Metallophilic interactions in closed-shell copper(I) compounds--a theoretical study

Cuprophilic interactions in neutral perpendicular model dimers of the type (CH3CuX)2 (X = OH2, NH3, SH2, PH3, N2, CO, CS, CNH, CNLi) were analyzed by ab initio quantumchemical methods. The basis set superposition error for the weakly interacting CH3CuX subunits is significant and is discussed in detail. A new correlation-consistent pseudopotential valence basis set for Cu. derived at the second-order M?ller-Plesset level suppresses considerably the basis set superposition error in Cu-Cu interactions compared to the standard Hartree-Fock optimized valence basis set. This allowed the first accurate predictions of cuprophilicity, which has been the subject of considerable debate in the past. The dependence of the strength of cuprophilic interactions on the nature of the ligand X was addressed. The Cu-Cu interaction increases with increasing sigma-donor and pi-acceptor capability of the ligand and is approximately one-third of the well-documented aurophilic interactions. By fitting our potential-energy data to the Hershbach-Laurie equation, we determined a relation between the Cu-Cu bond length and the Cu-Cu force constant; this is important for future studies on vibrational behaviour. The role of relativistic effects on the structure and the interaction energy is also discussed. Finally we investigated cuprophilic interactions in (CH3Cu)4 as a model species for compounds isolated and characterized by X-ray diffraction.     

Effects of pressure and temperature on the carrier transports in organic crystal: a first-principles study

By employing density-functional theory coupled with Holstein-Peierls model, we investigate the pressure and temperature dependence of the hole and electron mobilities in naphthalene single crystal from atmospheric pressure up to 2.1 GPa (at room temperature) and from 5 to 296 K (at ambient pressure). It is found that the pressure reduces the electron-phonon coupling strength and enhances the mobilities. Importantly, we point out that only when temperature-dependent structure modifications are taken into account can one better describe the temperature-dependent transport behavior. Especially, the band to hopping crossover transition temperature for the electron transport in the c'-axis is calculated to be around 153 K, which is close to the experimental result of between 100 and 150 K. If this temperature-dependent structure modifications were neglected, the transition temperature would be only about 23 K, as previously obtained [L. J. Wang et al., J. Chem. Phys. 127, 044506 (2007)].     

Photochemical generation of electrons and holes in germanium-containing ITQ-17 zeolite

Laser flash photolysis of germanium-containing ITQ-17 zeolite (Ge/ITQ-17, a single polymorph of beta zeolite) at 266 nm generates a transient spectrum decaying in the sub-millisecond time scale that is compatible with the formation of two transient species. The shorter lived transient (tau approximately 45 micros under nitrogen) has been assigned to trapped electrons due to the characteristic spectroscopic absorption (single band at 480 nm) and its quenching by typical electron scavengers such as N(2)O and CH(2)Cl(2). The second longer lived transient (lambda(max) = 500, 540, and 600 nm; tau approximately 390 micros) is not quenched by O(2) or electron scavengers, but it is quenched by methanol as hole scavenger and has been assigned to positive holes. Also there is a remarkable similarity of the transient spectrum of the Ge/ITQ-17 with the optical spectrum reported previously for electron-hole pairs in ZSM-5 zeolite. Under the same irradiation conditions, photoejection of electrons and photogeneration of positive holes has not been observed for conventional aluminosilicate zeolites, all-silica zeolites, or GeO(2)-impregnated zeolites. Therefore this photochemical behavior has been ascribed to the presence of framework germanium atoms opening the way for photoresponsive zeolites. The ability of Ge/ITQ-17 to generate photochemically electrons and holes has been confirmed by adsorbing naphthalene and propyl viologen sulfonate as electron donor and acceptor, respectively, and observing the generation of the corresponding radical ions.     

Recombination rate constant of free electrons and holes in thin CdSe films

The decay kinetics of electrons generated in thin CdSe films by laser pulse (wavelength 337 nm, pulse duration 8 ns) at 295 K was studied by the microwave photoconductivity method (36 GHz). Based on analysis of the photoresponse decay kinetics and the reactions of free and trapped electrons, holes, and ions, a model for the processes was proposed and the recombination rate constant of free electrons and holes in cadmium selenide was determined, being (4–6)· 10–11 cm³ s⁻¹. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 740–743, May, 2006. The samples were kindly presented by Yu. V. Meteleva.     

Organic mixed-valence compounds: a playground for electrons and holes

Mixed-valence (MV) compounds are excellent model systems for the investigation of basic electron-transfer (ET) or charge-transfer (CT) phenomena. These issues are important in complex biophysical processes such as photosynthesis as well as in artificial electronic devices that are based on organic conjugated materials. Organic MV compounds are effective hole-transporting materials in organic light emitting diodes (OLEDs), solar cells, and photochromic windows. However, the importance of organic mixed-valence chemistry should not be seen in terms of the direct applicability of these species but the wealth of knowledge about ET phenomena that has been gained through their study. The great variety of organic redox centers and spacer moieties that may be combined in MV systems as well as the ongoing refinement of ET theories and methods of investigation prompted enormous interest in organic MV compounds in the last decades and show the huge potential of this class of compounds. The goal of this Review is to give an overview of the last decade in organic mixed valence chemistry and to elucidate its impact on modern functional materials chemistry.     

Nature and energies of electrons and holes in a conjugated polymer, polyfluorene

Electrons and holes were injected selectively into poly-2,7-(9,9-dihexylfluorene) (pF) dissolved in a tetrahydrofuran (THF) and a 1,2-dichloroethane (DCE) solution, respectively, using pulse radiolysis. Transient absorption spectra of monoions of both signs revealed two bands attributable to formation of polarons, one in the visible region (pF+* at 580 nm, pF-* at 600 nm) and another in the near-IR region. Additional confirmation for the identification of pF+* and pF-* comes from bimolecular charge-transfer reactions, such as bithiophene-* + pF --> pF-* or pF+* + TTA --> +TTA+* (TTA = tri-p-tolylamine), in which known radical ions transfer charge to pF or from pF. Difference absorption spectra of pF chemically reduced by sodium in THF provided a ratio of absorbance of anions formed to bleaching of the neutral band at 380 nm. In conjunction with pulse-radiolysis results, the data show that each polaron occupies 4.5 +/- 0.5 fluorene units, most probably contiguous units. Extensive reduction of pF by sodium also revealed resistance to formation of bipolarons: excess electrons reside as separate polarons when two or more electrons were injected. Redox equilibria with pyrene and terthiophene by pulse radiolysis established reversible one-electron redox potentials of E0(pF+/0) = +0.66 V and E0(pF0/-) = -2.65 V vs Fc+/0. Together with the excited-state energy, these results predict a singlet exciton binding energy of 0.2 eV for pF in the presence of 0.1 M tetrabutylammonium tetrafluoroborate. This binding energy would increase substantially without an electrolyte.     

On the localization of electrons and holes in Hg n and rare-gas clusters

We have used a microscopic theory to study the size dependence of the degree of localization of the valence electrons and holes in neutral an ionized rare-gas-and Hg _n clusters. We discuss under which circumstances localization of the electrons and holes is favoured. We have calculated the ionization potential of Xe _n , Kr _n and small Hg _n clusters. Good agreement with experiments is obtained. We have also determined the dependence of the ionization potential on cluster structure.     

A theoretical study of thermodynamics and kinetics of nitrosamines: a potential no carrier

In this theoretical study, several hybird DFT functionals and MP2 method are used to investigate the properties and the kinetics of a series of nitrosamines. The results show SN or NS transnitrosation reaction to be more favorable via an S_N2-like pathway. The stability is predicted to be in the order of H₂NNO > cis-MeHNNO > trans-MeHNNO > Me₂NNO > trans-PhHNNO > cis-PhHNNO > cis-MeSNO > Ph₂NNO > N-methylenenitrous amide, in which Ph₂NNO and N-methylenenitrous amide will be potential candidates for the NO donor. For N-methylenenitrous amide, which has the strongest NO donating strength among the titled nitroamines, a nearly perpendicular configuration between H₂C=N and NO can plausibly be rationalized by the fact that lone pair of the nitrogen atom on the fragment H₂CN must be π-type, not σ-type, to form a mesomeric effect with π*N-O of the NO group. Using the polarizable continuum model to consider the water solvent effect, all the barriers and endothermicities of the transnitrosation reactions are decreased and the correlated %N–H and %N–S are decreased and increased.     

Self-trapping vs. non-trapping of electrons and holes in organic insulators: polyethylene

We show, by electronic structure based molecular dynamics simulations, that an extra electron injected in crystalline polyethylene should fall spontaneously into a self-trapped state, a shallow donor with a large novel distortion pattern involving a pair of trans-gauche defects. Parallel calculations show instead that a hole will remain free and delocalized. We trace the difference of behavior to the intrachain nature of the hole, as opposed to the interchain one of the electron, and argue that applicability of this concept could be more general. Thus electrons (but not holes) should tend to self-trap in saturated organic insulators, but not for example in aromatic insulators, where both carriers are intrachain.     

Photogeneration of holes and electrons in amorphous molecular semiconductors with neutral and ionic organic dyes

Photoconducting properties of amorphous molecular semiconductors based on polystyrene films doped with tetranitrofluorenone and merocyanine or an anionic polymethine dye, or with epoxypropylcarbazole and merocyanine or a cationic polymethine dye were studied. The former type of the films is characterized by electron conductivity, whereas the latter type by hole conductivity. The activation energy for photogeneration of mobile charge carriers increases on passing from a merocyanine dye to ionic dyes and decreases with a growth in quantum energy of excitation light for the films of the former type, but does not depend on the light quantum energy for the films of the latter type. It was concluded that the activation energy of photogeneration is determined by electrostatic interaction of a photogenerated charge carrier with the ionized photogeneration site or a counterion for neutral and ionic dyes, respectively. At low dissipation rates of the excess thermal energy of excited dye molecules via electron-nucleus interaction, photogenerated electrons have a possibility to travel over a long distance from the photogeneration site as compared with holes.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 3, 2005, pp. 195–203.Original Russian Text Copyright © 2005 by Davidenko, Derevyanko, Zabolotnyi, Ishchenko, Kuvshinskii, Studzinskii.     

Rate constant of the recombination of free electrons and holes in AgCl at 295 K

The transient kinetics of the loss of electrons generated by light pulses in powdered AgCl has been studied by the microwave photoconductivity method (36 GHz) at 295 K. At high light intensities,I ₀ > 10¹⁴ photon cm^–2 per pulse, the kinetics obeys the second-order law. The rate constant of the recombination of free electrons and holes is equal to 2·10^–12 cm³ s^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2234–2236, September, 1996.     

Probing redox photocatalysis of trapped electrons and holes on single Sb-doped titania nanorod surfaces

We used a fluorogenic reaction to study in conjunction the photocatalytic properties for both active sites (trapped photogenerated electrons and holes) on individual Sb-doped TiO(2) nanorods with single-molecule fluorescence microscopy. It was found that active sites around trapped holes show higher activity, stronger binding ability, and a different dissociation mechanism for the same substrate and product molecules in comparison with the active sites around trapped electrons. These differences could be elucidated by a model involving the charged microenvironments around the active sites.     

Electrochemistry, spectroscopy, and electrogenerated chemiluminescence of silole-based chromophores

We studied the electrochemical and spectroscopic properties of a series of extended silole-based chromophores to understand the effect of structure on behavior. By changing the substituents attached to the chromophore, we observed large variations in luminescence quantum efficiency (ca. 0-0.6), lambdamax for absorbance and photoluminescence (PL), and radical ion stability. The differences are related to the motion in the 2,5-substituents and the steric protection of both the chromophore and the reactive parts of the substituents. For several compounds the electrogenerated chemiluminescence (ECL) spectrum was also compared to the photoluminescence spectrum. In all cases, the ECL lambdamax and the PL lambdamax were about the same.     

On the accessibility of the bandgap on semiconductor interfaces by non-equilibrium Fermi energies of electrons and holes

Non-equilibrium surface Fermi energies of electrons and holes on semiconductor electrodes reach only a part of the bandgap. This phenomenon is proved in greater detail by calculation of Fermi energy profiles, for stationary irreversible anodic or cathodic as well as for reversible charge transfer processes. both in the dark and at illumination. The effect of limited bandgap accessibility upon surface charging is outlined briefly. Solid-state junctions are likewise taken into account.     

"Proton holes" in long-range proton transfer reactions in solution and enzymes: A theoretical analysis

Proton transfers are fundamental to chemical processes in solution and biological systems. Often, the well-known Grotthuss mechanism is assumed where a series of sequential "proton hops" initiates from the donor and combines to produce the net transfer of a positive charge over a long distance. Although direct experimental evidence for the sequential proton hopping has been obtained recently, alternative mechanisms may be possible in complex molecular systems. To understand these events, all accessible protonation states of the mediating groups should be considered. This is exemplified by transfers through water where the individual water molecules can exist in three protonation states (water, hydronium, and hydroxide); as a result, an alternative to the Grotthuss mechanism for a proton transfer through water is to generate a hydroxide by first protonating the acceptor and then transfer the hydroxide toward the donor through water. The latter mechanism can be most generally described as the transfer of a "proton hole" from the acceptor to the donor where the "hole" characterizes the deprotonated state of any mediating molecule. This pathway is distinct and is rarely considered in the discussion of proton-transfer processes. Using a calibrated quantum mechanical/molecular mechanical (QM/MM) model and an effective sampling technique, we study proton transfers in two solution systems and in Carbonic Anhydrase II. Although the relative weight of the "proton hole" and Grotthuss mechanisms in a specific system is difficult to determine precisely using any computational approach, the current study establishes an energetics motivated framework that hinges on the donor/acceptor pKa values and electrostatics due to the environment to argue that the "proton hole" transfer is likely as important as the classical Grotthuss mechanism for proton transport in many complex molecular systems.     

Glycosidic bond cleavage of pyrimidine nucleosides by low-energy electrons: a theoretical rationale

DNA damage by attachment of low-energy secondary electrons is a very interesting and important mechanism. Electron capture and subsequent base release are thought to be the elementary steps of this mechanism. The process of the N1-glycosidic bond breaking of anion radicals of pyrimidine nucleosides, specifically the 2'-deoxyribothymidine (dT) and 2'-deoxyribocytidine (dC) anions, has been investigated theoretically at the B3LYP/DZP++ level of theory. The release of nucleobases by the attachment of low-energy electrons depends on the formation of a stable anion radical of the nucleoside. The lower bond-breaking activation energy and the higher vertical electron detachment energy for dT enables the heterolytic cleavage of the N1-glycosidic bond. However, with the higher bond-breaking activation energy and the lower vertical electron detachment energy for dC, the release of cytosine might be impractical when the incident electrons have high kinetic energy. Furthermore, the release of cytosine would have a quantum yield much lower than that of dT when the incident electrons have lower kinetic energy. This study also demonstrates the importance of the proton at O5' of 2'-deoxyribose in the base release process. Extending this investigation from dT to dC advances the insight into the mechanism of the N1-glycosidic bond-breaking process. The information from this extensive investigation should be valuable for further experimental studies of cytosine release in irradiated DNA.     

Pulse radiolysis study of electrons in frozen alkaline solutions

The decay kinetics of e _t ^– optical absorption has been analyzed for 10 M frozen alkaline solutions of Na⁺, K⁺ and Rb⁺ cations. Samples were irradiated with 8 MeV electron pulses at temperatures in the range 92 to 160 K. The change of absorption with time depends on wavelength, temperature and cation used. To interprete the influence of cations and temperature on behaviour of electrons in the system examined a first-order kinetics with time dependent rate constant k(t)=B·t^–1 was used. The dependence of parameters and B on the kind of cation and irradiation conditions is discussed.