Acceptor Orbitals Research Articles

Fermi Level Equilibration at the Metal-Molecule Interface in Plasmonic Systems.

We highlight a new metal-molecule charge transfer process by tuning the Fermi energy of plasmonic silver nanoparticles (AgNPs) in situ. The strong adsorption of halide ions upshifts the Fermi level of AgNPs by up to ∼0.3 eV in the order Cl- < Br- < I-, favoring the spontaneous charge transfer to aligned molecular acceptor orbitals until charge neutrality across the interface is achieved. By carefully quantifying, experimentally and theoretically, the Fermi level upshift, we show for the first time that this effect is comparable in energy to different plasmonic effects such as the plasmoelectric effect or hot-carriers production. Moreover, by monitoring in situ the adsorption dynamic of halide ions in different AgNP-molecule systems, we show for the first time that the catalytic role of halide ions in plasmonic nanostructures depends on the surface affinity of halide ions compared to that of the target molecule.

Experimental spectroscopic and quantum computational analysis of pyridine-2,6-dicarboxalic acid with molecular docking studies

Pyridine-2,6-dicarboxylic acid (2,6-DPA), investigated experimental-spectroscopically and quantum chemically. The computational investigation was done using DFT (Density functional theory) approach. We have examined the titled molecule with experimental spectroscopy like FT-IR and U.V visible. Vibrational spectroscopy investigation has also been included in the present work. In order to get the optimized structure which was base on all the other calculations (vibrational frequency, NBO, NLO, etc), the B3LYP method with a 6-311++G(d,p) basis set was used. Atom in molecule theory (AIM) was investigated for the determination of binding energy, ellipticity, and isosurface projection by electron localization function of the titled molecule. The IR, Raman, and NMR spectra have also been calculated computationally and compared with the experimental spectra which showed a good match. Further, to determine the interaction between donor and acceptor orbitals NBO analysis was done. For the identification of the reactive areas of the molecule Molecular electrostatic potential (MEP) and Fukui Function were studied along with Hirshfled analysis. Thermodynamic properties like G, H, and S were determined with the variation of temperature. A good exchange of charges happening inside the molecule was proved by HOMO-LUMO energy results. The calculated UV-visible spectrum by TD-DFT/PCM method was also compared with the experimental spectra. To check the probability of the bioactive nature of the molecule theoretically, a parameter named electrophilicity index was calculated which showed the great possibility to be bioactive. Molecular docking and drug-likeness which were a part of biological examinations were also done on the titled molecule.

Synergism of dynamics of tetrahedral hydrogen bonds of liquid water

We used modified Arrhenius approximations to analyze known temperature dependences (TDs) of water microstructure parameters and its dielectric and dynamic characteristics. Analysis of activation energies showed a significant difference in the molecular dynamics of water in ranges 273–298 and 300–373 K. Features of TDs in the first range were associated with the dynamics of a metastable ice-like phase of water, in which hexagonal clusters with tetrahedral hydrogen bonds (HBs) predominate. Based on ratios of signs and values of activation energies of HBs' fluctuations and parameters of a microstructure, it was assumed that fluctuations of HBs' dipoles play a key role in the mechanism of resonant activation by a thermal energy of concertedly reactions of deformation, rupture, and formation of tetrahedral HBs in water clusters. Synergism of these reactions and interaction of charges of the vacant acceptor and donor tetrahedral orbitals of an oxygen atom trigger at 298 K an explosive transition of a metastable ice-like phase of water into an argon-like phase. Synergy of water dynamics above 298 K is adequately characterized by the product Dη, from which TDs follow the activation energies of reactions that determine the form of Stokes–Einstein relation in temperature ranges below and above 298 K point.

Molecular Structure Vibrational &amp; Electronic Properties of Some Isatin Derivatives

In this study, Natural Bonding Orbitals (NBOs) were applied to isatin, 5‑fluoroisatin, 5‑chloroisatin, 5‑methylisatin and 5‑methoxyisatin using the Lee-Yang-Parr correlation functional B3LYP with 6‑311++G(2d,2p) basis set. Natural bonding analysis was performed to consider the transfer interactions of intra-molecular charge, pre-hybridization and electron density within the isatin, 5‑fluoroisatin, 5‑chloroisatin, 5‑methylisatin and 5‑methoxyisatin. In natural bonding orbital analysis, the wave functions of the electrons were explicated in sets of occupied Lewis type terms, (bonds or lone pairs) and sets of unoccupied non‑Lewis (anti‑bond and Rydberg) localized natural bonding orbitals. The electron density between these orbitals was correlated to stabilize the interaction of donor-acceptor orbitals. Second-order Perturbation Theory was employed to evaluate the stabilization energies of all possible interactions between donor and acceptor orbitals in natural bonding orbitals. The aim was to study the properties of these series and to produce a molecular geometry description, hyper-conjugative interactions, natural bond orbital (NBO) analysis and the HOMO‑LUMO energy gap-dependent properties that are important for the stability of structures. From the results, it can be noted that for all forms, the HOMO‑LUMO energy gap values decreased by substitution for the 5‑fluoroisatin, 5‑chloroisatin, 5‑methylisatin and 5‑methoxyisatin compounds and, with the exception of the deprotonated form in all the compounds, leading to less stable molecules. The anion form of the 5‑methoxyisatin compound and the cation form of the 5‑fluoroisatin, in which the HOMO‑LUMO energy gap values increased, became more stable. It also appeared that the 5‑CH3- and 5‑Cl groups caused an increase in the linear polarizability and anisotropic polarizability values in all forms; however, the 5‑OCH3 and 5‑F groups caused different effects in different forms. Furthermore, we concluded that the intramolecular hyper-conjugative interactions between bonded atoms changed by groups. These changes caused changes in the electron density values in the atoms, which in turn led to a change in the stabilization energy values of the isatin, 5‑fluoroisatin, 5‑chloroisatin, 5‑methylisatin and 5‑methoxyisatin.

Ultrafast Luminescence Decay in Rhenium(I) Complexes with Imidazo[4,5-f]-1,10-Phenanthroline Ligands: TDDFT Method

The interpretation of the ultrafast luminescence decay in [Re(Br(CO)3(N^N)] complexes as a new group of chromophoric imidazo[4,5-f]-1,10-phenanthroline ligands, including 1,2-dimethoxy benzene, tert-butyl benzene (L4) and 1,2,3-trimethoxy benzene, tert-butyl benzene (L6), was studied. Fac-[Re(Br(CO)3L4 and L6] with different aryl groups were calculated in singlet and triplet excited states. The calculations were performed in the gas phase and in the presence of chloroform as a solvent. The absorption and luminescence wavelengths, transition energy, and oscillator strength for singlet and triplet states were calculated for entitled complexes with L4 and L6 ligands. The UV-Vis spectrum and the singlet and triplet energy diagrams show a good agreement between our computational results and the experimental results, particularly in the case of a triplet state in chloroform. Spin-orbit coupling calculations show sufficient intensity and short wavelength in the UV-Vis absorption after SOC corrected absorption. The results of NBO calculations in the solvent show that rhenium acts as an acceptor by means of E(2), the second-order interaction energies between donor and acceptor orbitals, at the B3LYP/lanl2mb level of theory.

Defining Direct Orbital Pathways for Intermolecular Electron Transfer Using Sensitized Semiconducting Surfaces.

High-performance electronic materials and redox catalysts often rely on fast rates of intermolecular electron transfer (IET). Maximizing IET rates requires strong electronic coupling (HDA) between the electron donor and acceptor, yet universal structure-property relationships governing HDA in outer-sphere IET reactions have yet to be developed. For ground-state IET reactions, HDA is reasonably approximated by the extent of overlap between the frontier donor and acceptor orbitals involved in the electron-transfer reaction. Intermolecular interactions that encourage overlap between these orbitals, thereby creating a direct orbital pathway for IET, have a strong impact on HDA and, by extension, the IET rates. In this Forum Article, we present a set of intuitive molecular design strategies employing this direct orbital pathway principle to maximize HDA for IET reactions. We highlight how the careful design of redox-active molecules anchored to solid semiconducting substrates provides a powerful experimental platform for elucidating how electronic structure and specific intermolecular interactions affect IET reactions.

Scrutinizing "Ligand Bands" via Polarized Single-Crystal X-ray Absorption Spectra of Copper(I) and Copper(II) Bis-2,2'-bipyridine Species.

High-energy resolution fluorescence-detected Cu K-edge X-ray absorption spectroscopy (XAS) and single-crystal polarized XAS data are presented toward refining the assignments of bands assigned as excitations from Cu 1s to ligand-localized molecular orbitals. These have been previously dubbed "XAS-metal-ligand charge transfer" (XAS-MLCT) bands. Data are presented for a series of [Cu(xbpy)2]n+ complexes (xbpy = 2,2'-bipyridine (1n+), 4,4'-bisamino-2,2'-bipyridine (2n+), and 4,4'-dimethoxy-2,2'-bipyridine (3n+); n = 1 and 2). Dipolar dependencies of these "XAS-MLCT" bands in both Cu1+ and Cu2+ species lead to reassignment of these features as owing their intensities primarily to Cu 1s → Cu 4p excitations. The transition densities are Cu-localized, highlighting that XAS-MLCT features in Cu XAS spectra are not "charge transfer" transitions but rather quasi-atomic transitions. Although scrutiny of the acceptor orbitals supports assignment as Cu 1s → ligand π* transitions, it ultimately appears that while the ligand orbital energetics govern the positions of these bands the intensity is conferred through a small degree of metal 4p mixing into otherwise ligand-dominated acceptor molecular orbitals.

Stereochemistry of Transition Metal Complexes Controlled by the Metallo-Anomeric Effect.

The use of stereoelectronic interactions to control reactivity and selectivity has a long history in chemistry. The anomeric effect, one of the fundamental concepts in organic chemistry, describes the preferences of a substituent at the anomeric carbon in glycosides to adopt axial configuration when the anomeric group is an electronegative element such as oxygen or a halogen. The origin of the anomeric effect has been the subject of intense debate. Explanations capitalizing on either the delocalization of the endocyclic oxygen lone pair into the antibonding σ*(C-X) orbital or the minimization of the dipole-dipole interactions are currently the two leading theoretical models. Although the majority of experimental and theoretical studies have focused on the elements from groups 6 and 7, little is known about conformational preferences of tetrahydropyran rings substituted with a transition metal at the anomeric carbon and the role of these interactions in stereoselective synthesis. Here, we report studies on conformational and configurational preferences of organometallic complexes stabilized by vicinal heteroatoms. We provide computational evidence that late transition metals adopt the axial position in heterocycles or synclinal geometry in acyclic systems. Furthermore, the anomeric preferences of late transition metals correlate with the oxidation state of the metal and can be explained by hyperconjugative interactions between endocyclic heteroatom and the σ* acceptor orbitals of the C-M bond. In a broader context, this discovery provides insight into the role of previously unanticipated stereoelectronic effects that can be harnessed in the design of stereoselective reactions, including chemical glycosylation and enantioselective catalysis.

Synthesis, quantum chemical calculations and molecular docking studies of 2-ethoxy-4[(2-trifluromethyl-phenylimino)methyl]phenol

In the present work, the FT-IR, FT-Raman and UV-Vis spectra of 2-Ethoxy-4[(2-trifluromethyl-phenylimino)methyl]phenol (2E42TP) were recorded and analysed. The vibrational assignments of the present molecule were made on the basis of total energy distribution (TED) by calculating using Veda 4 program. The experimental spectra of 2E42TP molecule had shown excellent agreement with the computed vibrational spectra. The hyper conjugative interaction energy (E)2 and electron densities (EDs) of donor (i) and acceptor (j) orbitals were calculated using natural bond orbital (NBO) analysis. The energy band gap of the molecule was calculated using highest occupied molecular orbital and lowest unoccupied molecular orbital. The electronic transition was studied using UV-Vis analysis of (2E42TP) with same level of basis set. The non-linear optical (NLO) behaviour, molecular electrostatic potential (MEP) and thermo dynamical properties of the title molecule were also calculated. Furthermore, the 1H and 13C NMR chemical shift values of 2E42TP in the ground state for B3LYP/ 6-311++G (d,p) basis set were also calculated using Gauge independent atomic orbital (GIAO) method. To establish the information about the interactions between the topoisomerase DNA gyrase enzymes and the novel compound theoretically, docking studies were carried out using Schrodinger software.

Adsorption of H2, N2, CO, H2S, NH3, SO2 and CH4 on Li-functionalized graphitic carbon nitride investigated by density functional theory

The interaction of small gas molecules as H2, N2, CO, H2S, NH3, SO2 and CH4 on Li-functionalized graphitic carbon nitride was investigated by using the density functional theory, to explore their gas adsorption properties. The calculated Eads values of all gas molecules on [Li g-C3N4]+ show that these gas molecules present favourable interaction with the lithium atom coordinated on the sheet. NH3 and SO2 molecules present strong interactions, with Eads values of −18.60 and −9.50 kcal mol−1. The natural bond orbital analysis indicates that donor orbitals belong to the lone pairs of oxygen, nitrogen, sulphur and carbon atoms from SO2, N2, NH3, H2S, CO molecules, and acceptor orbitals (LP*) from the lithium atom. Computational studies suggest that H2, N2, CO, H2S, NH3, SO2 and CH4 molecules on [Li g-C3N4]+ present physisorption.

A silicon-carbonyl complex stable at room temperature.

Main-group-element compounds with energetically high-lying donor and low-lying acceptor orbitals are able to mimic chemical bonding motifs and reactivity patterns known in transition metal chemistry, including small-molecule activation and catalytic reactions. Monovalent group 13 compounds and divalent group 14 compounds, particularly silylenes, have been shown to be excellent candidates for this purpose. However, one of the most common reactions of transition metal complexes, the direct reaction with carbon monoxide and formation of room-temperature isolable carbonyl complexes, is virtually unknown in main-group-element chemistry. Here, we show the synthesis, single-crystal X-ray structure, and density functional theory computations of a room-temperature-stable silylene carbonyl complex [L(Br)Ga]2Si:-CO (L = HC[C(Me)N(2,6-iPr2-C6H3)]2), which was obtained by direct carbonylation of the electron-rich silylene intermediate [L(Br)Ga]2Si:. Furthermore, [L(Br)Ga]2Si:-CO reacts with H2 and PBr3 with bond activation, whereas the reaction with cyclohexyl isocyanide proceeds with CO substitution.

Making triplets from photo-generated charges: observations, mechanisms and theory

Triplet formation by charge recombination is a phenomenon that is encountered in many fields of the photo-sciences and can be a detrimental unwanted side effect, but can also be exploited as a useful triplet generation method, for instance in photodynamic therapy. In this Perspective we describe the various aspects that play a role in the decay of charge separated states into local triplet states. The observations and structures of a selection of (pre-2015) molecular electron donor-acceptor systems in which triplet formation by charge recombination occurs are reported. An overview is given of some more recent systems consisting of BODIPY dimers, and BODIPYs attached to various electron-donor units displaying this same triplet formation process. A selection of polymer–fullerene blends in which triplet formation by (non-geminate) charge recombination has been observed, is presented. Furthermore, in-depth information regarding the mechanistic aspects of triplet formation by charge recombination is given on spin dephasing, through hyperfine interactions, as well as on spin–orbit coupling occurring simultaneously with charge recombination. The limits and constraints of these factors and their role in intersystem crossing are discussed. A pictorial view of the two mechanisms is given and this is correlated to aspects of the selection rules for triplet formation, the so-called El-Sayed rules. It is shown that the timescale of triplet formation by charge recombination is indicative for the mechanism that is responsible for the process. The relatively slow rates (CRkT ~ 1 × 108 s−1 or slower) can be correlated to proton hyperfine interactions (also called the radical pair mechanism), but substantially faster rates (CRkT ~ 1 × 109 up to 2.5 × 1010 s−1 or faster) have to be correlated to spin-orbit coupling effects. Several examples of molecular systems showing such fast rates are available and their electron donor and acceptor orbitals display an orthogonal relationship with respect to each other. This orientation of (the nodal planes of) the π-orbitals of the donor and acceptor units is correlated to the mechanisms in photodynamic agents and photovoltaic blends

Phosphorescent heteroleptic iridium(III) cyclometallates: Improved syntheses of acetylacetonate complexes and quantum chemical studies of their excited state properties

We have investigated methods to prepare cyclometallated iridium(III) complexes with efficient photoluminescence spanning a broad color palette. In particular, we find that addition of ancillary ligands to chloro-bridged iridium dimers proceeds cleanly in refluxing 1,2-dimethoxyethane (DME) without the need for additional product purification. This represents an improvement over the conventional use of 2-ethoxyethanol which requires column chromatographic separation. Our efforts in this work have focused on acetylacetonate complexes such as (F2ppy)2Ir(acac), where F2ppy = 2-(4′,6′-diflurophenyl)pyridinato. We have prepared fifteen compounds by the route, eight of which are newly reported; in four cases we were able to prepare complexes which were inaccessible via the conventional route. Nine of the complexes were characterized by single crystal x-ray diffraction and possess the same distorted octahedral geometry around the iridium with two bidentate phenylpyridine ligands and one bidentate acetylacetonate ligand. Seven of the complexes exhibited efficient photoluminescence with colors ranging from yellow to blue and quantum yields of 0.51–0.74. All of the compounds with trifluoromethyl or phenyl substituents on the acetylacetone displayed emission in the orange with low quantum efficiency. The use of TD-DFT calculations, along with natural transition orbitals (NTOs), permitted a detailed interpretation of the electronic structures for the complexes. The nature of the acceptor orbitals for the low energy triplet state NTOs proved to be an important predictor for the emission spectra of the compounds.

Iron(II) coordination complexes with panchromatic absorption and nanosecond charge-transfer excited state lifetimes.

Replacing current benchmark rare-element photosensitizers with ones based on abundant and low-cost metals such as iron would help facilitate the large-scale implementation of solar energy conversion. To do so, the ability to extend the lifetimes of photogenerated excited states of iron complexes is critical. Here, we present a sensitizer design in which iron(II) centres are supported by frameworks containing benzannulated phenanthridine and quinoline heterocycles paired with amido donors. These complexes exhibit panchromatic absorption and nanosecond charge-transfer excited state lifetimes, enabled by the combination of vacant, energetically accessible heterocycle-based acceptor orbitals and occupied molecular orbitals destabilized by strong mixing between amido nitrogen atoms and iron. This finding shows how ligand design can extend metal-to-ligand charge-transfer-type excited state lifetimes of iron(II) complexes into the nanosecond regime and expand the range of potential applications for iron-based photosensitizers.

Theoretical study of binuclear Cu-M complexes (M = Zn, Cu, Ni) with p-xylylene-bridged-bis(1,4,7-triazacyclononane) ligands: Possible CuZnSOD mimics

DFT calculations were used to study a series of binuclear complexes with the general formula [CuM(pxbtacn-R2)Cl4] (pxbtacn = p-xylylene-bridged-bis(1,4,7-triazacyclononane); M = Zn, Cu and Ni; R = -H, -CH3, -CH(CH3)2) as potential mimics of superoxide dismutase. The obtained results indicate that all complexes have similar structures to the experimental structure reported for [Cu2(pxbtacn)Cl4] complex. In the reduction process, the loss occurs of one of the Cl- ligands in which the generated Cu(I) show distorted tetrahedral geometry. An analysis of acceptor (oxidized complexes) and donor (reduced complexes) orbitals shows that these orbitals have significant participation from Cu (II) and Cu (I), respectively. This indicates that the reduction process occurs via Cu(II) receiving an electron and the oxidation process occurs with the removal of this electron from Cu(I), according to the process observed in the native enzyme. The Gibbs free energy shows that the catalysis is spontaneous for all of the complexes studied, and the reduction step is the predominant stage of the global process. For the three series studied here, the calculated values of the Gibbs free energy and electronic affinity indicate that complexes with R = -CH3 and -CH(CH3)2 have the greatest ability to reduce, in accordance with the reported experimental mimic activity. Complexes with these substituents should therefore have a higher mimic activity.

Accurate and efficient DFT-based diabatization for hole and electron transfer using absolutely localized molecular orbitals.

Diabatic states and the couplings between them are important for quantifying, elucidating, and predicting the rates and mechanisms of many chemical and biochemical processes. Here, we propose and investigate approaches to accurately compute diabatic couplings from density functional theory (DFT) using absolutely localized molecular orbitals (ALMOs). ALMOs provide an appealing approach to generate variationally optimized diabatic states and obtain their associated forces, which allows for the relaxation of the donor and acceptor orbitals in a way that is internally consistent in how the method treats both the donor and acceptor states. Here, we show that one can obtain more accurate electronic couplings between ALMO-based diabats by employing the symmetrized transition density matrix to evaluate the exchange-correlation contribution. We demonstrate that this approach yields accurate results in comparison to other commonly used DFT-based diabatization methods across a wide array of electron and hole transfer processes occurring in systems ranging from conjugated organic molecules, such as thiophene and pentacene, to DNA base pairs. We also show that this approach yields accurate diabatic couplings even when combined with lower tiers of the DFT hierarchy, opening the door to combining it with quantum dynamics approaches to provide an ab initio treatment of nonadiabatic processes in the condensed phase.

Resolving orbital pathways for intermolecular electron transfer

Over 60 years have passed since Taube deduced an orbital-mediated electron transfer mechanism between distinct metal complexes. This concept of an orbital pathway has been thoroughly explored for donor–acceptor pairs bridged by covalently bonded chemical residues, but an analogous pathway has not yet been conclusively demonstrated for formally outer-sphere systems that lack an intervening bridge. In our present study, we experimentally resolve at an atomic level the orbital interactions necessary for electron transfer through an explicit intermolecular bond. This finding was achieved using a homologous series of surface-immobilized ruthenium catalysts that bear different terminal substituents poised for reaction with redox active species in solution. This arrangement enabled the discovery that intermolecular chalcogen⋯iodide interactions can mediate electron transfer only when these interactions bring the donor and acceptor orbitals into direct contact. This result offers the most direct observation to date of an intermolecular orbital pathway for electron transfer.

Relevance of Orbital Interactions and Pauli Repulsion in the Metal-Metal Bond of Coinage Metals.

The importance of relativity and dispersion in metallophilicity has been discussed in numerous studies. The existence of hybridization in the bonding between closed shell d10–d10 metal atoms has also been speculated, but the presence of attractive MO interaction in the metal–metal bond is still a matter of an ongoing debate. In this comparative study, a quantitative molecular orbital analysis and energy decomposition is carried out on the metallophilic interaction in atomic dimers (M+···M+) and molecular perpendicular [H3P–M–X]2 (where M = Cu, Ag, and Au; X = F, Cl, Br, and I). Our computational studies prove that besides the commonly accepted dispersive interactions, orbital interactions and Pauli repulsion also play a crucial role in the strength and length of the metal–metal bond. Although for M+···M+ the orbital interaction is larger than the Pauli repulsion, leading to a net attractive MO interaction, the bonding mechanism in perpendicular [H3P–M–X] dimers is different due to the larger separation between the donor and acceptor orbitals. Thus, Pauli repulsion is much larger, and two-orbital, four-electron repulsion is dominant.

Photolabile Ru Model Complexes with Chelating Diimine Ligands for Light‐Triggered Drug Release

A series of water‐soluble photolabile model complexes of the general formula [Ru([9]aneS3)(chel)(py)]Cl2 ([9]aneS3 = 1,4,7‐trithiacyclononane, chel = chelating diimine) was prepared and fully characterized. The phototriggered release of pyridine with visible light as a function of the nature of the diimine {chel = 2,2′‐bipyridine (6) 1,10‐phenanthroline (7), 4,7‐diphenil‐1,10‐phenanthroline (8), dipyrido‐[3,2‐a:2′,3′‐c]phenazine (dppz, 9), 2,2′‐biquinoline (bq, 10)} was investigated. Our aim is to establish whether this type of complexes might be realistically used in the photo‐uncaging strategy of photoactivated chemotherapy (PACT) in the future. Compounds 6–9 present a MLCT absorption in the blue region of the visible spectrum. When irradiated with light at 470 nm, they rapidly and quantitatively release the coordinated pyridine. Complex 10 turned out to be quite different from the others in the series. Structure‐wise, in 10 the average plane of coordinated bq – owing to its steric demand – is remarkably tilted relative to the equatorial coordination plane [37.43(4)°, with the “front“ of the ligand pointing towards the axial py] and the orientation of py is nearly orthogonal compared to that found in 6 and 7 for minimizing steric clashes with bq. The low‐lying acceptor orbitals of bq induce a red‐shift of the MLCT absorption maximum to ca. 500 nm. Contrary to the expectations, complex 10 is more photostable compared to 6–9 and photo‐dissociation of both py and bq, in nearly equal amounts, occurs. A detailed theoretical investigation was performed on 10 (and on 6 for comparison), for explaining its peculiar spectral features and photochemical behavior.

Identification of Ultrafast Photophysical Pathways in Photoexcited Organic Heterojunctions

The exciton dissociation and charge separation occurring on subpicosecond time scales following the photoexcitation are studied in a model donor/acceptor heterojunction using a fully quantum approach. Higher-than-LUMO acceptor orbitals which are energetically aligned with the donor LUMO orbital participate in the ultrafast interfacial dynamics by creating photon-absorbing charge-bridging states in which charges are spatially separated and which can be directly photoexcited. Along with the states brought about by single-particle resonances, the two-particle (exciton) mixing gives rise to bridge states in which charges are delocalized. Bridge states open up a number of photophysical pathways that indirectly connect the initial donor states with states of spatially separated charges and compete with the efficient progressive deexcitation within the manifold of donor states. The diversity and efficiency of these photophysical pathways depend on a number of factors, such as the precise energy alignment of exciton states, the central frequency of the excitation, and the strength of carrier–phonon interaction.