A Mechanism for Photoinduced Effects In Tetracyanoethylene-Based Organic Magnets

The photoinduced magnetism in manganese-tetracyanoethylene (Mn-TCNE) molecule-based magnets is ascribed to charge-transfer excitations from manganese to TCNE. Charge-transfer energies are calculated using Density Functional Theory; photoinduced magnetization is described using a model Hamiltonian based on a double-exchange mechanism. Photoexciting electrons from the manganese core spin into the lowest unoccupied orbital of TCNE with photon energies around 3 eV increases the magnetization through a reduction of the canting angle of the manganese core spins for an average electron density on TCNE less than one. When photoexciting with a smaller energy, divalent TCNE molecules are formed. The delocalization of the excited electron causes a local spin flip of a manganese core spin.Comment: 4 pages, 4 figure

Marcus inverted region in the photoinduced electron transfer reactions of ruthenium(ii)-polypyridine complexes with phenolate ions.

Ruthenium (II)- polypyridy1 complexes of similar size but with variable reduction potential undergo efficient photoinduced electron- transfer reactions with phenolate ions in aqueous medium. All these reactions are exergonic and are in accordance with the Marcus theory of electron transfer. At high negative G° Marcus inverted region is observed in this bimolecular photoinduced charge separation reaction

Photoinduced melting of charge order in a quarter-filled electron system coupled with different types of phonons

Photoinduced melting of charge order is calculated by using the exact many-electron wave function coupled with classically treated phonons in the one-dimensional quarter-filled Hubbard model with Peierls and Holstein types of electron-phonon couplings. The model parameters are taken from recent experiments on (EDO-TTF)_2PF_6 (EDO-TTF=ethylenedioxy-tetrathiafulvalene) with (0110) charge order, where transfer integrals are modulated by molecular displacements (bond-coupled phonons) and site energies by molecular deformations (charge-coupled phonons). The charge-transfer photoexcitation from (0110) to (0200) configurations and that from (0110) to (1010) configurations have different energies. The corresponding excited states have different shapes of adiabatic potentials as a function of these two phonon amplitudes. The adiabatic potentials are shown to be useful in understanding differences in the photoinduced charge dynamics and the efficiency of melting, which depend not only on the excitation energy but also on the relative phonon frequency of the bond- and charge-coupled phonons.Comment: 7 pages, 5 figures, accepted for publication in PR

Ultrafast photoinduced electron transfer in coumarin 343 sensitized TiO2-colloidal solution

Photoinduced electron transfer from organic dye molecules to semiconductor nanoparticles is the first and most important reaction step for the mechanism in the so called “wet solar cells” [1]. The time scale between the photoexcitation of the dye and the electron injection into the conduction band of the semiconductor colloid varies from a few tens of femtoseconds to nanoseconds, depending on the specific electron transfer parameters of the system, e.g., electronic coupling or free energy values of donor and acceptor molecules [2–10]. We show that visible pump/ white light probe is a very efficient tool to investigate the electron injection reaction allowing to observe simultaneously the relaxation of the excited dye, the injection process of the electron, the cooling of the injected electron and the charge recombination reaction

Photoinduced coherent oscillations in the one-dimensional two-orbital Hubbard model

We study photoinduced ultrafast coherent oscillations originating from orbital degrees of freedom in the one-dimensional two-orbital Hubbard model. By solving the time-dependent Schr\"odinger equation for the numerically exact many-electron wave function, we obtain time-dependent optical response functions. The calculated spectra show characteristic coherent oscillations that vary with the frequency of probe light. A simple analysis for the dominant oscillating components clarifies that these photoinduced oscillations are caused by the quantum interference between photogenerated states. The oscillation attributed to the Raman-active orbital excitations (orbitons) clearly appears around the charge-transfer peak.Comment: 5 pages, 5 figure

Influence of Donor-Acceptor Distance Variation on Photoinduced Electron and Proton Transfer in Rhenium(I)-Phenol Dyads

A homologous series of four molecules in which a phenol unit is linked covalently to a rhenium(I) tricarbonyl diimine photooxidant via a variable number of p-xylene spacers (n = 0–3) was synthesized and investigated. The species with a single p-xylene spacer was structurally characterized to get some benchmark distances. Photoexcitation of the metal complex in the shortest dyad (n = 0) triggers release of the phenolic proton to the acetonitrile/water solvent mixture; a H/D kinetic isotope effect (KIE) of 2.0 ± 0.4 is associated with this process. Thus, the shortest dyad basically acts like a photoacid. The next two longer dyads (n = 1, 2) exhibit intramolecular photoinduced phenol-to-rhenium electron transfer in the rate-determining excited-state deactivation step, and there is no significant KIE in this case. For the dyad with n = 1, transient absorption spectroscopy provided evidence for release of the phenolic proton to the solvent upon oxidation of the phenol by intramolecular photoinduced electron transfer. Subsequent thermal charge recombination is associated with a H/D KIE of 3.6 ± 0.4 and therefore is likely to involve proton motion in the rate-determining reaction step. Thus, some of the longer dyads (n = 1, 2) exhibit photoinduced proton-coupled electron transfer (PCET), albeit in a stepwise (electron transfer followed by proton transfer) rather than concerted manner. Our study demonstrates that electronically strongly coupled donor–acceptor systems may exhibit significantly different photoinduced PCET chemistry than electronically weakly coupled donor–bridge–acceptor molecules

Photoinduced Proton Transfer as a Possible Mechanism for Highly Efficient Excited-State Deactivation in Proteins

CASSCF//CASPT2 pathways for a two-glycine minimal model system show that photoinduced electron-driven forward and backward proton transfer could play an important role for the stability of proteins against damage by UV radiation, when a hydrogen bond is located between the two amino acids. The overall photoinduced process involves two electron and proton transfer processes (forward and backward) and results in the reformation of the initial closed-shell electronic structure of the system.Ministerio de Educación y CienciaUniversidad de Alcal

Photoinduced electron transfer between a donor and an acceptor separated by a capsular wall

The efficient photoinduced electron transfer from a stilbene derivative incarcerated within a negatively charged organic nanocapsule to positively charged acceptors (methyl viologen and a pyridinium salt) adsorbed outside and the back electron transfer were controlled by supramolecular effects

Photoinduced electron transfer across ortho-oligo-phenylenes and novel luminophores based on earth-abundant metals

Long-range electron-transfer is of high interest for many fields, such as artificial photosynthesis and molecular electronics. Donor – bridge – Acceptor compounds, wherein photoinduced electron transfer takes place were thoroughly investigated in this regard. Especially para-phenylene systems were chosen due to their rigid, rod-like wire behaviour. Depending on the electronic coupling of the bridge, para-phenylenes reveal �-values ranging from 0.2 to 0.8 °A−1. Their ortho-connected relatives are completely unexplored until now. In chapter I of this thesis the motivation for this work and the theoretical background for electron transfer will be given. This will be followed by a few examples of electron transfers across para-phenylenes, to put the herein presented work into perspective. In chapter II photoinduced electron-transfer across an ortho-phenylene wire consisting of 2 to 6 phenyl units will be presented. A Ru(II)-photosensitiser and a triarylamine electron donor were chosen to investigate the kinetics of the charge-shift reaction. The photoinduced forward, as well as the thermal back-reaction, were explored with time resolved measurements. Due to the flexibility of the bridge and slowly interconverting conformers in solution, analysis of charge-separation remains turbid, but a coherent analysis can be made for charge-recombination. The main discovery is that ortho-phenylenes possess very low �-values for charge-transfer, with a �-value in acetonitrile of 0.04 °A−1. The mechanism for the hole transfer is coherent tunnelling and a relevant aspect seems to be the �-pathway, which is shorter for ortho-phenylenes than for para-phenylenes. Ortho-phenylenes can therefore be considered as a new class of molecular wires. Chapter III will then present the results of photoinduced long-range electron transfer through ortho-naphthalenes, which can form different atropisomers and electron transfer is studied in these systems for the first time. Photoinduced electron transfer would not be possible without photosensitisers, which allow for an enough long living excited state with enough reducing or oxidising power, that electron transfer reactions can take place. Most photosensitisers today which reach the photophysical goals for being applied, for example in photoredox catalysis or in long-range electron transfer, rely on noble metals, such as Ru(II) or Ir(III). Therefore it would be desirable, to shift from noble metals as centres to more earth abundant metals. Nickel(0) allows for an MLCT transition when the ligand orbitals are of the right energy. In chapter IV first preliminary results of a nickel(0) bis(diphenylphosphino)naphthalene complex will be presented, which were accompanied by DFT calculations. Following this approach of using more earth-abundant metals, in chapter V the possibility of titanium(IV) complexes is considered, which could possibly undergo LMCT emission transitions. Different possible approaches towards a titanium-based luminophore will be given

Auger-assisted electron transfer from photoexcited semiconductor quantum dots

Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remain unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors (anthraquinone, methylviologen, and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ∼0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Franck-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes