Charge Transfer Optical Absorption Research Articles

Optical Absorption by Charge-Transfer Molecules.

An analytical formalism is developed for the calculation of absorption of radiation by charge-transfer molecules with electronic density delocalized between the donor and acceptor parts. The theory consistently incorporates both the vibronic coupling to quantum intramolecular vibrations and electrostatic interactions with a classical polarizable medium. The formulation operates in terms of basis-invariant parameters and can be used for calculations based on both the localized diabatic states and delocalized adiabatic wave function produced by standard quantum-chemistry algorithms. The basis-invariant reorganization energy is particularly important, since it determines observable spectroscopic parameters, in contrast to the adiabatic reorganization energy based on the molecular charge distributions in the adiabatic vacuum states. Analytical formulas are derived for the charge-transfer absorption band and first two spectral moments connecting spectroscopy to properties of charge-transfer molecules. Electronic delocalization requires a new definition of the Huang-Rhys factor traditionally used to construct vibronic band shapes. Theory's performance is illustrated by application to experimental data for self-exchange charge-transfer optical absorption and for metal-to-ligand emission transitions.

Charge-transfer optical absorption mechanism of DNA:Ag-nanocluster complexes.

Optical properties of DNA:Ag-nanoclusters complexes have been successfully applied experimentally in Chemistry, Physics, and Biology. Nevertheless, the mechanisms behind their optical activity remain unresolved. In this work, we present a time-dependent density functional study of optical absorption in DNA:Ag_{4}. In all 23 different complexes investigated, we obtain new absorption peaks in the visible region that are not found in either the isolated Ag_{4} or isolated DNA base pairs. Absorption from red to green are predominantly of charge-transfer character, from the Ag_{4} to the DNA fragment, while absorption in the blue-violet range are mostly associated to electronic transitions of a mixed character, involving either DNA-Ag_{4} hybrid orbitals or intracluster orbitals. We also investigate the role of exchange-correlation functionals in the calculated optical spectra. Significant differences are observed between the calculations using the PBE functional (without exact exchange) and the CAM-B3LYP functional (which partly includes exact exchange). Specifically, we observe a tendency of charge-transfer excitations to involve purines bases, and the PBE spectra error is more pronounced in the complexes where the Ag cluster is bound to the purines. Finally, our results also highlight the importance of adding both the complementary base pair and the sugar-phosphate backbone in order to properly characterize the absorption spectrum of DNA:Ag complexes.

Charge transfer optical absorption and fluorescence emission of 4-(9-acridyl)julolidine from long-range-corrected time dependent density functional theory in polarizable continuum approach

A long-range-corrected time-dependent density functional theory (LC-TDDFT) in combination with polarizable continuum model (PCM) have been applied to study charge transfer (CT) optical absorption and fluorescence emission energies basing on parameterized LC-BLYP xc-potential. The molecule of 4-(9-acridyl)julolidine selected for this study represents typical CT donor–acceptor dye with strongly solvent dependent optical absorption and fluorescence emission spectra. The result of calculations are compared with experimental spectra reported in the literature to derive an optimal value of the model screening parameter ω. The first absorption band appears to be quite well predictable within DFT/TDDFT/PCM with the screening parameter ω to be solvent independent (ω≈0.245Bohr−1) whereas the fluorescence emission exhibits a strong dependence on the range separation with ω-value varying on a rising solvent polarity from about 0.225 to 0.151Bohr−1. Dipolar properties of the initial state participating in the electronic transition have crucial impact on the effective screening.

Charge transfer excitons in low band gap polymer based solar cells and the role of processing additives

Organic semiconductor blends yielding efficient charge generation and transport are key components for the development of high performance organic bulk-heterojunction solar cells. In this paper the effect of the processing additive octane-dithiol on the charge transfer emission in poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is investigated. Despite the fact that blends processed with and without additive show a ground state charge transfer optical absorption only the blend processed without additive shows a corresponding charge transfer emission. The presented experimental data show that the nano-morphology of the bulk-heterojunction blends plays an important role for the formation of emissive charge transfer excitons, and that it is a loss channel in the studied solar cells.

Correct interpretation of heme protein spectra allows distinguishing between the heme and the protein dynamics.

It is shown by using the vibronic approach that the iron displacement out of the porphyrin plane in deoxyheme proteins intermixes the porphyrin pi and axial iron-histidine sigma electronic subsystems. This intermixing explains the substantial coupling of the iron-histidine vibration to the heme Soret excitation, the appearance of the iron-histidine band in the corresponding resonance Raman spectra, and a number of other experimental data, including the dependence of the iron-histidine vibrational frequency on the extent of the iron displacement out of the porphyrin plane. This dependence implies that there is an anharmonic coupling between the corresponding vibrations, which is shown to be the cause of the specific temperature dependence of the iron-histidine band. The anharmonic coupling and the dependence of the dipole transition moment of the charge transfer optical absorption band III on the iron-porphyrin distance cause the anomalous temperature and pressure dependencies of this band. It is shown that the change in both the magnitude and the distribution of the iron-porphyrin distance is expected to affect the band III intensity. Consequently, the stationarity of the band III intensity can be considered as a signature of the stationarity of the iron-porphyrin distance and its distribution in deoxyheme proteins, whereas the band III position and width could be also affected by the change in the protein electric field, caused by the protein globule dynamics.

"Almost delocalized" intervalence compounds

Interest is high in +M-bridge-M systems that have very low barriers to intramolecular electron transfer giving M-bridge-M+ ("almost delocalized" systems). Hush showed how to evaluate the electronic coupling across the bridge (H) from the M-to-M charge-transfer optical absorption band, but did not point out that his classical model causes the extinction coefficient to suddenly drop to zero at a photon energy of 2 H. Ignoring this band cutoff leads to a low estimation of H.

Atmospheric pressure excimer lamp-assisted photoselective activation process for electroless plating

Photoselective activation technologies for electroless metallisation have significant potential in minimising the number of activation process steps. In this work, a novel photo-selective activation process for electroless plating is described. The process is based on the selective photodecomposition of a palladium based precursor using a purpose built 222 nm KrCl* excimer lamp contact mask aligner. The photoexposure conditions to produce a selective palladium activation are detailed. Previously reported maximum pressure thresholds (1 mbar) for an efficient photoprocess have been overcome. This has been achieved using a triple excimer lamp source to increase ultraviolet (UV) intensity, a substrate temperature of 70–85°C and an organometallic precursor having a ligand to metal charge transfer optical absorption peak at 224 nm corresponding to the peak output of the KrCl* excimer lamp. Activation is reported at atmospheric pressure in a N 2 ambient, removing the need for processing under vacuum. Electroless copper features with a lateral resolution of 50 μm are demonstrated. The electroless metallisations are characterised in terms of adhesion performance, resistivity and resistance to peel-off during sawing.

Charge-transfer optical absorption in linear magnetic insulators in a strong electric field.

The effect of a strong electric field on the charge-transfer optical absorption is studied in linear narrow-band Mott-Hubbard insulators (or semiconductors) such as alkali-TCNQ's (tetracyanoquinodimethanes) and antiferromagnetic Heisenberg chains, using the half-filled-band Hubbard model with strong on-site Coulomb repulsion. It is shown that, for an antiferromagnetic ground state, the main effect of the field is to severely (i.e., nonlinearly with the applied field) squeeze the unperturbed continuum absorption band into a narrower band of unevenly spaced discrete rungs of a Stark ladder. The intensity of the absorption into the Stark-ladder states has exponential tails both above and below the main band, showing a discrete version of the Franz-Keldysh effect. For random and ferromagnetic spin configurations, the absorption profile narrows and the discrete absorption resonances smear into sharp continuum structures.

Valence-band photoemission and optical absorption in nickel compounds

Photoemission, optical-absorption, and isochromat spectra of NiO and ${\mathrm{NiCl}}_{2}$ are studied theoretically by the consideration of configuration interactions within the metal-ligand cluster. It is shown that the satellites in the valence-band photoemission spectra contain significant ${d}^{7}$ final-state components produced by photoemission of a $d$ electron from the largely ${d}^{8}$-like ground state and that final states giving the main lines are predominantly ${d}^{8}$-like resulting from ligand $\ensuremath{\rightarrow}3d$ charge-transfer transitions following the $d$-electron emission. This identification differs markedly from the traditional one, according to which the main lines are due to ${d}^{7}$ final states and the satellites are produced by ligand $\ensuremath{\rightarrow}3d$ shakeup transitions. The crystal-field splitting and the apparent reduction of Racah parameters are shown to be due to hybridization between different configurations. The resonance enhancement of the satellites rather than the main lines at the $3p\ensuremath{\rightarrow}3d$ photoabsorption threshold is attributed partly to covalency and partly to the small number of $3d$ holes in the nickel compounds as compared to other $3d$ transition-metal compounds. Excitation energies for ligand $p\ensuremath{\rightarrow}\mathrm{Ni} 3d$ charge-transfer optical absorption are calculated and it is shown that the fundamental absorption edge of NiO at ~4 eV is not due to the $p\ensuremath{\rightarrow}d$ charge-transfer transitions. Instead, $d\ensuremath{\rightarrow}d$ charge-transfer transitions are proposed as the origin of the NiO fundamental edge. Energy levels involved in the intra-atomic $d\ensuremath{\rightarrow}d$ optical absorption are also calculated by the configurationinteraction approach and good agreement with experiment and energy levels calculated by the ligand-field theory is obtained. Finally the isochromat spectrum of NiO is discussed, based on the same approach.

Electronic properties of Bi 2O 3 and MoO 3 and relationships to oxidation catalysis

A semiempirical atom superposition and electron delocalization molecular orbital analysis of the bonding and electronic structure of MoO 3, oxygen deficient MoO 3, and the α, β, and δ phases of Bi 2O 3 has been made. It is found that both small — e.g. MoO 6 — and large — e.g. Mo 6O 24 — clusters are useful models for cation electronic structure within the theory used. From the calculations, an interpretation is given for all available optical and photoemission data for the oxides. The color, conductivity, and new photoemission peak of oxygen-deficient MoO 3 conducting bronzes are found to be due to the addition of electrons to the lowest of three Mo 5d bands which are empty in MoO 3. Weakly allowed d ← d transitions in the red are responsible for the color. Strongly allowed Mo 5d ← O 2p charge transfer excitations are responsible for the optical absorption above 3.2 eV. For the bismuth oxides, three occupied bands are found showing strong Bi 6s, 6p, 6d and O 2p hybridization. These bands have been seen experimentally. The highest band surprisingly has Bi 6p lone-pair character which is explained in terms of the relative Bi 6s and 6p and O 2p ionization potentials using perturbation theory. Rather similar electronic structures are found for the three phases despite their varying cation coordinations and structures. A charge transfer optical absorption edge at ∼ 2.6 eV for the β form agrees well with observations reported in the literature, and similar edges should occur for the other phases. The cubic δ form has an unusual low-lying band suggesting absorption in the infrared. Our results provide insight into the surface properties of these oxides.

Measurement of small charge transfer optical absorption bands by modulation spectroscopy

An optical modulation apparatus is described for the measurements of very weak (ε<1 M−1 cm−1) charge transfer optical absorption bands between electron donor–acceptor complexes. Such small bands, which are of interest in understanding the mechanism of electron transfer in physical, chemical, and biological systems, are undetectable by more traditional methods of optical spectroscopy due to their small size and their superposition on much larger background sample absorptions. An additional feature of the method is that the kinetics of the electron returning to its equilibrium state following photo-induced charge transfer can be measured.

Fundamental aspects of electron transfer: experimental verification of vibronically coupled electron tunneling.

A detailed investigation of the applicability of the vibronically coupled electron tunneling theory in biomolecules can be made by a quantitative study of a weak charge-transfer optical absorption band that has been predicted by this theory. The measurement of the position, width, and molar extinction coefficient of this band is examined in the bound model system cytochrome c-Fe(CN)6 at room temperature and demonstrates that the theory is quantitatively applicable in this system. The size of the parameters measured is typical of those relevant for biological electron transfers. The comparisons lend credibility to the generality of vibronically coupled electron theory in biomolecules and its short transfer distances.

Photo-induced charge transfer. A critical test of the mechanism and range of biological electron transfer processes

The vibronic coupling theory of electron tunneling between biomolecules requires that all such tunnelings involve vibronic coupling, finds temperature dependence to tunneling at finite temperatures, and predicts relatively short tunneling distances. This theory might be expected to apply to most electron transfers involved in the membrane-bound electron transfer reactions of photosynthesis and oxidative phosphorylation. This paper calculates the properties of a weak charge-transfer optical absorption band, whose predicted characteristics are a direct and simple consequence of the model that describes vibronically coupled tunneling. The new absorption band provides the basis for a critical experimental test of the constructs and parameters of the tunneling theory. If the tunneling theory is valid, the oscillator strength of such bands will be the most reliable measure of the tunneling matrix element and of the distance between the sites exchanging an electron.