Up-Conversion Fluorescent Oligo(2

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Kinetics, Catalysis, and Reaction Engineering

Novel synthesis of down-/up-conversion fluorescent oligo(2-pyrazoline)s Yan Li, Tao Li, Long-Qiang Xiao, and Yue-Fei Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02709 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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Industrial & Engineering Chemistry Research

Novel synthesis of down-/up-conversion fluorescent oligo(2-pyrazoline)s Yan Lia, Tao Lia, Long-Qiang Xiaob, Yue-Fei Zhang*a a

School of Chemistry and Biological Engineering, Changsha University of Science &

Technology, Changsha, 410114, P. R. China b

Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics,

South China Normal University, Guangzhou, 510631, P. R. China

ABSTRACT: The polycondensation of 2-pyrazolines is presented. The products are oligomers with the five-membered rings of the 2-pyrazoline as the backbone and the reaction is a polycondensation reaction by decomposition of ROH from the hydrogen atom in the –NH- group and the –OR in the ester group. Metal catalysts, reaction periods and the addition amounts of the catalyst are investigated to optimize the reaction conditions. The products are down-/upconversion fluorescence molecules that can significantly enhance cell imaging through the synchronized excitation of visible and near-infrared light and may be widely used in other optical fields.

INTRODUCTION The 1,3-dipolar cycloaddition reaction between 1,3-dipole and olefin is a classic reaction in organic chemistry, which usually is used to construct molecules of five-membered rings and is

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fundamentally important for both academia and industry.

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The discovery of 1,3-dipolar

cycloaddition reaction goes back to 1888 when Buchner firstly found the reaction between diazoacetate and α,β-unsaturated ester.3 It is noteworthy that his colleague Curtius discovered the first example of 1,3-dipole, diazoacetate, five years ago.4 And diazoacetate is an excellent green compound, which can be synthesized easily from the natural product glycine. 2-Pyrazoline is the important product of 1,3-dipolar cycloaddition reaction of diazo compound with olefin, which is five-membered heterocycle with two nitrogen atoms within the ring (–N1– N2=C3–C4–C5–N1–). Pyrazoline derivatives are known as a kind of fluorescent brightening agent. It is reported that in the conjugated part (–N1–N2=C3–) of the ring, the N1 atom and C3 atom are electron donor and acceptor, respectively. The other two carbon atoms of the ring do not conjugate with the above conjugated part.5-9 Jia et al. reported a type of efficient fluorescent pseudopeptide

oligo(3,4,5-triethoxycarbonyl-2-pyrazoline)

prepared

by

the

novel

cyclopolycondensation of 3,4,5-triethoxycarbonyl-2-pyrazoline, in which the oligomer exhibits a remarkable fluorescence enhancement compared with the monomer 2-pyrazoline.10 This is the first example of fluorescent oligomer constructed by pyrazoline as the backbone. Recently, several researches proved that diazo-contained polymers may have unique optical properties, and they could emit both down-conversion fluorescence excited by visible light and the upconversion fluorescence excited by near-infrared light, which is a potential field in optical materials.11-15 Up-conversion luminescent materials are current research focus with advantages of deep light penetration in tissues, lowautoﬂuorescence and reduced light scattering, and they can be excellent bioprobes and optical devices.16-18 Lanthanide ion-doped chemicals are most investigated up-conversion luminescent materials, but they usually have disadvantages of


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complicated synthetic steps and relying heavily on metal ions, which may limit their development and application.17,19,20 This work aims at simplifying the synthetic steps by simple polymerization reaction and replacing the complicated systems of up-conversion fluorescent compounds through simple polymer, and then making the polycondensation reaction of 2pyrazolines become an efficient and concise method to synthesize up-conversion luminescent polymers.

RESULTS AND DISCUSSION Synthesis of oligo(2-pyrazoline)s. Table 1 shows the oligomerization of different 2pyrazolines (Py-1, Py-2, Py-3 and Py-4). Four kinds of metal catalysts (Cu(OTf)2, Sn(Oct)2, Rh(OAc)2 and PdCl2) are chosen to study their efficiency for catalyzing the oligomerization of Py-1. Compared to the oligomerization of Py-1 without any catalyst (Entry 5), the yields of the products with the addition of metal catalysts improve obviously. Of the four catalysts (Entries 1 to 4), Rh(OAc)2 is optimal for the oligomerization of Py-1, yielding the oligomer with a relatively high average molar mass Mn, low Mw/Mn and high yield. Thus, Rh(OAc)2 is chosen as the best catalyst in this reaction. Reaction time has significant effects on the reaction. The oligomerization of Py-1 catalyzed by Rh(OAc)2 under 6h, 12h, 18h and 24h is conducted, from which we can conclude that the period of 24h is optimal for the reaction with higher molar mass and yield. As the reaction time grows, the yield of the product increases. The addition amount of the catalyst Rh(OAc)2 also influences the oligomerization of Py-1. As the ratio between the monomer and catalyst decreases, the Mn of the oligomer grows obviously,


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in which [M]:[C]=20:1 produces the largest Mn of PPy-1 with 2200 g mol-1. Thus, the optimal condition in the oligomerization is that the addition amount of Rh(OAc)2 is [M]:[C]=20:1 with reaction time of 24h. Table 1. The oligomerization of 2-pyrazolines.a

Entry

Momomer [M]

Catalyst [C]

Ratio ([M]:[C])

Temperature (oC)

Time (h)

Mn (g Mw/Mnc Yield %d mol-1)c

1

Py-1

Cu(OTf)2

50:1

210

24

400

2.17

17.9e

2

Py-1

Sn(Oct)2

50:1

210

24

1100

1.76

28.4f

3

Py-1

Rh(OAc)2

50:1

210

24

1100

1.46

30.0f

4

Py-1

PdCl2

50:1

210

24

800

1.43

9.7e

5

Py-1

-b

50:1

210

24

800

1.16

5.4e

6

Py-1

Rh(OAc)2

50:1

210

6

600

1.30

9.9e

7

Py-1

Rh(OAc)2

50:1

210

12

1000

1.26

15.6f

8

Py-1

Rh(OAc)2

50:1

210

18

700

1.43

27.3e

9

Py-1

Rh(OAc)2

100:1

210

24

900

1.43

25.6e

10

Py-1

Rh(OAc)2

20:1

210

24

2200

2.3

25.6f

11

Py-2

Rh(OAc)2

20:1

210

24

1200

1.38

39.8f

12

Py-3

Rh(OAc)2

20:1

160

24

800

1.44

18.6e

13

Py-4

Rh(OAc)2

20:1

210

24

600

1.37

25.2e

a

General reaction conditions: oligomerization period = 24 h; [M] = 1 mmol, [C]= 0.05 mmol. Without metal catalyst. cMn and Mw/Mn are obtained by SEC using polystyrene as standards in tetrahydrofuran (THF). dCalculated after reprecipitated, yield = (the mass of the obtained

b


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oligomer)/ (the mass of the 2-pyrazoline monomer)×100 %. eReprecipatated by n-hexane. f Reprecipatated by ethyl ether.

Other 2-pyrazolines (Py-2, Py-3, Py-4) are oligomerized under identical conditions (except that the oligomerization of Py-3 is under 160 oC, for which it has been decomposed under 210 oC).

Figure 1. a, 1H NMR spectrum of the PPy-1. b, 13C NMR spectrum of the PPy-1. Characterization of oligo(2-pyrazoline)s. The NMR spectra of the obtained oligomers are shown in Figure 1. As to the 1H NMR spectrum of the oligomer PPy-1 (Figure 1a), the peaks around 1.1 and 4.2 ppm are assigned to the resonance values of protons on the ethyl ester group (COOCH2CH3). The peaks from 6.5 to 7.9 ppm are attributed to the protons in the phenyl group. With respect to the 13C NMR spectrum of PPy-1 (Figure 1b), the peak at 173 ppm is belonging to the resonance of carbonyl group (-COO-). The resonance values appearing at 30, 38, 160 ppm


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are assigned to the carbon atoms located on the pyrazoline ring backbone (adjacent to the ester group and the phenyl group, and the C=N bond, respectively). 10 For phenyl group, the resonance values appear around 125-138 ppm. The peaks at 51 and 14 ppm are attributed to the -OCH2CH3 group. The FT-IR spectra of the oligomers are shown in Figure S7 (see the Supporting Information). The bands near 3000 and 1370-1450 cm-1 are assigned to the stretching and bending vibration of C-H respectively, which show a nature of different band appearance due to the variation of the ester group (-COOR). The bands near 1730 and 1590 cm-1 are ascribed to the stretching vibration of C=O and C=N, respectively.

Figure 2. a, MALDI-TOF-MS spectrum of PPy-1 with m/z 700-1400. b, MALDI-TOF mass spectrum of PPy-3 with m/z 800-2000.


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Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDITOF-MS). The constitutional units of the resulted oligomers are accurately identified by the MALDI-TOF-MS. Figure 2a shows the MALDI-TOF-MS spectrum of PPy-1 with m/z from 700 to 1400. It presents that 244 Da is the repeating unit between these three main peaks, m/z 801.2, 1045.3 and 1289.2, in which 244 Da is due to the decomposition of a molecule of ethanol (46 Da) from a molecule of Py-1 (290 Da). In this spectrum, the structure of m/z 801.2 can be resolved as m/z 801.2=3×244+46+23, that is, [H-(Py-1’)3-OEt]Na+ (Py-1’ refers to the structure of ethanoldecomposed Py-1). Thus, PPy-1 is a linear oligomer, and its terminal groups are hydrogen atom and ethoxy group, respectively. Figure 2b shows the MALDI-TOF-MS spectrum of PPy-3 with m/z from 800 to 2000. Herein, three series (A, B and C) are classified, in which a repeated m/z difference of 168 Da is recorded in every series and 168 Da (Py-3’) is due to the decomposition of a molecule of ethanol (46 Da) from a molecule of Py-3 (214 Da). Taking A8 for example, m/z 1367.3=8×168+23, so the general formula to calculate the mass of series A is 168n+23 Da (23 stands for Na+), which implies a cyclic structure 21-23 (i.e. n=8, the structure [(Py-3’)8]Na+ for m/z=1367.3). For B8, m/z 1385.3=8×168+18+23, and the general formula for series B is 168n+18+23 (e.g. n=8, the structure [H-(Py-3’)8-OH]Na+ for m/z=1385.3). In addition, C8 can be resolved as m/z 1413.3=8×168+46+23, so the general formula for series C could be expressed as 168n+46+23 (e.g. n=8, the structure [H-(Py-3’)8-OEt]Na+ for m/z=1413.3). In PPy-3, both cyclic (series A) and linear (series B and C) oligomers exist, and series B owns terminal groups of hydrogen atom and hydroxy group while series C has hydrogen atom and ethoxy group in two terminals of the backbone.


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The MALDI-TOF-MS spectrum of PPy-4 is shown in Figure S8 (see the Supporting Information), which is similar to the structure of PPy-1 and the repeating unit is 241 Da due to the decomposition of a molecule of ethanol (46 Da) from a molecule of Py-4 (287 Da). From MALDI-TOF-MS, it proves that the oligomerization of 2-pyrazolines is a polycondensation reaction by decomposition of ROH from the hydrogen atom in the –NH- group and the –OR in the ester group. Thermal properties. Differential scanning calorimetry (DSC) and thermogravimetry (TG) are utilized to detect the thermal properties of the obtained oligomers. As shown in Figure 3a, the glass transition temperature (Tg) of oligomers are 96.44, 89.57, 72.02 and 97.99 oC for PPy-1~4, respectively. What is more, there is no crystal or melting peak in the DSC curves, so the oligomers are armorphous phase.

Figure 3. Thermal properties of the oligomers. a, DSC curves of the oligomers. b, TGA curves of the oligomers under nitrogen atmosphere.


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According to the TGA curves (Figure 3b), the oligomers lose weight of around 10% at the first stage. In the literature, it is reported that pyrazoline is subjected to the thermal decomposition, resulting in cyclopropane via the loss of nitrogen.24 In addition, the oligomers show good thermal stability, with the temperature of T10% all upon 240 oC, in which the T10% of PPy-2 is 352.7 oC. Applications of oligo(2-pyrazoline)s with excellent down-conversion and up-conversion fluorescent properties. The fluorescent properties of the oligomers are elucidated in Figure 4. Under the irradiation of light around 480 nm, the solution of the oligomers in DMSO emits fluorescence from greenyellow to yellow. Among these four oligo(2-pyrazoline)s, PPy-3 shows the strongest fluorescence property, with which its concentration is just one tenth of the others’, and it emits the yellow light of 545 nm excited by 478 nm. For these oligomers, they not only can emit down-conversion fluorescence under short-wavelength light, but also can emit upconversion fluorescence by the excitation of long-wavelength light. For PPy-3, it is observed to emit 520-nm yellow light excited by 780-nm long-wavelength laser light, and the yellow light path can be observed in the cuvette. Thus, in these oligomers, the maximum excitation wavelengths excited by both down-conversion and up-conversion fluorescence are around the same region. To date, there have only been rare reports of organics with both down- and upconversion fluorescence properties. 11,25-32 HeLa cells are incubated for cell imaging. As shown in Figure 5a-d, fluorescence is clearly excited by both 458-nm single-photon light and a 780-nm two-photon laser. The fluorescence is discretely localized in the cellular cytoplasm in contrast to the bright-field image, which implies that the oligomer is efficiently internalized into HeLa cells. Figure 5d suggests that two-channel synchronized excitation can indeed increase the beneficial imaging effect of the specific downand up-conversion fluorescence.


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Figure 4. Single- photon and two-photon fluorescence spectra of the oligomers. a, PPy-1; b, PPy-2; c, PPy-3; d, PPy-4. Concentrations: 1 mg/mL of PPy-1, PPy-2 and PPy-4 and 0.1 mg/mL of PPy-3 in DMSO.


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Figure 5. In vitro cell experiments of PPy-3 under bright field (a), 458 nm (b), 780 nm (c) and overlapped two-channel (d) irradiation by Zeiss confocal microscope.

Figure 6. The optimized geometry (a) and molecular orbitals (b) of PPy-3 trimer by B3LYP/631G(d, p). DFT calculations. Density functional theory (DFT) is utilized at the level of B3LYP/631G(d,p) using GAMESS to calculate the optimized geometry and the molecular orbitals (MO)s. It is found that the electrons are mainly distributed in the delocalized electronic orbitals on the backbone of the 2-pyrazoline ring system forming the HOMO, HOMO-1 and HOMO-2 (Figure 6). However, the LUMO, LUMO+1 and LUMO+2 are generated from the π-π interaction between C=N and C=O. Hence, the much higher fluorescence intensity is mainly attributed to the electronic radiation transition from the conjugated N=C-C=O to the delocalized N-N=C system.


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CONCLUSIONS Herein, the polycondensation of 2-pyrazolines (Py-1, Py-2, Py-3 and Py-4) is presented, and a synthetic methodology is established. Metal catalysts (Cu(OTf)2, Sn(Oct)2, Rh(OAc)2 and PdCl2), reaction time and the addition amount of the catalyst are investigated to optimize the reaction conditions, in which the PPy-1 gets the larger mass weigh of 2200 g mol-1 and higher yield catalyzed by Rh(OAc)2 with [M]:[C]=20:1 for 24h. MALDI-TOF-MS spectrometry clearly demonstrates the structure of the oligomer based on a logical relationship among the main peaks, and the results prove that the oligomerization of 2-pyrazolines is an oligocondensation reaction by decomposition of ROH from the hydrogen atom in the –NH- group and the –OR in the ester group. The resulting oligomers possess both down- and up-conversion fluorescence, which has rarely been reported, and PPy-3 shows better fluorescence intensity. DFT calculations indicates that the fluorescence is mainly attributed to the electronic radiation transition from the conjugated N=CC=O to the delocalized N-N=C system. Because of these excellent properties, these materials are both fascinating and promising and are worthy of further exploration in a variety of fields.

MATERIALS AND METHODS Materials Ethyl diazoacetate (EDA) and methyl diazoacetate (MDA) are prepared based on the literature methods. The other chemicals are purchased commercially: ethyl cinnamate (AR), ethyl acrylate (AR), PdCl2 (AR) from Sinopharm Chemical Reagent Co., Ltd; N-phenylmaleimide (>99%), Cu(OTf)2 (AR) from Aladdin Industrial Inc.; Sn(Oct)2 (95%) from Sigma-Aldrich Inc.;


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Rh(OAc)2 (20.5 %-22.7 %C) from TCI Co., Ltd. The solvents are purchased from Sinopharm Chemical Reagent Co., Ltd, and dried by calcium hydride overnight and distilled before use. Instruments Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were performed using a 4800 MALDI TOF/TOF Analyser (ABSciex) equipped with a Nd:YAG 355 nm laser. Number average mass weights (Mn) and polydispersity index (PDI, Mw/Mn) of the oligomers were determined by size exclusion chromatography (SEC) calibrated with polystyrene standards with THF as eluent (1.0 ml min-1) at 40 oC of columns and 50 oC of detector, and equipped with a Waters 717 plus auto sampler, a Waters 1515 isocratic HPLC pump, a Waters 2414 refractive index detector, and Shodex K-805, K-804, and K-802.5 columns in series. Elemental analysis (EA) data were suggested on Vario EL. 1H and

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C NMR spectra were

recorded on Mercury VX-300 spectrometers using CDCl3 as solvent and TMS as the internal standard. Thermo gravimetric analysis (TGA) curves were measured with NETZSCH STA 449C thermal analyzer (NETZSCH, Germany). A few milligrams of sample were heated with a rate of 10 oC min-1 from room temperature to 800 oC under N2 atmosphere. Differential scanning calorimeter (DSC) was performed using Q20 (TA) with N2 as the protecting gas (50 ml min-1). The samples were heated from 0 oC to a proper temperature and held for 1 min to erase the thermal history, then cooled to 0 oC at a rate of 10 oC min-1, and finally heated to 120 oC and cooled to 0 oC at a rate of 10 oC min-1. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on Thermo iS10 spectrometer. Single-photon fluorescence excitation and emission spectra were recorded on RF-5301PC (Shimadzu) with slit width settled as 5.0 nm for both emission and excitation. Two-photon excited fluorescence data were measured by exciting


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with a mode-locked Ti:sapphire femtosecond pulsed laser (Chameleon Ultra I, Coherent Inc.) with a pulse width of 140 fs and repetition rate of 80 MHz.

Methods General synthetic procedure for the polycondensation of 2-pyrazoline: Py-1 (290 mg, 1.0 mmol) and Rh(OAc)2 (8.8 mg, 0.2 mmol) were mixed and heated at 210 oC with with stirring for 24 h. The obtained product was dissolved in DMSO or CHCl3 and precipitated by ethyl ether or n-hexane at least 3 times. Finally, it was dried under vacuum for 24 h. Other experiments were performed in the same manner.

ASSOCIATED CONTENT Supporting Information. Characterization of the oligomers and experimental details are listed. AUTHOR INFORMATION Corresponding Author * Yue-Fei Zhang, E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (Grant No. 21504025) and Scientific Research Fund of Hunan Provincial Education Department (Grant No. 16A004).

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Novel synthesis of oligo(2-pyrazoline)s with both down- and up-conversion fluorescent property by polycondensation of 2-pyrazolines. 82x36mm (300 x 300 DPI)


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Up-Conversion Fluorescent Oligo(2

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