Ytterbocene charge-transfer molecular wire complexes

A systematic study of the novel charge-transfer [(f)14-(pi)0-(f)14 --> (f)13-(pi)2-(f)13] electronic state found in 2:1 metal-to-ligand adducts of the type [(Cp)2Yb](BL)[Yb(Cp)2] [BL = tetra(2-pyridyl)pyrazine (tppz) (1), 6',6' '-bis(2-pyridyl)-2,2':4',4':2',2'-quaterpyridine (qtp) (2), 1,4-di(terpyridyl)-benzene (dtb) (3), Cp = (C5Me5)] has been conducted with the aim of determining the effects of increased Yb-Yb separation on the magnetic and electronic properties of these materials. The neutral [(f)13-(pi)2-(f)13], cationic [(f)13-(pi)1-(f)13] and dicationic [(f)13-(pi)0-(f)13] states of these complexes were studied by cyclic voltammetry, UV-vis-NIR electronic absorption spectroscopy, NMR, X-ray crystallography, and magnetic susceptibility measurements. The spectroscopic and magnetic data for the neutral bimetallic complexes is consistent with an [(f)13(pi)2(f)13] ground-state electronic configuration in which each ytterbocene fragment donates one electron to give a singlet dianionic bridging ligand with two paramagnetic Yb(III) centers. The voltammetric data demonstrate that the electronic interaction in the neutral molecular wires 1-3, as manifested in the separation between successive metal reduction waves, is large compared to analogous transition metal systems. Electronic spectra for the neutral and monocationic bimetallic species are dominated by pi-pi and pi-pi transitions, masking the f-f bands that are expected to best reflect the electronic metal-metal interactions. However, these metal-localized transitions are observed when the electrons are removed from the bridging ligand via chemical oxidation to yield the dicationic species, and they suggest very little electronic interaction between metal centers in the absence of pi electrons on the bridging ligands. Analysis of the magnetic data reveals that the qtp complex displays antiferromagnetic coupling of the type Yb(alpha)(alphabeta)Yb(beta) at approximately 13 K.     

New ionization potentials from charge-transfer spectra

New ionization potential values are reported for sixteen aromatic molecules. They are obtained from CT spectra of a new electron acceptor, 3,5-dinitro-phthalic anhydride with the aromatic electron donors in 1,2-dichloroethane. The results are correlated with molecular electronic structure using perturbation theory.     

Wannier spin excitons in charge-transfer complexes

Electron spin resonance in some charge-transfer complexes of 7,7,8,8-tetracyanoquinodimethane (TCNQ) with sulphanilamides and antibiotics has been investigated. The ESR spectra are caused by two types of paramagnetic centres: the impurity type and the thermally excited type (Wannier spin excitons).     

Intramolecular charge-transfer interactions in pi-extended tetrathiafulvalene derivatives

New electron-donor (D)-electron-acceptor (A) TTF architectures are presented in which two electron-donating 1,3-dithiole moieties are connected by a pi bridge to the weak electron-accepting quinoxaline moiety (D-pi-A compounds 9a and 9b and also two 1,3-dithiole-2-ylidene moieties are connected by a pi bridge to the electron-accepting thieno[3,4-b]quinoxaline bridge (D-pi-A-pi-D compounds 12a-c). There are through-bond intramolecular charge-transfer (ICT) interactions, predicted in theoretical calculations, and confirmed by UV-vis spectroscopy and cyclic voltammetry measurements. This work constitutes the first use of quinoxalines as electron-accepting moieties in D-pi-A compounds.     

Spin solitons in organic charge-transfer salts

Organic charge-transfer (CT) salts that crystallize in face-to-face stacks of π-electron donors (D) and acceptors (A) are one-dimensional electronic systems with strong coupling to lattice and molecular vibrations. Peierls–Hubbard models of CT salts resemble π-electron theory of conjugated polymers, although transfer integrals t along mixed DA stacks are considerably smaller. Strong D and A yield Peierls systems with dimerized ground states of ion radicals, D⁺ and A⁻. Topological spin solitons separate regions of opposite dimerization, similarly to solitons in trans-polyacetylene, but are thermally accessible in ionic CT salts due to small t. Salts with still smaller t have Peierls transitions at T_P < 300 K to undimerized stacks. A Peierls–Hubbard model, H_CT, describes both types of salts and estimates of t are consistent with T_P < 300 K in some salts and spin solitons in others with higher T_P. Electron paramagnetic resonance (epr) spectra of single crystals provide direct evidence for spin solitons and one-dimensional electronic states. Spin solitons and H_CT resolve longstanding conflicts between vibrational and magnetic data that indicate dimerized stacks in several prototypical CT salts, while structural data point to undimerized stacks with large thermal ellipsoids. The low energy scale, availability of single crystals and diversity of CT salts offer opportunities for detailed modeling.     

Interfacial charge-transfer absorption: semiclassical treatment

Optically induced charge transfer between adsorbed molecules and a metal electrode was predicted by Hush to lead to new electronic absorption features but has not been experimentally observed. However, Gerischer characterized photocurrents arising from such absorption between adsorbed metal atoms and semiconductor conduction bands. 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 provide a framework for modeling and predicting IFCTA spectra. The key feature of optical charge transfer to or from a band of electronic levels (taken to have a constant density of states and electronic coupling element) is that the absorption probability reaches half intensity at lambda + DeltaG(theta), where lambda and DeltaG(theta) are the reorganization energy and free-energy gap for the optical charge transfer, attains >90% intensity at lambda + DeltaG(theta) + 0.9 square root[4lambdak(B)T], and remains essentially constant until the top (bottom) level of the band is attained. However, when the electronic coupling and transition moment are assumed to be independent of photon energy (Mulliken-Hush model), a peaked, highly asymmetric absorption profile is predicted. We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that absorption is not detectable, whereas for semiconductors there may be intense features involving coupling to surface states.     

Supramolecular assemblies built with host-stabilized charge-transfer interactions

Host-stabilized charge-transfer (CT) interactions and supramolecular assemblies built with these interactions are described. A variety of supramolecular assemblies including polyrotaxanes, molecular necklaces, and rotaxane dendrimers were synthesized through the intramolecular or intermolecular host-stabilized CT complex formation using cucurbit[8]uril (CB[8]) and D-A molecules having both electron-donor and electron-acceptor units connected by various types of linkers. Applications, including the design and synthesis of redox-driven molecular machines such as molecular loop locks, development of redox-controllable vesicles and detection of biologically important molecules, are also described.     

Dynamic salt effect on intramolecular charge-transfer reactions

The dynamic salt effect in charge-transfer reactions is investigated theoretically in this paper. Free-energy surfaces are derived based on a nonequilibrium free-energy functional. Reaction coordinates are clearly defined. The solution of the reaction-diffusion equation leads to a rate constant depending on the time correlation function of the reaction coordinates. The time correlation function of the ion-atmosphere coordinate is derived from the solution of the Debye-Falkenhagen equation. It is shown that the dynamic salt effect plays an important role in controlling the rate of charge-transfer reactions in the narrow-window limit but is balanced by the energetics and the dynamics of the polar-solvent coordinate. The simplest version of the theory is compared with an experiment, and the agreement is fairly good. The theory can also be extended to charge-transfer in the class of electrolytes that has come to be called "ionic fluids."     

Toward a charge-transfer model of neuromolecular computing

A computational model of charge transfer through an associated polyenic system is presented. This model is based on proposed transient alignment of adjacent ethylenes of phospholipid diacyls in the neural membrane. Influx of anions and cations into the cytosol at ∼10⁸ ions/s at ligand-gated channels hypothetically establishes the conditions for charge transfer through adjacent diacyl ethylenes. It is suggested that this process produces interactions between phospholipid potential energy hypersurfaces. These interactions operating in many-dimensional (Hilbert) space represent a form of massively parallel computation. Basic theoretical principles of quantum computing relevant to the present model are briefly discussed. A preliminary computational model of charge transfer through stacked ethylenes is then presented. In this model molecules were aligned with planes parallel and perpendicular. Singly charged counterions were positioned at the ends of the stacks and ab initio Hartree–Fock calculations at the 6-31+G(d, p) level were carried out. Degree of charge transfer between counterions was monitored by Mulliken population analysis from which atomic charges and dipole moments were calculated. The results of these calculations are interpreted in a larger neurobiological context. Models are proposed which relate the charge-transfer process to ion channel dynamics (open/closed), changes in membrane potential, and macroscopic memory systems. A hypothetical feedback circuitry which could regulate membrane potential and prevent recurrent excitation or hyperpolarization is described. Potential tests of the model utilizing photoinduced charge transfer through a polyenic molecular wire are proposed. It is concluded that this research could lead to a better understanding of computational processes in neurophysiology and cognition. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 3–10, 1998     

Ultrafast charge-transfer dynamics of donor-substituted truxenones

The photophysics of two donor-substituted truxenone derivatives has been studied by femtosecond time-resolved transient absorption spectroscopy. The systems consist of a central truxenone acceptor with three triarylamine (TARA) branches which act as electron donors. Upon excitation in the visible regime an electron is transferred from the donor to the acceptor, generating a charge-separated state. This state can be probed via the characteristic absorption of the TARA radical cation around 700 nm. A second absorption band around 420 nm exhibits the same kinetics and is assigned to an absorption of the radical anion of the truxenone moiety. The back electron transfer and the recovery of the ground state can be interpreted within the framework of Marcus theory. To study the dependence of the back electron transfer on the electronic coupling, the distance between the donor and the acceptor was adjusted. Two solvents were employed, dimethylsulfoxide and dichloroethane. A biexponential decay of the bands assigned to the charge-separated state was observed, with time constants in the picosecond range. Surprisingly, the rates for electron back transfer do not follow the simple picture of the donor-acceptor distance being the determining factor. The observations are explained within a model that additionally takes steric interactions between the donor and the acceptor into account.     

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.     

Luminescent charge-transfer platinum(II) metallacycle

The photophysical and electrochemical properties of a platinum(II) diimine complex bearing the bidentate diacetylide ligand tolan-2,2'-diacetylide (tda), Pt(dbbpy)(tda) [dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine] (1), are compared with two reference compounds, Pt(dbbpy)(C[triple bond]CPh)(2) (2) and Pt(dppp)tda [dppp = 1,3-bis(diphenylphosphino)propane] (3), respectively. The X-ray crystal structure of 1 is reported, which illustrates the nearly perfect square planarity exhibited by this metallacycle. Chromophore 2 possesses low-lying charge-transfer excited states analogous to 1, whereas structure 3 lacks such excited states but features a low-lying platinum-perturbed tda intraligand triplet manifold. In CH(2)Cl(2), 1 exhibits a broad emission centered at 562 nm at ambient temperature, similar to 2, but with a higher photoluminescence quantum yield and longer excited-state lifetime. In both instances, the photoluminescence is consistent with triplet-charge-transfer excited-state parentage. The rigidity imposed by the cyclic diacetylide ligand in 1 leads to a reduction in nonradiative decay, which enhances its room-temperature photophysical properties. By comparison, 3 radiates highly structured tda-localized triplet-state phosphorescence at room temperature. The 77 K emission spectrum of 1 in 4:1 EtOH/MeOH becomes structured and is quantitatively similar to that measured for 3 under the same conditions. Because the 77 K spectra are nearly identical, the emissions are assigned as (3)tda in nature, implying that the charge-transfer states are raised in energy, relative to the (3)tda levels in 1 in the low-temperature glass. Nanosecond transient absorption spectrometry and ultrafast difference spectra were determined for 1-3 in CH(2)Cl(2) and DMF at ambient temperature. In 1 and 2, the major absorption transients are consistent with the one-electron reduced complexes, corroborated by reductive spectroelectrochemical measurements performed at room temperature. As 3 does not possess any charge-transfer character, excitation into the pipi* transitions of the tda ligand generated transient absorptions in the relaxed excited state assigned to the ligand-localized triplet state. In all three cases, the excited-state lifetimes measured by transient absorption are similar to those measured by time-resolved photoluminescence, suggesting that the same excited states giving rise to the photoluminescence are responsible for the absorption transients. ESR spectroscopy of the anions 1- and 2- and reductive spectroelectrochemistry of 1 and 2 revealed a LUMO based largely on the pi* orbital of the dbbpy ligand. Time-dependent density functional theory calculations performed on 1-3 both in vacuum and in a CH(2)Cl(2) continuum revealed the molecular orbitals, energies, dipole moments, and oscillator strengths for the various electronic transitions in these molecules. A DeltaSCF-method-derived shift applied to the calculated transition energies in the solvent continuum yielded good agreement between theory and experiment for each molecule in this study.     

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.     

Recent developements in charge-transfer theory

In this paper we discuss some recent theoretical developments of importance in the area of charge transfer between atoms and surfaces. Using the complex scaling method we have calculated the energy shift and broadening of atomic levels near metal surfaces. Two novel applications will be discussed. The first concerns the interaction of atomic Rydberg levels with clean metal surfaces. It is shown that as Rydberg atoms approach a surface, strong hybridization occurs that depends sensitively on both the atom-surface separation and the details of the surface potential. The widths of the hybridized states can differ by several orders of magnitude depending on their orientation with respect to the surface. The second application is an investigation of how dielectric overlayers adsorbed on metal surfaces can influence the energy shift and broadening of atomic levels. The calculations show that the energies and widths of atomic levels near metal surfaces can be influenced strongly by thin dielectric films adsorbed on the surface.     

Branched charge-transfer chromophores featuring a 4,5-dicyanoimidazole unit

Six branched and stable push-pull chromophores featuring 4,5-dicyanoimidazole as an acceptor moiety, an N,N-dimethylamino group as a donor and various π-conjugated linkers are reported. Systematic extension of the π-linker revealed that the optical and electrochemical properties of A-π-D chromophores are mainly affected by the nature of the π-conjugated backbone (length and planarity) as well as by the number of appended donors.     

Switching between ligand-to-ligand charge-transfer, intraligand charge-transfer, and metal-to-ligand charge-transfer excited states in platinum(II) terpyridyl acetylide complexes induced by pH change and metal ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2'-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.     

Interligand transmetallic charge-transfer transitions in mixed-ligand chelates

In chelates of divalent zinc which have two different aromatic ligands, a new absorption band is observed in the visible region. This new band is interpreted as arising from a π→π* interligand transmetallic charge-transfer (ITCT) transition. The nature of this type of charge-transfer transition in a chelate molecule, using a molecular orbital model, is examined. Experimental results for the chelates (3,4-toluenedithiolato)(1,10-phenanthroline)zinc(II), (3,4-toluenedithiolato)(2,2′-dipyridyl)zinc(II),and (3,4-toluenedithiolato)(2,2′-biquinoline)zinc(II) are presented.     

An intramolecular charge-transfer complex in hydrogenated quinolines

The IR spectra of 6R-2,2,4-trimethyl-1,2-dihydroquinolines (I) and 6-R2,2,4-ttimethyl-1,2,3,4-tetrahydroquinolines (II) with R=H, CH₃, OCH₃, and OC₂H₅ and those of their acetyl derivatives have been studied. The change in the intensity and the frequencies of the bands of the stretching vibrations of the C–N and C=C bonds and the ring with a change in the structure of the molecule have been considered. The considerable differences in the IR and UV spectra of I and II are explained by the fact that in the I series the interaction between the pelectrons of the nitrogen and the -electrons of the double bond (of the CTC type) and in the II series that of the -electrons of the nitrogen with the aromatic ring is predominant.     

Core hole induced charge-transfer reactions in disubstituted benzenes

The application of the charge-transfer concept on core electron ionization in donor–acceptor molecules is analyzed. A single excitation picture, involving the highest occupied and lowest unoccupied molecular orbitals is compared with a full π-valence orbital active space model. The connections between the notion of charge transfer in donor–acceptor species with the shake-up picture used for core photoelectron spectra of small molecules and with the dynamical screening concept applied on surface–adsorbate spectra are discussed.     

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.