Lowest Occupied Molecular Orbital Research Articles

Ultrafast Electron Transfer in Photosynthesis: Reduced Pheophytin and Quinone Interaction Mediated by Conical Intersections

The mechanism of electron transfer (ET) from reduced pheophytin (Pheo(-)) to the primary stable photosynthetic acceptor, a quinone (Q) molecule, is addressed by using high-level ab initio computations and realistic molecular models. The results reveal that the ET process involving the (Pheo(-) + Q) and (Pheo + Q(-)) oxidation states can be essentially seen as an ultrafast radiationless transition between the two hypersurfaces taking place via conical intersections (CIs). According to the present findings, an efficient ultrafast ET implies that the Pheo- and Q move toward each other in a given preferential parallel orientation, reaching the most effective arrangement for ET at intermolecular distances (R) around 5-3 Angstrom, where the lowest CIs are predicted. Favored donor/acceptor interactions are related to orientations with some overlap between the lowest occupied molecular orbitals (LUMO) of the two systems, and they lead to state-crossings at an earlier stage of the movement (larger R). Furthermore, when the topology of the interacting moieties does not make possible the LUMOs overlap, the corresponding diabatic potential energy curves do not intersect. Thus, it is anticipated that large scale motions, which are difficult to monitor experimentally, are actually occurring in the photosynthetic reaction centers of bacteria, algae, and higher plants, to fulfill the observed ultrafast ET processes.

Synthesis and characterization of phosphorescent cyclometalated iridium complexes containing 2,5‐diphenylpyridine based ligands

AbstractA series of phosphorescent cyclometalated iridium complexes with 2,5‐diphenylpyridine‐based ligands has been synthesized and characterized to investigate the effect of the simple ligand modification on photophysics, thermostability and electrochemistry. The complexes have the general structure (C∧N)2Ir(acac), where C∧N is a monoanionic cyclometalating ligand [e.g. 2,5‐diphenylpyridyl (dppy), 2,5‐di(4‐methoxyphenyl)pyridyl (dmoppy), 2,5‐di(4‐ethoxyphenyl)pyridyl (deoppy) and 2,5‐di(4‐ethylphenyl)pyridyl (deppy)]. The absorption, emission, cyclic voltammetry and thermostability of the complexes were systematically investigated. The (dppy)2Ir(acac) has been characterized using X‐ray crystallography. Calculation on the electronic ground state of (dppy)2Ir(acac) was carried out using B3LYP density functional theory. The highest occupied molecular orbital (HOMO) level is a mixture of Ir and ligand orbitals, while the lowest occupied molecular orbital (LUMO) is predominantly dppy ligand‐based. Electrochemical studies showed the oxidation potentials of (dmoppy)2Ir(acac), (deoppy)2Ir(acac), (deppy)2Ir(acac) were smaller than that of (ppy)2Ir(acac), while the oxidation potential of (dppy)2Ir(acac) was larger relative to (ppy)2Ir(acac). The 10% weight reduction temperatures of these complexes were above that of (ppy)2Ir(acac). All complexes exhibited intense green photoluminescence, which has been attributed to MLCT triplet emission. The maximum emission wavelengths in CH2Cl2 at room temperature were in the range 531–544 nm, which is more red‐shifted than that of (ppy)2Ir(acac). Copyright © 2005 John Wiley & Sons, Ltd.

Oxygen doping of TiO2/poly(phenylene–vinylene) bilayer solar cells

The change in electronic properties of poly[2-methoxy-5-(3,7dimethyloctyloxy), paraphenylene–vinylene] (MDMO–PPV) in oxygen-saturated SnO2:F/TiO2/MDMO–PPV/Hg cells caused by optical irradiation is the subject of the present study. During preparation, the samples are exposed to air with exclusion of light, or in a glovebox in the presence of light. When these samples are subjected to a well-controlled light dose in ambient atmosphere, their current–voltage behavior changes. If open-circuit conditions prevail during irradiation, the dark forward and reverse currents increase. However, when the cells are shorted during irradiation, in ambient atmosphere, virtually no change in the I–V response occurs. The changes in the I–V response are reversible and are not related to photodegradation. These findings can be accounted for by the occurrence of oxygen doping. When oxygen is present it can interact with electrons in the lowest occupied molecular orbital (LUMO) band of the polymer to form a charge transfer complex. Due to this electron transfer, holes are generated in the MDMO–PPV highest occupied molecular orbital band. This type of oxygen doping takes place upon illumination under open-circuit conditions, since in this case LUMO-band electrons are not transferred to TiO2. When cells are short circuited, the photogenerated electrons are injected into the conduction band of TiO2 and quickly escape from the interface. In this case the electron transfer to oxygen is inhibited. To evaluate the oxygen-doping effect quantitatively, capacitance–voltage scans have been recorded. From these scans the increase of the acceptor density due to the light dose is determined. It is found that soon after the onset of irradiation the acceptor density becomes so high that tunneling of electrons through the barrier at the TiO2/MDMO–PPV interface becomes possible.

Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements

The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq3)/N,N′-di-(3-methylphenyl)-N,N′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thick Alq3 substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be −0.13 eV from Alq3 to TPD. By including the known HOMO–lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be −0.33 eV from Alq3 to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq3 to TPD than for hole injection from TPD to Alq3.

Self‐returning walks and fractional electronic charges of atoms in molecules

AbstractThree hierarchically ordered topological factors, i.e., atom connectivity, centrality, and cyclicity, were found to control the number of self‐returning walks (SRWS) associated with every atom in the molecule. The reversal of their order was observed in a few cases where the central location of atoms had a stronger influence than did their connectivity on the number of SRWS. Three atomic topological indices, i.e., the Morgan extended connectivity, the Balaban, Mekenyan, and Bonchev hierarchical extended connectivity, the Randić atomic path code, were found to closely match the ordering of atoms in molecules determined by their number of SRWS. New atomic graph invariants fi = lim SRW/SRWn and ti = fi · SRW2 were specified and may find application in QSAR and QSPR. The ti indices are nonintegers close to atomic valence. The fi indices represent the limit of the number of SRWS of length n for the atom i, SRW, normalized by dividing it by the total number of SRWS for the molecule. In the case of Hückel MO considerations, these invariants were shown to be numerically equal to the partial electronic charges of the lowest occupied molecular orbital (LOMO). A new class of isocodal atoms (atoms having the same number of SRWns) was observed, i.e., atoms that become isocodal only at n ⩾ 1. A number of open questions following from these findings were formulated, including the possibility for a topological modeling of electron correlation. © 1993 John Wiley & Sons, Inc.

Am 1 Semiempirical Mo Calculations of Torsional Barriers and Structural Reorganization in Radical Ions of Pbo and Pbzt Model Structures

ABSTRACTAM 1 semiempirical calculations on model structures of neutral, anionic and cationic poly(p-phenylenebenzobisthiazole), PBZT, offer an explanation why the PBZT anion has been shown to be an electrical conductor while the PBZT cation is not. To wit, the highest occupied molecular orbital (HOMO) in the PBZT model structure is centered primarily on the sulfur atoms, resulting in concentration of the positive charge on the sulfur atom when the cation is formed. The lowest occupied molecular orbital (LUMO) is far more delocalized and consequently anion formation produces a species which has electronic charge delocalized over the entire model structure. Similar calculations on model structures of neutral, anionic and cationic poly(p-phenyienebenzobisoxazole), PBO, indicate that both frontier orbitals, i. e., the HOMO and the LUMO are delocalized.

Quantum mechanical interpretation of photodecomposition of di-tertiarybutyl mercury

The quantum mechanical “Extended Hückel Theory” is applied to the electronic structure calculation of compounds containing heavy elements like mercury. Di-tertiarybutyl mercury, which deposits elementary mercury when irradiated, shows an absorption band at 242 nm (ε = 4070). A quantitative discussion of the molecular orbitals calculated by the Extended Hückel Method, using a basis set of s-, p- and d-functions was made. The calculations indicate that the absorption band at 242 nm is attributed to a charge transfer between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Occupied Molecular Orbital (LUMO). It was found to be a symmetry allowed transition and its calculated energy is in good agreement with that observed. A quantitative discussion of the coefficients of different atomic orbitals of both the HOMO- and LUMO-calculated wave functions and calculation of bond orders between Hg- and C-atoms for the ground and first excited states explained reasonably, that it is this transition which is responsible for the decomposition of di-tertiarybutyl mercury with the liberation of elementary mercury.