Electrochemical behavior of the tetracationic porphyrins (py)ZnOEP(py)44+4PF6- and ZnOEP(py)44+4Cl-

Journal of Electroanalytical Chemistry 635 (2009) 20–28

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 Electrochemical behavior of the tetracationic porphyrins ðpyÞZnOEPðpyÞ4þ 4 4PF6  and ZnOEPðpyÞ4þ 4 4Cl

Delphine Schaming a, Alain Giraudeau b,*, Sylvie Lobstein b, Rana Farha c,d, Michel Goldmann c,e, Jean-Paul Gisselbrecht b, Laurent Ruhlmann a,* a

Laboratoire de Chimie Physique, UMR 8000 CNRS/Université Paris-Sud 11, Faculté des Sciences d’Orsay, Bâtiment 349, 91405 Orsay Cedex, France Laboratoire d’Electrochimie et de Chimie-Physique du Corps Solide, UMR 7177 CNRS/Université de Strasbourg, 4 rue Blaise Pascal, 67070 Strasbourg Cedex, France Institut des NanoSciences de Paris, UMR 7588 CNRS/Université Paris 6, 140 rue de Lourmel, 75015 Paris, France d Laboratoire d’Analyse de Contrôle des Systèmes Complexes, Ecole Centrale d’Electronique, 37 quai de Grenelle, 75015 Paris, France e Université Paris Descartes, 45 rue des Saint Pères, 75006 Paris, France b c

a r t i c l e

i n f o

Article history: Received 29 April 2009 Received in revised form 28 July 2009 Accepted 30 July 2009 Available online 5 August 2009 Keywords: Tetracationic porphyrins Pyridinium Electrochemistry Spectroelectrochemistry Saddle porphyrins

a b s t r a c t Electro-oxidation of b-octaethylporphyrinato zinc(II) (ZnOEP) in the presence of an excess of pyridine yields the water soluble salt [N-pyridyl-Zn-5,10,15,20-tetrakis (N-pyridinium)-2,3,7,8,12,13,17,18-octa4þ  ethyl-porphyrin]4+ 4PF 6 ððpyÞZnOEPðpyÞ4 4PF6 Þ. Its water solubility can be strikingly increased by replac   porphyrin without axial pyridine ing the counter anions PF6 by Cl yielding the ZnOEPðpyÞ4þ 4 4Cl  4þ  coordinated on the zinc. The electrochemical properties of ZnOEPðpyÞ4þ 4 4Cl and ðpyÞZnOEPðpyÞ4 4PF6 were investigated in CH3CN by cyclic voltammetry and spectroelectrochemistry. Reduction takes place first at the pyridinium sites in four well separated one-electron steps due to mutual interactions between the four substituents, followed by the reduction of the porphyrin ring occurring at a potential close to the unsubstituted parent porphyrin ZnOEP. The third and the fourth reduction steps led to the formation of coated electrodes. The morphology of the deposits was scrutinized by AFM and appeared in the form of tightly packed coils indicative of polymers formation. UV–visible absorption spectra of the films showed a significant broadening associated with a splitting of the Soret band characteristic of important excitonic interactions between the porphyrin subunits in the polymers. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Numerous ‘‘water-soluble” porphyrins have been studied with respect to their physicochemical and redox properties in both aqueous and nonaqueous media [1]. These compounds have also received considerable attention due to their possible applications in medicine. For example, 5,10,15,20-tetrakis-(1-methyl-4-pyridyl) porphyrins (TMpyP) with specific metal ions have been used in nuclear medicine [2], tested as tumor and liver contrast agents in mice using magnetic resonance imaging techniques [3], examined as mimics for superoxide dismutase [4,5], and demonstrated to act against the human immunodeficiency virus [6] as well as mad-cow disease [7]. More recently, the TMpyPs have been examined for the purpose of developing DNA-specific photosensitizers for photodynamic virus inactivation [8]. The porphyrin functionalisation was necessary to get the required properties and was often obtained by the addition of

* Corresponding authors. Tel.: +33 3 90 24 14 16; fax: +33 3 90 24 14 31 (A.G.), tel.: +33 1 69 15 44 38; fax: +33 1 69 15 61 88 (L.R). E-mail addresses: [email protected] (A. Giraudeau), laurent.ruhlmann@ lcp.u-psud.fr (L. Ruhlmann). 0022-0728/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2009.07.023

appropriate substituents on the porphyrin macrocycle. In turn, these multiple peripheral substitutions on porphyrins have been shown to induce significant conformational distortions of the porphyrin skeleton that minimize steric interactions between the substituents [9,10]. Therefore, the macrocyclic deformations could drastically affect the optical, redox, magnetic properties as well as radical and excited-state properties of the nonplanar porphyrins [9,10]. The reactions of electrogenerated porphyrin p-cation radicals and dications with nucleophiles to yield meso or b-substitutions are now well established and have been used to attach substituents via nitrogen, phosphorous and sulfur atoms at the meso or b positions of porphyrins [11–22]. Previous works [19,23] have demonstrated that the exhaustive oxidative electrolysis of ZnOEP in the presence of pyridine or bipyridine afford tetracationic porphyrins in which the four meso protons of ZnOEP are replaced by four pyridinium or bipyridinium groups linked via their nitrogens atoms. Even more recently, the corresponding free-base porphyrin H2 OEPðpyÞ4þ 4 , and its Fe and Mn complexes, were synthesized [24,25]. The latter are new biomimetic hydroxylation catalysts that exhibit good solubility in both polar aprotic solvents and water.

D. Schaming et al. / Journal of Electroanalytical Chemistry 635 (2009) 20–28

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ture dependent, we carried out the electrochemical and spectro electrochemical investigations of ðpyÞZnOEPðpyÞ4þ 4 4PF6 (1) and  4Cl (2) in CH the non-coordinated parent ZnOEPðpyÞ4þ 3CN. These 4 behaviors were compared with this of the monosubstituted moiety ZnOEPðpyÞþ PF 6 (3) [17] in the same experimental conditions. 2. Results and discussion 2.1. 1H NMR spectroscopy 

Fig. 1. Porphyrins investigated in this study.

This kind of dodeca-substituted porphyrins was the first representative of a new class of dodeca-substituted nonplanar metalloporphyrins bearing four positive charges less than 5 Å from the metal center. The porphyrin skeleton adopts a severely nonplanar saddle conformation in the solid state that minimizes steric hindrance between the 12 peripheral substituents as previously demonstrated by the crystallographic data of the [N-pyridyl-Zn-5,10, 15,20-tetrakis (N-pyridinium)-2,3,7,8,12,13,17,18-octaethyl-porph4þ  yrin]4+ 4PF 6 , (abbreviated ðpyÞZnOEPðpyÞ4 4PF6 , 1, Fig. 1) where an axial pyridine coordinated on the metal is observed [23].  Although this porphyrin ðpyÞZnOEPðpyÞ4þ 4 4PF6 is soluble both in polar protic and aprotic solvents as H2O and CH3CN, its weak solubility in water limits its interest in aqueous media. This solubility can be strikingly increased by replacing counter anions PF 6 by halides  (Cl for example) obtained just by passing ðpyÞZnOEPðpyÞ4þ 4 4PF6  (1) on a Cl exchange ion column. During chromatography, the axial  pyridine on Zn is lost leading to ZnOEPðpyÞ4þ 4 4Cl (2, Fig. 1). The study of the redox pattern of water soluble porphyrins in nonaqueous media allows to benefit from comparable conditions to correlate their redox properties to these of the main parent porphyrins which are known only in aprotic media due to their insolubility in water. A direct comparison of electrochemical properties obtained in protic and in aprotic media can be risky. As only the crystallographic structure of 1 has been resolved, and due to the fact that the redox behavior of porphyrins is struc-

The 1H NMR spectrum in CD3CN solution for ZnOEPðpyÞ4þ 4 4Cl (2, Fig. 2) showed that the four meso protons of ZnOEP were missing and replaced by pyridinium substituents whose protons appeared downfield as one doublet (8H), one triplet (4H) and one double doublet (8H) which clearly identified the ortho, para and meta protons, respectively. The b-ethyl groups appeared as a triplet (terminals methyls) and as a broad multiplet (methylene) in the 0.60–2.05 ppm range (not shown). The broad methylene peaks were indicative of a dynamic macrocycle inversion process, i.e. fast interconversion (on the NMR timescale) between nonplanar saddle conformations, a process that has been observed in several saddleshaped porphyrins as a function of the temperature [26,27]. The  1 ðpyÞZnOEPðpyÞ4þ 4 4PF6 porphyrin (1) presented a H NMR similar spectrum with three additional signals which appeared downfield as one triplet (1H), one double doublet (2H) and one doublet (2H) in the 4.46–6.93 ppm range. These additional signals corresponded to the para, meta and ortho protons of a single axially to Zn ligated pyridine, respectively. By comparison with the 1H NMR spectrum of the pyridine, the upfield shift of the ortho protons of the pyridiniums was consistent with an axially ligated pyridine to the Zn. 2.2. UV–visible absorption spectra The absorption UV–visible spectra of the two tetracationic porphyrins consisted in two absorption bands corresponding to two p–p* transitions. According to the admitted notation, these bands are called Q and B corresponding to S1 and S2 electronic states [28,29].



4þ  Fig. 2. 1H NMR of py, ðpyÞZnOEPðpyÞ4þ 4 4PF6 (1) and ZnOEPðpyÞ4 4Cl (2) (300 MHz, CD3CN) in the 10.5–4 ppm range.

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D. Schaming et al. / Journal of Electroanalytical Chemistry 635 (2009) 20–28

Table 1 UV–visible spectral data for 1, 2 in CH3CN and 3, ZnOEP in CH2Cl2. k, nm (e  103 L mol1 cm1). Compounds

Soret bands

Q bands

ZnOEP

400 (284.6) 472 (85.9)

531 (14.7) 567 (20.9) 599 (11.3)

476 (68.9)

600 (9.7)

411 (211.2)

542 (15.4) 578 (18.3)

 1, ðpyÞZnOEPðpyÞ4þ 4 4PF6

2, ZnOEPðpyÞ4þ 4 4Cl



3, ZnOEPðpyÞþ PF 6

By comparison with the parent porphyrin ZnOEP, the B Soret and visible Q bands were red shifted and decreased markedly in intensity (Table 1), in addition the Soret Band was broadened. Both effects were attributable to the four electron-withdrawing pyridinium groups as well as to the nonplanar saddle conformation of the complex, the magnitude of the red shift increased with the number of electron-withdrawing pyridinium groups, albeit in a nonlinear fashion, as well as with the magnitude of the distortion. Such optical red shifts induced by the nonplanarity of porphyrins are now well documented [9,10,26,27]. They were rationalized theoretically by a stronger destabilization of the highest occupied molecular orbitals (HOMOs, a1u(p) and a2u(p) orbitals) relative to the lowest unoccupied molecular orbitals (LUMOs, eg(p) orbitals) resulting in smaller HOMO to LUMO gaps and, therefore, an optical red shift [9,10,30–34]. The p–p aggregation and the complex formation were apparently disfavored for highly nonplanar porphyrins, or brought about unusual aggregation behavior. The formation of p–p dimers usually results in a flattening of the porphyrin macrocycle [35]. The p–p stacking did not occur for the main dodeca-substituted porphyrins, most likely because of the interference caused by the nonplanar structure and the bulky substituents surrounding the macrocycle.

cause surface or adsorption phenomena occurred on the Pt electrode and strongly distorted the shape of the recorded signals during the cathodic polarizations [23]. Unfortunately, the dme was not appropriate for oxidative experiments. In order to avoid any noticeable effect of inhibiting phenomena at the working electrode, the porphyrins were examined at a glassy carbon electrode, which has been shown to be well suited for studies of electrochemical reduction of pyridinium moieties in both aqueous and nonaqueous media [53].  The cyclic voltammetry curves of ZnOEPðpyÞ4þ 4 4Cl (2, Fig. 3A) recorded in the potential range 0 V to 1.8 V exhibited five distinct reduction steps (Table 2). The shape of the corresponding reduction peaks Epc showed that inhibiting phenomena were still present at the electrode surface. The ratios of the integrated peaks were 0.7:1.0:0.2:–:1.0. Nevertheless the deformation was not so pronounced than in the case of the Pt electrode where discrimination of the signals was not possible. As shown on Fig. 3B, the first electron transfer appeared as a partial reversible electron transfer occurring on the pyridinium substituent as demonstrated by spectroelectrochemical investigations, whereas the second one presented a poor reversibility. The two following reduction steps (III and IV) were irreversible electronic transfers. That could be attributed to a low electron density at the 4-position in particular for the first reduced pyridinium which was then not sufficient to induce further fast chemical reactions as dimerization processes. This observation is consistent as the consequence of the mutual interactions between the pyridini-

2.3. Cyclic voltammetry The electrochemistry of free bases and metalloporphyrins with nonelectroactive metals indicated that the porphyrin p-system undergoes two reversible one-electron oxidations and reductions to yield, respectively, p-cation radicals and dications, p-anion radicals and dianions [32]. Exceptions to this general behavior could be observed if catalytic processes occurred [36] or when the porphyrin was bearing substituents that may undergo electron transfers at similar potentials to those of the porphyrin ring [37–39]. Porphyrins substituted at the b or meso position with a single positively charged substituent such as a pyridinium, bipyridinium or phosphonium group did indeed undergo reduction of the substituent at potentials close to those of the porphyrin p system [16– 19,40–44]. In this case, the redox pattern of the substituted groups could be strongly modified by the influence of the porphyrin moiety [37–39]. That was related in studies devoted to the electrochemical reduction of various substituted pyridinium groups [45,46]. The latter generally proceeds through two successive one-electron transfers and numerous works were mainly oriented toward the conditions of reversibility of the first electron uptake [47–49]. The investigations on this subject have demonstrated the impact of the substituents attached to the pyridine ring. In particular, the instability of the free pyridyl radical could be correlated with the 4-position in the pyridine ring, being the most reactive site, its availability would enable dimerization to occur readily [47–52]. Various experiments devoted to the reduction of pyridinium groups or tetracationic porphyrins bearing pyridinium substituents were carried out on the mercury electrode. We used also a dropping mercury electrode (dme) in preliminary experiments be-



Fig. 3. CVs for ZnOEPðpyÞ4þ (2, c = 4.5  104 mol L1) in CH3CN and 4 4Cl 0.1 mol L1 TBAPF6 at 100 mV s1. (A) First anodic and cathodic scans. Dashed line: TBACl alone in CH3CN (c = 20  104 mol L1). (B) Scans with different negative potential limits.

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D. Schaming et al. / Journal of Electroanalytical Chemistry 635 (2009) 20–28 Table 2 Electrochemical data for 1, 2, 3. Compounds

Oxidation

Reduction

Ring Panel (a) ZnOEP Panel (b) 3, ZnOEPðpyÞþ PF 6 2, ZnOEPðpyÞ4þ 4 4Cl

0.94 1.28 

 1, ðpyÞZnOEPðpyÞ4þ 4 4PF6

Cl



Py+

Ring

0.68

1.60 irr

0.98

*1.06

*1.40irr

*1.40irr

*1.38irr

1.47

*0.54

*0.74

*0.99irr

*1.21irr

1.56

(6.3) *0.51

(6.3) *0.71

(2.5) *0.94irr

(1.8) *1.16irr

(6.3) 1.51

(6.4)

(6.4)

(4.5)

(5.0)

(6.4)

*1.21irr

TBACl

All potentials in V vs. SCE were obtained from cyclic voltammetry (scan rate = 100 mV s1). Panel (a) in CH3CN/1,2-C2H4Cl2 containing 0.1 M TBAPF6. WE: Pt electrode. Panel (b) in CH3CN containing 0.1 M TBAPF6. WE: glassy carbon electrode. * Peak potential values. Under brackets, current peak values Ipc (lA).

ums that allow delocalization of the electron density onto the four substituents [19,54]. A well defined reversible electron transfer (step V) was observed at 1.56 V. The ratio of the peak current values Ipc/Ipa  1 and the potential separation of the cathodic and anodic peak DEp = Epc  Epa = 60 mV indicated that the reaction was a reversible monoelectronic process. Fig. 4 illustrated the current function Ipc = f (v1/2) of (2) at the first and second reduction step. The peak currents Ipc for reduction steps I, II and V of (2) varied linearly with the square root of scan rate (see Supplementary materials) thus indicating a diffusion-controlled electron transfer process. When the four pyridinium substituents would be reduced first (steps I–IV), the porphyrin core may be expected to be reduced at a potential value close to this of the unsubstituted porphyrin (1.60 V) [17]. That would be consistent with the reversible reduction of the porphyrin ring at 1.56 V (Table 2). Moreover, the difference (0.35 V) in potentials between the addition of the electron in steps IV and V compared to the others (ca. 0.20 V), in steps I ? II, II ? III, III ? IV, suggested also a rather different reduction site. These results obtained at the glassy carbon electrode agree with previous related observations at the dropping mercury electrode [23]. In particular, this sequential reduction demonstrated the existence of strong electronic interactions between all the substituents, the reduction of the first pyridinium making the second one harder to reduce and so on. In absence of any interactions, a unique four-electron reduction signal would be expected. The effects of the electronic coupling between the



porphyrin p-system and the pyridinium moieties were also illustrated by the relative anodic potential shift (DE1/2  0.70 V) for the oxidation step of the substituted porphyrin compared to the unsubstituted parent ZnOEP (E1/2 = 0.68 V) [17]. On reversal of the potential sweep at 1.00 V (step III, Fig. 3B), an anodic sharp peak (a) appeared at +0.25 V whose height was increasing when the potential range explored shifted towards more negative potentials. The shape of the oxidation peak was similar to those observed during anodic redissolution processes, indicating the formation of insoluble species during reduction. A steady state polarization at 1.00 V before reversal of the sweep would induce also a similar behavior. This peak (a) was not shown upon direct anodic scan (Fig. 3A) where the oxidation peak of the chloride anions overlapped the signal corresponding to the oxidation of the porphyrin ring. This signal at +0.25 V could be attributed to the oxidation of dimeric or polymeric species resulting from a coupling at the 4-position of the pyridyl radicals generated from the reduction (Scheme 1) [47,50–52]. Indeed the usual electrochemical pattern for the cationic 1-substituted pyridinium compounds in nonaqueous media involved an initial one electron uptake which produced the first cathodic wave and was generally followed by rapid dimerization of the free-radical products when the 4-position was free. The generated dimers were oxidized at considerably more positive potentials to the original electroactive compound [45–47]. The coupling of pyridyl radicals generally yielded to unstable dimers which were rapidly altered even in nonaqueous media

Fig. 4. The two first reduction steps for ZnOEPðpyÞ4þ (2, c = 4.5  104 mol L1) in CH3CN and 0.1 mol L1 TBAPF6 recorded at different scan rates (between 20 and 4 4Cl 1000 mV s1). Inset: Ipc = f (v1/2).

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D. Schaming et al. / Journal of Electroanalytical Chemistry 635 (2009) 20–28

Scheme 1. Dimeric porphyrin formation scheme resulting from a coupling at the 4position of the neutral electrogenerated radical.

[45–50]. We can suggest that the relative stability of the dimers and/or polymers obtained here would be probably a consequence of their insolubility yielding coated electrodes. Many examples of porphyrin electropolymerizations were reported in the literature. The corresponding radical coupling process occurred usually via electro-oxydation [14,15,55–58]. Processes via electro-reduction, as the presented one, were less frequent. This redox behavior was strikingly different from that of the planar tetracationic N-methyl pyridinium porphyrins [(TMpyP)M]4+ [37,39]. Six electrons were added to the porphyrin ring of the [(TMpyP)Zn]4+ porphyrin which was reversibly reduced in three distinct two-electrons steps [39]. In this complex, the pyridiniums were reduced after the porphyrin ligand in only two waves. No chemical reaction occurred after the electronic exchange as the conjugation between the two electroactive sites stabilized the entire molecule. The presence of the porphyrin as substituent at the 4-position of the pyridinium hinders also any possibilities of dimerization. The cyclic voltammetry curves recorded for ðpyÞZnOEPðpyÞ4þ 4 4PF 6 , 1, (Fig. 5) were similar to those observed with 2. All the five reductions steps appeared at potential values close from the corresponding one of 2 (Table 2) whose ratios of the integrated peaks were 0.8:1.0:0.8:0.6:1.2. The main difference in the redox behavior of 1 compared to 2 consisted in the appearance of new additional oxidation peaks (peaks b and c), respectively, at ca. +0.5 V and ca. +0.6 V (Fig. 5A) when the reduced moieties were oxidized in the reversal scans. This result indicated that the neutral radicals generated after the reduction steps III and IV reacted to give more complex products (Fig. 5B) than in the case of 2. Moreover the shapes of these oxidation peaks were similar to those observed during anodic redissolution processes, indicating the formation of insoluble species at the surface of the working electrode. In addition the sharp shape of the cathodic peak Epc corresponding to the reduction of the last pyridinium substituent suggested a drastic decrease of the solubility of the porphyrin after the reduction step IV. This is also consistent with the shape of the reduction signal of the porphyrin ring, at 1.51 V, and the low separation of the corresponding cathodic and anodic peak potentials DEp = Epc  Epa = 40 mV. Upon direct anodic scan, the first oxidation step of the porphyrin 1 indicated the uptake of one electron (Fig. 5A) in an irreversible signal at 1.38 V. These similar redox behaviors of the tetracationic porphyrins 1 and 2 were consistent with similar structures for the two species [23]. In order to validate the behavior of the tetracationic porphyrins 1 and 2, it was appropriate to compare these results with the redox pattern of the monosubstituted parent ZnOEPðpyÞþ PF 6 (3). Fig. 6 illustrated the CV curves recorded in the potential range 2.0 V to +1.4 V. An initial voltage sweep toward positive potentials showed the two anodic reversible oxidation steps of the porphyrin ring at 0.98 V and 1.28 V. In reduction, we observed the reversible one-electron transfer onto the porphyrin ring at 1.47 V following the irreversible uptake of one electron by the pyridinium substituent at 1.06 V. Only after a steady state polarization past the

 4 Fig. 5. CVs for ðpyÞZnOEPðpyÞ4þ mol L1) in CH3CN and 4 4PF6 (1, c = 4.5  10 0.1 mol L1 TBAPF6 at 100 mV s1. (A) First anodic and cathodic scans. (B) Scans with different negative potential limits.

4 Fig. 6. CVs for ZnOEPðpyÞþ PF mol L1) in CH3CN and 0.1 mol L1 6 (3, c = 4.5  10 TBAPF6 at 100 mV s1. ? Start of the scan.

reduction of the pyridinium, the return sweep revealed a well defined oxidation signal (a0 ) at +0.73 V characteristic of the oxidation of generated dimers at the electrode. This pattern did not differ much from this observed for 1 and 2 where the presence of four electrogenerated pyridyl radicals induce larger possibilities for fast radical coupling and agree the proposal discussed above. The weaker anodic shift (0.30 V) of the oxidation potential of the porphyrin ring was the consequence expected from the electron-withdrawing effect of a single pyridinium substituent.

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2.4. Investigations of coated ITO electrodes In order to characterize the formation of the oligomeric or polymeric species, resulting from a coupling at the 4-position of the pyridyl radicals generated at steps III and IV, we prepared coated ITO electrodes by applying potentials corresponding to steps III (1.05 V) and IV (1.45 V). After coating, the electrodes were washed with CH3CN to remove traces of the conducting salt present on the film and the morphology of the deposits was scrutinized by AFM. Fig. 7 presented the electrochemical behavior of a coated electrode with 1. An initial voltage sweep toward negative potentials showed only the appearance of three cathodic signals at potentials corresponding to steps III to V. The lack of the two first reduction signals at ca. 0.50 V and 0.70 V strongly suggested the coupling of two pyridinium substituents per molecule and was in agreement with the formation of polymers. When repetitive scans were carried out the main signal at 0.25 V disappeared rapidly and we noticed that the film peeled off from the electrodes. In the scanning atomic force micrographs recorded on coated electrodes at 1.45 V, the deposits appeared in the form of tightly packed coils (Fig. 8). A quite similar morphology of the films was   (2) or ðpyÞZnOEPðpyÞ4þ obtained using ZnOEPðpyÞ4þ 4 4Cl 4 4PF6 (1). The deposits differ only by the size of the coils which were ca. 75 nm and ca. 125 nm, respectively (Fig. 8). UV–visible absorption spectra of both the monomer in solution and the film were compared. The Soret band of the film obtained after 60 s at applied potential 1.45 V, exhibited a significant broadening associated with a shoulder corresponding to some extent, to a splitting of the Soret band (Fig. 8C and F). This result suggested excitonic interactions which could be interpreted by the exciton-coupling theory involving intra- or intermolecular excitonic interaction between the different porphyrins subunits [17,59,60].

Fig. 7. CV (100 mV s1) of the film obtained using ðpyÞZnOEPðpyÞ4þ 4PF 6 (1) after 60 s at applied potential 1.45 V in CH3CN and 0.1 mol L1 TBAPF6.

Similar results were obtained by applying a potential corresponding to step III (1.05 V), a smaller thickness of the film was the only difference. 2.5. Spectroelectrochemistry In order to confirm the localization of the electron transfers, either the porphyrins or the pyridinium substituents, spectroelectrochemical studies were carried out with a Pt mini-grid as working electrode under the same experimental conditions as the electrochemical measurements. The reduced moieties were generated in situ, by application of suitable potentials, based on the redox potentials measured by cyclic voltammetry.

Fig. 8. Left: The atomic force micrograph (AFM) of the film deposited on an ITO electrode at applied potential 1.45 V vs. SCE (step IV) during 60 s in CH3CN and 0.1 mol L1  4þ  TBAPF6. (A and B) from the ZnOEPðpyÞ4þ 4 4Cl (2) solution. (D and E) from the ðpyÞZnOEPðpyÞ4 4PF6 (1) solution. Right: Normalized UV–vis spectra of the films (full line)  4þ  corresponding to the AFM pictures (A and B) and (D and E) compared with solutions (dotted line) of (C) ZnOEPðpyÞ4þ 4 4Cl (2) or (F) ðpyÞZnOEPðpyÞ4 4PF6 (1), respectively, in CH3CN.

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The two further irreversible reduction steps (III and IV) were different. Upon polarization, a decrease of the intensity of both the Soret and Q bands was observed (Fig. 9C). A complete flattening of the UV–vis spectrum occurred by polarization at the reduction potential of the step IV. These observations could be reasonably correlated to the solubility of the porphyrin in CH3CN where precipitation occurred when the porphyrin became neutral. The initial spectra were not recovered after reverse electrolysis at 0 V.  The UV–vis spectra of ZnOEPðpyÞ4þ 4 4Cl (2) exhibited also a typical porphyrin pattern along the first four reduction steps, indicating that the electrons were transferred to the pyridinium substituents and not to the porphyrin ring. Examination of the UV–vis spectrum recorded after the first reduction step (Fig. 10A) involving a pyridinium substituent

Fig. 9. Time-resolved electronic absorption spectra during thin layer controlled 1 TBAPF6. potential electrolysis of ðpyÞZnOEPðpyÞ4þ 4 4PF6 (1) in CH3CN and 0.1 mol L (A) First reduction (I) step, (B) second reduction (II) step and (C) third reduction (III) step.

 The UV–vis spectra of ðpyÞZnOEPðpyÞ4þ 4 4PF6 (1), after the first and the second electron transfers, exhibited a typical porphyrin pattern as depicted in Fig. 9A and B, confirming our initial assignment based on the voltammetric results. Blue shifts of ca. 33 nm and 15 nm for the Soret band and 19 nm and 23 nm for Q bands were observed, respectively, for the first (I) and second reduction (II) steps. Such blue shifts of the Soret and Q bands well agree with a decrease of the whole withdrawing character of the peripheral substituents on the porphyrin ring induced by the decrease of their net positive charge. The initial spectra were totally recovered after reverse electrolysis at 0 V in agreement with the cyclic voltammetry experiments and the two quasi successive reversible one electron steps observed (Fig. 5B).

Fig. 10. Time-resolved electronic absorption spectra during thin layer controlled 1 TBAPF6. (A) potential electrolysis of ZnOEPðpyÞ4þ 4 4Cl (2) in CH3CN and 0.1 mol L First reduction (I) step, (B) second reduction (II) step and (C) third reduction (III) step.

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showed only a slight red shift. A similar behavior occurred also with ZnOEP(py)+ (3) and was already noticed for the pyridinium reduction with ZnTPP(py)+ and H2TPP(py)+ [45]. That was then not discussed because it was not possible to support any clear answer to this behavior. However, this result can be tentatively explained, after the first reduction, by a fast protonation of the pyridyl radical which would maintain the overall positive charge of the molecule so that the position of the Soret band should not be affected. Moreover protonation is a common reaction with basic radicals even in nonaqueous media, and has been described in the case of pyridinium reductions [48–52]. The further electron transfer (second reduction step) (Fig. 10B) led to a consequential hypsochromic shift of the recorded UV–vis spectrum. The Soret band of 2 (471 nm) is blue shifted of 48 nm and agree with a decrease of the overall withdrawing effect of the substitution. Evolutions for reduction steps III (Fig. 10C) and IV were similar to those discussed above for 1. 3. Conclusions The quasi identical redox pattern of the tetracationic porphyrin  4þ 4þ ZnOEPðpyÞ4 4Cl (2) with this of ðpyÞZnOEPðpyÞ4 4PF 6 (1) strongly suggest that its structure would be similar, with a porphyrin skeleton in a nonplanar saddle conformation. Besides the anodic potential shifts of the porphyrin oxidation step due to the withdrawing character of the pyridinium substituents, one of the most important results of these studies is the particularly strong mutual interaction between the four pyridinium groups through the porphyrin p-ring current. This exceptional communication could be explained in part by the close proximity of the positive charge of the substituents to the porphyrin skeleton, allowing probably an extensive mixing of both pyridinium molecular orbitals with those of the porphyrin ring system. One interesting consequence lies in the offered possibilities of a transfer of the two first electrons onto these tetracationic porphyrins obtaining stable pyridyl radicals although the 4-positions of the pyridiniums were free. The other main result is the evidenced coupling reaction of the pyridyl radicals which was clearly demonstrated through the generation of characterized insoluble polymeric forms. This kind of soluble porphyrin with appropriate metallation has already focused interest as biomimetic hydroxylation catalyst. Its use as light harvesting component may be of peculiar interest in the actual main challenge of ‘‘green chemistry-type” condition with water. 4. Experimental 4.1. Materials All solvents and chemicals were of reagent grade quality, purchased commercially and used without further purification. þ ZnOEPðpyÞ PF 6 (3) was synthesised by a published method [17]. UV–visible spectra were recorded on a Perkin Elmer Lambda 19 spectrophotometer. 1 H NMR spectra were obtained in CD3CN on a Brucker VC-300 spectrometer (300 MHz). Elementary analyses were performed by the microanalysis services of the Chemical Department of IUT Sud (Strasbourg). 4.2. Electrochemistry All electrochemical measurements were carried out at room temperature on a glassy carbon disk electrode (d = 3 mm). The working electrode is a glassy carbon (GC, Tokai, Japan). The solutions were deaerated thoroughly for at least 30 min by bubbling ar-

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gon (Ar-U from Air Liquide) and kept under argon atmosphere during the whole experiment. Voltammetric data were obtained with a standard three-electrode system using a EG&G 2273A potentiostat driven by a personal computer. The auxiliary electrode was a platinum wire and the reference electrode was a saturated calomel electrode (SCE) electrically connected to the studied solution by a junction bridge filled with the corresponding solventsupporting electrolyte solution. 4.3. Spectroelectrochemistry Spectroelectrochemical measurements were carried out using a potentio/galvanostat (EI30 M Brucker) and a diode array spectrophotometer Hewlett–Packard 8453A. A homemade borosilicate glass cell with an optical pathway of 0.1 mm was used. The working electrode was a platinum grid (1000 mesh) placed in the optical pathway, the auxiliary electrode was a platinum wire, and the reference electrode was an aqueous Ag |AgCl| KCl (sat.) electrode. 4.4. Electrosynthesis 4þ (1): 40 mg of ZnOEP Synthesis of ðpyÞZnOEPðpyÞ4 4PF 6 (0.067 mmol, 1 equiv.) and 0.55 mL of pyridine (6.70 mmol, 100 equiv.) were dissolved in 50 mL of 1,2-C2H4Cl2/CH3CN (10:1) containing 0.1 M Et4NPF6. The solution was stirred and degassed with argon before and during electrolysis. The electrolysis was carried out at +1.30 V vs. SCE for one day. After 24 h, the initially red solution had turned green with the formation of a precipitate at the working electrode. This precipitate was collected by filtration. The solvent of the filtrate was removed by a rotary evaporator and the residue was dissolved in a minimum of CH2Cl2. The mixture was poured into water and the organic layer was washed twice. After washing, the organic layer was light pink while the aqueous phase was green and contained the target porphyrin that is obviously soluble in water. The aqueous phase was then washed three times with CH2Cl2 and the water was evaporated. The sample was dried under vacuum. The total amount collected of the new porphyrin was 80 mg (0.051 mmol) for a yield of 76%. ZnN9C61H65(PF6)4 (M = 1569.48 g mol1): UV–vis (CH3CN), kmax (nm) (e/M1 cm1) = 472 (85,900), 599 (11,300). 1H NMR (300 MHz, CD3CN, 25 °C) d = 10.00 (d, J = 6.0 Hz, 8 H, o-H of py+); 9.28 (t, J = 7.9 Hz, 4 H, p-H of py+); 8.69 (dd, J = 6.0 Hz, J = 7.9 Hz, 8 H, m-H of py+); 6.93 (t, J = 7.6 Hz, 1 H, p-H of py); 6.20 (dd, J = 5.6 Hz, J = 7.6 Hz, 2 H, m-H of py); 4.46 (d, J = 5.6 Hz, 2 H, o-H of py); 2.00 (m, 16 H, CH2 of CH2CH3); 0.56 (t, J = 7.4 Hz, 24 H, CH3 of CH2CH3). FAB-MS (NBA) m/z 1568.5 [C61H65N9Zn(PF6)4–H+], 100%. Elemental analysis (C61H65N9Zn(PF6)4) calcd. C 46.68, H 4.17, N 8.03; found C 46.64, H 4.21, N 7.97.  4þ Synthesis of ZnOEPðpyÞ4þ 4 4Cl (2): 100 mg of ðpyÞZnOEPðpyÞ4 4PF (0.063 mmol) were dissolved in 5 mL of CH CN. The solution 3 6 was eluted three times with H2O on a column prepared with DOWEXÒ ion-exchange resin (1H2, 100–200 mesh, Cl form) in a mixture H2O/CH3CN (1:1). The solvent of the recovered solution was evaporated under vacuum. The total collected amount of  ZnOEPðpyÞ4þ 4 4Cl was 57 mg (0.054 mmol) for a yield of 86%. ZnN8C56H60(Cl)4 (M = 1052.34 g mol1): UV–vis (CH3CN), kmax (nm) (e/M1 cm1) = 476 (68 900), 600 (9700). 1H NMR (300 MHz, CD3CN, 25 °C) d = 10.31 (d, J = 6.2 Hz, 8 H, o-H of py+); 9.22 (t, J = 7.8 Hz, 4 H, p-H of py+); 8.65 (dd, J = 6.2 Hz, J = 7.8 Hz, 8 H, m-H of py+); 1.94 (m, 16 H, CH2 of CH2CH3); 0.64 (t, J = 7.5 Hz, 24 H, CH3 of CH2CH3). FAB-MS (NBA) m/z 1051.3 [C56H60N8Zn(Cl)4-H+], 100%. Elemental analysis (C56H60N8ZnCl4) calcd. C 63.92, H 5.75, N 10.65; found C 63.85, H 5.81, N 10.55.

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