A Search for Blues Brothers: X-ray Crystallographic/Spectroscopic Characterization of the Tetraarylbenzidine Cation Radical as a Product of Aging of Solid Magic Blue

Magic blue (MB+˙ SbCl6− salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in the solid state upon storage to form so called ‘blues brothers’, which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of the main ‘blues brother’ as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (TAB) by a combined DFT and experimental approach, including isolation of TAB+˙ SbCl6− and its X-ray crystallography characterization. The formation of TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of MB followed by a loss of molecular bromine. The recognition of this fact led us to the rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as ‘blues cousin’ (BC: Eox1 = 0.78 V vs. Fc/Fc+, λmax(BC+˙) = 805 nm, εmax = 9930 cm−1 M−1), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of the aryl rings. A ready two-step synthesis of BC from triphenylamine and the high stability of its cation radical (BC+˙) promise that BC will serve as a ready replacement for MB and an oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.This article discusses recent progress by a combination of spectroscopy and quantum-chemical calculations in classifying and characterizing organic mixed-valence systems in terms of their localized vs.delocalized character. A recently developed quantum-chemical protocol based on non-standard hybrid functionals and continuum solvent models is evaluated for an extended set of mixed-valence bis-triarylamine radical cations, augmented by unsymmetrical neutral triarylamine-perchlorotriphenylmethyl radicals. It turns out that the protocol is able to provide a successful assignment to class II or class III Robin-Day behavior and gives quite accurate ground- and excited-state properties for the radical cations. The limits of the protocol are probed by the anthracene-bridged system8, where it is suspected that specific solute–solvent interactions are important and not covered by the continuum solvent model. Intervalence charge-transfer excitation energies for the neutral unsymmetrical radicals are systematically overestimated, but dipole moments and a number of other properties are obtained accurately by the protocol.DFG, GRK 1221, Steuerung elektronischer Eigenschaften von Aggregaten pi-konjugierter Molekül

A Triarylamine-Triarylborane Dyad with a Photochromic Dithienylethene Bridge

A molecular triad composed of a triarylamine donor, a triarylborane acceptor, and a photoisomerizable dithienylethene bridge has been synthesized and explored by cyclic voltammetry, UV–vis, and luminescence spectroscopy. The effects of irradiation with UV light and fluoride addition on the electrochemical and optical spectroscopic properties of the donor–bridge–acceptor molecule were investigated. Photoisomerization of the dithienylethene bridge affects the triarylboron reduction potential, but not the triarylamine oxidation potential. UV–vis experiments reveal that the association constant for fluoride binding at the triarylborane site is independent of the isomerization state of the bridge. Irradiation of a THF solution of our donor–bridge–acceptor molecule with UV light, followed by F– addition, leads to a different color of the sample than UV irradiation alone or F– addition alone

Photoinduced Electron Transfer in Linear Triarylamine-Photosensitizer-Anthraquinone Triads with Ruthenium(II), Osmium(II), and Iridium(III)

A rigid rod-like organic molecular ensemble comprised of a triarylamine electron donor, a 2,2′-bipyridine (bpy) ligand, and a 9,10-anthraquinone acceptor was synthesized and reacted with suitable metal precursors to yield triads with Ru(bpy)32+, Os(bpy)32+, and [Ir(2-(p-tolyl)pyridine)2(bpy)]+ photosensitizers. Photoexcitation of these triads leads to long-lived charge-separated states (τ = 80–1300 ns) containing a triarylamine cation and an anthraquinone anion, as observed by transient absorption spectroscopy. From a combined electrochemical and optical spectroscopic study, the thermodynamics and kinetics for the individual photoinduced charge-separation and thermal charge-recombination events were determined; in some cases, measurements on suitable donor–sensitizer or sensitizer–acceptor dyads were necessary. In the case of the ruthenium and iridium triads, the fully charge-separated state is formed in nearly quantitative yield

Charge Transfer Emission in Oligotriarylamine-Triarylborane Compounds

Donor–acceptor compounds exhibiting charge transfer emission are of interest in a variety of different contexts, for example, for nonlinear optical processes and for sensor applications. Recently investigated triarylamine–triarylborane compounds represent an important class of donor–acceptor systems, and we explored to what extent their charge-transfer properties can be further improved by using stronger amine donors and borane acceptors than prior studies. The oligotriarylamine employed here is a much stronger donor than previously used triarylamines containing single nitrogen centers. In order to increase the acceptor strength, the electron-accepting unit was equipped with two (instead of one) dimesitylboron substituents. In our comparative study, six donor–acceptor compounds were synthesized and investigated by cyclic voltammetry and optical spectroscopy. An increase of the donor strength through replacement of an ordinary triarylamine by an oligotriarylamine unit leads to the expected energetic stabilization of charge transfer (CT) excited states, but the emission solvatochromism is not more pronounced. The attempted increase of the acceptor strength by substitution of the acceptor moiety by two (instead of one) dimesitylboron groups leads to a drastic decrease of emission quantum yields. On the basis of these results, our purely experimental study provides fundamental guidelines for the design of new triarylamine–triarylborane donor–acceptor compounds with favorable charge-transfer emission properties

Komplexchemie perhalogenierter Cyclopentadiene und Alkine, VII

Coordination Chemistry of Perhalogenated Cyclopentadienes and Alkynes, VII1). - Synthesis of Several Tetrachlorometalloles of Cobalt, Rhodium, and Iridium; Structure of a Iridacyclopentadiene Derivative The reaction of dichloroethyne with CpCo(PPh3)2, RhCl(EPh3)3 (E = P, As, Sb), and IrCl(N2)(PPh3)2 leads to the formation of complexes containing a tetrachloro-1-metallacyclopentadiene unit. The crystal-structure determination of (Ph3P)2(Cl)- is reported

Crossing Bridges: Synthetic and Charge Transfer Studies of Linear and Cross-Conjugated Systems

This thesis sets out to explore the electronic structure and properties of organometallic complexes. The mixed valence states generated on one-electron oxidation are investigated based on the theory developed in Chapter 1, which provides an introduction to the general area and electron transfer. Further Chapters in this thesis set out to explore the communication between redox states spanning linear and branched architectures and draw on the ideas presented in Chapter 1 to draw conclusions as to the degree of communication between the sites. To this extent a series of platinum-acetylide complexes bearing the unusual triarylamine ligand, containing one to four redox centres, were prepared and the electronic properties probed with IR and UV-vis NIR spectroelectrochemistry. The results show that there is only very weak communication between the centres. In Chapter 3 the work looks at symmetrical analogues of these compounds and builds on previously published work to explore in more detail the electronic structure of a series of trans-[Pt(C≡CC6H4NAr’2)2(PR3)2] complexes to explore the effect of the triarylamine and ancillary phosphine ligands. The results show modest communication between the centres and little effect of the phosphine ligand. The later chapters in this thesis focus on developing synthetic routes and analyses of branched-conjugated materials. Chapter 4 develops the synthesis of a range of compounds based on the FcCH=C(CCR)2 with organic R groups. The results show that, despite proposals of such systems being able to act as transistors, the ferrocene moiety is electronically isolated. Chapter 5 builds on this work and develops the synthesis of FcCH=C(CCC6H4CCMLn)2, MLn= Ru(PPh3)2Cp or Ru(dppe)Cp*. Spectroelectrochemical analysis has shown weak communication between the –Ru-(CC)- centres. Finally, Chapter 6 addresses the interactions within multi-ferrocenyl compounds. Analysis by IR SEC studies have shown that in compounds bearing either 2, 3 or 4 ferrocene groups that each centre is essentially electronically isolated despite electrochemical studies highlighting that in the right conditions a degree of through-space communication can be observed

Antiferromagnetic spin-coupling between MnII and amminium radical cation ligands: models for coordination polymer magnets

One and two electron oxidation of the manganese(II) complex [L2Mn(hfac)2] {L = 4'',4'''-di-tert-butyl-2',2'',2'''trimethoxy-{4-(4'-diphenylaminophenyl)pyridine} were studied by ultra violet/ visible/ near infra red spectroscopy, cyclic voltammetry and magnetometry. A one-electron oxidation converts the triarylamine ligand to its radical cation and gives a complex in which the antiferromagnetic coupling between the spin on the ligand and that on the metal J/kb is -1.5 K. In a dilute frozen matrix and at low temperature this behaves as an S = 2 system. A two electron oxidation gives [L2Mn(hfac)2]2.+ which at low enough temperatures behaves as an S = 3/2 system but the spin-coupling between the metal and the ligand is weaker (J/kb = -0.3 K). The weakness of these spin-couplings mean that MnII/amminium radical cation complexes are not promising systems on which to base coordination polymer magnets. The equivalent copper(II) complex [L2Cu(hfac)2] was also investigated but this decomposes when an attempt is made to oxidise the ligand to its amminium radical cation

Utilisation des méthodes de localisation multi-référence pour les systèmes quasi-dégénérés

La structure électronique et les propriétés de transfert de charge intramoléculaire de certains composés à valence mixte ont été étudiés au niveau ab-initio multi-référence, en utilisant des orbitales moléculaires canoniques et localisées. Les composés chimiques étudiés sont : un cation de spiro pi-sigma-pi : 5,5'(4H,4H')-spirobi-[cyclopenta[c]pyrrole]2,2',6,6'tetrahydro cation, ("Spiro" dans le texte suivant); une série des chaînes cationiques linéaires composées d'atomes de béryllium : BeN, (avec N = 6, ..., 12); et deux molécules bis-triarylamines : N,N,N',N'-Tetra(4-méthoxyphényl)-1,4phénylènediamine cation, et bis{4-[N,N-di(4-méthoxyphnyl)amino]phényl}butadiyne cation. Les modèles théoriques et les méthodes de calcul utilisées dans ce travail sont les suivants : CAS-SCF, CAS+S, CAS+SD (MRCI), et CAS+SD en utilisant des orbitales localisées. Des différents bases contractées ont été utilisés. Les surfaces d'énergie potentielles adiabatiques de l'état électronique fondamentale et les trois états excités les plus bas du cation de Spiro, ont été calculés, au sein d'un modèle à deux états, et un potentiel de double-puits a été obtenu pour l'état électronique fondamental. En suivant la localisation/délocalisation de charge dans le système p de valence du Spiro à travers la coordonnée de réaction du transfert de charge intramoléculaire, nous avons montré un faible couplage électronique entre les deux moitiés d'oxydoréduction de ce cation moléculaire. L'effet de la corrélation dynamique, en utilisant des orbitales localisées ou canoniques, a été jugé cruciale pour une description quantitative de la structure électronique et les autres paramètres importantes de transfert d'électron de ce système modèle à valence mixte. Les résultats des chaînes linéaires de béryllium montrent une évolution progressive de classe III (couplage électronique fort) envers classe II (couplage électronique faible) en fonction de nombre d'atomes de béryllium. En effet, dans les cas où (N > 10), les chaînes cationiques ont été trouvés se rapprocher de la classe I, où le couplage disparaît. Les paramètres de transfert d'électrons intramoléculaire Vab, Ea, et Eopt ont été calculés pour chaque chaîne atomique. Il à été montré que la baisse des valeurs de Vab avec l'augmentation de N suit une courbe exponentielle.The electronic structure and the intramolecular charge transfer properties of a selected number of mixed valence compounds were investigated at multireference ab-initio level, using both canonical and localized molecular orbitals. The chemical compounds studied are: a spiro p-s-p molecular cation: 5,5_(4H,4H_)- spirobi[cyclopenta[c]pyrrole]2,2',6,6'tetrahydro cation (the "Spiro" molecule in the following); a series of cationic linear chains composed of beryllium atoms: BeN, (with N = 6, ..., 12); and two bis-Triaryl amines molecules: namely N,N,N',N'-Tetra(4-methoxyphenyl)-1,4-phenylenediamine cation, and bis{4-[N,N-di(4-methoxyphenyl)amino]phenyl}butadiyne cation. The theoretical models and computational methods used in this work are: CAS-SCF, CAS+S, CAS+SD (MRCI), and CAS+SD using localized orbitals. Different basis sets contractions were used. For Spiro cation, The potential energy surfaces of the adiabatic ground and the lowest three excited electronic states have been computed, within a two-state model, and a double-well potential has been obtained for the ground electronic state. We have demonstrated a low coupling interaction between the two redox moieties of this molecular cation by following the charge localization/delocalization in the valence _ system through the reaction coordinate of the intramolecular charge transfer. The effect of dynamical correlation, using either localized or canonical orbitals, was found to be crucial for a quantitative description of the electronic structure and some important electron transfer parameters of this model mixed-valence system. The results of the linear beryllium chains show a consistent gradual shift between different classes of mixed-valence compounds as the number of beryllium atoms increases, from class-III strong coupling toward class-II valence trapped. Indeed, in the largest cases (N > 10), the cationic chains were found to be closer to class I, where the coupling vanishes. The intramolecular electron transfer parameters Vab, Ea, and Eopt were calculated for each atomic chain. It is shown that the decrease of Vab with increasing N follows an exponential pattern

新規カチオン性窒素ラジカル化合物の合成と性質

広島大学(Hiroshima University)博士(理学)Doctor of Sciencedoctora