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The Effect of Imidazole on the Electrochemistry of Zinc Porphyrins: An Electrochemical and Computational Study Thai Thi Ha Tran, Guan-Ling Chen, Tuan K. A. Hoang, Ming-Yu Kuo, and Yuhlong Oliver Su J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05002 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 24, 2017

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The Journal of Physical Chemistry

The Effect of Imidazole on the Electrochemistry of Zinc Porphyrins: An Electrochemical and Computational Study Thai T. H. Tran,1 Guan-Ling Chen,1 Tuan K. A. Hoang,2 Ming-Yu Kuo*,1 and Yuhlong O. Su*1 1

Department of Applied Chemistry, National Chi Nan University, 1 University Road, Puli,

Nantou, Taiwan 545 2

Department of Chemical Engineering, University of Waterloo, 200 University Avenue,

Waterloo ON N2L 3G1, Canada

ABSTRACT: In this study, the electrochemical behavior of zinc meso-substituted porphyrins in the presence of imidazole is examined by using both cyclic voltammetry (CV) and density functional theory (DFT) methods. The results show that the first half-wave oxidation potentials (1st E1/2) of zinc porphyrins complexed with imidazole all move to negative side while the second ones (2nd E1/2) move positive side, resulting in larger half-wave oxidation potential splittings of the two oxidation states (∆E= second E1/2 - first E1/2) comparing with the zinc porphyrins. By employing DFT calculations, we have found that both sterically controlled inter π-conjugation between porphyrin rings and meso-substituted phenyl groups and deformation of porphyrin rings

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do play important roles in contributing to the half-wave oxidation potentials. Imidazole exhibits strong effects on the deformation of porphyrin rings which is dominant in determining the 1st E1/2 while the inter π-conjugation between porphyrin rings and meso-substituted phenyl groups mainly contributes to the 2nd E1/2. Without imidazole, the inter π-conjugation between porphyrin rings and meso-substituted phenyl groups is the only important criterion which effects both 1st E1/2 and 2nd E1/2 of zinc porphyrins.

INTRODUCTION The rich electrochemistry of porphyrinoids is the key for their wide applications in catalysis,1-4 electron-transfer systems,5-7 and photoelectric devices.8-11 Applications of metalloporphyrins in biological systems have been reported in many studies last few decades, notably the effects of axial ligands on metalloporphyrin structures.12-17 The electrochemical behavior of metalloporphyrins depends on many factors, such as the planarity of porphyrin rings, the electron-donating/withdrawing substituents, the central metal ions, the solution conditions, and the axial ligands.18-19 With the development of computational chemistry, employing Density Functional Theory (DFT) calculations in studying electrochemistry of metallopophyrins gains much research interests.13-15, 20-21 In 2012, we reported the electrochemical properties of various substituted free base meso-tetraphenylporphyrins (H2T(o,o-X)PP, H2T(o-X)PP, and H2T(p-X)PP, where X = OCH3, CH3, H, F, or Cl on the phenyl rings) exploiting a combination of experimental cyclic voltammetry (CV) and theoretical DFT methods.22 We have found that the effect of porphyrin ring deformation on the half-wave oxidation potential splitting (∆E = 2nd E1/2 - 1st E1/2) values of the free base prophyrins is less than the effect of sterically controlled πconjugation of the meso-phenyl groups to the central porphyrin ring, defined by the dihedral


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angles (ψ) between meso-phenyl groups and porphyrin ring. Recently, we have published a study on zinc porphyrins with meso-substituted five-membered heterocylic rings.23 We found that ∆E can be easily predicted by calculating spin density distribution of zinc porphyrin cation radicals on meso-carbon positions. Furthermore, dihedral angles (ψ) are an important factor in explaining the variation of half-wave oxidation potentials. The effects of substituent and axial ligands on the electrochemical behavior of zinc porphyrin complexes have been examined previously.12 The electrochemistry of three kinds of zinc porphyrins were studied by focusing on equilibrium constants of electron transfer processes.12 In this work, we continue to employ the combination of CV and DFT calculations to study the electrochemical behavior of zinc(II) meso-tetraphenylporphyrin (TPP), zinc(II) mesotetramesitylporphyrin (TMP), and zinc(II) meso-tetra(2,6-diclorophenyl)porphyrin (TDCPP) (Chart 1) with a particular interest in the effects of imidazole on the first and the second halfwave oxidation potentials of zinc meso-substituted porphyrins. CV was used to observe the oxidation processes of zinc porphyrins while DFT calculations were employed to study zinc porphyrins at molecule level, particularly identifying factors which affect the half-wave oxidation potentials. We find out that the first half-wave oxidation potentials, 1st E1/2, are smaller while second half-ware oxidation potentials, 2nd E1/2, are larger than those without imidazole. In our previous paper, for meso-free-base porphryins, dihedral angles (ψ) are mainly attributed to the half-wave oxidation potentials due to inter π-conjugation between porphyrin rings and mesosubstituted phenyl groups rather than the often discussed deformation of porphyrin.22 However, in this paper, we find that in the presence of imidazole, both porphyrin deformation and inter πconjugation contributed differently to half-wave oxidation potentials of zinc porphyrins at different oxidation states. To our knowledge, this examination has not been addressed before


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although there have been many publications regarding the effects of axial ligands on the electrochemistry of zinc porphyrins.12-17

Chart 1. Zinc Porphyrins Studied in This Work

EXPERIMENTAL SECTION General Synthesis. Meso-porphyrins were synthesized by using the reaction between pyrrole and carboxaldehyde of phenyl rings as previously reported.12 Zinc meso-porphyrins were synthesized by using the reaction between their respective bare porphyrin with zinc acetate (Zn(CH3COO)2.2H2O) in dichloromethane and methanol solvent mixtures (Scheme 1).12,24

Scheme 1. Synthesis Pathways of Zinc meso-Substituted Porphyrins


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Electrochemical Measurements. Tetra-n-butylammonium perchlorate (TBAP) was employed as the electrolyte for cyclic voltametry. TBAP was purchased from ACROS, then recrystallized twice from ethyl acetate (EtOAc) and dried in vacuum before use. Dichloromethane (CH2Cl2) was degassed by purging with prepurified nitrogen gas and dried prior to use. The CHI Model 760 series electroanalytical workstation was employed to collect electrochemical data. The three-electrode cell, in which the glassy carbon (area = 0.07 cm2) was the working electrode. A platinum wire was used as the auxiliary electrode. A homemade Ag|AgCl(KClsat) was employed as the reference electrode. The meso-substituted zinc porphyrins were dissolved in CH2Cl2 containing 0.1 M TBAP at room temperature. All CVs were recorded at a scan rate of 0.1 V/s and ferrocene was used as a calibration standard (+0.54 V versus Ag/AgCl). THEORETICAL CALCULATIONS DFT calculations were conducted by employing the Gaussian 09 software.25 All studied zinc porphyrin structures were optimized with B3LYP density functional. The LANL2DZ effectivecore potential basis set was employed for the zinc atom while the 6-31G(d) one was used for nonmetal atoms.26 All geometries were optimized without symmetry restrictions in the gas phase and checked their minima by their real vibrational frequencies. Because oxidation potential and ionization potential are correlated,22, 27we first calculated the first ionization potential (IP1), the second ionization potential (IP2). Also, spin densities of their cationic radicals and dihedral angles between porphyrin and meso-substituted rings were


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obtained. For comparison, we further calculated meso-substituted zinc porphyrins with planar porphyrin rings and orthogonal meso-phenyl groups. Experimental data and DFT calculations from our previous work have indicated that solvation effect is not a determining factor in our case.22, 23 Therefore, all DFT calculations were done in gas phase. The details of computational calculations are displayed in the Supporting Information (Table S1, S2 and S3). RESULTS AND DISCUSSION Electrochemical Behavior. The zinc meso-porphyrins are grouped into series 1 (zinc mesoporphyrins without imidazole) and series 2 (zinc meso-porphyrin complexes in the presence of imidazole). The cyclic voltammograms (CVs) of the two series in CH2Cl2 containing 0.1 M TBAP are displayed in Figures 1, 2, 3, S1, S2, and S3 while the half-wave oxidation potentials and their ∆E values are listed in Table 1. The CVs of the zinc meso-substituted porphyrins in series 1 exhibit distinctly two reversible oxidations (Figures S1, S2, and S3). When imidazole is present in the solution, the redox chemistry of the zinc porphyrins is still retained but the energies associated with this process are altered. The first oxidation potentials (1st E1/2) of zinc meso-substituted porphyrins gradually shift toward negative side while the second oxidation potentials (2nd E1/2) move toward positive side (Figures 1, 2, and 3). This results in a smaller 1st E1/2, a larger 2nd E1/2, and thus a larger ∆E values compared to the data of the original porphyrin materials (Table 1). For example, the 1st E1/2 of ZnTPP is about 0.84 V and the 2nd E1/2 is 1.13 V. However, the 1st E1/2 of HIm….ZnTPP is 0.70 V while its 2nd E1/2 is 1.19 V. Electron (e)-donating/withdrawing really affects to the halfwave oxidation potentials in both two series. When e-donating groups are available (ZnTMP), both 1st E1/2 and 2nd E1/2 are smaller while e-withdrawing groups (ZnTDCPP) deliver larger half-


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wave oxidation potentials compared to those without e-donating/withdrawing groups (ZnTPP). This is consistent with formation constants (Kf) for the binding of imidazole to zinc porphyrins (Table 1). The binding constants decrease in the order ZnTDCPP > ZnTPP > ZnTMP, which means zinc porphyrin with electron-withdrawing groups is more favorable in binding with imidazole, and this results in a larger in half-wave oxidation potentials than others.12

Figure 1. Cyclic voltammograms of 1.0 × 10-3 M ZnTPP in CH2Cl2 containing 0.1M TBAP and various equivalents of HIm. Scan rate = 0.1 V/s. The working electrode was glassy carbon


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Figure 2. Cyclic voltammograms of 1.0 × 10-3 M ZnTMP in CH2Cl2 containing 0.1M TBAP and various equivalents of HIm. Scan rate = 0.1 V/s. The working electrode was glassy carbon.

Figure 3. Cyclic voltammograms of 1.0 × 10-3 M ZnTDCPP in CH2Cl2 containing 0.1M TBAP and various equivalents of HIm. Scan rate = 0.1 V/s. The working electrode was glassy carbon Table 1. Half-Wave Oxidation Potentials (V vs. Ag/AgCl) and the Splitting between the First and the Second Oxidation Processes of Zinc meso-Substituted Porphyrins with/without Imidazole in CH2Cl2/TBAP (Fc+/0 = +0.54 V) CH2Cl2/TBAPa 2nd E1/2

1st E1/2

CH2Cl2/TBAPb ∆E c

2nd E1/2

Series 1 – without Imidazole ZnTPP 1.13 0.83 0.30 1.13 ZnTMP 1.11 0.78 0.33 1.11 ZnTDCPP 1.36 1.06 0.30 1.36 Series 2 – with Imidazole HIm…ZnTPP 1.19 0.70 0.49 1.40 HIm…ZnTMP 1.25 0.61 0.64 1.26 HIm…ZnTDCPP 1.42 0.90 0.52 1.43 a This work b Reference [12] c ∆E = 2nd E1/2 - 1st E1/2 d Formation constant for binding Imidazole with zinc porphyrins [12]

1st E1/2

Kf d

∆E c

0.84 0.78 1.06

0.29 0.33 0.30

0.70 0.63 0.91

0.70 0.63 0.52

5.13 x 104 1.92 x 104 5.11 x 105

DFT calculations. The DFT calculated ionization potentials (IP), spin densities, and dihedral between porphyrin ring and meso-substituted rings are listed in Table 2. The results show that the


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variation of the first ionization potentials (IP1) is consistent with the trend of the 1st E1/2 obtained from experiments. Complexation of imidazole (series 2) leads to smaller IP1 values, which means less energy is required to oxidize the materials. However, the calculated second ionization potentials (IP2) are not in good agreement with experimental results. The calculated IP2 in series 2 are still smaller than those in series 1 while the 2nd E1/2 obtained from experiments in series 2 are larger than those in series 1. This implies that IP calculations cannot be applied in this case to interpret the half-wave oxidation potentials. This fact shows the limitations of this parameter which should be applied in case of treating small organic molecules that are closely related in structures.22, 27 Table 2. Calculated Ionization Potentials (IP, eV) of Zinc meso-Substituted Porphyrins, Highest Spin Density Distribution of their Cation Radicals (ρmeso), and Dihedral Angles (ψ) between Central Porphyrin Ring and meso- Substituted Rings at Different Oxidation States* Ionization Potentials (eV) IP1

IP2

5.88 5.89 6.19

9.04 8.94 9.33

Highest Spin Densities (ρmeso)

Dihedral Angles (ψ) n=0

n=+1

n=+2

66.02 89.82 90.00

55.86 89.41 89.81

45.36 67.65 80.21

HIm…ZnTPP 5.47 8.48 0.256 63.23 HIm…ZnTMP 5.48 8.60 0.293 88.74 HIm…ZnTDCPP 5.72 8.92 0.295 88.54 (*) n = 0, +1, +2 for neutral, cation radical, and dicationic states, respectively

55.11 87.51 88.29

46.01 69.52 82.80

Series 1 ZnTPP ZnTMP ZnTDCPP

0.247 0.292 0.198

Series 2

The differences in the molecule orbitals of the two series explains the effect of imidazole on the electronic structures of zinc porphyrins as well as clarifies the differences in their electrochemistry. Both HOMOs and LUMOs of the two series are mainly located on porphyrin rings (Figure S4 and S5) and the highest spin density distribution is located on meso-carbon positions at cationic states (Figure 4). However, complexation with imidazole leads to higher


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spin densities on meso carbons of zinc porphyrins (ρmeso) (Table 2), thus, it requires less energy to remove an electron from the material to convert it from neutral state material to cation radical. This is consistent with smaller calculated first ionization potential IP1 values and a negative shift in the 1st E1/2 obtained from experiments. The increase in ρmeso of cationic radicals of zinc porphyrins complexed with imidazole is also correlated to a larger ∆E.23 While our previous papers show that diatonic zinc porphyrins at the singlet state have lower electronic energies than those at the triplet state,22, 23 using ρmeso cannot explain why it is harder to take an electron from cation radicals of imidazole complexes to form dicationic states and this results in upper moves of the 2nd E1/2 values.

Figure 4. Spin density distribution of cationic radical meso-substituted zinc porphyrins in gas phase: (a) without Imidazole and (b) in the presence of Imidazole. Isovalue was set to 0.004


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While steric hindrance between meso-substituents and porphyrin ring is an important factor in intriguing the electrochemical behavior of porphyrins22-23, comparison of the calculated dihedral angles (ψ) between porphyrin ring and meso-substituted groups at different oxidation states with/without imidazole possibly examine the half-wave oxidation potentials (Table 2). Compared with the zinc-porphyrin materials, the dihedral angle ψ of the radical cation of the imidazole-zinc porphyrin complexes are smaller while they are larger in the case of dications. This is consistent with the smaller 1st E1/2 and the larger 2nd E1/2. For example, the ψ of ZnTPP is 55.86o at cation radical state, but it is 55.11° for its imidazole complex. Meanwhile, ψ of ZnTPP at dicationic state is 45.36o but that of HIm…ZnTPP is 46.01o. The results show that there is a good agreement between calculated ψ and the half-wave oxidation potentials obtained from experiments. Imidazole axial ligand makes zinc porphyrin structures less steric hindrance at neutral and cation radical states, resulting in easier way to remove an electron from neutral state and a negative shift of 1st E1/2 in CV. Meanwhile, it makes zinc porphyrins more steric hindrance at dicationic state and this explains a harder way to take an electron from cationic radical state and a positive shift of 2nd E1/2. The geometry changes in zinc meso-substituted porphyrins in two series at different oxidation states are displayed in Figures 5. Imidazole binds to zinc ion center to form 5-coordinate metalloporphyrins in which the zinc ion center is moved above the porphyrin-four-nitrogen-atom plane and this delivers higher deformed porphyrin ring than the 4-coordinate zinc porphyrins. For the oxidized 5-coordinate metalloporphyrins, the bond length between N atom of imidazole and Zn ion center is shorter, and the deformation of porphyrin rings is larger (Figures 5, S6 and S7). At neutral state (n=0) and at cation radical state (n=+1), the 5-coordinate zinc mesosubstituted porphyrins are more deformed than the 4-coordinate ones, their dihedral angles are


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also smaller. However, at the dicationic state (n=+2), the dihedral angles of 5-coordinate zinc meso-substituted porphyrins are larger than those of 4-coordinate ones although their porphyrin ring deformation is larger. This is probably because of the effects of porphyrin deformation and/or inter-π conjugation between porphyrin rings and meso-substituted phenyl rings.

Figure 5. Optimized structures of ZnTPP, ZnTMP, and ZnTDCPP with and without Imidazole at neutral (n=0), cation radical (n=+1) and dicationic states (n=+2) in gas phase.


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To demonstrate the effects of porphyrin deformation and inter-π conjugation on zinc porphyrin molecules at different states of oxidation, we conceptualize and calculate several terms. These terms have been defined in our previous paper.22 Firstly, the saddle deformation energy Esaddle is defined as the difference between zinc porphyrins deformed with dihedral angle is 900 and orthogonal zinc porphyrins (Esaddle=E(deformation, ψ=90)-E(orthogonal). Secondly, the inter-π conjugation energy between porphyrin ring and meso-substituted phenyl groups, Eπ, is defined as the difference between optimized zinc porphyrin structures and deformed zinc porphyrins with dihedral angle is 900 (Eπ=E(deformation, ψ)-E(deformation, ψ=90). Finally, the total energy raising/lowering due to porphyrin deformation and the inter-π conjugation is Etotal=Eπ + Esaddle (Table 3). Table 3. Calculated Electronic Energy due to Porphyrin Deformation (Esaddle), Electronic Energy due to inter-π Conjugation (Eπ), and Total Electronic Energy due to Porphyrin Deformation and inter-π Conjugation (Etotal) at Different Oxidation States* Esaddlea (eV) n=0

n=+1

Eπb (eV) n=+2

n=0

n=+1

Etotal=Eπ + Esaddle (eV) n=+2

Series 1 0.037 0.188 0.300 ZnTPP -0.063 -0.325 -0.691 0.000 0.000 0.222 ZnTMP 0.000 0.000 -0.355 0.000 0.001 0.021 ZnTDCPP 0.000 0.000 -0.018 Series 2 HIm…ZnTPP -1.901 -1.920 0.501 +1.871 +1.799 -0.987 HIm…ZnTMP 0.000 0.010 0.165 0.000 +0.008 -0.217 HIm…ZnTDCPP 0.000 0.000 0.003 0.000 +0.001 -0.006 (*) n = 0, +1, +2 for neutral, cation radical, and dicationic states, respectively a Esaddle=E(deformation, ψ=90o)-E(orthogonal) (eV) b Eπ=E(ψ)-E(ψ=90o) (eV)

n=0

n=+1

n=+2

-0.026 0.000 0.000

-0.137 0.000 0.000

-0.390 -0.133 0.003

-0.030 0.000 0.000

-0.121 +0.018 0.000

-0.486 -0.052 -0.003

The calculated results in Table 3 show that, for ZnTPP, the deformation increases electronic energy of the neutral, cation radical, and dicationic states. Inter-π conjugation between central


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porphyrin ring meso-substituted rings leads to lower energy levels for ZnTPP at three different oxidation states. Because the inter-π conjugation energy Eπ is larger than the energy-rising (due to porphyrin deformation),22 the total electronic energy of ZnTPP, therefore, depends mainly on inter-π conjugation (Table 3). In the presence of imidazole, saddle deformation benefits in terms of energy for HIm…ZnTPP at both neutral and cation radical states, it also leads to enhancement of electronic energy of the dicationic state. Meanwhile, inter-π conjugation results in increases in electronic energy at neutral and cation radical states and a decrease in energy at the dicationic states (Table 3). Because the energy increase due to inter-π conjugation of HIm…ZnTPP (+1.871 and 1.799 eV at n = 0 and n=+1, respectively) is compensated by the energy decrease due to porphyrin deformation (-1.901 and -1.920 eV), the total electronic energy of HIm…ZnTPP is determined mainly by the saddle deformation energy (-0.030 and -0.121 eV) (Table 3). However, at the dicationic state, the energy decrease due to inter-π conjugation of HIm…ZnTPP (-0.987 eV) is offset by the energy increase due to porphyrin deformation (+0.501 eV), so the total electronic energy is determined by inter-π conjugation (-0.486 eV). The ZnTMP and ZnTDCPP possess nearly orthogonal structures, thus, the effects of both porphyrin deformation and inter-π conjugation cannot be seen clearly through the calculated total electronic energy. Beside the porphyrin ring deformation and the inter-π conjugation, electron donating/withdrawing groups also contribute significantly to their electrochemical behavior. The presence of e-donating groups on ZnTMP makes it more susceptible to oxidation than ZnTPP, resulting in smaller experimentally-derived 1st E1/2 and 2nd E1/2. On the other hand, with the presence of e-withdrawing groups in the ZnTDCPP structure makes it more inert to oxidation and results in larger in 1st E1/2 and 2nd E1/2.12


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Figure 6. Total electronic energy diagram of zinc porphyrin molecules: (a) without imidazole and (b) with imidazole

The correlation between the calculated total electronic energy and dihedral angles at different oxidation states is displayed in Figure 6. When zinc porphyrin is oxidized, the porphyrin ring is deformed. The more ZnTPP oxidized, the larger extent of the deformation is. Without imidazole, porphyrin deformation only leads to increases in electronic energy and decreases in dihedral


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angles ψ when oxidized but inter-π conjugation is dominant. In the presence of imidazole, saddle deformation is dominant and the 5-ordinate structure has smaller dihedral angle ψ at cation radical state, aligning smaller first oxidation potential. At the dicationic state, inter-π conjugation is dominant for the 5-ordinate structure with a larger dihedral angle ψ, which results in a larger second oxidation potential. This study confirms that the dihedral angles are consistent to oxidation potentials obtained from experiment. Both porphyrin deformation and inter-π conjugation attribute to the electrochemistry of 5-coordinate zinc porphyrins. Meanwhile, inter-π conjugation mainly affects to electrochemical behavior of 4-coordinate zinc porphyrin complexes. This deserves further research attention.

CONCLUSIONS Employing CV and DFT calculations, we have examined the effect of imidazole to the electrochemistry of zinc porphyrins. Imidazole binds to zinc ion center to form 5-coordinate metalloporphyrins and changes the electrochemistry of the materials significantly. The 5coordinate zinc porphyrins have lower first half-wave oxidation potentials but the second halfwave oxidation potentials are higher than the 4-coordinate zinc porphyrins. In the 5-coordinate zinc porphyrins, the deformation of porphyrin ring mainly contributes to the oxidation potentials at the neutral and cation radical states while the inter-π conjugation controls oxidation potentials at the dicationic state. In the 4-ordinate zinc porphyrins, only inter-π conjugation contributes to both first and second oxidation potentials. In this study, a good agreement between calculated results and experimental data is achieved and this suggests that both porphyrin deformation and inter-π conjugation between porphyrin ring and meso-phenyl groups attribute to the


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electrochemical behavior of 5-coordinate zinc porphyrins. This will benefit future studies about the underlying materials. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. CVs of zinc porphyrins and theoretical calculations. AUTHOR INFORMATION Corresponding Authors (Y.O.S.) E-mail: [email protected] (M.-Y.K.) E-mail: [email protected] ACKNOWLEDGMENTS The authors are grateful to the Ministry of Science and Technology for the support of this work. We thank Dr. Kuo-Yuan Chiu for supporting the experiment. The authors declare no competing financial interest.

REFERENCES [1] Paul, N. D.; Mandal, S.; Otte, M.; Cui, X.; Zhang, X. P.; de Bruin, B. Metalloradical Approach to 2H-Chromenes. J. Am. Chem. Soc. 2014, 136, 1090−1096.


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[2] Zhu, S-L.; Xu, X.; Ou, S.; Zhao, M.; He, W-L.; Wu, C-D.

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Assembly of a

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Table of Contents Graphic


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Effect of Imidazole on the Electrochemistry of Zinc ... - ACS Publications

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