Excited State Properties of Diiron Dithiolate Hydrides: Implications in the Unsensitized Photocatalysis of H₂ Evolution

Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to investigate how visible light photons can excite an asymmetrically substituted diiron hydride, [Fe₂(pdt)­(μ-H)­(CO)₄dppv]⁺ (<b>1</b>^<b>+</b>, dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂; pdt = 1,3-propanedithiolate), as well as the symmetric species [Fe₂(pdt)­(μ-H)­(CO)₄(PMe₃)₂]⁺ (<b>2</b>^<b>+</b>), which are the first photocatalysts of proton reduction operating without employing sensitizers (Wang, W.; Rauchfuss, T. B.; Bertini, L.; Zampella, G.; <i>J. Am. Chem. Soc.</i>, <b>2012</b>, <i>134</i>, 4525). Theoretical results illustrate that the peculiar reactivity associated to the excited states of <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> is compatible with three different scenarios: (i) it can arise from the movement of the hydride ligand from fully bridging to semibridging/terminal coordination, which is expected to be more reactive toward protons; (ii) reactivity could be related to cleavage of a Fe–S bond, which implies formation of a transient Fe penta-coordinate species that would trigger a facile turnstile hydride isomerization, if lifetime excitation is long enough; (iii) also in line with a Fe–S bond cleavage is the possibility that after excited state decay, a highly basic S center is protonated so that a species simultaneously containing S–H^δ+ and Fe–H^δ− moieties is formed and, once reduced by a suitable electron donor, it can readily afford H₂ plus an unprotonated form of the FeFe complex. This last possibility is consistent with ³¹P NMR and IR solution data. All the three possibilities are compatible with the capability of <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> to perform photocatalysis of hydrogen evolving reaction (HER) without sensitizer. Moreover, even though it turned out difficult to discriminate among the three scenarios, especially because of the lack of experimental excitation lifetimes, it is worth underscoring that all of the three pathways represent a novelty regarding diiron carbonyl photoreactivity, which is usually associated with CO loss. Results provide also a rationale to the experimental observations which showed that the simultaneous presence of donor ligands (dppv in the case of <b>1</b>^<b>+</b>) and a H ligand in the coordination environment of diiron complexes is a key factor to prevent CO photodissociation and catalyze HER. Finally, the comparison of photoexcitation behavior of <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> allows a sort of generalization about the functioning of such hydride species

Photophysical Properties of S, Se and Te-Substituted Deoxyguanosines: Insight into Their Ability To Act as Chemotherapeutic Agents

Guanine and guanosine derivatives have long been in use as anticancer drugs and recently have been proposed also as photosensitizers in photodynamic therapy. By means of density functional theory and its time-dependent formulation, the potential power as UVA chemotherapeutic agents has been investigated computing the photophysical properties (absorption spectra, excitation energies, and spin–orbit matrix elements) of sulfur, selenium, and tellurium-substituted deoxyguanosines. Different pathways for the population of the lowest triplet state have been considered. Results show that all the examined systems have the lowest triplet state lying above the energy required for the production of the highly cytotoxic excited molecular oxygen ¹Δ_g and that the heavy atom effect ensures an efficient intersystem spin crossing

Unsensitized Photochemical Hydrogen Production Catalyzed by Diiron Hydrides

The diiron hydride [(μ-H)­Fe₂(pdt)­(CO)₄(dppv)]⁺ ([H<b>2</b>]⁺, dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂) is shown to be an effective photocatalyst for the H₂ evolution reaction (HER). These experiments establish the role of hydrides in photocatalysis by biomimetic diiron complexes. Trends in redox potentials suggests that other unsymmetrically substituted diiron hydrides are promising catalysts. Unlike previous catalysts for photo-HER, [H<b>2</b>]⁺ functions without sensitizers: irradiation of [H<b>2</b>]⁺ in the presence of triflic acid (HOTf) efficiently affords H₂. Instead of sacrificial electron donors, ferrocenes can be used as recyclable electron donors for the photocatalyzed HER, resulting in 4 turnovers

DFT Dissection of the Reduction Step in H₂ Catalytic Production by [FeFe]-Hydrogenase-Inspired Models: Can the Bridging Hydride Become More Reactive Than the Terminal Isomer?

Density functional theory has been used to study diiron dithiolates [HFe₂(xdt)­(PR₃)_<i>n</i>(CO)_5–<i>n</i>X] (<i>n</i> = 0, 2, 4; R = H, Me, Et; X = CH₃S^–, PMe₃, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt = azadithiolate; pdt = propanedithiolate). These species are related to the [FeFe]-hydrogenases catalyzing the 2H⁺ + 2e^– ↔ H₂ reaction. Our study is focused on the reduction step following protonation of the Fe₂(SR)₂ core. Fe­(H)­s detected in solution are terminal (t-H) and bridging (μ-H) hydrides. Although unstable versus μ-Hs, synthetic t-Hs feature milder reduction potentials than μ-Hs. Accordingly, attempts were previously made to hinder the isomerization of t-H to μ-H. Herein, we present another strategy: in place of preventing isomerization, μ-H could be made a stronger oxidant than t-H (<i>E</i>°_μ‑H > <i>E</i>°_t‑H). The nature and number of PR₃ unusually affect Δ<i>E</i>°_{t‑H−μ‑H}: 4PEt₃ models feature a μ-H with a milder <i>E</i>° than t-H, whereas the 4PMe₃ analogues behave oppositely. The correlation Δ<i>E</i>°_{t‑H−μ‑H} ↔ stereoelectronic features arises from the steric strain induced by bulky Et groups in 4PEt₃ derivatives. One-electron reduction alleviates intramolecular repulsions only in μ-H species, which is reflected in the loss of bridging coordination. Conversely, in t-H, the strain is retained because a bridging CO holds together the Fe₂ core. That implies that <i>E</i>°_μ‑H > <i>E</i>°_t‑H in 4-PEt₃ species but not in 4PMe₃ analogues. Also determinant to observe <i>E</i>°_μ‑H > <i>E</i>°_t‑H is the presence of a Fe apical σ-donor because its replacement with a CO yields <i>E</i>°_μ‑H < <i>E</i>°_t‑H even in 4PEt₃ species. Variants with neutral NHC and PMe₃ in place of CH₃S^– still feature <i>E</i>°_μ‑H > <i>E</i>°_t‑H. Replacing pdt with (Hadt)⁺ lowers <i>E</i>° but yields <i>E</i>°_μ‑H < <i>E</i>°_t‑H, indicating that μ-H activation can occur to the detriment of the overpotential increase. In conclusion, our results indicate that the electron richness of the Fe₂ core influences Δ<i>E</i>°_{t‑H−μ‑H}, provided that (i) the R size of PR₃ must be greater than that of Me and (ii) an electron donor must be bound to Fe apically

Mechanistic Insight into Electrocatalytic H₂ Production by [Fe₂(CN){μ-CN(Me)₂}(μ-CO)(CO)(Cp)₂]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe₂(CN)­{μ-CN­(Me)₂}­(μ-CO)­(CO)­(Cp)₂] (<b>1</b>) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN^– and amino-carbyne ligands have been elucidated. After the first reduction, CN^– works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN^– that forms a Fe­(CNH) moiety: i.e., the acidic source for the H₂ generation. The hydride (formally 2e/H⁺), necessary to the heterocoupling with H⁺ is thus provided by the μ-CN­(Me)₂ ligand and not by Fe centers, as occurs in typical L₆Fe₂S₂ derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN^– plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN^– to CN­(Me)₂ is highlighted. The incompetence for the HER of the related species [Fe₂{μ-CN­(Me)₂}­(μ-CO)­(CO)₂(Cp)₂]⁺ (<b>2</b>^<b>+</b>) has been investigated and attributed to the loss of proton responsiveness caused by CN^– replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe₂ core without loss of HER, but the redox and acid–base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe₂ scaffold, could be exploited to develop more active and robust catalysts for the HER

Computational Investigation on the Spectroscopic Properties of Thiophene Based Europium β‑Diketonate Complexes

The adiabatic transition energies from the lowest triplet states of four Europium tris β-diketonate/phenantroline complexes have been determined in vacuo and in dicholomethane solution by the ΔSCF approach at the density functional theory level, using the PBE1PBE and the CAM-B3LYP hybrid functionals. The calculated adiabatic transition energies have been compared with the experimental 0–0 transitions of each complex determined from phosphorescence spectra of the corresponding Gd³⁺ complexes and followed by direct comparison between simulated and experimental spectra line shapes.For compound <b>1</b>, the Eu­(TTA)₃Phen system, triplet states other than the lowest one and conformational isomers other than the one present in the crystallographic structure have been considered. In the crystallographic structure, this compound presents three quasi-degenerate low energy triplet states, differing for the TTA ligand where the two unpaired electrons are localized and showing close adiabatic transition energies. For compound <b>1</b>, the lowest triplet states of the four investigated conformational isomers show similar characteristics and close adiabatic transition energies. On the basis of these results, an investigation of compounds <b>2</b>–<b>4</b> (Eu­(Br-TTA)₃Phen, Eu­(DTDK)₃Phen, and Eu­(MeT-TTA)₃) has been performed by considering only the isomer present in the crystallographic structure and only the lowest triplet state of each compound. For compounds <b>1</b>–<b>3</b>, the energies of the lowest triplet states calculated by both functionals in solution including zero-point energy corrections well reproduce the experimental trends as well as the values of the adiabatic transition energies: CAM-B3LYP, the best performing functional, provides energies of the lowest triplet state with deviations from experiments lower than 1200 cm^–1. Also, the calculated vibrationally resolved phosphorescence spectra and UV–vis absorptions well reproduce the main features of their experimental counterparts. Significant differences between calculated and experimental results are observed for compound <b>4</b>, for which difficulties in the experimental determination of the triplet state energy were encountered: our results show that the negligible photoluminescence quantum yield of this compound is due to the fact that the energy of the most stable triplet state is significantly lower than that of the resonance level of the Europium ion, and thus the energy transfer process is prevented.These results confirm the reliability of the adopted computational approach in calculating the energy of the lowest triplet state energy of these systems, a key parameter in the design of new ligands for lanthanide complexes presenting large photoluminescence quantum yields

Toward hydrophobic carminic acid derivatives and their inclusion in polyacrylates.

<p>Carminic acid, a natural hydrophilic dye extensively used in food and cosmetic industries, is converted in hydrophobic dyes by acetylation or pivaloylation. These derivatives are successfully used as biocolorants for polyacrylate objects. Spectroscopic properties of the carminic acid derivatives in DMSO and in polybutylacrylate are studied by means of Photoluminescence and Time Resolved Photoluminescence decays, revealing an hypsochromic effect due to presence of bulky substituents as the acetyl or pivaloyl groups. Molecular Mechanics and Density Functional Theory (DFT) calculations confirm the disruption of planarity between the sugar ring and the anthraquinoid system determined by the esterification. </p