Multidimensional Electron Transfer Pathways in a Tetrahedral Tetrakis{4-[N,N-di(4-methoxyphenyl)amino]phenyl}Phosphonium Salt: One-Step vs Two-Step Mechanism

Electron transfer across a thermal gradient

Charge transfer is a fundamental process that underlies a multitude of phenomena in chemistry and biology. Recent advances in observing and manipulating charge and heat transport at the nanoscale, and recently developed techniques for monitoring temperature at high temporal and spatial resolution, imply the need for considering electron transfer across thermal gradients. Here, a theory is developed for the rate of electron transfer and the associated heat transport between donor-acceptor pairs located at sites of different temperatures. To this end, through application of a generalized multidimensional transition state theory, the traditional Arrhenius picture of activation energy as a single point on a free energy surface is replaced with a bithermal property that is derived from statistical weighting over all configurations where the reactant and product states are equienergetic. The flow of energy associated with the electron transfer process is also examined, leading to relations between the rate of heat exchange among the donor and acceptor sites as functions of the temperature difference and the electronic driving bias. In particular, we find that an open electron transfer channel contributes to enhanced heat transport between sites even when they are in electronic equilibrium. The presented results provide a unified theory for charge transport and the associated heat conduction between sites at different temperatures.

A bis(terpyridine)ruthenium complex with three redox-active amine sites: electrochemical, optical, and computational studies

Two ruthenium complexes, [Ru(NN)(ttpy)](2+) and [Ru(NN)(daatpy)](2+), have been designed and prepared, where NN is bis(amine) ligand 4'-tolyl-5,5"-bis(di-p-anisylamino)-2,2':6',2"-terpyridine, ttpy is 4'-tolyl-2,2':6',2"-terpyridine, and daatpy is 4'-di-p-anisylamino-2,2':6',2"-terpyridine. Complex [Ru(NN)(daatpy)](2+) contains three redox-active amine groups and has been characterized by single-crystal X-ray analysis. These two complexes display much-enhanced light absorption capabilities with respect to the prototype compound [Ru(tpy)(2)](2+) (tpy = 2,2':6',2"-terpyridine), which has been rationalized on the basis of time-dependent density functional theory calculations. Electrochemical and optical studies showed that there was little electronic coupling between two amine sites in complex [Ru(NN)(ttpy)](2+). On the other hand, a ligand-to-ligand (N → N'(•+)) charge-transfer band has been observed at 1430 nm for singly and doubly oxidized forms of [Ru(NN)(daatpy)](2+), and an electronic coupling parameter of 1000 cm(-1) was derived using the Hush formula. This band is interpreted as a charge transfer from the neutral amine of the daatpy ligand to oxidized aminium units in the NN ligand.

A reliable quantum-chemical protocol for the characterization of organic mixed-valence compounds

Structures, dipole moments, electron-transfer barriers, and spin density distributions of a series of mixed-valent bistriarylamin radical cations have been studied systematically by hybrid density functional methods with variable exact-exchange admixture combined with a continuum solvent model. The chosen systems differ in their bridging units and are all relatively close, from both sides, to the class II/III borderline of the Robin-Day classification of mixed-valence systems. Solvent effects are found to have a dramatic influence on the localized vs delocalized character of these cations. While gas-phase calculations or computations in a nonpolar solvent place all systems on the delocalized class III side, a more polar solvent like acetonitrile enables observation of symmetry breaking and charge localization with moderate exact-exchange admixtures in a hybrid functional for the systems on the class II side (with diphenylbutadiyne and diphenylethyne bridges). In contrast, the cations with the shortest bridges (phenylene, biphenylene) are characterized as class III. The comparison of computed intervalence charge-transfer excitation frequencies with experiment confirms the system with the diphenylbutadiyne bridge, and probably the system with the diphenylethyne bridge, to be class II, whereas in the dichloromethane solvent employed for spectroscopic measurements, the two other systems are on the class III side. Nonstandard hybrid density functional calculations with 35% Hartree-Fock-like exchange combined with continuum solvent models are suggested as a practical protocol for the quantum-chemical characterization of organic mixed-valence systems. This approach should allow closer examinations and provides a basis for the evaluation of other computational methods.

Bis[bis-(4-alkoxyphenyl)amino] Derivatives of Dithienylethene, Bithiophene, Dithienothiophene and Dithienopyrrole: Palladium-Catalysed Synthesis and Highly Delocalised Radical Cations

Five diamines with thiophene-based bridges--(E)-1,2-bis{5-[bis(4-butoxyphenyl)amino]-2-thienyl}ethylene (1), 5,5'-bis[bis(4-methoxyphenyl)amino]-2,2'-bithiophene (2), 2,6-bis[bis(4-butoxyphenyl)amino]dithieno[3,2-b:2',3'-d]thiophene (3), N-(4-tert-butylphenyl)-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 a) and N-tert-butyl-2,6-bis[bis(4-methoxyphenyl)amino]dithieno[3,2-b:2',3'-d]pyrrole (4 b)--have been synthesised. The syntheses make use of the palladium(0)-catalysed coupling of brominated thiophene species with diarylamines, in some cases accelerated by microwave irradiation. The molecules all undergo facile oxidation, 4 b being the most readily oxidised at about -0.4 V versus ferrocenium/ferrocene, and solutions of the corresponding radical cations were generated by addition of tris(4-bromophenyl)aminium hexachloroantimonate to the neutral species. The near-IR spectra of the radical cations show absorptions characteristic of symmetrical delocalised species (that is, class III mixed-valence species); analysis of these absorptions in the framework of Hush theory indicates strong coupling between the two amine redox centres, stronger than that observed in species with phenylene-based bridging groups of comparable length. The strong coupling can be attributed to high-lying orbitals of the thiophene-based bridging units. ESR spectroscopy indicates that the coupling constant to the amino nitrogen atoms is somewhat reduced relative to that in a stilbene-bridged analogue. The neutral species and the corresponding radical cations have been studied with the aid of density functional theory and time-dependent density functional theory. The DFT-calculated ESR parameters are in good agreement with experiment, while calculated spin densities suggest increased bridge character to the oxidation in these species relative to that in comparable species with phenylene-based bridges.

Neutral Organic Mixed-Valence Compounds: Synthesis and All-Optical Evaluation of Electron-Transfer Parameters

In this paper we present the synthesis as well as a detailed study of the electrochemical and photophysical properties of a series of neutral organic mixed-valence (MV) compounds, 1-7, in which different amine donor centers are connected to perchlorinated triarylmethyl radical units by various spacers. We show that this new class of compounds are excellent model systems for the investigation of electron transfer due to their uncharged character and, consequently, their excellent solubility, particularly in nonpolar solvents. A detailed band shape analysis of the intervalence charge-transfer (IV-CT) bands in the context of Jortner's theory allowed the electron-transfer parameters (inner vibrational reorganization energy lambdav, outer solvent reorganization energy lambdao, and the difference in the free energy between the diabatic ground and excited states, DeltaG degrees degrees , as well as the averaged molecular vibrational mode v) to be extracted independently. In this way we were able to analyze the solvatochromic behavior of the IV-CT bands by evaluating the contribution of each parameter. By comparison of different compounds, we were also able to assign specific molecular moieties to changes in vv. For this class of molecules, we also demonstrate that the adiabatic dipole moment difference Deltamicroab and, consequently, the electronic coupling V12 can be evaluated directly from the absorption spectra by a new variant of the solvatochromic method. Furthermore, an investigation of the electrochemistry of compounds 1-7 by cyclic voltammetry as well as spectroelectrochemistry shows that, not only in the neutral MV compounds but also in their oxidized forms, a charge transfer can be optically induced but with exchanged donor-acceptor functionalities of the redox centers.

Intramolecular Charge Transfer in a Star-Shaped Oligoarylamine

The intervalence (IV) states of the monocationic states of the star-shaped nonaamine (3) and the triamine (2) as the branched unit in 3 have been examined by electrochemical, spectroelectrochemical, and temperature-dependent ESR spectroscopy. The oligoarylamines 2 and 3 were synthesized by using the successive palladium-catalyzed amination reaction. The redox property of 3 was basically the same as that of 2. However, there exist small potential differences between the first three one-electron oxidations for 3, indicating electronic coupling among the peripherally substituted triamine moieties via the central 1,3,5-benzenetriyl bridging unit. The observed ESR spectral pattern for 2+ remained unchanged over the measured temperature range. From the spectral simulation analyses, it was concluded that the unpaired electron in 2+ is fully delocalized over the whole molecule on the ESR time scale. This conclusion was corroborated by comparison of its optical absorption spectrum with TD-DFT-calculated results. In contrast, the peak-to-peak ESR line width (DeltaHPP) of 3+ exhibited temperature dependency. This behavior is ascribed to the thermally activated intramolecular charge transfer (ICT) among the branched three triamine moieties via the central 1,3,5-benzenetriyl bridging unit. From the spectral simulations based on the stochastic Liouville method, the first-order rate constant at each temperature and the parameters of the energy barrier for the ICT in 3+ were successfully determined.

High-Spin Radical Cations of a Dendritic Oligoarylamine

A new dendritic oligoarylamine, N,N,N',N',N",N"-hexakis[4-(di-4-anisylamino)phenyl]- 1,3,5-benzenetriamine (BTA) 2, which contains a 1,3,5-benzenetriamine molecular unit as an potential precursor of a high-spin molecule and three oligoarylamine moieties as spin-carrying units surrounding the core BTA, has been prepared by the sequential palladium-catalyzed amination reactions. The redox property has been investigated by cyclic voltammetry, and the highly charged states up to the hexacation are accessible to 2. The polycationic high-spin species have been generated by stepwise chemical oxidation, and the electronic structures have been examined in detail by the continuous wave (CW) and pulsed ESR spectroscopy in comparison with the previously studied 1. The pulsed ESR technique enabled us to determine the definite spin multiplicity of the generated polycationic species of 2. It was confirmed that the dominant oxidized species observed by the two- and three-electron oxidations were assigned to the spin triplet 2(2+) and the spin quartet 2(3+), respectively. Moreover, these high-spin polycationic species turned out to be far more stable as compared to 1, and the isolation of 2(3+) as the SbCl(6)(-) salt has been accomplished. The temperature dependence of the magnetic susceptibility for the 2(3+)(SbCl(6)(-))(3) salt revealed that the intramolecular ferromagnetic interaction exists in 2(3+), and moreover, the trication 2(3+) was found to be deformed in the solid state.

Dications of Bis-triarylamino-[2.2]paracyclophanes: Evaluation of Excited State Couplings by GMH Analysis

In this paper, we present the absorption properties of a series of bis-triarylamino-[2.2]paracyclophane diradical dications. The localized pi-pi and the charge-transfer (CT) transitions of these dications are explained and analyzed by an exciton coupling model that also considers the photophysical properties of the "monomeric" triarylamine radical cations. Together with AM1-CISD-calculated transition moments, experimental transition moments and transition energies of the bis-triarylamine dications were used to calculate electronic couplings by a generalized Mulliken-Hush (GMH) approach. These couplings are a measure for interactions of the excited mixed-valence CT states. The modification of the diabatic states reveals similarities of the GMH three-level model and the exciton coupling model. Comparison of the two models shows that the transition moment between the excited mixed-valence states mu(ab) of the dimer equals the dipole moment difference Delta of the ground and the excited bridge state of the corresponding monomer.

Triradical Cation of p-Phenylenediamine Having Two Nitroxide Radical Groups: Spin Alignment Mediated by Delocalized Spin

The cationic state of a p-phenylenediamine (PDA) molecule having two nitroxide radical groups was prepared and characterized using electrochemical, electron spin resonance (ESR) spectroscopic, and absorption spectroscopic methods. The delocalized intervalence state of the p-phenylendiamine (PDA) moiety was detected in the cationic state. From the pulsed ESR measurements, it was confirmed that the delocalized spin induces parallel spin alignment between the localized two nitroxide groups which are magnetically weakly coupled in the neutral state. It was found that the resulting high-spin alignment does not seriously affect the delocalized intervalence state of the PDA radical cation.

[2.2]Paracyclophane-Bridged Mixed-Valence Compounds: Application of a Generalized Mulliken−Hush Three-Level Model

A series of [2.2]paracylophane-bridged bis-triarylamine mixed-valence (MV) radical cations were analyzed by a generalized Mulliken-Hush (GMH) three-level model which takes two transitions into account: the intervalence charge transfer (IV-CT) band which is assigned to an optically induced hole transfer (HT) from one triarylamine unit to the second one and a second band associated with a triarylamine radical cation to bridge (in particular, the [2.2]paracyclophane bridge) hole transfer. From the GMH analysis, we conclude that the [2.2]paracyclophane moiety is not the limiting factor which governs the intramolecular charge transfer. AM1-CISD calculations reveal that both through-bond as well as through-space interactions of the [2.2]paracyclophane bridge play an important role for hole transfer processes. These electronic interactions are of course smaller than direct pi-conjugation, but from the order of magnitude of the couplings of the [2.2]paracyclophane MV species, we assume that this bridge is able to mediate significant through-space and through-bond interactions and that the cyclophane bridge acts more like an unsaturated spacer rather than a saturated one. From the exponential dependence of the electronic coupling V between the two triarylamine localized states on the distance r between the two redox centers, we infer that the hole transfer occurs via a superexchange mechanism. Our analysis reveals that even significantly longer pi-conjugated bridges should still mediate significant electronic interactions because the decay constant beta of a series of pi-conjugated MV species is small.

Intervalence Transitions in the Mixed-Valence Monocations of Bis(triarylamines) Linked with Vinylene and Phenylene−Vinylene Bridges

(E)-4,4'-Bis[bis(4-methoxyphenyl)amino]stilbene, 1, (E,E)-1,4-bis[4-[bis(4-methoxyphenyl)amino]styryl]benzene, 2, and two longer homologues, (E,E,E)-4,4'-bis[4-[bis(4-methoxyphenyl)amino]styryl]stilbene, 3, and (E,E,E,E)-1,4-bis(4-[4-[bis(4-methoxyphenyl)amino]styryl]styryl)benzene, 4, have been oxidized to their mono- and dications using tris(4-bromophenyl)aminium hexachloroantimonate. The intervalence charge-transfer (IVCT) band of 1(+) is narrow and asymmetric and exhibits only weak solvatochromism. Analysis of this band indicates that 1(+) is a class-III or class-II/III borderline mixed-valence species. In contrast, a broad, strongly solvatochromic IVCT band is observed for 2(+), indicating that this species is a class-II mixed-valence species. The assignment of 1(+ ) and 2(+) as symmetric class-III and unsymmetric class-II species, respectively, is also supported by AM1 calculations. Hush analysis of the IVCT bands of both 1(+) and 2(+) gives larger electronic couplings, V, than for their analogues in which the double bonds are replaced with triple bonds. The diabatic electron-transfer distance, R, in 1(+) can be estimated by comparison of the V estimated by Hush analysis and from the IVCT maximum; it is considerably less than the geometric N-N separation, a result supported by quantum-chemical estimates of R for 1(+)-4(+). In 3(+) and 4(+), the IVCT is largely obscured by an intense absorption similar to a band seen in the corresponding dications and to that observed in the monocation of a model compound, (E,E,E)-1-[bis(4-methoxyphenyl)amino]-4-[4-[4-(4-tert-butylstyryl)styryl]styryl]benzene, 5, containing only one nitrogen redox center; we attribute this band to a bridge-to-N(+) transition. The corresponding dications 1(2+)-4(2+) show a complementary trend in the coupling between redox centers: the shortest species is diamagnetic, while the dication with the longest bridge behaves as two essentially noninteracting radical centers.

Photoinduced Charge Transfer Processes along Triarylamine Redox Cascades

In this paper, we describe the synthesis and photophysical properties of a series of acridine-triarylamine redox cascades. These cascades were designed in order to promote photoinduced hole transfer from an acridine fluorophore into an adjacent triarylamine. The excited dipolar state then injects a hole into the triarylamine redox cascade. Subsequently, the hole migrates along the redox gradient which was tuned by the substituents attached to the triarylamine redox centers. The rate of hole migration was determined by fluorescence lifetime measurements and is in the ns regime and depends strongly on the solvent polarity. The photophysical processes were also investigated by femtosecond broadband pump-probe spectroscopy. Our studies reveal different dynamic processes in the cascades depending on the solvent polarity, e.g., direct charge separation after photoexcitation vs a two step hole transfer mechanism.

How Delocalized Is N,N,N‘,N‘-Tetraphenylphenylenediamine Radical Cation? An Experimental and Theoretical Study on the Electronic and Molecular Structure

The electronic and molecular structure of N,N,N',N'-tetraphenylphenylenediamine radical cation 1(+) is in focus of this study. Resonance Raman experiments showed that at least eight vibrational modes are strongly coupled to the optical charge resonance band which is seen in the NIR. With the help of a DFT-based vibrational analysis, these eight modes were assigned to symmetric vibrations. The contribution of these symmetric modes to the total vibrational reorganization energy is dominant. These findings are in agreement with the conclusions from a simple two-state two-mode Marcus-Hush analysis which yields a tiny electron-transfer barrier. The excellent agreement of the X-ray crystal structure analysis and the DFT computed molecular structure of 1(+) on one hand as well as the solvent and solid-state IR spectra and the DFT-calculated IR active vibrations on the other hand prove 1(+) adopts a symmetrical delocalized Robin-Day class III structure both in the solid state and in solution.