Zwitterionic Ground State Research Articles

Fully zwitterionic diaminobenzoquinonediimines promoted by cyanoaromatic N-substituents

2,5-Diamino-1,4-benzoquinonediimine (DABQDI) derivatives possessing two electron-withdrawing aromatic N-substituents can potentially exhibit a zwitterionic ground state in lieu of the expected canonical structure. It was previously shown that the use of nitroaromatics electron-withdrawing groups (EWG) could yield the quantitative formation of zwitterionic tautomer of DABQDI in polar solvents, while a mixture of canonical and zwitterionic forms was present in low polarity solvents. In this work, we report that the replacement of nitro-containing EWG by weaker nitrile ones prevents the formation of canonical species in apolar solvents for the benefit of the zwitterionic tautomer. This counterintuitive observation is rationalized with theoretical calculations, which points out that the fully zwitterionic electronic structure of the nitrile-containing DABQDI arises from the absence of possible intramolecular hydrogen bonding between the cyanoaromatic N-substituent and the N–H proton of the bridge in the canonical form.

Tuning the magnetic properties of a diamagnetic di-Blatter's zwitterion to antiferro- and ferromagnetically coupled diradicals.

In the quest of obtaining organic molecular magnets based on stable diradicals, we have tuned the inherent zwitterionic ground state of tetraphenylhexaazaanthracene (TPHA), a molecule containing two Blatter's moieties, by adopting two different strategies. In the first strategy, we have increased the length of the coupler between the two radical moieties and observed a transition from the zwitterionic ground state to the diradicalized state. With a larger coupler, ferromagnetic interactions are realized based on density functional theory (DFT) and wave-function theory (WFT) based complete active space self-consistent field (CASSCF)-N-electron valence state perturbation theory (NEVPT2) methods. An analysis based on the extent of spin contamination, diradical character, CASSCF orbital occupation number, Head-Gordon's index, HOMO-LUMO and SOMOs energy gaps is demonstrated that marks the transition of the ground state in these systems. In another approach, we systematically explore the effect of push-pull substitution on the way to obtain molecules based on a TPHA skeleton with diradicaloid state and, in some cases, even a triplet ground state.

A nitrogen-doped asymmetric phenalenyl with a zwitterionic ground state.

Herein, an asymmetric N-doped phenalenyl (BTAP) compound has been synthesized and carefully studied for the first time. The synthesis of BTAP reveals a planar configuration and an unexpected zwitterionic ground state with the negative charge delocalized around the circumjacent part and the positive charge mainly localized on the center.

Loose Canon: Tunable Quiniodal Macrocycles with Canonical and Zwitterionic Ground States

Loose Canon: Tunable Quiniodal Macrocycles with Canonical and Zwitterionic Ground States

Density Functional Theory and ab Initio Computational Evidence for Nitrosamine Photoperoxides: Hammett Substituent Effects in the Photogeneration of the Nitrooxide Intermediate.

Little attention has been focused on diradical and zwitterionic photoperoxides formed from nitrosamine compounds. Here, an attempt is made to probe the electronic character of the nitrooxide intermediate formed in photochemical reactions with triplet oxygen (3 O2 ). Theoretical studies have been conducted to screen para-substituted phenyl nitrosamines. In particular, we find that electron-withdrawing substituents produce low-lying triplet nitrooxide diradicals. A clear electronic dependence in the S0 - T1 and S0 - S1 energy gaps of nitrooxides was found using Hammett plots. Computed geometries show a twisted diradical triplet nitrooxide moiety, which contrasts to the nearly flat singlet zwitterionic ground state nitrooxide moiety; analyses of charges (natural bond order), molecular orbitals (HOMO/LUMO) and spin densities enable these assignments. Calculations predict the former triplet species is photogenerated initially from nitrosamine with O2 . The conversion of the triplet nitrooxide diradical to the singlet ground state is an example where longer-lived zwitterionic nitrooxide structures become possible. The reaction mechanism is consistent with a zwitterionic ground state nitrooxide playing an important role in the bimolecular oxygen-transfer reaction with phosphine and phosphite trapping agents as has been observed experimentally.

Ultra-High-Response, Multiply Twisted Electro-optic Chromophores: Influence of π-System Elongation and Interplanar Torsion on Hyperpolarizability.

The systematic synthesis, structural, optical spectroscopic, and second-order nonlinear optical (NLO) characterization of a series of donor-acceptor poly-arylene chromophores which have heretofore unachieved π-extension and substantial twisting from planarity, are reported: specifically, two-ring 2TTMC, dicyano(4-(3,5-dimethyl-1-(2-propylheptyl)pyridin-1-ium-4-yl)-3-methylphenyl)methanide; three-ring 3TTMC, dicyano(4'-(3,5-dimethyl-1-(2-propylheptyl)pyridin-1-ium-4-yl)-2,2',3',5',6'-pentamethyl[1,1'-biphenyl]-4-yl)methanide; and four-ring 4TTMC, dicyano(4″-(3,5-dimethyl-1-(2-propylheptyl)pyridin-1-ium-4-yl)-2,2',3″,6,6'-pentamethyl[1,1':4',1″-terphenyl]-4-yl)methanide. Single-crystal X-ray diffraction, DFT-optimized geometries, and B3LYP/INDO-SOS analysis identify three key features underlying the very large NLO response: (1) For ring catenation of three or greater, sterically enforced π-system twists are only essential near the chromophore donor and acceptor sites to ensure large NLO responses. (2) For synthetic efficiency, deletion of one ortho-methyl group from o,o',o″,o‴-tetramethylbiaryl junctures, only slightly relaxes the biaryl twist angle from 89.6° to ∼80°. (3) Increased arylene catenation from two to three to four rings (2TTMC→ 3TTMC → 4TTMC) greatly enhances NLO response, zwitterionic charge localization, and thus the ground-state dipole moment, consistent with the contracted antiparallel solid-state π-π stacking distances of 8.665 → 7.883 → 7.361 Å, respectively. This supports zwitterionic ground states in these chromophores as do significant optical spectroscopic solvatochromic shifts, with aryl-aryl twisting turning on significant intra-subfragment absorption. Computed molecular hyperpolarizabilities (μβ) approach an unprecedented 900,000 × 10(-48) esu, while estimated chromophore figures of merit, μβ(vec)/M(w), approach 1500 × 10(-48) esu, 1.5 times larger than the highest known values for twisted chromophores and >33 times larger than that of planar donor-acceptor chromophores.

Manipulating the singlet–triplet energy gaps of arene-fused bis(1,2,3-dithiazoles): A computational study

Benzo- and azino-fused bis(1,2,3-dithiazoles) were investigated computationally using density functional theory (DFT) in an attempt to relate structure to ground state multiplicity preference. Structural changes on the central arene by substitution with electron donating (EDG) and electron withdrawing groups (EWG) or replacement of the central arene by pyridino or pyrazino rings, were studied by optimizing the structures as singlet and triplet ground states. The aromaticity of each ring was probed using nucleus independent chemical shift (NICS) calculations. Molecular orbital analysis identified the substituent effects on the energy of the frontier orbitals. Calculations show that the ground state multiplicity can be effectively controlled with strategic substitutions. EWG directly attached on the negative cyanine of the central arene stabilize zwitterionic singlet states whereas EDG attached at the same position favor a triplet ground state. NICS calculations indicate that the central arenes sacrifice their aromaticity and become non-aromatic for molecules with zwitterionic ground states as an effective way to avoid their overall 4n π antiaromaticity.

CHAMELEON GROUND STATE AND EXCITED STATES OF EDT-TTF-IM-F4TCNQ RADICAL DYAD IN DIFFERENT ENVIRONMENTS

We have performed a systematic density functional study on the ground-state electronic structure and excited states of a representative D-σ-A dyad, i.e. EDT-TTF-Im-F4TCNQ π-radical, in vacuo and in different conventional solvents (toluene, THF, DMF and DMSO) by using some popular hybrid density functionals (B3LYP, M05, M05-2X, PBE0 and BMK). It has been shown that the M05 and B3LYP functionals perform the best in predicting the intramolecular charge-transfer (ICT) pertaining to both the ground state and excited states of the dyad. The amphoteric dyad is liable to solvent-promoted ICT from its EDT-TTF-Im donor (D) to F4TCNQ acceptor (A), adopting a charge-unseparated ground state D-A• in vacuo, a partially zwitterionic ground state [D-A]• in nonpolar toluene solvent, and a fully zwitterionic ground state D•+-A- in such polar solvents as THF, DMF and DMSO. Owing to its solvent-dependent chameleon ground state, excited states of the dyad in solvents also exhibit remarkable dependence on solvent polarity, as revealed by TDDFT calculations. Furthermore, cluster model calculations revealed that intermolecular charge-transfer readily occurs between the dyads, accounting for the observed zwitterionic charge state in solid state and solid-state semiconductivity.

Benzothiazolium-π-thiazole-dicyanomethanides: new nonlinear optical chromophores

A series of merocyanines with a benzothiazolylidene donor and a quinoidal thiazole moiety have been synthesized. Experimental results and theoretical calculations show that these compounds have zwitterionic ground states and negative second-order optical nonlinearities. Unlike most merocyanines, the degree of charge transfer does not decrease on increasing the separation between the donor and the acceptor groups, an unexpected feature related to the presence of the proaromatic thiazole fragment.

4H-Pyran-4-ylidenes: Strong Proaromatic Donors for Organic Nonlinear Optical Chromophores

Merocyanines where a polyenic spacer separates a 4H-pyran-4-ylidene moiety and different strong organic acceptors have been synthesized. According to NMR studies and X-ray diffraction data, these compounds have weakly alternated structures and remarkably zwitterionic ground states, with a partial aromatic character that is compared to those of other pyran derivatives. The proaromaticity of the 4H-pyran-4-ylidene donor lies behind the cyanine-like behavior and low (positive or negative) second-order optical nonlinearities of the shorter derivatives. On the other hand, lengthening the pi-spacer gives rise to rapidly increasing mubeta(1907) values up to 17,400 x 10(-48) esu.

In Situ Spectroscopic Characterization of Rectifying Molecular Monolayers Self‐Assembled on Gold

We report visible, Raman, and infrared spectra of self-assembled monolayers (SAMs) formed by the donor-(pi-bridge)-acceptor chromophore, Z-beta-[N-(omega-acetylthioalkyl)-4-quinolinium]-alpha-cyano-4-styryldicyanomethanide (CH3CO-S-CnH2n-Q3CNQ where n=8, 10), on gold-coated substrates. The data are compared with the spectra collected for the same compound in solution and in the solid state, and with those obtained for a Langmuir-Blodgett (LB) monolayer of C16H33-Q3CNQ deposited on gold. Spectral analysis confirms that in solution, in the solid state and in the LB film the chromophore has a zwitterionic (D+-pi-A-) ground state. At variance with this well-known result, our data show that in SAMs deposited on gold the chromophore has a more neutral, quinoid ground state. We relate this difference to the different packing of the molecules in the two different films: in SAMs in fact the chromophores stand almost vertical with respect to the substrate, whereas in LB films they make an angle of about 45 degrees. The Q3CNQ molecule is a well-known molecular rectifier, and for SAMs we were able to check the direction of electron flow at forward bias on the same samples that have been characterized spectroscopically, shedding light on the rectification mechanism.

Exceptional Molecular Hyperpolarizabilities in Twisted π‐Electron System Chromophores

Nonlinear optics: Twisted π-electron system electrooptic (EO) chromophores with exceptional molecular hyperpolarizabilities (−488 000×10−48 esu at 1907 nm) were designed and synthesized. Crystallographic analysis reveals large twist angles and a highly charge-separated zwitterionic ground state. Experiments on poled host–guest polymers containing these chromophores show them to be promising candidates for EO applications.

Efficient Synthesis and Structural Characteristics of Zwitterionic Twisted π-Electron System Biaryls

A series of unconventional twisted pi-electron system molecules has been synthesized via Suzuki cross-coupling of two sterically hindered arenes. Crystallographic analysis of these molecules reveals a large ring-ring dihedral twist angle (87 degrees ) and a highly charge-separated zwitterionic ground state. An efficient conversion of phenols into aryl halides is also reported. [reaction: see text]

Tuning First Molecular Hyperpolarizabilities through the Use of Proaromatic Spacers

In this paper, we describe the second-order nonlinear optical properties of a series of 1,3-dithiole-based electron donor-acceptor systems incorporating proaromatic donor and spacer groups. Modification of the proaromaticity of the quinoid spacer gives rise to NLO-phores with mubeta values ranging from -2000 x 10(-)(48) esu to +3000 x 10(-)(48) esu. Quite surprisingly, compounds with a p-benzoquinoid spacer and a strong acceptor group show negative mubeta values, usually associated to zwitterionic ground states, and yet they are largely quinoid, as evidenced by crystallographic data and theoretical calculations. Progressive benzoannulation of the spacer and introduction of alkylsulfanyl substituents on the dithiole donor unit result in a shift to more positive mubeta values. DFT and ab initio calculations verify these empirical trends.

A novel three-component reaction designed by the combinatorial method: Heteroarenes, isothiocyanates and isocyanides

The novel three-component reaction of isoquinoline with isothiocyanates and isocyanides leads to a variety of new imidazoisoquinolines. The zwitterionic ground state of these new ring systems is established by means of NMR and X-ray analysis. Use of phthalazine instead of isoquinoline gives access to imidazole annulated phthalazines.

Electrical rectification by monolayers of three molecules

AbstractRectification, i.e. asymmetrical electrical conduction by a Langmuir‐Blodgett monolayer of dicyano{4‐[1‐cyano‐2‐(1‐hexadecylquinolin‐1‐ium‐4‐yl)vinyl]phenyl} methanide (1), occurs between both Al and Au electrodes: this rectification arises from the asymmetry of the molecule, and the interplay between the zwitterionic ground state D+‐π‐A− and the less dissociated first electronic excited state D0‐π‐A0. Two more monolayer rectifiers have been found: 1‐butyl‐2,6‐bis{2‐[4‐(dibutylamino)phenyl] vinyl}pyridin‐1‐ium iodide (2) is an interionic rectifier with back charge transfer between the iodide ion and the pyridinium ring. 1a‐[4‐(dimethylamino)phenyl]‐1aH‐1a‐aza‐1(2)a‐homo(C60‐Ih)[5,6]fullerene (3) is a moderate rectifier, with a rectification ratio of 2.

One-Molecule-Thick Devices: Rectification of Electrical Current by Three Langmuir-Blodgett Monolayers

Electrical rectification by a Langmuir-Blodgett monolayer of y-hexadecylquinolinium tricyanoquinodimethanide, C 16 H 33 Q-3CNQ, 1 has been verified first between Al electrodes, then between Au electrodes: this rectification arises from the asymmetry of the molecule, and the interplay between the zwitterionic ground state D + -π-A - and the less dissociated first electronic excited state D 0 -π-A 0 . Two more rectifiers have been found. 2,6-Di[dibutylamino-phenylvinyl]-1-butylpyridinium iodide, (Bu 2 NφV) 2 BuPy + I - , 2 is an interionic rectifier which involves back charge transfer between the iodide ion and the pyridinium ring. Dimethylanilino-aza[C 60 ]fullerene, DMAn-NC 60 , 3, has a rectification ratio close to 2.

Entrapping of Push-Pull Zwitterionic Chromophores in Hybrid Matrices for Photonic Applications

A new class of heterocycle-based push-pull chromophores showing enhanced nonlinear properties characterized by an aromatic and highly zwitterionic ground state and a quinoid/neutral excited state have been synthesized to be incorporated in sol-gel hybrid systems. This class of compounds shows very large first molecular hyperpolarizabilities (βμ as high as 27000 × 10−48 esu) and is a promising candidate for photonic applications where large second order non-linearities are required. In spite of their very large hyperpolarizability and chemical and thermal stability, these chromophores are decomposed in presence of light and oxygen (photobleaching) and are sensitive to acidic environments due to the carbanionic nature of the donor moiety. A hybrid matrix, based on N-[(3-trimethoxysilyl)propyl]ethylenediamine and 3-glycidoxypropyltrimethoxysilane, has been specifically designed to allow the incorporation of such zwitterionic compounds assuring at the same time a good temporal stability of the optical properties. Amine functionalisation has been found very effective in reducing the photobleaching by acting on these chromophores via the singlet oxygen. Second harmonic generation has been observed on poled films, and an order parameter, Φ, of 0.17 has been estimated. The nonlinear coefficient deff of the samples has been thus estimated at a value two times larger than for d11 of quartz that, from literature data, is about 0.335 pm/V.

Structural and solvatochromic studies of a series of tricyanoquinodimethane-based zwitterions

The synthesis and characterization of three new solvatochromic tricyanoquinodimethane zwitterionic systems is reported. (Z)-{β-[N-(1-Propyl)benzothiazolium-2-yl]-α-cyano-4-styryl}dicyanomethanide, C3H7(2)BT3CNQ, (Z)-{β-[N-(1-propyl)benzothiazolium-2-yl]-α-cyano-4-(2,3,5,6-tetrafluorostyryl)}dicyanomethanide, C3H7(2)BT3CNQF4, and (Z)-{β-[N-(1-butyl)benzoselenazolium-2-yl]-α-cyano-4-styryl}dicyanomethanide, C4H9(2)BS3CNQ, exhibit negative solvatochromism, consistent with a transition from an ionic ground state to a significantly less polar excited state. X-Ray crystallography measurements confirm the zwitterionic ground state of these novel T3CNQ-based chromophores. The solvatochromic properties of these compounds are compared to those of a series of related quinolinium/pyridinium adducts, and also with those of the Reichardt betaine dye.

Charge-density study of the nonlinear optical precursor DED-TCNQ at 20 K

A charge-density study of the nonlinear-optical (NLO) precursor {4-[bis(diethylamino)-methylium] phenyl}dicyanomethanide (DED-TCNQ), space group ${P2}_{1}/c,$ $a=11.174(2)\AA{};$ $b=12.859(2)\AA{};$ $c=12.486(2)\AA{};$ $\ensuremath{\beta}=112.00(1)\ifmmode^\circ\else\textdegree\fi{},$ is presented. The results derive from a suitable combination of complementary 20 K x-ray and neutron diffraction data, the latter being important for locating the hydrogen atoms precisely. The compound is one in a series of TCNQ derivatives that exhibit varying degrees of quinoidal and zwitterionic character, these two electronic states being very close energetically. Bond-length-alternation type calculations show that the molecule at 20 K exists in a mixture of the two states, the zwitterionic ground state being dominant (63:37% zwitterionic: quinoidal). A topological analysis of the bonding density within the benzenoid ring provides for a more direct, alternative method to calculate this ratio which utilizes ellipticity values derived from the charge-density study. Results are identical thus corroborating the validity of the ``strength-length'' relationship implicitly assumed in bond-length-alternation type calculations. The ratio determined corresponds well to the electronic configuration needed to meet the requirements of the general rule for obtaining a maximum value of \ensuremath{\beta} (a measure of the NLO response on the molecular scale) as a function of bond-length alternation. The promise of this class of compounds for nonlinear optics also lies partly in their high molecular dipole moments and so the pseudoatomic charges derived from this study were used to evaluate the nature of the molecular charge transfer in detail and the solid-state dipolar vector moment \ensuremath{\mu}. Such measurements of \ensuremath{\mu} are otherwise difficult in the solid state. A value of $|\ensuremath{\mu}|=91\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}\mathrm{Cm}$ was deduced which compares with liquid and gas phase theoretical calculations of $\ensuremath{\mu}=66.71\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}\mathrm{Cm}$ and $\ensuremath{\mu}=33.36\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}\mathrm{Cm},$ respectively. This comparison, combined with an analysis of the sense of this vector, show that local crystal-field effects are highly influential in the solid state.