Redox potential tuning of s-tetrazine by substitution of electron-withdrawing/donating groups for organic electrode materials

Article abstract of DOI:10.3390/molecules26040894

Herein, we tune the redox potential of 3,6-diphenyl-1,2,4,5-tetrazine (DPT) by introducing various electron-donating/withdrawing groups (methoxy, t-butyl, H, F, and trifluoromethyl) into its two peripheral benzene rings for use as electrode material in a

Full text of DOI:10.3390/molecules26040894

molecules
Article
Redox Potential Tuning of s-Tetrazine by Substitution of
Electron-Withdrawing/Donating Groups for Organic
Electrode Materials
Dong Joo Min 1 , Kyunam Lee 1, Hyunji Park 1, Ji Eon Kwon 2,
*
and Soo Young Park 1,
*
1
Lab for Supramolecular Optoelectronic Materials (LSOM), Department of Materials Science and Engineering,
Research Institute of Advanced Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu,
Seoul 08826, Korea; mdj88@snu.ac.kr (D.J.M.); lgn0427@snu.ac.kr (K.L.); 2018-20150@snu.ac.kr (H.P.)
Functional Composite Materials Research Center, Institute of Advanced Composite Materials,
Korea Institute of Science and Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun,
Jeonbuk 55324, Korea
2
*
Correspondence: jekwon@kist.re.kr (J.E.K.); parksy@snu.ac.kr (S.Y.P.)
Abstract: Herein, we tune the redox potential of 3,6-diphenyl-1,2,4,5-tetrazine (DPT) by introducing
various electron-donating/withdrawing groups (methoxy, t-butyl, H, F, and trifluoromethyl) into its
two peripheral benzene rings for use as electrode material in a Li-ion cell. By both the theoretical
DFT calculations and the practical cyclic voltammetry (CV) measurements, it is shown that the redox
potentials (E1/2) of the 1,2,4,5-tetrazines (s-tetrazines) have a strong correlation with the Hammett
constant of the substituents. In Li-ion coin cells, the discharge voltages of the s-tetrazine electrodes are
successfully tuned depending on the electron-donating/withdrawing capabilities of the substituents.
Furthermore, it is found that the heterogeneous electron transfer rate (k0) of the s-tetrazine molecules
and Li-ion diffusivity (DLi) in the s-tetrazine electrodes are much faster than conventional electrode
active materials.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Min, D.J.; Lee, K.; Park, H.;
Kwon, J.E.; Park, S.Y. Redox Potential
Tuning of s-Tetrazine by Substitution
of Electron-Withdrawing/Donating
Groups for Organic Electrode
Materials. Molecules 2021, 26, 894.
https://doi.org/10.3390/molecules
26040894
Keywords: s-tetrazine; organic electrode; Li ion battery; potential tuning
1. Introduction
Recently, interest in organic electrode materials has been growing rapidly due to
the abundance and light weight of their elements, prospect of low cost, environmental
friendliness, and structural diversity [1–3]. Moreover, in contrast to inorganic electrode
materials based on metal oxides, the organic electrode materials can be applied to various
types of metal-ion batteries such as Li-, Na-, K-, and divalent metal-ion batteries [4–6]. To
date, an enormous number of organic electrode materials has been reported; however, they
Academic Editors: Pierre Audebert
and Jean-Cyrille Hierso
Received: 31 December 2020
Accepted: 4 February 2021
Published: 8 February 2021
still mostly rely on only a few kinds of redox centers including quinone [
nitroxide [11 12], organosulfur [13 14], and carboxylate [15 16], which limits their practical
performance inferior to their counterparts.
In this context, our group very recently proposed 1,2,4,5-tetrazine (s-tetrazine) as a
new redox center for organic electrode materials for the first time [17]. The s-tetrazine
redox center is promising for electrode-active material of metal-ion batteries because it
can be reversibly reduced by one electron to form a stable radical anion due to its strong
7,8], imide [9,10],
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,
,
,
electron-deficiency [18,19]. Therefore, it is expected to theoretically deliver a high specific
Copyright:
© 2021 by the authors.
capacity as high as 327 mAh g−1 by virtue of its small molecular weight (Mw = 82 g mol−1).
Furthermore, there is also a chance to greatly increase its theoretical specific capacity up
to 654 mAh g−1 if the second one-electron reduction reaction of the s-tetrazine can be
induced to reversibly occur by optimizing electrolytes and/or chemical structure modifica-
tion [19–21].
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
In the previous study, we evaluated the applicability of s-tetrazine redox center for
electrode materials using four simple s-tetrazine derivatives as model compounds. Among
Molecules 2021, 26, 894. https://doi.org/10.3390/molecules26040894
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 894
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them, 3,6-diphenyl-1,2,4,5-tetrazine (DPT) showed a reversible one-electron redox reaction
with an insertion/de-insertion of a Li-ion in a coin cell, which was evidenced by ex-situ
XPS analysis [17]. It was ascribed to the fact that the two phenyl rings of the s-tetrazine core
at 3- and 6-position prevented the unwanted side reactions. However, its discharge voltage
was 2.24 V vs. Li/Li+, which lies in an intermediate voltage range between the typical
cathode and anode. Therefore, redox potential tuning of s-tetrazine is highly desired for its
practical use.
Fortunately, it is well known that the redox potential of organic materials can easily
be tuned by the introduction of electron-donating or -withdrawing substituents [22–25].
For instance, Vadehra et al. [22] achieved 0.09 V and 0.54 V of elevation in the redox
potential of naphthalene diimide (NDI)-based electrode materials by substitution of F and
CN groups, respectively. Similarly, Kim et al. [23] reported that the discharge voltage of
p-benzoquinone was elevated by 0.32 V through introducing four chlorine atoms. Park
et al. [24] also showed that a disodium terephthalate (Na2TP) substituted by electron-
donating amino-groups exhibited decreased redox potential in a Na-ion cell. Typically, the
energy level of frontier molecular orbitals (FMOs) of organic molecules can be modulated
depending on the electron-donating/withdrawing capability of the substituents, resulting
in a redox potential change.
In this study, we introduced various substituents possessing different electron-
withdrawing/donating capabilities into the s-tetrazine redox center to tune its redox
potential systematically. To this end, a series of DPT derivatives bearing methoxy (MeO),
t-butyl (t-Bu), H, F, and trifluoromethyl (CF3) groups was designed and synthesized. Us-
ing quantum chemical calculation methods based on density functional theory (DFT),
the lowest unoccupied molecular orbital (LUMO) levels and reduction potentials of the
s-tetrazines were theoretically calculated. Then, the electrochemical properties of the DPT
derivatives including the redox potential (E1/2), the diffusion coefficient (D), and the het-
erogeneous electron transfer rate constant (k0) were evaluated by cyclic voltammetry (CV)
measurements. Finally, the discharge voltages of the s-tetrazine electrodes measured by
galvanostatic charge/discharge test was found to correlate well with the predicted redox
potentials.
2. Result and Discussion
2.1. Materials Design and Synthesis
As shown in Figure 1a, four different electron-withdrawing/donating substituents
(1: methoxy, 2: tert-butyl, 4: fluoro, and 5: trifluoromethyl, respectively) were introduced
into 3,6-diphenyl-1,2,4,5-tetrazine (3, DPT). The designed s-tetrazines were synthesized
by a modified Pinner method using the corresponding 4-substituted benzonitriles as their
respective starting materials [26,
27], and their structures were fully characterized by 1H-
and 13C-NMR and elemental analysis. The detailed synthetic procedures with the structure
characterization data are described in the Experimental section.
To predict the change in the FMO energy levels of the s-tetrazines by the substitution,
computational calculations were conducted by using the DFT method with 6–31+G(d)
basis set and B3LYP functionals in the gas phase. The LUMO levels of the s-tetrazines were
calculated to be
(Table 1). As expected, the methoxy (
while F (4) and CF3 (5) groups stabilized it compared to that of DPT.
−
2.71,
−
2.80,
−
2.96,
−
3.17, and
) and t-butyl (
−
3.50 eV for
1, 2, 3, 4, and 5, respectively
1
2) groups destabilized the LUMO level,

Molecules 2021, 26, 894
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Figure 1.
(a) The synthesis scheme of the s-tetrazine derivatives. (b) The calculated frontier molecular orbital (FMO) energy
levels of the s-tetrazines calculated by DFT in solvated system.
Table 1. The HOMO, LUMO levels and the redox potentials of s-tetrazines.
c
b
Oxidation Potential
Reduction Potential
(E1/2) (V vs. Fc/Fc+)
a
a
d
d
HOMO (ev)
LUMO (ev)
HOMO (ev)
LUMO (ev)
(Eonset) (V vs. Fc/Fc+)
1
2
3
4
5
−6.02
−6.31
−6.46
−6.67
−7.01
−2.71
−2.80
−2.96
−3.17
−3.50
−1.34
−1.32
−1.28
−1.25
−1.14
1.29
1.42
1.48
1.51
1.60
−6.39
−6.52
−6.58
−6.61
−6.70
−3.76
−3.78
−3.82
−3.85
−3.96
The calculated FMO levels by DFT method. The measured half-wave potentials of the reduction reaction by CV. c The measured onset
a
b
potentials of the oxidation reaction by CV. d The measured HOMO and LUMO levels calibrated by HOMO level of ferrocene (5.1 eV).
2.2. Electrochemical Properties
The CV of the s-tetrazines was measured in acetonitrile (MeCN) solutions with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAHFP) as a supporting electrolyte. The
cathodic CV scans showed quasi-reversible one-electron redox reactions for all s-tetrazines,
and the reduction potentials (E1/2) were measured to be
−
1.34,
−
1.32,
−
1.28,
−
1.25 and
−
1.14 vs. Fc/Fc+ for , respectively, at a scan rate of 50 mV s−1 (Figure 2a).
1,
2,
3,
4
, and
5
The anodic CV scans were also performed, but the oxidation of the s-tetrazines was not
reversible (Figure S1). The LUMO and HOMO energy levels were obtained based on the
formal potential of the ferrocene/ferrocenium couple [28] and displayed in Figure S2 and
Table 1. It should be noted that the measured FMO levels correlated with the FMO trend
calculated by DFT as well as the Hammett constant (Figures 2c and S3) [29].

Molecules 2021, 26, 894
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Figure 2. (a) Cyclic voltammetry (CV) of the s-tetrazines in 0.1 M [NBu4][PF6] acetonitrile (CH3CN) solution. Scan rate
(v) wa−s 350 mV s−1 with a 3 mm glassy carbon working electrode. The concentration of the s-tetrazine derivatives was
−3
2 × 10
M
in CH3CN except for
1
(1
×
10 M), which has lower solubility in CH3CN than the others. The linear fitting of
(b) the calculated LUMO levels of the s-tetrazines in solvated system vs. the Hammett constant of substituents and (c) the
measured LUMO levels by CV vs. Hammett constant.
To examine the kinetics of the s-tetrazines, additional cathodic CV scans were per-
formed at various scan rates. As shown in Figures S4 and S5, the s-tetrazines showed larger
current values as the scan rate increased due to a decrease in the diffusion layer thickness.
It was found that the current densities at anodic and cathodic peaks were proportional to
square root of the scan rate (see Figure S6). Thus, the diffusion coefficients
were calculated from Randles–Sevcik equation [30] as follows (Equation (1)).
D )
(cm2 s−1
ꢀ
ꢁ
ip = 2.69 × 105 n3/2·A·D1/2·C·v1/2
(1)
where, ip (A) is the peak current, n is the number of electrons transferred in the redox
event, A (cm2) is the electrode surface area, (mol cm−3) is the bulk concentration of
C
the analyte, and
v
(V s−1) is the scan rate. The calculated D of the s-tetrazines were
1.63–3.63 × 10−5 cm2 s−1 (see Tables 2 and 3).

Molecules 2021, 26, 894
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Table 2. Diffusion coefficients (D) and heterogeneous electron transfer rate constants (k0) of tetrazines.
2
−1
−1
)
D (cm s
)
k0 (cm s
8.50 × 10
2.76 × 10
2.55 × 10
5.58 × 10
5.06 × 10
−5
−3
−3
−3
−3
−3
1
2
3
4
5
3.63 × 10
1.63 × 10
2.19 × 10
2.06 × 10
2.02 × 10
−5
−5
−5
−5
Table 3. Li-ion diffusion coefficients (DLi) and the discharge voltages of the s-tetrazine electrodes.
2
2
a
−1
−1
)
DLi (cm s
)
DLi (cm s
Discharge Voltage
(V vs. Li/Li+)
for Cathodic Peak
for Anodic Peak
−9
−9
1
2
3
4
5
2.17
2.27
2.26
2.27
2.28
1.51 × 10
7.30 × 10
1.34 × 10
2.50 × 10
2.02 × 10
9.66 × 10
1.60 × 10
3.43 × 10
−10
−10
−9
−9
−9
−9
−9
−9
1.45 × 10
3.08 × 10
a
The discharge voltages obtained from the dQ/dV plots.
Then, the heterogeneous electron transfer rate constant (k0) of the s-tetrazines were
estimated using the following equations based on the Nicholson method [30,31].
(DO/DR)α/2k0
ϕ =
(2)
(3)
(πDO f v)12
f = F/RT
where
ϕ is the Nicholson dimensionless number, which is a function of the peak-to-peak
separation from a CV curve, Do and DR is the diffusion coefficient of the oxidized and
reduced species, respectively,
α is the transfer coefficient, v is the scan rate, F is the Faraday
constant (96,485 C mol−1), R is the ideal gas constant (8.314 J mol K−1), and T is the absolute
temperature in K. Typically, the reduced species of the analyte were assumed to have the
same diffusion coefficient with the oxidized one due to negligible molecular weight change
during the redox reaction. Then, Equation (2) can be simplified as follows:
ϕ = k0(πD f v)(−1/2)
(4)
The k0 calculated from the plots of
ϕ
versus v(−1/2) (see Figures S4 and S6) was
2.55 × 10−3 cm s−1 for
3, and the other s-tetrazines also showed high k0 values
(
2–8 × 10−3 cm s−1, see Table 3). It is noteworthy that the s-tetrazines have several or-
ders of magnitude faster k0 values than V2+/V3+ (k0 = 10−4–10−6 cm s−1) redox couple [32],
which is a commercialized anolyte for a redox flow battery (RFB), suggesting great promise
of s-tetrazines not only as an active material for a metal-ion battery but also as an anolyte
for a RFB.
2.3. Reduction Potential Prediction by DFT Calculation
To predict the change in the reduction potential depending on the substituents, we con-
ducted computational calculations on the one-electron reduction reaction of the s-tetrazines
in the solvated phase using the PCM model of DFT calculation method (The detailed
calculation process is fully described in the Experimental section). We first investigated
the free energy changes of the reduction reaction to form anions without a Li-ion insertion.

Molecules 2021, 26, 894
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The calculated Gibbs free energies of the s-tetrazines at the optimized geometries in the
neutral and anion states are summarized in Tables S1–S3.
In the neutral states, it was found that the tetrazine core and the two peripheral ben-
zene rings were coplanar regardless of the substituents (Figures S7–S11 in Supplementary
Materials). Interestingly, upon reduction, the s-tetrazine anions showed no significant
changes in the geometries. The LUMO orbitals of the neutral states and the spin densities
of anion states were located on the tetrazine core regardless of the substituent (Figure S15).
The reduction potentials were calculated to be
−
2.52,
−
2.46,
−
2.40,
−
2.27, and
−
2.10 V vs.
Fc/Fc+ for
1
,
2,
3,
4, and , respectively. As expected, the calculated reduction potentials
5
showed the same trend with the measured reduction potentials from the solution CV (see
Figure 3c).
Next, we considered the insertion of a Li-ion to the reduced s-tetrazines for charge
compensation. In the optimized geometries, the inserted Li cation was located at the one
side of the tetrazine core in the same plane to form coordinate bonds with the two nitrogen
atoms of the core ring (r N-Li = 1.95–1.96 Å, Figures S7–S11). However, in contrast to the
anion case above, it was found that slight distortion was generated between the tetrazine
core and the two peripheral phenyl rings. The distorted angles were different depending on
the substituents. The t-butyl-substituted s-tetrazine (
θ1,2 = 12.5◦), while the CF3-substituted one ( ) had virtually planar geometry (θ1,2 = 0.8◦)
, and were
2) exhibited the largest distorted angle
(
5
.
When considering the Li-ion insertion, the reduction potentials of
1
,
2,
3,
4
5
calculated to be 2.28, 2.30, 2.40, 2.43, and 2.50 V vs. Li/Li+, respectively. However, it should
be noted that the trend of reduction potential change depending on the substituents was
unaffected by the Li-ion insertion.
2.4. CV and Galvanostatic Test in Coin Cells
To evaluate the electrochemical properties of the s-tetrazines in Li-ion cells, we fab-
ricated composite electrodes composed of the s-tetrazines as an active material, Super-P
as a conductive additive, and PVDF as a binder, respectively. In Li-ion cells, CV scans
of the s-tetrazine electrodes were performed at various scan rates (Figures S12 and S13).
Based on the Randles–Sevcik equation, the diffusion coefficient of Li-ion (DLi) in the
s-tetrazine electrodes were calculated to be 7.30
cathodic peaks and 9.66
10−10–3.43 10−9 cm2 s−1 for the anodic peaks, respectively
(see Figure S13 and Table 3), which are much higher values than those of conventional
cathode materials for the Li-ion battery such as LCO (10−11–10−13 cm2 s−1) [33
34] and
×
10−10–2.50
×
10−9 cm2 s−1 for the
×
×
,
LFP (10−13–10−14 cm2 s−1
) [35,36]. Typically, many organic electrode materials including
the s-tetrazines were found to have high DLi (>10−9 cm2 s−1) [
8,37] than the inorganic
materials.
Then, galvanostatic charge/discharge tests were carried out for the s-tetrazine elec-
trodes in Li-ion coin cells. As shown in Figure 3a, the s-tetrazine electrodes showed a
clear charge/discharge plateau with specific discharge capacities of 63, 39, 68, 84, and
73 mAh g−1 for
1–5 in the first cycle at 0.1 C, respectively. The C-rate for each electrode
was calculated by their respective theoretical specific capacities (Ctheo = 91, 77, 144, 99,
and 72 mAh g−1 for 1–5, respectively). The different capacity utilization of the s-tetrazine
electrodes is likely attributed to their solubility difference in the electrolyte. Unfortunately,
all s-tetrazine electrodes showed a gradual capacity decrease during the cycle test due to
the dissolution of active materials into the electrolyte. However, their coulombic efficien-
cies were maintained to 100% after the first cycle (Figure S14), indicating that their redox
reactions were reversible in Li-ion cells.

Molecules 2021, 26, 894
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Figure 3.
of the s-tetrazine electrodes. The correlation graphs about (
reduction potentials without considering Li-ion insertion by the DFT method and (
Li-ion cells vs. the calculated reduction potentials considering Li-ion insertion. The dashed red lines in (
linear fitting curves and the regression coefficients are presented in the graphs. In graph ( ), the point for the
(
a
) The galvanostatic charge/discharge profiles for the s-tetrazine electrodes at 0.1 C rate. (
) the measured reduction potentials in CV vs. the calculated
) the measured discharge voltages in
b) The dQ/dV plots
c
d
c
) and (
d
) indicate
d
2 electrode
was excepted as an outlier for the linear regression because of the statistical reasons. See Figure S15 for the detailed analysis.
In the differential analysis (dQ/dV) graphs (Figure 3b), each s-tetrazine electrode
showed a sharp charge/discharge peak. As expected, the discharge voltage of the s-
tetrazine electrodes was gradually shifted from 2.17 V (
on the electron-donating/withdrawing capabilities of the substituents except
to the fact that the lowered redox potential was predicted by the DFT calculation (vide
supra), the electrode bearing t-butyl substituents showed 0.01 V higher discharge voltage
than that of the electrode (2.26 V). It is also contrary to the solution CV results measured
1) to 2.28 V (
5
) vs. Li/Li+ depending
. In contrast
2
2
3

Molecules 2021, 26, 894
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with the TBA-salt electrolyte (see Section 2.2). This exception is most likely attributed
to the fact that the
2 molecules may have different crystal structure from the other s-
tetrazine derivatives due to its bulky substituents [38]. In the bulk electrode, there have
been a few reports that the interaction between active materials and Li-ions as well as the
crystal structure changes during redox reactions could affect the discharge voltage of active
materials [4,39,40], which will be further studied in the future.
Nevertheless, excepting the electrode of molecule 2, which comprises the exceptionally
largest substituent group among others, the measured discharge voltages of the other four
s-tetrazine electrodes clearly showed a linear relationship (Pearson’s r = 0.93) with the
calculated reduction potentials by DFT method (see Figure S15 for the detailed analysis).
It means that the redox potential of s-tetrazine can be tuned by substitution of electron-
donating or withdrawing groups and also can be predicted by DFT calculation.
3. Experimental Section
Chemicals were purchased from commercial suppliers and used without further purifi-
cation unless otherwise stated. 3,6-diphenyl-1,2,4,5-tetrazine (DPT), 4-(tert-butyl)benzonitrile
and 4-fluorobenzonitrile were purchased from TCI (Seoul, Korea). Hydrazine mono-
hydrate was purchased from Sigma-Aldrich (Seoul, Korea). 4-methoxy benzonitrile, 4-
(trifluoromethyl)benzonitrile and sulfur were purchased from Thermo Fisher Scientific
(Incheon, Korea). Reactions were monitored using thin-layer chromatography (TLC) with
commercial TLC plates (silica gel 60 F254) and silica gel column chromatography was
performed with silica gel 60 (particle size 0.063–0.200 mm) from Sigma-Aldrich (Seoul,
1
Korea). H-NMR and 13C-NMR spectra were recorded on a Bruker Avance III HD (300 Hz)
and Avance III 500 (500 Hz), respectively. Elemental analyses were carried out using a
Flash2000 elemental analyzer (Thermo Fisher Scientific).
3.1. Synthesis of 1
Hydrazine monohydrate (22 mL, 421 mmol) was added dropwise to a solution of
4-methoxy benzonitrile (8 g, 60 mmol) and sulfur (1.92 g, 60 mmol) in ethanol (15 mL). The
reaction mixture was heated at reflux during overnight. After the reaction finished, the
mixture was cooled to room temperature and the precipitate was quickly filtered and dried
to afford crude dihydro tetrazines. The crude material (4.59 g, 15 mmol) was dissolved
in dichloromethane (15 mL) and a solution of sodium nitrite (5.34 g, 77 mmol) in water
◦
(100 mL) was added to the mixture. The mixture was cooled to 0 C and acetic acid (2.5 mL,
43 mmol) was added dropwise. The reaction mixture was stirred at room temperature
during overnight. The organic layer was extracted and dried with MgSO4. The crude
product was purified by column chromatography (dichloromethane:n-Hex = 1:2, silica
1
gel), yielding a dark red solid. Yield: 2.3 g (26%). H NMR (300 MHz, CDCl3,
δ
): 8.58
(d, J = 8.70 Hz, 4H), 7.10 (d, J = 8.73 Hz, 4H), 3.93 (s, 6H); 13C NMR (125 MHz, CDCl3,
δ
):
163.43, 163.40, 129.77, 124.60, 114.94, 55.73. Anal. Calcd for C16H14N4O2: C, 65.30; H, 4.79;
N, 19.04; O, 10.87. Found: C, 65.28; H, 4.77; N, 19.04; O, 10.93.
3.2. Synthesis of 2
Hydrazine monohydrate (11 mL, 209 mmol), was added dropwise to a solution of
4-(tert-butyl)benzonitrile (4.75 g, 30 mmol) and sulfur (0.96 g, 30 mmol) in ethanol (15 mL).
The reaction mixture was heated at reflux during overnight. After the reaction finished, the
mixture was cooled to room temperature and the precipitate was quickly filtered and dried
to afford crude dihydro tetrazines. The crude material (3.89 g, 11 mmol) was dissolved
in dichloromethane (15 mL) and a solution of sodium nitrite (3.84 g, 56 mmol) in water
◦
(150 mL) was added to the mixture. The mixture was cooled to 0 C and acetic acid (1.79 mL,
31 mmol) was added dropwise. The reaction mixture was stirred at room temperature
during overnight. The organic layer was extracted and dried with MgSO4. The crude
product was purified by column chromatography (dichloromethane:n-Hex = 1:19 to 1:4,
1
silica gel), yielding a pink solid. Yield: 0.47 g (10%). H NMR (300 MHz, CDCl3,
δ): 8.58

Molecules 2021, 26, 894
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(d, J = 8.61 Hz, 4H), 7.64 (d, J = 8.59 Hz, 4H), 1.41 (s, 18H); 13C NMR (125 MHz, CDCl3,
δ):
163.95, 156.51, 129.25, 127.92, 126.54, 35.36, 31.36. Anal. Calcd for C22H26N4: C, 76.27; H,
7.56; N, 16.17. Found: C, 76.31; H, 7.54; N, 16.18.
3.3. Synthesis of 4
Hydrazine monohydrate (8.93 mL, 173 mmol), was added dropwise to a solution of
4-fluorobenzonitrile (3 g, 25 mmol) and sulfur (0.8 g, 25 mmol) in ethanol (10 mL). The
reaction mixture was heated at reflux during overnight. After the reaction finished, the
mixture was cooled to room temperature and the precipitate was quickly filtered and dried
to afford crude dihydro tetrazines. The crude material (3.29 g, 12 mmol) was dissolved
in dichloromethane (20 mL) and a solution of sodium nitrite (4.17 g, 60 mmol) in water
◦
(200 mL) was added to the mixture. The mixture was cooled to 0 C and acetic acid (1.94 mL,
34 mmol) was added dropwise. The reaction mixture was stirred at room temperature
during overnight. The organic layer was extracted and dried with MgSO4. The crude
product was purified by column chromatography (dichloromethane:n-Hex = 1:3, silica
1
gel), yielding a purple solid. Yield: 1.2 g (36%). H NMR (300 MHz, CDCl3,
δ
): 8.65–8.69
(m, 4H), 7.28–7.34 (m, 4H); 13C NMR (125 MHz, CDCl3,
δ): 167.07, 165.05, 163.34, 130.50,
128.14, 116.84. Anal. Calcd for C14H8F2N4: C, 62.22; H, 2.98; F, 14.06; N, 20.73. Found: C,
62.29; H, 3.00; N, 20.74.
3.4. Synthesis of 5
Hydrazine monohydrate (8.43 mL, 164 mmol), was added dropwise to a solution
of 4-(trifluoromethyl)benzonitrile (4 g, 23 mmol) and sulfur (0.74 g, 23 mmol) in ethanol
(10 mL). The reaction mixture was heated at reflux during overnight. After the reaction
finished, the mixture was cooled to room temperature and the precipitate was quickly
filtered and dried to afford crude dihydro tetrazines. The crude material (3.55 g, 10 mmol)
was dissolved in dichloromethane (15 mL) and a solution of sodium nitrite (3.29 g, 48 mmol)
in water (150 mL) was added to the mixture. The mixture was cooled to 0 ◦C and acetic
acid (1.53 mL, 27 mmol) was added dropwise. The reaction mixture was stirred at room
temperature during overnight. The organic layer was extracted and dried with MgSO4.
The crude product was purified by column chromatography (dichloromethane:n-Hex =
1
1:5, silica gel), yielding a purple solid. Yield: 1.1 g (24%). H NMR (300 MHz, CDCl3,
δ
):
8.82 (d, J = 8.39 Hz, 2H), 7.91 (d, J = 8.41 Hz, 2H); 13C NMR (125 MHz, CDCl3,
δ): 163.69,
134.98, 134.85, 134.60, 128.70, 126.59. Anal. Calcd for C16H8F6N4: C, 51.90; H, 2.18; F, 30.79;
N, 15.13. Found: C, 52.04; H, 2.32; N, 15.14.
3.5. Electrochemical Measurement
Cyclic voltammetry (CV) was performed on a Princeton Applied Research Model
273a using a three-electrode beaker cell with an Ag wire in 0.01 M AgNO3 solution as a
reference electrode, a glassy carbon disc (diameter = 3 mm) as a working electrode, and
a platinum wire as a counter electrode, respectively. The redox potential of the reference
electrode was calibrated using ferrocene/ferrocenium (Fc/Fc+) as an internal standard.
0.1 M tetrabutylammonium hexafluorophosphate (TBAHFP) was used as a supporting
electrolyte. The concentration of the s-tetrazines solutions for the CV measurements was
2 × 10−3 M in acetonitrile (MeCN) except
1 used for the CV due to its low solubility.
1
. An MeCN solution containing 1
×
10−3 M of
3.6. Cathode Fabrication and Galvanostatic Test
Slurries of the active materials (1, 2, 3, 4, and 5), carbon black (Timcal Super P),
and polyvinylidene fluoride (PVDF, Sigma Aldrich) in dimethylformamide (DMF, 99.5%,
JUNSEI) were prepared with a weight ratio of 4:4:2. The slurries were stirred overnight at
room temperatu◦re and then spread on aluminum foils by doctor blading. The electrodes
were dried at 25 C for 8 h in a vacuum oven and punched into circular discs to a diameter
of 14 mm. Coin type CR2032 (Hohsen) cells were assembled with the fabricated cathodes,

Molecules 2021, 26, 894
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a Li-metal anode, and a polypropylene separator (Celgard 2400) in an Ar-filled glove box
(Korea Kiyon KK-011-AS) in which moisture and oxygen levels were tightly regulated
under 0.5 ppm. 2 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in a 1:1 (v/v)
mixture of 1,3-dioxolane (DOL) and dimethoxy ethane (DME) with 1% LiNO3 is used for
the electrolyte. The galvanostatic discharge/charge tests of the coin cells were performed
on a battery cycler (Wonatech WBCS3000L) at 30 ◦C.
3.7. Theoretical Calculation
All density functional theory (DFT) calculations were carried out using Gaussian
09 quantum chemical package [41]. The geometry optimizations were performed using
Becke-Lee-Yang-Parr (B3LYP) functionals and the 6–31G+(d) basis set. Vibrational fre-
quency calculations were performed for the obtained structures at the same level to confirm
the stable minima. The frontier molecular orbital (FMO) levels of the s-tetrazine molecules
were calculated in the gas phase.
To calculate the reduction potential of the s-tetrazines, geometries of the neutral and
the reduced molecules were optimized in the solvated phase using PCM model with the
dielectric constant (
ε
) of 7.155 for DOL/DME (εDOL = 7.13 and εDME = 7.18) [4,42–46]. Note
that, to reduce the calculation cost, an average
ε
value was employed for the 1:1 DOL/DME
mixture solvent without solvation cavity modification. Then, the reduction potential was
calculated using the following formula:
Ered = − (Ganion − Gneutral)/nF,
(5)
where Gneutral and Ganion are the Gibbs free energies of the s-tetrazines at the neutral and
anion state, respectively; n is the number of electrons involved in the reaction; F is the
Faraday constant.
To consider the effect of cation insertion, a Li cation was added to the optimized anion
form of the s-tetrazines. The reduction potentials were calculated using the following
formula:
Ered = − (Glithiated form − Gneutral − GLi cation)/nF,
(6)
where Glithiated form and GLi cation are Gibbs free energies of the lithiated s-tetrazine molecules
and a Li cation, respectively. The calculated reduction potentials were shifted by 1.917 V
with respect to Li/Li+ as calculated.
4. Conclusions
In summary, we investigated the substituent effect on the redox potentials and dis-
charge voltages of the s-tetrazine derivatives. The theoretical DFT calculation and the
practical CV measurement clearly revealed that the electron-donating substituents (i.e.,
MeO and t-Bu) destabilized the FMO energy levels to lower the redox potential of the
s-tetrazine, while the electron-withdrawing groups (i.e., F and CF3) stabilized their FMO
energy levels to elevate their redox potential. It should also be noted that the potential
changes depended on the electron-donating/withdrawing capabilities of the substituents,
which was shown by a correlation with the Hammett constant. Most importantly, the dis-
charge voltages of the s-tetrazine electrodes were also varied by the substituents, and the
trend of voltage change well correlated with the calculated values from the DFT except one
outlier. Although the tuned voltage of the DPT derivatives in this study was rather small,
it is worth noting that introducing substituents clearly affected the discharge voltages and
the change could be well predicted by theoretical calculation.
On the other hand, it was revealed that the s-terazines possess high heterogenous
electron transfer rate constants (k0) by the solution CV measurements. In addition, in the
Li-ion coin cells, it was observed that the Li-ion diffusion in the s-tetrazine electrodes was
much faster than conventional LCO and LFP electrodes. These results clearly indicate that
the s-tetrazine redox center is a promising candidate for an active material with high power
capability in RFBs as well as metal-ion batteries.

Molecules 2021, 26, 894
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Supplementary Materials: The following are available online. Figure S1: The anodic CV of the
s-tetrazines measured in acetonitrile solutions with 0.1 M tetrabutylammonium hexafluorophosphate
(TBAHFP) as a supporting electrolyte and an Ag wire in 0.01 M AgNO3 solution as a reference
electrode at a scan rate of 50 mV s−1, Figure S2. The HOMO and LUMO energy levels of the s-
tetrazines obtained from solution CV, Figure S3. The linear fitting of the measured (a) LUMO level
and (b) HOMO level by CV vs. the calculated FMO levels of the s-tetrazines by DFT. (c) The linear
fitting of the measured HOMO levels by CV vs. Hammett Constant, Figure S4. (a) The scan-rate-
dependent CV of
square root of the scan rate from CV. (c) The graph of
standard rate constant (k0), Figure S5. The scan-rate-dependent CV of (a)
3
in 0.1 M [NBu4][PF6] acetonitrile solution. (b) The peak current density of 3 vs. the
−1/2
ϕ
vs.
v
by Nicholson method to calculate
, (b) , (c) and (d) in
1
2
4
5
0.1 M [NBu4][PF6] acetonitrile solution, Figure S6. The peak current density vs. the square root of the
−1/2
by Nicholson method to calculate
scan rate of (a)
standard rate constant of (b)
energies in Hartrees of (a) the neutral, (b) the reduced, and (c) the lithiated
The top and side views are shown at left and right panels, respectively, Figure S8. The optimized
geometries and total energies in Hartrees of (a) the neutral, (b) the reduced, and (c) the lithiated in
1
, (c)
2
, (e)
4
and (g)
5
, (f)
. The graph of
ϕ
vs.
v
1
, (d)
2
4
and (h) 5, Figure S7. The optimized geometries and total
in the solvated phase.
1
2
the solvated phase. The top and side views are shown at left and right panels, respectively, Figure
S9. The optimized geometries and total energies in Hartrees of (a) the neutral, (b) the reduced, and
(c) the lithiated
respectively, Figure S10. The optimized geometries and total energies in Hartrees of (a) the neutral,
(b) the reduced, and (c) the lithiated in the solvated phase. The top and side views are shown at left
and right panels, respectively, Figure S11. The optimized geometries and total energies in Hartrees of
(a) the neutral, (b) the reduced, and (c) the lithiated in the solvated phase. The top and side views
are shown at left and right panels, respectively, Figure S12. (a) Cyclic Voltammetry of the electrode
3 in the solvated phase. The top and side views are shown at left and right panels,
4
5
3
with different scan rates and (b) the plot of peak current density vs. the square root of the scan rate
for obtaining Li-ion diffusion coefficient, Figure S13. Cyclic Voltammetry of the tetrazine electrodes
with different scan rates and the plot of cathodic and anodic peak current density vs. the square root
of the scan rate. (a) and (b) for
1 electrode, (c) and (d) for 2 electrode, (e) and (f) for 4 electrode, (g)
and (h) for electrode, respectively, Figure S14. The cycle retention and the corresponding coulomb
5
efficiency of the s-tetrazine electrodes at 0.1 C, Figure S15. The correlation graphs about the measured
discharge voltages in Li-ion cells vs. the calculated reduction potentials considering Li-ion insertion.
The dashed red lines indicate linear fitting curves, and the corresponding regression coefficients are
presented in the graphs. In the linear regression, all five points were used for (a), but the point 1 and
the point 2 were excluded as an outlier for (b) and (d), respectively. The graph (c) is a magnified plot
of the circled region in (b), Figure S16. The LUMO orbital of neutral s-tetrazines (left) and the spin
1
density distribution of radical s-tetrazines (right) ((a)
1, (b)
2
, (c)
3, (d)
4
and (e)
5
), Figure S17. H
and 13C NMR spectra of 1, Figure S18. H and 13C NMR spectra of 2, Figure S19. H and 13C NMR
1
1
spectra of 4, Figure S20. H and 13C NMR spectra of 5, Table S1. Gibbs Free energies of a Li atom
1
and Li cation, Table S2. Redox potentials of tetrazines without a Li-ion insertion, Table S3. Redox
potentials of tetrazines with Li cation insertion.
Author Contributions: Conceptualization, D.J.M., J.E.K. and S.Y.P.; software, D.J.M. and K.L.; val-
idation, D.J.M. and H.P.; formal analysis, D.J.M.; investigation, D.J.M.; resources, D.J.M., K.L. and
H.P.; writing—original draft preparation, D.J.M.; writing—review and editing, J.E.K. and S.Y.P.;
visualization, D.J.M.; supervision, J.E.K. and S.Y.P.; project administration, J.E.K. and S.Y.P.; funding
acquisition, S.Y.P. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by Ministry of Science, ICT and Future Planning, grant num-
ber 2017R1E1A1A01075372[RIAM0417-20200051], Ministry of Education, grant number 2016R1
A6A3A04008134, Ministry of Science and ICT, grant number 2017M3A7B4041699.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available in article.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the s-tetrazines are available from the authors.

Molecules 2021, 26, 894
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