# Thiophene or Pyridine-Substituted Quinoline Derivatives: Synthesis, Properties, and Electropolymerization for Energy Storage

Authors: Dan Wang; Qi Yao; Tianfen Shao; Zhenxin Wang; Yangmin Ma; Chao Wang
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Designing novel electropolymerizable monomers is essential in developing electron donor-electron acceptor (D-A) type conjugated polymers for applications in sensing, catalysis, and energy storage. Thiophene and pyridine-substituted quinoline-based molecules (Th-Q, DTh-Q, and Py-Q) are synthesized in this work, and their crystal structures, optical properties, and electrochemical properties are investigated. The potentiostatic electropolymerization of these molecules are successfully carried out on carbon cloth substrate, with the thiophene and quinoline as the possible electropolymerizable units. The PTh-Q shows the best charge storage performance (1.12×10−3 C at 0.02 mA cm-2 in the potential range of 0-1 V) in 0.1 M Na2SO4, and maintains 77% of the initial capacity after 3000 charge-discharge cycles at 0.02 mA cm-2. The morphology and structure of the PTh-Q does not alter significantly after the repetitive charge-discharge cycling. The superior charge storage performance of the PTh-Q than PDTh-Q and PPy-Q is originated from the exposed highest amount of electrochemically active sites and the participation of redox processes of both hydroxyl/ketone structures on quinoline and thiophene groups.


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## Body

## 1. Introduction

Energy storage technologies are crucial in the utilization of energy harvested from the renewable sources. Supercapacitors store energy based on charge separation on the electro-electrolyte double layer (EDLC), or based on the redox process of the electrode material (pseudocapacitors and battery-type capacitors) [1]. Compared to EDLC where carbon-based materials [2] are frequently adopted, pseudocapacitors and battery-type capacitors usually employ redox active species, including metal oxides [3, 4] and conducting polymers as electrode material and generally show larger capacitances [5]. Among them, conducting polymers feature flexibility and easily-tailored redox properties by functionalization [6], and are utilized as electrode materials to effectively tune the achievable voltage of supercapacitors [7, 8].Electron donor (D)-electron acceptor (A) type conjugated polymers have gained increasing interests as the electrode material for supercapacitors. Owing to the tunable interaction of donor and acceptor groups, the redox properties of the polymer can be effectively tuned, and the polymer can be both p-doping and n-doping. Therefore, when a D-A polymer is used as the electrode material for supercapacitors, the potential window and energy density can be enhanced compared to other conjugated polymers [9]. In addition, D-A polymers are conductive in nature, and intramolecular charge transfer between electron donors and electron acceptors as well as intermolecular interactions between polymer chains brings more ordered stacking to increase conductivity [9].Polythiophene, as a type of conductive polymer, features highly reversible doping/dedoping process, coupled with good stability as the electrode material for supercapacitors [10]. Substituting thiophene unit at 3-position with phenyl, alkoxyl, and ethyl groups has been reported to enhance the stability [11]. Further enhancement in the stability has been achieved through adding electron withdrawing groups to these substituents, like poly(3,4-ethylenedioxythiophene) [12] and poly(3-(4-fluorophenyl)thiophene) [13]. Therefore, structural design of the thiophene-based molecules as monomer offers the route to prepare stable substituted polythiophene as electrode material for supercapacitors. We designed and synthesized the mono- and dithiophene substituted quinoline-based molecules as monomers for D-A type polymer. The thiophen groups serve as the electron donor, while the quinoline as the acceptor [9, 14]. Pyridine substituted quinoline-based molecules was also synthesized. The crystal structures, optical properties, and redox properties of these novel molecules are systematically investigated. Electropolymerization of these molecules was carried out to directly form D-A polymers on conductive electrodes. The redox properties and charge storage abilities of these polymers in aqueous neutral solutions are also investigated.


## 2. Experimental

### 2.1. Materials and Apparatus

All reagents and chemicals were purchased from Energy Chemical, Klamar or Macklin Biochemical Co., Ltd., and were used directly as received.1H NMR and 13C NMR spectra were acquired on a Bruker Ascend 400 (400 MHz) spectrometer. FT-IR spectra were obtained on a Bruker INVENIO Fourier transform infrared spectrometer using KBr pellets. ESI-HRMS were acquired on a Xevo G2-XS Qtof high-resolution mass spectrometry. UV-Vis absorption spectra were recorded on an UV-2600 spectrophotometer. All electrochemical tests were carried out using a CHI660E electrochemical workstation. The morphologies and elemental distributions of films were captured using a ZEISS Gemini 300 field emission scanning electron microscope (SEM) equipped with an Oxford Xplore EDS detector. XPS measurements were performed on a Thermo Fisher Scientific ESCALAB 250Xi X-ray photoelectron spectrometer.


### 2.2. Synthesis

The detailed synthetic routes of pyridine and thiophene-substituted quinoline derivatives (Py-Q, Th-Q, and DTh-Q) are outlined in Scheme1. All of them were synthesized by Suzuki coupling reaction between bromo-substituted intermediates with commercially available aromatic boronic acids under the catalysis of Pd(PPh3)4. The synthetic procedures of intermediates (compounds 1–4) shown in Scheme 1 are provided in Supporting Information. The synthesis details of target products (Py-Q, Th-Q, and DTh-Q) are described as follows. Although it requires three steps to synthesize Py-Q, Th-Q, and DTh-Q, the yield of each step approximately reaches 90%, which could result in high ultimate yields. In addition, the ingredients are commercially available and cheap, the synthetic routs have the characteristics of mild reaction conditions and convenient operations. Therefore, it is possible to produce the three compounds in large scales with low preparation costs.Scheme 1
Synthetic routes of Py-Q, Th-Q, and DTh-Q.


#### 2.2.1. Synthesis of Compound Py-Q

2-Methyl-5-bromo-8-methoxyquinoline (5.0 mmol, 1.255 g), 4-pyridine boric acid (8.0 mmol, 0.983 g), K2CO3 (15.0 mmol, 2.073 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) were mixed in 40 mL DME and 20 mL H2O under the protection of N2. The mixture was stirred and heated at 95°C overnight. After cooling to room temperature, the reaction mixture was extracted with DCM. The organic layer was dried (MgSO4) and concentrated in vacuum. The crude product was purified by column chromatography using DCM/methanol 60 : 1 as eluent to give Py-Q (1.126 g, yield: 90%). The Py-Q crystals suitable for X-ray single-crystal diffraction were obtained by cooling recrystallization in DMSO. 1H NMR (400 MHz, DMSO-d6) (Figure S1(a)) δ 8.71 (d, J=4.8 Hz, 2 H), 8.09 (d, J=8.8 Hz, 1 H), 7.49–7.44 (m, 4 H), 7.27 (d, J=8.0 Hz, 1 H), 4.02 (s, 3 H), 2.68 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S1(b)) δ 157.79, 155.57, 150.22, 147.13, 139.61, 133.44, 128.84, 127.27, 125.24, 124.81, 123.35, 108.46, 56.12, 25.13 ppm. IR (KBr pellet, cm-1): 3024, 2921, 1595, 1470, 1100, 820, 709. HRMS (ESI) (Figure S2): m/z [M+H]+ Calcd. for C16H14N2O: 251.11843, Found: 251.11780.


#### 2.2.2. Synthesis of Compound Th-Q

Th-Q was prepared by the same method as the synthesis of Py-Q, using 2-methyl-5-bromo-8-methoxyquinoline (5.0 mmol, 1.260 g), 3-thiopheneboronic acid (6.0 mmol, 0.767 g), K2CO3 (15.0 mmol, 2.073 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) as the reagents. The crude product was purified by silica gel column chromatography using DCM as eluent to afford Th-Q (1.149 g, yield: 90%). The obtained Th-Q was recrystallized from ethyl acetate, affording faint yellow flake crystals suitable for X-ray single-crystal diffraction. 1H NMR (400 MHz, DMSO-d6) (Figure S3(a)) δ 8.23 (d, J=8.0 Hz, 1 H), 7.74 (s, J=4.0 Hz, 1 H), 7.64 (s, 1 H), 7.47 (t, J=8.0 Hz, 2 H), 7.33 (d, J=4.0 Hz, 1 H), 7.23 (d, J=8.0 Hz, 1 H), 4.02 (s, 3 H), 2.69 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S3(b)) δ 157.45, 154.68, 139.91, 139.61, 134.12, 129.80, 126.83, 126.79, 126.52, 125.65, 124.09, 123.04, 108.56, 56.08, 25.14 ppm. IR (KBr pellet, cm-1): 3090, 2935, 2830, 1562, 1465, 1310, 1100, 790. HRMS (ESI) (Figure S4): m/z [M+H]+ Calcd. for C15H13NOS: 256.07960, Found: 256.07886.


#### 2.2.3. Synthesis of Compound DTh-Q

DTh-Q was also synthesized by the same method as the synthesis of Py-Q, using 2-methyl-5,7-dibromo-8-methoxyquinoline (5.0 mmol, 1.660 g), 3-thiopheneboronic acid (15.0 mmol, 1.919 g), K2CO3 (37.0 mmol, 5.106 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) as the reagents. The crude product was purified by silica gel column chromatography using DCM/methanol 60 : 1 as eluent to give DTh-Q (1.468 g, yield: 87%). The obtained DTh-Q was recrystallized from ethyl acetate, affording yellow flake crystals suitable for X-ray single-crystal diffraction. 1H NMR (400 MHz, DMSO-d6) (Figure S5(a)) δ 8.24 (d, J=8.4 Hz, 1 H), 8.07 (d, J=2.8 Hz, 1 H), 7.78–7.75 (m, 3 H), 7.73 (d, J=5.2 Hz, 1 H), 7.69 (dd, J=4.8, 2.8 Hz, 1 H), 7.45 (d, J=8.8 Hz, 1 H), 7.42 (dd, J=4.4, 1.6 Hz, 1 H), 4.07 (s, 3 H), 2.74 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S5(b)) δ 158.30, 151.66, 142.97, 139.56, 138.04, 134.49, 130.35, 129.82, 128.98, 127.66, 127.24, 126.96, 126.21, 125.42, 125.01, 124.82, 122.57, 61.88, 25.55 ppm. IR (KBr pellet, cm-1): 3100, 2976, 2927, 1595, 1450, 1350, 1237, 1075, 840, 790. HRMS (ESI) (Figure S6): m/z [M+H]+ Calcd. for C19H15NOS2: 338.06732, Found: 338.06702.


### 2.3. Molecular Structure Determination

Crystallographic data were acquired on a Bruker D8 Venture diffractometer (Germany) with graphite monochromated Mo-Kα radiation (λ=0.71073 Å) at 193 K for Py-Q, Th-Q, and DTh-Q. The structures were solved by the direct methods and refined by full matrix least squares technique using the SHELX-2018/3 program [15, 16] in the Olex2 package [17]. X-ray crystallographic data for Py-Q, Th-Q, and DTh-Q are summarized in Table 1, the main bond lengths and bond angles for Py-Q, Th-Q, and DTh-Q are listed in Table S1. CCDC 2195077 (Py-Q), CCDC 2195078 (Th-Q), and CCDC 2195079 (DTh-Q) contain the supplementary crystallographic data for this paper.Table 1
Crystal data and structure refinement for Py-Q, Th-Q, and DTh-Q.
Identification codePy-QTh-QDTh-QEmpirical formulaC16H14N2OC15H13NOSC19H15NOS2Formula weight250.29255.32337.44Crystal systemMonoclinicOrthorhombicMonoclinicSpace groupP21/cPbcaP21/ca/Å12.3179(8)16.7658(10)7.9619(8)b/Å14.3024(9)7.1651(6)21.603(3)c/Å7.3404(6)20.5747(16)9.9230(14)α/°909090β/°98.241(3)90106.173(4)γ/°909090Volume/Å31279.85(16)2471.6(3)1639.2(3)Z484ρcalcg/cm31.2991.3721.367μ/mm-10.0830.2480.328F (000)528.01072.0704.0Index ranges−14≤h≤14, −15≤k≤17, −6≤l≤8−22≤h≤22, −9≤k≤8, −25≤l≤27−10≤h≤10, −28≤k≤22, −13≤l≤13Reflections collected92743640716074Independent reflections2233 [Rint=0.0487, Rsigma=0.0419]3063 [Rint=0.0916, Rsigma=0.0364]4051 [Rint=0.0445, Rsigma=0.0384]Data/restraints/parameters2233/0/1743063/80/2024051/122/284Goodness-of-fit onF21.0521.0421.022Final R indexes [I>=2σI]R1=0.0674, wR2=0.1652R1=0.0399, wR2=0.0959R1=0.0468, wR2=0.1283Final R indexes [all data]R1=0.0788, wR2=0.1739R1=0.0543, wR2=0.1096R1=0.0699, wR2=0.1472Largest diff. Peak/hole/e Å-30.79/-0.230.27/-0.200.25/-0.24CCDC number219507721950782195079R1=ΣFo−Fc/Fo; wR2=ΣwFo2−Fc22/ΣwFo221/2.


### 2.4. Electrochemistry

The electrochemical properties of the Py-Q, Th-Q, and DTh-Q are investigated in DCM, MeCN, and DMSO solutions with 0.1 M NBu4PF6 as the supporting electrolyte. The working electrode is a glassy carbon (GC) electrode with 3 mm in diameter. A silver wire was used as the pseudo reference electrode and a cleaned graphite rod as the counter. The ferrocene (Fc+/Fc) was used as the internal standard for potential calibration of the Ag wire. Electropolymerization of the Py-Q, Th-Q, and DTh-Q are carried out in DCM with 0.1 M NBu4PF6 at 1.20 V (vs. Fc+/Fc) for 600 s. The carbon cloth (CC) was used as the substrate for electropolymerization, and the area immersed in the electrolyte is controlled to 1 cm2. The electropolymerized Py-Q, Th-Q, and DTh-Q on the CC substrate are named as PPy-Q, PTh-Q, and PDTh-Q, respectively. The charge storage abilities are measured in 0.1 M Na2SO4, with a saturated calomel electrode (SCE) as the reference, and a graphite rod as the counter. The potentials are reported relative to SCE unless otherwise stated.


## 2.1. Materials and Apparatus

All reagents and chemicals were purchased from Energy Chemical, Klamar or Macklin Biochemical Co., Ltd., and were used directly as received.1H NMR and 13C NMR spectra were acquired on a Bruker Ascend 400 (400 MHz) spectrometer. FT-IR spectra were obtained on a Bruker INVENIO Fourier transform infrared spectrometer using KBr pellets. ESI-HRMS were acquired on a Xevo G2-XS Qtof high-resolution mass spectrometry. UV-Vis absorption spectra were recorded on an UV-2600 spectrophotometer. All electrochemical tests were carried out using a CHI660E electrochemical workstation. The morphologies and elemental distributions of films were captured using a ZEISS Gemini 300 field emission scanning electron microscope (SEM) equipped with an Oxford Xplore EDS detector. XPS measurements were performed on a Thermo Fisher Scientific ESCALAB 250Xi X-ray photoelectron spectrometer.


## 2.2. Synthesis

The detailed synthetic routes of pyridine and thiophene-substituted quinoline derivatives (Py-Q, Th-Q, and DTh-Q) are outlined in Scheme1. All of them were synthesized by Suzuki coupling reaction between bromo-substituted intermediates with commercially available aromatic boronic acids under the catalysis of Pd(PPh3)4. The synthetic procedures of intermediates (compounds 1–4) shown in Scheme 1 are provided in Supporting Information. The synthesis details of target products (Py-Q, Th-Q, and DTh-Q) are described as follows. Although it requires three steps to synthesize Py-Q, Th-Q, and DTh-Q, the yield of each step approximately reaches 90%, which could result in high ultimate yields. In addition, the ingredients are commercially available and cheap, the synthetic routs have the characteristics of mild reaction conditions and convenient operations. Therefore, it is possible to produce the three compounds in large scales with low preparation costs.Scheme 1
Synthetic routes of Py-Q, Th-Q, and DTh-Q.


### 2.2.1. Synthesis of Compound Py-Q

2-Methyl-5-bromo-8-methoxyquinoline (5.0 mmol, 1.255 g), 4-pyridine boric acid (8.0 mmol, 0.983 g), K2CO3 (15.0 mmol, 2.073 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) were mixed in 40 mL DME and 20 mL H2O under the protection of N2. The mixture was stirred and heated at 95°C overnight. After cooling to room temperature, the reaction mixture was extracted with DCM. The organic layer was dried (MgSO4) and concentrated in vacuum. The crude product was purified by column chromatography using DCM/methanol 60 : 1 as eluent to give Py-Q (1.126 g, yield: 90%). The Py-Q crystals suitable for X-ray single-crystal diffraction were obtained by cooling recrystallization in DMSO. 1H NMR (400 MHz, DMSO-d6) (Figure S1(a)) δ 8.71 (d, J=4.8 Hz, 2 H), 8.09 (d, J=8.8 Hz, 1 H), 7.49–7.44 (m, 4 H), 7.27 (d, J=8.0 Hz, 1 H), 4.02 (s, 3 H), 2.68 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S1(b)) δ 157.79, 155.57, 150.22, 147.13, 139.61, 133.44, 128.84, 127.27, 125.24, 124.81, 123.35, 108.46, 56.12, 25.13 ppm. IR (KBr pellet, cm-1): 3024, 2921, 1595, 1470, 1100, 820, 709. HRMS (ESI) (Figure S2): m/z [M+H]+ Calcd. for C16H14N2O: 251.11843, Found: 251.11780.


### 2.2.2. Synthesis of Compound Th-Q

Th-Q was prepared by the same method as the synthesis of Py-Q, using 2-methyl-5-bromo-8-methoxyquinoline (5.0 mmol, 1.260 g), 3-thiopheneboronic acid (6.0 mmol, 0.767 g), K2CO3 (15.0 mmol, 2.073 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) as the reagents. The crude product was purified by silica gel column chromatography using DCM as eluent to afford Th-Q (1.149 g, yield: 90%). The obtained Th-Q was recrystallized from ethyl acetate, affording faint yellow flake crystals suitable for X-ray single-crystal diffraction. 1H NMR (400 MHz, DMSO-d6) (Figure S3(a)) δ 8.23 (d, J=8.0 Hz, 1 H), 7.74 (s, J=4.0 Hz, 1 H), 7.64 (s, 1 H), 7.47 (t, J=8.0 Hz, 2 H), 7.33 (d, J=4.0 Hz, 1 H), 7.23 (d, J=8.0 Hz, 1 H), 4.02 (s, 3 H), 2.69 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S3(b)) δ 157.45, 154.68, 139.91, 139.61, 134.12, 129.80, 126.83, 126.79, 126.52, 125.65, 124.09, 123.04, 108.56, 56.08, 25.14 ppm. IR (KBr pellet, cm-1): 3090, 2935, 2830, 1562, 1465, 1310, 1100, 790. HRMS (ESI) (Figure S4): m/z [M+H]+ Calcd. for C15H13NOS: 256.07960, Found: 256.07886.


### 2.2.3. Synthesis of Compound DTh-Q

DTh-Q was also synthesized by the same method as the synthesis of Py-Q, using 2-methyl-5,7-dibromo-8-methoxyquinoline (5.0 mmol, 1.660 g), 3-thiopheneboronic acid (15.0 mmol, 1.919 g), K2CO3 (37.0 mmol, 5.106 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) as the reagents. The crude product was purified by silica gel column chromatography using DCM/methanol 60 : 1 as eluent to give DTh-Q (1.468 g, yield: 87%). The obtained DTh-Q was recrystallized from ethyl acetate, affording yellow flake crystals suitable for X-ray single-crystal diffraction. 1H NMR (400 MHz, DMSO-d6) (Figure S5(a)) δ 8.24 (d, J=8.4 Hz, 1 H), 8.07 (d, J=2.8 Hz, 1 H), 7.78–7.75 (m, 3 H), 7.73 (d, J=5.2 Hz, 1 H), 7.69 (dd, J=4.8, 2.8 Hz, 1 H), 7.45 (d, J=8.8 Hz, 1 H), 7.42 (dd, J=4.4, 1.6 Hz, 1 H), 4.07 (s, 3 H), 2.74 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S5(b)) δ 158.30, 151.66, 142.97, 139.56, 138.04, 134.49, 130.35, 129.82, 128.98, 127.66, 127.24, 126.96, 126.21, 125.42, 125.01, 124.82, 122.57, 61.88, 25.55 ppm. IR (KBr pellet, cm-1): 3100, 2976, 2927, 1595, 1450, 1350, 1237, 1075, 840, 790. HRMS (ESI) (Figure S6): m/z [M+H]+ Calcd. for C19H15NOS2: 338.06732, Found: 338.06702.


## 2.2.1. Synthesis of Compound Py-Q

2-Methyl-5-bromo-8-methoxyquinoline (5.0 mmol, 1.255 g), 4-pyridine boric acid (8.0 mmol, 0.983 g), K2CO3 (15.0 mmol, 2.073 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) were mixed in 40 mL DME and 20 mL H2O under the protection of N2. The mixture was stirred and heated at 95°C overnight. After cooling to room temperature, the reaction mixture was extracted with DCM. The organic layer was dried (MgSO4) and concentrated in vacuum. The crude product was purified by column chromatography using DCM/methanol 60 : 1 as eluent to give Py-Q (1.126 g, yield: 90%). The Py-Q crystals suitable for X-ray single-crystal diffraction were obtained by cooling recrystallization in DMSO. 1H NMR (400 MHz, DMSO-d6) (Figure S1(a)) δ 8.71 (d, J=4.8 Hz, 2 H), 8.09 (d, J=8.8 Hz, 1 H), 7.49–7.44 (m, 4 H), 7.27 (d, J=8.0 Hz, 1 H), 4.02 (s, 3 H), 2.68 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S1(b)) δ 157.79, 155.57, 150.22, 147.13, 139.61, 133.44, 128.84, 127.27, 125.24, 124.81, 123.35, 108.46, 56.12, 25.13 ppm. IR (KBr pellet, cm-1): 3024, 2921, 1595, 1470, 1100, 820, 709. HRMS (ESI) (Figure S2): m/z [M+H]+ Calcd. for C16H14N2O: 251.11843, Found: 251.11780.


## 2.2.2. Synthesis of Compound Th-Q

Th-Q was prepared by the same method as the synthesis of Py-Q, using 2-methyl-5-bromo-8-methoxyquinoline (5.0 mmol, 1.260 g), 3-thiopheneboronic acid (6.0 mmol, 0.767 g), K2CO3 (15.0 mmol, 2.073 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) as the reagents. The crude product was purified by silica gel column chromatography using DCM as eluent to afford Th-Q (1.149 g, yield: 90%). The obtained Th-Q was recrystallized from ethyl acetate, affording faint yellow flake crystals suitable for X-ray single-crystal diffraction. 1H NMR (400 MHz, DMSO-d6) (Figure S3(a)) δ 8.23 (d, J=8.0 Hz, 1 H), 7.74 (s, J=4.0 Hz, 1 H), 7.64 (s, 1 H), 7.47 (t, J=8.0 Hz, 2 H), 7.33 (d, J=4.0 Hz, 1 H), 7.23 (d, J=8.0 Hz, 1 H), 4.02 (s, 3 H), 2.69 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S3(b)) δ 157.45, 154.68, 139.91, 139.61, 134.12, 129.80, 126.83, 126.79, 126.52, 125.65, 124.09, 123.04, 108.56, 56.08, 25.14 ppm. IR (KBr pellet, cm-1): 3090, 2935, 2830, 1562, 1465, 1310, 1100, 790. HRMS (ESI) (Figure S4): m/z [M+H]+ Calcd. for C15H13NOS: 256.07960, Found: 256.07886.


## 2.2.3. Synthesis of Compound DTh-Q

DTh-Q was also synthesized by the same method as the synthesis of Py-Q, using 2-methyl-5,7-dibromo-8-methoxyquinoline (5.0 mmol, 1.660 g), 3-thiopheneboronic acid (15.0 mmol, 1.919 g), K2CO3 (37.0 mmol, 5.106 g), and Pd(PPh3)4 (0.02 mmol, 0.024 g) as the reagents. The crude product was purified by silica gel column chromatography using DCM/methanol 60 : 1 as eluent to give DTh-Q (1.468 g, yield: 87%). The obtained DTh-Q was recrystallized from ethyl acetate, affording yellow flake crystals suitable for X-ray single-crystal diffraction. 1H NMR (400 MHz, DMSO-d6) (Figure S5(a)) δ 8.24 (d, J=8.4 Hz, 1 H), 8.07 (d, J=2.8 Hz, 1 H), 7.78–7.75 (m, 3 H), 7.73 (d, J=5.2 Hz, 1 H), 7.69 (dd, J=4.8, 2.8 Hz, 1 H), 7.45 (d, J=8.8 Hz, 1 H), 7.42 (dd, J=4.4, 1.6 Hz, 1 H), 4.07 (s, 3 H), 2.74 (s, 3 H) ppm. 13C NMR (100 MHz, DMSO-d6) (Figure S5(b)) δ 158.30, 151.66, 142.97, 139.56, 138.04, 134.49, 130.35, 129.82, 128.98, 127.66, 127.24, 126.96, 126.21, 125.42, 125.01, 124.82, 122.57, 61.88, 25.55 ppm. IR (KBr pellet, cm-1): 3100, 2976, 2927, 1595, 1450, 1350, 1237, 1075, 840, 790. HRMS (ESI) (Figure S6): m/z [M+H]+ Calcd. for C19H15NOS2: 338.06732, Found: 338.06702.


## 2.3. Molecular Structure Determination

Crystallographic data were acquired on a Bruker D8 Venture diffractometer (Germany) with graphite monochromated Mo-Kα radiation (λ=0.71073 Å) at 193 K for Py-Q, Th-Q, and DTh-Q. The structures were solved by the direct methods and refined by full matrix least squares technique using the SHELX-2018/3 program [15, 16] in the Olex2 package [17]. X-ray crystallographic data for Py-Q, Th-Q, and DTh-Q are summarized in Table 1, the main bond lengths and bond angles for Py-Q, Th-Q, and DTh-Q are listed in Table S1. CCDC 2195077 (Py-Q), CCDC 2195078 (Th-Q), and CCDC 2195079 (DTh-Q) contain the supplementary crystallographic data for this paper.Table 1
Crystal data and structure refinement for Py-Q, Th-Q, and DTh-Q.
Identification codePy-QTh-QDTh-QEmpirical formulaC16H14N2OC15H13NOSC19H15NOS2Formula weight250.29255.32337.44Crystal systemMonoclinicOrthorhombicMonoclinicSpace groupP21/cPbcaP21/ca/Å12.3179(8)16.7658(10)7.9619(8)b/Å14.3024(9)7.1651(6)21.603(3)c/Å7.3404(6)20.5747(16)9.9230(14)α/°909090β/°98.241(3)90106.173(4)γ/°909090Volume/Å31279.85(16)2471.6(3)1639.2(3)Z484ρcalcg/cm31.2991.3721.367μ/mm-10.0830.2480.328F (000)528.01072.0704.0Index ranges−14≤h≤14, −15≤k≤17, −6≤l≤8−22≤h≤22, −9≤k≤8, −25≤l≤27−10≤h≤10, −28≤k≤22, −13≤l≤13Reflections collected92743640716074Independent reflections2233 [Rint=0.0487, Rsigma=0.0419]3063 [Rint=0.0916, Rsigma=0.0364]4051 [Rint=0.0445, Rsigma=0.0384]Data/restraints/parameters2233/0/1743063/80/2024051/122/284Goodness-of-fit onF21.0521.0421.022Final R indexes [I>=2σI]R1=0.0674, wR2=0.1652R1=0.0399, wR2=0.0959R1=0.0468, wR2=0.1283Final R indexes [all data]R1=0.0788, wR2=0.1739R1=0.0543, wR2=0.1096R1=0.0699, wR2=0.1472Largest diff. Peak/hole/e Å-30.79/-0.230.27/-0.200.25/-0.24CCDC number219507721950782195079R1=ΣFo−Fc/Fo; wR2=ΣwFo2−Fc22/ΣwFo221/2.


## 2.4. Electrochemistry

The electrochemical properties of the Py-Q, Th-Q, and DTh-Q are investigated in DCM, MeCN, and DMSO solutions with 0.1 M NBu4PF6 as the supporting electrolyte. The working electrode is a glassy carbon (GC) electrode with 3 mm in diameter. A silver wire was used as the pseudo reference electrode and a cleaned graphite rod as the counter. The ferrocene (Fc+/Fc) was used as the internal standard for potential calibration of the Ag wire. Electropolymerization of the Py-Q, Th-Q, and DTh-Q are carried out in DCM with 0.1 M NBu4PF6 at 1.20 V (vs. Fc+/Fc) for 600 s. The carbon cloth (CC) was used as the substrate for electropolymerization, and the area immersed in the electrolyte is controlled to 1 cm2. The electropolymerized Py-Q, Th-Q, and DTh-Q on the CC substrate are named as PPy-Q, PTh-Q, and PDTh-Q, respectively. The charge storage abilities are measured in 0.1 M Na2SO4, with a saturated calomel electrode (SCE) as the reference, and a graphite rod as the counter. The potentials are reported relative to SCE unless otherwise stated.


## 3. Results and Discussion

### 3.1. Structural Description of Py-Q, Th-Q, and DTh-Q

Py-Q, Th-Q, and DTh-Q were characterized by1H NMR, 13C NMR, FT-IR, and ESI-HRMS (Figures S1–S6). In order to further visualize their structure, suitable single crystals were cultured through recrystallization and their crystal structures were determined by X-ray single-crystal diffraction. Crystal structure determination suggested that both Py-Q and DTh-Q crystallized in monoclinic space group P21/c, Th-Q was assigned to orthorhombic space group Pbca (Table 1). The molecular structures of Py-Q, Th-Q, and DTh-Q are displayed in Figures 1(a)–1(c). Details of their crystal packing are presented in Figures 1(d)–1(f). As depicted in Figure 1(d), in Py-Q, the shortest aromatic centroid-to-centroid distance (3.468 Å) were observed between pyridine rings of quinoline units and quinoline rings in adjacent Py-Q molecules, indicating the existence of face-to-face π-π stacking interactions. Such interactions contributed to the regular arrangements of molecules along c-axis. The 1D chain structure was connected through C-H⋅⋅⋅O (0.980 Å, 2.527 Å, 149.62°) hydrogen bonds. Additionally, the intermolecular hydrogen bonds generated 6-membered rings. In the crystal structure of Th-Q, no intermolecular π-π interaction was observed, but we found several weak intermolecular interactions, including C-H⋅⋅⋅π and C-H⋅⋅⋅O interactions. As shown in Figures 1(e) and 1(c)–1(h)⋅⋅⋅π interactions with a distance of 2.886 Å existed between neighboring Th-Q molecules, resulting in zigzag-shaped chain arrangements along b-axis. Moreover, the formed 1D chains were linked via weak hydrogen bonds (C-H⋅⋅⋅O: 0.950 Å, 2.457 Å, 152.73°) into 2D sheets. For DTh-Q, as shown in Figure 1(f), the benzene rings and pyridine rings of quinoline units in adjacent molecules partially overlapped with a distance of 3.855 Å, suggesting the presence of successive π-π interactions. Moreover, it is notable that two types of C-H⋅⋅⋅π interactions with distances of 2.725 Å and 2.852 Å also existed between 7-thiophene ring and 5-thiophene group, benzene ring and 5-thiophene group, respectively. Hence, DTh-Q monomers adopted well-organized head-to-tail stacking pattern along c-axis via π-π and C-H⋅⋅⋅π interactions.Figure 1
Crystal structure of (a) Py-Q, (b) Th-Q, and (c) DTh-Q; and the molecular packing of (d) Py-Q, (e) Th-Q, and (f) DTh-Q.
(a)(b)(c)(d)(e)(f)


### 3.2. Optical Properties

The UV-vis absorption spectra of Py-Q, Th-Q, and DTh-Q are shown in Figure2. Py-Q shows the absorption peaks at 247, 274, and 306 nm. In Th-Q, 5-position of quinoline ring is substituted with thiophene, instead of pyridine. This results in a higher electron cloud density than Py-Q. Therefore, a slight red shift is observed for Th-Q. The two absorption peaks located at 249 and 316 nm correspond to π-π∗ and n-π∗ transitions, respectively [18]. For the DTh-Q, the corresponding processes are at 268 and 320 nm, respectively. Compared to Th-Q, the processes for DTh-Q are red-shifted, which is related to the extension of π-conjugated system. The optical band gap (Egopt) of these molecules are estimated from the onset of the absorption, and are summarized in Table 2.Figure 2
The UV–vis absorption spectra of Py-Q, Th-Q, and DTh-Q in DCM.Table 2
Summary of the optical parameters acquired from UV-vis.
Compoundλmax (nm)λonset (nm)Egopt (eV)Py-Q3063533.51Th-Q3163623.42DTh-Q3203643.41∗The optical parameters acquired from UV-vis in DCM solution. The optical band gap (Egopt) is calculated from the onset value of the UV spectrum (Egopt=1240/λonset) [19].


### 3.3. Electrochemical Properties

The electrochemical properties of the Th-Q, DTh-Q, and Py-Q are investigated using cyclic voltammetry (CV) in MeCN with 0.1 M NBu4PF6 as the supporting electrolyte and GC as the working electrode. Figure 3(a) shows the CV at 100 mV s-1 of the Th-Q, DTh-Q, and Py-Q in MeCN. All three compounds show irreversible reduction peaks. The reduction peak potentials are at -1.20 V, -1.19 V, and -0.95 V (vs. Fc+/Fc), for Th-Q, DTh-Q, and Py-Q, respectively. The Py-Q shows the highest potential for reduction, which indicates its strong electron-deficient nature. For oxidation, the Py-Q shows only one oxidation peak, which is originated from the oxidation of quinoline structure [20, 21], while the Th-Q and DTh-Q show an extra peak at higher potential that is originated from the oxidation of thiophene [22]. The potentials of the oxidation peaks are summarized in Table 3, and the Py-Q exhibits the highest oxidation potential, followed by Th-Q and DTh-Q. This trend is consistent with the stronger electron donating ability of thiophene than pyridine that increases the electron densities and leads to decreased oxidation potential. Both the reduction and oxidation peaks are irreversible, which indicates that the formed radical cations by oxidation and the radical anions by reduction undergo irreversible reactions following the electrochemical step [23]. From the onset of the oxidation and reduction peak potential, the HOMO and LUMO levels of these compounds are estimated, and the results are listed in Table 3. The electrochemical band gaps (Egele) of the Th-Q, DTh-Q, and Py-Q are calculated to be 1.60, 1.64, and 1.77 eV, respectively. We note that lower bandgaps are estimated from the CV than the UV-vis spectra. The differences arise from the fact that the bandgap estimated by CV reflects the adiabatic charge transfer process, which could be affected by the solvent, while the Egopt results from the lowest spin-allowed transition that could be significantly larger than the Egele [24].Figure 3
(a) CV of the Th-Q, DTh-Q, and Py-Q in MeCN with 0.1 M NBu4PF6 at 100 mV s-1. CV at different scan rates of the (b) Th-Q, (c) DTh-Q, and (d) Py-Q in MeCN with 0.1 M NBu4PF6.
(a)(b)(c)(d)Table 3
Summary of the electrochemical parameters acquired from cyclic voltammetry.
CompoundEox (V)Ered (V)HOMO (eV)LUMO (eV)Egele (eV)αD (cm2 s-1)∗∗Py-Q1.12-0.95-6.64-4.481.771122.83Th-Q1.14-1.20-6.43-4.731.601163.29DTh-Q0.98-1.19-6.45-4.641.64859.67∗∗Calculated from the first oxidation peak.Figures3(b)–3(d) display the CV of Th-Q, DTh-Q, and Py-Q at different scan rates in MeCN. The CV in DMSO and in DCM of the monomers can be found in Figure S7. The CV current increases with the increased sweep rates. The peak current Ip of the first oxidation or the first reduction peak is plotted versus logarithm of the scan rate. Figures 4(a)–4(c) show the results. The slopes of Ip to log v are all around 0.5, which indicates that the oxidation or reduction processes obey semi-infinite linear diffusion. For a totally irreversible peak, the peak current follows
(1)Ip=2.99×105αD0.5ACv0.5where α is the transfer coefficient, D is the diffusion coefficient, A is the geometric surface area of the electrode, and C is the bulk concentration. Based on this equation, the αD values of Th-Q, DTh-Q, and Py-Q can be acquired. The values are listed in Table 3. The Th-Q shows the highest value of αD, which suggests that the diffusion is the most facile among the three molecules assuming that the α for the three molecules are the same. This is probably originated from the small molecular size of the Th-Q molecule. When the heterocyclic molecules are oxidized or reduced, the resulting cationic or anionic radicals undergo following chemical reactions like coupling, and the behavior follows EC mechanism [25–27]. Figures 4(d)–4(f) show the oxidation or reduction peak potentials plotted against log v. The slopes for the oxidation peaks of Th-Q and DTh-Q and reduction peaks of DTh-Q and Py-Q are around 59 mV dec-1. This indicates that the rate of chemical reactions following to the electrochemical oxidation or reduction of the molecule is fast, and that the electrochemical behavior of these molecules fall in the IR region (kinetics controlled by charge transfer) [23, 28]. This phenomenon agrees well with the electropolymerization of thiophene and other heterocyclic compound, where the initiation is rate-limiting, and the generated radicals would easily couple to another neutral molecules to form dimers in solution [22, 23]. The oxidation peak for Py-Q and the reduction peak for Th-Q show the slope~29 mV dec-1, which falls in the KP zone in the zone diagram of the EC mechanism [22, 28, 29]. This indicates that the chemical step following the charge transfer is rate-limiting. Therefore, the Th-Q, DTh-Q, and Py-Q are possible to be electropolymerized at anodic potentials, and that the Th-Q and DTh-Q would exhibit higher kinetics towards coupling after oxidation.Figure 4
LogIp against log v of the (a) Th-Q, (b) DTh-Q, and (c) Py-Q. Peak potential against log v of (d) Th-Q, (e) DTh-Q, and (f) Py-Q. The indicated slopes are absolute values.
(a)(b)(c)(d)(e)(f)The potentiostatic electropolymerization of the Th-Q, DTh-Q, and Py-Q are carried out at 1.20 V (vs. Fc+/Fc) for 600 s in DCM with 0.1 M NBu4PF6 as the supporting electrolyte on CC. The potential is selected higher than the Eox, and the I-t curve for the electropolymerization of Th-Q is shown in Figure 5(a). A transient current drop is observed, and the current is stabilized at 0.29 mA. However, the mass of the deposited film is too low to be accurately measured by the laboratory balance. The I-t curves for the potentiostatic electropolymerization of DTh-Q, and Py-Q are shown in Figure S8. In these molecules, the thiophene group can be electropolymerized following the well-known radical mechanism [22]. Besides, under anodic potentials, carbonyl/hydroxyl type structures would be introduced to the quinoline ring, and the hydroxyl-substituted quinolines are also reported to be electropolymerized under anodic potential via radical mechanism [20]. The SEM of the PTh-Q formed by electropolymerization are shown in Figures 5(b) and 5(c). A homogeneous polymer coating is formed on the carbon fibre surface, with aggregated particle-like structure. Figure 5(d) shows the EDS elemental mapping of the PTh-Q. The C, N, O, and S are homogeneously dispersed throughout the carbon fibre.Figure 5
(a) TheI-t curve for the electropolymerization of Th-Q on CC in DCM with 0.1 M NBu4PF6. (b, c) The SEM images of the PTh-Q; (d–g) EDX elemental mapping of PTh-Q.
(a)(b)(c)(d)(e)(f)(g)Figure6 shows the XPS results of the PTh-Q. In Figure 6(a), the deconvoluted C 1 s peaks at 284.50 eV, 285.12 eV, and 287.18 eV are originated from the C-C or C=C, C-S, and C-O structures, respectively [30, 31]. The N 1 s peak (Figure 6(b)) can be deconvoluted into peaks at 400.22 eV and 401.45 eV that correspond to the C-N in the quinoline ring and oxidized N, respectively [30, 31]. Moreover, the O 1 s peak shown in Figure 6(c) can be deconvoluted into the peaks at 532.54 eV (O-H structure) and 533.37 eV (C-O structure) [30, 31]. In Figure 6(d), the deconvoluted S 2 p peaks match the thiophene structure (164.49 eV and 166.21 eV) and the S-O bond (168.81 eV and 169.79 eV) [32–34]. The existence of S-O and oxidized N indicates that the electropolymerized Th-Q is partially oxidized during the anodic electropolymerization. Both the results of XPS and SEM indicate the successful electropolymerization of the Th-Q on CC.Figure 6
The deconvoluted high-resolution XPS spectra of the (a) C 1 s region, (b) N 1 s region, (c) O 1 s region, and (d) S 2p region of the PTh-Q.
(a)(b)(c)(d)The PTh-Q is then investigated in the monomer-free CH3CN with 0.1 M NBu4PF6 and in 0.1 M Na2SO4. In CH3CN, the PTh-Q shows redox peaks at 0.57 V and -1.05 V (vs. Fc/Fc+), and an oxidation peak starts at 0.20 V (vs. Fc/Fc+, Figure S9b) [35]. This also indicates the Th-Q is successfully polymerized onto CC substrate. The PTh-Q does not dissolve in organic solvent, as the CV of the PTh-Q after immersing in CH3CN for 6 h shows enlarged redox peaks, probably results from polymer swelling, and the UV-vis spectra of the CH3CN used for soaking PTh-Q does not show any absorbance attributed to PTh-Q nor Th-Q (Figure S9(a)). The CV of the PTh-Q in 0.1 M Na2SO4 are displayed in Figure 7(a). Three pairs of consecutive redox peaks are shown at 0.13 and -0.06 V, 0.33 and 0.09 V, and 0.57 and 0.23 V. The charge storage is investigated using galvanostatic charge-discharge (GCD) at different current densities as shown in Figure 7(b). At low current densities, the GCD curve deviates from triangular shape, with longer charging time than the discharging. This suggests that at high potentials (0.8-1 V), irreversible oxidation happens. However, this irreversible process is largely suppressed by charging with higher current densities, and the GCD curve becomes symmetric. Also, a change in slope is observed at 0.13 V, indicative of the redox process contribution of charge storage [36]. The charge stored by the PTh-Q are calculated to be 1.12×10−3 C, 3.50×10−4 C, 4.00×10−4 C, and 4.00×10−4 C at 0.02, 0.05, 0.1, and 0.2 mA cm-2, respectively. From the shape of CV and the GCD curve, the charge storage mechanism is proposed to be battery-type [37].Figure 7
(a) CV of PTh-Q from 5 to 100 mV s−1 in 0.1 M Na2SO4. (b) GCD profiles of PTh-Q from 0.02 to 0.2 mA cm-2 in 0.1 M Na2SO4.
(a)(b)The PDTh-Q and PPy-Q can also be electropolymerized at potential higher thanEox. The potentiostatic electropolymerizaton curves of the PDTh-Q and PPy-Q in DCM are shown in Figure S8. For PDTh-Q and Ppy-Q, the CV (Figure S9(c)) shows redox peaks at -1.14 V (vs. Fc/Fc+), and oxidation peaks at -0.73 V and 0.53 V (vs. Fc/Fc+) [22]. In Na2SO4, three pairs of redox peaks occur, similar to the PTh-Q. However, the peak current and peak area are smaller than the PTh-Q, which indicates that fewer electrochemically active sites are exposed in aqueous solution for the PDTh-Q. As the PDTh-Q has two thiophen groups in the monomer, cross-linking is possible during the polymerization. This interconnected polymer structure might not be swelled by aqueous solution, which leads to relatively poor electrolyte penetration to the electrochemically active sites, and to lower charge capacity. Using GCD (Figure 8(b)), the charge stored by the PDTh-Q are calculated to be 4.0×10−4 C, 3.5×10−4 C, 4.0×10−4 C, and 4.0×10−4C at 0.02, 0.05, 0.1, and 0.2 mA cm-2. This shows that the charge stored by the PDTh-Q is lower than the PTh-Q in the aqueous solution.Figure 8
CV of the PDTh-Q (a) in 0.1 M Na2SO4 from 5 to 100 mV s−1; (b) GCD profiles of PDTh-Q from 0.02 to 0.2 mA cm-2 in 0.1 M Na2SO4; CV of the PPy-Q (c) in 0.1 M Na2SO4 from 5 to 100 mV s−1; (d) GCD profiles of PPy-Q from 0.02 to 0.2 mA cm-2 in 0.1 M Na2SO4.
(a)(b)(c)(d)For PPy-Q, the CV in CH3CN (Figure S9(d)) shows redox peaks at -0.97 V (vs. Fc/Fc+), and oxidation peaks at -0.63 and 0.53 V (vs. Fc/Fc+) (Figure S9(d)). In Na2SO4 (Figure 8(c)), two pairs of redox couples are observed at 0.29 and -0.38 V, and 0.16 and 0.04 V. Compared to the PTh-Q and PDTh-Q, the redox peaks at 0.57 and 0.23 V are missing. This implies that the redox peaks at 0.57 and 0.23 V are originated from the redox process of thiophene groups [22], while the rest of the peaks are originated from the hydroxyl/carbonyl redox processes of quinoline-based structures [21]. Further decrease in potential leads to hydrogen evolution. Using GCD (Figure 8(d)), the charge stored by the PPy-Q is calculated to be 7.6×10−4C, 7.0×10−4C, 6.0×10−4 C, and 5.0×10−4C at 0.02, 0.05, 0.1, and 0.2 mA cm-2, respectively. Since the charge for electropolymerization of PPy-Q is the lowest, and only hydroxyl/carbonyl redox processes of quinoline-based structures in PPy-Q contribute to the faradaic process during charge storage, lower charge storage capacity is expected for PPy-Q. These results indicate that the PTh-Q stores highest amount of charge at low charge-discharge current densities, owing to the enhanced number of electrochemically active sites exposed.We further probed the stability of the PTh-Q by GCD at 0.2 mA cm-2. Figure 9(a) shows the result. The capacity quickly drops after initial 60 cycles, then stabilized. After 3000 cycles, the capacity retention is 77% of its original value. The SEM after the 3000 cycles of the PTh-Q is shown in Figures 9(b) and 9(c). The polymer still covers the carbon fibre homogeneously, with particle-like structure exists. This suggests that the morphology of the PTh-Q does not alter significantly. EDX elemental mapping shows that the S and N still exit (Figures 9(d)–9(g)).Figure 9
(a) The cycle stability of PTh-Q at 0.2 mA cm−2 in 1 M Na2SO4. (b, c) The SEM images after repetitive cycling of the PTh-Q. (d–g) Elemental mapping of the PTh-Q after repetitive cycling.
(a)(b)(c)(d)(e)(f)(g)The XPS spectra of the PTh-Q after repetitive cycling are shown in Figure10. In Figure 10(a), the C-S bond (thiophene structure) at 285.38 eV still exists. Also, in the N 1 s region (Figure 10(b)), the C-N bond and the oxidized N both present. In the O 1 s region (Figure 10(c)), the C-O and O-H exist. In the S 2 p region (Figure 10(d)), the C-S peak still presents. Compared to the PTh-Q before cycling, the percentage of oxidized species (i.e., C-O and N+) increases (Tables S3–S5). This indicates that the PTh-Q is partially oxidized during the repetitive cycling that possibly leads to the capacity loss.Figure 10
The deconvoluted high-resolution XPS spectra after repetitive cycling of the (a) C 1 s region, (b) N 1 s region, (c) O 1 s region, and (d) S 2 p region of the PTh-Q.
(a)(b)(c)(d)


## 3.1. Structural Description of Py-Q, Th-Q, and DTh-Q

Py-Q, Th-Q, and DTh-Q were characterized by1H NMR, 13C NMR, FT-IR, and ESI-HRMS (Figures S1–S6). In order to further visualize their structure, suitable single crystals were cultured through recrystallization and their crystal structures were determined by X-ray single-crystal diffraction. Crystal structure determination suggested that both Py-Q and DTh-Q crystallized in monoclinic space group P21/c, Th-Q was assigned to orthorhombic space group Pbca (Table 1). The molecular structures of Py-Q, Th-Q, and DTh-Q are displayed in Figures 1(a)–1(c). Details of their crystal packing are presented in Figures 1(d)–1(f). As depicted in Figure 1(d), in Py-Q, the shortest aromatic centroid-to-centroid distance (3.468 Å) were observed between pyridine rings of quinoline units and quinoline rings in adjacent Py-Q molecules, indicating the existence of face-to-face π-π stacking interactions. Such interactions contributed to the regular arrangements of molecules along c-axis. The 1D chain structure was connected through C-H⋅⋅⋅O (0.980 Å, 2.527 Å, 149.62°) hydrogen bonds. Additionally, the intermolecular hydrogen bonds generated 6-membered rings. In the crystal structure of Th-Q, no intermolecular π-π interaction was observed, but we found several weak intermolecular interactions, including C-H⋅⋅⋅π and C-H⋅⋅⋅O interactions. As shown in Figures 1(e) and 1(c)–1(h)⋅⋅⋅π interactions with a distance of 2.886 Å existed between neighboring Th-Q molecules, resulting in zigzag-shaped chain arrangements along b-axis. Moreover, the formed 1D chains were linked via weak hydrogen bonds (C-H⋅⋅⋅O: 0.950 Å, 2.457 Å, 152.73°) into 2D sheets. For DTh-Q, as shown in Figure 1(f), the benzene rings and pyridine rings of quinoline units in adjacent molecules partially overlapped with a distance of 3.855 Å, suggesting the presence of successive π-π interactions. Moreover, it is notable that two types of C-H⋅⋅⋅π interactions with distances of 2.725 Å and 2.852 Å also existed between 7-thiophene ring and 5-thiophene group, benzene ring and 5-thiophene group, respectively. Hence, DTh-Q monomers adopted well-organized head-to-tail stacking pattern along c-axis via π-π and C-H⋅⋅⋅π interactions.Figure 1
Crystal structure of (a) Py-Q, (b) Th-Q, and (c) DTh-Q; and the molecular packing of (d) Py-Q, (e) Th-Q, and (f) DTh-Q.
(a)(b)(c)(d)(e)(f)


## 3.2. Optical Properties

The UV-vis absorption spectra of Py-Q, Th-Q, and DTh-Q are shown in Figure2. Py-Q shows the absorption peaks at 247, 274, and 306 nm. In Th-Q, 5-position of quinoline ring is substituted with thiophene, instead of pyridine. This results in a higher electron cloud density than Py-Q. Therefore, a slight red shift is observed for Th-Q. The two absorption peaks located at 249 and 316 nm correspond to π-π∗ and n-π∗ transitions, respectively [18]. For the DTh-Q, the corresponding processes are at 268 and 320 nm, respectively. Compared to Th-Q, the processes for DTh-Q are red-shifted, which is related to the extension of π-conjugated system. The optical band gap (Egopt) of these molecules are estimated from the onset of the absorption, and are summarized in Table 2.Figure 2
The UV–vis absorption spectra of Py-Q, Th-Q, and DTh-Q in DCM.Table 2
Summary of the optical parameters acquired from UV-vis.
Compoundλmax (nm)λonset (nm)Egopt (eV)Py-Q3063533.51Th-Q3163623.42DTh-Q3203643.41∗The optical parameters acquired from UV-vis in DCM solution. The optical band gap (Egopt) is calculated from the onset value of the UV spectrum (Egopt=1240/λonset) [19].


## 3.3. Electrochemical Properties

The electrochemical properties of the Th-Q, DTh-Q, and Py-Q are investigated using cyclic voltammetry (CV) in MeCN with 0.1 M NBu4PF6 as the supporting electrolyte and GC as the working electrode. Figure 3(a) shows the CV at 100 mV s-1 of the Th-Q, DTh-Q, and Py-Q in MeCN. All three compounds show irreversible reduction peaks. The reduction peak potentials are at -1.20 V, -1.19 V, and -0.95 V (vs. Fc+/Fc), for Th-Q, DTh-Q, and Py-Q, respectively. The Py-Q shows the highest potential for reduction, which indicates its strong electron-deficient nature. For oxidation, the Py-Q shows only one oxidation peak, which is originated from the oxidation of quinoline structure [20, 21], while the Th-Q and DTh-Q show an extra peak at higher potential that is originated from the oxidation of thiophene [22]. The potentials of the oxidation peaks are summarized in Table 3, and the Py-Q exhibits the highest oxidation potential, followed by Th-Q and DTh-Q. This trend is consistent with the stronger electron donating ability of thiophene than pyridine that increases the electron densities and leads to decreased oxidation potential. Both the reduction and oxidation peaks are irreversible, which indicates that the formed radical cations by oxidation and the radical anions by reduction undergo irreversible reactions following the electrochemical step [23]. From the onset of the oxidation and reduction peak potential, the HOMO and LUMO levels of these compounds are estimated, and the results are listed in Table 3. The electrochemical band gaps (Egele) of the Th-Q, DTh-Q, and Py-Q are calculated to be 1.60, 1.64, and 1.77 eV, respectively. We note that lower bandgaps are estimated from the CV than the UV-vis spectra. The differences arise from the fact that the bandgap estimated by CV reflects the adiabatic charge transfer process, which could be affected by the solvent, while the Egopt results from the lowest spin-allowed transition that could be significantly larger than the Egele [24].Figure 3
(a) CV of the Th-Q, DTh-Q, and Py-Q in MeCN with 0.1 M NBu4PF6 at 100 mV s-1. CV at different scan rates of the (b) Th-Q, (c) DTh-Q, and (d) Py-Q in MeCN with 0.1 M NBu4PF6.
(a)(b)(c)(d)Table 3
Summary of the electrochemical parameters acquired from cyclic voltammetry.
CompoundEox (V)Ered (V)HOMO (eV)LUMO (eV)Egele (eV)αD (cm2 s-1)∗∗Py-Q1.12-0.95-6.64-4.481.771122.83Th-Q1.14-1.20-6.43-4.731.601163.29DTh-Q0.98-1.19-6.45-4.641.64859.67∗∗Calculated from the first oxidation peak.Figures3(b)–3(d) display the CV of Th-Q, DTh-Q, and Py-Q at different scan rates in MeCN. The CV in DMSO and in DCM of the monomers can be found in Figure S7. The CV current increases with the increased sweep rates. The peak current Ip of the first oxidation or the first reduction peak is plotted versus logarithm of the scan rate. Figures 4(a)–4(c) show the results. The slopes of Ip to log v are all around 0.5, which indicates that the oxidation or reduction processes obey semi-infinite linear diffusion. For a totally irreversible peak, the peak current follows
(1)Ip=2.99×105αD0.5ACv0.5where α is the transfer coefficient, D is the diffusion coefficient, A is the geometric surface area of the electrode, and C is the bulk concentration. Based on this equation, the αD values of Th-Q, DTh-Q, and Py-Q can be acquired. The values are listed in Table 3. The Th-Q shows the highest value of αD, which suggests that the diffusion is the most facile among the three molecules assuming that the α for the three molecules are the same. This is probably originated from the small molecular size of the Th-Q molecule. When the heterocyclic molecules are oxidized or reduced, the resulting cationic or anionic radicals undergo following chemical reactions like coupling, and the behavior follows EC mechanism [25–27]. Figures 4(d)–4(f) show the oxidation or reduction peak potentials plotted against log v. The slopes for the oxidation peaks of Th-Q and DTh-Q and reduction peaks of DTh-Q and Py-Q are around 59 mV dec-1. This indicates that the rate of chemical reactions following to the electrochemical oxidation or reduction of the molecule is fast, and that the electrochemical behavior of these molecules fall in the IR region (kinetics controlled by charge transfer) [23, 28]. This phenomenon agrees well with the electropolymerization of thiophene and other heterocyclic compound, where the initiation is rate-limiting, and the generated radicals would easily couple to another neutral molecules to form dimers in solution [22, 23]. The oxidation peak for Py-Q and the reduction peak for Th-Q show the slope~29 mV dec-1, which falls in the KP zone in the zone diagram of the EC mechanism [22, 28, 29]. This indicates that the chemical step following the charge transfer is rate-limiting. Therefore, the Th-Q, DTh-Q, and Py-Q are possible to be electropolymerized at anodic potentials, and that the Th-Q and DTh-Q would exhibit higher kinetics towards coupling after oxidation.Figure 4
LogIp against log v of the (a) Th-Q, (b) DTh-Q, and (c) Py-Q. Peak potential against log v of (d) Th-Q, (e) DTh-Q, and (f) Py-Q. The indicated slopes are absolute values.
(a)(b)(c)(d)(e)(f)The potentiostatic electropolymerization of the Th-Q, DTh-Q, and Py-Q are carried out at 1.20 V (vs. Fc+/Fc) for 600 s in DCM with 0.1 M NBu4PF6 as the supporting electrolyte on CC. The potential is selected higher than the Eox, and the I-t curve for the electropolymerization of Th-Q is shown in Figure 5(a). A transient current drop is observed, and the current is stabilized at 0.29 mA. However, the mass of the deposited film is too low to be accurately measured by the laboratory balance. The I-t curves for the potentiostatic electropolymerization of DTh-Q, and Py-Q are shown in Figure S8. In these molecules, the thiophene group can be electropolymerized following the well-known radical mechanism [22]. Besides, under anodic potentials, carbonyl/hydroxyl type structures would be introduced to the quinoline ring, and the hydroxyl-substituted quinolines are also reported to be electropolymerized under anodic potential via radical mechanism [20]. The SEM of the PTh-Q formed by electropolymerization are shown in Figures 5(b) and 5(c). A homogeneous polymer coating is formed on the carbon fibre surface, with aggregated particle-like structure. Figure 5(d) shows the EDS elemental mapping of the PTh-Q. The C, N, O, and S are homogeneously dispersed throughout the carbon fibre.Figure 5
(a) TheI-t curve for the electropolymerization of Th-Q on CC in DCM with 0.1 M NBu4PF6. (b, c) The SEM images of the PTh-Q; (d–g) EDX elemental mapping of PTh-Q.
(a)(b)(c)(d)(e)(f)(g)Figure6 shows the XPS results of the PTh-Q. In Figure 6(a), the deconvoluted C 1 s peaks at 284.50 eV, 285.12 eV, and 287.18 eV are originated from the C-C or C=C, C-S, and C-O structures, respectively [30, 31]. The N 1 s peak (Figure 6(b)) can be deconvoluted into peaks at 400.22 eV and 401.45 eV that correspond to the C-N in the quinoline ring and oxidized N, respectively [30, 31]. Moreover, the O 1 s peak shown in Figure 6(c) can be deconvoluted into the peaks at 532.54 eV (O-H structure) and 533.37 eV (C-O structure) [30, 31]. In Figure 6(d), the deconvoluted S 2 p peaks match the thiophene structure (164.49 eV and 166.21 eV) and the S-O bond (168.81 eV and 169.79 eV) [32–34]. The existence of S-O and oxidized N indicates that the electropolymerized Th-Q is partially oxidized during the anodic electropolymerization. Both the results of XPS and SEM indicate the successful electropolymerization of the Th-Q on CC.Figure 6
The deconvoluted high-resolution XPS spectra of the (a) C 1 s region, (b) N 1 s region, (c) O 1 s region, and (d) S 2p region of the PTh-Q.
(a)(b)(c)(d)The PTh-Q is then investigated in the monomer-free CH3CN with 0.1 M NBu4PF6 and in 0.1 M Na2SO4. In CH3CN, the PTh-Q shows redox peaks at 0.57 V and -1.05 V (vs. Fc/Fc+), and an oxidation peak starts at 0.20 V (vs. Fc/Fc+, Figure S9b) [35]. This also indicates the Th-Q is successfully polymerized onto CC substrate. The PTh-Q does not dissolve in organic solvent, as the CV of the PTh-Q after immersing in CH3CN for 6 h shows enlarged redox peaks, probably results from polymer swelling, and the UV-vis spectra of the CH3CN used for soaking PTh-Q does not show any absorbance attributed to PTh-Q nor Th-Q (Figure S9(a)). The CV of the PTh-Q in 0.1 M Na2SO4 are displayed in Figure 7(a). Three pairs of consecutive redox peaks are shown at 0.13 and -0.06 V, 0.33 and 0.09 V, and 0.57 and 0.23 V. The charge storage is investigated using galvanostatic charge-discharge (GCD) at different current densities as shown in Figure 7(b). At low current densities, the GCD curve deviates from triangular shape, with longer charging time than the discharging. This suggests that at high potentials (0.8-1 V), irreversible oxidation happens. However, this irreversible process is largely suppressed by charging with higher current densities, and the GCD curve becomes symmetric. Also, a change in slope is observed at 0.13 V, indicative of the redox process contribution of charge storage [36]. The charge stored by the PTh-Q are calculated to be 1.12×10−3 C, 3.50×10−4 C, 4.00×10−4 C, and 4.00×10−4 C at 0.02, 0.05, 0.1, and 0.2 mA cm-2, respectively. From the shape of CV and the GCD curve, the charge storage mechanism is proposed to be battery-type [37].Figure 7
(a) CV of PTh-Q from 5 to 100 mV s−1 in 0.1 M Na2SO4. (b) GCD profiles of PTh-Q from 0.02 to 0.2 mA cm-2 in 0.1 M Na2SO4.
(a)(b)The PDTh-Q and PPy-Q can also be electropolymerized at potential higher thanEox. The potentiostatic electropolymerizaton curves of the PDTh-Q and PPy-Q in DCM are shown in Figure S8. For PDTh-Q and Ppy-Q, the CV (Figure S9(c)) shows redox peaks at -1.14 V (vs. Fc/Fc+), and oxidation peaks at -0.73 V and 0.53 V (vs. Fc/Fc+) [22]. In Na2SO4, three pairs of redox peaks occur, similar to the PTh-Q. However, the peak current and peak area are smaller than the PTh-Q, which indicates that fewer electrochemically active sites are exposed in aqueous solution for the PDTh-Q. As the PDTh-Q has two thiophen groups in the monomer, cross-linking is possible during the polymerization. This interconnected polymer structure might not be swelled by aqueous solution, which leads to relatively poor electrolyte penetration to the electrochemically active sites, and to lower charge capacity. Using GCD (Figure 8(b)), the charge stored by the PDTh-Q are calculated to be 4.0×10−4 C, 3.5×10−4 C, 4.0×10−4 C, and 4.0×10−4C at 0.02, 0.05, 0.1, and 0.2 mA cm-2. This shows that the charge stored by the PDTh-Q is lower than the PTh-Q in the aqueous solution.Figure 8
CV of the PDTh-Q (a) in 0.1 M Na2SO4 from 5 to 100 mV s−1; (b) GCD profiles of PDTh-Q from 0.02 to 0.2 mA cm-2 in 0.1 M Na2SO4; CV of the PPy-Q (c) in 0.1 M Na2SO4 from 5 to 100 mV s−1; (d) GCD profiles of PPy-Q from 0.02 to 0.2 mA cm-2 in 0.1 M Na2SO4.
(a)(b)(c)(d)For PPy-Q, the CV in CH3CN (Figure S9(d)) shows redox peaks at -0.97 V (vs. Fc/Fc+), and oxidation peaks at -0.63 and 0.53 V (vs. Fc/Fc+) (Figure S9(d)). In Na2SO4 (Figure 8(c)), two pairs of redox couples are observed at 0.29 and -0.38 V, and 0.16 and 0.04 V. Compared to the PTh-Q and PDTh-Q, the redox peaks at 0.57 and 0.23 V are missing. This implies that the redox peaks at 0.57 and 0.23 V are originated from the redox process of thiophene groups [22], while the rest of the peaks are originated from the hydroxyl/carbonyl redox processes of quinoline-based structures [21]. Further decrease in potential leads to hydrogen evolution. Using GCD (Figure 8(d)), the charge stored by the PPy-Q is calculated to be 7.6×10−4C, 7.0×10−4C, 6.0×10−4 C, and 5.0×10−4C at 0.02, 0.05, 0.1, and 0.2 mA cm-2, respectively. Since the charge for electropolymerization of PPy-Q is the lowest, and only hydroxyl/carbonyl redox processes of quinoline-based structures in PPy-Q contribute to the faradaic process during charge storage, lower charge storage capacity is expected for PPy-Q. These results indicate that the PTh-Q stores highest amount of charge at low charge-discharge current densities, owing to the enhanced number of electrochemically active sites exposed.We further probed the stability of the PTh-Q by GCD at 0.2 mA cm-2. Figure 9(a) shows the result. The capacity quickly drops after initial 60 cycles, then stabilized. After 3000 cycles, the capacity retention is 77% of its original value. The SEM after the 3000 cycles of the PTh-Q is shown in Figures 9(b) and 9(c). The polymer still covers the carbon fibre homogeneously, with particle-like structure exists. This suggests that the morphology of the PTh-Q does not alter significantly. EDX elemental mapping shows that the S and N still exit (Figures 9(d)–9(g)).Figure 9
(a) The cycle stability of PTh-Q at 0.2 mA cm−2 in 1 M Na2SO4. (b, c) The SEM images after repetitive cycling of the PTh-Q. (d–g) Elemental mapping of the PTh-Q after repetitive cycling.
(a)(b)(c)(d)(e)(f)(g)The XPS spectra of the PTh-Q after repetitive cycling are shown in Figure10. In Figure 10(a), the C-S bond (thiophene structure) at 285.38 eV still exists. Also, in the N 1 s region (Figure 10(b)), the C-N bond and the oxidized N both present. In the O 1 s region (Figure 10(c)), the C-O and O-H exist. In the S 2 p region (Figure 10(d)), the C-S peak still presents. Compared to the PTh-Q before cycling, the percentage of oxidized species (i.e., C-O and N+) increases (Tables S3–S5). This indicates that the PTh-Q is partially oxidized during the repetitive cycling that possibly leads to the capacity loss.Figure 10
The deconvoluted high-resolution XPS spectra after repetitive cycling of the (a) C 1 s region, (b) N 1 s region, (c) O 1 s region, and (d) S 2 p region of the PTh-Q.
(a)(b)(c)(d)


## 4. Conclusion

Thiophene- and pyridine-substituted quinoline-based molecules are synthesized. Their crystal structures, optical properties, and electrochemical properties are investigated. Changing substituents in the quinoline group affects the band gap and electrochemical redox potentials of these molecules. Irreversible oxidation and reduction in organic solvents are observed for these molecules. The potentiostatic electropolymerization of these molecules are successfully carried out on CC substrate, with the thiophene and quinoline as the possible electropolymerizable units. The electropolymerized films do not dissolve in organic solvents, and can be used to store charge in aqueous solutions. The PTh-Q shows the highest charge storage performance (1.12×10−3 C at 0.02 mA cm-2 in the potential range of 0-1 V), and maintains 77% of the initial capacity after 3000 charge-discharge cycles at 0.02 mA cm-2. The morphology and structure of the PTh-Q do not alter significantly after the repetitive cycling, and the PTh-Q exhibits battery-type charge storage behavior. The superior charge storage performance of the PTh-Q than PDTh-Q, and PPy-Q is originated from the exposed highest amount of electrochemically active sites and the participation of redox processes of both hydroxyl/ketone structures on quinoline and thiophene groups. This study offers fundamental properties of these novel molecules. Further rational design of electropolymerizable monomers is essential in enhancing the charge capacity and stability of conducting polymers.



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