A high quantum yield two-way conversion luminescent oligomer: 1,4-butanediol-bis(5-carbonyl-3-carbethoxy-2-pyrazoline)

Abstract

Oligomers as fluorescent macromolecules are of significant importance and are fairly attractive for high quantum yield two-way conversion luminescence. We report a high conversion rate (Φ_F = 0.5184) for a fluorescent oligomer, 1,4-butanediol-bis(5-carbonyl-3-carbethoxy-2-pyrazoline). It was prepared by the novel cyclopolycondensation of 1,4-butanediol-bis(5-carbonyl-3-carbethoxy-2-pyrazoline), a chromophore with good fluorescence (Φ_F = 0.0573). After polymerization, a green fluorescent protein (GFP)-mimetic structure was formed, in which a methylene chain (–CH₂–CH₂–CH₂–CH₂–) as a flexible group is beneficial due to its folding, and an azo group sits in the rigid pyrazoline ring because of a resonance effect. These factors collectively contribute to the excellent efficiency of the fluorescence performance of the oligomer. Additionally, the oligomer exhibited a weak red shift and a significant fluorescence enhancement compared with the monomer, and we achieved good quality live-cell imaging. The oligomer also allowed for the enhancement of fluorescence intensity through the two-wavelength excitation of a cell rather than increasing the dosage of the dye or the power of the laser.

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Introduction

Quantum yield (Φ_F) is the ratio of the number of emitted photons to the number of absorbed photons per unit time,¹ and it is a major parameter for fluorescent materials that indicates the efficiency of the fluorescence process. Thus, improvement of the quantum yield for fluorescent materials has desirable effects, such as more sensitive chemical detection and analysis,^2,3 brighter fluorescence imaging for cells,^4,5 and higher efficiency utilization of solar energy.⁶ In recent decades, 2-pyrazoline and its derivatives have been frequently used as pharmaceutical intermediates^7,8 and biological fluorescence dyestuffs^9–12 because of their considerable biological activities, including antimicrobial, antifungal, anti-inflammatory, analgesic, antidepressant, and anticancer activities.^13–18 To increase its quantum yield, much effort has been made to expand the conjugated structure by utilizing reactants containing conjugated rings or by adopting tedious synthesis steps.¹⁰ However, Jia et al. increased the fluorescence intensity and quantum yield of 2-pyrazoline by oligomerizing the 2-pyrazoline monomer.¹⁹ The fluorescence intensity was enhanced significantly, but the quantum yield of the oligomers remained relatively low. Thus, there is a significant need to develop a facile and low-cost method for the preparation of pyrazoline derivatives with a high quantum yield.

Two-way conversion luminescence (TWCL) is the combination of down-conversion (DC) and up-conversion (UC) luminescence properties by which ultraviolet (UV) light and near-infrared (IR) light can be converted to visible light. Hence, the UV and IR portions of sunlight can be converted to the visible spectral region, and by increasing the intensity of the visible spectrum using TWCL materials, the utilization efficiency of solar energy for visible light-absorbing solar cells may be improved. However, the reported conversion rates of TWCL materials are very low.^20,21 Thus, improving the quantum yield of TWCL materials is extremely urgent for a number of applications. Here, we develop 1,4-butanediol-bis(5-carbonyl-3-carbethoxy-2-pyrazoline) (BBP), with a quantum yield of 0.0573 as measured with the method of Williams and Winfield.²² After polymerization (Scheme 1), the quantum yield improved to 0.5184 under optimized conditions. Compared to other reported pyrazolines, the oligomers that were polymerized from BBP have an azo group inserted in the rigid pyrazoline ring, and the facilitated folding geometry provides an excellent TWCL performance.

Scheme 1 The whole synthesis process of the oligomers.

Experimental section

Materials

Ethyl diazoacetate (95+%) was purchased from Energy Chemical. Tin powder and cobalt oxide were purchased from Aladdin Industrial Corporation, and 1,4-butanediol diacrylate (99+%, reactive esters) with 50–105 ppm of hydroquinone was purchased from Alfa Aesar. Rhodium(II) acetate dimer was purchased from Adamas Reagent Co., Ltd. Other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

Methods

¹H NMR and ¹³C NMR spectra were recorded using a Mercury VX-300 spectrometer (300 MHz) using CDCl₃ as the solvent and TMS as the internal standard. The number-average molecular weights (M_n) and polydispersity indices (PDI, M_w/M_n) of the oligomer samples were determined using gel permeation chromatography (GPC) that was calibrated with polystyrene standards, and tetrahydrofuran (THF) was used as the eluent (1.0 mL min⁻¹) at 30 °C (both the columns and detector). Fourier transform infrared (FT-IR) spectra were recorded using a Thermo iS10 spectrometer. Elemental analysis data were collected using a Vario EL instrument. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) spectra were obtained using a Bruker Biflex III mass spectrometer equipped with a 337 nm nitrogen laser. The melting point and glass transition temperature data were collected using a Q20 (TA Instruments), with nitrogen as the protecting gas (50 mL min⁻¹). Fluorescence emission spectra were recorded on an RF-5301PC (Shimadzu) fluorescence spectrophotometer with a 150 W Xe lamp as the light source. Ultraviolet-visible (UV-vis) spectra were obtained using a UV-3600 (Shimadzu) spectrometer. Confocal fluorescence micrographs were recorded using confocal laser scanning microscopy (CLSM; Nikon C1-si, Japan, BD Laser) at 405 nm, 760 nm, and 405 + 760 nm.

Preparation of BBP

In a double-necked flask (250 mL) fitted with a stirrer and a thermometer, we placed 1,4-butanediol diacrylate (1.9822 g, 0.01 mol), EDA (1.1440 g, 0.01 mol), and ethyl acetate (20 mL) and stirred the mixture for 24 h at 30 °C. Precipitation occurred as the temperature of the mixture was adjusted to 0 °C, and centrifugation collected the solid product. Mp 100 °C. ¹H NMR (300 MHz, CDCl₃, TMS): δ 6.75 (s, 2H, NH), 4.50–4.45 (q, J = 5.0 Hz, CH, 2H), 4.34–4.21 (m, OCH₂, 8H), 3.36–3.16 (m, CH₂, 4H), 1.80 (t, J = 2.25 Hz, CH₂, 4H), 1.38–1.29 ppm (m, CH₃, 6H); ¹³C NMR (75 MHz, CDCl₃, TMS): δ 171.83 (COO), 162.24, 143.09 (C=N), 65.71 (CH–NH), 64.81, 62.43 (CH₂O), 61.67, 34.99 (CH₂–C=N), 25.36, 14.51 ppm (CH₃); FT-IR (KBr): ν = 3354 (m; ν_N–H), 2922 (s; νs_CH3), 2947 (s; νs_CH2), 1732 (s; ν_C=O), 1648 (m; ν_C=N), 1570 cm⁻¹ (w; δ_N–H); HRMS (ESI) m/z: [M + H]⁺ calcd for C₁₈H₂₇N₄O₈, 427.4; found, 427.0. Elemental analysis: calcd for C₁₈H₂₆N₄O₈, C 50.70, H 6.15, N 13.14; found, C 50.92, H 6.11, N 13.28%.

General procedure for preparation of oligomers

Approximately 10.2 mg of Al(OCH(CH₃)₂)₃ (10 mol%) was added to 213.2 mg BBP, and the mixture was heated to 160 °C while stirring for 22 h at a reduced pressure of 500 Pa. The product was dissolved in 2 mL of THF, and the catalyst was removed via filtration. Re-precipitation with 50 mL of petroleum ether and centrifugation yielded the oligomers, which were subsequently dried under vacuum.

Cell culture and cell imaging

Human cervical carcinoma (HeLa) cells and African green monkey kidney (Cos7) cells were purchased from China Center for Type Culture Collection and incubated in Dulbecco’s Modified Eagle’s Medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% antibiotics at 5% CO₂ and 37 °C. The cells were seeded in a 6-well plate at a density of 3 × 10⁵ cells per well and cultured in 1 mL of DMEM with 10% FBS for 24 h. Next, a solution of the oligomer (run 11, 50 μg) was dissolved in DMEM (1 mL), and 10% FBS, 1% antibiotics and 5% dimethyl sulfoxide were added. The solution was incubated for another 4 h at 37 °C. The cells were then washed with PBS (1 mL) upon medium removal, and CLSM was performed at 405 nm, 760 nm, and 405 + 760 nm.

Results and discussion

Design thinking of the oligomers

Because of the special spatial configuration of folded GFP, it forms specific contacts with 4-hydroxybenzylidene imidazolinone (HBI), which is the fluorophore of GFP, and thereby limits the intramolecular motion of HBI, making fluorescence the major pathway available for dissipating the energy of the excited state fluorophore.^23,24 Hence, the HBI chromophore of GFP exhibits a strong fluorescence property. Based on the mechanism of GFP fluorescence, Jia et al. synthesized oligomers using 2-pyrazoline monomers, and the oligomers displayed a 100-fold enhanced fluorescence intensity compared with monomeric 2-pyrazoline. However, the fluorescence intensity of monocyclic 2-pyrazoline is very weak, and the oligomers that are polymerized from monocyclic 2-pyrazoline lack a flexible group that prevents the formation of a folding conformation in the oligomers. Thus, the fluorescence intensity of the oligomers was significantly enhanced, but without a high quantum yield. We designed a monomer BBP that has two pyrazoline rings and good fluorescence properties. Additionally, it bears a methylene chain (–CH₂–CH₂–CH₂–CH₂–), which acts as a flexible group that allows the formation of a complex and an easily folding spatial configuration when it is polymerized.

Data analysis of different catalyzed polymerization

Six metals and metal compounds were used to catalyze the polymerization of BBP (Table 1). The M_n values of most of the polymerization products ranged from 401 to 1756 g mol⁻¹, which indicates that the polymerization products were oligomers. For polymerization without a catalyst, temperature had a facilitative effect on the molecular weight under reduced pressure, as shown by a comparison of runs 10 and 11. Although the molecular weights of runs 10 and 11 were lower than those of the others, their quantum yields (47.91% and 51.84%, respectively) were very high. For aluminum isopropoxide (Al(OCH(CH₃)₂)₃), a higher M_n was achieved with 10% catalyst, and reduced pressure was beneficial for the high molecular weight, as shown by comparing runs 4 and 5. Among the catalysts that we employed, Sn showed advantages over the others in that it gave a higher yield (73%), while Co₃O₄ showed advantages with respect to a higher quantum yield. The nitrogen content of the oligomers ranged from 4.12 to 8.00%, as determined by elemental analysis. These oligomers exhibited a reverse glass transition temperature between 85.46 and 123.63 °C (Fig. S1†).

Table 1 The oligomerization of BBP^a

Run	Catalyst	[Cat.]/[M]^b	Yield^c (%)	Mn^d (g mol^−1)	Mw^d (g mol^−1)	PDI^d	Elemental analysis (%)			Quantum yield^e (%)
							C	H	N
a At 160 °C for 22 h at a reduced pressure of 500 Pa.b [Cat.]/[M]: the molar ratio between catalyst and monomer.c Yield = (the weight of oligomer after re-precipitation once)/(the weight of monomer) × 100%.d Mn, Mw, and PDI (Mw/Mn) were obtained by GPC calibration using standard polystyrenes in THF.e The quantum yield (QY) is measured with quinine sulfate as the standard (0.1 M H2SO4 at 22 °C, QY = 58% with excitation at 350 nm), using the method of Williams and Winfield.f At 160 °C for 22 h at atmospheric pressure.g At 150 °C for 22 h at a reduced pressure of 500 Pa without catalyst.h At 160 °C for 22 h at a reduced pressure of 500 Pa without catalyst.
1	Rh2(OAc)4	0.06	35.6	920	2020	2.18	52.69	5.87	4.74	10.17
2	PdCl2	0.06	9.80	960	1680	1.76	53.65	5.89	4.26	16.01
3	Sn	0.12	73.0	650	980	1.51	51.57	5.64	8.00	22.02
4	Al(OCH(CH3)2)3	0.10	51.9	1760	13 130	7.47	53.20	5.82	6.38	19.72
5^f	Al(OCH(CH3)2)3	0.10	68.9	1040	4260	4.12	53.87	5.70	6.34	17.76
6^f	Al(OCH(CH3)2)3	0.06	59.5	1330	6050	4.45	53.80	5.52	6.61	30.62
7^f	Al(OCH(CH3)2)3	0.16	72.4	1420	8780	6.16	53.45	5.84	5.94	10.02
8	Zn	0.18	13.5	680	1290	1.81	54.85	5.67	5.31	25.42
9	Co3O4	0.06	10.5	820	1370	1.67	56.15	6.04	4.22	32.10
10^g	—	—	62.9	400	480	1.18	57.21	6.88	4.16	47.91
11^h	—	—	65.4	770	1250	1.62	55.40	6.24	4.12	51.84

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The characterization of the resulting oligomers

Nuclear magnetic resonance (NMR) and FT-IR spectra. The transformation from BBP to oligopyrazoline was confirmed by ¹H NMR, ¹³C NMR, FT-IR (Fig. 1, S2, and S3†), and MALDI-TOF-MS (Fig. 2). As shown in Fig. 1a, the peak in the BBP spectrum at δ 6.75 ppm disappeared completely, and new peaks representing the oligomer spectrum appeared at δ 7.07–6.95 and 5.91–5.96 ppm, indicating that the N–H partly reacts with another group and that the peaks of the remaining N–H groups were largely unchanged. Because of the oligomerization, the multiple discrete peaks were transformed into a single broad peak. The FT-IR spectrum of the obtained oligomer is shown in Fig. 1b, and the weak broad bands at 3358 cm⁻¹ are attributed to N–H, which was weakened compared with that for BBP. The bands near 2980–2850 cm⁻¹ are attributed to CH₃, CH₂, and CH stretching vibrations. The bands near 1725 and 1174 cm⁻¹ are characteristic of ester groups, and the bands near 1660 cm⁻¹ are attributed to the C=N stretching vibration.^25,26 As shown in Fig. 1c, the peaks near δ 14.09 ppm indicate the methyl group (CH₃). The peaks near δ 60.35–61.04 ppm indicate the methylene groups (OCH₂) that are adjacent to the oxygen atom. The peaks near δ 64.51–63.77 ppm are attributed to the methane group (CH) that is connected with the nitrogen atom. The peaks at δ 139.27–139.90 ppm are ascribed to the C=N group,²⁵ and the peaks near δ 165.63, 169.69, and 171.67 ppm are assigned to carbonyl groups.

Fig. 1 NMR and FT-IR characteristics of the monomer and the oligomer. (a) The ¹H NMR spectrum, (b) the FT-IR spectrum, and (c) the ¹³C NMR spectrum of BBP (black line) and the oligomer (run 11, red line).

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). The primary peaks in the MALDI-TOF-MS spectrum can be sorted in the following series: Na⁺ + 380 × 2 + 114 + 168n (n = 0–5), K⁺ + 380 × 2 + 114 + 168n (n = 0–5), and H⁺ + 380 × 2 + 168n (n = 1–6). The main repeated m/z difference in the spectrum of the polymerized product from the Al(OCH(CH₃)₂)₃-catalyzed reaction was the peak at m/z 168 (Fig. 2, series A1 to A6, B1 to B6, and C1 to C6), which is half the mass of de-butanediolized BBP, as well as the run 11 product (Fig. S4†). Additionally, the mass of de-ethanolized BBP is 380, and the mass of dimeric BBP (de-diethanolized) is 760. The deviation of the calculated segment mass from the measured m/z for series A and B is 114 (for series C, it is 0), which is the mass of ethyl diazoacetate (EDA). This segment originates from the ring-opened 2-pyrazoline, revealing that there are no end groups in the oligomers and that they must be cyclic.²⁷ Thus, the repeat subunit in the oligomer that is catalyzed by Al(OCH(CH₃)₂)₃ powder is de-butanediolized BBP, which is cyclized with 2 de-ethanolized BBP molecules and 1 EDA molecule. Moreover, the proposed mechanism of the oligomerization has been reported previously by us.¹⁹

Fig. 2 MALDI-TOF-MS spectra of the oligomer (run 4) catalyzed by Al(OCH(CH₃)₂)₃.

UV-vis absorption and fluorescence properties

The ultraviolet-visible (UV-vis) spectra of the obtained oligomers in Fig. 3 were recorded in chloroform (CHCl₃) with a concentration of 0.05 mg mL⁻¹ at room temperature. The absorption intensity of each oligomer was significantly different from the absorption bands in the range of 250–350 nm, which were assigned to the π–π* transition and n–π* transition that are localized on the pyrazoline ring system.²⁸ Combining the DC fluorescence spectra (Fig. 3b) with the UV-vis spectra, we found that at very low concentrations, the UV absorption of the oligomers (run 10 and 11) was extremely weak, near 0.05. However, the fluorescence was strongly increased, indicating that the absorbed photons were mainly converted to emitted fluorescence.

Fig. 3 UV-vis absorption and fluorescence properties of the oligomers. (a) UV-vis spectra of oligomers (run 3, 6, 8, 9, 10, and 11) with 0.05 mg mL⁻¹ in CHCl₃. (b) The down-conversion emission spectra of BBP and oligomers (run 3, 6, 8, 9, 10, and 11) with 0.08 mg mL⁻¹ in CHCl₃. (c) The up-conversion emission spectra of BBP and oligomers (run 3, 6, 8, 9, 10, and 11) with 0.08 mg mL⁻¹ in CHCl₃. (d) I/N of BBP and obtained oligomers (run 3, 6, 8, 9, 10, and 11) at 0.08 mg mL⁻¹ in CHCl₃. The down-conversion excitation wavelengths of run 3, 6, 8, 9, 10, and 11 are 386, 358, 387, 384, 374, and 376 nm respectively, and the up-conversion excitation wavelengths of run 3, 6, 8, 9, 10, and 11 are 741, 698, 713, 741, 703, and 704 nm respectively. I: the maximum fluorescence emission intensity; N: the concentration of nitrogen atoms, where N = (the content of nitrogen obtained by elemental analysis) × (the concentration of the solution); I/N: the maximum fluorescence emission intensity for a nitrogen atom of 1 mg mL⁻¹, named as the molecular fluorescence emission efficiency. A: absorbance; a.u.: arbitrary units.

The TWCL properties of the oligomers and BBP were elucidated, as shown in Fig. 3b and c. Under UV irradiation at approximately 370 nm and near-infrared irradiation at approximately 710 nm (the excitation spectra are provided in Fig. S5†), the fluorescence emitted by the oligomers and the emission wavelengths were the same. Additionally, a solution of the oligomers in CHCl₃ emitted a fluorescence that was red-shifted slightly, and there was a significant enhancement to the emission intensity when compared with BBP. The maximum enhancement can be calculated using the molecular fluorescence emission efficiency (defined as I/N, which is the maximum emission intensity for a nitrogen atom of 1 mg mL⁻¹), and the oligomer (run 10) showed nearly a 30-fold increase at the same concentration of 0.08 mg mL⁻¹ (Fig. 3d).

High quantum yield mechanism study

To explore the mechanism that results in the fluorescence enhancement and high quantum yield, we utilized density functional theory (DFT) at the level of B3LYP/6-31G (d, p), using GAMESS to calculate the optimized geometry (Fig. 4a) and the molecular orbitals (MOs). An oligomer that consisted of two de-ethanolized BBP molecules and one EDA was used as an example, and we found that the electrons were mainly distributed in the delocalized electronic orbitals on the backbone of the 2-pyrazoline ring system, forming a highest occupied molecular orbital (HOMO), HOMO−1, and HOMO−2 (Fig. 4b). However, the lowest unoccupied molecular orbital (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 primarily attributed to the electronic radiation transition from the conjugated N=C–C=O to the delocalized N–N=C system, and compared with BBP (Fig. S6†) the difference value of the orbital energy level for the oligomer (run 11) is less. Additionally, we speculate that the high quantum yield of the emitted fluorescence may be oligomerization-induced and related to the azo group formation. After oligomerization, the oligomers formed a GFP-mimetic structure, including a conformation that folds easily. The methylene chain (–CH₂–CH₂–CH₂–CH₂–) that was introduced by BBP acted as a flexible group that enhanced the folding property of the oligomers, and the fluorescent pyrazoline ring acted as a fluorophore that was encased within the folded conformation. These features made a significant contribution to the high quantum yield. Additionally, azo group formation was also significant to the oligomers. By comparing the ¹H NMR and ¹³C NMR spectra of the oligomer (run 11) with those of BBP, we found that the double bond of the pyrazoline ring changed. As shown in Fig. 1a, a significant double peak at 5.96 and 5.91 ppm appeared in the oligomer ¹H NMR spectrum, and another double peak at 124.68 and 124.29 ppm appeared in the oligomer ¹³C NMR spectrum. However, those NMR peaks were not present in the BBP NMR spectrum. If the oligomer is comprised of only the de-butanediolized BBP, de-ethanolized BBP, and one EDA, the existence of those NMR peaks cannot be explained. Thus, C=N has a resonance effect with the adjacent nitrogen atom (Scheme 2), as shown by the resonance effect in the ¹³C NMR spectrum in which the peaks for C=N shift from 139.83 to 124.29. Additionally, in the ¹H NMR spectrum, the peaks for the CH group that is adjacent to the nitrogen atom shift from 4.48 to 5.91 ppm. The heteronuclear single quantum correlation (HSQC) correlates the coupled heteronuclear spins across a single bond and identifies directly connected nuclei. As shown in Fig. S7,† the cross-peaks at approximately 139.27–139.90/6.95–7.06 ppm were assigned to the C=N and N–H of structures 1 or 3. The cross-peaks at approximately 124.29–124.68/5.90–5.95 ppm were assigned to the N=N and C–H of structure 2. As supported by the HSQC spectrum, we reasonably concluded that the azo groups exist in the oligomer. Additionally, the inter-system crossing of the azo group was suppressed by the rigid pyrazoline ring and the structure of the oligomer, resulting in the enhancement of the fluorescence property of the azo group^29–31 and also contributing substantially to the high quantum yield.

Fig. 4 (a) The optimized geometry and (b) molecular orbitals of the oligomer (run 11) comprised of 2 de-ethanolized BBP molecules and 1 EDA molecule.

Scheme 2 The C=N–NH and N=N–CH resonance transition of the pyrazoline ring.

Cell experiment

We conducted experiments with living cells to evaluate in vivo fluorescence images with the obtained oligomers. As shown in Fig. 5, we observed strong fluorescence in HeLa cells imaged using single-laser excitation at 405 nm (Fig. 5b), two-photon laser excitation at 760 nm (Fig. 5c), and multi-photon laser excitation at 405 + 760 nm (Fig. 5d). Additionally, we clearly observed that the luminescence in Fig. 5d was brighter than that in Fig. 5b or Fig. 5c; the same phenomenon appeared in Cos7 cell imaging (Fig. 5f–h), indicating that the TWCL property of the oligomer provides a simple way to enhance luminescence for cell imaging. This method is an improvement over increasing the dye dosage or the laser power because it is less harmful to living cells. Meanwhile, the oligomer shows no obvious cytotoxicity (Fig. S8†), so is capable of usage in bioimaging, especially for multi-photon excitation at UV-vis and near-infrared light. Thus, the oligomers are great candidates for application in fluorescence imaging and probes.

Fig. 5 Confocal images of living cells in the presence of the oligomer solution. (a to d) Fluorescence imaging of HeLa cells after being incubated with the oligomer (run 11) for 4 h (multi-photon laser scanning confocal microscopy). (a) Bright field, (b) excited at 405 nm, (c) excited at 760 nm, and (d) excited at 405 nm + 760 nm. (e to h) Fluorescence imaging of Cos7 cells after being incubated with the oligomer (run 11) for 4 h. (e) Bright field, (f) excited at 405 nm, (g) excited at 760 nm, and (h) excited at 405 nm + 760 nm.

Conclusions

In summary, the oligomer from the cyclopolycondensation of 1,4-butanediol-bis(5-carbonyl-3-carbethoxy-2-pyrazoline) is a new cyclic N-hetero-macromolecule that exhibits a significant fluorescence enhancement compared with 1,4-butanediol-bis(5-carbonyl-3-carbethoxy-2-pyrazoline) itself, which comes from the folding effect of the flexible group –CH₂–CH₂–CH₂–CH₂– and the resonance effect of the azo group in the rigid pyrazoline ring. The advantage of the oligomer in live-cell imaging is that the high-quality images are achieved conveniently by its two-way conversion fluorescence, which is also potentially attractive for solar harvesting.

Acknowledgements

We acknowledge the National Natural Science Foundation of China (no. 21074097 and 21274112) for supporting this project. We thank Jinlong Wu (Wuhan University) and Wuhan National Laboratory for Optoelectronics for assistance with the cell culture and cell imaging.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The differential scanning calorimetry curves of the oligomers; additional H¹ NMR spectrum and FT-IR spectrum of the oligomers; HSQC spectrum of the oligomer. See DOI: 10.1039/c6ra15007a

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