Electrochemical Behavior of meso-Substituted Porphyrins: The Role of

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Electrochemical Behavior of meso-Substituted Porphyrins: The Role of Cation Radicals to the Half-Wave Oxidation Potential Splitting Thai Thi Ha Tran, Yan-Ru Chang, Tuan K. A. Hoang, Ming-Yu Kuo, and Yuhlong Oliver Su J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b03538 • Publication Date (Web): 05 Jul 2016 Downloaded from http://pubs.acs.org on July 12, 2016

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The Journal of Physical Chemistry

Electrochemical Behavior of meso-substituted Porphyrins: The Role of Cation Radicals to the HalfWave Oxidation Potential Splitting Thai T. H. Tran,1 Yan-Ru Chang,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 free base and zinc meso-substituted porphyrins is examined by cyclic voltammetry (CV) and density functional theory (DFT). The results show that the half-wave oxidation potential splitting of the two oxidation states (∆E= 2nd E1/2 - 1st E1/2) of tetraphenylporphyrin (H2TPP) and its zinc complex (ZnTPP) are higher than those of porphyrins and their zinc complexes with meso-substituted five-membered heterocylic rings. The ∆E values follow the trend of TPP > T(3’-thienyl)P > T(3’-furyl)P > T(2’-thienyl)P for both meso-porphyrins and their respective zinc complexes. By employing DFT calculations, we have found that the trend of ∆E values is consistent with that of highest spin density (HSD)

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distribution and HOMO-LUMO energy gaps of cationic radicals as well as the π-conjugation between central porphyrin and meso-substituted rings. Also, they exhibit the better resonance between the porphyrin ring with meso-substituted rings as moving from porphyrins and their zinc complexes with phenyl rings to five-membered heterocyclic rings. A good agreement between calculated and experimental results indicates that cationic radicals, especially their spin density distribution, do play an important role in half-wave oxidation potential splitting of mesoporphyrins and their zinc complexes.

INTRODUCTION Porphyrinoids and their electrochemistry have been attracting much attention due to their wide applications in catalysis,1 electron-transfer systems,2 and photoelectric devices.3 Porphyrins and metalloporphyrins contain extensive conjugated π- ring systems. The electron transfer behavior of porphyrins and their metal complexes depends on the degree of delocalization of electronic structures.4 The higher the π system delocalizes, the easier the electron uptakes or releases thanks to minor changes in the structure upon on electron transfer.4 The extent of structural changes in the reversible electron-transfer reactions can be examined via the half-wave oxidation potential difference between the first and the second oxidation state (∆E = 2nd E1/2 - 1st E1/2).5 The relationships between experiment and calculated ∆E values have been elaborated in the litterature.6,7,8,9 By combining experimental work and computational calculations, Lambert and Nöll found that the half-wave redox potential splitting (∆E) of the two redox processes of a mixed-valence compounds has a linear correlation to electronic coupling (V) of the reactant state and the product state,6 which can be described as a function of the overlap of the donor and


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acceptor orbitals.4 While the calculation methods of electronic coupling are still being developed.6,7,8 Winter stated in his review of relationship between half-wave potential splitting and the degree of electronic coupling that the ∆E only provides a qualitative, not quantitative, measurement of electronic coupling of mixed-valence compounds.9 There are cases of small ∆E values despite of strong electronic coupling and others of no coupling at all although their ∆E are significantly acknowledged.9 Another parameter, found by Gosh and co-workers on the electrochemical behavior of Mg (II) porphyrins, is the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)10. This energy gap decreases in the order of six-membered phenyl rings > five-membered thienyl > furyl rings, which are associated with a decrease in ∆E values. In another study on the prediction of redox potentials of diverse organic molecules and free radicals, Yu et al. found that there is a good correlation between redox potentials (E1/2) and ionization potentials (IP).11 However, their studies were restricted in some small groups of closely related organic compounds. Furthermore, there was no clear relationship between the ∆E and key parameters established. While cyclic voltammetry (CV) is a popular method to study electrochemical behavior of new systems5 and density functional theory (DFT) is the promising approach to calculate the electronic states of chemical compounds,12 we, recently, have published the study on the electrochemical behavior of various substituted free base meso-tetraphenylporphyrins (H2T(o,oX)PP, H2T(o-X)PP, and H2T(p-X)PP, where X = OCH3, CH3, H, F, or Cl on the phenyl rings) by combining CV and DFT methods. We have found that ∆E values of these free base prophyrins are associated with the sterically controlled π-conjugation of the meso-phenyl groups to the central porphyrin ring, defined by the dihedral angles (ψ) between meso-phenyl groups and


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porphyrin ring.13 Our study also revealed that the trend of ∆IP follows the trend of ∆E by changing positions of the substituents at the phenyl ring. In this work, we continue to employ the combination of CV and DFT calculations to study the electrochemical behavior of porphyrins and their metal complexes with meso-substituted phenyl and five-membered rings (Chart 1) with a particular interest in exploiting the effect of the highest spin densities of cation radicals toward chemical properties. These free base porphyrin and zinc porphyrins are among the most studied in dye-sensitized solar cells (DSSCs). Zinc porphyrins exhibit higher short circuit currents than free base porphyrin analogue because of difference in their excited state levels. Zinc porphyrins exhibit a longer-lived singlet excited state (> 1ns) without singlet/triplet mixing.3 While the delocalization of electron is still considered as an important factor on electrochemical properties of porphyrins,4,6 we herein focus on the effect of the spin density distribution of their cationic radicals based on DFT calculations on the ∆E values obtained from CV and the dihedral angles (ψ) defined in our previous paper.13 The relationship between oxidation potentials of the free base and metal-porphyrins have been studied widely in the literature, which delivers some established relationship between the HOMO-LUMO energy gaps and the oxidation potentials.14 In this work, energy gaps between HOMO and LUMO of porphyrin cation radicals are calculated to elucidate the effect of cation radical intermediates to the half-wave oxidation potential splitting. It is revealed that cation radical intermediates uncover important aspects in the electrochemical behavior of porphyrins. Cation radicals whose larger values in the highest spin densities, which are observed on the meso-carbon atoms of the cation radicals, and in energy gaps are associated with larger values in ∆E and ψ. Studies on porphyrin cation radicals, especially calculations of electronic densities on these intermediates, can predict quickly the half-wave oxidation potential splitting of their


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neutral compounds as well as decrease the computation resources compared to previous studies.6,10,13 Series 1

O

S

S

NH N

NH

N HN

S

N

NH O

HN

N

N

S

S

HN

N

NH

O

N

S

HN

N

S O

S

H2TPP

O

S

Series 2

H2T(2'-thienyl)P

H2T(3'-furyl)P

H2T(3'-thienyl)P

S

N

N

N

Zn N

N

S

N Zn N

N

N

S

N

N O

Zn

O

N

N

N

S

Zn S

N

N

S S

ZnTPP

ZnT(3'-thienyl)P

O

ZnT(3'-furyl)P

ZnT(2'-thienyl)P

Chart 1. Free base and zinc porphyrins studied in this work.

EXPERIMENTAL SECTION General Synthesis. Free base meso-porphyrins were synthesized by using the reaction between pyrrole and carboxaldehyde of five-membered heterocylic rings as previously reported (Scheme 1.a).15-17 Zinc meso-porphyrins were synthesized by using the reaction between their respective free bases with zinc acetate (Zn(CH3COO)2.2H2O) in dichloromethane and methanol solvent mixtures (Scheme 1.b).10 The obtained porphyrins were characterized by UV-Vis and 1H NMR spectroscopies. UV-vis spectra were obtained by using a Hewlett Packard Model 8453 spectrophotometer. 1H NMR spectra were recorded on a Varian Unity Inova 300 WB. The


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details of synthesis, UV-Vis results, and 1H NMR spectra are displayed in the Supporting Information.

Scheme 1. Synthesis pathway of (a): free base meso-substituted porphyrins, and (b): zinc mesosubstituted porphyrins.

Electrochemical Measurements. Tetra-n-butylammoniumperchlorate (TBAP) and tetra-n-butyl ammonium hexafluorophosphate (TBAF6) were employed as electrolytes for electrochemical analysis. Both TBAP and TBAF6 were purchased from ACROS. They were recrystallized twice from ethyl acetate (EtOAc) and then dried in vacuum before use. Dichloromethane (CH2Cl2) was degassed by purging with prepurified nitrogen gas and also dried prior to use. Electrochemical data were collected by using a CHI Model 760 series electroanalytical workstation. Cyclic voltammetry was conducted by employing a three-electrode cell, in which a BAS glassy carbon electrode (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


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electrode. The meso-substituted porphyrins were dissolved in CH2Cl2 containing 0.1 M TBAP or TBAPF6 at room temperature. All CVs were recorded at a scan rate of 0.1 V/s. For calibration, ferrocene was used in all experiments and served as the external reference. THEORETICAL CALCULATIONS All theoretical calculations were conducted by using the Gaussian 09 package.18 Structures of free base meso-porphyrins were optimized with B3LYP density functional12 and 6-31G(d,p) basis set. Symmetry restrictions, C2v, C2 or C1, and atrop-isomers were also considered for freebase porphyrins. Optimized structures were characterized by frequency analysis to confirm the true minima. If imaginary frequencies were found, the structures were re-optimized in lower point groups, such as C2 or C1. For zinc meso-porphyrins, geometries were optimized without symmetry restrictions. B3LYP density functional was still used in optimizing zinc-porphyrin complexes. LANL2DZ effectivecore potential basis set was employed for zinc atom while 6-31G(d) one was used for non-metal atoms.19 All minima of zinc porphyrins were characterized by their real vibrational frequencies. Due to the correlation between oxidation potential and ionization potential,11,13 we firstly calculated the first ionization potential (IP1), the second ionization potential (IP2), and the difference between the two ionization potentials (∆IP) of meso-substituted porphyrins. Also, spin densities of their cationic radicals and dihedral angles between porphyrin and meso-substituted rings were then obtained. The energy gaps between HOMO and LUMO were calculated to elucidate the role of porphyrin cation radical intermediates. All geometries of studied porphyrins were optimized both in the gas phase and in dichloromethane solvent (CH2Cl2). The details of computational calculations are displayed in the Supporting Information.


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RESULTS AND DISCUSSION Meso-porphyrins are grouped into series 1 (free base meso-porphyrins) and series 2 (zinc mesoporphyrin complexes). The cyclic voltammograms (CVs) of the porphyrin and their complexes in two types of electrolytes, TBAP and TBAPF6, are displayed in Figure 1 and Figure 2 while the half-wave oxidation potentials and their ∆E values are listed in Table 1. The CVs of the meso-substituted porphyrins in both series clearly exhibit two reversible oxidations (see Figure 1 and 2), except for that of the H2T(2’-thienyl)P, whose CV in CH2Cl2 containing 0.1M TBAPF6 displays two peaks that nearly overlap. This thus has proved the higher resonance between the porphyrin ring with the 2’-thienyl rings in comparison with porphyrins possessing five-membered rings and phenyl groups. Under the same acquisition condition, the CVs of ZnT(2’-thienyl)P reveal reversible peak potentials which are more distinctive than those of the H2T(2’-thienyl)P. This shows the role of the central zinc atom in affecting the electrochemical oxidation and reduction processes. The zinc ion can lock the porphyrin ring, rendering the ability of structural changes in zinc porphyrins less likely. Thus, the splitting between the first and second oxidation states (∆E values) of zinc porphyrins are relatively larger than those of the pristine ones (Table 1).


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Figure 1. Cyclic voltammograms of (a) Series 1 and (b) Series 2 in CH2Cl2 containing 0.1M TBAP (Fc+/0 = +0.54 V). Scan rate = 0.1 V/s.

Figure 2. Cyclic voltammograms of (a) Series 1 and (b) Series 2 in CH2Cl2 containing 0.1M TBAPF6 (Fc+/0 = +0.54 V). Scan rate = 0.1 V/s.


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Table 1. Half-wave Oxidation Potentials (V vs. Ag/AgCl) and Splitting between the First and the Second Oxidation Processes of meso-substituted Porphyrins and their Zinc Complexes in CH2Cl2/TBAP (Fc+/0 = +0.54 V) and CH2Cl2/TBAPF6 (Fc+/0 = +0.50 V) CH2Cl2/TBAP nd

st

CH2Cl2/TBAPF6 2 E1/2

1st E1/2

∆Ea

― 0.16 0.16 0.23

+1.12 +1.21 +1.24 +1.36

+1.01 +0.97 +0.97 +1.01

0.11 0.24 0.27 0.35

0.21 0.28 0.29 0.30

+1.13 +1.04 +1.07 +1.17

+0.93 +0.78 +0.80 +0.90

0.20 0.26 0.27 0.27

2 E1/2

1 E1/2

∆E

H2T(2'-thienyl)P H2T(3’-furyl)P H2T(3'-thienyl)P H2TPP

+1.10* +1.19 +1.19 +1.30

+1.50* +1.03 +1.03 +1.07

ZnT(2'-thienyl)P ZnT(3'-furyl)P ZnT(3'-thienyl)P ZnTPP

+1.11 +1.08 +1.08 +1.13

+0.90 +0.80 +0.79 +0.83

a

nd

Series 1

Series 2

a

∆E = 2nd E1/2 - 1st E1/2 *: irreversible peak potential

The ∆E values of porphyrin-substituents and their respective zinc complexes with phenyl rings are higher than those with five-member heterocyclic rings (Table 1). The results indicate the better resonance between the porphyrin ring and the meso-substituted rings when altering from the six-membered phenyl rings to the five-membered heterocyclic rings. The trend of ∆E variation generally follows TPP >T(3’-thienyl)P > T(3’-furyl)P > T(2’-thienyl)P for a given series in both types of electrolytes. Taking the ∆E values of series 2 in CH2Cl2/TBAP as an example, the ∆E of ZnTPP, ZnT(3’-thienyl)P, ZnT(3’-furyl)P, and ZnT(2’-furyl)P are 0.30, 0.29, 0.28, and 0.21 V, respectively (see Table 1). It is noted that the first and the second half-wave oxidation potentials (1st E1/2 and 2nd E1/2) of porphyrin-substituents and their respective zinc complexes also follow the trend of TPP > T(3’-thienyl)P > T(3’-furyl)P in both series, except the H2T(2’-thienyl)P and the ZnT(2’-thienyl)P. The 1st E1/2values of H2T(2’-thienyl)P and ZnT(2’thienyl)P are the highest ones while the 2nd E1/2 values are at the lowest ones for H2T(2’-


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thienyl)P and the second largest ones for ZnT(2’-thienyl)P. However, the ∆E values of H2T(2’thienyl)P and ZnT(2’-thienyl)P are still at the smallest ones, compared to other porphyrinsubstituents (Table 1), proving that the extent of structural changes during their oxidation processes is relatively small due to their better resonance between the porphyrin ring and the 2’thienyl rings or higher extent of delocalization of electron over their redox centers.4,6 Employing DFT calculations for ionization potentials (IP) and the splitting between the first and the second ionization processes (∆IP), it has been found that there is nearly no difference in the ∆IP values under C2v, C2, and C1 symmetry restrictions in gas phase among the porphyrins in series 1, except the H2TPP (see Theoretical Calculations in the Supporting Information, Figure S1, Table S1, S2 and S3). The calculations on atrop-isomers of porphyrins with five-membered heterocyclic rings were also conducted. The data are presented on Figure S2 and Table S4. The results show that the energies of atrop-isomers are nearly unchanged. Furthermore, the data show that these results have meanings within a particular compound. In the series of porphyrins possessing five-membered heterocyclic rings, the energy differences in atrop-isomers are much smaller than the energy differences between porphyrins. We therefore chose the calculated ∆IP without symmetries in both gas phase and CH2Cl2 to consider the trend of ∆IP comparing with the trend of ∆E. The results of calculated IP and ∆IP values, listed in Table 2, show that ∆IP values follow the trend of T(3’-furyl)P > TPP > T(3’-thienyl)P > T(2’-thienyl)P, which is quite different from that of ∆E obtained from CVs, for a given series in both gas phase and solution. IP and ∆IP values in solution are significantly lower than those in gas phase. Notably, the trend of IP in solution is consistent to that of 1st E1/2 and 2nd E1/2 as moving from porphyrin-substituents and their complexes with six-member phenyl rings to 3’-thienyl and 3’-furyl rings, but the trend of ∆IP is


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inconsistent. This, however, somewhat clarifies the limitation mentioned in Yu et al.’s study, in which ionization potentials (IPs) have a linear relationship with oxidation potentials (Es) in case of treating small organic molecules that are closely related in structures.11,20 In our previous study,13 we found that the trend of ∆IP follows that of ∆E when varying positions of substituents at the phenyl rings among ortho, meta, and para ones in a meso-substituted free base porphyrin. Table 2. Calculated Ionization Potentials (IP, eV) and Difference of the Two Ionization Potentials (∆IP) of meso-substituted Porphyrins and their Zinc Complexes

Gas phase geometriesa IP1

IP2

∆IPc

Solution geometriesb IP1

IP2

∆IPc

Series 1 H2T(2'-thienyl)P H2T(3’-furyl)P H2T(3'-thienyl)P H2TPP

5.88 5.87 5.89 5.90

8.51 8.87 8.77 8.96

2.64 3.00 2.88 3.06

4.95 4.90 4.95 5.05

5.50 5.72 5.71 6.03

0.55 0.82 0.77 0.98

ZnT(2'-thienyl)P ZnT(3'-furyl)P ZnT(3'-thienyl)P ZnTPP a Optimized in gas phase, b Optimized in CH2Cl2 c ∆IP= IP2 - IP1

5.89 5.87 5.90 5.88

8.54 8.85 8.76 9.04

2.65 2.98 2.86 3.16

4.87 4.79 4.85 4.90

5.49 5.62 5.63 5.96

0.62 0.82 0.79 1.06

Series 2

While delocalization of electronic structures is still considered an important factor on the determination of electrochemical properties of porphyrins,4,6 we observed the selected HOMOs, LUMOs of meso-substituted porphyrins and their complexes (see Figure 3, S3 and S4 ). It can be seen that HOMOs are mainly located on porphyrin rings, especially on meso-C positions and the extent of delocalization seems to increase when changing from porphyrins and their complexes


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with phenyl rings to those with five-membered heterocyclic rings, and from a free base porphyrin to its respective zinc complex (Figure 3).

Figure 3. HOMOs of free base and zinc porphyrins in gas phase. (Isovalue set to 0.02 a.u.)

To obtain the qualitative delocalization of electrons, we calculated spin densities of cation radicals and dications formed during the respective oxidation processes (Table S5 and Table 3). The calculation results show that diatonic porphyrins at the singlet state are more stable than those at triplet ones (see Theoretical Calculations, Table S2). The electron density distribution therefore depends on cationic radicals of porphyrins formed at the first oxidation state. Figure 4 shows that meso-C positions are always of highest spin densities (HSD), compared with other ones of porphyrins and their zinc complexes. In gas phase, the HSD values range from 0.199 to 0.245 in series 1, and from 0.202 to 0.247 in series 2. These values are quite higher in CH2Cl2,


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from 0.200 to 0.248 for free base porphyrins, and from 0.211 to 0.255 for complexes of porphyrins (Table 3). However, whether cation radicals are optimized in gas phase or solution, the trend of HSD values all follows TPP >T(3’-thienyl)P > T(3’-furyl)P > T(2’-thienyl)P in both series, and it is the same to the trend of ∆E obtained from experimental data. This result is consistent with the output of one of the previous work which confirms that solvation effect is not a determining factor to the trend of ∆E13 and acknowledges the correlation between ∆E and calculated HSD values of cation radicals.

Figure 4. Spin density distribution of cationic radical meso-substituted porphyrins in gas phase: (a) H2TPP+•, and (b) ZnTPP+•. The numbers show the calculated spin density values on the respective atoms. Isovalue: 0.004

Since zinc ion locks the porphyrin ring, the electrochemical properties of the compounds in series 2 depend on the existence of zinc ions. This may result in larger ∆E of the zinc porphyrin complex than that of its respective free base. The calculated spin densities (ρ) on Zn atoms, shown in series 2 calculation, follows the trend of ∆E in both gas and solution phase geometries


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(Table 3). This means that spin densities (ρ) at both meso-C and Zn positions do have contribution to the electrochemical behavior of the studied porphyrins and their zinc respective complexes. The large calculated- spin densities (ρ) at these positions are associated with the large values of ∆E. Table 3.Calculated Spin Density Distribution (ρ) and HOMO-LUMO Gaps of Cation Radicals of Porphyrins and their Complexes Solution geometriesb

Gas phase geometriesa HOMOLUMO gap

ρCmeso

0.19905

2.26

0.20019

2.24

0.22015

2.40

0.22226

2.41

0.22420

2.42

0.22662

2.43

0.24469

2.67

0.24832

2.70

ρCmeso

ρZn

ρZn

HOMOLUMO gap

Series 1 H2T(2'-thienyl)P+ H2T(3’-furyl)P

+.

H2T(3'-thienyl)P H2TPP

.

+.

+.

Series 2 .

ZnT(2'-thienyl)P+ ZnT(3'-furyl)P+. . ZnT(3'-thienyl)P+ ZnTPP

+.

0.20225 0.22235

0.00583 0.00755

2.36 2.54

0.21133 0.23088

0.00320 0.00450

2.36 2.55

0.22757

0.00781

2.57

0.23714

0.00477

2.59

0.24668

0.00883

2.74

0.25513

0.00554

2.75

Calculated ρ of meso-C and Zn atoms shows that ρ is well correlated to dihedral angles (ψ) between porphyrin ring and meso-substituents (Figure S5, Table S6, and Table 3). The larger calculated ρ at these positions, the larger the ψ measured in both neutral, cation radical, and dicationic states (Table 4). The larger ψ means the higher level of steric hindrance between meso-substituents and the central porphyrin ring, and this results in larger possibility of structure changes (∆E) there is a reduction of electron transfer between neutral, cation radical and dicationic states. In this case, the electronic distribution is mainly at meso-C positions rather than


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spreading over the porphyrin structures, and thus ρ values of meso-C and Zn atoms (also HSD) are larger. Because porphyrins and their phenyl-ring-containing complexes exhibit larger HSD values than those possessing five-membered heterocyclic rings, such as 3’-thienyl, 3’-furyl, and 2’-thienyl, respectively. It can be explained why the trend of HSD (also ψ, and ∆E) follows TPP > T(3’-thienyl)P > T(3’-furyl)P > T(2’-thienyl)P in both series. Notably, ψ of the porphyrin and its complex with 2’-thienyl rings are relatively larger than those possessing 3’-furyl rings, even those with 3’-thienyl rings, at neutral states, which are correlated to their larger 1st E1/2 and 2nd E1/2 values (Table 1). At cationic radical and dicationic states, their ψ values are among the smallest ones, which are apparently associated with a larger extent of deformation of porphyrin rings (Figure 5)13. The smallest ∆E values of H2T(2’-thienyl)P and its respective complex show the role of cation radicals and dications in the contribution to the half-wave oxidation potential splitting ∆E. Table 4. Calculated Dihedral Angles (ψ) between Central Porphyrin ring and meso- substituted Rings at Different Oxidation States* Gas phase geometries

Solution geometries

n=0

n=+1

n=+2

n=0

n=+1

n=+2

64.99 56.71 61.74 68.22

43.70 43.96 49.19 61.01

31.82 33.35 37.27 52.19

62.59 55.39 61.23 68.25

41.15 43.16 47.53 61.12

30.58 32.76 35.69 53.77

ZnT(2'-thienyl)P 65.26 44.78 34.46 63.57 44.74 ZnT(3'-furyl)P 58.37 44.52 35.53 57.13 44.20 ZnT(3'-thienyl)P 61.86 49.12 39.24 61.18 49.04 ZnTPP 66.01 55.86 45.36 65.43 56.00 *n = 0, +1, +2 for neutral, cation radical, and dicationic states, respectively

34.46 34.61 39.32 55.09

Series 1 H2T(2'-thienyl)P H2T(3-furyl)P H2T(3'-thienyl)P H2TPP Series 2

a b

Optimized in gas phase Optimized in CH2Cl2


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Figure 5. Optimized structures of H2T(3’-furyl)P, H2T(2’-thienyl)P and their respective complexes at neutral, cation radical and dicationic states in gas phase.

Charge distribution of porphyrin cation radicals was calculated and the data are presented on Figure S6, S7, and Table S7. The calculated results show that for the porphyrin cation radicals, the positive charges are located on Calpha and Cmeso positions. It is noted that the charges on the Cmeso are less positive than on the Calpha ones (Table S7), resulting in the probability of finding electron density at the Cmeso positions are higher than the Calpha ones. So, the spin density distribution at Cmeso atoms is high. In other words, there is a correlation between the positive charge and the spin density distribution in affecting half-wave oxidation potential splitting, ∆E. The better the positive charges delocalize on porphyrin ring, the better the spin densities


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delocalize, and this leads to smaller ∆E. Therefore, spin density distribution can be used as an easy computational parameter which can be correlated with ∆E. The calculated results also show that for porphyrin cation radicals, there is a correlation between charge distribution on Cmeso positions and HOMO-LUMO gaps (see Table S8 and S11), and the data are correlated with the ∆E obtained from the experiments. This further clarifies the role of cation radicals in the electrochemical behaviors of porphyrins compared to other states of porphyrins.

Energy gaps between HOMOs and LUMOs of porphyrin cation radicals and their respective complexes were calculated (Table S9, S10, S11, Figure S8, and Table 3). The data show a decrease trend from porphyrins and their complexes containing phenyl rings to five-membered heterocylic rings, 3’-thienyl, 3’-furyl, and 2’-thienyl, respectively. The trend of HOMO-LUMO gaps is consistent to that of experimental ∆E values. This explains the good agreement between calculated and experimental data as well as confirming HOMO-LUMO gap as a parameter, found in work of Gosh and his co-workers,10 related to the half-wave oxidation potential splitting (∆E). However, it is also noted that among neutral, cation radical, and dicationic states of mesoporphyrins and their complexes, only cation radical states whose trend of LUMO-HOMO gaps is consistent to that of ∆E obtained from experiment (see Table S11). This clearly demonstrates the special role of cation radicals in affecting half-wave oxidation potential splitting rather than that of other states. This fact is nearly ignored in the previous studies.10 The correlation between calculated parameters and experimental data is displayed in Figure 6, showing the role of porphyrin cation radicals to the half-wave oxidation potential of their neutral compounds. An increase in the highest spin density distribution (ρ) on meso-C positions or/and


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dihedral angles ψ, and/or HOMO-LUMO energy gaps of cation radicals is correlated to an upward trend in ∆E. Studying on these cation radicals, therefore, can explain qualitatively the trend of ∆E compared to other electrochemical studies.4,10,12

Figure 6.Correlation diagram between the HOMO-LUMO gaps, spin density distribution on meso-C positions (ρ), and dihedral angles (ψ)of porphyrincation radicals and the experimental ∆E of neutral porphyrins.

CONCLUSIONS


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Employing CV and DFT calculations, we have clarified that the cation radicals play important roles in the half-wave oxidation potential splitting of meso-porphyrins and their zinc complexes. When changing phenyl rings to five-membered heterocylic rings, such as 3’-thienyl, 3’-furyl, and 2’-thienyl, respectively, the half-wave oxidation potential splitting (∆E) of the porphyrins and their zinc complexes decreases, which is consistent to a downward trend in spin density distribution (ρ) on meso-C positions and/or Zn atoms and in HOMO-LUMO gaps of their cationic radicals. In addition, spin density distribution of porphyrin cationic radicals is related to the conjugated π-ring systems, defined by dihedral angles ψ between porphyrin rings and mesosubstituted rings, demonstrating the better resonance between the porphyrin ring with meso-five membered rings than that with phenyl groups. A good agreement between calculated cation radicals and experimental data in this study suggests that the spin density distribution should be added as a parameter in elucidating electrochemistry of porphyrins and other mixed-valence compounds in future studies.

ASSOCIATED CONTENT Supporting Information. Synthesis of meso-porphyrins and theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected].


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ACKNOWLEDGMENTS The authors are grateful to the Ministry of Science and Technology for the support of this work.

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Electrochemical Behavior of meso-Substituted Porphyrins: The Role of

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