiety as a linker, which was beneficial to the enhancement of luminescence intensity at aggregate state. After replacing the fluorene moiety in P-1 with the stronger electron-donating carbazole moiety, the AIE emission of the P-2 displayed an obvious red-shift of 25 nm and relatively weaker AIE intensity than P-1 due to the stronger ICT mechanism. The strong electron-donating carbazole moiety made the electrochemical oxidation of the polymer easier, and the ECL emission from corresponding Pdots


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modified electrode significantly stronger, in both annihilation and coreactant ECL emission. In annihilation route, the presence of carbazole moiety declined the anodic ECL peak potential by 553 mV. In coreactant route, the ECL emission originated from the band gap emission of corresponding Pdots. This work provides a novel strategy for developing AIE-active ECL luminphor with low-potential and high ECL intensity via adjusting D-A electronic structure.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ . Experimental section; synthesis of P-1, P-2 and model polymers 1 to 4; AFM and DLS of P-1 and P-2 Pdots; UV-vis absorbance and fluorescence of P-1 and P-2; CV and ECL of P-1 and P2 Pdots, fluorescence lifetime and 1H NMR spectra of P-1, P-2 and model polymer 1 to 4.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]; [email protected]; [email protected] Author Contributions Z.W. and Y.F. contributed equally to this work. ORCID Yixiang Cheng: 0000-0001-6992-4437 Yiwu Quan: 0000-0001-6017-1029 Huangxian Ju: 0000-0002-6741-5302



Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (21674046, 51673093, 21635005, 21361162002), and State Key Laboratory of Analytical Chemistry for Life Science (SKLACLS1807).


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