Elena Jakubikova

Calculation of one-electron redox potentials revisited. Is it possible to calculate accurate potentials with density functional methods?

Density Functional calculations have been performed to calculate the one-electron oxidation potential for ferrocene and the redox couples for a series of small transition metal compounds of the first-, second-, and third-row elements. The solvation effects are incorporated via a self-consistent reaction field (SCRF), using the polarized continuum model (PCM). From our study of seven different density functionals combined with three different basis sets for ferrocene, we find that no density functional method can reproduce the redox trends from experiment when referencing our results to the experimental absolute standard hydrogen electrode (SHE) potential. In addition, including additional necessary assumptions such as solvation effects does not lead to any conclusion regarding the appropriate functional. However, we propose that if one references their transition metal compounds results to the calculated absolute half-cell potential of ferrocene, they can circumvent the additional assumptions necessary to predict a redox couple. Upon employing this method on several organometallic and inorganic complexes, we obtained very good correlation between calculated and experimental values (R(2) = 0.97), making it possible to predict trends with a high level of confidence. The hybrid functional B3LYP systematically underestimates the redox potential; however, the linear correlation between DFT and experiment is good (R(2) = 0.96) when including a baseline shift. This protocol is a powerful tool that allows theoretical chemists to predict the redox potential in solution of several transition metal complexes a priori and aids in the rational design of redox-active catalysts.

Low-Spin versus High-Spin Ground State in Pseudo-Octahedral Iron Complexes

Pseudo-octahedral complexes of iron find applications as switches in molecular electronic devices, materials for data storage, and, more recently, as candidates for dye-sensitizers in dye-sensitized solar cells. Iron, as a first row transition metal, provides a weak ligand-field splitting in an octahedral environment. This results in the presence of low-lying (5)T excited states that, depending on the identity of iron ligands, can become the ground state of the complex. The small energy difference between the low-spin, (1)A, and high-spin, (5)T, states presents a challenge for accurate prediction of their ground state using density functional theory. In this work, we investigate the applicability of the B3LYP functional to the ground state determination of first row transition metal complexes, focusing mainly on Fe(II) polypyridine complexes with ligands of varying ligand field strength. It has been shown previously that B3LYP artificially favors the (5)T state as the ground state of Fe(II) complexes, and the error in the energy differences between the (1)A and (5)T states is systematic for a set of structurally related complexes. We demonstrate that structurally related complexes can be defined as pseudo-octahedral complexes that undergo similar distortion in the metal-ligand coordination environment between the high-spin and low-spin states. The systematic behavior of complexes with similar distortion can be exploited, and the ground state of an arbitrary Fe(II) complex can be determined by comparing the calculated energy differences between the singlet and quintet electronic states of a complex to the energy differences of structurally related complexes with a known, experimentally determined ground state.

Reactions of Sulfur Dioxide with Neutral Vanadium Oxide Clusters in the Gas Phase. II. Experimental Study Employing Single-Photon Ionization

Single-photon ionization through vacuum ultraviolet (VUV, 10.5 eV) and soft X-ray (extreme ultraviolet, EUV, 26.5 eV) laser radiation is successfully employed for the study of the reactions of neutral vanadium oxide clusters (V(m)O(n)) with sulfur dioxide (SO2) in the gas phase. V(m)O(n) clusters are generated by reaction of a laser-generated vanadium plasma with O2 in a supersonic expansion. The clusters are cooled in the expansion and are reacted with SO2 in a fast-flow reactor. Detection of neutral clusters and products is through ionization employing VUV and EUV laser radiation and time-of-flight mass spectrometry. Many association reaction intermediates [V(m)O(n)SO2 and V2O4(SO2)2] are observed. Isolated SO is also observed, as a product as predicted by theoretical studies presented in part I (J. Phys. Chem. A 2007, 111, 13339). A weak feature at the SO3 mass channel (80 amu) is suggested to be present in the product mass spectra. Further reactions of the intermediates with O2 are positively identified for VO2SO2, V3O7SO2, and V5O10SO2. Reaction mechanisms are interpreted on the basis of the observations and preliminary theoretical calculations. Molecular level reaction mechanisms for oxidation of SO2 to SO3 facilitated by condensed-phase vanadium oxides as catalysts are suggested.

Comparison of the Reactivity of 2‐Li‐C6H4CH2NMe2 with MCl4 (M=Th, U): Isolation of a Thorium Aryl Complex or a Uranium Benzyne Complex

Why do U react like that? Reaction of 2-Li-C6H4CH2NMe2 with [MCl4(DME)n] (M=Th, n=2; M=U, n=0) results in the formation of a thorium aryl complex, [Th(2-C6H4CH2NMe2)4] or a uranium benzyne complex, [Li][U(2,3-C6H3CH2NMe2)(2-C6H4CH2NMe2)3]. A DFT analysis suggests that the formation of a benzyne complex with U but not with Th is a kinetic and not thermodynamic effect.

Electron injection from copper diimine sensitizers into TiO2: Structural effects and their implications for solar energy conversion devices

Copper(I) diimine complexes have emerged as low cost replacements for ruthenium complexes as light sensitizers and electron donors, but their shorter metal-to-ligand-charge-transfer (MLCT) states lifetimes and lability of transient Cu(II) species impede their intended functions. Two carboxylated Cu(I) bis-2,9-diphenylphenanthroline (dpp) complexes [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(COOH)2)](+) and [Cu(I)(dpp-O(CH2CH2O)5)(dpp-(Φ-COOH)2)](+) (Φ = tolyl) with different linker lengths were synthesized in which the MLCT-state solvent quenching pathways are effectively blocked, the lifetime of the singlet MLCT state is prolonged, and the transient Cu(II) ligands are stabilized. Aiming at understanding the mechanisms of structural influence to the interfacial charge transfer in the dye-sensitized solar cell mimics, electronic and geometric structures as well as dynamics for the MLCT state of these complexes and their hybrid with TiO2 nanoparticles were investigated using optical transient spectroscopy, X-ray transient absorption spectroscopy, time-dependent density functional theory, and quantum dynamics simulations. The combined results show that these complexes exhibit strong absorption throughout the visible spectrum due to the severely flattened ground state, and a long-lived charge-separated Cu(II) has been achieved via ultrafast electron injection (<300 fs) from the (1)MLCT state into TiO2 nanoparticles. The results also indicate that the TiO2-phen distance in these systems does not have significant effect on the efficiency of the interfacial electron-transfer process. The mechanisms for electron transfer in these systems are discussed and used to develop new strategies in optimizing copper(I) diimine complexes in solar energy conversion devices.

Cyclometalated Fe(II) Complexes as Sensitizers in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) often utilize transition metal-based chromophores for light absorption and semiconductor sensitization. Ru(II)-based dyes are among the most commonly used sensitizers in DSSCs. As ruthenium is both expensive and rare, complexes based on cheaper and more abundant iron could serve as a good alternative. In this study, we investigate Fe(II)-bis(terpyridine) and its cyclometalated analogues, in which pyridine ligands are systematically replaced by aryl groups, as potential photosensitizers in DSSCs. We employ density functional theory at the B3LYP/6-31G*,SDD level to obtain the ground state electronic structure of these complexes. Quantum dynamics simulations are utilized to study interfacial electron transfer between the Fe(II) photosensitizers and a titanium dioxide semiconductor. We find that cyclometalation stabilizes the singlet ground state of these complexes by 8-19 kcal/mol but reduces the electron density on the carboxylic acid attached to the aryl ring. The results suggest that cyclometalation provides a feasible route to increasing the efficiency of Fe(II) photosensitizers but that care should be taken in choosing the substitution position for the semiconductor anchoring group.

Electronic Structure and Absorption Properties of Strongly Coupled Porphyrin-Perylene Arrays.

Porphyrin-perylene arrays are ideal candidates for light-harvesting systems capable of panchromatic absorption. In this work, we employ density functional theory (DFT) and time-dependent DFT to investigate the unique UV-vis absorption properties exhibited by a series of ethynyl-linked porphyrin-perylene arrays that were previously synthesized and characterized spectroscopically [Chem. Commun. 2014, 50, 14512-5]. We find that the ethynyl linker is responsible for strong electronic coupling of porphyrin and perylene subunits in these systems. Additionally, these arrays exhibit a low barrier to rotation around the ethynyl linker (<1.4 kcal/mol per one perylene substituent), which results in a wide range of molecular conformations characterized by different porphyrin-perylene dihedral angles being accessible at room temperature. The best match between the calculated and experimental UV-vis spectra is obtained by averaging the calculated UV-vis spectra over the range of conformations defined by the porphyrin-perylene dihedral angles. Finally, our calculations suggest that the transitions in the lower energy region (550-750 nm) can be assigned to the excitations originating from the porphyrin subunit; the mid-energy region transitions (450-550 nm) are assigned to the perylene-centered excitations, while the high-energy transitions (350-450 nm) involve contributions from both porphyrin and perylene subunits.

Effects of Peripheral and Axial Substitutions on Electronic Transitions of Tin Naphthalocyanines

Tin naphthalocyanine molecules display strong absorption in the infrared region (IR), making them ideal as components of organic photodiodes and solar cells. We use density functional theory and time-dependent density functional theory (TD-DFT) at the B3LYP level to study the influence of axial and peripheral ligands on the absorption wavelength of tin naphthalocyanines. We find that TD-DFT is successful at reproducing the experimental absorption spectra of free base naphthalocyanine and tin naphthalocyanine molecules and can be used as a reliable tool to predict absorption spectra of substituted naphthalocyanines. Functional groups attached axially to tin (-F, -Cl, -Br, -I) and peripherally to the inner ring (-F, -Cl, -Br, -Ph, -OH, -COCH(3), -O(CH(2))(3)CH(3)) of the tin naphthalocyanine molecule tune the excitation wavelength in the near-infrared region between 770 and 940 nm. While substituents to the outer naphthalocyanine ring (-Cl, -Br) affect the intensity of the absorption peaks in the NIR region, they do not influence their absorption wavelength. Asymmetric substitution of naphthalocyanine pendant arms can be employed to decrease intensity of the absorption peaks in the visible region with respect to the intensity of the peaks in the NIR.

Quantum Dynamics Simulations of Excited State Energy Transfer in a Zinc–Free-Base Porphyrin Dyad

Rational design of artificial light-harvesting molecular architectures entails building systems that absorb strongly in the visible and near-IR region of the electromagnetic spectrum and also funnel excited state energy to a single site. The ability to model nonadiabatic processes, such as excited-state energy transfer (EET), that occur on a picosecond time scale can aid in the development of novel artificial light-harvesting arrays. A combination of density functional theory (DFT), time-dependent DFT, tight-binding molecular dynamics, and quantum dynamics is employed here to simulate EET in the ZnFbΦ dyad, a model artificial light-harvesting array that undergoes EET with an experimentally measured rate constant of (3.5 ps)-1 upon excitation at 550 nm in toluene [ Yang et al. J. Phys. Chem. B 1998 , 102 , 9426 - 9436 ]. We find that to successfully simulate the EET process, it is important to (1) include coupling between nuclear and electronic degrees of freedom in the QD simulation, (2) account for Coulomb coupling between the electron and hole wavepackets, and (3) parametrize the extended Hückel model Hamiltonian employed in the QD simulations with respect to the DFT.

Role of noncoplanar conformation in facilitating ground state hole transfer in oxidized porphyrin dyads.

We employ density functional theory to investigate ground state hole transfer in covalently linked oxidized zinc-zinc porphyrin ([ZnZn](+)) and zinc-free-base porphyrin ([ZnFb](+)) dyads in both coplanar and noncoplanar (tilted) conformations. We obtain reactant, product, and transition state (TS) for the hole transfer reaction in the [ZnZn](+) system. The hole is localized on a single porphyrin unit in the reactant and product states while delocalized in the TS, implying the dominance of superexchange mechanism in the hole transfer reaction. A metastable as well as stable states are located for the [ZnFb](+) system while no TS is found, indicating a barrierless hole transfer reaction. The hole lifetimes are calculated to be 15.80 and 0.034 ns for [ZnZn](+) in the coplanar and tilted conformation, respectively, and 14.45 and 0.313 ns for [ZnFb](+). The hole transfer rates are found to be several orders of magnitude faster in the tilted conformation than in the coplanar conformation for both dyads, showing the importance of noncoplanar conformation between the two porphyrin pigments in facilitating the hole transfer process. We also show that inclusion of solvent effects in calculations plays an important role in the proper ground state hole localization in oxidized dyads. These results provide an unconventional insight into the hole transfer mechanism in porphyrin arrays and are relevant to design of artificial photoharvesting materials.