Stable Radical Isoporphyrin Copolymer Prepared with Di(phenylphosphane)

Two diphosphanes with variable-length ligands tested as nucleophiles to prepare isoporphyrin copolymers in the presence of ditolylporphyrin of zinc (ZnT2P) prevented the oxidation of the diphosphine ligand. This paper demonstrates the power of this approach and describes the photoelectrocatalytic properties. The obtained copolymers were characterized by UV–vis spectroscopy, X-ray photoelectron spectroscopy, atomic force micrograph (AFM), EQCM (Electrochemical Quartz Cristal Microbalance) and electrochemistry. Their impedance properties (EIS) were studied and their photovoltaic performances were also investigated by photocurrent transient measurements under visible light irradiation.


Introduction
Over the years, several extended porphyrin structures have been prepared and studied with a particular interest in the dependence of the geometric factors on their physical properties.Structural flexibility is determined by the nature of the bridge.To observe efficient interactions between the different chromophores, a large variety of bridging chains have been used as spacers in the building of dimers, but only a few papers were devoted to phosphanylporphyrins. Electrochemistry has been shown to be an appropriate synthetic tool in the coupling of porphyrins.
Phosphorus ligands are useful nucleophiles and could nucleophilic attack the π cation radicals of metalloporphyrins.
In an original study, Smith and colleagues demonstrated that triphenylphosphine (PPh 3 ) functions as a nucleophile in the meso position when reacting with the cation radical of the Zn(II) octaethylporphyrin complex (ZnOEP), initially formed via chemical oxidation [1,2].Additionally, Shine et al. described the reactivity at the meso-position of the zinc(II) tetraphenylporphyrin cation radical ZnTPP +• , which was also chemically generated, in the presence of PPh 3 [3,4].Subsequently, Latos-Gra ży ński et al. reported the chemical β-functionalization of preformed Fe III TPP(ClO 4 ) 2 with PPh 3 , resulting in the formation of the dimer [Fe III TPP-β-+ PPh 3 ] 2 O(ClO 4 ), albeit with modest yields in both mesoand β-phosphonium-based porphyrins [5].Giraudeau et al. made significant strides in improving the yield of phosphonium-based porphyrins by continuously generating the porphyrin π-cation radical in the presence of PPh 3 through controlled potential electrolysis.Under these conditions, Zn II TPP-β-+ PPh 3 ClO 4 − was produced with a 50% yield [6]. Lastly, the electrochemical oxidation of zinc 5,15-bis(p-tolyl)-10-phenylporphyrin in the presence of PPh 3 resulted in the formation of meso-substituted triphenylphosphonium porphyrin, coordinated by one O=PPh 3 molecule, with a good yield of 84% [7].
In a direct one-pot electrochemical reaction, the bis(diphenylphosphino)acetylene ligand acts as a nucleophile allowing the electrosynthesis of diporphyrins with a diphosphonium spacer bridged directly at their meso position upon oxidation of the zinc mesotetraphenyl porphyrin (ZnTPP) at an applied potential of 0.80 V vs. SCE [8].We also described the efficient synthesis of various diphosphonium-bridged homometallated porphyrin dimers and trimers by controlled potential electrolysis in a direct one-pot electrochemical reaction using more or less rigid diphosphanes with variable length [9,10].It clearly illustrates that phosphorus ligands act as powerful nucleophiles which allow electrochemical β-substitutions on meso-tetra-phenylporphyrins.However, such diphosphane compounds are easily oxidized which can limit the use of such ligands for the electrosynthesis, which is conditioned by the applied potential as well as the potential of the first oxidation of the porphyrin.
It must be noted that not only diphosphonium ligands can act as bridges between porphyrins.In the case of an aminophenyl derivative with -NH 2 groups at the para position of the phenyl substituents, for example in 5,10,15,20-tetrakis(4-amino-phenyl)porphyrin or 5,15-bis(4-aminophenyl)-10,20-bis(4-carbomethoxyphenyl)porphyrin, oxydation gives a highly interconnected nanofibrous network and a linear porphyrin polymer [11][12][13].The mechanism is considered analogous to that found in aniline polymerization, except that the attack of the electrophilic nitrogen must occur exclusively to the meta-positions of the aminophenyl substituents of another porphyrin molecule.Furthermore, oxidative electrodeposition of CoTSPP (tetrakis 4-sulfonatophenyl Co porphyrin) on ITO electrode from NaOH 0.1 M aqueous solution has been also studied [14], following the procedure previously described by Bedioui et al. for Ni-porphyrins electrodeposition [15].
Recently, we also discovered the formation of stable isoporphyrins copolymers linked by pyridyl ligands [16,17].Our group has shown that stable isoporphyrins copolymers could be prepared during porphyrin electro-oxidation when sterically less hindered porphyrin is used, such as the zinc-meso-5,15-ditolyl-porphyrin (ZnT 2 P), which presents only two meso positions occupied by one substitutable proton at positions C10 and C20 (Scheme 1).
of the dimer [Fe III TPP-β-+ PPh3]2O(ClO4), albeit with modest yields in both meso-and βphosphonium-based porphyrins [5].Giraudeau et al. made significant strides in improving the yield of phosphonium-based porphyrins by continuously generating the porphyrin π-cation radical in the presence of PPh3 through controlled potential electrolysis.Under these conditions, Zn II TPP-β-+ PPh3ClO4 − was produced with a 50% yield [6].
In a direct one-pot electrochemical reaction, the bis(diphenylphosphino)acetylene ligand acts as a nucleophile allowing the electrosynthesis of diporphyrins with a diphosphonium spacer bridged directly at their meso position upon oxidation of the zinc mesotetraphenyl porphyrin (ZnTPP) at an applied potential of 0.80 V vs. SCE [8].We also described the efficient synthesis of various diphosphonium-bridged homometallated porphyrin dimers and trimers by controlled potential electrolysis in a direct one-pot electrochemical reaction using more or less rigid diphosphanes with variable length [9,10].It clearly illustrates that phosphorus ligands act as powerful nucleophiles which allow electrochemical β-substitutions on meso-tetra-phenylporphyrins.However, such diphosphane compounds are easily oxidized which can limit the use of such ligands for the electrosynthesis, which is conditioned by the applied potential as well as the potential of the first oxidation of the porphyrin.
It must be noted that not only diphosphonium ligands can act as bridges between porphyrins.In the case of an aminophenyl derivative with -NH2 groups at the para position of the phenyl substituents, for example in 5,10,15,20-tetrakis(4-amino-phenyl)porphyrin or 5,15-bis(4-aminophenyl)-10,20-bis(4-carbomethoxyphenyl)porphyrin, oxydation gives a highly interconnected nanofibrous network and a linear porphyrin polymer [11][12][13].The mechanism is considered analogous to that found in aniline polymerization, except that the attack of the electrophilic nitrogen must occur exclusively to the meta-positions of the aminophenyl substituents of another porphyrin molecule.Furthermore, oxidative electrodeposition of CoTSPP (tetrakis 4-sulfonatophenyl Co porphyrin) on ITO electrode from NaOH 0.1 M aqueous solution has been also studied [14], following the procedure previously described by Bedioui et al. for Ni-porphyrins electrodeposition [15].

Electropolymerization of Isoporphyrin Copolymers
As already reported, in the presence of various nucleophiles, the second oxidation step of the tetraphenyl porphyrin of zinc (ZnTPP) became irreversible and a new cathodic peak appeared, as observed in the reduction of electrochemically generated isoporphyrins [18][19][20][21][22].
In the case of the applied potential close to the second oxidation of the ZnTPP porphyrin (1.16 V vs. SCE) in the presence of tertiary diphosphane cis-1,2-bis(diphenylphosphino)ethene (1) or 1,2-bis(diphenylphosphino)benzene (2), oxidation of the di(phenylphosphane) occurs generating first the diphosphine cation radical, which is fully converted to the diphenylphosphino monooxide without formation of the dimer of porphyrin.The only product formed after purification was the diphenylphosphino monooxide.If using ZnTPP, no copolymer is obtained applying the potential at the first oxidation of the ZnTPP (0.80 V vs. SCE) where only the dimer of porphyrin with diphosphonium spacer is obtained as already published [8].It must be noted that at this potential first oxidation of the porphyrin, the oxidation of the diphosphane ligand does not occur.Indeed, irreversible oxidation waves have been measured for cis-1,2-Scheme 2. Representation of electropolymerization reaction from zinc-meso-5,15-ditolyl-porphyrin (ZnT 2 P), cis-1,2-bis-(diphenyl-phosphino)ethene (1), and 1,2-bis-(diphenylphosphino)-benzene (2).

Electropolymerization of Isoporphyrin Copolymers
As already reported, in the presence of various nucleophiles, the second oxidation step of the tetraphenyl porphyrin of zinc (ZnTPP) became irreversible and a new cathodic peak appeared, as observed in the reduction of electrochemically generated isoporphyrins [18][19][20][21][22].
In the case of the applied potential close to the second oxidation of the ZnTPP porphyrin (1.16 V vs. SCE) in the presence of tertiary diphosphane cis-1,2-bis(diphenylphosphino)ethene (1) or 1,2-bis(diphenylphosphino)benzene (2), oxidation of the di(phenylphosphane) occurs generating first the diphosphine cation radical, which is fully converted to the diphenylphosphino monooxide without formation of the dimer of porphyrin.The only product formed after purification was the diphenylphosphino monooxide.If using ZnTPP, no copolymer is obtained applying the potential at the first oxidation of the ZnTPP (0.80 V vs. SCE) where only the dimer of porphyrin with diphosphonium spacer is obtained as already published [8].It must be noted that at this potential first oxidation of the porphyrin, the oxidation of the diphosphane ligand does not occur.Indeed, irreversible oxidation waves have been measured for cis-1,2-bis(diphenylphosphino)ethene (1) and 1,2bis(diphenylphosphino)benzene (2) at +1.37 V and +1.68 V vs. SCE (Table 1 and Figure 1A) corresponding to the oxidation of the -P(Ph) 2 -phosphane group.This result is probably due to the steric hindrance of the ZnTPP porphyrin which leads to the slower kinetic of the nucleophilic attack at the meso positions (Scheme 1) [11,12].bis(diphenylphosphino)ethene (1) and 1,2-bis(diphenylphosphino)benzene (2) at +1.37 V and +1.68 V vs. SCE (Table 1 and Figure 1A) corresponding to the oxidation of the -P(Ph)2phosphane group.This result is probably due to the steric hindrance of the ZnTPP porphyrin which leads to the slower kinetic of the nucleophilic attack at the meso positions (Scheme 1) [11,12].In the case of the zinc-meso-5,15-ditolyl-porphyrin (ZnT 2 P), two sides occupied by hydrogen at the meso position 5 and 15 and at β positions are present (Scheme 1), leading to the decrease in the steric hindrance with faster nucleophilic attack at meso positions 5 and 15.Thus, oxidation at the first oxidation of ZnT 2 P should be more favorable to generate copolymers of isoporphyrin [16].
Upon the increase in the iterative scans n, the oxidation waves continue to increase, showing the growth of copolymer materials (Figure 1B,C).Finally, brown deposited films could be observed on the ITO surface (Figure 2D) showing the electrodeposition of the film.
Electropolymerization occurs in the presence of diphosphane ligand and ZnT2P via the oxidation.Further, the trace of the first scan in Figure 2A shows a significant decrease in the resonance frequency and thus an increase in the deposited mass at the first oxidation of the porphyrin, i.e., electropolymerization occurs upon the formation of the radical cation porphyrin ZnT2P +• in the presence of 1,2-bis(diphenylphosphino)benzene (2).
The resonance frequency displays a significant decrease meaning an increase in the deposited mass during the electropolymerization as shown in Figure 2B.Indeed, accord- The electrochemical synthesis of the polyisoporphyrin copolymers relies on the previously reported E ox1 C N1 E ox2 C B1 (E ox3+n C B1+n ) n process of nucleophilic substitution on porphyrins [16].The mechanism of electropolymerization can be explained as follows: First, the porphyrin radical cation (ZnT 2 P +• , electrochemical step E ox1 ) is electrogenerated at 0.79 V vs. SCE.Then, ZnT 2 P +• can be attacked by a diphenylphosphane groups (chemical step C Nmeso ) at meso-carbon position to yield an isoporphyrin.This later compound can be oxidized (electrochemical step E ox2 ) and the hydrogen atom initially located on the meso-carbon is released inducing the rearomatization of the porphyrin (chemical step C B1 ) which give the mono-substituted porphyrin ZnT 2 P-meso-(Ph) 2 P + -Ph-P(Ph) 2 PF 6 − .At this stage, we have already obtained the monosubstituted porphyrin with one phosphonium covalently connected to the porphyrin and one pendant phosphane, which is still active for nucleophilic attack.Thus, the oxidation of the monosubstituted porphyrin will generate the β radical cation porphyrin (E ox3 step) which can react itself (nucleophilic attack of the pendant phosphane group -P(Ph 2 )), forming isoporphyrin radical copolymer (C N2 step) with diphosphonium spacers (Scheme 2 and Supplementary Materials Schemes S1 and S2).Thus, the first oxidation of the ZnT 2 P generated the β-radical cation porphyrin ZnT 2 P +• , the di(phenylphosphane) ligand could react with ZnT 2 P +• to form stable poly-isoporphyrin radical copolymers.Indeed, isoporphyrin radical films were found to be highly stable in the presence of dioxygen (still stable in the solid state even after four years).
Electropolymerization occurs in the presence of diphosphane ligand and ZnT 2 P via the oxidation.Further, the trace of the first scan in Figure 2A shows a significant decrease in the resonance frequency and thus an increase in the deposited mass at the first oxidation of the porphyrin, i.e., electropolymerization occurs upon the formation of the radical cation porphyrin ZnT 2 P +• in the presence of 1,2-bis(diphenylphosphino)benzene (2).
The resonance frequency displays a significant decrease meaning an increase in the deposited mass during the electropolymerization as shown in Figure 2B.Indeed, according to the Sauerbrey's equation, the decrease in resonance frequency in Figure 2 corresponds to the increase in deposited mass.The mass of the copolymer film increased with the increase in number of potential cycle n (Figure 2B).Similar results have been obtained in the case of poly-ZnT 2 isoP1 (Figure S1).Note that a change in slope is observed for n > 15 in the case of poly-ZnT 2 isoP2 (Figure 2B).It might be related to the incorporation of supporting electrolyte, monomers as well as solvent molecules.
The mass change ∆m calc can also be calculated from Faraday's law by using the anodic charge Q as well as the molecular mass of the repeat unit of the copolymer.The calculated values ∆m calc are close to the experimental measurement ∆m EQCM obtained from EQCM study matching well to the formation of the 1D copolymer (Figure 3B and Table S1).

X-ray Photoelectron Spectroscopy (XPS)
The X-Ray photoelectron spectroscopy of poly-ZnT 2 isoP2 was also investigated as shown in Figure 4.The analysis of the survey spectra confirms the presence of the isoporphyrin and diphosphonium subunits (Zn 2p3 at 1022.0 eV, N 1s, and C 1s peaks), while the signals for F 1s (686.7 eV) and for P 2p (136.9 eV and 136.3 eV) electrons stem from the incorporated counterion PF 6 -as well as the phosphonium linkers (for the second P 2p signal at 136.3 eV).The C1s peaks are composed of two signals, 286.5 eV and 285.0 eV attributed to homo and hetero (connected to nitrogen) carbon atoms, respectively.The N1s peaks at 402.5 eV, 400.6 eV and 398.5 eV reveal the presence of three chemically different nitrogens due to the counter cation tetrabutylammonium, and the inner iminic nitrogen of the porphyrin.The whole XPS spectra are shown in Figure S3.Similar results have been obtained in the case of the second copolymer poly-ZnT 2 isoP1 (Figure S2).

Electron Paramagnetic Resonance (EPR)
The conspicuous central signal within these spectra confirms the presence of isoporphyrin-based free radicals (Figure 5), exhibiting a g-factor of 2.003 and a linewidth of approximately 0.6 mT [18].Notably, an increase in the deposition concentration of the copolymers on the ITO led to a commensurate rise in the EPR signal strength.Typically, in the literature, isoporphyrins and their radical cationic forms are known to be highly reactive, prone to decomposition [19,20].Analysis from the XPS data for the two copolymers can give the zinc-to-phosphorus atomic ratio, which is approximately 0.24 for the two copolymers.It is in agreement and very close to the theoretical ratio (0.25, one Zn per four P atoms, corresponding to the two phosphonium of the linker and the two PF 6 − counter anions) in the case of the formation of the 1D copolymer where the repeat units are (isoZnT 2 P-+ PPh 2 -HC=CH-PPh 2 + •2PF 6 − ) and (isoZnT 2 P-+ PPh 2 -Ph-PPh 2 + •2PF 6 − ) for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2, respectively.Thus, it confirms the 1D copolymer formation as suggested by the EQCM measurements.

Electron Paramagnetic Resonance (EPR)
The conspicuous central signal within these spectra confirms the presence of isoporphyrinbased free radicals (Figure 5), exhibiting a g-factor of 2.003 and a linewidth of approximately 0.6 mT [18].Notably, an increase in the deposition concentration of the copolymers on the ITO led to a commensurate rise in the EPR signal strength.Typically, in the literature, isoporphyrins and their radical cationic forms are known to be highly reactive, prone to decomposition [19,20].

Electron Paramagnetic Resonance (EPR)
The conspicuous central signal within these spectra confirms the presence of isoporphyrin-based free radicals (Figure 5), exhibiting a g-factor of 2.003 and a linewidth of approximately 0.6 mT [18].Notably, an increase in the deposition concentration of the copolymers on the ITO led to a commensurate rise in the EPR signal strength.Typically, in the literature, isoporphyrins and their radical cationic forms are known to be highly reactive, prone to decomposition [19,20].However, in this study, the isoporphyrin free radical films of poly-ZnT2isoP1 and poly-ZnT2isoP2 exhibited surprising stability in a solid state, even when exposed to the atmosphere, enduring up to 4 years.This remarkable stability can likely be attributed to the following factors: the electron-withdrawing groups playing a pivotal role in However, in this study, the isoporphyrin free radical films of poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 exhibited surprising stability in a solid state, even when exposed to the atmosphere, enduring up to 4 years.This remarkable stability can likely be attributed to the following factors: the electron-withdrawing groups playing a pivotal role in facilitating the reduction and stabilization of the radical cations, and the free radicals benefiting from the electronic delocalization effects of the porphyrin, in addition to the π-π interactions between molecules and the π-π stacking effects between the macrocycles.

UV-Visible-NIR Spectroscopy
Figure 6A showed UV-visible-NIR spectra on ITO electrodes coated with the polyisoporphyrin measured at various thicknesses.The absorption intensity increases with iterative scan number which showed same trend with EQCM results (Figure S4).
A typical UV-visible-NIR spectrum of poly-ZnT 2 isoP1 obtained after electropolymerization between −1.1 and +1.0 V vs. SCE exhibited a split Soret absorption band at λ = 432 nm and λ = 505 nm.The bands were red shifted by 18 and 90 nm, respectively, compared to the ZnT 2 P monomer (Table 2).
interactions between molecules and the π-π stacking effects between the macrocycles.
The visible bands (Q bands), observed at 587 and 633 nm, are also red-shifted by 41 and 49 nm compared to ZnT 2 P, while for this poly-isoporphyrin, additional band in the NIR region were detected at 810 nm (Figure 6A).For poly-ZnT 2 isoP2, Soret absorption bands are at λ = 432 nm and λ = 497 nm, and Q bands are at 574 nm, 632 nm, also one additional absorption band in NIR region at 805 nm.This bathochromic shift may result from the electron withdrawing effect of the positive charge on the phosphonium cations.
The red shift observed in the Soret (B) and Q bands should be attributed to the nonplanar conformation of the macrocycle, as well as the electron-withdrawing effects of the pyridinium groups.Indeed, the red shift observed in the Soret (B band) and Q bands caused by the non-planarity of porphyrins are well documented in the literature [21][22][23][24].Optical red shifts may be also attributed to the electron-withdrawing effects of the pyridinium groups.This phenomenon is explained by a greater destabilization of the highest occupied molecular orbitals (HOMOs) compared to the lowest unoccupied molecular orbitals (LU-MOs), leading to narrower HOMO-LUMO gaps [25][26][27][28].These spectral changes can also be interpreted by considering the intra-and intermolecular exciton coupling between the porphyrin macrocycles within the copolymer [29,30].
The splitting of the Soret band as well as the additional NIR band could be the result to the formation of the related radical isoporphyrin [16].The weak absorption band in NIR region more pronounced for poly-ZnT 2 isoP2 (Figures 6A and S4) may because of the high delocalization of π-cation radicals of poly-isoporphyrin as well as the presence of the isoporphyrin subunits [16].Similar bands have been already observed in the case of stable isoporphyrin monomer [31].
In Figure S4C, we observe results which seem contradictory to those obtained by the EQCM measurements if we only compare the maximum absorption intensity at λ = 432 nm.However, it can be noted that for poly-ZnT 2 isoP1, the splitting of the Soret absorption bands is more marked with a more marked widening of the bands.This is undoubtedly due to greater flexibility for poly-ZnT 2 isoP1 where the spacer is the cis-1,2bis(diphenylphosphonium)ethene.It is therefore difficult in this case to compare only the intensity of the Soret band at λ = 432 nm.
Interestingly, the broadening and the splitting of the Soret band as well as the presence of one additional band in the NIR between 750 nm and 1000 nm are expected to be advantageous to photovoltaic applications by extending the domain of solar light absorption.

Film Morphology (Atomic Force Microscopy, AFM)
The obtained films were studied by scanning atomic force microscopy (AFM) (Figure 7A,C).
In a typical image of poly-ZnT 2 isoP 1 , the copolymer is seen on the surface as densely packed coils with an average diameter of approximately 80-150 nm and a height approximately 22.0 nm for the film produced after 10 iterative scans between −1.1 V and +1.0 V (Figure 6A).The rms surface roughness of the two films was estimated to be 14.2 nm for poly-ZnT 2 isoP 1 , based on calculations from an area of 1.0 µm 2 with 10 scans (Figure 6).
The poly-ZnT 2 isoP1 copolymer films obtained after higher iterative scan number exhibited comparable morphology but showed in several positions some aggregation of the coils accompanied by a larger value of the rms surface roughness (24.9 nm for n = 15).The formation of coil aggregates might be related to the change in slope of the deposited mass, observed from EQCM measurements in Figure 2B.
The poly-ZnT 2 isoP2 films were also studied by AFM (Figure 7C,D).The poly-ZnT 2 isoP2 copolymer appears on the surface yet again as tightly packed coils with an average diameter of ca.80-100 nm, the height being approximately 24.0-33.0nm for the film.The rms surface roughness of the films has been estimated at 7.6 nm for n = 10 scans (calculated from an area of 1.0 µm 2 ).tion.

Film Morphology (Atomic Force Microscopy, AFM)
The obtained films were studied by scanning atomic force microscopy (AFM) (Figure 7A,C).In a typical image of poly-ZnT2isoP1, the copolymer is seen on the surface as densely packed coils with an average diameter of approximately 80-150 nm and a height approximately 22.0 nm for the film produced after 10 iterative scans between −1.1 V and +1.0 V (Figure 6A).The rms surface roughness of the two films was estimated to be 14.2 nm for poly-ZnT2isoP1, based on calculations from an area of 1.0 µm 2 with 10 scans (Figure 6).
The poly-ZnT2isoP1 copolymer films obtained after higher iterative scan number exhibited comparable morphology but showed in several positions some aggregation of the coils accompanied by a larger value of the rms surface roughness (24.9 nm for n = 15).The formation of coil aggregates might be related to the change in slope of the deposited mass, observed from EQCM measurements in Figure 2B.
The poly-ZnT2isoP2 films were also studied by AFM (Figure 7C,D).The poly-ZnT2isoP2 copolymer appears on the surface yet again as tightly packed coils with an average diameter of ca.80-100 nm, the height being approximately 24.0-33.0nm for the film.The rms surface roughness of the films has been estimated at 7.6 nm for n = 10 scans (calculated from an area of 1.0 µm 2 ).

Electrochemistry of the Copolymer
The cyclic voltammetry of electroactive polymers deposited on ITO surfaces have been characterized by electrochemical methods (Table 2).The CV curves shown in Figures S5 and S6 have been recorded for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 with copolymer grown on the electrode surface for various potential scans (n = 1, 2, 3, 5, 10, 15, 20, 25 cycles) in a clean electrolytic solution containing only the solvent and the supporting electrolyte.
When porphyrin was substituted by such electron-withdrawing groups, the oxidation of the substituted isoporphyrins was more difficult than the oxidation of the corresponding unsubstituted porphyrins ZnT 2 P (Table 2).
In the anodic part, three successive waves are observed, the first wave is reversible while the two last waves are irreversible (Table 2).The first reversible processes were measured at +0.86 V and +0.88 V for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2, respectively.This corresponded to the wave which increases in intensity during electropolymerization (Figure 1B,C).The reversible wave corresponds to the reversible oxidation of the isoporphyrin radical isoP • giving the oxidized isoporphyrin isoP + .
The two irreversible oxidation peaks were observed after the reversible wave for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 at 1.15 V and 1.39 V vs. SCE and at 1.17 V and 1.40 V vs. SCE, respectively (Figure S5).
In the cathodic domain, not well-resolved wave observed approximately -0.91 V vs. SCE and -0.87 V vs. SCE (peaks I or I') for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2, respectively, are attributed to the reduction of the phosphonium forming the corresponding radical (-• P(Ph) 2 -).
Furthermore, after the reduction of the phosphonium units, the additional wave measured at +0.42 V and +0.38 V for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2, respectively (peak a or a'), which were also observed during electropolymerization (peaks a or a') might correspond to the reaction between the radical phosphine -(Ph) 2 P • -formed after reduction of the phosphonium unit and the radical present on to the isoporphyrin isoP • .Thus, the reduction of the diphosphonium generates one biradical which can react rapidly via radical coupling with the radical present onto the isoporphyrin (formation of the internal C-C bound between (-• P(Ph) 2 -and the IsoP • ) as proposed in Scheme S3.
The peaks a or a' might be due to the oxidation of the resulting compound obtained to regenerate the starting isoporphyrin radical-phosphonium material.
Poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 are highly stable even after 4 years, but at a higher applied potential than the first oxidation wave, which is reversible, at 0.86 V and 0.88 V, respectively, for instance at an applied potential of 1.60 V, fully decomposition of the film occurs leading to the fully removal of the film from the ITO slides (Figure S5, right).

Electrochemical Impedance Spectroscopy (EIS) and Photoelectrochemical Properties
The electrochemical and photoelectrochemical properties of this two poly-isoporphyrins are investigated and compared (Figures 6B,C, S7 and S8).
The typical current-potential curves were obtained by photocurrent measurements (Figures S7 and S8).For the best photocurrent value of both poly-ZnT 2 isoP1 (20 scans) and poly-ZnT 2 isoP2 (10 scans) samples, the amount of deposited copolymer on ITO substrate is approximately the same (ca.6.5 µg/cm 2 ), thus enabling the comparison of their (photo)electrochemical properties.
Compared with Figure S7, the photocurrent response depends on the number of deposition scans.The magnitude of the photocurrent increased from n = 1 to n = 10, and then as decreased with higher deposition scans of poly-ZnT 2 isoP1.The same trend is observed for poly-ZnT 2 isoP2.
The electrical properties of copolymer films were studied by electrochemical impedance spectroscopy (EIS), which is widely used in the investigation of dye sensitized solar cell.Under light illumination, the poly-ZnT 2 isoP2 showed a better conductivity than poly-ZnT 2 isoP1 according to the smaller semi-circle (Figures 6D and S9).This appearance seems to be correlated with the magnitude of the photocurrent measured under visible illumination.All the results showed that the poly-ZnT 2 isoP2 had a better photocurrent generation under visible illumination (Figures 6C and S9).
The Nyquist plots of a typical impedance spectra measured for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 at 0 V in 5 mM I 3 − /0.5 M I − aqueous solution with different scan numbers (n = 1, 2, 3, 5, 10, 15, 20 and 25) are plotted in Figure S9.The Nyquist diagrams can be fitted with an equivalent circuit composed of two parallel RC circuits in series.One RC circuit can be attributed to the charge transfer resistance R ct of the I − /I 3 − species at the ITO/solution interface in parallel with the interfacial capacitance C i , while the other RC circuit also is due to the ion transport resistance R f within the copolymer film in parallel with the C f capacitance of the film.The evolution of the R ct and R f values with the number of polymerization scan number under and without illumination shown in Figure S10.
Under illumination, the charge transfer resistance decreased compared to ones measured in the dark conditions, illustrating the shift of the open circuit potential and then the photochemical activity of copolymers when illuminated.The charge transfer resistance (R ct ) decreases with scan number indicating that the presence of the copolymer film is catalyzing the electron transfer kinetics between I − /I 3 − and the ITO electrode.Meanwhile, the R f tends to increase with the number of electrodeposition scans n due to the increase in film thickness due to the ionic resistance into the copolymer film.Thus, the lowest total charge transfer resistance is obtained with 20 scans for poly-ZnT 2 isoP1 and 15 scans for poly-ZnT 2 isoP2, which showed almost the same trend with photocurrent.It suggests that the photocurrent efficiency is dependent on charge transfer resistance.For poly-ZnT 2 isoP1 at 20 scans under light illumination, R ct is 397 ohms versus R f of 534 ohms.For poly-ZnT 2 isoP2, at 15 scans under light illumination, R ct is 456 ohms versus R f of 18 ohms.Thus, polypoly-ZnT 2 isoP2 possessed better conductivity with similar deposited mass than poly-ZnT 2 isoP1.
To investigate the electron transfer mechanism in the system, the energies of the relevant electronic states have been estimated.In the case of copolymer poly-ZnT 2 isoP1, an energy level diagram can be built using the oxidation potential of isoporphyrin subunit and the reduction potential of phosphonium subunits, together with the optical absorption spectra of copolymer films (Figure 8).It describes the thermodynamics for spectral sensitization of the ITO electrode.The LUMO levels of the excited poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 species can be roughly estimated by subtracting the excitation energy of the Soret or Q bands from the HOMO level energies.The levels of the excited isoporphyrin, isoPorph + /isoPorph • *, are calculated from the absorption spectra of the film and the wavelength of the Soret (B), Q and NIR bands.The ground state isoporphyrin (isoPorph + /isoPorph • ) is calculated from the oxidation potential of the isoporphyrin giving for instance in the absolute scale -5.60 eV for the poly-ZnT 2 isoP1 film.The energy level of I 3 − /I -on the absolute scale is -5.00 eV.The energy Level of poly-ZnT 2 isoP2 corresponding to the band in the NIR region has also been indicated in the energy diagram (Figure 8).Under illumination at 0 V, the photon absorption by the (iso)porphyrin entities generate an electron hole pair in the copolymer.The electron is transferred from the excited (iso)porphyrins to the phosphonium subunit acting as a relay for the electron transfer forming the radical phosphane.This radical phosphane gives in turn the electron to the I 3 − , which is reduced into I − which, is reoxidized at the counter-electrode.The oxidized (iso)porphyrins are regenerated by an electron transfer from the ITO substrate.The I 3 − reduction is in competition with the recombination of the photogenerated electron-hole pair.Thus, fast electron transfer kinetics between the ITO and the oxidized (iso)porphyrins and/or between the excited (iso)porphyrins and the I 3 − are crucial for the efficient photocurrent generation.

Electrochemistry
Voltametric data were collected using a standard three-electrode setup with a STAT 2273 potentiostat.The electrolyte solution was a mixture of CH3CN and 1,2-C2

Electrochemistry
Voltametric data were collected using a standard three-electrode setup with a PARSTAT 2273 potentiostat.The electrolyte solution was a mixture of CH 3 CN and 1,2-C 2 H 4 Cl 2 (3:7) containing 0.1 mol•L −1 of tetrabutylammonium hexafluorophosphate (NBu4PF 6 ).Singleside-coated indium-tin-oxide (ITO, SOLEMS, 25-35 Ω/cm 2 ) electrodes with a surface area of 1 cm 2 served as the working electrode, while a platinum wire functioned as the auxiliary electrode.The reference electrode was a saturated calomel electrode, connected to the solution via a junction bridge filled with the electrolyte.Electrochemical impedance spectroscopy (EIS) measurements were conducted with a sinusoidal voltage excitation of 10 mV amplitude across a frequency range of 100 kHz to 100 mHz.Additionally, ITO electrodes were employed to obtain UV-vis spectra of the electrochemically deposited copolymers using an Agilent 8453 spectrophotometer.
The formation of the copolymer can be carried out either by applying a constant potential (chronoamperometry), or by iterative scanning between two potentials (cyclic voltammetry).Chronoamperometry works but gives less controlled deposits and less homogeneous materials in terms of film thickness.It must be noted that electropolymerization at controlled potential is only possible beyond the first oxidation potential of the porphyrin ZnT 2 P, i.e., 0.79 V vs. SCE and below the oxidation potentials of the diphosphine ligand, namely the cis-1,2-bis(diphenylphosphino)ethene (1) or 1,2-bis(diphenylphosphino)benzene (2), at 1.37 V and 1.68 V vs. SCE, respectively.Furthermore, by applying a constant potential between 0.79 V and 1.37 V or 1.69 V vs. SCE for cis-1,2-bis(diphenylphosphino)ethene (1) or 1,2-bis(diphenylphosphino)benzene (2), respectively, an accumulation of the released H + occurs which can induce a demetalation of the metalloporphyrin ZnT 2 P, follow by a protonation of the free base obtained H 2 T 2 P, leading to the formation of protonated porphyrin which will stop the reaction, these protonated porphyrins being much more difficult to be oxidized.
The cyclic voltammetry method over a voltage range of −1.1 to 1.0 V vs. SCE gave better results from the deposit quality point of view with a more homogeneous deposit thickness across the entire sample.The potential of 1.0 V was chosen to be sufficiently higher than the first oxidation potential of ZnT 2 P (0.79 V) and sufficiently lower than the oxidation potentials of cis-1,2-bis(diphenylphosphino)ethene (1) and 1,2bis(diphenylphosphino)benzene (2).The scan rate v = 100 mV•s −1 allowed to obtain good-quality films with a preparation time for each sample that is not too long.The advantage of iterative scanning between −1.1 V and 1.0 V vs. SCE in cyclic voltammetry mode also makes it possible to consume and reduce the H + released during the substitution by forming H 2 (g).
Below −1.1 V, a reduction in porphyrin macrocycle also occurs, inducing the formation of poor-quality film.Control of the optical spectra of the solution shows that no demetallation of the ZnT 2 P porphyrin is noticed in the case of iterative scan between −1.1 V to 1.0 V vs. SCE.
ITO electrodes with a surface area of 1 cm 2 were used as the working electrode.Each copolymer underwent 20 iterative scans.Following electropolymerization, the modified working electrodes were rinsed with CH 3 CN and then with CH 2 Cl 2 to remove any residual monomers and conducting salt from the deposited films.

Electrochemical Quartz Cristal Microbalance (EQCM)
A QCA-922 system (SEIKO EG&G instrument) combined with a Versa STAT 3 was employed for simultaneous electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry measurements.The electrochemical cell was assembled inside a glove box, using an ITO AT-cut quartz crystal resonator (mirror-finished, resonant frequency: 9.08 MHz ± 50 kHz, A = 0.2 cm 2 , SEIKO EG&G., Ltd.) as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl wire as a quasi-reference electrode.The solution used for electropolymerization was identical to that used for the copolymers with the larger ITO electrode.Iterative scans were performed at a scan rate of 100 mV•s −1 at room temperature, with simultaneous recording of the quartz resonance frequency.The change in quartz resonance frequency (∆f) was converted into mass change (∆m) using Sauerbrey's equation (Equation ( 1)): where µ is the shear modulus of quartz (2.947 × 10 11 g•cm −1 •s −2 ), A is the working area (0.2 cm 2 ) of the ITO quartz crystal resonator, and f 0 is the resonant frequency of the fundamental mode, and ρ is the density of the crystal (2.684 g/cm 3 ).

X-ray Photoelectron Spectroscopy
XPS experiments were conducted using a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) with Mg Kα radiation (hν = 1253.6eV) or Al Kα radiation (hν = 1486.6eV).Typically, the X-ray anode operated at 250 W, with a high voltage of 14.0 kV and a detection angle of 54 • .The pass energy was set at 23.5, 46.95, or 93.90 eV to ensure adequate sensitivity and resolution.The base pressure in the analyzer chamber was approximately 5 × 10 −8 Pa.Samples were pressed into self-supported disks (10 × 10 mm), mounted on a sample holder, and then transferred into the analyzer chamber.Both the full spectra (0-1100 eV) and the high-resolution narrow spectra of all elements were recorded using an RBD 147 interface (RBD Enterprises, Briarwood, NY, USA) with Auger Scan 3.21 software.Binding energies were calibrated using contaminant carbon (C1s = 284.6 eV).Data analysis was performed using either RBD Auger Scan 3.21 software from RBD Enterprises or XPS Peak 4.1 provided by Raymund W.M. Kwok (The Chinese University of Hong Kong, China).

Atomic Force Microscopy (AFM)
Atomic force micrographs (AFM) measurements have been conducted directly on the ITO surfaces using a NX10 Park Instrument in the tapping mode under ambient conditions.Silicon cantilevers Bruker with a spring constant of 40 N/m and a resonance frequency in the range of 300 kHz have been used.The scanning rate was 1.0 Hz.

EPR Measurements
CW EPR experiments were performed using an X-band (9-10 GHz) Bruker ESP300 spectrometer equipped with a high-sensitivity resonator (ER4119HS, Bruker) at room temperature.

Photoelectrochemistry
Photoelectrochemical measurements were conducted in a three-electrode electrochemical cell, with glass-ITO electrodes covered by the copolymer thin film as the photocathode, a Pt plate as the counter-electrode, and a Pt wire as the pseudo-reference electrode.The photoelectrochemical responses of the films were obtained through on-off light illumination in a 0.5 M I − /5 mM I 3 − aqueous solution using a 300 W Xe arc lamp.The I − and I 3 − anions worked as redox mediators.A water filter was used to block IR radiation, and long pass filters at either λ > 405 nm or λ > 800 nm were employed to isolate visible-NIR or NIR radiation, respectively.Current-potential curves were recorded both in the dark and under illumination using a PARSTAT 2273 potentiostat with linear sweep voltammetry at a scan rate of 10 mV•s −1 .EIS spectra were acquired under illumination by applying a sinusoidal potential modulation (10 mV over the Open Circuit Photopotential) across frequencies ranging from 0.1 to 100,000 Hz.
In photoelectrochemistry and voltametric measurements have been performed with a standard three-electrode system using a PARSTAT 2273 potentiostat.Glass carbon or single-side-coated indium-tin-oxide (ITO, SOLEMS, 25-35 Ω•cm −2 ) electrodes, with a surface of about 1 cm 2 has been used as working electrode.A platinum wire has been used as an auxiliary electrode.The reference electrode was a saturated calomel electrode.It was electrically connected to the solution by a junction bridge filled with the electrolyte.Photoelectrochemical responses for films were obtained by on-off light illumination of a 300 W Xe Arc lamp (with λ > 405 nm long pass filter).Solutions were prepared by bubbling with argon (Ar-U, from Air Liquide) in order to have deaerated prior to illumination.All experiments were conducted at room temperature using a water filter to block IR radiation, ensuring that the solution temperature did not increase by more than 1 degree during light exposure.UV-vis absorption spectra were recorded using a Hewlett-Packard HP 8453 diode array spectrophotometer, which operated with a resolution of 2 nm.

Conclusions
In summary, copolymer films have been prepared by the electro-oxidation of 5,15ditolylporphyrin (ZnT 2 P) in the presence of various types of di(phenylphosphane) ligands.Electrogenerated radical cation porphyrin is a powerful electrophile which can rapidly react to form copolymer containing stable isoporphyrin subunits (poly-ZnT 2 isoP • ).The achieved copolymers were characterized by UV-vis-NIR spectroscopy, and X-ray photoelectron spectroscopy.Mechanisms of electrochemical routes to these electroactive copolymers have been discussed as well as the description of the unusual redox properties of copolymers containing stable isoporphyrin.
The photocurrent measurements under visible-NIR light irradiation show that poly-ZnT 2 isoP2 thin films exhibit significantly better performance than poly-ZnT 2 isoP1.However, the photovoltaic performances reach an optimum depending on the number of electropolymerization potential scan number.The best performances for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 are obtained for n = 20 and 10, respectively.Therefore, the control of the thickness of the copolymer films is of great importance to optimize the generation of photocurrent under visible illumination.The copolymer in the solid state could be stored for over several months and even one year without any degradation under ambient conditions in air.

Figure 5 .
Figure 5. X-Band EPR spectrum in DMF of poly-ZnT2isoP1 and poly-ZnT2isoP2 at room temperature.The solution of copolymer was prepared by washing with 160 mL of DMF the covered ITO obtained using 25 scans between 1.1 V and +1.0 V versus SCE and -, v = 100 mV.s −1 .In order to have enough solution, the operation was repeated three times.

Figure 5 .
Figure 5. X-Band EPR spectrum in DMF of poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2 at room temperature.The solution of copolymer was prepared by washing with 160 mL of DMF the covered ITO obtained using 25 scans between 1.1 V and +1.0 V versus SCE and -, v = 100 mV•s −1 .In order to have enough solution, the operation was repeated three times.

Figure 7 .
Figure 7. Tapping mode AFM topography of (A,C) of poly-ZnT2isoP1 and poly-ZnT2isoP2, respectively.(B,D) Section analysis of the coils marked by a white line of poly-ZnT2isoP1 and poly-ZnT2isoP2, respectively.

Figure
Figure S9: (A) ESI Nyquist of poly-ZnT 2 isoP1 and (B) poly-ZnT 2 isoP2.Measurements have been obtained in the dark and in H 2 O containing I 3 − 5 mmol•L −1 and I − 0.5 mol•L −1 .BIAS potential: 0.00 V vs. OCP.Figure S10: Plot of the Log(R) versus n (number of iterative scan, v = 100 mVs −1 ) for (A) poly-ZnT 2 isoP1 and (B) poly-ZnT 2 isoP2.Measurements have been obtained in H 2 O under on (red line) and off light (black line) illumination from a 300W Xe arc lamp (with λ > 385 nm long pass filter) in containing I 3 − 5 mmol•L −1 and I − 0.5 mol•L −1 .BIAS potential: 0.00 V vs. OCP.(R ct = charge transfer resistance at the ITO/copolymer interface (triangle plot with solid line), R f = the charge transfer resistance of films (square plot with dotted line).Table S1: Mass change calculated from CV and Faraday's law compared with the masse change measured from EQCM during CV measurements during electropolymerization for poly-ZnT 2 isoP1 and poly-ZnT 2 isoP2.Author Contributions: Y.L., investigation and writing-review and editing.G.D. and M.G., AFM studies, writing-review and editing.S.C. and N.L.B., EPR studies and writing-review and editing.C.B., formal analysis and writing-review and editing.V.B., formal analysis and writing-review and editing.A.B., writing-review and editing.L.R., conceptualization, formal analysis, investigation, writing-review and editing and funding acquisition.All authors have read and agreed to the published version of the manuscript.