Electron Acceptor Tetracyanoethylene Research Articles

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.

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.

Strongly Reshaped Organic-Metal Interfaces: Tetracyanoethylene on Cu(100)

The interaction of the strong electron-acceptor tetracyanoethylene with the Cu(100) surface is studied with scanning tunneling microscopy experiments and first-principles density functional theory calculations. We compare two different adsorption models with the experimental results and show that the molecular self-assembly is caused by a strong structural modification of the Cu(100) surface rather than the formation of a coordination network by diffusing Cu adatoms. Surface atoms become highly buckled, and the chemisorption of tetracyanoethylene is accompanied by a partial charge transfer.

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.

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.

UV absorption spectra of polystyrene

Polystyrene (PSt) exhibits a new UV absorption band at around 290nm in comparison with ethylbenzene, which is attributed to associative interaction between pendant phenyl groups. Similar absorption bands are also observed for polymers of styrene derivatives. The UV spectra suggest charge-transfer complex formation between the polymeric and monomeric donors with electron-acceptor tetracyanoethylene (TCNE) and the equilibrium constants were estimated to be in the sequence of ethylbenzene > PSt > copolymer of St with methylmethacrylate.

Electron spin resonance study of phosphorescent centres in biphenyl, biphenyl–tetracyanoethylene, and biphenyl–tetracyanoethylene–naphthalene molecular crystals

Phosphorescent centres in single crystals of zone-purified biphenyl have been examined by their triplet e.s.r. spectra. These centres are attributed to impurities, possibly naphthalene, trapped at crystal defect sites, rather than conformationally strained biphenyl molecules. The strong electron acceptor tetracyanoethylene is found to enter the lattice isomorphically, and to have negligible, if any, contribution to the triplet through its own triplet state or that of a donor–acceptor charge-transfer complex. Naphthalene in the presence of an excess of tetracyanoethylene forms a donor–acceptor charge-transfer complex which crystallises out of biphenyl solid solution. Insofar as a residue of this complex is retained in the lattice there is, within the limits of the experimental technique, no detectable charge-transfer contribution to the triplet of the phosphorescent centres of the host biphenyl.

Cationic and free radical propagation of copolymerization by the N-vinyl carbazole cation-radical

Copolymerization of N-vinyl carbazole (NVC) and p-methoxy styrene in ethylene dichloride (EtCl2) at 25°C gives copolymers of virtually identical composition whether the reaction is initiated by the cationic reagent BF3OEt2 or by the typical electron-acceptor tetracyanoethylene (TCNE), provided the latter system is shielded from daylight. The propagation mechanism is concluded to be cationic in both cases. The derived reactivity ratios are: r1(NCV)=21·4±0·81;r2=0·13±0·005(BF3OEt2)r1=20·1±1·5;r3=0·09±0·01(TCNE)A photochemically induced copolymerization is found in daylight with TCNEĒtCl2 and with EtCl2 alone giving copolymers slightly richer in p-MeO styrene. Evidence is offered that under these conditions, both cationic and a parallel free radical propagation mechanism can proceed. The reactivity ratios for the cationic copolymerization by BF3OEt2 show virtually no change over the temperature range 25° to 0°C.

π-Charge-Transfer Complexes of Hexamethylborazole

BORAZOLE has been described as inorganic benzene. It might be expected1 that appropriately substituted borazoles would form complexes similar to the well-known π-molecular complexes of benzene and its derivatives2. Recently it has been observed3 that mixtures of hexamethylborazole and tetracyanoethylene in chloroform show a broad featureless absorption band (λmax = 561 mµ) not possessed by either component alone. It was suggested that hexamethylborazole, acting as an electron donor, forms a π-π-charge-transfer complex with the electron acceptor tetracyanoethylene. Other electron acceptors should also form such complexes with hexamethylborazole. Unfortunately complexes with hexamethylborazole are relatively weak: the association constant (K) for the tetracyanoethylene complex is 0.7 l.mole−1 in chloroform at 21°, compared with K = 27.6 l.mole−1 for the corresponding hexamethylbenzene complex. The only other similar interaction which was reported was with the electron acceptor chloranil. In this case, instability of the complex, and the position of the charge-transfer band which overlapped a transition of ehloranil itself, made the definite determination of the charge-transfer band impossible.