Elucidating the Electroreduction Mechanism of the Monoprotonated Octaethylporphyrin. A Comparative Study with the Diprotonated Octaethyl- and meso-Tetraphenyl-porphyrins

2,3,7,8,12,13,17,18-octaethylporphyrin (H₂OEP 95%) from Sigma-Aldrich was purified by chromatography using silica gel with toluene/ethyl acetate. Pyrrole, benzaldehyde and propionic acid were purchased from Sigma-Aldrich, in the case of pyrrole it was distilled prior to use. Electrochemical grade tetrabutylammonium perchlorate (Bu₄NClO₄ ≥ 99%) was purchased from Aldrich and dried in a vacuum oven for 48 h prior to use. Perchloric acid, used to protonate the free base porphyrin, H₂TPP and H₂OEP, was purchased from Aldrich (HClO₄ ACS reagent 69 −72). Previous to use, this solution was standardized using sodium carbonate (Na₂CO₃ ≥ 99.5%, Sigma-Aldrich); the actual concentration was 11.5 M (69.44% purity). An aqueous tetrabutylammonium hydroxide solution from Sigma-Aldrich (40%, Bu₄NOH ∼ 1.5 M) was also standardized with phthalic acid monopotassium salt from Merck (KHP 99.8%) yielding a 1.9 M concentration. The standards KHP and Na₂CO₃ were dried for 4 h at 120 °C prior to use. 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU 98%, Sigma-Aldrich) was used as received. Benzonitrile (PhCN 99% extra pure, ACROSS Organics) was distilled under reduced pressure with phosphorous pentoxide (P₂O₅ ≥ 98%, Aldrich). The solutions for electrochemical experiments were bubbled with ultra-high purity nitrogen (99.999%, Infra).

NMR spectra were recorded using a Bruker ARX 300 spectrometer, the experiments were done with concentrations between 15 and 20 mg ml⁻¹ at 25 °C. Chemical shifts (ppm) are relative to Si(CH₃)₄ for ¹H and ¹³C (77.23 ppm) in CDCl₃, all coupling constants are reported in Hertz.

meso-Tetraphenylporphyrin was prepared by placing 200 ml of propionic acid in a round-bottomed flask equipped with a condenser and a magnetic stirrer, followed by heating under reflux, and simultaneous addition of 5.5 g (5.3 ml, 51.8 mmol) of pyrrole and 3.5 g (3.6 ml, 32.9 mmol) of benzaldehyde. The reaction mixture was refluxed for 30 min and allowed to cool.^23,24 The product was filtered over celite and purified by flash chromatography column using a mixture of hexane/CHCl₃ (60/40). Finally, the product was washed with methanol. The macrocycle H₂TPP was obtained as a purple solid, 2.7 g (4.40 mmol, 34%). P.f. > 300 °C. ¹H NMR (CDCl₃, 300 MHz): δ 8.85 (s, 8H, H-β), 8.22 (dt, 8H, J = 7.8, 1.7 Hz, H_o-Ph), 7.76 (m, 12H, H_m-Ph and H_p-Ph), −2.77 (s, 2H, N–H) ppm. ¹³C NMR (CDCl₃, 75.5 MHz) δ: 142.3 (C_ipso-Ph), 134.7 (C_o-Ph), 131.2 (C-β), 127.8 (C_m-Ph), 126.8 (C_p-Ph), 120.3 (C-meso) ppm. Spectral data for H₂TPP is in agreement with those previously reported.^25,26

An Autolab PGSTAT 302 potentiostat controlled by the GPES software was used for the voltammetry and coulometry experiments. For the spectroelectrochemical measurements, a BASi Epsilon potentiostat-galvanostat was coupled to a UV–vis Agilent 8453 spectrophotometer. A three-electrode jacketed cell was used for the cyclic voltammetry and controlled potential coulometry. The counter and reference electrode were a Pt wire and the Ag/AgNO₃ system (a silver wire immersed in 0.10 M Bu₄NClO₄/0.01 M silver nitrate/acetonitrile, separated from the main solution by a porous Vycor frit from Bioanalytical Systems). For the coulometry experiments, the Pt counter electrode was placed inside a fritted glass chamber isolated from the main solution. A glassy-carbon disc (0.3 and 0.1 cm diameter) and a porous carbon mesh (0.3 × 0.2 cm) were used as working electrodes for the cyclic voltammetry and coulometry, respectively. A thin-layer quartz electrochemical cell with a three-electrode arrangement (BASi) was employed for the spectroelectrochemistry measurements. Depending on the concentration of the solutions, two different path lengths where used, 0.5 and 1.0 mm. The working electrode consisted of an optically transparent gold minigrid, the counter electrode was a Pt wire, and the Ag/AgNO₃ system as reference.

The redox potential of the ferrocenium/ferrocene couple (Fc⁺/Fc) was regularly measured, all potential values in this work are referred to the Fc⁺/Fc couple. All the experiments were performed in Bu₄NClO₄ 0.1 M/PhCN at 25 °C; except for the spectroelectrochemistry that was carried out at room temperature. Prior to every experiment, the solutions were bubbled with N₂ for approximately 50 min Also, the working electrode was polished with alumina paste (0.05 μm, Buehler) before each measurement.

The total uncompensated resistance in Bu₄NClO₄ 0.1 M/PhCN at 25 °C (880 and 2800 Ω for the 0.3- and 0.1-cm diameter electrode) was determined as previously described.²⁷ Approximately 80% of the total resistance was electronically compensated during the experiment. For the voltammetry simulation, the remaining ∼20% resistance was added during the simulation (DigiElch 8, http://digielch.de).²⁸

Free base H₂TPP shows the typical UV–vis porphyrin spectrum: an intense Soret band at 422 nm, and four Q bands of lower intensity at 517, 551, 592 and 648 nm. When adding increasing amounts of HClO₄ to the H₂TPP benzonitrile solution, the Soret band gradually decreases, and a new signal appears and increases at 442 nm. Additionally, the bands at 517 and 551 nm gradually decrease until they disappear, and that at 648 nm increases and shows a 10-nm bathochromic shift (Fig. S1, Supplementary material is available online at stacks.iop.org/JES/167/155507/mmedia). The reduction from four to two Q bands is due to a symmetry change from D_2h to D_2d.^14,29 These characteristics are typical of the direct formation of the diprotonated porphyrin, H₄TPP²⁺, i.e., without the formation of the monoprotonated species.^30,31

The free base H₂OEP spectrum also presents the characteristic pattern, with the Soret band at 402 nm and four Q bands at 498, 532, 568 and 622 nm (Fig. 1, black line). Unlike H₂TPP, the addition of acid induces spectral changes that are consistent with the two-step protonation where both the mono- and diprotonated species (H₃OEP⁺ and H₄OEP²⁺) are observed. When adding 0.2–1.0 HClO₄ equivalents (Fig. 1 top), there is a slight hypsochromic shift of the Soret band (7 nm), and the shoulder at 372 nm disappears. In addition, Q-IV (498 nm) gradually decreases until it is no longer observed, and Q-II increases and is hypsochromically shifted (13 nm). At 622 nm, Q-I decreases while a new band gradually grows at 602 nm. Band Q-III, 532 nm, shows only very slight changes during this first protonation step. It is worth noting that the formation of the monoprotonated species is characterized by two isosbestic points at 515 and 610 nm. A similar behavior has been observed for other porphyrins.^8,22,32,33 When adding between 1.2 and 2.0 HClO₄ equiv (Fig. 1 bottom), a Soret band appears at 407 nm. Also, Q-III (532 nm) decreases and then disappears when two equivalents of acid have been added; the band at 571 nm decreases while that at 555 nm increases and shifts to the blue. Band Q-I has a similar behavior to that observed during monoprotonation, i.e., the signal at 602 nm decreases and one at 594 nm rises. The spectra measured in-between the mono- and diprotonaded species (those with 1.2–1.8 acid equivalents) show three isosbestic points at 400, 541 and 584 nm. Based on the spectral pattern for the acid titration of H₂TPP and H₂OEP, the following equilibria were determined:

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{2}}}{\mathrm{TPP}+\mathrm{2H}}^{+}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{{\rm{TPP}}}^{2+}\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{2}}}{\rm{OEP}}+{{\rm{H}}}^{+}\rightleftharpoons {{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}+{{\rm{H}}}^{+}\rightleftharpoons {{\rm{H}}}_{4}{{\rm{OEP}}}^{2+}\end{eqnarray} \tag{ 3 }$$

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The reduction of H₂TPP in dry benzonitrile (PhCN) shows the typical porphyrin behavior; two reversible one-electron reductions, E_1/2 = –1.65 and –2.03 V (Fig. 2a, black curve), corresponding to the formation of the radical anion and the dianion.^34–39 In both processes, the plot of peak current vs square root of the sweep rate shows a linear relation with the intercept at zero, indicating the electron transfers are diffusion-controlled.

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Increasing additions of HClO₄ induces the rise of two new contiguous reduction processes, Ic and IIc, that gradually increase. Simultaneously, the current intensity of the neutral H₂TPP reduction process decreases (Fig. 2a). When the potential is switched right after IIc, we observe the oxidation processes Ia and IIa, which are coupled to Ic and IIc (Fig. 2b). This indicates that the reduced species are stable enough to allow for their oxidation at the scan rates used during these experiments. Based on the results of section 2.1, the addition of 2 equiv of HClO₄ provokes the full disappearance of the H₂TPP bands, indicating that the equilibrium of reaction 1 is mostly shifted toward the H₄TPP²⁺ species, and almost no free protons are in solution. Therefore, Ic/Ia and IIc/IIa must correspond to one-electron reduction processes of H₄TPP²⁺ that yield the radical cation H₄TPP^•+ and isophlorin H₄TPP, reactions 4 and 5. The direct reduction of free protons at these potentials was ruled out by recording voltammograms of HClO₄ in the electrolytic media. A similar behavior has been reported for the reduction of meso-tetra(4-N-methyl-pyridyl)porphyrin diacid in aqueous HCl 1.0 M.¹⁷

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{4}}}{{\rm{TPP}}}^{2+}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{4}{{\rm{TPP}}}^{\cdot +}\,{E}_{{\rm{4}}}^{^\circ },{k}_{s,4,}{\alpha }_{{\rm{4}}}\,{\rm{Ic}}/{\rm{Ia}}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{4}}}{{\rm{TPP}}}^{\bullet +}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{\rm{TPP}}\,{E}_{5}^{^\circ },{k}_{s,5,}{\alpha }_{5}\,{\rm{IIc}}/{\rm{IIa}}\end{eqnarray} \tag{ 5 }$$

Digital simulation was performed for the cyclic voltammograms obtained for H₄TPP²⁺ at three concentrations (0.27, 0.47 and 0.73 mM), and scan rates from 0.1 to 5.0 V s⁻¹. For the simulation, reactions 4 and 5 with ${E}_{4}^{^\circ }$ = –0.72 V and ${E}_{5}^{^\circ }$ = –0.82 V, and k_s,4 and k_s,5 = 0.04 cm s⁻¹ were used. Examples of the fit are shown in Fig. 3. At low scan rates, 0.1 V s⁻¹, the current intensity of Ia and IIa is slightly larger in the simulated voltammogram than that observed in the experiments. This indicates that the isophlorin, H₄TPP, formed at IIc undergoes some kind of chemical reactions.

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Coulometry of H₄TPP²⁺ at –0.89 V (process IIc) consumed 1.4 ± 0.15 e⁻ per H₄TPP²⁺ molecule in approximately 50 min (the experiment was performed in triplicate). The voltammetric analysis of the solution after electrolysis demonstrates that Ic and IIc completely disappeared, indicating that H₄TPP²⁺ was depleted (Fig. 4a, green line). The voltammogram also shows a new peak, IIIc, and the partial recovery of the reduction peaks of free base H₂TPP. In addition, the anodic sweep (blue line) does not show the oxidation processes of isophlorin (Ia and IIa, reverse of reactions 4 and 5), but a new oxidation process is observed at 0.07 V, peak IVa. The presence of IIIc and IVa confirms the existence of chemical reactions coupled to the formation of H₄TPP.

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The electroreduced solution was further analyzed by UV–vis spectroscopy (Fig. 5, blue line). Compared to the solution before electrolysis (pink line), a new band rises at 773 nm. This agrees with previous reports for phlorin species, which present absorption bands in the 700–850 nm region.^40–47 It is worth noting that the H₂TPP Q bands at 517 and 551 nm before electrolysis (pink line) had disappeared but they partially reappear after electroreduction (blue line).

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Phlorins are two-electron reduced porphyrin species that have been protonated at one of the meso- positions of the main ring (Scheme 1). Since there are no free protons during the electrolysis of H₄TPP²⁺, a self-protonation process is suggested. In it, H₄TPP²⁺ transfers a proton to isophlorin H₄TPP, reaction 6. Such reactions between a reduced species and its parent compound are known as self-protonation or father-son reactions and are fairly common in organic compounds with acidic properties.^48–52 The global process, reaction 7, corresponds to the combination of reactions 4 to 6, and implies the transfer of 1.33 e⁻ per H₄TPP²⁺ molecule with the formation of the phlorin cation H₄TPFH⁺ and of H₂TPP, agreeing with the experimental results. Accordingly, IIIc and IVa should correspond to the reduction and oxidation of H₄TPFH⁺.

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{4}}}{\rm{TPP}}+1/2{{\rm{H}}}_{{\rm{4}}}{{\rm{TPP}}}^{2+}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{{\rm{TPFH}}}^{+}+1/2{{\rm{H}}}_{{\rm{2}}}{\rm{TPP}}\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&3/2{{\rm{H}}}_{{\rm{4}}}{{\rm{TPP}}}^{2+}+2{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{{\rm{TPFH}}}^{+}+1/2{{\rm{H}}}_{{\rm{2}}}{\rm{TPP}}\end{eqnarray} \tag{ 7 }$$

To further confirm the protonation of isophlorin H₄TPP, cyclic voltammetry was carried out for H₄TPP²⁺ solutions with 0.5 and 1.0 equiv of HClO₄. Figure 6a shows that the presence of free protons results in the loss of reversibility for Ic/Ia and IIc/IIa, and also induces a new oxidation process IVa. Additionally, when the inversion potential is –1.30 V (Fig. 6b), a new reversible reduction process, IIIc/IIIa, appears. This matches the observations for the electroreduced solution (Fig. 4), and indicates the formation of phlorin cation H₄TPFH⁺, which is feasible because of the presence of free protons from HClO₄, reaction 8. The formation of phlorins has also been reported during the reduction of metallated porphyrins in aprotic media and in the presence of proton-donating species.^43,53–55

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{4}}}{\rm{TPP}}+{{\rm{H}}}^{+}\to {{\rm{H}}}_{{\rm{4}}}{{\rm{TPFH}}}^{+}\end{eqnarray} \tag{ 8 }$$

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A process similar to IIIc/IIIa has been observed during the reduction of H₂TPP in CH₂Cl₂ with TFA, which was attributed to the one-electron reduction of phlorin species, reaction 9.¹⁵ Moreover, the experimental results show that the oxidation peak IVa corresponds to the two-electron oxidation of phlorin cation, reverse of reaction 10 (section A.1, Figs. S2 and S3, Supplementary material)

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{4}}}{{\rm{TPFH}}}^{+}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{{\rm{TPFH}}}^{{\rm{\bullet }}}\,{\rm{IIIc}}/{\rm{IIIa}}\end{eqnarray} \tag{ 9 }$$

Based on the above and to unequivocally confirm the existence of the self-protonation reaction, it was assumed that the presence of 1 equivalent of HClO₄ during the electroreduction of H₄TPP²⁺ should suppress self-protonation, reaction 6. This, in turn, would result in a two-electrons global process, reaction 10, encompassing reactions 4, 5 and 8.

$$\begin{eqnarray}&&{{\rm{H}}}_{{\rm{4}}}{{\rm{TPP}}}^{2+}+{{\rm{H}}}^{+}+{{\rm{2e}}}^{-}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{{\rm{TPFH}}}^{+}\,\mathrm{Ic} \mbox{-} \mathrm{IIc}\end{eqnarray} \tag{ 10 }$$

Exhaustive electrolysis of a H₄TPP²⁺ + 1 equiv HClO₄ solution at –0.89 V (process IIc), consumed 2.1 e^–/H₄TPP²⁺, which corresponds to what is expected from the global process 10. Similar to the electroreduction of H₄TPP²⁺ with no HClO₄ excess (Fig. 4a), the voltammograms of the electrolyzed solution (Fig. 4b) show the reduction (IIIc) and oxidation (IVa) of the phlorin cation species. In addition to the main oxidation peak, IVa, the anodic sweep (Fig. 4b, blue line) presents a small oxidation peak Va that was barely present in Fig. 4a. The addition of Bu₄NOH to the electroreduced H₄TPP²⁺ solution proves that IVa and Va are related to the oxidation of less protonated phlorin species, e.g. the neutral phlorin H₃TPFH (Scheme 1, and section A.2, Fig. S4, Supplementary material). The absorption spectrum of the electroreduced solution (Fig. 5, green line) does not show the band of H₂TPP at 517 nm, which also agrees with the global process 10. Unlike the global process 7, the only reduction product obtained under these conditions is phlorin cation.

To obtain experimental evidence of intermediate species, such as H₄TPP^•+ and H₄TPP, UV–vis spectroelectrochemistry was carried out. We acquired the absorption spectra of 0.26 mM H₄TPP²⁺ at different reduction potentials (from –0.10 to –1.25 V) and in presence and absence of 1 additional equiv of HClO₄ (Figs. 7b and 7c). It can be observed that, as the potential reaches Ic and IIc, the Q band at 658 nm decreases while two bands at 773 and 859 nm gradually increase. It is worth noting that the band at 859 nm appears before that at 773 nm, the former is barely perceptible at −0.74 V (Ic) but becomes clearer as the potential reaches that of IIc (−0.87 V, Fig. 7b). This band was not observed in the spectra of electroreduced H₄TPP²⁺ (Fig. 5) and should thus correspond to short-lived species, such as isophlorin H₄TPP, formed during the second electroreduction process and proven to be unstable in the time-scale of the electrolysis experiment. However, there were no bands that could be assigned to H₄TPP^•+.

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The spectra at Fig. 7b show that as the potential departs from that of IIc, the band at 773 nm increases and that at 859 nm decreases. This agrees with the fact that more H₄TPP is consumed as time increases in the self-protonation reaction 6, producing more phlorin cation (which is responsible for the band at 773 nm). When the sweep direction is switched, the band at 859 nm drastically decreases at –0.06 V because at that point all the unreacted H₄TPP is oxidized back to H₄TPP²⁺ (reverse of reactions 4 and 5). After IVa (0.60 V), phlorin cation is oxidized, regenerating the diprotonated porphyrin, reverse reaction 10, as evidenced by the decrease in the phlorin cation band at 773 nm, and the increase of the H₄TPP²⁺ Q band at 653 nm.

Spectroelectrochemistry experiments were performed for the solution of H₄TPP²⁺ + 1 equiv of HClO₄. A similar behavior was observed, however the increase of the band at 773 nm is more evident than when there are no excess protons, and the isophlorin band at 859 nm is smaller at all the potentials analyzed (Fig. 7c). These results agree with reaction 8, where the electrogenerated H₄TPP is promptly consumed by the reaction in presence of free protons.

In acid-free media, H₂OEP is reduced via two consecutive one-electron processes, E_1/2 = –1.91 and –2.34 V, that correspond to the formation of the radical anion and the dianion (Fig. 8a, black line).^19,36 When adding HClO₄ to the H₂OEP solution, the behavior is different from that of H₂TPP. Increasing amounts of HClO₄ (from 0 to 1 equiv) lead to the appearance and increase of a reduction process at –1.06 V, peak Ic' (Fig. 8a). Simultaneously, the processes corresponding to the non-protonated H₂OEP decrease. Based on the spectrophotometric titration from Fig. 1, which shows that with 1 equiv of HClO₄ only the monoprotonated species H₃OEP⁺ is present, peak Ic' can be assigned to the reduction of H₃OEP⁺.

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The addition of 1.2 and 2.0 equiv of HClO₄ (Fig. 8b), leads to three reduction processes (Ic'', IIc'' and IIIc'') that increase in intensity as the acid concentration increases. Peak IIc'' seems to be located at the same potential than Ic', suggesting the presence of the monoprotonated species H₃OEP⁺, e.g. at 2.0 equiv of HClO₄. However, voltammograms of H₃OEP⁺ and H₄OEP²⁺ solutions obtained at different scan rates (Fig. 9), demonstrate that Ic' and IIc'' are actually different: at 0.1 V s⁻¹ Ic' (H₃OEP⁺) and IIc'' (H₄OEP²⁺) overlap; at 5.0 V s⁻¹, Ic' is cathodically shifted by 0.131 V from the value obtained at 0.1 V s⁻¹ and IIc'' barely shifted (0.049 V). A detailed analysis of the H₃OEP⁺ voltammograms is provided below.

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In the H₄OEP²⁺ voltammograms obtained when the potential is switched right after the second reduction process (IIc''), coupled oxidation peaks can be observed for both Ic'' and IIc'' (Fig. 9, dashed-dotted lines, peaks Ia'' and IIa''). Similar to the behavior of H₄TPP²⁺, Ic''/Ia'' and IIc''/IIa'' can be assigned to consecutive one-electron transfers that yield the radical cation and isophlorin, reactions 11 and 12. It is worth noting that at low scan rates (Fig. 9, pink line), the reversibility of these processes decreases and peaks IIIc'' and IVa'', due to the phlorin cation H₄OEFH⁺, become evident. This contrasts with the H₄TPP²⁺ behavior, for which the formation of phlorin cation H₄TPFH⁺ is only observed in longer timescale experiments, i.e., coulometry, or when excess HClO₄ is added (Fig. 6). Moreover, the presence of IVa'' at 5.0 V s⁻¹ (Fig. 9, green line), proves that the H₂OEP isophlorin is more reactive than the H₂TPP isophlorin, which is why self-protonation with the initial porphyrin occurs even at short timescales, reaction 13. Also, when comparing the experimental voltammogram at 5.0 V s⁻¹ with a simulation based on reactions 11 and 12, the loss of reversibility of processes Ic''/Ia'' and IIc''/IIa'' is evident (Section A.3, Fig. S5, Supplementary material).

$$\begin{eqnarray}&&{{\rm{H}}}_{4}{{\rm{OEP}}}^{2+}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{{\rm{OEP}}}^{\cdot +}\,{E}_{11}^{^\circ },{k}_{s,11},{\alpha }_{11}\,{\rm{Ic}}^{\prime\prime} /{\rm{Ia}}^{\prime\prime} \end{eqnarray} \tag{ 11 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{4}{{\rm{OEP}}}^{{\rm{\bullet }}+}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{4}{\rm{OEP}}\,{E}_{12}^{^\circ },{k}_{s,12},{\alpha }_{{\rm{12}}}\,{\rm{IIc}}^{\prime\prime} /{\rm{IIa}}^{\prime\prime} \end{eqnarray} \tag{ 12 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{4}{\rm{OEP}}+1/2{{\rm{H}}}_{4}{{\rm{OEP}}}^{2+}\rightleftharpoons {{\rm{H}}}_{4}{{\rm{OEFH}}}^{+}+1/2{{\rm{H}}}_{2}{\rm{OEP}}\end{eqnarray} \tag{ 13 }$$

The charge consumed after exhaustive electrolysis of a H₄OEP²⁺ solution corresponds to 1.35 e^–/H₄OEP²⁺ molecule. Voltammograms obtained after the electrolysis show signals of phlorin cation H₄OEFH⁺, i.e., oxidation IVa'' during the anodic sweep and reduction IIIc'' during the cathodic sweep (Fig. S6a, Supplementary material). The UV–vis absorption spectrum of the electroreduced solution also presents signals from phlorin cation (band at 745 nm), and bands assigned to the formation of H₂OEP at 498 and 622 nm (Fig. 10, orange line).

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These results agree with the global reaction 14, which is obtained from the combination of reactions 11–13, demonstrating that the reduction of H₄OEP²⁺ follows the same behavior than H₄TPP²⁺.

$$\begin{eqnarray}&&3/2{{\rm{H}}}_{{\rm{4}}}{{\rm{OEP}}}^{2+}+{{\rm{2e}}}^{-}\rightleftharpoons {{\rm{H}}}_{{\rm{4}}}{{\rm{OEFH}}}^{+}+1/2{{\rm{H}}}_{{\rm{2}}}{\rm{OEP}}\end{eqnarray} \tag{ 14 }$$

The reduction of H₃OEP⁺ was also studied by performing an exhaustive electrolysis of H₂OEP with 1.0 equiv of HClO₄ at –1.11 V (Ic') for 1 h. The charge consumed was equivalent to 0.7 e^–/H₃OEP⁺ molecule. Both the UV–vis spectrum and the voltammograms before electrolysis confirm the presence of H₃OEP⁺ with no interference from the H₂OEP and H₄OEP²⁺ species. After electroreduction, the spectrum presents bands at 745 nm and at 498 and 622 nm, corresponding to phlorin cation H₄OEFH⁺ and to H₂OEP as products (Fig. 10, blue line). The voltammograms show the same behavior than that of electroreduced H₄OEP²⁺, i.e., peaks corresponding to phlorin cation (Figs. S6, S7 and S8, Supplementary material). These results can be explained by the global reaction 15. Unlike H₄OEP²⁺ reduction, where H₄OEFH⁺ and H₂OEP are obtained in a ratio 1:0.5 (reaction 14); during H₃OEP⁺ reduction, the ratio becomes 0.5:1 (reaction 15). This can be qualitatively observed in the spectra of the electrolyzed solutions (Fig. 10): the H₄OEFH⁺ band at 745 nm is larger than that of H₂OEP at 498 nm for the H₄OEP²⁺ solution (orange line), and the opposite occurs in the H₃OEP⁺ solution (blue line).

$$\begin{eqnarray}&&3/2{{\rm{H}}}_{{\rm{3}}}{{\rm{OEP}}}^{+}+{{\rm{e}}}^{-}\rightleftharpoons 1/2{{\rm{H}}}_{{\rm{4}}}{{\rm{OEFH}}}^{+}+{{\rm{H}}}_{{\rm{2}}}{\rm{OEP}}\end{eqnarray} \tag{ 15 }$$

To better explain the mechanism involved in the H₃OEP⁺ reduction, the voltammetric behavior was analyzed for different H₃OEP⁺ concentrations and scan rates. Representative curves at two different concentrations (Fig. 11) show that the reduction process Ic' cathodically shifts as the scan rate increases, indicating the existence of coupled chemical reactions. Looking first at the 0.61 mM voltammogram, it is evident that, in addition to the cathodic shift, the current of Ic' changes. This is consistent with the presence of a coupled chemical reaction that yields a species whose reduction potential is similar to that of H₃OEP⁺. Also, at 5 and 10 V s⁻¹, two-reduction processes with similar potential values are observed (Fig. 11a). For the 1.85 mM solution, the current and shape of Ic' remain unmodified with increasing scan rates (Fig. 11b). Since the current of Ic' depends on the scan rate at low (0.61 mM) but not at higher concentrations (1.85 mM), it can be inferred that the coupled chemical reaction is of bimolecular nature. This behavior is characteristic of organic molecules with acid protons in their structure and is due to typical father-son⁵⁰ or self-protonation^49,56 reactions. Such process is feasible because the product of the first electronic transfer is more electronegative than the starting molecule.

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Based on the above, the reduction of H₃OEP⁺ likely proceeds via a one-electron transfer, yielding H₃OEP^• (reaction 16) that is protonated in presence of the initial H₃OEP⁺. For H₃OEP^•, there are two sites where protonation can occur: the free electron pair of the pyrrolic nitrogen (yielding H₄OEP^•+, reaction 17a) or in one of the meso- positions of the macrocycle^54,55 (yielding the phlorin radical cation H₃OEFH^•+, reaction 17b), Scheme 2. Regardless of the protonation site, the electronic affinity of the protonated species is larger than that of H₃OEP⁺, thus it is reduced at the same potential than H₃OEP⁺ (reactions 18a and 18b) (E°₁₆ < E°_18a(b)), or from the neutral radical H₃OEP^• in homogeneous phase (reactions 18a' and 18b').

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{3}{{\rm{OEP}}}^{{\rm{\bullet }}}\,{E}_{16}^{^\circ }\,{\rm{Ic}}^{\prime} \end{eqnarray} \tag{ 16 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEP}}}^{{\rm{\bullet }}}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\rightleftharpoons {{\rm{H}}}_{4}{{\rm{OEP}}}^{{\rm{\bullet }}+}+{{\rm{H}}}_{2}{\rm{OEP}}\end{eqnarray} \tag{ 17a }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEP}}}^{{\rm{\bullet }}}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\rightleftharpoons {{\rm{H}}}_{3}{{\rm{OEFH}}}^{{\rm{\bullet }}+}+{{\rm{H}}}_{2}{\rm{OEP}}\end{eqnarray} \tag{ 17b }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{4}{{\rm{OEP}}}^{{\rm{\bullet }}+}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{4}{\rm{OEP}}\,{E}_{18}^{^\circ }\,I{\rm{c}}^{\prime} \end{eqnarray} \tag{ 18a }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{4}{{\rm{OEP}}}^{{\rm{\bullet }}+}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{{\rm{\bullet }}}\rightleftharpoons {{\rm{H}}}_{4}{\rm{OEP}}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\end{eqnarray} \tag{ 18a’ }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEFH}}}^{{\rm{\bullet }}+}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{3}{\rm{OEFH}}\,{{\rm{E}}}_{18}^{^\circ }\,{\rm{Ic}}^{\prime} \end{eqnarray} \tag{ 18b }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEFH}}}^{{\rm{\bullet }}+}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{{\rm{\bullet }}}\rightleftharpoons {{\rm{H}}}_{3}{\rm{OEFH}}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\end{eqnarray} \tag{ 18b’ }$$

Voltammograms at 5 and 10 V s⁻¹ for H₃OEP⁺ 0.61 mM (Fig. 11a) indicate that the rate of the H₃OEP^• protonation can be partially overcome so that a second reduction process is observed as a small shoulder (peak IIc') at a slightly more negative potential than that of Ic', and is attributed to the reduction of the remaining H₃OEP^• that did not react with H₃OEP⁺, reaction 19. The oxidation peaks observed during the reverse sweep of H₃OEP⁺ coincide with peaks Ia'', IIa'' and IVa'' from Fig. 9, which were attributed to the oxidation of isophlorin H₄OEP (IIa'' and Ia''; reverse reactions 12 and 11) and of phlorin cation H₄OEFH⁺ (IVa''; reverse reaction 10). Therefore, it can be inferred that the protonation of the neutral radical H₃OEP^• occurs at the pyrrolic nitrogen electron free pair, yielding H₄OEP^•+ that is then reduced at Ic' to produce H₄OEP (Scheme 2) and is then oxidized at Ia'' and IIa''. An additional amount of H₄OEP is obtained from the processes indicated in reactions 19 and 20.

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEP}}}^{{\rm{\bullet }}}+{{\rm{e}}}^{-}\rightleftharpoons {{\rm{H}}}_{3}{{\rm{OEP}}}^{-}\,\mathrm{II}{\rm{c}}^{\prime} \end{eqnarray} \tag{ 19 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{{\rm{OEP}}}^{-}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\rightleftharpoons {{\rm{H}}}_{4}{\rm{OEP}}+{{\rm{H}}}_{2}{\rm{OEP}}\end{eqnarray} \tag{ 20 }$$

It is worth noting that, as the scan rate increases, the current of IVa'' decreases and a new oxidation process is observed, Va''. This is even more remarkable in the voltammograms obtained for the 0.61 mM solution than for that at 1.85 mM (Fig. 11). According to the acid-base behavior of the meso-tetraphenylporphyrin phlorin cation (Fig. S4, Supplementary material), it can be assumed that Va'' corresponds to the oxidation of neutral phlorin species H₃OEFH, which was confirmed by experiments where DBU was added to a solution of H₄OEFH⁺. The presence of neutral phlorin H₃OEFH as intermediate in the mechanism for the formation of phlorin cation H₄OEFH⁺ suggests that the protonation of H₃OEP^• occurs not only in the pyrrolic nitrogen, but also in the meso-position of the macrocycle so that H₃OEFH^•+ is obtained and immediately reduced to the neutral phlorin H₃OEFH.

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In summary, experimental results indicate that both of the reaction routes shown in Scheme 2 are possible. The protonation of both H₄OEP and H₃OEFH leads to phlorin cation H₄OEFH⁺, and, since H₃OEP⁺ is the proton-donating species, the free base H₂OEP is also a product, reactions 21−22. Section A.4 in Supplementary material presents how the two reaction routes lead to the global reaction 15.

$$\begin{eqnarray}&&{{\rm{H}}}_{4}{\rm{OEP}}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\rightleftharpoons {{\rm{H}}}_{4}{{\rm{OEFH}}}^{+}+{{\rm{H}}}_{2}{\rm{OEP}}\end{eqnarray} \tag{ 21 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{3}{\rm{OEFH}}+{{\rm{H}}}_{3}{{\rm{OEP}}}^{+}\rightleftharpoons {{\rm{H}}}_{4}{{\rm{OEFH}}}^{+}+{{\rm{H}}}_{2}{\rm{OEP}}\end{eqnarray} \tag{ 22 }$$

Spectroelectrochemical measurements, similar to those performed for H₄TPP²⁺ (Fig. 7), were carried out for H₃OEP⁺ and H₄OEP²⁺. As the potential moved towards negative values, a band corresponding to phlorin cation was observed at 745 nm; however, no isophlorin bands were detected. This agrees with the voltammetry results in which the existence of oxidation and reduction processes of phlorin cation indicate the isophlorin H₄OEP is a highly reactive species.