University of Southern Denmark

Computational Electrochemistry Study of Derivatives of Anthraquinone andPhenanthraquinone Analoguesthe Substitution EffectWang, Zhen; Li, Anyang; Gou, Lei; Ren, Jingzheng; Zhai, Gaohong

Published in:RSC Advances

DOI:10.1039/C6RA19128B

Publication date:2016

Document version:Final published version

Citation for pulished version (APA):Wang, Z., Li, A., Gou, L., Ren, J., & Zhai, G. (2016). Computational Electrochemistry Study of Derivatives ofAnthraquinone and Phenanthraquinone Analogues: the Substitution Effect. RSC Advances, 6(92), 89827-89835.https://doi.org/10.1039/C6RA19128B

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Computational e

aKey Laboratory of Synthetic and Natural Fu

Education, College of Chemistry and Mater

710127, China. E-mail: liay@nwu.edu.cn; zbSchool of Materials Science and Engineeri

ChinacDepartment of Technology and Innovat

Campusvej 555230, Odense M, Denmark

† Electronic supplementary informa10.1039/c6ra19128b

Cite this: RSC Adv., 2016, 6, 89827

Received 28th July 2016Accepted 8th September 2016

DOI: 10.1039/c6ra19128b

www.rsc.org/advances

This journal is © The Royal Society of C

lectrochemistry study ofderivatives of anthraquinone andphenanthraquinone analogues: the substitutioneffect†

Zhen Wang,a Anyang Li,*a Lei Gou,b Jingzheng Renc and Gaohong Zhai*a

The substituent effect on fused heteroaromatic anthraquinone and phenanthraquinone are investigated by

density functional calculations to determine some guidelines for designing potential cathode materials for

rechargeable Li-ion batteries. The calculated redox potentials of the quinone derivatives change

monotonically with increasing number of substitutions. Full substitution with electron-withdrawing

groups brings the highest redox potential; however, mono-substitution results in the largest mass energy

density. Carbonyl groups are the most favorable active Li-binding sites; moreover, intramolecular lithium

bonds can be formed between Li atoms and electronegative atoms from the substituent groups. The

lithium bonds increase the redox potential by improving the thermodynamic stabilization of the lithiation

derivatives. Furthermore, the calculation of nucleus-independent chemical shift indicates that the

derivatives with Li-bound carbonyl groups are more stable than the bare derivatives.

1. Introduction

Driven by the requirement of sustainable development, theresearch focus has shied from conventional inorganic elec-trode materials based on the lithium metal oxides to organicelectrode materials in the past decades.1–3 Organic conjugatedcarbonyl compounds such as quinone,4–6 anhydride,7,8 oxo-carbon salt,9,10 Li-carboxylate salt11,12 and diimide dilithiumsalt13 are considered to be promising candidates as electrodematerials because of their excellent electrochemical redoxcharacteristics. Among them, quinone and its derivativesattract considerable attention due to their superior electrontransfer properties, stability, lower cost for electricity storage,and the ability to tune their electronic properties by makingmodications to the structure and substituents.14–23 Forexample, the incorporation of heteroaromatic structures isrecommended to improve electrical conductivity,15 and theredox potential can be increased via chemical substitutions.19

The large number of candidate quinone derivatives posesa challenge for the identication of optimal redox couples.

nctional Molecule Chemistry, Ministry of

ials Science, Northwest University, Xi'an

gh@nwu.edu.cn

ng, Chang'an University, Xi'an, 710061,

ion, University of Southern Denmark,

tion (ESI) available. See DOI:

hemistry 2016

Experimental searching is extremely time-consuming andexpensive. A reliable computational method could be viablealternative approach for rapid screening with minimal invest-ment. Several previous computational studies have examinedthe redox properties of quinone derivatives and provide someunderstanding of electrochemical windows at the molecularlevel. Assary and co-workers investigated the inuence offunctional groups on the redox windows of anthraquinonederivatives and aromatic nitrogen-containing quinones.16,17

They found a relationship between the computed lowest unoc-cupied molecular orbital (LUMO) energy and the redox poten-tial used to screen reduction windows of novel molecules.Hernandez-Burgos et al. studied the heteroatom and substit-uent effects in carbonyl-based organic molecules and identiedthe most promising candidate materials for electrical energystorage.18 Er et al. predicted the redox potentials and solvationfree energies of quinone derivatives using high-throughputcomputational screening with the aim of designing quinonemolecules for use in organic-based aqueous ow batteries.4 Veryrecently, Kim et al. reported the Li-binding thermodynamicsand redox potentials of quinone derivatives.6 Most of theseworks are based on density functional theory (DFT), which hasproven a robust and useful method for the design of newmolecules capable of participating in redox processes.24

Anthraquinone and phenanthraquinone and their deriva-tives are environmentally friendly and possess fast reactionkinetics, which are required in organic cathode materials.However, their redox potentials are lower than the potentials ofconventional inorganic electrode materials. As recommended

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Fig. 1 Molecular scheme of the anthraquinone and phenanthraquinone analogues used in this study.

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by previous studies, the addition of substituent groups could bean attractive strategy to increase the potentials of these mole-cules. In this work, we focus on the effects of substituentgroups on fused heteroaromatic anthraquinone- andphenanthraquinone-bearing thiophene, furan and pyridinerings (i.e., anthraquinone analogues benzo[1,2-b:4,5-b0]dithiophene-4,8-dione (BDTD), benzofuro[5,6-b]furan-4,8-dione(BFFD), and pyrido[3,4-g]isoquinoline-5,10-dione (PID) andphenanthraquinone analogues benzo[1,2-b:4,3-b0]dithiophene-4,5-quinone (BDTQ), benzo[1,2-b:4,3-b0]difuran-4,5-dione(BDFD), and benzo[2,1-b;3,4-b0]dithiophene-4,5-dione (TQ),1,10-phenanthroline-5,6-dione (PhenQ)) which are illustrated inFig. 1. The effects of the incorporated functional groups on theredox potentials of the quinone derivative couples are investi-gated for single substitution to full substitution. The Li-bindingproperties between Li atoms and substituent groups aresystematically studied to understand the inuence of the posi-tion of the substituent on the reduction potential. In addition,aromaticity is calculated using nucleus-independent chemicalshi (NICS) as a proxy to characterize the stability of the system.

2. Computational details

The reduction of a quinone was treated as a single-step, two-electron, two-Li+ process as suggested by Er et al.4 The redoxpotential was calculated by the Nernst equation E ¼ �DG/nF,where n ¼ 2 is the number of electrons transferred and F is theFaraday constant. Similar to in recent studies,16,23 the Gibbs freeenergy DG of a species M was computed by a thermodynamiccycle:

Mgas þ 2egas� þ 2Ligas

þ�����!DrGgas

MLi2gasYDGsolðMÞYDGsolðe�ÞYDGsolðLiþÞYDGsolðMLi2ÞMsol

þ 2esol� þ 2Lisol

þ�����!DG

MLi2sol (1)

Thus DG ¼ DrGgas + DGsol(MLi2) � DGsol(M) � 2DGsol(e�) �

2DGsol(Li+), where DrGgas is the reaction Gibbs free energy in the

gas phase. The gas-phase geometries of each species were

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optimized at the B3LYP/6-31+G(d,p) level,25,26 and the Gibbsenergies were calculated at T ¼ 298.15 K and P ¼ 1 atm. Basedon the optimized gas-phase geometries, single-point energycalculations were performed using the SMD model27 to obtainthe solvation free energy of each species, DGsol. The SMDmodelwas employed using dimethylsulfoxide as the solvent with thedielectric constant (3 ¼ 46.7) t to model the bulk solvent effectof a ethylene carbonate-dimethyl carbonate (EC-DMC) mixture8

(3 ¼ 85.1 and 3.1, respectively), which is typically employed asthe electrolyte solvent.14 This is an effective approximation forcomputing the free energies of redox-active species in solution.In addition, the energy of electrons DGsol(e

�) was approximatedas zero. All calculations were carried out using the Gaussian 09soware.28

With respect to the reference potential of Li/Li+, the calcu-lated redox potential Ecal is shied by 1.24 (Ecal¼ E� 1.24) sincethe standard hydrogen electrode (SHE) is �4.28 V versusvacuum and Li/Li+ is�3.04 V versus SHE. The ab initio Ecal mightagree quantitatively with the experimental value; however, thetwo are not directly equivalent18,23 due to cell resistance inmeasurements or a lack of a better description of the electrolytesolvent in calculations. In addition to B3LYP, other functionalssuch as B98,29 PBE1PBE,30 BHandHLYP,31 mPW1PW91,32

uB97XD33 and M06 34 are used to check the accuracy of thecalculations. Fig. 2 compares the calculated values using B3LYPto the experimental redox potentials of 12 molecules (seeFig. S1†) containing both anthraquinone and phenan-thraquinone analogues investigated in this paper. It is clear thatthe linear correlation between the calculated and experimentalvalues is excellent, conrming the validity of our calculations.The linear regression coefficients of determination (R2) ob-tained using other functionals are also close to one, but nonewere better than that obtained using B3LYP (see Fig. S2†). In thediscussion below, the linear t shown in Fig. 2 is used to predictthe formal potentials.

In addition, the aromaticities of these derivatives wereevaluated by NICS.35,36 The NICS values were calculated usingthe gauge-independent atomic orbital method at the B3LYP/6-31+G(d,p) level with the same solvent parameters based on the

This journal is © The Royal Society of Chemistry 2016

Fig. 2 The fitting line of calculated redox potentials (vs. Li/Li+) at theB3LYP/6-31+G* level to experimental values.

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optimized structures. The NICS values 1 A above the center ofring place (NICS(1)),are recommended as good measures of peffects. A more negative NICS value corresponds to a higheraromatic character of the molecule, while a positive NICS valuecorresponds to antiaromaticity.

3. Results and discussion3.1 Substituent effects on the redox potential and energydensity

The redox potentials (Ecal) of BDTD with typical electron-withdrawing substituent CN and electron-donating substit-uent NH2 are listed in Table 1. It is clear that when thesubstituent is CN, all the nine derivatives increase the Ecal of thepure parent BDTD. However, the functionalization of BDTDwith NH2 shows the opposite effect, regardless of the number orsite of the substituent. During the reduction process, a lower-energy LUMO could lead to a higher redox potential sinceelectrons prefer to ll in the orbital with lowest energy. Ingeneral, electron-withdrawing groups (EWGs) are supposed todecrease the energy of the LUMO, whereas electron-donatinggroups (EDGs) increase it.16,23 As shown in Table 1, the LUMOenergies continue to decline with increasing number ofelectron-withdrawing groups (CN groups). Consequently, thecorresponding redox potential grows. For mono-substitutions,the LUMO energies decrease by 0.4 to 0.5 eV, and the valuesof Ecal increase by 0.2 to 0.3 V. The potential growth of the fourdi-substitutions derivatives are over 0.5 V to be 3.0 V or greater.When three H atoms are replaced by CN, the LUMO energies arereduced by more than 1.0 eV, and the potentials reach 3.2 to 3.3V. In the last row of Table 1, the full CN-substituted derivativehas the highest Ecal (3.460 V) and the lowest LUMO energy(�5.05 eV). On the other hand, increasing the number ofelectron-donating groups (NH2) tends to increase the LUMOenergies and lower the redox potentials.

To verify the effect of substitution on redox potential, morefunctional groups (CH3, PO3H2, F, COOH, SO3H and NO2) wereinvestigated at the same calculation level. The Ecal values of

This journal is © The Royal Society of Chemistry 2016

BFFD derivatives incorporating all eight substituents were alsocalculated for comparison. The horizontal red and blue lines inFig. 3 indicate the calculated redox potentials Ecal of unsub-stituted BDTD and BFFD. The mean values of Ecal for allpossible single, double, triple and quadruple substitutions foreach of the functional groups are indicated. As shown in Fig. 3,the EDGs such as NH2 and CH3 are expected to decrease theredox potentials with the values below the lines, while theEWGs, such as PO3H2, F, COOH, SO3H, NO2 and CN, areeffective in promoting the Ecal. Conrming the trend discussedabove, Ecal changes monotonically with increasing number ofsubstitutions. This is in accordance with previous works.4,16

It should be noted that the redox potentials of mono-substituted derivatives of BFFD with all the functional groupsare larger than those of BDTD derivatives. Comparing thesemolecules with molecules containing sulfur as the heteroatom(BDTD derivatives), the oxygen-containing molecules have lowerLUMO energy levels, corresponding to higher redox potentials.However, the multi-substituted derivatives do not follow thisrule. For di-, tri- and tetra-substituted molecules with F and CN,the calculated potentials of BFFD derivatives are lower thanthose of BDTD derivatives. More substituents bring complexstructures and different Li atom binding effects, which is dis-cussed in the next part of this section.

The redox potential is one of the parameters to improve inthese organic cathode materials. Mass energy density, namelystored electrochemical energy per unit mass, is also a veryimportant parameter when choosing the electrode material forlithium ion batteries. The calculation of mass energy density (r¼ nFE/3.6M) depends on molecular mass (M), redox potential(E), and the number of electrons transferred (n). When the Hatoms are substituted by functional groups, additional mass isadded to the molecule, resulting in a decrease in the massenergy density. Though the redox potential is the highest whenBDTD is fully substituted with CN groups, the theoretical energydensity is reduced relative to the parent BDTD (see Table 1).However, the mono-, di- and tri-substituted molecules all havelarger energy densities, and the largest one is the derivative witha single CN group substituted at a specic site. Aer the CN-substituted molecule, all the mono-substituted BDTD mole-cules with EWGs have the largest energy densities.

3.2 Effect of substituent position on the redox potential

As mentioned above, the positions of the substituted groupshave signicant effects on the redox potentials. Takingaccount of the mass energy density, we focus on the anthra-quinone and phenanthraquinone analogues incorporatingsingle EWGs. All possible substitution sites on a quinone aresystematically studied. The substitution of EWGs close to thecarbonyl group in anthraquinone analogues, such as 1-b, 2-b,3-b and 3-c (seen in Fig. 4a), increase the Ecal more effectivelythan substitution with remote ring hydrogen. Carbonyl groupsare the most favorable active Li-binding sites.6 Indeed, two Liatoms bind strongly at each carbonyl group of all the deriva-tives, as shown in Fig. S3.† However, when the functionalgroups are closed to the carbonyl group, such as in the b-type

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Table 1 The LUMO energies (LUMO), predicted redox potentials (Ecal) and mass energy densities (r) of BDTD with CN and NH2

Numbers of substituent Molecular structure

R ¼ CN R ¼ NH2

LUMO (eV) Ecal (V) r (W h kg�1) LUMO (eV) Ecal (V) r (W h kg�1)

0 �3.54 2.474 602.79 �3.54 2.474 602.79

1

�4.03 2.678 585.98 �3.30 2.372 541.06

�3.93 2.885 631.27 �3.45 2.428 553.74

2

�4.34 3.051 605.51 �3.28 2.356 503.87

�4.39 3.082 611.47 �3.22 2.315 495.29

�4.30 3.170 627.35 �3.37 2.384 510.30

�4.47 2.994 593.60 �3.03 2.246 480.28

3

�4.76 3.222 585.09 �3.02 2.241 453.10

�4.69 3.322 603.26 �3.21 2.310 465.23

4 �5.05 3.460 579.58 �3.01 2.235 427.87

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or c-type derivates, one Li can bind with the electronegativeatoms (N, O and F) of the EWGs. To provide deep insight intothis feature, the binding energies and Wiberg bond orders37

between Li with other atoms are calculated based on theoptimized geometries of the lithiation states of mono-substituted derivatives. For instance, Fig. 5 shows the stablestructures and binding energies of the CN-, NO2- and F-substituted BDTD derivatives with lithium, and some bondlengths and Wiberg bond orders are also marked. For the a-

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type derivatives, the negative binding energies indicate favor-able binding, the carbonyl oxygen forms a chemical bond witha Li atom since the bond lengths are about 1.7 A less than thesum of the covalent bond radii (1.94 A),38 and the bond ordersof the Li–O bonds are even close to 1. For the b-type deriva-tives, the orders of two Li–O bonds show almost no change;however, one Li–O bond deects, and other binding sites areactivated. The newly formed bonds of Li–N (or O, F), where thelone-pair electrons of the N, O and F atoms of the EWGs act as

This journal is © The Royal Society of Chemistry 2016

Fig. 3 Effects of functional group substitutions on the redox potential. The horizontal red and blue lines indicate the predicted redox potentialsof unsubstituted BDTD and BFFD, respectively. The symbols indicate potentials for all possible single and multi-substitutions for each of thefunctional groups. The 1,2,3-substitution value was obtained from the average of the isomers.

Fig. 4 Positional effects of functional group on the Ecal for single electron-withdrawing group (NO2, SO3H, PO3H2, COOH, CN and F) substi-tution of (a) anthraquinone analogues and (b) phenanthraquinone analogues.

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Fig. 5 Structures and total binding energies of the CN-, NO2- and F-substituted BDTD (1-a and 1-b) derivatives with lithium. Total bindingenergies (BE) are defined as BE ¼ E(MLi2) � E(M) � 2E(Li). The values of bond length and Wiberg bond orders of Li–O (N or F) are also marked byan underline and in italic, respectively.

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electron donors, could be considered as intramolecularlithium bonds (ILBs),39 which are similar to the intramolecularhydrogen bonds in hydroquinone derivatives40 but muchstronger. As shown in Fig. 5, the Wiberg bond orders of theILBs are about 0.5, while the hydrogen bond orders mostly areless than 0.1.40 Moreover, the bond lengths of the ILBs are onlyslightly larger than the distance between Li and carbonyloxygen, and six- or seven-membered cyclic structures areformed with bridging Li atoms. Accordingly, the binding

Fig. 6 Structures and total binding energies of the CN-, NO2- and F-subond orders of Li–O (N or F) are also marked in italic.

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energies are more negative as a result of electronic delocal-ization in the more stable quasi-ring. Therefore, the presenceof the intramolecular lithium bonds makes the moleculesmore difficult to oxidize, which enhances the redox potential.

For the derivatives of phenanthraquinone analogues, thesimilar position dependence of the redox potentials is pre-sented for TQ and PhenQ in Fig. 4b. The Ecal values of 6-b and 7-c are signicantly larger than those of 6-a and 7-a or b,respectively. However, the position effects are not obvious when

bstituted BDTQ (4-a) and BDFD (5-a) derivatives with lithium. Wiberg

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fused oxygen or sulfur atoms are close to the carbonyl group,such as in BDTQ and BDFD. As shown in Fig. 6, the 1Li atomlocates between two carbonyl oxygen atoms, and a ve-membered ring is formed by including two lithium bonds.One of the lithium bonds is a little stronger with bond ordermore than 0.6, and the weaker one could be affected by the 2Liatom, since the 2Li atom just binds one carbonyl oxygen in theside way. The weak position effects of BDTQ and BDFD could beexplained as there is no intramolecular lithium bond formedaround the substituent, due to the substituent is far from thecarbonyl group. On the other hand, since the electronegativityof the O atom is larger than that of the S atom, ILBs formedbetween the 2Li atoms and the fused oxygen atoms of the siderings of BDFD derivatives. Although the bond order of the ILB is

Fig. 7 NICS(1) values in ppm of each ring of the mono-substituted BDTD

This journal is © The Royal Society of Chemistry 2016

less than those of the other three Li–O bonds, the additionalbonds lead to more stable structures, which are reected by themore negative binding energies.

3.3 Stability studies using NICS

The substitution of EWGs adjacent to the carbonyl group couldmake the lithiated anthraquinone and phenanthraquinoneanalogues more stable, which is benecial for the battery cyclelife and safety of the battery system. In addition to structuralfeatures, aromaticity is a simple and efficient measurement ofthe stability of such cyclic organic molecules. In terms of theelectronic nature of the molecule, aromaticity describes conju-gated systems allowing for the electrons to be delocalized

and BDTQ derivatives with six different electron-withdrawing groups.

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around the ring, increasing the molecule's stability. Here, wechose NICS, which is based on magnetic properties, to deter-mine the aromaticity. A more negative NICS value indicatesgreater electron shielding via a stronger ring current andcorresponds to greater aromatic character. NICS values of eachring of the mono-substituted derivatives and their lithiationstates are calculated. Fig. 7 shows the NICS(1) values (ppm) forthree rings of BDTD and BDTQ derivatives with six EWGs as anexample (more NICS(1) results are found in Table S1†). Ingeneral, the NICS values of the two rings containing hetero-atoms on both sides are negative for each molecule. However,the NICS values of center rings attached to carbonyl groups arepositive. When the molecules are lithiated, a negative NICSvalue is obtained in the center ring. This result indicates thatthe derivatives with Li-bound carbonyl groups are more stablethan the bare derivatives, which agrees with previous studies oncarbonyl-based organic molecules.18 In addition, the NICSvalues of the center lithiated quinone rings of b-type BDTDmolecules are more negative than those of the a-type isomers.The formation of intramolecular lithium bonds could increasethe stability of the lithiated quinone ring by extending the rangeof electron delocalization. This idea is also supported by themore negative NICS values of the BDFD derivatives compared tothe BDTQ derivatives (see Table S1b†).

4. Conclusion

As cathode materials for lithium-ion batteries, anthraquinoneand phenanthraquinone derivatives show similar electro-chemical characteristics as other quinone compounds. Theredox potential increases with increasing number of EWGs;thus, full substitution could bring the highest redox potential.Moreover, in term of the mass energy density, single substitu-tion is the best choice. Based on the DFT calculation results ofseven anthraquinone and phenanthraquinone analoguesincorporating eight functional groups, we found that thesubstitution effects could be enhanced by the formation of ILBsbetween Li atoms and the electronegative atoms of thesubstituent group as well as intramolecular hydrogen bonds inhydroquinone derivatives. The lithium bonds increase theredox potentials by improving the thermodynamic stabilizationof the lithiation derivatives. For example, when the substituentsite is close to the carbonyl group, as in the b-type structure ofBDTD, its redox potential is higher than that of the a-typeisomer since more stable six- or seven-membered cyclic struc-tures are formed with bridging Li atoms. To the best of ourknowledge, this study is the rst to systematically study the Li-binding properties between Li atoms and substituent groups.Moreover, we studied the stabilities of the systems usingaromaticity, which was calculated with NICS. The NICS values ofthe side rings of these derivatives are always negative andundergo reversible redox processes, while the NICS values of thecenter rings change from positive to negative aer the moleculeis lithiated. The results of this study imply that EWGs havea favorable effect on the redox potentials of organic electrodesand may be useful in establishing guidelines for designingbetter electrode materials.

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Acknowledgements

This work is supported by the National Natural Science Foun-dation of China (No. 21103013), the Natural Science BasicResearch Plan in Shaanxi Province of China (Program No.2016JM2027), and the Scientic Foundation of NorthwestUniversity (No. 338050068).

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