An excess electron binding to the `purple' zwitterion quinonoid

Tuning graphene transistors through ad hoc electrostatics induced by a nanometer-thick molecular underlayer

We report on the modulation of the electrical properties of graphene-based transistors that mirror the properties of a few nanometers thick layer made of dipolar molecules sandwiched in between the 2D material and the SiO₂ dielectric substrate. The chemical composition of the films of quinonemonoimine zwitterion molecules adsorbed onto SiO₂ has been explored by means of X-ray photoemission and mass spectroscopy. Graphene-based devices are then fabricated by transferring the 2D material onto the molecular film, followed by the deposition of top source-drain electrodes. The degree of supramolecular order in disordered films of dipolar molecules was found to be partially improved as a result of the electric field at low temperatures, as revealed by the emergence of hysteresis in the transfer curves of the transistors. The use of molecules from the same family, which are suitably designed to interact with the dielectric surface, results in the disappearance of the hysteresis. DFT calculations confirm that the dressing of the molecules by an external electric field exhibits multiple minimal energy landscapes that explain the thermally stabilized capacitive coupling observed. This study demonstrates that the design and exploitation of ad hoc molecules as an interlayer between a dielectric substrate and graphene represents a powerful tool for tuning the electrical properties of the 2D material. Conversely, graphene can be used as an indicator of the stability of molecular layers, by providing insight into the energetics of ordering of dipolar molecules under the effect of electrical gating.

Changes in molecular film metallicity with minor modifications of the constitutive quinonoid zwitterions

The electronic properties of molecular films formed by quinonoid zwitterions deposited on gold are highly dependent on the nature of the N-substituent.

Reactivity of TCNE and TCNQ derivatives of quinonoid zwitterions with Cu(I)

The reactions of TCNE- and TCNQ-functionalized (TCNE: tetracyanoethylene and TCNQ: 7,7',8,8'-tetracyanoquinodimethane) zwitterionic benzoquinonemonoimines with a Cu(I)-BIAN complex (BIAN = bis(o,o'-bisisopropylphenyl)acenaphthenequinonediimine) have been investigated and found to follow a diversity of interesting patterns. The complexes [Cu(BIAN)(NCMe)(L2)]BF4 (2) and [Cu(BIAN)(L2)2]BF4 (4) were obtained by reacting [Cu(BIAN)(NCMe)2]BF4 (1) with one and two equivalents of L2, respectively. Following similar procedures, the complexes [Cu(BIAN)(NCMe)(L3)]BF4 (6) and [Cu(BIAN)(L3)2]BF4 (7) were obtained by reaction of 1 with L3. The reaction of 2 with 0.5 equiv. of 4,4'-bipyridine afforded [{Cu(BIAN)(L2)}2(μ-4,4'-bipyridine)](BF4)2 (3). The complexes were characterized by multinuclear NMR, IR and UV-Vis spectroscopic techniques, mass spectrometry, cyclic voltammetry and elemental analysis. The molecular structures of complexes 3 ·4CH2Cl2 and 4 ·CH2Cl2 were determined by single crystal X-ray diffraction. An unexpected coordination polymer [Cu((L2-))2]∞ (5) was also structurally characterized, which contains Cu(II) centres chelated by two N,O-bound ligands resulting from the monodeprotonation of L2.

Dipole driven bonding schemes of quinonoid zwitterions on surfaces

The permanent dipole of quinonoid zwitterions changes significantly when the molecules adsorb on Ag(111) and Cu(111) surfaces. STM reveals that sub-monolayers of adsorbed molecules can exhibit parallel dipole alignment on Ag(111), in strong contrast with the antiparallel ordering prevailing in the crystalline state and retrieved on Cu(111) surfaces, which minimizes the dipoles electrostatic interaction energy. DFT shows that the rearrangement of electron density upon adsorption is a result of donation from the molecular HOMO to the surface, and back donation to the LUMO with a concomitant charge transfer that effectively reduces the overall charge dipole.

Altering the static dipole on surfaces through chemistry: molecular films of zwitterionic quinonoids

The adsorption of molecular films made of small molecules with a large intrinsic electrical dipole has been explored. The data indicate that such dipolar molecules may be used for altering the interface dipole screening at the metal electrode interface in organic electronics. More specifically, we have investigated the surface electronic spectroscopic properties of zwitterionic molecules containing 12π electrons of the p-benzoquinonemonoimine type, C(6)H(2)(···NHR)(2)(···O)(2)(R = H (1), n-C(4)H(9) (2), C(3)H(6)-S-CH(3) (3), C(3)H(6)-O-CH(3) (4), CH(2)-C(6)H(5) (5)), adsorbed on Au. These molecules are stable zwitterions by virtue of the meta positions occupied by the nitrogen and oxygen substituents on the central ring, respectively. The structures of 2-4 have been determined by single crystal X-ray diffraction and indicate that in these molecules, two chemically connected but electronically not conjugated 6π electron subunits are present, which explains their strong dipolar character. We systematically observed that homogeneous molecular films with thickness as small as 1 nm were formed on Au, which fully cover the surface, even for a variety of R substituents. Preferential adsorption toward the patterned gold areas on SiO(2) substrates was found with 4. Optimum self-assembling of 2 and 5 results in ordered close packed films, which exhibit n-type character, based on the position of the Fermi level close to the conduction band minimum, suggesting high conductivity properties. This new type of self-assembled molecular films offers interesting possibilities for engineering metal-organic interfaces, of critical importance for organic electronics.

The interface bonding and orientation of a quinonoid zwitterion

We have investigated the interaction and orientation of a strongly dipolar zwitterionic p-benzoquinonemonoimine-type molecule, with a large intrinsic dipole of 10 Debye, on both conducting and on polar insulating substrates. Specifically, we deposited (6Z)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate C(6)H(2)([horiz bar, triple dot above]NHR)(2)([horiz bar, triple dot above]O)(2) where R = n-C(4)H(9), on both gold and ferroelectric lithium niobate surfaces. An influence of both transient and static electric dipoles on the zwitterionic adsorbate has been observed. For adsorption on gold, we find that the molecule bonds to the surface through the nitrogen atoms, forming films that remain fairly uniform down to thicknesses in the 1 nm range. Adsorption of this zwitterionic compound from solution on insulating, periodically poled ferroelectric lithium niobate substrates, showed preferential adsorption on one type of ferroelectric domain. For both gold and the lithium niobate substrates, the zwitterion adopts a preferential orientation with the plane of its "C(6) core" along the surface normal. This simplified geometry of strong dipole alignment provides a symmetry simplification allowing better identification of the vibrational modes responsible for Frank-Condon scattering revealed in the fine spectroscopic signature in the photoemission spectrum.

Molecular anions

The experimental and theoretical study of molecular anions has undergone explosive growth over the past 40 years. Advances in techniques used to generate anions in appreciable numbers as well as new ion-storage, ion-optics, and laser spectroscopic tools have been key on the experimental front. Theoretical developments on the electronic structure and molecular dynamics fronts now allow one to achieve higher accuracy and to study electronically metastable states, thus bringing theory in close collaboration with experiment in this field. In this article, many of the experimental and theoretical challenges specific to studying molecular anions are discussed. Results from many research groups on several classes of molecular anions are overviewed, and both literature citations and active (in online html and pdf versions) links to numerous contributing scientists' Web sites are provided. Specific focus is made on the following families of anions: dipole-bound, zwitterion-bound, double-Rydberg, multiply charged, metastable, cluster-based, and biological anions. In discussing each kind of anion, emphasis is placed on the structural, energetic, spectroscopic, and chemical-reactivity characteristics that make these anions novel, interesting, and important.

Tunable N-substitution in Zwitterionic Benzoquinonemonoimine Derivatives: Metal Coordination, Tandemlike Synthesis of Zwitterionic Metal Complexes, and Supramolecular Structures

Full details on a very efficient transamination reaction for the synthesis of zwitterionic N,N-dialkyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium derivatives [C6H2(=NHR)2(=O)2] 5-16 are reported. The molecular structures of zwitterions 5 (R=CH3) in 5.H2O, 13 (R=CH2CH2OMe), 15 (R=CH2CH2NMe2), and of the parent, unsubstituted system [C6H2(=NH2)2(=O)2] 4 in 4.H2O have been established by single-crystal X-ray diffraction. This one-pot preparation can be carried out in water, MeOH, or EtOH and allows access to new zwitterions with N-substituents bearing functionalities such as -OMe (13), -OH (9-12), NR1R2 with R1 = or not equal R2 (14-16) or an alkene (8), leading to a rich coordination chemistry and allowing fine-tuning of the supramolecular arrangements in the solid state. As previously described for 15, which reacted with Zn(acac)2 to afford the octahedral Zn(II) complex [Zn[C6H2(NCH2CH2NMe2)O(O)(NHCH2CH2NMe2)]2] (20), ligands 13 and 16 with coordinating "arms" afforded with Zn(acac)2 the 2:1 adducts [Zn[C6H2(NCH2CH2X)O(=O)(NHCH2CH2NX)]2] 19 (X=OMe) and 21 (X=NHEt), with N2O4 and N4O2 donor sets around the octahedral Zn(II) center, respectively. Furthermore, zwitterions 15 and 16 reacted with ZnCl2 to give the stable, crystallographically characterized Zn(II) zwitterionic complexes [ZnCl2[C6H2(NCH2CH2NR1R2)O(=O)(NHCH2CH2NHR1R2)]] 22 (R1=R2=Me) and 23 (R1=Et, R2=H) by means of an unprecedented, tandemlike synthesis in which 1) the two pendant amino groups of the organic benzoquinonemonoimine zwitterionic precursor favor metal coordination and proton transfer and 2) the saturated linker prevents pi-conjugation between the charges. The nature of the structural arrangements in the solid state for both inorganic (20, 22, 23) and organic (5, 9, 13, and 15) molecules is determined by subtle variations in the nature of the N-substituent on the zwitterion precursor.

First transamination reactions for the one-pot synthesis of substituted zwitterionic quinones

A new and very efficient one-pot synthesis, in water, of 6 pi + 6 pi electron zwitterionic benzoquinonemonoimines involves the first examples of transamination reactions in quinonoid chemistry and allows the introduction of functionalities to, and the fine-tuning of the properties of, a large family of molecules with interesting potential, both in supramolecular chemistry and as multidentate ligands in coordination chemistry.

Charge Separation in Ground-State 1,2,4,5-Tetra-Substituted Benzene Derivatives

The conditions required for a formal biradical to exist in a zwitterionic form in the ground state are discussed following the recent experimental observation of zwitterionic structure in the ground state of a quinoid molecule (di-tert-butyl derivative of 2,5-diamino-1,4-benzoquinonediimine, I). A unique characteristic of molecules of this class is the fact that they may be considered as being formed by the union of two radicals, each having an odd number of pi electrons. In the case of I, one fragment carries the two amino group having 7 pi electrons; it acts as the electron donor. The other fragment carries the two oxygen atoms (carrying 5 pi electrons) and acts as an electron acceptor. A model that predicts the properties of these systems is presented, based on previous work on non-Kekule hydrocarbons(2,3) and on the electron donating and attracting properties of the donor and acceptor groups, respectively. The zwitterion is formed by an electron transfer leading to two subunits carrying 6 pi electrons each and may become more stable than the triplet biradical even in the gas phase (i.e., in the absence of an external field) if the ionization potential of the donor is small (of the order of 3-4 eV). In some cases solvation in a polar solvent is required to make the zwitterionic form the lowest energy species on the ground-state surface. The 'spacer' between the donor and acceptor groups (which need not be necessarily derived from an aromatic structure) can be varied and influences the overall dipole moment that is calculated in some cases to be quite large (over 20 D in the gas phase).

A 6π + 6π Potentially Antiaromatic Zwitterion Preferred to a Quinoidal Structure: Its Reactivity Toward Organic and Inorganic Reagents

A straightforward synthesis of the zwitterionic benzoquinonemonoimine 8 is reported. This molecule is a rare example of a zwitterion being more stable than its canonical forms. It is shown that 8 is best described as constituted of two chemically connected but electronically not conjugated 6 pi electron subunits. Its reactivity with electrophiles such as H(+), CH(3)(+), and metal salts leads to the synthesis of new 12 pi electron molecules 12 (H(+)), 14 (CH(3)(+)), and 20 (Pd(2+)), respectively, in which one or both 6 pi electron subsystems localize into an alternation of single and double bonds, as established by X-ray diffraction. The acidity of the N[bond]H protons of 8 can be modulated by an external reagent. Dependent on the electrophile used, the control of the pi system delocalization becomes possible. When the electrophile simply adds to the zwitterion as in 12, 14, or 15, there is no more negative charge to be delocalized and only the positive charge remains delocalized between the nitrogen atoms. Furthermore, when a reaction with the electrophilic reagent results in deprotonation, as in 17-21, there remains no charge in the system to be delocalized. DFT calculations were performed on models of 8, 12, 14, 20, and on other related zwitterions 9 and 10 in order to examine the influence of the fused cycles on the charge separation and on the singlet-triplet energy gap. An effect of the nitrogen substituents in 8 is to significantly stabilize the singlet state. The dipole moment of 8 was measured to be 9.7 D in dichloromethane, in agreement with calculated values. The new ligands and complexes described in this article constitute new classes of compounds relevant to many areas of chemistry.