This chapter describes the electrochemistry of the porphyrins at solid‐liquid and liquid‐liquid interfaces. The fundamental electrochemical approach toward the porphyrin molecules in estimating their HOMO and LUMO energy levels is given. Various factors such as the effect of central metal ion, the periphery of the aromatic ring and axial ligands on the redox potentials of porphyrins have been discussed. Electrochemical sensing application of porphyrin molecules is described with few examples in brief. Much focus has been given on the electrochemistry of the self‐assembled monolayer (SAM) of thiol‐porphyrins on the gold electrode. Structural characterization and charge transfer across the SAM using cyclic voltammetry and electrochemical impedance spectroscopy are discussed. Theory and methodologies developed to study photoinduced charge transfer kinetics of porphyrin molecules using scanning electrochemical microscope at the solid‐liquid and liquid‐liquid interface have been described. Use of porphyrin molecules as luminophores in electrochemiluminescence sensing applications and the mechanisms involved are described through representative examples.
In the natural photosynthesis process, chlorophyll converts incident light into chemical energy with nearly 100% quantum yield through many complex steps. This excellent phenomenon inspired many scientists to study porphyrin derivatives and their metallated forms extensively for many decades and continue to be so. Substantial information has been gathered on the synthesis, structural characterization, and dependence of their property on the structure and applications of porphyrins . Porphyrins can be tailored by modifying the aromatic ring at the β and
As mentioned above, crafting the redox potentials of the porphyrins by modifying the periphery or the core of the aromatic ring remains the key strategy behind its multifunctional behavior. Most of such compounds are electroactive, exhibit multiple redox couples, and have been investigated for their electrochemical properties, generally, in nonaqueous solvents. Various factors such as a type of metal ion and its oxidation state present at the core, nature of the macrocyclic aromatic ring, and an axial ligand attached to the metal ion will affect the electrochemistry of the molecule.
Porphyrins exhibit outstanding absorption of electromagnetic radiation in the visible region. Upon light illumination, electrons present in the HOMO will get excited to LUMO of the porphyrin. Photoexcitation followed by various relaxation processes and charge separation is shown in Figure 1. Long‐lived radical ion pairs of porphyrins can be observed by stabilizing the charge separated states. Generally, the basic electrochemistry of the porphyrins is related to its electron donating or accepting behavior in the ground state. Electrochemistry of porphyrins under conditions similar to that of photovoltaic devices, artificial photosynthetic systems involves the other states depicted in Figure 1. In the following sections, we discuss the fundamental and applied electrochemistry of the porphyrins and its derivatives. Without going for the exhaustive citation of all the reported literature, representative examples have been chosen to support our discussion.
2. HOMO and LUMO energy levels of porphyrins
Electrochemical techniques such as cyclic voltammetry and differential pulse voltammetry are generally used to estimate the HOMO and LUMO energy levels of the organic compounds. Oxidation onset potential, that is, the energy required to take out the first electron from the HOMO of the molecule will give the HOMO energy level of the molecule under study in eV versus the reference electrode used. In a similar way, reduction onset potential, that is, the energy required to add the first electron to LUMO of the molecule will give information about the energy level of the LUMO of the molecule. Ferrocene (Fc) or other common references used as an internal standard to complete the calculation by using following equations. Such electrochemical studies generally carried out in organic solvents with a suitable electrolyte.
By knowing HUMO and LUMO energy levels, one can calculate the energy gap (
2.1. Effect of metal ion
Cheng et al. calculated the HOMO and LUMO energy levels of the tetraphenylporphyrin (TPP) and Cu, Zn, Ni, Pd, and Pt metallated porphyrins (MTPP) . Cyclic voltammograms (CVs) were recorded for TPP and MTPP in acetonitrile using tetra‐n‐butylammonium hexafluorophosphate as an electrolyte. Quinoxalinoporphyrin and its zincated form were studied in chlorobenzene with tetrabutylammonium tetrafluoroborate as an electrolyte versus Fc/Fc+ by recording the CVs.
2.2. Effect of modifying the periphery of aromatic ring
Factors such as electron donating or withdrawing nature of the substituent, where it has been located on the ring and its number will affect the oxidation and reduction potentials. Influence of π‐extension of the aromatic ring on the electrochemistry has been reported for platinum (II) porphyrin derivatives . Dependence of the oxidation and reduction half‐wave potentials of the porphyrins on the planarity of the molecule has been discussed by Shelnutt et al. in their review with various examples .
2.3. Effect of axial ligand
Coordination of nitrogenous bases is generally used in the axial ligation of metalloporphyrins. Type of ligand participated in the axial ligation can manipulate the oxidation and reduction half‐wave potentials of the metalloporphyrins. Kadish et al. have discussed this in detail with exemplifying a large number of ligands possessing nitrogen as a donor atom with iron and cobalt porphyrins . Basic electrochemistry of metalloporphyrins has been discussed in detail with numerous examples by Kadish et al. in a series of book volumes and also in reviews .
3. Porphyrins at solid‐liquid interface
Because of the multifunctional property and their interaction with the various analyte molecules, porphyrins deliver different signal outputs. Porphyrin molecules have been used for sensing applications through optical, electrochemical, different spectral modes. Electrochemical sensing methods developed using porphyrin molecules by our group is discussed here briefly. Porphyrin monolayer was used to electrochemically sense the phosphate anion based on the hydrogen bonding interaction. Upon hydrogen bonding of PO42⁻ with ⁻NH, ease of charge transfer between the redox mediator and monolayer on the electrode was increased. Taking the advantage of this, decrease in the charge transfer resistance,
3.2. Monolayers of porphyrin derivatives
Organosulfur compounds are well studied for the formation of self‐assembled monolayers (SAMs) on the gold substrate. SAMs have been studied for their effect on the interfacial properties. A molecule which involved in the formation of SAM can be divided into three parts. Head group of the molecule will interact with the gold substrate, a free end of the molecule can be considered as a tail group and the thickness and structure of the SAM will be decided by the spacer or linker moiety present between the head and tail. SAMs provide an ideal system for the electrochemical study of heterogeneous charge transfer. Effect of the length of linker molecule on the adsorption kinetics of 5‐[p‐(mercaptoalkoxy)‐phenyl]‐10,15,20‐triphenylporphyrin molecules denoted as H2TPPO(CH2)nSH was studied by varying the n from 3 to 12 . Cyclic voltammetry and electrochemical impedance spectroscope were used to observe the time dependence of the surface coverage and orientation of H2TPPO(CH2)nSH on the gold electrode. Adsorption rate constant was found to decrease with the increase in the length of the linker molecule. Though the bulky porphyrin molecules are present at the terminal, adsorption steps were similar to that of bare alkanethiols. The monolayers formed as a result of interaction between thiol and gold substrate are compact. Still, there will be imperfections in the form of pinholes. Hence, molecules or ions will reach electrode surface through them to result in a charge transfer. Hence, it is important to have information about such imperfections. The same set of different alkyl length thiol‐porphyrin molecules was used to study the surface coverage, size, and distribution of pinholes present in the monolayer. The size of the pinholes estimated using EIS was ranged between 4 and 6 µM with 40–70 µM separation between them. Randles equivalent circuit and pore size distribution model were used in the analysis of monolayer structure . Electrocatalytic activity of the metalloporphyrins depends on their orientation in the film produced on the electrode [18, 19]. Cobalt tetraphenylporphyrin (CoTPP) monolayers were prepared on the gold electrode using two different linker molecules such as a 3‐mercaptopropionic acid (MPA), 4‐mercaptopyridine (MPY). Free base porphyrin, tetra‐[
Generally, porphyrin monolayer formed by assembling each porphyrin molecule on the gold electrode through one thio‐ or thioacetate‐group, that is, through one clip. Studies on the formation of SAMs using multi clips are seldom [24, 25]. Our group investigated the formation of SAM of tetra[
A monolayer of base porphyrin 5‐[p‐(mercaptopropyloxy)‐phenyl]‐10,15,20‐triphenylporphyrin, H2MPTPP and its Co and Ni metallated forms were produced on the gold electrode to study their interaction with the DNA at the electrode/electrolyte interface. The magnitude of interaction was understood by calculating the heterogeneous rate constant values from SECM and EIS. Electrostatic attraction between DNA and Ni‐MPTPP found highest and it was least in the case of H2MPTPP . Based on the strong interaction between iron porphyrin and DNA, a composite film prepared out of this combination was used for electrochemical sensing of p‐nitrophenol .
Moving on from electrochemical properties of a monolayer of porphyrins, multilayered films were constructed. Gold nanoparticles (AuNPs) and 5,15‐di‐[p‐(6‐mercaptohexyl)‐phenyl]‐10,20‐diphenylporphyrin (trans‐PPS2) were used as inorganic and organic materials, respectively, to form hybrid multilayer film on the gold electrode. Electrochemical property particularly heterogeneous charge transfer constant,
So far, electrochemistry of porphyrin molecules at the solid‐liquid interface studied using basic techniques such as CV and EIS was discussed. Deducing charge transfer constants using CV is simple and straightforward. But, the factors such as resistive potential drop and double‐layer charging current pose an ambiguity on the reliability of the results obtained. SECM measures the steady state current using micro‐ or submicro‐size of the tip. Hence, the measured current will also be very small that in turn minimizes the influence of resistive potential drop and double‐layer charging current. Hence, SECM emerged as a versatile experimental technique to study the adsorption kinetics of films on various substrates, charge transfer kinetics across the thin films [30, 31]. Very few studies have also been reported on the investigation of porphyrin films using SECM [32, 33]. Understanding and optimizing the long‐range charge transfer across the nanometer thickness films is of prime technological importance in various research fields. Theoretical and experimental approaches have been developed to study such cases using SECM .
SAMs of thiol‐porphyrins, H2TPPO(CH2)nSH with varied alkyl chain length were formed on the gold electrode to investigate the electron transfer between the electrode and the redox mediator, [Fe(CN)6]3- present in the electrolyte. Three pathways for the electron transfer were proposed. (I) Mediated electron transfer, in this case, product formed at the tip will be regenerated by the bimolecular reaction. That is the film is also redox active. (II) Tunneling through the film, in this case, film is electro‐inactive. Hence, the product formed at the tip of the SECM is regenerated at the electrode by tunneling through the film. (III) Imperfections of the SAM such as pinholes and defects give a way for charge transfer. Theoretical approximations were deduced for all the three situations to calculate the SECM tip current. Experimental approach curves were recorded for the SAMs of different thickness and fitted with the theoretically simulated curves to extract the heterogeneous charge transfer constant,
In all the above examples, the charge transfer across the porphyrin film/electrolyte interface was originated as a result of the applied potential. But, the photo‐excitation of the porphyrins followed by charge transfer in the dyads and triads systems is well known and widely adopted for the construction of photovoltaic devices and artificial photosynthetic systems. Hence, a model system to study the photoelectrochemical properties of the porphyrin molecules is of prime importance. Our group proposed a novel experimental methodology to study the photoinduced charge transfer kinetics of the porphyrin films using SECM . Porphyrin coated on the transparent electrode will be excited by illuminating light from the bottom to result in the porphyrin cation. The reduced form of the redox mediator at the SECM tip will diffuse toward the film and reduce the porphyrin cation back to its original form by undergoing oxidation. Redox mediator oxidized at the film will travel back to the tip, thereby diffusion cone of redox mediator was generated between the SECM tip and porphyrin‐coated electrode (Figure 4). Hence, the tip current was increased as it moves close to substrate, that is, positive feedback. Through this bimolecular reaction, SECM tip was used to capture the photoinduced charge by performing the probe approach curve experiment. Such diffusion cone does not form when the light was not illuminated according to the above mechanism. Therefore, negative feedback was obtained when the probe approach curve was recorded (Figure 5).
Theoretical equations were proposed for tip current (
Where, and denote the tip current in case of insulating and conducting substrates, respectively. In this case, the transparent electrode is a conducting substrate, hence
where = , is the heterogeneous charge transfer constant, is the radius of the electroactive part of the SECM tip and is the diffusion coefficient of the mediator used. One will arrive with the value of after fitting experimental approach curves with theoretical ones. Then by substituting the value of
Above methodology was adopted to study the influence of various parameters on the PCT kinetics of zinc porphyrin across the solid/liquid interface using benzoquinone (BQ) as a redox mediator. The family of approach curves was recorded by varying the parameters such as wavelength, the intensity of the light source and for the different concentration of the mediator. The favorable condition for the PCT resulted in the greater
4. Electrochemiluminescence (ECL) of porphyrin
Electrochemiluminescence (ECL) involves a conversion of electrical energy into radiative energy. Fundamental principles, various luminophore systems, applications, and recent advances of ECL have been discussed in detail elsewhere [45, 46]. Polypyridyl complexes are the excessively studied luminophores so far. Because of the rich photo and electrochemical properties, porphyrins have also been used as luminophores in ECL. There are two well‐established mechanisms through which ECL can be produced. First one is the annihilation mechanism: In this, a potential of the electrode alternatively pulsed between the two values to produce the oxidized and reduced species of the luminophore. These electrogenerated species at the vicinity of the electrode will interact with each other to produce the excited species, which will return back to the ground state by emitting the radiation .
Another way of generating the ECL is by coreactant mechanism: in this, coreactant species upon oxidation or reduction will generate an intermediate, which will further react with the luminophore to cause the excitation. For example, oxalate ion upon oxidation produces the strong reductant. Hence, it is also called as “oxidative‐reductive” coreactant .
ECL luminophore present in the system also undergoes oxidation at the same potential. For example, tetrakis(3‐sulfonatomesityl)porphyrin (H2TSMP) .
Then the reaction takes place between the oxidized porphyrin and CO2-• produced from the coreactant to result in the excited porphyrin, which will emit radiation.
There is another category of coreactant referred to as “reductive‐oxidative,” that is, reduction of the coreactant will produce the strong oxidant species. Peroxydisulfate (S2O82-) can be mentioned as an example.
Let us consider luminophore, meso‐tetra(4‐sulfonatophenyl)porphyrin (TSPP) which is also undergoing reduction to produce radical anion under the same conditions .
Based on the above mechanisms, ECL sensors have been developed from our group to quantify pheophorbide, Cu2+, meso‐tetra(4‐carboxyphenyl) porphyrin [50–52]. ECL behavior of ruthenium and zinc porphyrins has been electrochemically investigated [53, 54]. Different porphyrin molecules have been studied in combination with clay, carbon nitride, and graphene to improve the intensity of the ECL signal and to achieve the applicability [55–57].
5. Porphyrins at liquid‐liquid interface
Investigation of the charge transfer process at the liquid‐liquid interface, that is, interface between the two immiscible electrolytes (ITIES) has got significance because of its mimicking nature of various fields such as phase transfer catalysis, biomembranes, and drug delivery systems. One of the important outcomes from ITIES studies is the dependence of charge transfer on the driving force. Three important charge transfer processes have been studied at the ITIES.
Electron transfer across the ITIES
Ion transfer across the ITIES facilitated by the complexing agent
More detailed information about theory, techniques used to study, and applications of ITIES have been discussed in detail in the review by Pekka and Hubert . Generally, in ITIES system, one out of the two phases will be rich of redox species and can be considered metal‐like. The potential drop across the ITIES is referred to as the difference of the Galvani potentials of the organic and aqueous phases, . At low overpotentials, the exponential dependence of electron transfer (ET) rate constant on the can be expected (Butler‐Volmer theory). Whereas at very high overpotentials, the ET rate constants will level off to follow Marcus theory .
The theory has been developed to split the complex multistep ET reaction into several one‐step processes and the ET rate constant of each step can be calculated. Taking ZnTPP/[Fe(CN)6]4- as a model system, effect of concentration of the species in the two phases, a thickness of the thin layer on the multistep ET processes was studied using thin layer cyclic voltammetry (TLCV). Experimentally obtained results were found to be in good correlation with the theoretical simulations . Owing to the close resemblance of iron porphyrin with the heme, ET kinetics of the various substituents bearing iron porphyrin was studied by constructing the artificial membrane in the form of ITIES (Figure 6). Effect of substituent with different electron affinity on the ET kinetics was understood by recording the approach curves using SECM. Good agreement between the experimentally obtained rate constant values and the electronic structure and molecular orbital energies calculated by the density functional theory was observed. More the number of electron donating substituent, more easily iron porphyrin will tend to lose the electron(s) at the ITIES .
Encouraged by these results, ET kinetics of iron porphyrin substituted with a range of electron accepting and donating groups was investigated by choosing the benzoquinone as a redox mediator. Dependence of the ET kinetics at the nitrobenzene‐water ITIES of the porphyrins was envisaged using SECM. Both Butler‐Volmer and Marcus inverted region ET kinetics were observed with the increase of low and high overpotentials as driving force, respectively . We extended our study for the complex two‐step electron transfer process, that is, zinc porphyrin substituted with the electron withdrawing groups. As expected, the oxidation potentials of the zinc porphyrin were positively shifted in the voltammograms recorded by TLCV. But, the ET kinetic data for the three different zinc porphyrin at the ITIES were not in line with the theoretical approximation . Consecutive ET kinetics of zinc porphyrin was investigated by extending its π‐conjugation using phenyl, naphthyl, and pyrenyl groups. ET rate estimated from the TLCV experiments were found to be slow if the substituted molecule is small and that of larger molecules was fast . From the SECM experiments, it was revealed that, increase in the driving force lead to the slowdown of the ET rate. This was explained by taking the stereostructures of the molecules into consideration. The structure of the molecule has a dominating effect on the ET rate over that of Galvani potential of the ITIES .
Desired changes in the electronic properties of the porphyrin molecules achieved by tailoring the macrocyclic ring are investigated in detail using electrochemical techniques. Thiol‐derivatized porphyrins are used to produce the self assembled monolayer (SAM) on the gold electrode to study their behavior at the solid/liquid interface. The SAMs composed of porphyrin molecules have been electrochemically investigated for the arrangement of molecules in it, imperfections and charge transfer across it. Simple and straightforward electrochemical methodologies have been developed to estimate the heterogeneous charge transfer constant at the solid/liquid and liquid/liquid interfaces using scanning electrochemical microscope (SECM). Porphyrin molecules have also been used in the electrochemical and electrochemiluminescence sensing applications.
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