**3. Technological applications**

structure [111, 112] that in turn influences the photosensitizer and the generation of longlived singlet oxygen. In cells, the positively charged TMPyP accumulates in the nucleus and mitochondria and could attack DNA, mitochondrial DNA and cardiolipin. The association of TMPyP with the inner mitochondrial membranes due to the affinity to cardiolipin favors the generation of singlet oxygen in situ with a high efficiency since its concentration is higher in the hydrophobic core of the lipid bilayers. Metalloporphyrins have also been studied as potential sensitizers for PDT. However, the results were less promising than those obtained

The synthetic analogs of porphyrins are widely used in therapy of diseases connected to oxidative stress processes. A quantitative structure-activity relationship (QSAR) studies have been performed to identify the optimal active molecule within a series of analog structure characteristics to diversify the biological action of the compound. The QSAR studies can correlate the physicochemical characteristics that affect the compound's activity in biological systems. These studies assumed that the binding affinity of the compound to the target receptor could determinate the biological activity [115]. The biological effects of two meso-tetrakis porphyrins, TPPS4 (anionic) and TMPyP (cationic) demonstrated that the cationic porphyrin has affinity to the inner mitochondrial membrane [99]. Therefore, in mitochondria, Mn3+TMPyP has been used as an antioxidant against superoxide ions. The replacement of manganese by an iron ion in TMPyP makes this porphyrin a prooxidant agent [116]. Au-porphyrins have been reported as excellent antiproliferative agents, showing cytotoxic effects on cancer cells. Regarding to the mimetic SOD activity of porphyrins, the correlation between the metal-centered reduction potential and the catalytic

ture-activity relationships have been established over the years by the rate-limiting step of metal reduction of this class of compounds [117]. Modulation of SOD activity has been achieved by decreasing the electron density of the groups at the meso and β-pyrrile positions, thus increasing the Mn3+/Mn2+ potential and facilitating its reduction [118–120].

the metal-centered positive charges via electrostatic facilitation [118, 119]. The manganese (III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl) porphyrin (Mn3+TE-2-PyP5+, E½ = +228 mV *vs* NHE, log kcat = 7.76) and manganese (III) 5,10,15,20-tetrakis(N-n-hexylpyridinium-2-yl) porphyrin (MnTnHex-2-PyP5+, E½ = +314 mV *vs* NHE, log kcat = 7.48), alkylated manganese

and electrostatic optimizations and yielded compounds because they exhibit the E½ close to the reduction potential of the SOD enzyme and are excellent mimetics of the SOD activity

that the para isomer (E½ = +60 mV *vs* NHE) of Mn3+TMPyP is less efficient as a SOD mimic

In a cell redox balance, the association of Mn3+TMPyP to membrane lipid bilayers can be intrinsically related to the redox potential of the Mn2+/Mn3+ couple. In homogeneous systems, Batinić-Haberle et al. [19] had reported the effect of Mn3+TMPyP in a CL-containing inner

•− dismutation was found for Fe and Mn porphyrins. The struc-

· −

is directed to the catalytic site by

), combined the thermodynamic

M−1 s−1) [19, 117–122]. Recently, it has been reported

with the free-base species [113, 114].

10 Phthalocyanines and Some Current Applications

**2.2. Porphyrins in chemical therapy**

rate constant for the O2

Either the mimetic SOD activity can occur when the O2

(III) 5,10,15,20-tetrakis(2-pyridyl)porphyrin (MnT-2-Pyp<sup>+</sup>

relative than the *ortho* isomer (E½ = +260 mV *vs* NHE) [12, 19, 123, 124].

(E½ ≅ +300 mV *vs* NHE, kcat ≅ 2 × 10<sup>9</sup>

Porphyrins free base are extensively applied in solar cells and sensor due to their photophysical characteristics. The intense absorption bands covering a significant range of the visible region of the electromagnetic spectrum and due to the relatively low cost of these compounds as compared with inorganic semiconductors make these molecules appropriate for application in solar cells. These characteristics experimentally observed are consistent with the results obtained by density functional theory (DFT). Therefore, DFT/time-dependent (TD)DFT calculation is a useful strategy for the molecular design of porphyrins with the more appropriate characteristics for application is dye-sensitizer solar cells (DSSCs) [125–129]. As an example, Santhanamoorthi et al. [129] have presented the theoretical study of newly designed porphyrin dyes (1−5) for DSSC applications. In this study, the authors calculated seven different structures of porphyrins and found the best characteristics for use in solar cells for two calculated molecules that were named Dyes 2 and 4. Dyes 2 and 4 presented smaller HOMO-LUMO energy gaps and absorption in Q band significantly stronger. Equally, DFT/TDDFT can be used for conceiving porphyrin derivatives for a diversity of technological applications. Theoretical calculations allow the prediction of the best characteristics for porphyrins to be used in technological applications and optimize the subsequent efforts for the synthesis.

#### **3.1. Porphyrins in solar cells**

Solar energy is an important source of energy (~3 × 1024 J year−1) that sustains the life on the Earth [130–132], and it can be an alternative to using fossil fuels due to be a clean, inexhaustible and sustainable source of energy [133–139]. The utilization of solar energy as solar fuel or electricity is fundamental for the maintenance of development and live on Earth and has attracted the attention of various members of the scientific community.

O'Regan and Grätzel [140] have discussed dye-sensitized solar cells (DSSC), a viable and promising technology which have low-cost production and high power conversion efficiency [141–148]. To build an efficient system of the solar cell is necessary [149–152] three components: (1) dye (light-absorber); (2) a hole transport agent; and (3) an electron-transport agent. **Figure 9** shows the schematic representation of components and representative operational principles of DSSC.

**Figure 9.** Schematic representation of components and representative operational principles of DSSC.

A typical DSSC device consists of a dye-sensitizer photoanode (TiO2 , anode) and a platinum counter electrode (Pt-coated, cathode) sandwiching an electrolyte that contains a redox mediator (iodine-based or cobalt complexes, redox mediator). Upon light illumination, the photoexcited dye in the LUMO level of sensitizer injects an electron into a conduction band (CB) of TiO2 , and then, the resultant oxidized dye is reduced by I<sup>−</sup> species (or Co2+ complex). The injected electrons move through an external circuit to the platinized counter electrode. Finally, the I<sup>−</sup> species (or Co2+ complex) is regenerated to produce the I<sup>3</sup> − species (or Co3+ complex) at the surface of the platinized counter electrode, and the circuit is completed [133]. The efficiency of conversion of light to electric power (η) increases when a light-absorbing the dye, and therefore, the choice of a suitable dye is essential to a high η [127, 144, 153–156].

Despite to the intense absorption band, typical porphyrins have poor light-harvesting ability in the Q bands, being necessary the introduction of a push-pull structure [157–160] and the elongation of porphyrin π-conjugated system into *meso* or β-positions to improve the lightharvesting property of porphyrins [158].

Porphyrin also can be used as a dye in thin layers on the porous TiO2 film. However, this system results in weak absorption of irradiated light, being essential the development of a way to strongly absorb the light in the dye layer. Gold layer can have been used in these systems due to its surface plasmon resonance (SPR) that offers an enhanced optical field with increased short-circuit current, which can be corroborated by theoretical calculations [161].

#### **3.2. Porphyrins in catalysis and sensing**

A typical DSSC device consists of a dye-sensitizer photoanode (TiO2

Porphyrin also can be used as a dye in thin layers on the porous TiO2

, and then, the resultant oxidized dye is reduced by I<sup>−</sup>

**Figure 9.** Schematic representation of components and representative operational principles of DSSC.

(CB) of TiO2

trode. Finally, the I<sup>−</sup>

12 Phthalocyanines and Some Current Applications

η [127, 144, 153–156].

harvesting property of porphyrins [158].

num counter electrode (Pt-coated, cathode) sandwiching an electrolyte that contains a redox mediator (iodine-based or cobalt complexes, redox mediator). Upon light illumination, the photoexcited dye in the LUMO level of sensitizer injects an electron into a conduction band

The injected electrons move through an external circuit to the platinized counter elec-

(or Co3+ complex) at the surface of the platinized counter electrode, and the circuit is completed [133]. The efficiency of conversion of light to electric power (η) increases when a light-absorbing the dye, and therefore, the choice of a suitable dye is essential to a high

Despite to the intense absorption band, typical porphyrins have poor light-harvesting ability in the Q bands, being necessary the introduction of a push-pull structure [157–160] and the elongation of porphyrin π-conjugated system into *meso* or β-positions to improve the light-

tem results in weak absorption of irradiated light, being essential the development of a way to strongly absorb the light in the dye layer. Gold layer can have been used in these systems due

species (or Co2+ complex) is regenerated to produce the I<sup>3</sup>

, anode) and a plati-

species (or Co2+ complex).

film. However, this sys-

−

species

The application of metalloporphyrins in bioinorganic chemistry has attracted interest in catalytic reactions. Synthetic metalloporphyrins are mimetic models inspired two heme proteins: cytochrome P450 (biosynthesis and degradation of biomolecules) and peroxidases as lignin peroxidases (degrades the lignin-cell wall). In 1970, Groves et al. [162] designed the firstgeneration of metalloporphyrin chlorine (5,10,15,10-tetraphenyl-porphyrinato)iron(III), or [Fe3+TPPCl], activated by iodosylbenzene (PhIO) revealed a catalytic activity in the epoxidation of alkenes and the hydroxylation of alkanes. About 30 years ago, Traylor and Tsuchyia [163] presented the first synthesis of porphyrins with more stability and more efficient catalytic activity due to the introduction of electronegativity and/or bulky auxiliaries groups such as halogen, nitro or sulfonate at the *meso* and/or β-pyrrolic positions, to obtain the second and third generation of porphyrin catalysts. Lately, the metal complexes like *meso*-tetrakis(penta fluorophenyl)porphyrin H<sup>2</sup> (TPFPP) represent alternative possibilities to structural modification of porphyrins by nucleophilic substitution of its fluorine atoms [164, 165]. The second generation of porphyrins, especially the manganese (II) and iron(III) porphyrins is the most important representatives as catalysts in the epoxidation of alkenes (cyclohexane, adamantane, or n-hexane). In this case, during the epoxidation reactions, Mn and Fe ions can accept active species from different substrates and oxygen atom donors that result in metal-oxo species formation. In some conditions, the catalytic efficiency of iron(III) porphyrins can be limited due to the presence of some by-products resulted from the epoxidation of alkenes, for example to the allylic oxidation reactions. In anadamant oxidation reaction, the catalytic reaction of manganese porphyrins (MnPor) derived from 5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin had an increased product yield of 1-adamantanol than those obtained with [Fe3+TPPCl] catalyst [166]. The MnPors can exhibit different behaviors regarding the electron-withdrawing substituents in the macrocycle structure. Doro et al. [167] revealed that MnPors had lower catalysis efficient than the second generation of catalyst [Mn3+PFTDCPP]Cl due the high-valence active species caused by the electronegativity of the substituents (fluoro and chlorine) at the *meso*-aryl positions of the macrocycle in [Mn3+PFTDCPP]Cl. Consistently with this observation, Rayati et al. [168] made a comparative catalytic study of two partially brominated MnPs, namely [Mn3+Br4 TPP]Cl and [Mn3+Br4 T4 (-OME)PP]Cl revealing that the electron-deficient Mnps were a better catalyst than electron-rich MnPs. Lately, new materials of metalloporphyrin catalysts supported on mesoporous silica have shown a high efficiency of stability and reaction conditions. Poltowicz et al. [169] have studied the supported MnTMPyP catalysts on aluminated MCM-41 and SBA-15 mesoporous to investigate the oxidation of cyclooctane with molecular oxygen (as air) without the use of sacrificial co-reductant. Due to the existence diffusion limitations within the pore inner space, the supported MnTMPyP had increased the catalysis activity in the SBA-15 mesoporous because it exhibits increased-size pore. The catalytic activity of porphyrins, including TMPyP, allows the use of these compounds in sensing. Porphyrins can form complexes with almost all metals, and consequently, a broad diversity of catalytic properties can be achieved. The central metal in porphyrins determines the affinity for additional ligands. In general, the complex of Cu2+ and Ni2+ has low affinity for additional ligands. The Mg3+, Cd2+ and Zn2+ porphyrins form pentacoordinate complexes with square-pyramidal structure. The metalloporphyrins with (Fe2+, Co2+, Mn2+) in the central position produce distorted octahedral structure with two axial ligands. Metallo *meso* tetrakis porphyrins have been extensively used in the voltammetric determination of oxygen, NO, sugars, organohalides, DNA, alcohols, dopamine and others. Therefore, due to their switchable structures and a diversity of catalytic properties, porphyrins are widely used in analytical chemistry. A diversity of porphyrins can be applied biosensors and as stationary phases in HPLC.
