Application of TD-DFT Theory to Studying Porphyrinoid-Based Photosensitizers for Photodynamic Therapy: A Review

An important focus for innovation in photodynamic therapy (PDT) is theoretical investigations. They employ mostly methods based on Time-Dependent Density Functional Theory (TD-DFT) to study the photochemical properties of photosensitizers. In the current article we review the existing state-of-the-art TD-DFT methods (and beyond) which are employed to study the properties of porphyrinoid-based systems. The review is organized in such a way that each paragraph is devoted to a separate aspect of the PDT mechanism, e.g., correct prediction of the absorption spectra, determination of the singlet–triplet intersystem crossing, and interaction with molecular oxygen. Aspects of the calculation schemes are discussed, such as the choice of the most suitable functional and inclusion of a solvent. Finally, quantitative structure–activity relationship (QSAR) methods used to explore the photochemistry of porphyrinoid-based systems are discussed

Application of TD-DFT Theory to Studying Porphyrinoid-Based Photosensitizers for Photodynamic Therapy: A Review

An important focus for innovation in photodynamic therapy (PDT) is theoretical investigations. They employ mostly methods based on Time-Dependent Density Functional Theory (TD-DFT) to study the photochemical properties of photosensitizers. In the current article we review the existing state-of-the-art TD-DFT methods (and beyond) which are employed to study the properties of porphyrinoid-based systems. The review is organized in such a way that each paragraph is devoted to a separate aspect of the PDT mechanism, e.g., correct prediction of the absorption spectra, determination of the singlet–triplet intersystem crossing, and interaction with molecular oxygen. Aspects of the calculation schemes are discussed, such as the choice of the most suitable functional and inclusion of a solvent. Finally, quantitative structure–activity relationship (QSAR) methods used to explore the photochemistry of porphyrinoid-based systems are discussed

Comparison of Catalytic Properties of Vanadium Centers Introduced into BEA Zeolite and Present on (010) V2O5 Surface–DFT Studies

Vanadium-based catalysts, in which vanadium is present either as bulk V2O5 or as isolated species, are active in numerous oxidation reactions. In the present study, vanadium speciation and the possibility of its introduction in various forms (V=O, V–OH, V(=O)(–OH)) into the structurally different crystallographic positions in BEA zeolite was considered by means of Density Functional Theory (DFT). Out of nine nonequivalent positions, T2 and T3 positions are the most preferred. The former may accommodate V=O or V–OH, the latter V–OH or V(=O)(–OH). The structural and electronic properties of all possible centers present in the BEA zeolite are then compared with the characteristics of the same species on the most abundant (010) V2O5 surface. It is demonstrated that they exhibit higher nucleophilic character when introduced into the zeolite, and thus, may be more relevant for catalysis

Effects of heavy central metal on the ground and excited states of chlorophyll

Chlorophylls, owing to their adjustable p-electron system and intense, well-separated electronic transitions, can serve as convenient intrinsic spectroscopic probes of ligand–metal center interactions. They are also interesting for their photosensitizing properties. In order to examine the heavy-atom effects on the chlorophyll triplet state, a key intermediate in chlorophyll–photosensitized reactions, the synthesis of a novel Pt(II)-substituted chlorophyll a was carried out, and the effects of the substitution on steady-state and transient photophysical properties of chlorophyll were studied by absorption and fluorescence spectroscopies, and by laser flash photolysis. The presence of highly electronegative platinum as the central ion increases the energies of the chlorophyll main absorption transitions. As laser flash photolysis experiments show, in air-equilibrated solutions, chlorophyll triplets are efficiently quenched by molecular oxygen. Interestingly, this quenching by oxygen is more effective with metal-containing pigments, in spite of the increased spin–orbit coupling, introduced with the central metals. This points to occurrence of nonspecific interactions of molecular oxygen with metallochlorophylls. The differences in the effects exerted on the pigment triplet by the central metal become distinct after the removal of oxygen. The lifetime of a Ptchlorophyll triplet remains very short, in the range of only a few microseconds, unlike in the free-base and Mg- and Zn-substituted chlorophylls. Such drastic shortening of the triplet lifetime can be attributed to a large heavy-atom effect, implying that strong interactions must occur between the central Pt(II) ion and the chlorophyll macrocycle, which lead to a more efficient spin–orbit coupling in Pt-chlorophyll than in Pt-porphyrins

Mechanistic insight into peroxo-shunt formation of biomimetic models for compound II, their reactivity toward organic substrates and the influence of N-methylimidazole axial ligation

High-valent iron-oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)FeIII(OH) porphyrin ([meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H2O2 with (TMPS)FeIII(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)FeIV[DOUBLE BOND]O(OH) point to the overall associative nature of the process. A pH-dependence study on the formation of (TMPS)FeIV[DOUBLE BOND]O(OH) revealed a very high reactivity of OOH− toward (TMPS)FeIII(OH) in comparison to H2O2. The influence of N-methylimidazole (N-MeIm) ligation on both the formation of iron(IV)-oxo species and their oxidising properties in the reactions with 4-methoxybenzyl alcohol or 4-methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)FeIII(H2O)(N-MeIm) is highly reactive toward H2O2 to form the iron(IV)-oxo species, (TMPS)FeIV[DOUBLE BOND]O(N-MeIm). The latter species can also be formed in the reaction of (TMPS)FeIII(N-MeIm)2 with H2O2 or in the direct reaction of (TMPS)FeIV[DOUBLE BOND]O(OH) with N-MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)FeIV[DOUBLE BOND]O(OH) and (TMPS)FeIV[DOUBLE BOND]O(N-MeIm) do not display a pronounced effect of the N-MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH− substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH− or N-MeIm) in the trans position to the oxo group in the iron(IV)-oxo species does not significantly affect the activation barriers calculated for C[BOND]H dehydrogenation of the selected organic substrates

Temperature and Pressure Effects on C–H Abstraction Reactions Involving Compound I and II Mimics in Aqueous Solution

The presented results cover a comparative mechanistic study on the reactivity of compound (Cpd) I and II mimics of a water-soluble iron­(III) porphyrin, [<i>meso</i>-tetrakis­(2,4,6-trimethyl-3-sulfonatophenyl)­porphinato]­iron­(III), Fe^III(TMPS). The acidity of the aqueous medium strongly controls the chemical nature and stability of the high-valent iron­(IV) oxo species. Reactivity studies were performed at pH 5 and 10, where the Cpd I and II mimics are stabilized as the sole oxidizing species, respectively. The contributions of Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧ to the free energy of activation (Δ<i>G</i>^⧧) for the oxidation of 4-methoxybenzaldehyde (4-MB-ald), 4-methoxybenzyl alcohol (4-MB-alc), and 1-phenylethanol (1-PhEtOH) by the Cpd I and II mimics were determined. The relatively large contribution of the Δ<i>H</i>^⧧ term in comparison to the −<i>T</i>Δ<i>S</i>^⧧ term to Δ<i>G</i>^⧧ for reactions involving the Cpd II mimic indicates that the oxidation of selected substrates by this oxidizing species is clearly an enthalpy-controlled process. In contrast, different results were found for reactions with application of the Cpd I mimic. Depending on the nature of the substrate, the reaction at room temperature can be entropy-controlled, as found for the oxidation of 4-MB-alc, or enthalpy-controlled, as found for 1-PhEtOH. Importantly, for the first time, activation volumes (Δ<i>V</i>^⧧) for the oxidation of selected substrates by both reactive intermediates could be determined. Positive values of Δ<i>V</i>^⧧ were found for reactions with the Cpd II mimic and slightly negative ones for reactions with the Cpd II mimic. The results are discussed in the context of the oxidation mechanism conducted by the Cpd I and II mimics