Interfacial charge-transfer absorption: 3. Application to semiconductor-molecule assemblies

Interfacial charge-transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the energy levels of a metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. Here we utilize a previously published model (J. Phys. Chem. B 2005, 109, 10251) to predict the energetics of IFCTA spectra for semiconductors and compare literature observations to these predictions for n-type semiconductors (largely TiO2). In contrast to metals, where IFCTA has been only rarely observed, new absorption features due to IFCTA are common for semiconductors such as TiO2. At issue is whether the electron accepting states in the TiO2 are localized or delocalized over the conduction band.     

Interfacial charge-transfer switch: ruthenium-dppz compounds anchored to nanocrystalline TiO2

The compound Ru(bpy)2(dppz-R)(PF6)2, where bpy is 2,2'-bipyridine and dppz-R is 11-(diethoxyphosphorylmethyl)dipyrido[3,2-a:2',3'-c]phenazine, was prepared and anchored to mesoporous nanocrystalline (anatase) TiO2 thin films as a probe of the effects of interfacial water on excited-state charge transfer processes at semiconductor interfaces. In nitrogen-saturated fluid acetonitrile, the Ru(bpy)2(dppz-R)(PF6)2 compound was found to be highly photoluminescent. Water was found to quench the excited state by a mechanism adequately described by the Perrin model, from which the radius of quenching was abstracted, 75 +/- 2 A. The Ru(bpy)2(dppz-R)(PF6)2 compounds were found to bind to the TiO2 thin films in high surface coverages, 5 x 10(-8) mol cm(-2). When these films were immersed in acetonitrile, long-lived excited states (tau = 825 ns) that were quenched by the addition of water were observed. About 30% of the excited states could not be quenched by water. Efficient electron injection, phi(inj) = 0.8, was observed after light excitation of Ru(bpy)2(dppz-R)/TiO2 in a 0.1 M LiClO4/acetonitrile solution. The addition of large concentrations of water, >0.5 M, was found to decrease the injection yield to phi(inj) = 0.3.     

Simple MO-calculations of the charge-transfer absorption in quinhydrones

HMO-calculations of charge-transfer absorptions are reported for the “pseudoortho” and “pseudogeminal” orientations of 1,4-benzoquinone and hydroquinone. This approach is used to explain the difference in the charge-transfer absorptions which have been observed for the corresponding intramolecular quinhydrones 1 and 2.     

Interfacial charge transfer absorption: Application to metal–molecule assemblies

Optically induced charge transfer between adsorbed molecules and a metal electrode was predicted by Hush to lead to new electronic absorption features, but has been only rarely observed experimentally. Interfacial charge transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. Here, we utilize a previously published model [C. Creutz, B.S. Brunschwig, N. Sutin, J. Phys. Chem. B 109 (2005) 10251] to predict IFCTA spectra of metal–molecule assemblies and compare the literature observations to these predictions. We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that IFCTA is not detectable. However, few experiments designed to detect IFCTA have been done. We suggest approaches to optimizing the conditions for observing the process.     

Visible-light photocurrent response of TiO2-polyheptazine hybrids: evidence for interfacial charge-transfer absorption

We investigated photoelectrodes based on TiO(2)-polyheptazine hybrid materials. Since both TiO(2) and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO(2) (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO(2) gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs. NHE, pH 7) allows for photooxidation of water (+0.82 V vs. NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO(2) nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO(2)-polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.     

Solvent effects on the three-photon absorption of a symmetric charge-transfer molecule

We present a theoretical study of the solvent-induced three-photon absorption cross section of a highly conjugated fluorene derivative, performed using density functional (DFT) cubic response theory in combination with the polarizable continuum model. The applicability of the often used two-state model is examined by comparison against the full DFT response theory results. It is found that the simplified model performs poorly for the three-photon absorption properties of our symmetric charge-transfer molecule. The dielectric medium enhances the three-photon absorption cross section remarkably. The effects of solvent polarity and geometrical distortions have been carefully examined. A detailed comparison with experiment is presented.     

Vibronic induced one- and two-photon absorption in a charge-transfer stilbene derivate

Both the electronic and the vibronic contributions to one- and two-photon absorption of a D-pi-D charge-transfer molecule (4-dimethylamino-4'-methyl-trans stilbene) are studied by means of density functional response theory combined with a linear coupling model. Vibronic profiles of the first four excited states are fully explored. The dominating vibrational modes for both Franck-Condon and Herzberg-Teller contributions are identified. The Franck-Condon contribution dominates the spectra of first, second, and fourth excited states. The Herzberg-Teller contribution is on the other hand of comparable size for the third excited state, where its inclusion leads to a blueshift with respect to the vertical transition. A similar vibronic coupling behavior is found for both one- and two-photon absorptions.     

Quantum effects in semiclassical treatment of molecular reactions

By the use of the known Coulomb wavefunctions, “quantal” Coulomb streamlines are defined for the eikonal of the system. Comparisons of such streamlines with the classical Coulomb trajectories are made. Use of such “quantum” Coulomb streamlines is proposed for reaction systems containing the Coulomb interaction U. The derived equations of motion are of particular use for cases where the residual potential V₁ (V₁ = V ? U) has either no classical forbidden region or has relatively small effects on trajectories.     

Strong two-photon absorption in new asymmetrically substituted porphyrins: interference between charge-transfer and intermediate-resonance pathways

We study two-photon absorption (2PA) in two series of new free-base porphyrins with 4-(diphenylamino)stilbene or 4,4'-bis-(diphenylamino)stilbene (BDPAS) attached via pi-conjugating linkers at the porphyrin meso-position. We show that this new substitution modality increases the 2PA cross section in the Soret band region (excitation wavelength 750-900 nm) of the core porphyrin by nearly 2 orders of magnitude, from sigma(2) approximately 10 GM for the meso-phenyl-substituted analogue to sigma(2) approximately 10(3) GM for the ethynyl-linked BDPAS-porphyrin dyad. The 2PA properties are quantitatively described by considering two different and interfering 2PA quantum transition pathways. The first path involves virtual transition via intermediate one-photon resonance. The second path bypasses the intermediate resonance and occurs due to a large permanent dipole moment difference between the ground and the final electronic states. To our best knowledge, this is the first experimental observation of the combined effect of these two pathways on one particular two-photon transition, resulting in quantum-interference-modulated 2PA strength.     

Ligand-ligand inter-valence charge-transfer absorption in reduced ruthenium(II) bipyridine complexes

An electronic absorption band at 4000 cm^?1 in incompletely reduced Ru(II)-bipyridine and Ru(II)-bipyride-pyridine complexes is characteristic of co-existing bpy^o and bpy^?1 ligands and assigned to bpy^?/bpy⁰ inter-valence charger transfer.     

Interfacial effects on moisture absorption in thin polymer films

Moisture absorption in model photoresist films of poly(4-hydroxystryene) (PHOSt) and poly(tert-butoxycarboxystyrene) (PBOCSt) supported on silicon wafers was measured by X-ray and neutron reflectivity. The overall thickness change in the films upon moisture exposure was found to be dependent upon the initial film thickness. As the film becomes thinner, the swelling is enhanced. The enhanced swelling in the thin films is due to the attractive nature of the hydrophilic substrate, leading to an accumulation of water at the silicon/polymer interface and subsequently a gradient in concentration from the enhancement at the interface to the bulk concentration. As films become thinner, this interfacial excess dominates the swelling response of the film. This accumulation was confirmed experimentally using neutron reflectivity. The water rich layer extends 25 +/- 10 A into the film with a maximum water concentration of approximately 30 vol %. The excess layer was found to be polymer independent despite the order of magnitude difference in the water solubility in the bulk of the film. To test if the source of the thickness dependent behavior was the enhanced swelling at the interface, a simple, zero adjustable parameter model consisting of a fixed water rich layer at the interface and bulk swelling through the remainder of the film was developed and found to reasonably correspond to the measured thickness dependent swelling.     

Anti-[3.3]metacyclophane quinhydrones: synthesis,charge-transfer absorption and thermal rearrangement to syn-stereoisomers

The synthesis of $\text{anti}$-[3.3]metacyclophane quinhydrones and their thermal rearrangement to $\text{syn}$-stereoisomers are described. Charge-transfer absorptions are discussed under the aspect of different donor-acceptor orientations.     

Electronic absorption spectra of charge-transfer complexes and effects of substituents in radical cations

The effects of X substituents on the energies of charge-transfer bandshv _CT in electronic absorption spectra of charge-transfer complexes of π-, n-, or σ-donors (DX) with π- or σ-acceptors (A) as well as on the ionization potentialsI _D of individual DX molecules are described by the equationhv _CT(l _D)=a +bσ₁ +cσ _R ⁺ +dσ_α. When DX and A are fixed, the inductive (bσ₁), resonance (cσ _R ⁺ ), and polarization (dσ_α) contributions tohv _CT andI _D are virtually identical. The electronic structure of the D^.+X donor component of the compact [A^.−, D^.+X] radical-ionic pair in a solution is similar to that of the radical cation generated upon photoionization of the individual DX molecule in the gaseous phase. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1002–1006, June, 2000.     

Solvent effect on the charge-transfer absorption bands of iodine complexes with aromatic hydrocarbons

The charge-transfer absorption bands of iodine complexes with aromatic hydrocarbons such as benzene, toluene, xylenes (p-, o- and m-), mesitylene and hexamethylbenzene have been measured in different solvents and different values have been obtained in each set of the complexes. The charge-transfer energies of each complex are plotted against the refractive index function and inverse dielectric constant of the solvents separately and the extrapolated values obtained at unit refractive index or dielectric constant are considered to be the corresponding values of the complexes in the gas phase.     

The charge-transfer absorption band of Eu3+ in oxides

The position of the charge-transfer band of Eu³⁺ in the absorption spectra of a number of oxides is discussed. It is shown that this position is more or less fixed in octahedral VI coordination and that it varies in VIII and XII coordinations as a function of the effective ionic radius of the relevant host lattice ion.     

Photochromism of a diarylethene charge-transfer complex: photochemical control of intermolecular charge-transfer interaction

A diarylethene derivative bearing a phenylenediamine group formed radical ions with an electron acceptor molecule in solution, and the concentration of the radical ions was modulated by the photochromic reaction of the diarylethene, reflecting the difference in the electron-donating character between the open- and closed-ring isomers.     

硝基芴桐类化合物与芳香胺相互作用的电子吸收光谱研究

本文研究不同结构的芳香胺和硝基芴酮形成的基态电荷转移复合物, 讨论结构和溶剂对基吸收光谱的影响.