Intervalence Band Research Articles

Comparisons of Measured Rate Constants with Spectroscopically Determined Electron-Transfer Parameters

This work involves comparison of rate constants measured for an intervalence (IV) compound with electron-transfer parameters derived from its optical absorption spectrum. The temperature-dependent rate constants for the radical cation having 3-tert-butyl-2,3-diazabicyclo[2.2.2]oct-2-yl (hydrazine) charge-bearing units attached para to a tetramethylbenzene bridge (1+) were previously measured. In this study, resonance Raman is used to calculate the magnitudes of the distortions of normal modes of vibration caused by excitation into the intervalence absorption band. These data produce a vibrational reorganization energy lambdavsym of 9250 cm(-1), and averaged single-mode omegav for use in the Golden Rule equation of 697 cm(-1). Zhu-Nakamura theory has been used to calculate preexponential factors for analysis of the previously measured variable temperature optical spectra using quartic-enhanced intervalence bands to extract the total reorganization energy and the intramolecular electron-transfer rate constants for intramolecular electron transfer using electron spin resonance. In contrast to using the Golden Rule equation, separation of lambda into solvent and vibrational components is not significant for these data. The Zhu-Nakamura theory calculations produce ln(k/T) versus 1/T slopes that are consistent with the experimental data for electronic couplings that are somewhat larger than the values obtained from the optical spectra using Hush's method.

Perturbation of the Structure of P680 and the Charge Distribution on Its Radical Cation in Isolated Reaction Center Complexes of Photosystem II as Revealed by Fourier Transform Infrared Spectroscopy

The structure and the electronic properties of P680 and its radical cation in photosystem II (PSII) were studied by means of Fourier transform infrared spectroscopy (FTIR). Light-induced P680+/P680 FTIR difference spectra in the mid- and near-IR regions were measured using PSII membranes from spinach, core complexes from Thermosynechococcus elongatus, and reaction center (RC) complexes (D1-D2-Cytb559) from spinach. The spectral features of the former two preparations were very similar, indicating that the structures of P680 and its radical cation are virtually identical between membranes and cores and between plants and cyanobacteria. In sharp contrast, the spectrum of the RC complexes exhibited significantly different features. A positive doublet at approximately 1724 and approximately 1710 cm-1 due to the 131-keto C=O stretches of P680+ in the membrane and core preparations were changed to a prominent single peak at 1712 cm-1 in the RC complexes. This observation was interpreted to indicate that a positive charge on P680+ was extensively delocalized over the chlorophyll dimer in RC, whereas it was mostly localized on one chlorophyll molecule (70-80%) in intact P680. The significant change in the electronic structure of P680+ in RC was supported by a dramatic change in the characteristics of a broad intervalence band in the near-IR region and relatively large shifts of chlorin ring bands. It is proposed that the extensive charge delocalization in P680+ mainly causes the decrease in the redox potential of P680+/P680 in isolated RC complexes. This potential decrease explains the well-known phenomenon that YZ is not oxidized by P680+ in RC complexes.

Strong and weak coupling in mixed-valence complexes bridged by 4,4′-azo-di(phenylcyanamido)

The complexes [{Ru(tpy ∗)(bpy)} 2(μ-adpc)][PF 6] 2 where tpy ∗ is 4,4′,4″-tri-( tert-butyl)-2,2′:6′,2″-terpyridine, bpy is 2,2′-bipyridine, and adpc 2− is 4,4′-azo-diphenylcyanamide dianion and trans, trans-[{Ru(tpy ∗)(pc)} 2(μ-adpc)] where pc − is 2-pyrazine-carboxylato were prepared and characterized by cyclic voltammetry and spectroelectrochemical methods. Intervalence band properties and IR spectroelectrochemistry of the mixed-valence complexes [{Ru(tpy ∗)(bpy)} 2(μ-adpc)] 3+ and trans, trans-[{Ru(tpy ∗)(pc)} 2(μ-adpc)] + are consistent with delocalized and valence-trapped mixed-valence properties respectively. The reduction in mixed-valence coupling upon substituting a bipyridine ligand with 2-pyrazine carboxylato strongly suggests that hole-transfer superexchange is the dominant mechanism for metal–metal coupling in these complexes.

Continuum of Outer- and Inner-Sphere Mechanisms for Organic Electron Transfer. Steric Modulation of the Precursor Complex in Paramagnetic (Ion-Radical) Self-Exchanges

Transient 1:1 precursor complexes for intermolecular self-exchange between various organic electron donors (D) and their paramagnetic cation radicals (D+*), as well as between different electron acceptors (A) paired with their anion radicals (A-*), are spectrally (UV-NIR) observed and structurally (X-ray) identified as the cofacial (pi-stacked) associates [D, D+*] and [A-*, A], respectively. Mulliken-Hush (two-state) analysis of their diagnostic intervalence bands affords the electronic coupling elements (HDA), which together with the Marcus reorganization energies (lambda) from the NIR spectral data are confirmed by molecular-orbital computations. The HDA values are found to be a sensitive function of the bulky substituents surrounding the redox centers. As a result, the steric modulation of the donor/acceptor separation (rDA) leads to distinctive electron-transfer rates between sterically hindered donors/acceptors and their more open (unsubstituted) parents. The latter is discussed in the context of a continuous series of outer- and inner-sphere mechanisms for organic electron-transfer processes in a manner originally formulated by Taube and co-workers for inorganic (coordination) donor/acceptor dyads-with conciliatory attention paid to traditional organic versus inorganic concepts.

Growth of short-period InAs∕GaSb superlattices

The purpose of this work is to explore materials for midinfrared detectors that can operate at room temperature. Shorter-period InAs∕GaSb superlattices (SLs) have larger intervalance band separations, which are beneficial for reducing Auger recombination and tunneling current, thus making room temperature operation possible. To test these possibilities, several short-period SLs ranging from 50to11Å were grown and their morphological properties were carefully monitored by transmission electron microscopy. The effect of structural degradation caused by the period reduction on the optical properties was studied using low-temperature photoluminescence (PL). The samples with larger periods (50–32Å) showed excellent structural qualities and produced narrow full width at half maximum (FWHM) of the PL peak (5meV). As the period approached 24Å, slight layer thickness undulations within the SLs were observed and these undulations intensified as the period further reduced to 17Å. These structural degradations strongly influence optical properties causing significant broadening in the FWHM and higher residual carrier concentration in the shorter-period SLs. With slower growth rates, samples with periods as thin as 19Å were grown without significant layer undulations.

Asymmetric redox reactions of C3-symmetric starburst compound with six arylamines

The redox behavior of the titled species, arranged concentrically with three inner arylamines and three outer ones, was examined voltammetrically and electrospectroscopically in order to obtain the redox interaction between six redox active phenylamines. Our attention was paid to whether the nearly C 3 -symmetric species showed the symmetric or the asymmetric multi-redox reactions The steady-state voltammogram at a microelectrode exhibited asymmetric features of three waves of which oxidation currents had the ratio of 1:3:6. These numbers of the electrons transferred, n , were confirmed with the analysis of the Cottrell plots with non-zero intercept. Visible spectra for n = 1 and 3 showed intervalence bands. The voltammetric asymmetry was explained in terms of four interaction energies between N and N + and between N + and N + through a phenyl group at the meta - and the para -positions. The n vs. E curve calculated statistic mechanically for adequate values of the interaction energy agreed with the experimental one. The most stable chemical structure of an mixed-valence species at each n was suggested, for example, one two N + s in the inner arylamines and one N + in the outer ones for n = 3.

Multicomponent Supramolecular Devices: Synthesis, Optical, and Electronic Properties of Bridged Bis‐dirhodium and ‐diruthenium Complexes

AbstractFour ruthenium‐ and rhodium‐based metal–metal‐bonded multicomponent systems have been synthesized, and their absorption, redox, spectroelectrochemical and structural properties have been studied. The absorption spectra of the four bis‐dimetallic compounds M2LM2, where L is a bridging ligand and M is rhodium or ruthenium, exhibit very strong bands in the UV, visible and, for the diruthenium species, near‐IR region. The low‐energy absorption bands are assigned to charge‐transfer transitions involving a metal–metal bonding orbital as the donor and an orbital centered on the bis‐tetradentate aromatic ligands as the acceptor (metal–metal to ligand charge transfer, M2LCT). Each compound exhibits reversible bridging‐ligand‐centered reductions at mild potentials and several reversible oxidation processes. The oxidation signals of the two equivalent dimetallic centers of each bis‐dimetallic compound are split, with the splitting — a measure of the electronic coupling — depending on both the metal and bridging ligand. The mixed‐valence species of the dirhodium species was investigated, and the electronic coupling matrix element calculated from the experimental intervalence band parameters for one of them (86 cm–1) indicates a significant inter‐component electronic interaction which compares well with good electron conducting anionic bridges such as cyanides. Although none of these compounds is luminescent, the M2LCT excited state of one of the bis‐dirhodium complexes is relatively long‐lived (about 6 μs) in degassed acetonitrile at room temperature. The results presented here are promising for the development of linear poly‐dimetallic complexes built on longer naphthyridine‐based strands, with significant long‐range electronic coupling and molecular‐wire‐like behavior. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Intervalence band solitary waves in semiconductor quantum wells

We show theoretically the possibility of excitation of a special class of two-frequency solitons, sustained by the nonlinearity provided by the nonradiative intervalence band coherence between heavy-hole and light-hole excitonic transitions in semiconductor quantum wells. The formation of such resonant solitons excited by femtosecond pulses in InxGa1‑xAsyP1‑y quantum wells is investigated numerically using a phenomenological model based on the local-field approximation, and a realistic configuration for their experimental observation is proposed. The pumping conditions for using complex resonant soliton dynamics as an efficient source of continuum generation are also discussed.

Free-carrier absorption and growth temperature of highly Be-doped InGaAs in molecular beam epitaxy

The current–voltage characteristics of p ++-InGaAs/n-InP diodes grown by molecular beam epitaxy have been studied. When the substrates are directly radiatively heated during the growth (the so-called In-free mounting), a displacement of the electrical junction towards the n-type InP is observed for high doping ( [ Be ] = 9 × 1 0 19 cm - 3 ). This effect comes from an increased Be diffusion, induced by an anomalous growth temperature increase. The growth temperature set point had to be decreased by 80 °C to recover the expected current–voltage characteristic. Optical absorption measurements carried out in single p ++-InGaAs layer have shown the high strength of the inter-valence band absorption, which might be responsible for this significant temperature increase.

Study of Intervalence Bands due to Interchain Electron Transfer Phenomena in Purely Organic Conductive Polymers

not Available.

Charge-Localized Naphthalene-Bridged Bis-hydrazine Radical Cations

Electron transfer (ET) in four symmetrically substituted naphthalene-bridged bis-hydrazine radical cations (1,4; 1,5; 2,6; and 2,7) is compared within the Marcus-Hush framework. The ET rate constants (k(ET)) for three of the compounds were measured by ESR; the 2,7-substituted compound has an intramolecular ET that is too slow to measure by this method. The k(ET) values are significantly dependent upon the substitution pattern of the hydrazine units on the naphthalene bridge but do not correlate with the distance between them. This is contrary to an assumption that is frequently made about intervalence compounds that the bridge serves only as a spacer that fixes the distance between the charge-bearing units. The internal vibrational and solvent portions (lambda(v) and lambda(s)) of the total reorganization energy (lambda) have been separated using solvent effects on the intervalence band maximum, resulting in a lambda(v) that is the same, 9900 cm(-1), for the differently substituted naphthalenes. This is in accord with the general assumption that lambda(v) is primarily dependent upon the charge bearing unit and not the bridge. However, the trends in lambda(s) cannot be explained by dielectric continuum theory.

Interfaces as design tools for short-period InAs/GaSb type-II superlattices for mid-infrared detectors

AbstractThe effect of interface anisotropy on the electronic structure of InAs/GaSb type-II superlattices is exploited in the design of thin-layer superlattices for mid-IR detection threshold. The design is based on a theoretical envelope function model that incorporates the change of anion and cation species across InAs/GaSb interfaces, in particular, across the preferred InSb interface. The model predicts that a given threshold can be reached for a range of superlattice periods with InAs and GaSb layers as thin as a few monolayers. Although the oscillator strengths are predicted to be larger for thinner period superlattices, the absorption coefficients are comparable because of the compensating effect of larger band widths. However, larger intervalence band separations for thinner-period samples should lead to longer minority electron Auger lifetimes and higher operating temperatures in p-type SLs. In addition, the hole masses for thinner-period samples are on the order the free-electron mass rather than being effectively infinite for the wider period samples. Therefore, holes should also contribute to photoresponse. A number of superlattices with periods ranging from 50.6 to 21.2 Å for the 4 μm detection threshold were grown by molecular beam epitaxy based on the model design. Low temperature photoluminescence and photoresponse spectra confirmed that the superlattice band gaps remained constant at 330 meV although the period changed by the factor of 2.5. Overall, the present study points to the importance of interfaces as a tool in the design and growth of thin superlattices for mid-IR detectors for room temperature operation.

Spectroscopic and Electrochemical Study of the Adsorption of [Co(en)2Cl2]Cl on γ-Alumina: Influence of the Alumina Ligand on Co(III)/(II) Redox Potential

UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co(III)(en)2Cl2]Cl (en=ethylenediamine) complexes and the gamma-alumina-supported cis-Co((III)) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on gamma-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the gamma-alumina-supported cis-Co(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively.

On the temperature sensitivity of 1.5-/spl mu/m GaInNAsSb lasers

We analyze the temperature sensitivity of 1.5-/spl mu/m GaInNAsSb lasers grown on GaAs. Building on the method of Tansu and coworkers, we find evidence that the characteristic temperatures for the threshold current T/sub 0/ and external efficiency T/sub 1/ are balanced by a combination of monomolecular recombination and temperature destabilizing mechanism(s) near room temperature. At elevated temperatures, the destabilizing process(es) dominate, due to increased threshold current density J/sub th/. While it is difficult to definitively identify carrier leakage, Auger recombination, or a combination of the two as the responsible mechanism(s), results indicate that carrier leakage certainly plays a role. Evidence of intervalence band absorption was also found; T/sub 1/ was reduced, but J/sub th/ and T/sub 0/ were not significantly degraded. Conclusions are corroborated by supporting measurements of the Z-parameter with bias, spontaneous emission spectrum, and band-offsets. Spontaneous emission measurements show evidence of weak Fermi-level pinning within the active region at threshold, indicating a form of carrier leakage. This is consistent with the characteristic temperature analysis and a leakage mechanism is proposed. This process is partially responsible for the greater temperature sensitivity of device parameters and the poor internal efficiency. Methods for reducing the effects of each parasitic mechanism are also described.

Below bandgap optical absorption in tellurium-doped GaSb

Enhancement in below bandgap room temperature infrared transmission has been observed in tellurium (Te)-doped GaSb bulk crystals. The effect of Te concentration on the transmission characteristics of GaSb has been experimentally and theoretically analysed. Undoped GaSb is known to exhibit p-type conductivity with residual hole concentration of the order of (1–2) × 1017 cm−3 at room temperature due to the formation of native defects. For such samples, inter-valence band absorption has been found to be the dominant absorption mechanism. The residual holes could be compensated by n-type dopants such as Te. With increasing Te concentration, free carrier absorption due to electrons and inter-valley transitions in the conduction subband become significant. The dependences of various absorption mechanisms as a function of wavelength have been discussed in this paper.

Hole spin relaxation: optical deformation potential scattering

The scattering of hole spin during hole intra- and intervalence band transitions due to optical deformation potential interaction in tetrahedral semiconductors is considered. Matrix elements for transitions mediated by longitudinal and transverse optical phonons are presented. It is shown that for this type of scattering, the spin relaxation time can be found from hole momentum scattering time.

The effect of mixed alkali on EPR and optical absorption spectra in mixed alkali borate xNa 2O–(30 − x)K 2O–70B 2O 3 glasses doped with iron ions

Electron paramagnetic resonance (EPR) and optical absorption studies of iron doped mixed alkali borate glasses, xNa 2O–(30 − x)K 2O–70B 2O 3 (5 ⩽ x ⩽ 25) have been investigated as a function of alkali content to look for the ‘mixed alkali effect’ on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Fe 3+ ions. The EPR spectrum exhibits an intense resonance signal at g = 4.02 ± 0.1, a moderately intense signal at g = 2.02 ± 0.1 and a shoulder in the region of g = 7.20 ± 0.5. The existence of the resonances at g = 4.02 and g = 7.20 have been attributed to Fe 3+ ions in rhombic and axial symmetry sites respectively. The g = 2.02 resonance is due to Fe 3+ ions coupled by exchange interactions. The number of spins ( N) participating in resonance and its paramagnetic susceptibility ( χ) have been evaluated. It is observed that N and χ decrease with x up to x = 10 and then increase up to x = 15 exhibiting a minimum at x = 10 and a maximum at x = 15 and thereafter it gradually decreases. The optical absorption spectrum exhibits three bands assigned to 6A 1g(S) → 4A 1g(G), 4E g(G); 6A 1g(S) → 4T 2g(G) of Fe 3+ and the other band is assigned to Fe 2+–Fe 3+ inter-valence charge transfer band. From ultraviolet absorption edges, the optical bandgap and Urbach energies have been evaluated. The optical band gap energy obtained in the present work varies 2.74–3.77 eV for both the direct and indirect transitions. The physical parameters of all the glasses have been evaluated with respect to x.

Deformation potential limited spin relaxation in cubic semiconductors

The scattering of valence band hole spin by acoustical and optical phonons due to deformation potential interaction is considered. Six-band deformation potential matrix, which takes into account doubly degenerate heavy-mass, light-mass, and split-off energy bands, is used to evaluate the matrix elements for spin conserving and flipping transitions. The concept of the spin surface was addressed to define the initial and final states of the hole spin. It was found that, in agreement with experiment, the spin conserving and flipping transitions are of comparable magnitude in both intravalence and intervalence band scattering mediated by acoustical as well as optical phonons.

Correlations between electrochemical and spectral properties of alkyl-substituted diphthalocyanine lanthanide complexes

The electrochemical behavior and spectral properties of a series of symmetrical 2,3,9,10,16,17,23,24,2’,3’,9’, 10’,16’,17’,23’,24’-hexadecaalkyl-substituted lanthanide complexes (R8Pc)2Ln (R = H, Me, Et, Bun; Ln = Eu, Dy, Lu) were studied. Regularities of changing the parameters under study were established, depending on the nature of lanthanide and substituents in the phthalocyanine macroligands. The position of the intervalence band of the complexes in the near-IR region depends on the effective distance between the macroligands and also on the electronic effect of the substituents. Correlations between the electrochemical and spectral properties of the complexes were found.

Temperature dependence of polarized absorption bands in p-type CdGeAs2

The temperature and polarization behaviors of four absorption bands in p-type CdGeAs2 bulk crystals have been studied from 5 to 300 K. One band, peaking near 0.22 eV (5.5 μm), is the V2→V1 intervalence band transition, and its intensity taken with E‖c is about 2.8 times larger than that taken with E⊥c. Three additional absorption bands can be separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E‖c. A second broad band peaking near 0.52 eV is detected using E⊥c. These two bands are assigned to transitions from the top two valence bands to a deep acceptor. Infrared photoluminescence studies of a 0.35 eV emission confirm the presence of the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E‖c. This band is assigned to transitions from a shallow 120 meV acceptor to shallow donor states and/or conduction band states. At room temperature, the normally observed broad absorption feature extending from the band edge to beyond 0.2 eV is a result of the superposition of the four bands. These absorption bands presently limit the use of p-type CdGeAs2 crystals as nonlinear optical materials in infrared high-power laser systems.