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.     

Low temperature kinetics and energetics of the electron and hole traps in irradiated TiO2 nanoparticles as revealed by EPR spectroscopy

We have monitored exclusively the dynamics of photogenerated charge carriers trapping in deep traps and trapped electron-hole recombination in UV irradiated anatase TiO2 powders by electron paramagnetic resonance (EPR) spectroscopy at 10 K. The results reveal that the strategy of using low temperatures contributes to the stabilization of the charged pair states for hours by reducing the rate of electron-hole recombination processes. Since only the localized states such as holes trapped at oxygen anions and electrons trapped at coordinatively unsaturated cations are accessible to EPR spectroscopy, the time-dependent population and depopulation of these EPR signals reflect the kinetics and energetics of these trap states. The data support a model of sequential accumulation of deep trap site populations in which the initial fast direct trapping into a deep trap site is followed by slower carrier trap-to-trap hopping until a deep trap is encountered for both photogenerated electrons and holes. Effective modeling of the subsequent decay of trapped-holes is achieved by employing a first-order kinetics, whereas the decay of either surface- or inner-trapped electrons has both a fast and a slow component. The fast component is attributed to a trapped-electron and a free-hole recombination, and the slow component is attributed to trapped electron-hole recombination. The activation energies for the process of diffusion of trapped electrons from their Ti3+ trapping sites are estimated.     

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.     

Semiconducting polymers based on charge-transfer complexes. IV. Electrical properties and structure of polymeric charge-transfer complexes

Model electron donor molecules, 10-methylphenothiazine and 4-(methylthio)anisole, and polymeric electron donors which contained these molecules on the side chains of N-acyl-substituted polyethylenimines, were complexed with the electron acceptors, dichlorodicyanoquinone (DDQ), tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), and tetranitrofluorenone (TNF). The model donors formed 1:1 complexes with all the acceptors except TCNE. The polymeric donors formed amorphous complexes with DDQ, TCNQ, and TCNE. Crystalline complexes were formed with TNF which had low melting points (lower than the model complexes and the pure polymer). This is apparently due to poor lateral packing of the polymer chains. Electrical resistivities were lower for all the polymer complexes than for the corresponding model complexes. Electrical resistivity also decreased with increase in complex crystallinity. In the best case the polymer complex was two hundred times as conducting as the model. The concentration of unpaired electrons measured by EPR was nearly independent of temperature. Most of the electrons seen are trapped and do not participate in conduction. Thermal activation energies for conduction were in the range of 0.5–1.8 eV and were nearly equal for the model and corresponding polymeric complexes. Elongation of polymer complex with TCNQ by rolling produces a decrease in resistivity in the roll direction, although the complex is amorphous. This reinforces the hypothesis that conduction is parallel to the polymer backbone. A polymer–tetranitrofluorenone complex was photoconducting, though the photoconductivity was smaller than the dark conductivity at the level of illumination used. Dember and Seebeck effects indicated that the major carrier in the complex was holes.     

EXPONENTIAL TRAPPING AND CONDUCTION PARAMETERS IN POLYMERS

Abstract Application of nearest-neighbour band theory to the case of charges trapped on polymer chains shows that the trapping states can have an exponential energy distribution. Comparison with existing photoconduction data suggests that Fermi level in polymers is about 1eV below the conduction band, that the mobility activation energy is 0.1eV or less in commercial polymers and that the conduction bands are narrow.     

SENSITIZATION OF PHOTOCONDUCTIVE EFFECTS IN ZINC OXIDE*,†

Abstract— The current status of research on sensitized photoconductive effects in zinc oxide is reviewed. The various types of chemical and spectral photosensitization processes observed in polycrystalline specimens can be explained qualitatively with a generalized electron acceptor molecule (EAM) model. in which electronegative surface agents trap zinc oxide conduction electrons in the dark and illumination causes a release of the trapped electrons. The EAM may be an adsorbed gas or vapor, adsorbed dye molecules, or an aggregated dye film, depending on the particular photoeffect.     

Mapping of the Photoinduced Electron Traps in TiO2 by Picosecond X‐ray Absorption Spectroscopy

Titanium dioxide (TiO₂) is the most popular material for applications in solar‐energy conversion and photocatalysis, both of which rely on the creation, transport, and trapping of charges (holes and electrons). The nature and lifetime of electron traps at room temperature have so far not been elucidated. Herein, we use picosecond X‐ray absorption spectroscopy at the Ti K‐edge and the Ru L₃‐edge to address this issue for photoexcited bare and N719‐dye‐sensitized anatase and amorphous TiO₂ nanoparticles. Our results show that 100 ps after photoexcitation, the electrons are trapped deep in the defect‐rich surface shell in the case of anatase TiO₂, whereas they are inside the bulk in the case of amorphous TiO₂. In the case of dye‐sensitized anatase or amorphous TiO₂, the electrons are trapped at the outer surface. Only two traps were identified in all cases, with lifetimes in the range of nanoseconds to tens of nanoseconds.     

Study of molecular trapping inside gold nanofinger arrays on surface-enhanced Raman substrates

The binding of trans-1,2-bis(4-pyridyl)-ethylene (BPE) molecules on substrates arrayed with flexible gold nanofingers has been studied by surface-enhanced Raman spectroscopy (SERS) and angle-resolved X-ray photoelectron spectroscopy (AR-XPS). On the basis of the SERS and XPS results, BPE molecules are found to interact with the gold nanofingers through the lone pair electrons of pyridyl nitrogens, not through delocalized π electrons. Furthermore, after comparing the AR-XPS spectra of finger arrays preclosed before exposure to BPE with the spectra of arrays that closed after exposure to BPE, we observed in the latter case, at grazing takeoff angles, an increase in the component of the nitrogen photoelectron peak associated with pyridyl nitrogen atoms residing on bridging sites. These results demonstrate that a small percentage of BPE molecules was trapped between the neighboring gold finger tips during the finger closing process. However, because these trapped BPE molecules coincidently resided in the hot spots formed among the touching finger tips, the substantial increase in the observed SERS signal was dominated by the contribution from this small minority of BPE molecules.     

Dissociation of electrolytes in a nano-aqueous system within single-wall carbon nanotubes

Research on material incorporation within single-wall carbon nanotubes (SWNTs) through aqueous solutions of various electrolytes is performed for the purpose of providing a foundation for future application of SWNTs to, for example, drug delivery systems. We have determined that the optical spectra of SWNTs were significantly affected when SWNTs that had opened holes or removed caps were treated through immersion in an aqueous solution of electrolytes, followed by drying at room temperature; however, the spectra of SWNTs without opened holes or removed caps were not subjected to such treatment. We infer that when the sucked solutions remained inside the tubes, even after drying (the nano-aqueous system), the electrolyte was dissociated into ions, which was likely to change the electronic states of SWNTs. On the other hand, when the SWNTs were well-dried under vacuum, no remarkable changes in their optical spectra were observed.     

Inside powders: a theoretical model of interfaces between MgO nanocrystallites

The electron- and hole-trapping and optical properties of a wide variety of interfaces between MgO nanocrystallites are investigated for the first time using a quantum-mechanical embedded-cluster method and time-dependent density functional theory. We conclude that delocalized holes can be transiently trapped at a large number of places within a powder. However, it is more energetically favorable for holes to trap on low-coordinated anions on the nanocrystallite surface, forming O- species. Electrons are trapped at few interfaces but are readily trapped by surface kink and corner sites. Contrary to common perception, our calculations of optical absorption spectra indicate that a variety of features buried within a powder can be exited with photon energies less than 5 eV, usually used to selectively excite low-coordinated surface sites.     

Electronic gas-phase spectra of larger polyacetylene cations

The origin bands of the A 2Pi-X 2Pi electronic transition for three new linear polyacetylene cation chains, HC12H+, HC14H+, and HC16H+, have been recorded in the gas phase at approximately 30 K, located at 924.7, 1034.6, and 1144.0 nm. The absorption spectra were observed using a two-color two-photon ion-photodissociation experiment that utilizes the cooling capabilities of a 22-pole ion trap. Such spectra allow a direct comparison between laboratory and astrophysical data; however, no matches were found between the experimentally determined origin bands and the known diffuse interstellar bands.     

Quasi-elastic light scattering studies on T7 DNA in the presence of a sinusoidal electric field

The mobilities and lifetimes of electrons and holes as well as the triplet exciton lifetime have been measured in a range of samples originating from a long anthracene crystal grown from the vapour phase in the zone refining tube. The results have demonstrated the influence of chemical and physical imperfections on these parameters. The triplet lifetime has been found to be fairly insensitive to impurities, as opposed to the lifetime of electrons and holes. The measurements of the charge-carrier mobility as a function of temperature have revealed that hole transport in a defective anthracene crystal is controlled by structural traps 0.26–0.27 eV deep, whereas the electron mobility. contrary to theoretical predictions. is lattice controlled over the investigated temperature range. These localized states can be photochemically eliminated. which suggests that paired anthracene molecules (“incipient” dimers) are responsible for the observed trapping asymmetry. These observations, by analogy with dimeric species forming excimers while excited, have been interpreted in terms of new chemical species formed as a result of trapping at “incipient” dimer pairs.     

Solvatochromism of anthraquinone and symmetrical dihydroxy derivatives. Local interactions

The solvent effects on the electronic absorption spectra of 9,10-anthraquinone (AQ) and its symmetric dihydroxy derivatives namely 1,5-dihydroxyanthraquinone (1,5-DHAQ) and 2,6-dihydroxyanthraquinone (2,6-DHAQ) have been studied in pure solvents and some binary solvent mixtures. The frequencies of the absorption for AQ and 2,6-DHAQ are quite solvent sensitive while those for 1,5-DHAQ are not. Due to the intramolecular hydrogen bond between the CO and OH groups, no influence of solvent hydrogen bond acceptors is observed in 1,5-DHAQ. This hydrogen bond gives a stable six member cycle which is not broken even by the strongest hydrogen bond acceptor solvents used in this work, such as DMSO and DMF. The Taft and Kamlet's solvatochromic comparison method was applied for AQ and 2,6-DHAQ. Aromatic solvents and aliphatic amines were not included in the correlations since they strongly deviate suggesting another type of interactions. All the π→π^* bands of AQ and 2,6-DHAQ show strong influence of π^* despite the fact that their dipole moment is zero. Although it would be reasonable to expect that in the absence of a solute dipole moment there is not significant orientation of solvent molecules around the solute molecules, in this case dipolar interactions between solute and solvent due to local effects might be expected. AQ may be considered as formed by two carbonyl groups weakly interacting with the benzene rings; that means that the carbonyl group can behave as an isolated dipole and independently of the other. To detect possible specific interactions between the AQ and aliphatic amines and aromatic hydrocarbons, preferential solvation in mixed solvent was investigated. It is concluded that EDA interactions are important in the solvation of AQ with these compounds as solvents.     

Formation and dynamics of van der Waals molecules in buffer-gas traps

We show that weakly bound He-containing van der Waals molecules can be produced and magnetically trapped in buffer-gas cooling experiments, and provide a general model for the formation and dynamics of these molecules. Our analysis shows that, at typical experimental parameters, thermodynamics favors the formation of van der Waals complexes composed of a helium atom bound to most open-shell atoms and molecules, and that complex formation occurs quickly enough to ensure chemical equilibrium. For molecular pairs composed of a He atom and an S-state atom, the molecular spin is stable during formation, dissociation, and collisions, and thus these molecules can be magnetically trapped. Collisional spin relaxation is too slow to affect trap lifetimes. However, (3)He-containing complexes can change spin due to adiabatic crossings between trapped and untrapped Zeeman states, mediated by the anisotropic hyperfine interaction, causing trap loss. We provide a detailed model for Ag(3)He molecules, using ab initio calculation of Ag-He interaction potentials and spin interactions, quantum scattering theory, and direct Monte Carlo simulations to describe formation and spin relaxation in this system. The calculated rate of spin-change agrees quantitatively with experimental observations, providing indirect evidence for molecular formation in buffer-gas-cooled magnetic traps. Finally, we discuss the possibilities for spectroscopic detection of these complexes, including a calculation of expected spectra for Ag(3)He, and report on our spectroscopic search for Ag(3)He, which produced a null result.     

The solvent trapping or co-adsorbing effect during the self-assembly monolayer studied by surface-enhanced Raman scattering

The possibility of solvent molecules being trapped within the monolayer interior during the self-assembly of 2,5-dimercapto-1,3,4-thiadiazole (DMTD) molecules from alcoholic solution, or co-adsorbing together with the solute molecules onto the silver surface was verified by means of surface-enhanced Raman scattering (SERS). Variations of the relative intensity of the solvent bands to the concentrations of the solution were investigated. To a certain range of the concentration, the smaller the concentration is, the larger the relative intensity of the solvent bands is. A new method for obtaining the SERS spectra of the organic compounds that are insoluble in water was developed in our laboratory, which also provides further evidence for the solvent trapping or co-adsorbing effect.     

Resonance Raman study of free-base tetraphenylporphine and its dication

Resonance Raman spectra of free-base tetraphenylporphine and its dication obtained with 441.6, 476.5, 488.0 and 514.5 nm excitation lines in the frequency region 100-1625 cm(-1) are reported. Some bands due to in-plane and out-of-plane vibrational modes, which are symmetry forbidden under ideal D(2h), are also seen in the Raman spectra of these molecules. These bands arise due to dynamic and/or static coupling of out-of-plane modes with the allowed in-plane modes. Dynamic coupling may be facilitated by the proton tunneling, while static coupling is due to out-of-plane distortion in the geometrical structure of the molecule. Shift in the positions for certain bands in the Raman spectra of dication are interpreted on the basis of electronic changes due to sharing of electrons of the B(1u) orbital by the two added protons.     

Hole-trapping effect on thermoelectric power of mixed ionic electronic conductor BaTiO3

The effect of hole-trapping on the thermopower of a mixed ionic electronic conductor, e.g., BaTiO3, is analyzed in terms of irreversible thermodynamics by taking trapped holes as a fourth kind of electronic charge carrier in addition to free electrons, free holes and mobile oxide ions. It is found that the effect manifests itself in two ways: thermostatically in the ionic thermopower via the thermodynamic factor and dynamically in the electronic thermopower via the electrical conductivity contribution of the trapped holes. The thermopowers of both 99.995% pure, undoped and 1.8 m/o Al-doped BaTiO3, that were measured against oxygen activity in the range of -18 < log aO2 < or = 0 at elevated temperatures of 800 degrees to 1100 degrees C [H.-I. Yoo and C. R. Song, J. Electroceram., 2001, 6, 61, ref. 6], are reanalyzed by taking into account the hole-trapping for the doped case. It is found that while the reduced heats-of-transport of free electrons and holes are, respectively, close to their thermal energy k(B)T (k(B) being the Boltzmann constant), that of trapped holes is close to their migration energy that is essentially the same as the trapping energy onto the acceptors doped, 1.04 eV.     

The electronic structure of free water clusters probed by Auger electron spectroscopy

(H2O)(N) clusters generated in a supersonic expansion source with N approximately 1000 were core ionized by synchrotron radiation, giving rise to core-level photoelectron and Auger electron spectra (AES), free from charging effects. The AES is interpreted as being intermediate between the molecular and solid water spectra showing broadened bands as well as a significant shoulder at high kinetic energy. Qualitative considerations as well as ab initio calculations explain this shoulder to be due to delocalized final states in which the two valence holes are mostly located at different water molecules. The ab initio calculations show that valence hole configurations with both valence holes at the core-ionized water molecule are admixed to these final states and give rise to their intensity in the AES. Density-functional investigations of model systems for the doubly ionized final states--the water dimer and a 20-molecule water cluster--were performed to analyze the localization of the two valence holes in the electronic ground states. Whereas these holes are preferentially located at the same water molecule in the dimer, they are delocalized in the cluster showing a preference of the holes for surface molecules. The calculated double-ionization potential of the cluster (22.1 eV) is in reasonable agreement with the low-energy limit of the delocalized hole shoulder in the AES.     

The nature of the field dependence of drift mobility in molecularly doped polymers

A direct experimental comparison is performed for the field dependences of drift and effective mobilities of holes in a polar molecularly doped polymer (polycarbonate containing 30 wt % p-diethylaminobenzaldehyde diphenylhydrazone), which are measured by the time-of-flight method during the bulk irradiation of polymer samples with a pulse of fast electrons. A numerical simulation of the time-of-flight experiments is conducted with the use of the multiple-trapping model with the Gaussian energy trap distribution. The parameters of the model are determined during independent measurements. It is shown that, because of the nonequilibrium state of the transport of holes under the conditions of the time-of-flight experiment, the true Poole–Frenkel constant (0.27 (μm/V)^1/2) makes up only a part of the experimentally measured value (0.39 (μm/V)^1/2).     

Electron trapping by polar molecules in alkane liquids: cluster chemistry in dilute solution

Experimental observations are presented on condensed-phase analogues of gas-phase dipole-bound anions and negatively charged clusters of polar molecules. Both monomers and small clusters of such molecules can reversibly trap conduction band electrons in dilute alkane solutions. The dynamics and energetics of this trapping have been studied using pulse radiolysis-transient absorption spectroscopy and time-resolved photoconductivity. Binding energies, thermal detrapping rates, and absorption spectra of excess electrons attached to monomer and multimer solute traps are obtained, and possible structures for these species are discussed. "Dipole coagulation" (stepwise growth of the solute cluster around the cavity electron) predicted by Mozumder in 1972 is observed. The acetonitrile monomer is shown to solvate the electron by its methyl group, just as the alkane solvent does. The electron is dipole-bound to the CN group; the latter points away from the cavity. The resulting negatively charged species has a binding energy of 0.4 eV and absorbs in the infrared. Molecules of straight-chain aliphatic alcohols solvate the excess electron by their OH groups; at equilibrium, the predominant electron trap is a trimer or a tetramer, and the binding energy of this solute trap is ca. 0.8 eV. Trapping by smaller clusters is opposed by the entropy that drives the equilibrium toward the electron in a solvent trap. For alcohol monomers, the trapping does not occur; a slow proton-transfer reaction occurs instead. For the acetonitrile monomer, the trapping is favored energetically, but the thermal detachment is rapid (ca. 1 ns). Our study suggests that a composite cluster anion consisting of a few polar molecules imbedded in an alkane "matrix" might be the closest gas-phase analogue to the core of solvated electron in a neat polar liquid.