Quinonoid oligothiophenes as electron-donor and electron-acceptor materials. A spectroelectrochemical and theoretical study

Two quinonoid bis(dicyanomethylene) oligothiophenes, terthiophene and quaterthiophene analogues of TCNQ, have been investigated by spectroelectrochemical experiments and density functional theory calculations. Electrochemical data show that the molecules can be both reduced and oxidized at relatively low potentials, and that the quaterthiophene derivative forms four stable redox species, the dianion, neutral, cation radical, and dication. The neutral oligomers are characterized by a strong electronic absorption in the red or near-infrared region and can be viewed as structural and electronic analogues of aromatic oligothiophenes in the dication or bipolaron state. Upon reduction, dianions, not anion radicals, are formed which absorb in the visible region. The theoretical calculations show that the dianions have aromatic oligothiophene moieties with two anionic dicyanomethylene groups. The transition from a quinonoid to an aromatic structure is fully supported by UV-vis-near-IR and vibrational spectroscopic data. Oxidation, generating cation radicals and dications, occurs at rather low potentials similar to those reported for oligothiophenes. The electronic spectra of these cations are understood from the calculations, which suggest that the oxidized species are stabilized by the partial aromatization of the oligothiophene backbone. IR spectra of the species, especially the CN stretching frequencies, confirm the structural conclusions and allow comparison with TCNQ and the TCNQ dianion.     

Radical ions : XXVII. Radical ions of tetrakis(trimethylsilyl)butatriene

One-electron oxidation and one-electron reduction of the electron-rich acetylene derivative, hexakis(trimethylsilyl)-2-butyne [H₃C₃)₃Si]₃CCCC[Si(CH₃)₃]₃, unexpectedly produce the persistent radical cation and radical anion of the hitherto unknown neutral compound, tetrakis(trimethylsilyl)butatriene (R₃Si)₂CCCC(SiR₃)₂. The radical anion can also be generated from the corresponding diacetylene, bis(trimethylsilyl)-1,3-butadiyne R₃SiCCCCSiR₃ and potassium metal, obviously via disproportionation. Photoelectron and electron spin resonance spectroscopic data permit the detection and characterization of the individual species. The stability of both the radical anion and the radical cation of the same molecule can be rationalized by the unique combination of the twofold butatriene π-system with 4 R₃Si substituents, which can act either as electron donors or electron acceptors and thus stabilize the ground state of either the cation or the anion.     

Isomeric separation in donor-acceptor systems of Pd(II) and Pt(II) and a combined structural, electrochemical and spectroelectrochemical study

Compounds of the form [(pap)M(Q(2-))] (pap = phenylazopyridine; Q = 3,5-di-tert-butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl-benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a and 4b) were synthesized in a one-pot reaction. The geometrical isomers, which are possible because of the built in asymmetry of these ligands, have been separated by using different temperatures and variable solubility. Structural characterization of 1b shows that the metal centers are in a square planar environment, the pap ligand is in the unreduced neutral state and the quinones are in the doubly reduced, Q(2-) catecholate form. Cyclic voltammetric measurements on the complexes display two one-electron oxidations and two one-electron reductions. EPR and vis-NIR spectra of the one-electron oxidized forms of the complexes indicate that the first oxidation takes place on the Q(2-) ligands to produce a metal bound semiquinone (Q˙(-)) radical. Reduction takes place on the pap ligand, generating metal bound pap˙(-) as seen from the (14)N (I = 1) coupling in their EPR spectrum. All the complexes in their [(pap)M(Q(2-))] neutral forms show strong absorptions in the NIR region which are largely LLCT (ligand to ligand charge transfer) in origin. These NIR bands can be tuned over a wide energy range by varying the metal center as well as the Q ligand. In addition, the intensity of NIR bands can be switched on and off by a simple electron transfer at relatively low potentials. DFT studies were used to corroborate these findings.     

Preparation and characterization of alpha,alpha'-bisdiarylamino-capped oligothiophenes

[reaction: see text] A series of alpha,alpha'-bisdiarylamino-capped oligothiophenes C(n) were prepared by the palladium-catalyzed reaction of the dibromo compounds A(i) with diarylamines, N,N-diarylamino-substituted thiophenes or 2,2'-bithiophenes BX(j). These easily oxidizable compounds exhibit a high tendency to form amorphous glasses and characteristic electrochemical and spectroscopic properties that depend significantly on the number of their thiophene moieties.     

Charge injection and transfer tuning of a series of Pt complexes through oligothiophenes: A theoretical study

The geometries, frontier molecular orbitals, charge injection and transfer, as well as spectroscopic properties of a series of novel bis(8-hydroxyquinolinato) (Ptq₂) derivatives with oligothiophenes moiety were investigated theoretically. The calculated results indicated that incorporating of oligothiophene units into the Ptq₂ had major role in the tailoring the optical properties. Furthermore, from the analyses of ionization potentials (IP), electron affinity (EA), reorganization energy (λ), it was found that these Pt derivatives have excellent hole and electron injection and transfer ability by introducing the oligothiophenes at the 7-positions of the quinoline ligand. These results are favorable to establish the structure-photophysical property relationship for enhancing the carrier transfer abilities of Ptq₂.     

Preparation and structural study of the enantiomers of alpha,alpha'-bis(trifluoromethyl)-9,10-anthracenedimethanol and its perdeuterated isotopomer, highly effective chiral solvating agents

Enantiopure forms of alpha,alpha'-bis(trifluoromethyl)-9,10-anthracenedimethanol and the corresponding perdeuterated isotopomers were prepared. The conformational study was carried out by (1)H NMR, and the absolute configuration was determined by the X-ray study of the crystallized diastereoisomeric carbamate derivative. This compound was tested as a chiral solvating agent (CSA). The results showed very good discrimination for several racemic mixtures that improved other classical methods. The study of diastereomeric complexes was carried out by determination of the stoichiometry of the complex and the binding constant of the equilibrium.     

Radical ring expansion reactions of methylenecyclopropane derivatives: a theoretical study

The evolution of the primary radicals from 1-(3-bromopropyl)-2-ethyl-3-methylenecyclopropane, 1-(3-bromopropyl)-1-trimethylsilyl-2-methylenecyclopropane, 1-(3-bromobutyl)-2-ethyl-3-methylenecyclopropane, and 1-(3-bromobutyl)-1-trimethylsilyl-2-methylenecyclopropane was theoretically studied at the ROMP2/6-311++G(d,p)//UB3LYP/6-31G(d,p) theory level taking into account the effect of solvent through a PCM-UAHF model. For the propyl-substituted radicals, the attack of the radical center on the double bond takes place most favorably in an exo fashion. The subsequent ring expansions yield the product corresponding to the rupture of the endo C-C bond as the most favorable one in accordance with the experimental results. In the case of 1-(3-bromobutyl)-2-ethyl-3-methylenecyclopropane, the Gibbs energy barriers for the endo and exo attacks are the same, and the subsequent reversible evolution yields the product corresponding to the rupture of the exo C-C bond as the most favorable one through thermodynamic control in agreement with experiment. Finally, for 1-(3-bromobutyl)-1-trimethylsilyl-2-methylenecyclopropane, our calculations predict that the endo attack is 0.8 kcal/mol more favorable than the exo one. In the subsequent reversible ring expansion, the product corresponding to the rupture of the endo C-C bond is kinetically the most favored one in reasonable agreement with the experimental observations.     

Gas-phase bimolecular reactions between (.)CH(2)-(CH(2))(n)-C(+)=O distonic ions and pyridine: a combined experimental and theoretical study

The gas-phase reactivities of the well-known (.)CH(2)CH(2)C(+)=O and (.)CH(2)CH(2)CH(2)C(+)=O distonic ions towards neutral pyridine were studied both experimentally (six sector hybrid mass spectrometer) and theoretically (density functional theory and M?ller-Plesset ab initio calculations). Competitively to the charge exchange and protonation processes, both radical cations react with pyridine by an initial bonding between the positive charge site of the ion and the lone electron pair of the neutral molecule. At variance with previously reported studies in which such a nucleophilic interaction was proposed to play only a transient catalytic role, the initial C-N bond is likely to remain in the observed ion-molecule reaction products. The structures of the ion-molecule reactions products were probed by collisional activation at high kinetic energy and the reaction pathways were tentatively proposed on the basis of labeling experiments and ab initio molecular orbital calculations.     

Cyclic oligothiophenes: novel organic materials and models for polythiophene. A theoretical study

Cyclic oligothiophenes (CnT, n = 6-30, even only) in syn- and anti-conformations are studied theoretically at the B3LYP/6-31G(d) level of theory. Strain energies, ionization potentials, HOMO-LUMO gaps, bond length alternations, NICS values, and IR and Raman spectra have been studied. The properties of anti-conformers change systematically with increasing ring size and were studied in detail in neutral, radical cation, and dication forms. In syn-conformation, the oligomers lose their nearly planar ring shape and bend significantly for n > 14, and thus properties cannot be related to ring size. The HOMO-LUMO gap in C14T-syn is even lower than polythiopehene. IR and Raman spectra calculated at the B3LYP/6-31G(d) level are used to differentiate syn- from anti-conformations. The properties of cyclic oligomers are compared to those of the linear system, and cyclic oligothiophenes are revealed as good models for polythiophene. To assist the experimental study of known cyclic oligomers having dibutyl substituents on alternate thiophene rings, the corresponding dimethyl-substituted oligomers are also studied. The experimentally evaluated HOMO-LUMO gaps for alternately dibutyl-substituted cyclic oligomers match the calculated values; however, they are significantly higher than those of the unsubstituted analogues.     

Ca, Cd, Zn, and their ions interacting with Cytosine: a theoretical study

Metal atoms play a major role in the chemical behavior of biological systems. In this work, known issues of the metal-base interactions, such as the stabilization of different tautomers of cytosine that could be incompatible with the DNA double helix, are researched using DFT methods. Ca-, Zn-, and Cd-cytosine in neutral and ionic forms were studied at the B3LYP/LANL2DZ level. Several neutral and ionic isomers were found within an interval of 10 kcal/mol of relative stability, with the most stable isomer in each group being a compound derived from the canonical isomer of cytosine, except for the dications where two iso-energetic isomers were found. Interatomic lengths from each metal atom to the nearest atoms in cytosine's ring were larger than 2 A, discouraging the possibility of a covalent interaction, as supported by additional evidence from molecular orbitals. The interaction between metal and cytosine, electrostatic in nature, is reinforced with the increase of the metal's nuclear charge. Additionally, the ionization energies of the metal-cytosine compounds exhibit a significant reduction (below 6 eV) compared with that of plain cytosine (8.7 eV), posing an interesting possibility with respect to the experimental determination of the photoelectron spectra of these compounds. Analyses of the energetics of the global reactions to form cationic species show that metal cations bind more strongly to neutral cytosine than to neutral metals. Metal dications form the most stable compounds with neutral cytosine, and the stabilities of these systems decrease as (Zn-cyt)2+ > (Cd-cyt)2+ > (Ca-cyt)2+. Aromaticities computed via the HOMA indexes also support the observation regarding the greater affinity of cytosine for metal cations.     

2-Bromophospholide ions: synthesis and theoretical study

DFT calculations at the B3 LYP/6-311++G(3df,2p) level indicate that the 2-bromophospholide ion could be stable toward self-arylation as a result of the lowered nucleophilicity of the in-plane phosphorus lone pair (the corresponding sigma(P) orbital is lowered by 0.7 eV compared with the corresponding orbital of the parent phospholide ion, and the negative charge at P is reduced from -0.435 to -0.369 e). Accordingly, the synthesis of 2-bromo-3,4-dimethylphospholide was successfully carried out by quantitative base-induced dealkylation of 2-bromo-1-(2-ethoxycarbonylethyl)-3,4-dimethylphosphole. This ion reacts with FeCl2 to give the corresponding 2,2'-dibromo-3,3',4,4'-tetramethyl-1,1'-diphosphaferrocene as a poorly stable mixture of meso- and rac-diastereomers in 18% yield.     

Electrochemical and spectroelectrochemical properties of neptunium(VI) ions in nitric acid solution

A benchmark study was carried out to verify whether MCNP is useful in the design stage of a PGNAA facility for large samples up to 1 m length and 0.15 m diameter, using a 2.54 cm diameter thermal neutron beam. For this facility neutron self-shielding and gamma-attenuation correction methods have to be developed. The relative spatial neutron-density distributions within three samples with different macroscopic scattering and absorption cross sections were studied in a comparison between an MCNP simulation and an irradiation experiment using copper wires as neutron monitors. The neutron density in the sample was within statistical agreement between experiment and simulation. Typically the relative spatial neutron-density distributions agreed to within 1%. Therefore, MCNP can be used in design studies for the development of a large sample PGNAA facility as specified. This revised version was published online in July 2006 with corrections to the Cover Date.     

The mechanism of the (bispidine)copper(II)-catalyzed aziridination of styrene: a combined experimental and theoretical study

Experimental and DFT-based computational results on the aziridination mechanism and the catalytic activity of (bispidine)copper(I) and -copper(II) complexes are reported and discussed (bispidine=tetra- or pentadentate 3,7-diazabicyclo[3.1.1]nonane derivative with two or three aromatic N donors in addition to the two tertiary amines). There is a correlation between the redox potential of the copper(II/I) couple and the activity of the catalyst. The most active catalyst studied, which has the most positive redox potential among all (bispidine)copper(II) complexes, performs 180 turnovers in 30 min. A detailed hybrid density functional theory (DFT) study provides insight into the structure, spin state, and stability of reactive intermediates and transition states, the oxidation state of the copper center, and the denticity of the nitrene source. Among the possible pathways for the formation of the aziridine product, the stepwise formation of the two N-C bonds is shown to be preferred, which also follows from experimental results. Although the triplet state of the catalytically active copper nitrene is lowest in energy, the two possible spin states of the radical intermediate are practically degenerate, and there is a spin crossover at this stage because the triplet energy barrier to the singlet product is exceedingly high.     

FT-IR and NMR investigation of 2-(1-cyclohexenyl)ethylamine: a combined experimental and theoretical study

FT-IR and (1)H, (13)C, DEPT, HETCOR, COSY, and NOESY NMR spectra of 2-(1-cyclohexenyl)ethylamine (CyHEA) have been reported for the first time. The vibrational frequencies and (1)H, (13)C NMR chemical shifts of CyHEA (C(8)H(15)N) have been calculated by means of the Hartree-Fock (HF), Becke-Lee-Yang-Parr (BLYP) and Becke-3-Lee-Yang-Parr (B3LYP) density functional methods with 6-31 G(d) and 6-31 G(d,p) basis sets, respectively. The comparison between the experimental and the theoretical results indicates that density functional B3LYP method is superior to the scaled HF and BLYP approach for vibrational frequencies and predicting NMR properties.     

Electronic states of o-nitrobenzaldehyde: a combined experimental and theoretical study

The experimental UV/vis absorption spectrum of ortho-nitrobenzaldehyde (o-NBA) has been assigned by means of MS-CASPT2/CASSCF, TD-DFT, and RI-CC2 theoretical computations. Additional information on the nature of the absorbing bands was obtained by comparing the o-NBA spectrum with that of related compounds, as, e.g., nitrobenzene and benzaldehyde. For wavelengths larger than approximately 280 nm, the absorption spectrum of o-NBA is dominated by a series of weak n pi* absorptions from the NO2 and CHO groups. These weak transitions are followed in energy by a more intense band, peaking at 250 nm and arising from charge transfer pi pi* excitations involving mainly benzene and nitro orbitals. Finally, the most intense band centered at 220 nm has its origin in the overlap of two different absorptions: the first one localized in the NO2 substituent and the second one arising from a charge transfer excitation involving the NO2 and the CHO fragments, respectively.