The present computational study aims to unravel the competitive photoinduced intermolecular energy transfer and electron transfer phenomena in a light-harvesting antenna with potential applications in dye-sensitized solar cells and photo catalysis. A series of three thiazole dyes with hierarchically overlapping emission and absorption spectra, embedded in a methacrylate-based polymer backbone, is employed to absorb light over the entire visible region. Intermolecular energy transfer in such antenna proceeds via energy transfer from dye-to-dye and eventually to a photosensitizer. Initially, the ground and excited state properties of the three push pull-chromophores (e.g., with respect to their absorption and emission spectra as well as their equilibrium structures) are thoroughly evaluated using state-of-the-art multiconfigurational methods and computationally less demanding DFT and TDDFT simulations. Subsequently, the potential energy landscape for the three dyads, formed by the pi-stacked dyes as occurring in the polymer environment, is investigated along linear-interpolated internal coordinates to elucidate the photoinduced dynamics associated with intermolecular energy and electron transfer processes. While energy transfer among the dyes is highly desired in such antenna, electron transfer, or rather a light-induced redox chemistry, leading to the degradation of the chromophores, is disadvantageous. We performed quantum dynamical wavepacket calculations to investigate the excited state dynamics following initial light-excitation. Our calculations reveal for the two dyads with adjusted optical properties exclusively efficient intermolecular energy transfer within 200 fs, while in the case of the third dyad intermolecular electron transfer dynamics can be observed. Thus, this computational study reveals that statistical copolymerization of the individual dyes is disadvantageous with respect to the energy transfer efficiency as well as regarding the photostability of such antenna.

The present computational study aims at unraveling the competitive photoinduced electron transfer (ET) kinetics in a supramolecular photocatalyst model. Detailed understanding of the fundamental processes is essential for the design of novel photocatalysts in the scope of solar energy conversion that allows unidirectional ET from a light-harvesting photosensitizer to the catalytically active site. Thus, the photophysics and the photochemistry of the bimetallic complex RuCo, [(bpy)(2)Ru-II(tpphz)-Co-III(bpy)(2)](5+), where excitation of the ruthenium(II) moiety leads to an ET to the cobalt(III), were investigated by quantum chemical and quantum dynamical methods. Time-dependent density functional theory (TDDFT) allowed us to determine the bright singlet excitations as well as to identify the triplet states involved in the photoexcited relaxation cascades associated with charge-separation (CS) and charge-recombination (CR) processes. Diabatic potential energy surfaces were constructed for selected pairs of donor-acceptor states leading to CS and CR along linear interpolated Cartesian coordinates to study the intramolecular ET via Marcus theory, a semiempirical expression neglecting an explicit description of the potential couplings and quantum dynamics (QD). Both Marcus theory and QD predict very similar rate constants of 1.55 x 10(12) - 2.24 x 10(13) s(-1) and 1.21 x 10(13)-7.59 x 10(13) s(-1) for CS processes, respectively. ET rates obtained by the semiempirical expression are underestimated by several orders of magnitude; thus, an explicit consideration of electronic coupling is essential to describe intramolecular ET processes in RuCo.

The plasmon-mediated azo-coupling of p-aminothiophenol (PATP) and p-nitrothiophenol (pNTP) was recently achieved using gold and silver nanostructures with the help of hot electrons. To further investigate the underlying mechanisms the azo-coupling of dibenzo-1,2-dithiine-3,8-diamine (D3ATP) was investigated under distinct environmental conditions. Depending on the presence or absence of oxygen, either a plasmon-mediated or a photochemical reaction can be inferred. O2 plays an active role in the case of photoinduced azo-coupling of D3ATP but is not required under plasmonic conditions. Consequently, two different reaction channels are proposed. In the photoreaction, NH2 needs to react with oxygen to finally form the coupling-molecule and H2O, while under the plasmonic conditions the −NH2 groups can directly couple to form the azo compound and H2. This study suggests that plasmon-induced hot electrons provide the necessary activation energy for the azo-coupling of D3ATP without the need for O2.

Experimental evidence of extremely high spatial resolution of tip-enhanced Raman scattering (TERS) has been recently demonstrated. Here, we present a full quantum chemical description (at the density functional level of theory) of the non-resonant chemical effects on the Raman spectrum of an adenine molecule mapped by a tip, modeled as a single silver atom or a small silver cluster. We show pronounced changes in the Raman pattern and its intensities depending on the conformation of the nanoparticle-substrate system, concluding that the spatial resolution of the chemical contribution of TERS can be in the sub-nm range.

Nonadiabatic ab initio molecular dynamics simulations are carried out to monitor the attack of CH3 (+) on aniline in the gas phase to form the corresponding σ complexes. The reaction is ultrafast and is governed by a single electron transfer within 30 fs, which involves two sequential conical intersections and finally produces a radical pair. Positive-charge allocation in the aromatic compound is found to govern the substitution pattern in ortho, meta, or para position. Although the major products in the first step of the electrophilic aromatic substitution are the ortho and para σ complexes, initially 26 % of the simulated trajectories also form meta complexes, which then undergo H shifts, mainly to the para position.

The concept of nuclear spin isomers was already introduced in the early days of quantum mechanics. Despite its importance, not much work has been done to separate them experimentally by pushing the ratio away from its equilibrium value. We propose to use ultrashort laser pulses in a pump-dump-like experiment to enhance the ratio between different nuclear spin isomers. Exemplary wave packet simulations with optimized femtosecond pump and dump laser pulses are shown on a quinodimethane derivative to illustrate that the ratio between two different groups of nuclear spin isomers is enhanced.

Nonadiabatic molecular dynamics simulations are used to monitor the attack of CH3+ on aniline. On p. 2366, D. Kinzel et al. show that in the gas phase this reaction is governed by an ultrafast electron transfer, giving rise to ortho and para σ complexes.

Ab initio non-adiabatic dynamics is used to monitor the attack of CH3(+) to benzene. The results show that in the gas phase the reaction is ultrafast and is governed by a single electron transfer producing a radical pair.

Despite the concept of nuclear spin isomers (NSIs) exists since the early days of quantum mechanics, only few approaches have been suggested to separate different NSIs. Here, a method is proposed to discriminate different NSIs of a quinodimethane derivative using its electronic excited state dynamics. After electronic excitation by a laser field with femtosecond time duration, a difference in the behavior of several quantum mechanical operators can be observed. A pump-probe experimental approach for separating these different NSIs is then proposed.

We consider a class of molecules with C-2 symmetry axis and three segments A, B, C which can rotate independently about that axis, corresponding to two independent torsions (B vs. A and C vs. B). The torsions may be feasible either in the electronic ground or in the excited states. We determine the corresponding molecular symmetry group, i.e. the Abelian group G(16)(A) representing 16 feasible permutations and permutation-inversions, and its permutation subgroup with eight permutations, together with their properties, e.g. their character tables and the corresponding 16 or 8 irreducible representations (IREPs), respectively. Accordingly, the molecules which belong to this class have at most eight different nuclear spin isomers (NSIs). A subset of them "survives" at low temperature, T -> 0. The corresponding NSI selective wavefunctions contain products of torsional times nuclear wavefunctions with specific IREPs. The NSIs are characterized by these IREPs. As an example, we determine the molecular symmetry adapted torsional wavefunctions of the model 2-[4-(cyclopenta-2,4-dien-1-ylidene)cyclohexa-2,5-dien-1-ylidene]-2H-1,3-dioxole, abbreviated as CCD. In order to demonstrate the principles of the derivations, we employ a simple model, with the C-2 symmetry axis oriented along the laboratory Z-axis, and with all degrees of freedom frozen in the equilibrium structure of CCD, except the two torsional degrees of freedom. The resulting torsional wavefunctions represent different NSIs of CCD, ready for subsequent applications, e.g. for demonstrations of NSI selective dynamics.

The structures, redshifts, binding energies, Bader analysis, and shared-electron numbers (SENs) of 1,3-dihydrobenzimidazole-2-thione (DBS) derivatives hydrogen bonded to glycinamide were calculated by the means of DFT methods. The DBSglycinamide complex serves as a model for human immunodeficiency virus reverse transcriptase inhibitors of the N-dimethylallyl-6-methyl-4,5,6,7-tetrahydroimidazo-[4,5,1-jk][1,4]-benzodiazepin-2(1H)-thione family. A correlation between experimental Gibbs free energies, associated biological activities and the energy of the hydrogen bond obtained with the SEN method showed a linear relationship for different substitution patterns. Our results suggest that efficient inhibitors are those substituted in the 8-position with electron-withdrawing small substituents. (c) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 112:17861795, 2012

Non-adiabatic molecular dynamics simulations have been performed in the fluoro-olefin (4-methylcyclohexylidene) fluoromethane (4MCF) using multiconfigurational CASSCF (complete active space self-consistent field) on-the-fly calculations. As an olefin containing a C[double bond, length as m-dash]C double bond, 4MCF is expected to undergo cis-trans isomerization after light irradiation. However, ab initio molecular dynamics shows that a preferential dissociation of atomic hydrogen is taking place after population transfer to the bright ππ* state. This state is strongly mixed with πσ* states allowing dissociation in the electronic excited state before deactivation to the ground state occurs. A minor amount of trajectories experiences F-dissociation, followed by pyramidalization at the sp(2) carbons and CHF dissociation. In contrast, the amount of trajectories undergoing torsion around the double bond, and therefore cis-trans isomerization, is marginal. The H-abstraction reaction is ultrafast, taking place in less than 60 fs.

Hydrogen dissociation is an unwanted competing pathway if a torsional motion around the C=C double bond in a chiral fluoroethylene derivative, namely (4-methylcyclohexylidene) fluoromethane (4MCF), is to be achieved. We show that the excited state H-dissociation can be drastically diminished on timescales long enough to initiate a torsion around the C=C double bond using the non-resonant dynamic Stark effect. Potential energy curves, dipoles and polarizabilities for the regarded one-dimensional reaction coordinate are calculated within the CASSCF method. The influence of the excitation and the laser control field is then simulated using wavepacket dynamics.

Multiconfigurational ab initio calculations of the excited states and potential energy curves of the chiral fluoroethylene derivative (4-methylcyclohexylidene)fluoromethane provide evidence that pi sigma* states play an important role in the abstraction of HF. We show that more than the ground and valence pi pi* states are necessary to correctly describe the relaxation of the title molecule upon excitation to the bright valence pi pi* state. A conical intersection between the pi sigma* and pi pi* states has been identified at the FC geometry which makes dissociation of HF in the electronic excited state possible. This conclusion is different from all the previous studies on ethylenic systems where dissociation is postulated as a ground state reaction. (C) 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3394-3404,2011

Chemicals and Materials: Europium(III) acetate hydrate (Eu(OAc)3·H2O, 99.9 %, Alfa Aesar), gadolinium (III) acetate tetrahydrate (Gd(OAc)3·4 H2O, 99.9 %, ABCR), dichloromethane (99.9 %, Fisher Scientific), ethylene glycol (99 %, J.T. Baker), and ethanol (p.a., Riedel de Häen) were used as received. 1-Butyl-pyridinium tetrafluoroborate ([C4mim][BF4]), (2hydroxyethyl)-trimethylammonium tetrafluoroborate ([choline][BF4]), trishexyltetradecylphosphonium tetrafluoroborate ([P66614][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) were purchased from Iolitec, Heilbronn, D.

The thermal rearrangement reactions of cis-pinane, 1, and trans-pinane, 2, into beta-citronellene, 3, and isocitronellene, 4, have been investigated using ab initio multiconfigurational CASSCF and CASSCF MP2 calculations. Concerted as well as stepwise retro-[2+2]-cycloaddition conversion mechanisms are discussed and the corresponding stationary points along the relevant reaction paths from the bicyclic starting compounds into their acyclic isomers have been optimized. Our calculations show that the stepwise retro-[2+2]-cycloaddition via biradicals is energetically favoured with respect to the concerted mechanism. In the biradical pathways to 3 and 4, it was found that a gauche ring opening of the cyclobutane ring in 1 and 2, respectively, shows significantly lower activation barriers than the competing anti ring opening. With the predicted reaction paths, the calculated activation energies are in very good agreement with experimental values. The reaction mechanisms can explain the differences in the reactivity of 1 and 2, as well as the selectivity differences with respect to the formation of 3 and 4, reported in previous kinetic studies.

The photoisomerization around the C=C double bond and the competing elimination of hydrogen fluoride (HF) are studied in (4-methylcyclohexylidene) fluoromethane. Both reactions are mediated by twisted conical intersections (CI) around the C=C bond. Potential energy surfaces (PES) for the electronic ground state and first bright excited state of pi pi* character are calculated using the CASSCF method along two reaction coordinates: the torsion around the C=C bond and the distance between the center of masses of the hydrocarbon moiety and the HF fragment. Non-adiabatic couplings between both PES are obtained at the same level of theory. Wavepacket dynamics on the coupled surfaces show that after light irradiation torsion in the pi pi* state dominates over HF dissociation, although the system starts with enough kinetic energy to reach the CI leading to HF-elimination. (c) 2010 Elsevier B.V. All rights reserved.

On the basis of pyrolysis experiments with cis-pinane ( 1a), trans-pinane ( 1b), beta-citronellene ( 2), and isocitronellene ( 3), rate constants and activation parameters for the thermal rearrangement of the title compounds were calculated. Combining these with experimental parameters (residence time) allows for the kinetic modeling of the thermal rearrangement of 1a, 1b, 2, and 3. The chosen model of competitive first-order reactions describes the thermal behavior of the title compounds in a very good manner over a wide temperature range.

Abstract,iii 1 Introduction,1 2 Theory,4

Die vorliegende Arbeit erzielte detaillierte Erkenntnisse bezuglich der elektronischen Struktur und Dynamik des chiralen Fluoroethylenderivats, 4-Methylcyclohexyliden-Fluoromethan (4MCF), nach Anregung in den spektroskopisch hellen Zustand mit * Charakter als Modell fur einen lichtinduzierten molekularen Rotor bzw. Schalter. Der Hauptfokus liegt in der Betrachtung der miteinander konkurrierenden Relaxationspfade der Torsion um die C-C Doppelbindung und Dissoziation. Es konnte gezeigt werden, dass die Einbeziehung von repulsiven angeregten Zustanden mit * Charakter eine sehr wichtige Rolle in der Beschreibung der Dynamik nach Anregung in das olefinische System spielt. Die Eliminierung von atomarem Wasserstoff stellt dabei den Hauptreaktionspfad nach der Anregung von 4MCF in den * Zustand dar. Es konnte gezeigt werden, dass mit Hilfe des nicht-resonanten dynamischen Stark Effekts die dafur verantwortliche konische Uberschneidung auf der Potenzialhyperflache bewegt und somit weniger leicht erreichbar gemacht werden kann, sodass die unerwunschte Dissoziation von 4MCF effektiv verhindert bzw. verlangsamt wird.