# Post-Functionalization of Pyridine-Modified Electrodes via Anodic C-N Coupling

Fatima Akhssas, Sophie Fournier, Moad Bouzid, Abhishek Kumar, Rita

Meunier-Prest, Hélène Cattey, Régis Parvaud, Eric Lesniewska, Anna

Krystianiak, Olivier Heintz, et al.

► To cite this version:

Fatima Akhssas, Sophie Fournier, Moad Bouzid, Abhishek Kumar, Rita Meunier-Prest, et al., Post-Functionalization of Pyridine-Modified Electrodes via Anodic C-N Coupling. ACS Electrochemistry, 2026, 10.1021/acselectrochem.6c00014. [https://doi.org/10.1021/acselectrochem.6c00014](https://doi.org/10.1021/acselectrochem.6c00014)

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# Post-Functionalization of Pyridine Modified Electrodes via Anodic C-N Coupling

Fatima Akhssas, || Sophie Fournier, || Moad Bouzid, || Abhishek Kumar, ||† Rita Meunier-Prest, ||

Hélène Cattey, || Régis Parvaud,‡ Eric Lesniewska, ‡ Anna Krystianiak, ‡ Olivier Heintz, ‡ Charles

H. Devillers*||

|| Université Bourgogne Europe, CNRS, ICMUB UMR 6302, 21000 Dijon, France

‡ Université Bourgogne Europe, CNRS, ICB UMR 6303, 21000 Dijon, France

## Abstract

The oxidation of 2,7,12,17-tetra-tert-butylporphyrin Zn(II) (Zn-1) on phenyl-pyridinyl modified glassy carbon (GC) or Indium Tin Oxide (ITO) electrode resulted in the formation of a new C-N bond linking the porphyrin and pyridine units. This innovative post-functionalization grafting strategy was investigated using electrochemical, photophysical, structural and surface characterizations, validating the expected structure and the involved mechanism. Additionally, a reference pyridinium-porphyrin compound was electrosynthesized by oxidizing Zn-1 in the presence of a slight excess of the corresponding phenyl-4-pyridinyl nucleophile. The physico-
chemical properties of this reference compound matched those of the functionalized porphyrin material.

## Keywords

Pyridine-based Material, Porphyrin-based Material, Electrografting of Aryldiazonium, Oxidative Post-Functionalization, Electrosynthesis of Pyridinium-Porphyrin

## Introduction

Porphyrins are key players in fundamental biological processes such as photosynthesis¹ and oxygen transport², and they have found widespread applications in photoluminescent biosensors³,⁴, molecular electronics⁵, optical devices⁶, energy conversion⁷⁻⁹ and storage systems.¹⁰,¹¹ Owing to their versatility, the development of porphyrin-based materials through immobilization on electrode surfaces has attracted significant attention. Two main strategies have emerged: direct functionalization of electrodes with porphyrins and post-functionalization of pre-modified electrodes.

Direct approaches include physisorption on carbon¹²,¹³ or metal electrodes¹⁴, electropolymerization¹⁵⁻¹⁸, covalent attachment via oxidative coupling of terminal alkynes¹⁹,²⁰, and electrochemical or spontaneous reduction of diazonium salts²¹⁻²⁹. Alternatively, post-functionalization strategies rely on pre-grafted reactive groups such as carboxylic acids³⁰,³¹,¹³, azides³²⁻³⁵, or alkynes³⁶, which require complementary functionalization of the porphyrin to form amides, esters, or triazoles. Another explored route involves coordination of metalated porphyrins (Zn(II), Co(II), Fe(II)) to ligands such as imidazole³⁷,³⁸, amines³⁹, pyridines⁴⁰⁻⁴², or cyano groups⁴³.

Building on previous work by Ruhlmann and our group on the electrosynthesis of pyridinium-porphyrins via electrochemical oxidation of porphyrins in the presence of pyridine derivatives⁴⁴⁻
⁵⁰, we asked whether this anodic nucleophilic substitution could be extended to surfaces bearing pre-grafted pyridine groups. This heterogeneous/homogeneous C–N coupling strategy offers a key advantage: it eliminates the need for porphyrin pre-functionalization, enabling direct electrochemical C–H activation. If successful, this approach could be generalized to other oxidizable aromatic systems and other nucleophilic surfaces. To the best of our knowledge, such an immobilization strategy has never been reported for porphyrins or any aromatic compound.

Herein, we present a two-step electrochemical method for immobilizing aromatic molecules, using Zn(II) 2,7,12,17-tetra-tert-butylporphyrin (Zn-1, Scheme 3) as a proof of concept. First, 4-pyridin-4-yl-benzenediazonium (4-PBD, Scheme 2) is electrochemically grafted onto a conductive substrate⁵¹, allowing precise control over surface coverage. In the second step, Zn-1 is oxidized in solution using the pyridine-modified electrode, promoting C(porphyrin)–N(pyridine) bond formation and covalent anchoring of the macrocycle. The resulting pyridinium–porphyrin materials were thoroughly characterized by multiple analytical techniques. For comparison, a phenylpyridinium-porphyrin reference compound was also electrosynthesized to validate the proposed structure.

## Experimental Section

**General Comments.** Unless otherwise noted, all reactions were carried out without protection from air. CH₂Cl₂ and MeCN were dried over alumina cartridges using a solvent purification system PureSolv PS-MD-5 model from Innovative Technology and kept under argon. All other solvents used for reactions were obtained from commercial suppliers and used as received. The free base 2,7,12,17-tetra-tert-butylporphyrin (H₂-1) was synthesized following the protocol patented by Nickel and Liebeskind⁵²,⁵³ and purified according to Whitlock.⁵⁴ The zinc(II) complex Zn-1 was prepared following common metalation procedure.⁵⁵ TLC were performed with TLC Silica gel 60
F254 from Merck. The spots were directly visualized by naked eyes or through illumination with a UV lamp (λ = 254/365 nm). Column chromatography purifications were performed manually with Silica gel 60 (40-63 µm) from VWR.

**Instruments and methods.** ¹H-, and ¹³C- spectra were recorded either on a Bruker Avance NEO 400, on a Bruker Avance NEO 500 or on a Bruker Avance 600 III HD spectrometer. Chemical shifts are expressed in parts per million (ppm) from the residual non-deuterated solvent signal.⁵⁶ *J* values are expressed in Hz. Spectra were calibrated to TMS on the basis of the relative chemical shift of the residual non-deuterated solvent signal as an internal standard.

High-resolution mass spectra (HRMS) were recorded either on a Thermo Orbitrap Exploris 240 apparatus equipped with an ESI source and low resolution mass spectrometry was recorded on a Bruker Microflex LRF MALDI-TOF mass spectrometer (matrix: dithranol).

UV-visible absorption spectra were recorded on a VARIAN Cary 50 UV-Visible spectrophotometer using quartz cells (Hellma).

Atomic Force Microscope (AFM) images of the aryls coated ITO/HOPG and the bare ITO/HOPG were recorded on a Bruker Icon 2 nanoDMA equipment. AFM was operating in nanoDMA peak force mode, which is sensitive to local mechanical properties. Measurements were made at a peak force frequency of 2 kHz and at amplitude in the range 50–100 nm, using silicon probes (DNP SA or ScanAsyst-HR) (tip radius < 5 nm). Images were analyzed by Nanoscope analysis software.

Scanning electron microscope (SEM) images were acquired on Hitachi SU8230 electron microscope, equipped with cold field emission gun and operating at an accelerating voltage of 2 kV.

XPS analyses were performed using Versaprobe 5000 spectrometer (ULVAC-PHI apparatus), utilizing a monochromatic and focused Al Kα X-ray source (1486.6 eV). For each aryl coating as
well as bare ITO, a survey spectrum and a high-resolution core level spectrum of carbon 1s,
nitrogen 1s, fluorine 1s, oxygen 1s, chlorine 2p, indium 3d and tin 3d were recorded over a 200
µm spot size. The pass energies were 187.5 eV and 58.7 eV for the survey and core-level spectra,
respectively. The raw data were fitted and analyzed by CasaXPS software.⁵⁷

**Electrochemistry.** All manipulations were performed using Schlenk techniques in an
atmosphere of dry oxygen-free argon at room temperature (T = 20°C ± 3°C), unless otherwise
mentioned. The supporting electrolyte (tetraethylammonium tetrafluoroborate, TEABF₄) was
degassed under vacuum before use and then dissolved in CH₂Cl₂ (unless otherwise noted) to a
concentration of 0.1 mol L⁻¹. The following electrolyte volumes (unless otherwise noted) were
used for voltammetric analyses and bulk electrolysis: anodic compartment: V = 20 mL; cathodic
compartment: V = 5 mL; reference electrode compartment: V = 5 mL.

*Voltammetric analyses.* Voltammetric analyses were carried out in a standard three-electrode cell, with Biologic SP-300 potentiostat, connected to an interfaced computer that employed EC-Lab (v. 11.43) software. The reference electrode was a saturated calomel electrode (SCE) separated from the analyzed solution by a sintered glass disk filled with the background solution. The auxiliary electrode was a platinum foil separated from the analyzed solution by a sintered glass disk filled with the background solution. For all voltammetric measurements, the working electrode was a platinum electrode (Ø = 1.6 mm, from ALS). Before each voltammetric analysis, the Pt electrode was polished with a diamond suspension (1 µm, diamond suspension Mono, alcohol based, from ATM Qness GmbH). In these conditions, when operating in CH₂Cl₂ (0.1 M TEABF₄) or CH₃CN (0.1 M TEABF₄), the formal potential for the ferrocene (+/0) couple was +0.46 V or +0.40 V vs SCE, respectively. The ITO electrodes (75 × 25 × 1 mm), Indium tin oxide coated glass slide, surface resistivity 8-12 Ω/sq, reference: 578274, Sigma-Aldrich) were manually cut with a
diamond tip, cleaned *via* sonication for 10 min. in CH<sub>2</sub>Cl<sub>2</sub>, then 10 min. in acetone, then 10 min. in EtOH, then the solvent was evaporated at room temperature. The carbon electrodes (Ø = 1.0, 1.6 or 3.0 mm, from ALS) were cleaned by polishing with a diamond solution and then rinsed with distilled water and acetone.

*Bulk electrolyzes.* Bulk electrolyzes were performed in CH<sub>3</sub>CN (0.1 M TEABF<sub>4</sub>) at controlled potential in a cell with three compartments separated with glass frits of medium porosity with a Biologic SP-300 potentiostat, connected to an interfaced computer that employed EC-Lab (v. 11.43) software. The reference electrode was a saturated calomel electrode (SCE) separated from the analyzed solution by a sintered glass disk filled with the background solution. The auxiliary electrode was a platinum foil (ca. 30 cm<sup>2</sup>) separated from the analyzed solution by a sintered glass disk filled with the background solution. Two platinum wire spirals (*l* = 50 cm, Ø = 1 mm for each spiral, S<sub>tot</sub> ≈ 2×15 ≈ 30 cm<sup>2</sup>) were used as working electrodes. Electrolyzes were followed by TLC analyses and CV analyses and were stopped when the starting porphyrins were consumed.

**Synthesis.**

*Synthesis of 4-(pyridin-4-yl) aniline (4-PA)*<sup>58</sup>. 4-Bromoaniline (1.000 g, 5.81 mmol, 1.0 eq.), 4-pyridylboronic acid (1.559 g, 12.68 mmol, 2.2 eq.), allylpalladium(II) chloride dimer (103.5 mg, 0.28 mmol, 0.05 eq.), PPh<sub>3</sub> (228.4 mg, 0.87 mmol, 0.15 eq.), and Na<sub>2</sub>CO<sub>3</sub> (4.267 g, 40.26 mmol, 6.93 eq.) were dissolved in 157 mL of DMF/H<sub>2</sub>O (7:2 v/v), the solution was degassed by bubbling argon for 15 min, and then heated at 100 °C for 72 hours, the solvent was removed by rotary evaporation, and then the crude product was dissolved by CH<sub>2</sub>Cl<sub>2</sub> and washed three times with water. The organic layer was separated and the solvent removed by rotary evaporation. The purification was performed by column chromatography (SiO<sub>2</sub>, CH<sub>2</sub>Cl<sub>2</sub>/CH<sub>3</sub>OH (97:3 v/v). The fractions containing the 4-(pyridinyl-4-yl)aniline (4-PA) were collected, evaporated and
recrystallized in CH₂Cl₂/n-heptane mixture affording **4-PA** in 38% yield (377.0 mg, 2.21 mmol).

¹H NMR (CDCl₃, 400 MHz, 298 K): δ (ppm) 8.57 (d, ³Jₘ-ₘ = 6.2 Hz, 2H), 7.49 (d, ³Jₘ-ₘ = 8.6 Hz, 2H), 7.45 (d, ³Jₘ-ₘ = 6.2 Hz, 2H), 6.76 (d, ³Jₘ-ₘ = 8.6 Hz, 2H), 3.79 (br s, 2H, NH₂).

*Electrosynthesis of Zn-1-PPy+, BF₄⁻. Zn-1* (100.1 mg, 1.67×10⁻¹ mmol, 1 eq.) was dissolved in 100 mL CH₃CN (0.1 M TEABF₄) and 4-phenylpyridine (42.8 mg, 2.76×10⁻¹ mmol, 1.6 eq.) was added. Electrolysis was carried out under an argon atmosphere in a three compartments cell under vigorous stirring at room temperature and at controlled potential (*E*<sub>app</sub> = 0.75 V/SCE). The reaction was stopped after consumption of 2.0 F per mol of porphyrin. After evaporation of the solvent, the resulting residue was dissolved in 50 mL of CH₂Cl₂ then washed with 3×100 mL of water to remove the supporting electrolyte. After evaporation of the organic layer, the crude product was purified by column chromatography (SiO₂, *h* = 19 cm, *Ø* = 3 cm; CH₂Cl₂/pyridine (95:5) *v*/*v*). The second fraction was evaporated until only pyridine remains. Then recrystallization was performed by slow addition of 2 mL of CH₃CN, 0.1 mL pyridine then 10 mL of H₂O followed by 10 mL of CH₃OH. The grown crystals were filtrated providing **Zn-1-PPy+, BF₄⁻** in 71% yield (101.0 mg, 11.9×10⁻² mmol). ¹H NMR ((CD₃)₂SO, 500 MHz, 298 K): δ (ppm) 10.71 (s, 1H, Hf), 10.63 (s, 2H, Hj and Hh), 10.47 (d, 3Jₘ-ₘ = 6.3 Hz, 2H, Ha), 9.80 (s, 1H, He), 9.49 (s, 1H, Hg), 9.43 (s, 1H, Hi), 9.02 (d, 3Jₘ-ₘ = 6.3 Hz, 2H, Hb), 8.85 (d, 3Jₘ-ₘ = 6.2 Hz, 2H, Hc), 7.89-7.83 (m, 3H, Hd and Hp), 7.64 (s, 1H, Hk), 2.29 (s, 9H, Hm), 2.28 (s, 9H, Hn), 2.10 (s, 9H, Ho), 1.62 (s, 9H, Hl). ¹³C{¹H} NMR ((CD₃)₂SO, 125 MHz, 298 K): δ (ppm) 157.4, 155.8, 155.5, 155.0, 152.2, 152.0, 148.7, 148.4, 147.2, 147.1, 147.0, 146.1, 144.4, 143.6, 136.0, 133.3, 133.1, 130.5, 130.06, 130.00, 129.0, 123.3, 121.6, 116.5, 105.87, 105.80, 33.9, 33.7, 33.6, 33.59, 33.56, 33.54, 33.34. λmax (DMSO)/nm (log ε): 417 (5.58), 547 (4.31), 586 (4.20). HRMS (ESI⁺): m/z cald for C₄₇H₅₂N₅Zn [M-BF₄⁻]⁺ 750.3509, found 750.3501.
## Electrografting.

Electrodeposition of in situ generated 4-(pyridin-4-yl)benzenediazonium (4-PBD) from 4-(pyridin-4-yl)aniline (4-PA) leading to 4-phenylpyridine (4-PP) modified ITO or GC electrodes. Under argon atmosphere, and protection from light, in the electrochemical three compartments cell, the diazonium salt precursor of 4-(pyridin-4-yl)benzenediazonium (4-PBD) was prepared by mixing 5 mM of 4-PA (4-(pyridin-4-yl)aniline), 2 equivalents of NaNO₂ and 0.5 M HBF₄ in 0.1 M TEABF₄ in acetonitrile at 0°C. The electroreduction of diazonium species was performed by multiscan cyclic voltammetry (CV) cycles between 0.86 and -0.2 V for GC, and 0.70 to -0.14 V for ITO, at 50 mV·s⁻¹. The formed organic films were washed via sonication in a solution of 1 mM of NH₃ in CH₃CN for 30 s, and then in pure CH₃CN for 30 s.

Electrodeposition of Zn-1 on 4-phenylpyridine (4-PP) modified ITO or GC electrodes. In the electrochemical three compartment cell, a solution of 10⁻³ M of Zn-1 in CH₂Cl₂ 0.1 M TEABF₄ containing 2 equivalents of Cs₂CO₃ was prepared and stirred for 10 min at room temperature under argon. Then, the 4-PP modified working electrode was introduced in the cell. The porphyrin oxidation was carried out by multiscan cyclic voltammetry in the range of 0.0 V and +0.9/1.0 V vs SCE. The modified electrodes were then thoroughly rinsed with CH₂Cl₂, then by sonication in CH₃CN for 30 seconds.

## Results and Discussion

Electrografting attempts with 4-aminopyridine. Our initial approach involved 4-aminopyridine (Scheme 1), aiming to electrograft the *in situ* generated pyridine-4-diazonium species. However, the electrografting failed, yielding almost no deposit, despite previous reports of successful grafting of pyridine-4-diazonium and its 3-substituted isomer on carbon substrates. ⁵⁹⁻⁶⁷
Interestingly, Bélanger and co-workers demonstrated by ¹H NMR that diazotization of 3-aminopyridine in 0.5 M DCl with NaNO₂ rapidly forms pyridine-3-diazonium, which then evolves into 3-hydroxypyridine.⁶³ Under similar conditions, we observed no pyridine-4-diazonium by ¹H NMR; instead, 4-hydroxypyridine formed progressively, likely via nucleophilic substitution of the transient diazonium by water (see Figure S1).

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Unsuccessful electrografting of pyridine-4-diazonium.</figcaption></figure>

To overcome this limitation, we introduced an additional aniline ring at the 4-position of the pyridine moiety, yielding 4-(pyridin-4-yl)aniline (4-PA), a precursor to the corresponding diazonium salt (4-PBD, Scheme 2). While the synthesis of this diazonium has been reported,⁶⁸,⁶⁹ its electrografting has, to our knowledge, never been described.

*Electrografting of 4-phenylpyridine*

4-(Pyridin-4-yl)aniline (4-PA, Scheme 2) was synthesized via Suzuki cross-coupling between pyridin-4-ylboronic acid and 4-bromoaniline, following the protocol reported by Gunderson *et al.* (Scheme S1).⁵⁸ The corresponding 4-pyridin-4-yl-benzenediazonium salt (4-PBD) was then generated *in situ* from 4-PA (5 mM) by sequential addition of HBF₄ (0.5 M, 50% in water) and 2
equivalents of NaNO₂ in acetonitrile containing 0.1 M TEABF₄ (Scheme 2), under argon, vigorous stirring, and at 0 °C in the dark.

<figure><img src="image_3.png" /><figcaption>**Scheme 2.** Electrochemical grafting of diazonium (4-PBD).</figcaption></figure>

Electrochemical reduction of **4-PBD** on Glassy Carbon (GC) or Indium Tin Oxide (ITO) electrodes was carried out by multi-scan cyclic voltammetry (Figure 1A, on ITO and Figure 1B, on GC) at 50 mV·s⁻¹ (all potentials vs KCl Saturated Calomel Electrode (SCE)). During the first scan, a clear reduction peak of the diazonium group appeared at Epc = 0.05 V on ITO. On GC, two reduction peaks were observed at 0.00 (peak 2) and 0.35 V (peak 1), as already reported by Downard,⁷⁰ Bélanger,⁷¹ González⁷² and co-workers. According to these works, peak 1 corresponds to a surface catalyzed reduction that proceeds at a clean surface only while peak 2 is attributed to an uncatalyzed reduction at less positive potential. On both electrodes, these peaks rapidly decreased in subsequent scans, indicating the formation of a passivating organic layer that hinders further reduction of diazonium cations at the electrode surface. After electrografting, the modified electrodes were ultrasonically cleaned in 0.1 M NH₃/CH₃CN to regenerate the neutral phenylpyridine moiety, then rinsed with pure acetonitrile.
<figure><img src="image_4.png" /><figcaption>**Figure 1.** A and B: Cyclic voltammograms corresponding to the reduction of the diazonium salt **4-PBD** generated in situ from a 5 mM **4-PA** solution in the presence of 2 eq. of NaNO₂ and 0.5 M HBF₄, in CH₃CN 0.1 M TEABF₄, for 18 scan cycles (Nc(**4-PBD**) = 18). A: on ITO (S ≈ 0.3 cm²) and B: on GC (S = 0.07 cm²) at v = 50 mV/s. C and D: Cyclic voltammograms performed in a 1 mM ferrocene solution (CH₃CN 0.1 M TEABF₄) on ITO (C) and GC (D), number of electrografting cycles in the **4-PBD** solution (Nc(**4-PBD**)) = 0 (bare electrode, black curve), 2 (blue curve), 6 (green curve), 12 (yellow curve), and 18 (red curve) scans of electrografting in the solution containing diazonium salt **4-PBD**. v = 100 mV/s).</figcaption></figure>
The blocking properties of the resulting organic films on ITO and GC were evaluated by cyclic voltammetry in 1 mM ferrocene solution (0.1 M TEABF₄ in acetonitrile). Figures 1C and 1D show CVs of bare and modified electrodes for different numbers of electrografting cycles (Nc(**4-PBD**) = 0, 2, 6, 12, 18). As typically observed for aryldiazonium electrografting,⁷³ the redox peak current of ferrocene decreased with increasing cycle number, accompanied by a shift of peak potentials to more positive (oxidation) and less positive (reduction) values. After 18 cycles, the ferrocene signal nearly disappeared, confirming the formation of a highly passivating and compact organic layer which effectively blocks the redox probe access to the electrified electrodes. Electrografted and bare ITO surfaces were investigated by scanning electron microscopy (SEM) in secondary electron imaging mode at low accelerating voltage, revealing micrographs as shown in Figures 2 and S6.

The image presents darker and brighter zones. The dark region can be assigned to the surface covered with organic layer, owing to its insulating nature, producing fewer secondary electrons to be detected by the detector. On the other hand, the bright zone is conducting ITO, producing higher secondary electrons. Moreover, the increased roughness of the surface because of the electrografting can result in lower secondary electron yield in SEM imaging, resulting in a dark image.⁷⁴

<figure><img src="image_5.png" /></figure>
**Figure 2.** SEM image of the frontier between bare and 4-PPy modified ITO electrode (Nc(4-PBD) = 2).

The elemental composition of the bare ITO electrodes and the electrografted surfaces were analyzed by X-ray Photoelectron (XPS, Figures S7 and S8). For the modified electrodes, the survey XPS spectra revealed the presence of carbon and nitrogen in addition to the constituent elements of the ITO substrate. High resolution C1s XPS spectra of the electrografted electrodes exhibited a peak at 284.8 eV, characteristic of C-C/C-H bonds. A distinct N1s peak at ~398.9 eV appeared exclusively for the pyridine-modified electrodes, confirming the successful grafting of the phenylpyridine moiety. The N1s envelope was deconvoluted into two components: the main peak at 398.9 eV, assigned to neutral pyridine nitrogen,⁷⁵ and a secondary peak at 400.5 eV, attributed to azo bridges⁷⁶,⁷⁷ formed during electrodeposition. Based on the ratio of azo to pyridine nitrogen (n<sub>azo</sub>/2n<sub>pyr</sub>), the azo content was estimated at 5-10%. For all modified electrodes, the C/N ratio remained nearly constant (9-11), in good agreement with the theoretical value for a single phenylpyridine unit (C/N = 11).

*Electrochemical Immobilization of Zn-1 on Pyridine-Modified Electrodes*

The immobilization of **Zn-1** on phenylpyridine-modified electrodes was achieved via electrochemical oxidation. Grafting efficiency was significantly enhanced when multi-scan cyclic voltammetry (CV) was performed in dichloromethane with 2 equivalents of Cs<sub>2</sub>CO<sub>3</sub> (Figure 1), compared to acetonitrile with 2 equivalents of 2,6-lutidine. CVs of **Zn-1** on bare and 4-PPy/ITO electrodes are shown in Figure 3. On bare ITO electrode, the first oxidation, corresponding to formation of the porphyrin cation radical,⁷⁸ occurs at E<sub>1/2</sub> = 0.78 V, with peak currents and potentials remaining constant over 10 cycles (Figure 3, left). This stability indicates the absence
of follow-up chemical reactions and confirms the reversibility of the first oxidation on the CV timescale.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Left: cyclic voltammograms (10 cycles) corresponding to the oxidation of **Zn-1** on bare ITO electrode. Right: cyclic voltammograms (20 cycles) corresponding to the oxidation of **Zn-1** on phenylpyridine-modified ITO electrodes (Nc(**4-PBD**) = 12. [**Zn-1**] = 1 mM, 2 eq. Cs₂CO₃, CH₂Cl₂ 0.1 M TEABF₄, v = 50 mV/s, A ≈ 0.3 cm²).</figcaption></figure>

In contrast, multi-scan CV recorded on **4-PPy**-modified electrodes (Figure 3, right) exhibit irreversible behavior during the first scan, consistent with a rapid chemical reaction following cation radical formation. The progressive decrease in the first oxidation peak with successive scans strongly supports porphyrin electrografting, likely via oxidative C(porphyrin)-N(pyridine) coupling. The proposed mechanism involves generation of an electrophilic cation radical upon **Zn-1** oxidation, which undergoes nucleophilic attack by grafted pyridine moieties, forming a new C-N bond. The proton released during this step is scavenged by the added base, preventing **Zn-1** demetalation and pyridine protonation. The resulting modified electrode is expected to feature a pyridinium-porphyrin motif (Scheme 3).
<figure><img src="image_7.png" /><figcaption>**Scheme 3.** Immobilization of Zn-1 on phenylpyridine-modified electrodes by electrochemical oxidation.</figcaption></figure>

*Preparation of Zn-1-PPy⁺,BF₄⁻ as a Reference Compound*

Since the molecular structure of the pyridinium-porphyrin material (Zn-1-PPy⁺,BF₄⁻/GC or Zn-1-PPy⁺,BF₄⁻/ITO) was expected to resemble that proposed in Scheme 4, the reference compound Zn-1-PPy⁺,BF₄⁻ was electrosynthesized under the conditions described in Scheme 4 (see experimental section and SI for details). Electrochemical oxidation of Zn-1 at its first oxidation potential in the presence of 4-phenylpyridine (4-PPy) afforded Zn-1-PPy⁺,BF₄⁻ in 71% isolated yield. As expected, Zn-1-PPy⁺,BF₄⁻ exhibits a more positive first oxidation potential compared to Zn-1 ($E_{1/2}(ox1) = 1.01$ V vs 0.71 V), consistent with the electron-withdrawing effect of the pyridinium substituent (Figure 4). Additionally, the characteristic pyridinium reduction was observed at $E_{1/2}(red1) = -0.88$ V, while the porphyrin radical anion formed at $E_{1/2}(red2) = -1.13$ V.
<figure><img src="image_8.png" /><figcaption>**Scheme 4.** Electrosynthesis of Zn-1-PPy+, BF4- by anodic nucleophilic substitution of Zn-1 with 4-PPy.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4.** Cyclic voltammograms of Zn-1 (top) and Zn-1-PPy+, BF4- (bottom) in CH3CN 0.1 M TEABF4 ([Zn-1] = 1.6 mM, [Zn-1-PPy+,BF4-] = 1.0 mM, WE: Pt (Ø = 1.6 mm), v = 100 mV/s).</figcaption></figure>

The molecular structure of Zn-1-PPy+,BF4- was confirmed by 1D and 2D NMR analyses (see SI for details). The ¹H NMR spectrum displayed only three meso singlets, consistent with substitution at one meso position by the phenylpyridinium fragment (Figure 5). The α-protons of the pyridinium moiety (Ha) appeared as highly deshielded doublets at δ = 10.48 ppm, due to the positive charge and their position outside the porphyrin shielding cone. Conversely, the β-pyrrolic
proton Hk, located inside the pyridinium shielding cone, was strongly shielded ($\delta$ = 7.65 ppm) compared to other $\beta$-pyrrolic singlets (He, Hg, Hi) at 9.43, 9.49, and 9.81 ppm.

<figure><img src="image_10.png" /><figcaption>**Figure 5.** $^1$H NMR spectrum of Zn-1-PPy$^+$,BF$_4^-$ (DMSO-d$_6$, 500 MHz, 298 K).</figcaption></figure>

Definitive structural confirmation was obtained by single-crystal X-ray diffraction of a crystal grown by slow evaporation of a concentrated $CH_2Cl_2$ solution of Zn-1-PPy$^+$,BF$_4^-$ containing two drops of pyridine (Figure 6). Under these conditions, Zn-1-PPy$^+$,BF$_4^-$ was coordinated by one pyridine molecule (Zn-1-PPy$^+$,BF$_4^-$·Py). The cationic nature of the porphyrin was confirmed by the presence of the $BF_4^-$ anion. The $C_{meso}-N_{pyridinium}$ distance was 1.470(3) Å, very close to that previously reported for Zn-1 functionalized with a simple pyridinium fragment at the meso position (1.466(5) Å).$^{48}$
<figure><img src="image_11.png" /><figcaption>**Figure 6.** X-ray crystallographic structure of Zn-1-PPy+, BF4-.Py (CCDC 2499674, front and side Mercury<sup>79</sup> views). Thermal ellipsoids are scaled to the 50% probability level.</figcaption></figure>

*Characterization of Porphyrin Modified Electrodes*

Electrochemical immobilization of the porphyrin on semi-transparent ITO electrodes was monitored by UV-visible absorption spectroscopy. Porphyrins exhibit strong visible absorption, with an intense Soret band near 400 nm and weaker Q bands between 500-700 nm. For a fixed number of diazonium electrografting cycles, absorbance increased with the number of CV cycles during the porphyrin oxidation step, reaching a maximum for 20 cycles. Additional scans led to decreased absorbance; thus, 20 oxidative cycles were selected for subsequent studies. Similarly, absorbance increased with the number of reductive cycles for **4-PBD**, indicating a higher density of reactive pyridine moieties (Figure 7).
<figure><img src="image_12.png" /><figcaption>**Figure 7.** UV-visible absorption spectra of Zn-1-PPy+, BF₄⁻/ITO after Nc = 2, 6, 12 and 18 scanning cycles during reduction of 4-PBD and 20 oxidative scanning cycles in the solution of Zn-1 in dichloromethane ([Zn-1] = 1 mM, 2 eq. Cs₂CO₃, CH₂Cl₂ 0.1 M TEABF₄, v = 50 mV/s, A ≈ 0.3 cm²).</figcaption></figure>

To confirm porphyrin immobilization, reference analyses were performed (Figure 8 and Figure S13). A solution of Zn-1 in CH₂Cl₂ exhibited λmax at 400/529/564 nm (Figure 8A). A bare ITO electrode immersed for 15 min in Zn-1 solution and dried showed λmax at 409/533/566 nm (Figure 8B). Likewise, a phenylpyridine-modified ITO film (Nc(4-PBD) = 18) treated similarly gave λmax at 408/531/565 nm, suggesting no chemical interaction between grafted pyridine and Zn(II) (Figure 8C). In contrast, Zn-1-PPy+, BF₄⁻ in CH₂Cl₂ exhibited λmax at 412/546/583 nm, consistent with meso-substitution by phenylpyridinium (Figure 8D).⁴⁴ When Zn-1-PPy+, BF₄⁻ was deposited on bare ITO and dried, λmax shifted to 422 nm (Soret) and 550/585 nm (Q bands), nearly identical to those of the electrodeposited material (425 ± 1/549/585 nm), strongly supporting structural similarity (Figure 8E and F).
<figure><img src="image_13.png" /><figcaption>**Figure 8.** Top: UV-visible absorption spectra of A) **Zn-1** or D) **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>** in CH<sub>2</sub>Cl<sub>2</sub>. UV-visible absorption spectra of **Zn-1** deposited on B) bare ITO electrode or C) phenylpyridine modified electrode (Nc(**4-PBD**) = 18 scans) by immersion for 15 min in a solution of **Zn-1** in CH<sub>2</sub>Cl<sub>2</sub>. UV-visible absorption spectra of E) **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>** deposited on bare ITO electrode by immersion for 15 min in a solution of **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>** in CH<sub>2</sub>Cl<sub>2</sub> and F) porphyrin material electrodeposited by oxidation of **Zn-1** (20 oxidative grafting cycles) on ITO electrode modified by **PPy** (Nc(**4-PBD**) = 18 scans) in CH<sub>2</sub>Cl<sub>2</sub>.</figcaption></figure>

Modified **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>/ITO** electrodes were analyzed by CV (Figure 9 and Figure S10) in acetonitrile with 0.1 M TEABF<sub>4</sub>. CV showed two chemically irreversible oxidation processes at E<sub>pa</sub>(O1) = 0.98/1.00 V and E<sub>pa</sub>(O2) = 1.4/1.6 V. In the negative potential range, two reduction peaks at E<sub>pc</sub>(R1) = -1.07 ± 0.02 V (pyridinium reduction) and E<sub>pc</sub>(R2) = -1.45 ± 0.05 V (porphyrin reduction) were observed, close to those of **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>**, reinforcing structural similarity. The
irreversibility of these four redox processes was not expected since the solute Zn-1-PPy+,BF4- exhibits four chemically reversible redox reactions under the same experimental conditions. This behavior may result from possible irreversible C-C or C-N intermolecular couplings occurring upon oxidation (C(porphyrin)-C(porphyrin), C(porphyrin)-N(non-functionalized pyridine)) or reduction (C(pyridinium)-C(pyridinium)) of the porphyrin-pyridinium-based material.

<figure><img src="image_14.png" /><figcaption>**Figure 9.** Cyclic voltammogram corresponding to the analysis of Zn-1-PPy+,BF4-/ITO2 (Nc(4-PBD) = 6 scans) and porphyrin (Nc(Zn-1) = 20 scans) in a 0.1 M TEABF4 acetonitrile solution (v = 100 mV/s).</figcaption></figure>

Under the same electrodeposition conditions as in Figure 8, CV peak currents increased with the number of electroreduction cycles. The porphyrin surface coverage ΓCV was estimated from the first oxidation peak, considering a monoelectronic transfer using:

$$ \Gamma_{CV} = \frac{Q}{n_{FA}} \quad (1) $$

where Q is the integrated charge, n the number of electrons, A the electrode area (cm²), and F the Faraday constant (96485 C·mol⁻¹).
Surface coverage ($Γ_{UV}$) was also determined from UV-visible spectra using:³⁷

$$Γ_{UV} = \frac{A(λ)}{1000×ε(λ)} \quad (2)$$

Assuming similarity to **Zn-1-PPy⁺,BF₄⁻**, its molar extinction coefficient at the Soret band was used ($λ_{max}$ = 409 nm, $ε$ = 239000 M⁻¹·cm⁻¹ in CH₂Cl₂). The absorbance maximum of the electrodeposited material ($λ_{max}$ = 423 nm) was considered for $Γ_{UV}$ calculation. Under the conditions of Figure 7, $Γ_{UV}$ ranged from 1.9 × 10⁻¹⁰ to 3.3 × 10⁻¹⁰ mol·cm⁻², close to the expected monolayer value ($Γ$ ≈ 2.2 × 10⁻¹⁰ mol·cm⁻²).²³ Surface coverage values from CV and UV-visible measurements are summarized in Table 1. Notably, $Γ_{UV}$ and $Γ_{CV}$ were similar, though $Γ_{UV}$ appeared slightly overestimated.

**Table 1.** $Γ_{CV}$ and $Γ_{UV}$ for ITO1, ITO2, ITO3 and ITO4 modified electrodes prepared by electroreduction of **4-PBD** (Nc(**4-PBD**) = 2, 6, 12 or 18, respectively) then oxidation of **Zn-1** (C = 1 mM) in CH₂Cl₂ 0.1 M TEABF₄ with 2 eq. of Cs₂CO₃ (Nc(**Zn-1**) = 20 cycles for all electrodes, v = 50 mV/s).

<table><thead><tr><th>ITO sample</th><th>ITO1</th><th>ITO2</th><th>ITO3</th><th>ITO4</th></tr></thead><tbody><tr><td>Nc(**4-PBD**)</td><td>2</td><td>6</td><td>12</td><td>18</td></tr><tr><td>Nc(**Zn-1**)</td><td>20</td><td>20</td><td>20</td><td>20</td></tr><tr><td>Γ<sub>cv</sub> (10<sup>-10</sup> mol·cm<sup>-2</sup>)</td><td>1.2</td><td>2.6</td><td>3.1</td><td>3.3</td></tr><tr><td>Abs (a.u.) (Soret band)</td><td>0.056</td><td>0.073</td><td>0.083</td><td>0.090</td></tr><tr><td>Γ<sub>uv</sub> (10<sup>-10</sup> mol·cm<sup>-2</sup>)</td><td>2.3</td><td>3.1</td><td>3.5</td><td>3.8</td></tr></tbody></table>

## Surface Morphology.

The surface morphology and topography of **Zn-1-PPy⁺,BF₄⁻**/ITO were examined by SEM (Figures 10A and S12) and AFM imaging (Figure S22-S23). SEM images revealed a homogeneous surface, with a clear boundary between the low conducting porphyrin film (dark) and the uncovered highly conducting electrode (bright). AFM imaging was performed on bare ITO, ITO1
(Nc(**4-PBD**) = 2; Nc(**Zn-1**) = 20), and ITO4 (Nc(**4-PBD**) = 18; Nc(**Zn-1**) = 20) to investigate surface topography (Figure S22). Profilometry along 1.5-1.6 µm of the X-axis showed that increasing the amount of electrodeposited porphyrin led to more surface peaks and a reduced difference between maximum and minimum Z values. Bare ITO exhibited significant roughness with protruding grains (35-41 nm diameter), whereas ITO1 and ITO4 displayed smaller grains (11-25 nm), consistent with porphyrin deposition filling gaps between ITO grains.

<figure><img src="image_15.png" /><figcaption>**Figure 10.** A: SEM picture showing the frontier between the bare ITO electrode and the electrodeposited porphyrin film **Zn-1-PPy⁺,BF₄⁻/ITO** (this electrode was prepared with Nc(**4-PBD**) = 18 and Nc(**Zn-1**) = 20. B and C: AFM images of **Zn-1-PPy⁺,BF₄⁻/HOPG1** (Nc(**4-PBD**) = 2 and Nc(**Zn-1**) = 20, B) and **Zn-1-PPy⁺,BF₄⁻/HOPG4** (Nc(**4-PBD**) = 18 and Nc(**Zn-1**) = 20, C). D and E: profilometry of **Zn-1-PPy⁺,BF₄⁻/HOPG1** (D) and **Zn-1-PPy⁺,BF₄⁻/HOPG4** (E).</figcaption></figure>

To estimate film thickness, AFM analyses were performed on HOPG substrates under the same grafting conditions, yielding **Zn-1-PPy⁺,BF₄⁻/HOPG1** (Nc(**4-PBD**) = 2; Nc(**Zn-1**) = 20) and **Zn-1-PPy⁺,BF₄⁻/HOPG4** (Nc(**4-PBD**) = 18; Nc(**Zn-1**) = 20) (Figures 10B and 10C). The organic film thickness was 2-3 nm for **Zn-1-PPy⁺,BF₄⁻/HOPG1** and 4-5 nm for **Zn-1-PPy⁺,BF₄⁻/HOPG4**. For **Zn-1-PPy⁺,BF₄⁻/HOPG4**, this exceeds the expected monolayer thickness of **Zn-1-PPy⁺,BF₄⁻** (≈ 2.0 nm from X-ray data). Since Nc(**Zn-1**) was constant (20 cycles), the thickness difference arises
from the first electrografting step: increasing **4-PBD** cycles from 2 to 18 produces a thicker pyridine-based layer, leading to a thicker porphyrin-based material (Scheme 5).

*Chemical Composition Analysis*

XPS analysis of ITO-modified electrodes confirmed the presence of C1s, N1s, F1s and Zn2p<sub>3/2</sub> (Figures S16-S21). Detection of Zn validated successful porphyrin immobilization. The reference compound **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>** exhibited two N1s contributions at 398.4 eV (porphyrin core) and 401.6 eV (pyridinium nitrogen), with an experimental ratio of 1:4, matching theoretical expectations (Figure S17). For **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>/ITO** (Nc(**4-PBD**) = 2 and 18; Nc(**Zn-1**) = 20), three N1s components were observed at 398.5, 399.8, and 401.8 eV, assigned to porphyrin nitrogen atoms (merged with residual pyridine), azo bridge nitrogen, and pyridinium nitrogen, respectively. These values closely match those of the reference compound, supporting the proposed structure. The Zn2p<sub>3/2</sub> peak appeared at 1021.5 eV for **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>** and 1021.8 eV for **Zn-1-PPy<sup>+</sup>,BF<sub>4</sub><sup>-</sup>/ITO** (Figure S18).
<figure><img src="image_16.png" /><figcaption>**Scheme 5.** Proposed molecular structure of the Zn-1-PPy+, BF₄⁻/ITO or GC porphyrin-based materials for Nc(4-PBD) = 2 and 18, with Nc(Zn-1) = 20 for both materials.</figcaption></figure>

Because XPS is less sensitive to boron, TOF-SIMS analyses were performed on bare ITO and porphyrin-based samples (Figures S24 and S25). In negative mode, the fluorine peak (¹⁹F) was detected in all samples, while the BF₄⁻ parent ion (m/z = 87 for ¹¹BF₄, with isotopic pattern including ¹⁰B at m/z = 86) appeared only in porphyrin-containing samples. Positive mode confirmed the presence of ¹⁰B and ¹¹B isotopes in porphyrin materials and Zn-1-PPy+, BF₄⁻, but
not in bare ITO. These results confirm $BF_4^-$ incorporation, consistent with the cationic nature of
the pyridinium-porphyrin material.

Conclusions

In summary, pyridine-functionalized electrodes were successfully prepared via electroreduction of 4-pyridin-4-yl-benzenediazonium These original surfaces were thoroughly characterized by CV, SEM, and XPS. Subsequently, 2,7,12,17-tetra-tert-butylporphyrin Zn(II) was immobilized through direct oxidative coupling on the pyridine-modified electrodes, yielding a novel porphyrin-pyridinium-based interface. The porphyrin loading was tuned by both oxidative and reductive grafting cycles, and the resulting material was analyzed using CV, UV-Vis. spectroscopy, SEM, AFM, XPS, and TOF-SIMS. Comparison with the electrosynthesized reference compound **Zn-1-PPy+,BF4⁻** confirmed strong structural similarity, validating the proposed structure. This proof-of-concept study demonstrates a powerful strategy for anchoring aromatic molecules - here, a porphyrin - via direct oxidative C-H activation on pyridine-functionalized electrodes. The key advantages of this approach are: 1) *No pre-functionalization required* for the aromatic molecule; 2) *High versatility*, as pyridine's high oxidation potential (>2.0 V) enables extension to numerous oxidizable aromatic systems (porphyrinoids, PAHs, heterocycles, biomolecules...); 3) *Broad applicability*, allowing integration with other pre-grafted nucleophiles (imines, phosphines,...). This innovative method opens new avenues for designing functional molecular interfaces for electrochemical, biological and optoelectronic applications.

ASSOCIATED CONTENT
**Supporting Information.** Full NMR characterization ($^{1}$H, $^{13}$C and 2D COSY, NOESY, HSQC and HMBC) of Zn-1-PPy$^{+}$,BF$_{4}$$^{-}$, additional SEM and AFM images, XPS and TOF-SIMS spectra and crystallographic data are provided.

## AUTHOR INFORMATION

### Corresponding Author

* Charles H. Devillers. email: charles.devillers@u-bourgogne.fr.

### Present Addresses

† Univ. Lyon, CNRS, UMR 5307 LGF, Centre SPIN, 158 cours Fauriel-42023 Saint-Etienne, France

### Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## ACKNOWLEDGMENT

This work has been supported by the Université Bourgogne Europe, CNRS (Centre National de la Recherche Scientifique), EIPHI Graduate school (contract "ANR-17-EURE-0002") and by the Bourgogne Franche-Comté Region. F.A. and C.H.D. acknowledge the Conseil Régional de Bourgogne Franche-Comté for a PhD grant (PORFELEC project) and the French "Investissement d'Avenir" program, project ISITE-BFC (contract ANR-15-IDEX-03) for funding. The authors are thankful to Marie-José Penouilh, Quentin Bonnin, Myriam Heydel and Michel Picquet from the "Pôle Chimie Moléculaire" (http://www.wpcm.fr).

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Table of Contents Graphic

Anodic nucleophilic substitution of a meso-free porphyrin was achieved on a pyridine-modified
electrode yielding a C-N coupled pyridinium-porphyrin material. This latter was comprehensively
characterized by CV, UV-vis., XPS, SEM, TOF-SIMS and AFM. This innovative post-
functionalization strategy paves the way for the design of functional molecular interfaces tailored
for electrochemical, biological and optoelectronic applications.

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