Backward Charge Transfer Research Articles

Bidirectional Charge Transfer at the Heme Iron in Reversible and Quasi-irreversible Cytochrome P450 Inhibition.

The coordination bonding between inhibitor ligands and heme iron plays a critical role in disrupting the essential catalytic functions of cytochrome P450 enzymes (P450s). Despite its intrinsic importance and consequential implications for human health, our current understanding of coordination bonding in P450 inhibition remains limited. To address this knowledge gap, we conducted a systematic theoretical analysis of the complexes between a ferric or a ferrous heme model and representative inhibitor ligands. Specifically, we evaluated the charge-transfer (CT) effect within these complexes by employing a series of theoretical methods based on density functional theory (DFT). Through a comprehensive analysis, we unveiled the relative significance of ligand-to-heme forward CT in the ferric and ferrous complexes of reversible inhibitors. In contrast, backward CT dominates over forward CT in the ferrous heme complexes of quasi-irreversible inhibitors. Further analysis using the compact frontier orbital method underscores the elevated electron-accepting abilities of quasi-irreversible inhibitors for π backdonation, which greatly amplifies their binding affinity for the ferrous heme. This study sheds light on the intricate mechanisms underlying P450 inhibition and provides valuable insights for future inhibitor design and development.

Identification and Quantification of Charge Transfer in CaAl2O4:Eu2+,Nd3+ Persistent Phosphor

AbstractReversible charge transfer between lanthanide ions is identified as storage mechanism for the current workhorse persistent phosphors. Present evidence relies on sophisticated X‐ray absorption spectroscopy to detect the reversible charge transfer. Here, simple optical spectroscopy is used to study the charge transfer in one of the benchmark persistent phosphors CaAl2O4:Eu2+,Nd3+. Based on the observation that both the trapping and de‐trapping processes in CaAl2O4:Eu2+,Nd3+ are thermally activated, forward charge transfer from Eu2+ to Nd3+ and backward charge transfer from Nd2+ to Eu3+ are identified. The percentages of Eu2+ and Nd3+ involved in the charge transfer are >10%, which exceeds the previous estimates in other persistent phosphors. Furthermore, this strategy offers additional advantage of site selectivity, which enables the identification of the distinct contributions of different Nd3+ sites to the charge transfer. These findings underline the significance of reversible charge transfer in persistent phosphors and move towards a complete understanding of persistent luminescence mechanism.

Photocatalytic water splitting with a quantum efficiency of almost unity.

Overall water splitting, evolving hydrogen and oxygenin a 2:1 stoichiometric ratio, using particulate photocatalysts is a potential means of achieving scalable and economically viable solar hydrogen production. To obtain high solar energy conversion efficiency, the quantum efficiency of the photocatalytic reaction must be increased over a wide range of wavelengths and semiconductors with narrow bandgaps need to be designed. However, the quantum efficiency associated with overall water splitting using existing photocatalysts is typically lower than ten per cent1,2. Thus, whether a particulate photocatalyst can enable a quantum efficiency of 100 per cent for the greatly endergonic water-splitting reaction remains an open question. Here we demonstrate overall water splitting at an external quantum efficiency of up to 96 per cent at wavelengths between 350 and 360 nanometres, which is equivalent to an internal quantum efficiency of almost unity, using a modified aluminium-doped strontium titanate (SrTiO3:Al) photocatalyst3,4. By selectively photodepositing the cocatalysts Rh/Cr2O3 (ref. 5) and CoOOH (refs. 3,6) for the hydrogen and oxygen evolution reactions, respectively, on different crystal facets of the semiconductor particles using anisotropic charge transport, the hydrogen and oxygen evolution reactions could be promoted separately. This enabled multiple consecutive forward charge transfers without backward charge transfer, reaching the upper limit of quantum efficiency for overall water splitting. Our work demonstrates the feasibility of overall water splitting free from charge recombination losses and introduces an ideal cocatalyst/photocatalyst structure for efficient water splitting.

Variational Forward-Backward Charge Transfer Analysis Based on Absolutely Localized Molecular Orbitals: Energetics and Molecular Properties.

To facilitate the understanding of charge-transfer (CT) effects in dative complexes, we propose a variational forward-backward (VFB) approach to decompose the overall CT stabilization energy into contributions from forward and backward donation in the framework of energy decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA). Such a decomposition is achieved by introducing two additional constrained intermediate states in which only one direction of CT is permitted. These two "one-way" CT states are variationally relaxed such that the associated nuclear forces can be readily obtained. This allows for a facile integration into the previously developed adiabatic EDA scheme, so that the molecular property changes arising from forward and back donation can be separately assigned. Using ALMO-EDA augmented by this VFB model, we investigate the energetic, geometric, and vibrational features of complexes composed of CO and main group Lewis acids (BH3, BeO/BeCO3) and complexes of the N2, CO, and BF isoelectronic series with [Ru(II)(NH3)5]2+. We identify that the shift in the stretching frequency of a diatomic π-acidic ligand (XY), such as CO, results from a superposition of the shifts induced by permanent electrostatics and backward CT: permanent electrostatics can cause an either red or blue shift depending on the alignment of the XY dipole in the dative complex, and this effect becomes more pronounced with a more polar XY ligand; the back-donation to the antibonding π orbital of XY always lowers the X-Y bond order and thus red-shifts its stretching frequency, and the strength of this interaction decays rapidly with the intermolecular distance. We also reveal that while σ forward donation contributes significantly to energetic stabilization, it affects the vibrational feature of XY mainly by shortening the intermolecular distance, which enhances both the electrostatic interaction and backward CT but in different rates. The synergistic effect of the forward and backward donations appears to be more significant in the transition-metal complexes, where the forward CT plays an essential role in overcoming the strong Pauli repulsion. These findings highlight that the shift in the XY stretching frequency is not a reliable metric for the strength of π back-donation. Overall, the VFB-augmented EDA scheme that we propose and apply in this work provides a useful tool to characterize the role played by each physical component that all together lead to the frequency shift observed.

Controlling Magnetic Properties at BiFe1−xMnxO3/La2/3Ca1/3MnO3 Interfaces by Tuning the Spatial Distribution of Interfacial Electronic States

AbstractOne major challenge in engineering the magnetism of oxide heterostructures is controlling orbital reconstruction by tuning the charge transfer effect. This paper investigates the forward and backward charge transfer effect and the induced magnetic properties of BiFe1−xMnxO3/La2/3Ca1/3MnO3 heterostructures. Interfacial ferromagnetism is found to be qualitatively tunable by tuning the spatial distribution of interfacial electronic states. The subtle balance of magnetic coupling between FeOMn and MnOMn changed as the electronic structures are modified by charge transfer. The ions' valence is strongly correlated with the interfacial magnetic properties, extending the concept of the charge degrees of freedom affecting the magnetic coupling properties.

Molecular structure, spectroscopy, and photoinduced kinetics in trinuclear cyanide bridged complex in solution: a first-principles perspective.

We investigate the molecular structure of the solvated complex, [(NC)6Fe-Pt(NH3)4-Fe(CN)6](4-), and related dinuclear and mononuclear model complexes using first-principles calculations. Mixed nuclear complexes in both solution and crystal phases were widely studied as models for charge transfer (CT) reactions using advanced spectroscopical and electrochemical tools. In contrast to earlier interpretations, we find that the most stable gas phase and solvated geometries are substantially different from the crystal phase geometry, mainly due to variance in the underlying oxidation numbers of the metal centers. Specifically, in the crystal phase a Pt(IV) metal center resulting from Fe ← Pt backward electron transfers is stabilized by an octahedral ligand field, whereas in the solution phase a Pt(II) metal complex that prefers a square planar ligand field forms a CT salt by bridging to the iron complexes through long-range electrostatic interactions. The different geometry is shown to be consistent with spectroscopical data and measured CT rates of the solvated complex. Interestingly, we find that the experimentally indicated photoinduced process in the solvated complex is of backward CT (Fe ← Pt).

Novel Si Networks in the Ca/Si Phase Diagram under Pressure

The rich Ca/Si binary phase diagram is explored here theoretically and over a range of pressures, through four stoichiometries: Ca2Si, CaSi, CaSi2, and CaSi3. The calculations are calibrated by recovering the known P = 1 atm structures (even as they suggest metastable alternatives for some) and by predicting correctly the range of pressures at which CaSi3, which has a positive heat of formation at P = 1 atm, can be synthesized. Ca2Si, in accord with its Zintl compound nature, features isolated Si4– ions then chains until decomposition. CaSi, which begins as a Zintl phase (albeit metallic) chain structure evolves to rings and zeolitic-channel structures, while CaSi2 explores a variety of triply connected Si polytypes to Laves phases with increasing pressure. CaSi3 has a calculated narrow range of stability of 8–18 GPa. Interestingly, all the phases studied are calculated to decompose to the elements at P ∼ 250 GPa. This unusual behavior is traced to the special compressibility of elemental Ca and the backward charge transfer from Si to Ca. All the phases studied are metallic, except that Ca2Si is a semiconductor at 1 atm.

Charge Transfer from Regularized Symmetry-Adapted Perturbation Theory.

The charge-transfer (CT) together with the polarization energy appears at second and higher orders in symmetry-adapted perturbation theory (SAPT). At present there is no theoretically compelling way of isolating the charge-transfer energy that is simultaneously basis-set independent and applicable for arbitrary intermolecular separation. We argue that the charge-transfer can be interpreted as a tunneling phenomenon and can therefore be defined via regularized SAPT. This leads to a physically convincing, basis-independent definition of the charge-transfer energy that captures subtleties of the process, such as the asymmetry in the forward and backward charge transfer, as well as secondary transfer effects. With this definition of the charge-transfer the damping needed for polarization models can be determined with a level of confidence hitherto not possible.

Dipole-Assisted Charge Separation in Organic–Inorganic Hybrid Photovoltaic Heterojunctions: Insight from First-Principles Simulations

It has been observed that the external quantum efficiency (EQE) of a hybrid organic/inorganic (P3HT/ZnO) solar cell can be tripled by inserting a monolayer of fullerene (PCBA) at the donor (P3HT) and acceptor (ZnO) interface. First-principles simulations are performed to understand the origin of the increased EQE, offering a complementary perspective to the experiment. The interfacial atomic and electronic structure as well as energy alignment are examined for both the bare and PCBA-modified interface. The presence of PCBA induces an interfacial dipole and shifts up the lowest-unoccupied-molecular-orbital (LUMO) level of P3HT relative to the conduction band edge of ZnO. Interfacial electron dynamics including forward and backward charge transfer time scales are estimated. We find that the two interfaces have similar forward electron transfer rates but their transfer mechanisms differ. The PCBA-modified interface exhibits a weaker adiabatic but a stronger nonadiabatic charge transfer. In contrast, the charge recombination time scale of the modified interface is 20 times slower than that of the bare interface, thanks to reduced donor/acceptor wave function coupling as well as increased donor/acceptor energy offset. By slowing down the charge recombination, the PCBA-modified heterojunction offers the superior performance observed in the experiment.

Analysis of charge transfer effects in molecular complexes based on absolutely localized molecular orbitals

A new method based on absolutely localized molecular orbitals (ALMOs) is proposed to measure the degree of intermolecular electron density delocalization (charge transfer) in molecular complexes. ALMO charge transfer analysis (CTA) enables separation of the forward and backward charge transfer components for each pair of molecules in the system. The key feature of ALMO CTA is that all charge transfer terms have corresponding well defined energetic effects that measure the contribution of the given term to the overall energetic stabilization of the system. To simplify analysis of charge transfer effects, the concept of chemically significant complementary occupied-virtual orbital pairs (COVPs) is introduced. COVPs provide a simple description of intermolecular electron transfer effects in terms of just a few localized orbitals. ALMO CTA is applied to understand fundamental aspects of donor-acceptor interactions in borane adducts, synergic bonding in classical and nonclassical metal carbonyls, and multiple intermolecular hydrogen bonds in a complex of isocyanuric acid and melamine. These examples show that the ALMO CTA results are generally consistent with the existing conceptual description of intermolecular bonding. The results also show that charge transfer and the energy lowering due to charge transfer are not proportional to each other, and some interesting differences emerge which are discussed. Additionally, according to ALMO CTA, the amount of electron density transferred between molecules is significantly smaller than charge transfer estimated from various population analysis methods.

Backward Charge Transfer in Conjugated Polymers

It has been known that the static polarizability of apolymer chain with a biexciton is negative.In order to understand this peculiar fact, this paper studies thedynamical process of the charge transfer in the polymer chaininduced by an external electric field E during formingthe biexciton. The time dependence of the charge distribution inthe chain reveals that the charge transfer is backward: thepositive charge shifts in the opposite direction of the externalelectric field. Such a backward charge transfer (BCT) produces anopposite dipole, which makes the polarization negative. The effectof electron interaction on the BCT is illustrated.

Interaction of Self-Localized Excitations with C60 Weak Dopants in Conducting Polymers

Abstract Weak dopants, like C60, (which have LUMO level within the band gap of conducting polymer (CP)) may provide various types of interactions with self-localized (SL) excitations of CP chain: solitons (S˚, S), polaron/bipolarons (P/BP) or excitonpolarons (Ex), whose electronic levels are also located deeply inside the gap. In the present paper we analyse some types of interactions: 1) “Chemical reaction” type charge transfer (CT) processes between charged (P/BP, S) and neutral (S˚, Ex) excitations with C60 molecules intercallated between chains of CP. We stress the important role of SL states of C60 molecule itself: string type polarons (Pc) and bipolarons BP2- c in ttie stabilisation of this reactions through reorientations of string P− c/BP2- c relatively to CP chains resulting ui suppression of backward CT recombination. 2) The C60 mediated interchain hoping of electrons is considered withitin Holstein-Rashba molecular chain polaron approximation, which allows to find exact analytical solutions for...

Simplified sum-over-states calculations and missing-orbital analysis on hyperpolarizabilities of benzene derivatives

The simplified sum-over-states (SOS) and missing-orbital analysis methods are introduced. The former aims to calculate the molecular polarizabilities in the framework of ab initio SCF theory, and the latter to analyze the calculated polarizabilities. The utility of the combination of these methods is demonstrated for the benzene derivatives, e.g. p-nitroaniline (PNA) and p-cyanoaniline (PCA). The calculated first-order hyperpolarizabilities (β) agree well with those of both CPHF calculations and experimental results. The missing-orbital analysis shows that β for PCA is contributed by both forward and backward charge transfer (CT) related to “out-of-plane” and “in-plane” orbitals, respectively. In contrast, β for PNA is confirmed to be explained fully by the conventional two-state model characterized by strong forward CT.