Plasmonic Resonant Intercluster Coulombic Decay.

Light-induced energy confinement in nanoclusters via plasmon excitations influences applications in nanophotonics, photocatalysis, and the design of controlled slow electron sources. The resonant decay of these excitations through the cluster's ionization continuum provides a unique probe of the collective electronic behavior. However, the transfer of a part of this decay amplitude to the continuum of a second conjugated cluster may offer control and efficacy in sharing the energy nonlocally to instigate remote collective events. With the example of a spherically nested dimer Na_{20}@C_{240} of two plasmonic systems we find that such a transfer is possible through the resonant intercluster Coulombic decay (RICD) as a fundamental process. This plasmonic RICD signal can be experimentally detected by the photoelectron velocity map imaging technique.

Ultrafast Transfer and Transient Entrapment of Photoexcited Mg Electron in Mg@C_{60}.

Electron relaxation is studied in endofullerene Mg@C_{60} after an initial localized photoexcitation in Mg by nonadiabatic molecular dynamics simulations. Two approaches to the electronic structure of the excited electronic states are used: (i) an independent particle approximation based on a density-functional theory description of molecular orbitals and (ii) a configuration-interaction description of the many-body effects. Both methods exhibit similar relaxation times, leading to an ultrafast decay and charge transfer from Mg to C_{60} within tens of femtoseconds. Method (i) further elicits a transient trap of the transferred electron that can delay the electron-hole recombination. Results shall motivate experiments to probe these ultrafast processes by two-photon transient absorption or photoelectron spectroscopy in gas phase, in solution, or as thin films.

Solar Cell Materials by Design: Hybrid Pyroxene Corner-Sharing VO4 Tetrahedral Chains.

Hybrid organic-inorganic frameworks provide numerous combinations of materials with a wide range of structural and electronic properties, which enable their use in various applications. In recent years, some of these hybrid materials-especially lead-based halide perovskites-have been successfully used for the development of highly efficient solar cells. The large variety of possible hybrid materials has inspired the search for other organic-inorganic frameworks that may exhibit enhanced performance over conventional lead halide perovskites. In this study, a new class of low-dimensional hybrid oxides for photovoltaic applications was developed by using electronic structure calculations in combination with analysis from existing materials databases, with a focus on vanadium oxide pyroxenes (tetrahedron-based frameworks), mainly due to their high stability and nontoxicity. Pyroxenes were screened with different cations [A] and detailed computational studies of their structural, electronic, optical and transport properties were performed. Low-dimensional hybrid vanadate pyroxenes [A]VO3 (with molecular cations [A] and corner-sharing VO4 tetrahedral chains) were found to satisfy all physical requirements needed to develop an efficient solar cell (a band gap of 1.0-1.7 eV, strong light absorption and good electron-transport properties).

Enhancing the carrier thermalization time in organometallic perovskites by halide mixing.

Hybrid metal-organic halide perovskites have recently attracted a great deal of attention because of their interesting electronic, optical and transport properties, which make them promising materials for high-performance, low-cost solar cells. Fundamental understanding of the formation mechanisms and dynamics of photoinduced charge carriers is essential for improving the performance of perovskite solar cell devices. For example, a significant amount of absorbed solar energy is lost as a result of carrier thermalization. This energy could be harnessed by extracting hot carriers before they cool down to the band edges. Although such hot carrier collection is experimentally challenging, theoretical investigations based on time-dependent methods can guide future experimental research by providing insights into the thermalization process. Here, we perform ab initio nonadiabatic molecular dynamics simulations to study non-radiative relaxation dynamics of charge carriers in hybrid halide perovskites. We find that the carrier relaxation time can be considerably increased by mixing halogen atoms in the perovskite materials. These findings show that simple approaches could be adopted to slow down the thermalization process of hot carriers in perovskite materials.

Time delay in the recoiling valence photoemission of ar endohedrally confined in C60.

The effects of confinement and electron correlations on the relative time delay between the 3s and 3p photoemissions of Ar confined endohedrally in C60 are investigated using the time-dependent local density approximation--a method that is also found to mostly agree with recent time delay measurements between the 3s and 3p subshells in atomic Ar. At energies in the neighborhood of 3p Cooper minimum, correlations with C60 electrons are found to induce opposite temporal effects in the emission of Ar 3p hybridized symmetrically versus that of Ar 3p hybridized antisymmetrically with C60. A recoil-type interaction model mediated by the confinement is found to best describe the phenomenon.

Structure-based modeling of energy transfer in photosynthesis.

We provide a minimal model for a structure-based simulation of excitation energy transfer in pigment-protein complexes (PPCs). In our treatment, the PPC is assembled from its building blocks. The latter are defined such that electron exchange occurs only within, but not between these units. The variational principle is applied to investigate how the Coulomb interaction between building blocks changes the character of the electronic states of the PPC. In this way, the standard exciton Hamiltonian is obtained from first principles and a hierarchy of calculation schemes for the parameters of this Hamiltonian arises. Possible extensions of this approach are discussed concerning (i) the inclusion of dispersive site energy shifts and (ii) the inclusion of electron exchange between pigments. First results on electron exchange within the special pair of photosystem II of cyanobacteria and higher plants are presented and compared with earlier results on purple bacteria. In the last part of this mini-review, the coupling of electronic and nuclear degrees of freedom is considered. First, the standard exciton-vibrational Hamiltonian is parameterized with the help of a normal mode analysis of the PPC. Second, dynamical theories are discussed that exploit this Hamiltonian in the study of dissipative exciton motion.

Ultrafast hydrogen migration in acetylene cation driven by non-adiabatic effects.

Non-adiabatic dynamics of the acetylene cation is investigated using mixed quantum-classical dynamics based on trajectory surface hopping. To describe the non-adiabatic effects, two surface hopping methods are used, namely, Tully's fewest switches and Landau-Zener surface hopping. Similarities and differences between the results based on those two methods are discussed. We find that the photoionization of acetylene into the first excited state A(2)Σg(+) drives the molecule from the linear structure to a trans-bent structure. Through a conical intersection the acetylene cation can relax back to either the ground state of acetylene or vinylidene. We conclude that hydrogen migration always takes place after non-radiative electronic relaxation to the ground state of the monocation. Based on the analysis of correlation functions we identify coherent oscillations between acetylene and vinylidene with a period of about 70 fs after the electronic relaxation.

Non-Born-Oppenheimer dynamics of the photoionized Zundel cation: a quantum wavepacket and surface-hopping study.

The ultrafast fragmentation of the Zundel cation H(+)(H2O)2 after photoionization is studied by quantum-dynamics with the multiconfiguration time-dependent Hartree method and with surface-hopping approaches. A picture emerges in which the correlated motion of the electron hole and the shared proton leads to localization of the two positively charged entities at opposite sides of the Zundel dication in less than 10 fs followed by Coulomb explosion. Electronic non-adiabatic effects play a crucial role in the fragmentation dynamics. The photoionization spectrum of the cluster between 20 and 24 eV is calculated quantum-dynamically and its features explained. Two- and three-body fragmentation channels accessible by outer-valence ionization are also calculated and the branching ratios as a function of ionization energy are discussed. A good agreement between the quantum-dynamical treatment and surface-hopping is obtained for observables for which both methods are applied.

Correlated dynamics of the motion of proton-hole wave packets in a photoionized water cluster.

We explore the correlated dynamics of an electron hole and a proton after ionization of a protonated water cluster by extreme ultraviolet light. An ultrafast decay mechanism is found in which the proton-hole dynamics after the ionization are driven by electrostatic repulsion and involve a strong coupling between the nuclear and electronic degrees of freedom. We describe the system by a quantum-dynamical approach and show that nonadiabatic effects are a key element of the mechanism by which electron and proton repel each other and become localized at opposite sides of the cluster. Based on the generality of the decay mechanism, similar effects may be expected for other ionized systems featuring hydrogen bonds.

Structure-based simulation of linear optical spectra of the CP43 core antenna of photosystem II.

The linear optical spectra (absorbance, linear dichroism, circular dichroism, fluorescence) of the CP43 (PsbC) antenna of the photosystem II core complex (PSIIcc) pertaining to the S(0) â†’ S(1) (Q(Y)) transitions of the chlorophyll (Chl) a pigments are simulated by applying a combined quantum chemical/electrostatic method to obtain excitonic couplings and local transition energies (site energies) on the basis of the 2.9 Å resolution crystal structure (Guskov et al., Nat Struct Mol Biol 16:334-342, 2009). The electrostatic calculations identify three Chls with low site energies (Chls 35, 37, and 45 in the nomenclature of Loll et al. (Nature 438:1040-1044, 2005). A refined simulation of experimental spectra of isolated CP43 suggests a modified set of site energies within 143 cm(-1) of the directly calculated values (root mean square deviation: 80 cm(-1)). In the refined set, energy sinks are at Chls 37, 43, and 45 in agreement with earlier fitting results (Raszewski and Renger, J Am Chem Soc 130:4431-4446, 2008). The present structure-based simulations reveal that a large part of the redshift of Chl 37 is due to a digalactosyldiacylglycerol lipid. This finding suggests a new role for lipids in PSIIcc, namely the tuning of optical spectra and the creation of an excitation energy funnel towards the reaction center. The analysis of electrostatic pigment-protein interactions is used to identify amino acid residues that are of potential interest for an experimental approach to an assignment of site energies and energy sinks by site-directed mutagenesis.

Ultrafast dynamics of photoionized acetylene.

Acetylene cations [HCCH](+) produced in the A(2)Σ(g)(+) state by extreme ultraviolet (XUV) photoionization are investigated theoretically, based on a mixed quantum-classical approach. We show that the decay of the A(2)Σ(g)(+) state occurs via both ultrafast isomerization and nonradiative electronic relaxation. We find a time scale for hydrogen migration and electronic decay of about 60 fs, in good agreement with recent XUV-pump/XUV-probe time-resolved experiments on the same system [Phys. Rev. Lett. 105, 263002 (2010)]. Moreover, we predict an efficient vibrational energy redistribution mechanism that quickly transfers excess energy from the isomerization coordinates to slower modes in a few hundred femtoseconds, leading to a partial regeneration of acetylenelike conformations.

Structure-based identification of energy sinks in plant light-harvesting complex II.

The local S(0) → S(1) transition energies (site energies) and corresponding excitonic couplings of chlorophyll a (Chla) and b (Chlb) pigments bound to trimeric, major light-harvesting complex II (LHCII) of higher plants are calculated on the basis of the two crystal structures (Liu et al. Nature 2004, 428, 287-292; Standfuss et al. EMBO J. 2005, 24, 919-928) by using a combined quantum chemical/electrostatic method (Müh et al. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 16862-16867) that has been modified to cover membrane proteins and to account more realistically for the behavior of protonatable groups under the conditions of low-temperature optical spectroscopy. The obtained exciton levels are in reasonable agreement with experimental information (including linear absorption, linear dichroism, circular dichroism, fluorescence spectra of native as well as wild-type-minus-mutant difference absorption spectra of recombinant LHCII) and differ from earlier treatments based on fitted site energies (Novoderezhkin et al. J. Phys. Chem. B 2005, 109, 10493-10504) mainly by assigning a lower energy level to Chla 604 (in the nomenclature of Liu et al.) and Chlb 608 and a higher energy level to Chlb 605 and 609. The energy sink at cyrogenic temperatures is located at Chla 610 in the stromal layer of pigments, but structural changes at elevated temperatures may change the nature of the terminal emitter domain (including Chla 610/611/612). The site energy red-shift of Chla 610 is calculated to be significantly larger on the basis of the crystal structure of Standfuss et al. compared to that of Liu et al. due to conformational differences in the neighborhood of this pigment. A possible conformational change in the vicinity of Chla 604 involving tyrosine 112 and neoxanthin is found to strongly affect the site energy of this Chla and render it an alternative energy sink in the lumenal layer. A detailed, structure-based analysis of electrostatic pigment-protein interactions is performed to identify amino acid residues that are of interest for future mutagenesis experiments with the aim to further characterize the energy sinks, putative "bottleneck" states for excitation energy transfer, and potential sites of nonphotochemical quenching.

Determining the three-dimensional electronic transition dipole moment orientation: influence of an isomeric mixture.

A new mixed experimental and theoretical approach for determining the exact three-dimensional orientation of electronic transition dipole moments (tdms) within the molecular frame is discussed. Results of applying this method on Chlorophyll a and the dye Coumarin 314 (C314) are presented. For C314 the possible influence of a mixture of E- and Z-isomers in the sample on the determined electronic tdm is investigated. Moreover, the robustness of the method is investigated with the C314 data.

Structure-based calculations of optical spectra of photosystem I suggest an asymmetric light-harvesting process.

Optical line shape theory is combined with a quantum-chemical/electrostatic calculation of the site energies of the 96 chlorophyll a pigments and their excitonic couplings to simulate optical spectra of photosystem I core complexes from Thermosynechococcus elongatus. The absorbance, linear dichroism and circular dichroism spectra, calculated on the basis of the 2.5 A crystal structure, match the experimental data semiquantitatively allowing for a detailed analysis of the pigment-protein interaction. The majority of site energies are determined by multiple interactions with a large number (>20) of amino acid residues, a result which demonstrates the importance of long-range electrostatic interactions. The low-energy exciton states of the antenna are found to be located at a nearest distance of about 25 A from the special pair of the reaction center. The intermediate pigments form a high-energy bridge, the site energies of which are stabilized by a particularly large number (>100) of amino acid residues. The concentration of low energy exciton states in the antenna is larger on the side of the A-branch of the reaction center, implying an asymmetric delivery of excitation energy to the latter. This asymmetry in light-harvesting may provide the key for understanding the asymmetric use of the two branches in primary electron transfer reactions. Experiments are suggested to check for this possibility.

Femtosecond polarization resolved spectroscopy: a tool for determination of the three-dimensional orientation of electronic transition dipole moments and identification of configurational isomers.

A method is presented that combines femtosecond polarization resolved UV/visible pump-IR probe spectroscopy and density functional theory calculations in determining the three-dimensional orientation of an electronic transition dipole moment (tdm) within the molecular structure. The method is demonstrated on the approximately planar molecule coumarin 314 (C314) dissolved in acetonitrile, which can exist in two ground state configurations: the E- and the Z-isomer. Based on an exhaustive search analysis on polarization resolved measurement data for four different vibrational modes, it is concluded that C314 in acetonitrile is the E-isomer. The electronic tdm vector for the electronic S(0)-->S(1) transition is determined and the analysis shows that performing the procedure for four vibrational modes instead of the minimally required three reduces the 1sigma probability area from 2.34% to 2.24% of the solution space. Moreover, the fastest rotational correlation time tau(c) for the C314 E-isomer is determined to be 26+/-2 ps.

Deciphering the influence of short-range electronic couplings on optical properties of molecular dimers: application to "special pairs" in photosynthesis.

The excited states of chromophore dimers are, in general, delocalized, and the transition energies and transition dipoles are different from those of the monomers. The intermolecular interaction that is responsible for these effects has two contributions: Forster-type Coulomb coupling and a short-range coupling, which depends on the intermolecular overlap of electronic wave functions. The latter contains the Dexter-type exchange coupling and the coupling of excited states to intermolecular charge-transfer (CT) states. Recently, we developed a method (TrEsp) for an accurate and numerically efficient calculation of the Forster-type Coulomb part (Madjet et al. J. Phys. Chem. B 2006, 110, 17268). Here, we combine the latter with quantum chemical calculations to evaluate the short-range contribution, extending a method developed earlier by Scholes et al. (J. Phys. Chem. B 1999, 103, 2543). An effective two-state model is used, which relates the transition energies and transition dipole moments obtained by quantum chemical calculations of the monomers to those calculated for the dimer. From this relation, the short-range excitonic coupling and effective shifts of the local transition energies due to the coupling to intermolecular CT states can be inferred including a consistency check to evaluate quantum chemical methods that differ in the treatment of electron correlation. The method is applied to the special pairs of the reaction centers of purple bacteria (bRC) and photosystem I (PSI). We find that the short-range coupling represents the dominant contribution to the total excitonic coupling in both special pairs (80% in PSI and 70% in the bRC) and exhibits a monoexponential dependence on the distance between the pi-planes of the pigments with an attenuation factor of 2.8 A(-1). We obtain significant red-shifts of the local transition energies, which show a biexponential distance dependence with one attenuation factor being 2.8 A(-1) and another factor being in the range 0.3-0.7 A(-1) for PSI and 0.8-0.9 A(-1) for bRC. Both effects of the short-range coupling determine the excitation energy sink in the reaction centers at the special pairs.

Theory of solvatochromic shifts in nonpolar solvents reveals a new spectroscopic rule.

An expression of unexpected simplicity is derived for the shift in optical transition energies of solute molecules in nonpolar solvents. The expression reveals a new spectroscopic rule that says: The higher the excited state of the solute, the larger the solvatochromic red shift. A puzzle formulated >50 years ago by Bayliss is solved. Bayliss, based on arguments from classical physics, assumed that the shift scales with the oscillator strength of the solute transition, but noted strong quantitative deviations from this rule in experiments. As the present expression shows, the shift does not depend on the oscillator strength of the transition, but reflects the change in dispersive solute-solvent interactions between the ground and excited states of the solute, that are determined by the anisotropy of intramolecular electron correlation. The theory is applied to explain the solvatochromic shifts of the two lowest electronic excitations of bacteriochlorophyll a and bacteriopheophytin a.

Calculation of pigment transition energies in the FMO protein: from simplicity to complexity and back.

The Fenna-Matthews-Olson (FMO) protein of green sulfur bacteria represents an important model protein for the study of elementary pigment-protein couplings. We have previously used a simple approach [Adolphs and Renger (2006) Biophys J 91:2778-2797] to study the shift in local transition energies (site energies) of the FMO protein of Prosthecochloris aestuarii by charged amino acid residues, assuming a standard protonation pattern of the titratable groups. Recently, we have found strong evidence that besides the charged amino acids also the neutral charge density of the protein is important, by applying a combined quantum chemical/electrostatic approach [Müh et al. (2007) Proc Natl Acad Sci USA, in press]. Here, we extract the essential parts from this sophisticated method to obtain a relatively simple method again. It is shown that the main contribution to the site energy shifts is due to charge density coupling (CDC) between the pigments and their pigment, protein and water surroundings and that polarization effects for qualitative considerations can be approximated by screening the Coulomb coupling by an effective dielectric constant.

Alpha-helices direct excitation energy flow in the Fenna Matthews Olson protein.

In photosynthesis, light is captured by antenna proteins. These proteins transfer the excitation energy with almost 100% quantum efficiency to the reaction centers, where charge separation takes place. The time scale and pathways of this transfer are controlled by the protein scaffold, which holds the pigments at optimal geometry and tunes their excitation energies (site energies). The detailed understanding of the tuning of site energies by the protein has been an unsolved problem since the first high-resolution crystal structure of a light-harvesting antenna appeared >30 years ago [Fenna RE, Matthews BW (1975) Nature 258:573-577]. Here, we present a combined quantum chemical/electrostatic approach to compute site energies that considers the whole protein in atomic detail and provides the missing link between crystallography and spectroscopy. The calculation of site energies of the Fenna-Matthews-Olson protein results in optical spectra that are in quantitative agreement with experiment and reveals an unexpectedly strong influence of the backbone of two alpha-helices. The electric field from the latter defines the direction of excitation energy flow in the Fenna-Matthews-Olson protein, whereas the effects of amino acid side chains, hitherto thought to be crucial, largely compensate each other. This result challenges the current view of how energy flow is regulated in pigment-protein complexes and demonstrates that attention has to be paid to the backbone architecture.