Acceptor Tetracyanoethylene Research Articles

Strong Ground‐ and Excited‐State Charge Transfer in C3‐Symmetric Truxene‐Derived Phenothiazine‐Tetracyanobutadine and Expanded Conjugates

AbstractThe C3‐symmetric star‐shaped phenothiazene‐substituted truxene 1 was reacted with the electron acceptors tetracyanoethylene (TCNE) and 7,7,8,8‐tetracyanoquinodimethane (TCNQ). The cycloaddition–retroelectrocyclization reaction yields the conjugates 2 and 3. A combination of spectral, electrochemical, and photophysical investigations of 2 and 3 reveals that the functionalization of the triple bond has a pronounced effect on their ground and excited‐state interactions. Specifically, the existence of strong ground‐state interactions between phenothiazine and the electron‐accepting groups results in charge‐transfer states, while subsequent ultrafast charge separation yields electron transfer products. This is unprecedented not only in phenothiazine chemistry but also in tetracyanobutadiene‐ and dicyanoquinodimethane‐derived donor–acceptor conjugates. Additionally, by manipulating spectroelectrochemical data, a spectrum of the charge‐separated species is construed for the first time, and shown to be highly useful in interpreting the rather complex transient spectra.

Strong Ground- and Excited-State Charge Transfer in C3 -Symmetric Truxene-Derived Phenothiazine-Tetracyanobutadine and Expanded Conjugates.

The C3 -symmetric star-shaped phenothiazene-substituted truxene 1 was reacted with the electron acceptors tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). The cycloaddition-retroelectrocyclization reaction yields the conjugates 2 and 3. A combination of spectral, electrochemical, and photophysical investigations of 2 and 3 reveals that the functionalization of the triple bond has a pronounced effect on their ground and excited-state interactions. Specifically, the existence of strong ground-state interactions between phenothiazine and the electron-accepting groups results in charge-transfer states, while subsequent ultrafast charge separation yields electron transfer products. This is unprecedented not only in phenothiazine chemistry but also in tetracyanobutadiene- and dicyanoquinodimethane-derived donor-acceptor conjugates. Additionally, by manipulating spectroelectrochemical data, a spectrum of the charge-separated species is construed for the first time, and shown to be highly useful in interpreting the rather complex transient spectra.

Synthesis, structures and charge−transfer complexations of 1,n-di-[3,6-di-(9-carbazolyl)-9-carbazolyl]alkanes with tetracyanoethylene and tetranitromethane

Synthesis and charge−transfer (CT) complexations of amorphous 3,6-di-(9-carbazolyl)-9-ethylcarbazole, 3,6-di-(9-carbazolyl)-9-hexylcarbazole and a series of 1,n-di-[3,6-di-(9-carbazolyl)-9-carbazolyl]alkanes (n = 1–5) with electron acceptors tetracyanoethylene and tetranitromethane are reported. The molar extinction coefficients (ε), equilibrium constants (Keq), enthalpies (ΔH) and entropies (ΔS) of complexations have been determined. The low Keq values (1.83–3.82 M−1 for carbazole−TCNE and 0.28–0.45 M−1 for carbazole−TNM complexes) show weak donor−acceptor associations. The negative values of ΔH determined to be between −2.09 ± 0.08 and −3.10 ± 0.21 kcal mol−1 for carbazole−TCNE complexes and −0.91 ± 0.08 and −3.31 ± 0.28 kcal mol−1 for carbazole−TNM complexes indicate that complexations are driven by the exothermic enthalpies. Computational analysis using semi−empirical and DFT methods were applied to clarify the structures of the synthesized molecules and the nature of their CT complexations.

Tuning thermoelectric properties of graphene/boron nitride heterostructures

Using density functional theory combined with a Green’s function scattering approach, we examine the thermoelectric properties of hetero-nanoribbons formed from alternating lengths of graphene and boron nitride. In such structures, the boron nitride acts as a tunnel barrier, which weakly couples states in the graphene, to form mini-bands. In un-doped nanoribbons, the mini bands are symmetrically positioned relative to the Fermi energy and do not enhance thermoelectric performance significantly. In contrast, when the ribbons are doped by electron donating or electron accepting adsorbates, the thermopower S and electronic figure of merit are enhanced and either positive or negative thermopowers can be obtained. In the most favourable case, doping with the electron donor tetrathiafulvalene increases the room-temperature thermopower to −284 μv K−1 and doping by the electron acceptor tetracyanoethylene increases S to 210 μv K−1. After including both electron and phonon contributions to the thermal conductance, figures of merit ZT up to of order 0.9 are obtained.

Structure and properties of nitrogen-rich 1,4-dicyanotetrazine, C4N6: a comparative study with related tetracyano electron acceptors.

The crystal structure, redox electrochemical stability, and reaction chemistry of 1,4-dicyanotetrazine (DCNT) has been experimentally characterized. These experimental results were rationalized by the results of theoretical calculations of the electronic structure, spin and charge distributions, electronic absorption spectra, and electron affinity and compared with the results for related the tetracyano electron acceptors tetracyanoethylene (TCNE), 7,7,8,8-tetracyano-p-quinodimethane (TCNQ), and 2,3,5,6-tetracyanopyrazine (TCNP). DCNT is made from the dehydration of 1,2,4,5-tetrazine-3,6-dicarboxamide, and because of the unusual deep-magenta color of the dicarboxamide in the solid state, its hydrogen-bonded layered structure, electronic structure, and electronic absorption spectra were determined. The magenta color is attributed to its absorptions at 532 nm (18 800 cm(-1)), and this corresponds to normalized chromaticity coordinates of x = 0.42 and y = 0.31 in the pink/red/orange part of the 1931 CIE chromaticity diagram. In contrast with previous reports, DCNT exhibits an irreversible one-electron reduction at -0.09 V vs SCE (MeCN), and reduced forms of DCNT have yet to be isolated and characterized. In addition, the reactions of DCNT with V(CO)6, Fe(II)(C5Me5)2, and I(-) are discussed.

The adsorption and reconstruction of strong electron acceptor tetracyanoethylene (TCNE) on Si(001)-(2 × 1): A density functional theory investigation

The adsorption and reconstruction of strong electron-acceptor TCNE on the Si(001)-(2×1) surface are investigated by density functional theory (DFT). The results show that TCNE prefers to adsorb on the trench between two adjacent dimer rows and the CC double bond is parallel to dimer rows. Charge density difference calculation and Bader charge analysis indicate that abundant negative charge transfers from dimer Si to TCNE. Strong interaction makes it difficult for TCNE to move on Si(001), which is confirmed by nudged elastic band (NEB) analysis. In addition, the correlations between simulated STM images and molecular orbitals are discussed and two surface reconstructions of (2×1) and (4×2) are predicted at different TCNE coverages.

Effect of Charge Transfer Complex on Electrical Properties of 4-Cyano-4'-pentylbiphenyl

The dielectric constant, threshold voltage, and response time were measured for a nematic binary mixture of 4-cyano-4'-pentylbiphenyl (5CB) molecules and the doped electron acceptor tetracyanoethylene (TCNE). The doping of TCNE caused a large increase in the dielectric constant and dielectric anisotropy. However, no marked improvement was achieved for threshold voltage and response time. Considering that the large increase was due to mobile charges in the system, we proposed a theoretical model to discuss the singularity that appeared in experimental results obtained in an electric field.

Charge-transfer complexations of 1, n-di(9-ethylcarbazol-3-yl)alkanes with tetracyanoethylene and tetranitromethane

1, n-Di(9-ethylcarbazol-3-yl)alkanes, where n = 1–5, as the dichromophoric model compounds of poly-3-vinylcarbazoles were synthesized to examine their complexation behaviors with the electron acceptors tetracyanoethylene (TCNE) and tetranitromethane (TNM). 9,9′-Diethyl-3,3′-dicarbazolyl, di(3-ethylcarbazol-9-yl)methane, and three monomeric analogues were also included for comparison. In dichloromethane solution, the dicarbazoles formed stable 1:1 electron donor–acceptor complexes with TCNE having formation enthalpies around −3.5 kcal/mol. With TNM they formed more weakly bound complexes that showed little dependence on concentration and almost zero dependence on temperature changes having nearly 0 kcal/mol enthalpies of formation. The smaller gap between the two carbazole groups in 1, n-di(9-ethylcarbazol-3-yl)alkanes with n ≤ 2 affected complexation adversely, while such an effect was not observed in the dicarbazoles with n ≥ 3.

Raman Spectroscopy of Short‐Lived Terthiophene Radical Cations Generated by Photochemical and Chemical Oxidation

The Raman spectra of various terthiophene radical cations are investigated; namely those of unsubstituted terthiophene and two styryl-substituted terthiophenes. Transient pump-probe resonance Raman spectroscopy is used to measure the short-lived radical cation spectra of non-end-capped 2,2':5',2''-terthiophene (3T) and 3'-[(E)-2-(4-nitrophenyl)ethenyl]-2,2':5',2''-terthiophene (NO2-pe3T). For these two compounds, the radical cations are generated via either direct photogeneration or photochemically using the electron acceptor tetracyanoethylene. The radical cation of 5,5''-dimethyl-3'-[(E)-2-phenylethenyl]-2,2':5',2''-terthiophene (DM-pe3T) is stable for up to five minutes as a result of the two alpha end caps and continuous-wave resonance Raman spectroscopy and chemical oxidation is used to obtain the spectrum of this radical cation. The resonance Raman spectra of all three terthiophene radical cations are dominated by a group of very intense bands in the low-frequency region. These bands have been assigned, by density functional theory methods, to C-S stretching modes coupled to thiophene ring deformations. These modes are significantly less intense in the sigma-dimer of NO2-pe3T [i.e. the corresponding styryl sexithiophene (NO2-pe3T)2]. This observation is attributed to a smaller change in the C--S bond order in the sexithiophene compared to the analogous terthiophene. This bond order difference may be rationalised by consideration of the singly occupied molecular orbital and lowest unoccupied molecular orbital, which are involved in the electronic transition probed by the laser excitation wavelength.

Spectroscopic investigation of the novel charge-transfer complex [(phen)(TCNE) 12] formed in the reaction of phenacetin with tetracyanoethylene

The charge-transfer (CT) interaction of the electron donor phenacetin (phen) and the π-electron acceptor tetracyanoethylene (TCNE) has been studied in CH 2Cl 2. The results obtained indicate the formation of the novel CT-complex with the general formula [(phen)(TCNE) 12]. The 1:12 stoichiometry of the reaction was based on photometric titration, elemental analysis, infrared, thermal and cyclic voltametry measurements of the formed CT-complex.

Reversibly Altering Electronic Conduction through a Single Molecule by a Chemical Binding Event

This letter presents experimental evidence that the electrical conductance of a single molecule can be altered by a chemical binding event. Self-assembled monolayers of electron donor tetramethyl xylyl dithiol (TMXYL) have been synthesized and chemically switched to a conducting state by reaction with an electron acceptor tetracyanoethylene (TCNE). Low bias conductance measurements obtained by scanning tunneling spectroscopy under ultrahigh vacuum conditions show a change from insulating to ohmic behavior as a result of the electron donor/acceptor interaction.

A new ferrimagnetically ordered charge-transfer complex based on high-spin iron(III) chelate tetracyanoethenide with a Tc of 10 K

The reaction of the high-spin iron(II) quadridentate chelate complex [Fe(L)], (L=diethyl( E, E)-2,2′-[1,2-phenylenebis-(iminomethylidyne)]-bis-[3-oxobutanoate]), with the electron acceptor tetracyanoethylene (TCNE) leads to the formation of the 1:1 charge-transfer (CT) complex [Fe(L)] +[TCNE] −. The magnetic characterisation of the CT salt indicates a chain structure with alternating S= 5 2 high-spin iron(III) and S= 1 2 TCNE − radicals in which the spin centres are antiferromagnetic correlated. The antiferromagnetic intrachain interaction parameter is very weak ( J intra≈−8 cm −1) based on the spin Hamiltonian H=−J intra S i S j . At T=2 K and H=5.5 kG the magnetisation for the CT complex is about 83% of the saturation magnetisation value expected for a S total =( 5 2 − 1 2 )=2 system. A magnetic hysteresis loop was observed at T=2 K with a coercive field of 200 G. Below 10 K, the complex shows a weak spontaneous magnetisation characteristic for a canted ferrimagnet.

Ion Pairs from Photoexcited, “Random” Electron Donors and Acceptors: Alkylbenzenes and Tetracyanoethylene

Transient photocurrent experiments are used to measure the free radical ion quantum yield of a number of alkylbenzene electron donors with the electron acceptor tetracyanoethylene (TCNE). These experiments are performed at a variety of photoexcitation wavelengths in dichloromethane, a moderately polar solvent. It is found that the free ion yields often exhibit a very strong dependence on the excitation wavelength and may decrease markedly in the center of the charge-transfer band. For example, the free ion yield of the donor−acceptor system, pentamethylbenzene−TCNE, increases more than 100-fold when the excitation wavelength is switched from 532 to 397 nm! We show that this result and others are understandable from the following model. While closely associated electron donor−acceptor (EDA) complexes account for most of the absorption, there is an additional, usually small, absorption due to unassociated random donor and acceptor pairs. The Franck−Condon (vertical) excitation of these random pairs results in radical ion pairs which have center-to-center distances greater than contact and which have high probabilities for separation. Quantitative analysis based on Onsager theory indicates that only distantly separated radical ion pairs (ca. 1 nm or more) created by photoexcitation can escape each other's Coulombic attraction to produce the free ion yields observed in our experiments. The photoexcitation of ground-state EDA complexes plays little essential role in this process. The observed wavelength dependence then corresponds both to variation in the ratio of random pair to EDA complex absorption and to the distance distribution of radical ion pairs produced. Free ion yields calculated using Onsager theory and a simple excitation function for the random pairs fit our experimental results quite well and support this model.

Donor−Acceptor Complexes of Alkylcarbazole and Dicarbazolylalkane Donors with the Acceptors Tetracyanoethylene and Tetranitromethane

Electron donor−acceptor complexes of conformationally flexible 1,n-dicarbazolylalkanes (C12H8N−(CH2)n−NC12H8), where n = 1−5, were examined. Carbazole, methylcarbazole, ethylcarbazole, and cyanoethylcarbazole also were studied as monochromophoric analogues for comparison. In dichloromethane solution, the dicarbazolylalkanes form 1:1 complexes with the terminal carbazolyl chromophores acting as independent donors when n ≥ 2. With the acceptor tetranitromethane (TNM), the carbazoles form contact complexes displaying small positive enthalpies of formation. In contrast, stable complexes form with the acceptor tetracyanoethylene (TCNE). Crystalline TCNE complexes were isolated for the bichromophoric donors with n = 2−4. The solid complexes and their uncomplexed donor components were analyzed by single-crystal X-ray diffraction. The solid-state stoichiometries of (carbazolyl donor):(TCNE acceptor) were found to depend on the donor conformation. Dicarbazolylalkane donors separated by two or four methylene units ...

Interaction of N‐tert‐butylaziridine with π‐electron acceptors, 2. Polymerization initiated by tetracyanoethylene

AbstractThe interaction of N‐tert‐butylaziridine (TBA) with the strong π‐electron acceptor tetracyanoethylene (TCNE) was investigated, which results in ring‐opening polymerization of the aziridine. Charge‐transfer complex formation between TBA and TCNE was proved by means of UV/VIS spectroscopy and is assumed to be the first stage of the initiation. There is evidence that only linear TBA homopolymers are formed and TCNE is not covalently bound in the polymer chains as comonomer. The acceptor is present in the final products as a stable complex with the nitrogen atoms along the polyaziridine chains.

Spin polarization and ferromagnetic coupling in metallocenium charge transfer complexes

Spin polarization effects in metallocenium complexes across the transition metal series Cr, Mn, Fe, Co, Ni are discussed in a qualitative fashion using a restricted open-shell Hartree–Fock approach. The spin polarization is taken into account by a limited configuration interaction (CI) with singly excited configurations. In Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2, the unpaired electrons are localized in molecular orbitals with predominant metal 3d character. The polarization of the metal–cyclopentadiene π bonds then leads to a negative spin density on the cyclopentadiene (Cp) rings. The amount of negative spin density on Cp increases with increasing spin on the metal (S=1/2, S=1, and S=3/2 for ferrocenium, manganocenium, and chromocenium, respectively). The Cp spin density of nickelocenium on the other hand is positive because there is a direct delocalization of spin density to the Cp rings due to the considerable Cp contributions to the e*1g molecular orbitals in which the unpaired electron of nickelocenium is located. Correspondingly, the charge transfer complexes of these compounds with the acceptors tetracyanoethylene (TCNE) or tetracyanoquinodimethane (TCNQ) in an alternating arrangement ...D+A−D+A−... are expected to exhibit ferromagnetic donor–acceptor coupling for Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2 and antiferromagnetic coupling for Ni(C5Me5)+2 according to a model suggested previously by the authors and based on an idea of McConnell. This is in agreement with the experimental results. The observed nuclear magnetic resonance (NMR) shifts of some metallocenes in relation to these findings are discussed.

Functional Polymers. LVII. Electronic Absorption Spectra of Benzotriazole Derivative/Electron Acceptor Systems

Abstract The electronic absorption spectra of charge-transfer complexes (CTC) of benzotriazole derivatives including 2(2H-hydroxyphenyl)2H-benzotriazole-containing copolymers with various electron acceptors were investigated. While no charge-transfer interaction was observed with weak acceptors, strong acceptors such as trinitrofluorenone and pyromellitic anhydride exhibited an absorption of the contact charge-transfer type with these donors. When the very strong acceptor tetracyanoethylene was used as acceptor, new peaks of a CTC type appeared at longer wavelengths. From the wavelengths of the absorption maxima and the equilibrium constants of the CTC, the electron-donating ability of several related (2(2-hydroxyphenyl)2H-benzotriazole derivatives was estimated as follows: 2(2-Hydroxy-5-methylphenyl)2H-benzotriazole > 2H-benzotriazole > 2(4-hydroxyphenyl)2H-benzotriazole > 2(2-acetoxy-5-methylphenyl)2H-benzotriazole > copolymers containing 2(2-hydroxyphenyl)2H-benzotriazole groups.

Electron transfer reactions of bis[dicarbonyl(pentamethylcyclopentadienyl)iron] with organic compounds

[(C 5Me 5)Fe(CO) 2] 2 ( A) reacts with one equivalent of tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) to give the compounds {2[(C 5Me 5)Fe +(CO) 2]·TCNE 2−} ( B) and {2[(C 5Me 5)Fe +(CO) 2]·TCNQ 2−} ( D) respectively. The reaction of A with two equivalents of TCNE gives the polymeric compound [(C 5Me 5)Fe(CO) 2TCNE] n ( C) in which the TCNE 2− anion is coordinated to the metal through nitrogen of the CN group. Electronic, IR and ESR spectra indicate that compounds B and D are formed by two one-electron steps involving the rather elusive [(C 5Me 5)Fe(CO) 2 ] radicals. The electron transfer processes are discussed on the basis of the thermodynamic redox potentials of donor A and of two acceptors TCNE and TCNQ.

ChemInform Abstract: Azoalkanes as Electron Donors in Charge‐Transfer Complexes. Molecular Structures with Sacrificial (π*, σ*) and Increvalent (v) Electron Acceptors.

AbstractA series of EDA complexes of the azoalkanes (I)‐(IV) with the three unique types of electron acceptors tetracyanoethylene (TCNE) (π*), CBr4 (σ*), and Ag(I) (v) are characterized in solution from their charge‐transfer spectra.

Thermally stimulated currents in poly-p-phenylene sulphide doped with tetracyanoethylene

Localised states in poly-p-phenylene sulphide (PPS) with an organic acceptor tetracyanoethylene (TCNE) were studied by the analysis of thermally stimulated currents (TSC). It is shown that shallow localised states with energy depths between 0.12 and 0.45 eV are introduced by doping with TCNE. The shallow localised states effectively capture charge carriers at low temperatures (-170-0 degrees C), while they act as effective hopping centres above room temperature.