Electron Pair (De)Coupling in Aniline Radical Cation and Its Implications for Organic “Mixed Valence” Systems

Rational design and syntheses of aniline-based diradical dications: isolable congeners of quinodimethane diradicals

Two-electron oxidation of five aniline-based compounds 4,4′′-p/m-terphenyldiamines afforded the first isolable aniline-based diradical dications 1²⁺–5²⁺.

Selective oxidation of aniline contaminants via a hydrogen-abstraction pathway by Bi2·15WO6 under visible light and alkaline conditions

Conversion of aniline wastes to value-added products is always a promising method to treat aniline wastewater. In this study, a selective oxidation of aniline contaminants by Bi_2·15WO₆ was carried out under visible light and alkaline conditions. Kinetic results show that the oxidation rates of aniline increase with increasing pH values under visible light. UV-vis absorption spectra and GC-MS analysis confirm that azobenzene is the primary oxidation product with aminophenol and N,N'-diphenylhydrazine as the secondary products. The analyses from Mott-Schottky, electrochemical impedance spectroscopy (EIS), transient photocurrent and photoluminescence (PL) further indicate that OH^- promotes the separation and transfer of photogenerated electron-hole pairs on the surface of Bi_2·15WO₆, thus facilitating oxidation of aniline. Quenching experiments and electron spin resonance (ESR) analysis confirm that h⁺ is the predominant specie in the Bi_2·15WO₆ system and aniline radical cation (PhNH₂^•+) is an important intermediate. The Hammett and ΔBDE_N-H plots further reveal that e^- abstraction from aniline with the formation of PhNH₂^•+, followed by H⁺ abstraction from PhNH₂^•+ with the formation of anilino radicals (PhNH^•), is the prerequisite for the formation of N,N'-diphenylhydrazine, which is then oxidized to azobenzene via the hydrogen-abstraction pathway. This work provides a cost-effective method to selectively oxidize aniline to azobenzene.

Fine tuning of electron transfer and spin chemistry parameters in triarylamine-bridge-naphthalene diimide dyads by bridge substituents

The photoinduced charge separation and charge recombination in a set of four molecular dyads consisting of a triarylamine donor and a naphthalene diimide acceptor were investigated by time resolved transient absorption spectroscopy with fs and ns time resolution. In these dyads the donor and acceptor are bridged by a meta-conjugated diethynylbenzene bridge whose electronic nature was tuned by small electron donating (OMe, Me) or electron withdrawing (Cl, CN) substituents. While the formation of the transient charge separated states is complete within tens of ps, charge recombination is biphasic with a shorter component of several hundred ns and a longer component of several microseconds. This behaviour could be rationalized by assuming an equilibrium of singlet and triplet charge separated states. Magnetic field dependent measurements showed a strong influence on the biphasic decay kinetics and also a pronounced level crossing effect in the magnetic field affected reaction yield (MARY) spectra caused by a significant exchange coupling. An analysis of the observed kinetics using classical kinetic rate equations yields rate constants for charge separation and charge recombination as well as the exchange interaction splitting in the radical ion pair, all of them showing a delicate dependence on the bridge substituents.

An efficient generalized polyelectron population analysis in orbital spaces: the hole-expansion methodology

We present relations leading to an efficient generalized population analysis in orbital spaces of usual delocalized molecular orbital wave functions. Besides the calculation of the diagonal elements of the reduced density matrices of any order, one can also calculate efficiently the probabilities (or, in general, the weights) of various occupation schemes of local electronic structures, by using generalized density operators referring to both electrons and electron holes. Within this population analysis, correlated molecular orbital wave functions can be used, and there are no restrictions to the number of the analyzed electrons and electron holes. It is based on the hole-expansion methodology, according to which a given electronic population is expanded in terms involving only electron holes, which as shown, can be calculated very efficiently; usual difficulties arising from the necessity to handle extremely large local determinantal basis sets are avoided, without introducing approximations. Although an emphasis is given for populations in the basis of orthogonal orbital spaces (providing probabilities), the case of nonorthogonal ones is also considered in order to show the connection of the generalized populations and the traditional weights obtained from valence-bond wave functions. Physically meaningful populations can be obtained by using natural orbitals, such as the natural atomic orbitals (NAOs) (orthogonal orbitals) or the pre-NAO's (nonorthogonal orbitals); numerical applications for pyrrole molecule are presented in the basis of these natural orbitals.

Computation of pKaValues of Substituted Aniline Radical Cations in Dimethylsulfoxide Solution

A newly developed computation strategy was used to calculate the absolute pKa values of 18 substituted aniline radical cations in dimethylsulfoxide (DMSO) solution with the error origin elucidated and deviation minimized. The B3LYP/6-311++G(2df,2p) method was applied and was found to be capable of reproducing the gas-phase proton-transfer free energies of substituted anilines with a precision of 0.83 kcal/mol. The IEF-PCM solvation model with gas-phase optimized structures was adopted in calculating the pKa values of the substituted neutral anilines in DMSO, regenerating the experimental results within a standard deviation of 0.4 pKa unit. When the IEF-PCM solvation model was applied to calculate the standard redox potentials of anilide anions, it showed that the computed values agreed well with experiment, but the redox potentials of substituted anilines were systematically overestimated by 0.304 eV. The cause of this deviation was found to be related to the inaccuracy of the calculated solvation free energies of aniline radical cations. By adjusting the size of the cavity in the IEF-PCM method, we derived a reliable procedure that can reproduce the experimental pKa values of aniline radical cations within 1.2 pKa units to those from experiment.

Control of delocalization and structural changes by means of an electric field

The strength and, mainly, the direction of a static electric field can be used to control delocalization effects occurring in a non-polar pi-system. The delocalization energy, the weights, and the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations are considered and examined in cis-butadiene, used as model system. The effects of the electric field are detected and evaluated in the basis of natural orbital spaces appropriate to investigate the behavior of one- and poly-electron distributions. The consequences of modifying the delocalization effects on structural changes are also investigated. Full geometry optimizations in both Hartree-Fock and MP2 levels show that the changes in bond lengths, guided by the changes of the behavior of the electronic assembly, can be controlled by means of the electric field.

Orthogonal natural atomic orbitals form an appropriate one-electron basis for expanding CASSCF wave functions into localized bonding schemes and their weights

Localized bonding schemes and their weights have been obtained for the pi-electron system of nitrone by expanding complete active space self-consistent field wave functions into a set of Slater determinants composed of orthogonal natural atomic orbitals (NAOs) of Weinhold and Landis (Valency and Bonding: A Natural Bond Orbital Donor-Acceptor Perspective, 2005). Thus, the derived bonding schemes are close to orthogonal valence bond structures. The calculated sequence of bonding scheme weights accords with the sequence of genuine resonance structure weights derived previously by Ohanessian and Hiberty (Chem Phys Lett 1987, 137, 437), who employed nonorthogonal atomic orbitals. This accord supports the notion that NAOs form an appropriate orthogonal one-electron basis for expanding complete active space self-consistent field wave functions into meaningful bonding schemes and their weights.

Non-conjugated, phenyl assisted coupling in through bond electron transfer in a perylenemonoimide–triphenylamine system

Two donor-bridge-acceptor compounds containing triphenylamine (TPA) donors and perylenemonoimide (PMI) acceptors have been studied by spectroscopic techniques and quantum chemical computation. Both systems have been observed to emit prompt and delayed fluorescence under certain conditions indicating that forward and reverse electron transfer (ET) processes can occur between the locally excited and the charge separated states. The experimental and computational results show that the TPA and PMI chromophores are better coupled by almost 50% in the meta isomers which undergo ET more readily than the para isomers. Quantum chemical calculations indicate that this unexpected situation is the result of a phenyl group on the side of the bridge being advantageously positioned in the meta isomers. This leads to more extensive delocalisation of the TPA HOMO into the bridge enhancing the total through bond electronic coupling between the TPA and PMI chromophores. The calculations also indicate a strong angle dependence of the total coupling in both isomers. The experimental results are discussed in the context of the high temperature limit of Marcus's theory of non-adiabatic ET.

Electron Transfer at the Single-Molecule Level in a Triphenylamine-Perylene Imide Molecule

Photoinduced electron transfer (ET) processes in a donor-acceptor system based on triphenylamine and perylene imide have been studied at the single-molecule (SM) and ensemble levels. The system exists as two isomers, one of which undergoes forward and reverse ET in toluene with decay constants of 3.0 and 2.2x10(9) s(-1), respectively, resulting in the dual emission of quenched and delayed fluorescence while the other isomer remains ET-inactive. The fluorescence of both isomers is heavily quenched in the more polar solvent, diethyl ether, by ET. A broad range of ET dynamics is seen at the SM level in polystryene with the two isomers nonresolvable indicating that the local nanoenvironment of the SMs varies considerably throughout the polymer matrix. Both the electronic coupling and the driving force for ET are shown to influence the ET dynamics. Many fluorescence trajectories of SMs show long periods (tens of milliseconds to seconds) where the count rate is attenuated either partly (a "dim" state) or to the background level (an "off-time"). During these periods, the reduction or interruption of emission is attributed to cycles of rapid charge separation followed by charge recombination to the ground state reducing the fluorescence quantum yield of the SM.

Photoinduced electron transfer in a rigid first generation triphenylamine core dendrimer substituted with a peryleneimide acceptor

The electron-transfer process of a first generation dendrimer with a triphenylamine core substituted with one peryleneimide chromophore at the rim (N1P1) was investigated by steady-state and time-resolved spectroscopic techniques in two different solvents of medium and low polarity. Single photon counting experiments showed a fast charge separation and a thermally activated back reaction, which is uncommon for a polyaryl bridge or long-distance through-space electron transfer. The four exponential fluorescence decay can be traced to the presence of two subsets of molecules, which are constitutional isomers of N1P1. Although formally N1P1 resembles a donor-bridge-acceptor compound, detailed analysis of the data shows that the electron transfer occurs by a through-space mechanism. This amine core dendrimer has peculiar and unique characteristics resulting in the observation of efficient back transfer and delayed peryleneimide fluorescence in diethyl ether at 293 K and very long-lived charge recombination luminescence at 77 K.

Metal-metal interactions as a function of bridging ligand topology: an electrochemical, spectroelectrochemical, and magnetic study on dinuclear Oxo-Mo(V) complexes with various isomers of dihydroxynaphthalene as bridging ligand

Reaction of [MoV(TpMe,Me)(O)Cl2] with 1,3-, 1,5-, 1,6-, 2,6-, and 2,7-dihydroxynaphthalene affords the dinuclear complexes [[Mo(TpMe,Me)(O)Cl]2(mu-C10H6O2)], abbreviated as 1,3-Mo2, 1,5-Mo2, 1,6-Mo2, 2,6-Mo2, and 2,7-Mo2, according to the substitution pattern of the bridging ligand. Electrochemical, UV-vis/NIR spectroscopic, and variable-temperature magnetic susceptibility studies have been used to probe the effects of the bridging-ligand topology on the metal-metal electronic and magnetic interactions. The complexes can be split into two classes according to the properties of the bridging ligands. Complexes 1,3-Mo2, 1,6-Mo2, and 2,7-Mo2 all have bridging ligands that are topologically equivalent to meta-substituted bridging ligands such as 1,3-dihydroxybenzene, in that (i) there is an odd number of C atoms separating the two oxygen atoms, regardless of the pathway that is taken through the ligand skeleton, and (ii) the doubly oxidized from of the bridging ligand is a diradical. These complexes are classified as being "T-meta" (= topologically equivalent to meta). Complexes 1,5-Mo2 and 2,6-Mo2 have bridging ligands that are topologically equivalent to para-substituted groups such as 1,4-dihydroxybenzene, in that (i) there is an even number of C atoms separating the two oxygen atoms, whichever pathway is taken through the ligand skeleton, and (ii) the doubly oxidized form of the bridging ligand is a diamagnetic quinone. These complexes are classified as "T-para". Electrochemical studies show that the comproportionation constants for the Mo(V)/Mo(IV) mixed-valence states of the T-meta complexes are smaller than those for the T-para complexes. Spectroelectrochemical studies show that the Mo(V)/Mo(IV) mixed-valence states of the T-para complexes show pronounced Mo(IV)-->Mo(V) IVCT transitions, whereas those of the T-meta complexes do not show these transitions. Magnetic susceptibility studies show that the T-meta complexes all display ferromagnetic exchange between the metal centers, whereas the T-para complexes all display antiferromagnetic exchange. Thus, both the electronic and the magnetic properties of these complexes show a clear demarcation into two sets according to the bridging-ligand topology.