ABSTRACT
A central issue in molecular electronics in order to build functional devices is to assess whether changes in the electronic structure of isolated compounds by chemical derivatization are retained once the molecules are inserted into molecular junctions. Recent theoretical studies have suggested that this is not always the case due to the occurrence of pinning effects making the alignment of the transporting levels insensitive to the changes in the electronic structure of the isolated systems. We explore here this phenomenon by investigating at both the experimental and theoretical levels the I/ V characteristics of molecular junctions incorporating three different three-ring phenylene ethynylene derivatives designed to exhibit a significant variation of the HOMO level in the isolated state. At the theoretical level, our NEGF/DFT calculations performed on junctions including the three compounds show that, whereas the HOMO of the molecules varies by 0.61 eV in the isolated state, their alignment with respect to the Fermi level of the gold electrodes in the junction is very similar (within 0.1 eV). At the experimental level, the SAMs made of the three compounds have been contacted by a conducting AFM probe to measure their I/ V characteristics. The alignment of the HOMO with respect to the Fermi level of the gold electrodes has been deduced by fitting the I/ V curves, using a model based on a single-level description (Newns-Anderson model). The extracted values are found to be very similar for the three derivatives, in full consistency with the theoretical predictions, thus providing clear evidence for a HOMO level pinning effect.
ABSTRACT
The straightforward synthesis of a new Cu(i) metal-rich small metallacycle is presented. This compound is luminescent in the solid state with an emission quantum yield of 72% at room temperature and displays a pronounced reversible red-shift of its emission spectra upon cooling. Quantum chemical calculations reveal that these properties are governed by important geometrical relaxations that imply the formation of cuprophilic interactions at the excited states.
ABSTRACT
The combined time-resolved photoluminescence (PL) and theoretical study performed on luminescent [Mo6Br(i)8Br(a)6](2-)-based systems unambiguously shows that their NIR-luminescence is due to at least two emissive states. By quantum chemical studies, we show for the first time that important geometrical relaxations occur at the triplet states either by the outstretching of an apex away from the square plane of the Mo6 octahedron or by the elongation of one Mo-Mo bond. Experimental PL measurements demonstrate that the external environment (counter-ions, crystal packing) of the cluster has a noticeable impact on its relaxation processes. Temperature and excitation wavelength dependence of the two components of the luminescence spectra is representative of multiple competitive de-excitation processes in contradiction with Kasha's rule. Our results also demonstrate that the relaxation processes before and after emission can be tracked via fast time-resolved spectroscopy. They also show that the surroundings of the luminescent cluster unit and the excitation wavelength could be modulated for target applications.
ABSTRACT
The compounds Cp*Fe(dppe)X ([Fe]X) and the corresponding cation radicals [Fe*]X*+ are available for the series X = F, Cl, Br, I, H, CH3. This has allowed for a detailed investigation of the dependence of the nature of Fe-X bonding on the identity of X and the oxidation state (charge) of the complex. Cyclic voltammetry demonstrates that the electrode potentials for the [Fe]X0/+ couples decrease in the order I > Br > Cl > H > F > CH3. An "inverse halide order" is seen, in which the most electronegative X leads to the most easily oxidized complex. This suggests that F is the best donor among the halides. The halide trend is also reflected in NMR spectroscopic data. Mössbauer spectroscopy data also suggest that the F ligand is a strong donor (relative to H and CH3) in [Fe*]X*+. DFT calculations on CpFe(dpe)X ([Fe]X) model complexes nicely reproduce the trend in the electrode potentials for the [Fe*]X0/+ couples. Analysis of the theoretical data within the halogen series indicates that the energy of the [Fe]X HOMO does not correlate with the extent of its Fe(d(pi))-X(p(pi)) antibonding character, which varies in the order I > Br > Cl > F, but rather depends on the destabilizing electrostatic effect caused by X. This effect varies in the order F > Cl > Br > I. A thermochemical cycle that incorporates the [Fe*]X0/+ and [Fe*]0/+ electrode potentials was used to investigate the effect of the oxidation state of the complex on the homolytic bond dissociation energy (BDEhom), defined for the processes Fe-X --> Fe* + X* and Fe-X*+ --> Fe*+ + X*. For all X, it was found that a one-electron oxidation leads to a weakening of the Fe-X bond. This trend was reproduced by the DFT calculations. On the other hand, IR nu(Fe-X) spectroscopy data showed an increase in the stretching frequencies for X = H and Cl upon oxidation. X-ray crystallographic data showed a shortening of the Fe-Cl bond upon oxidation. The trends in IR and Fe-Cl bond distances were reproduced in the DFT calculations. The combined data therefore suggest that oxidation leads to weaker, but shorter, Fe-X bonds. A second thermochemical cycle was applied to investigate the effect of the one-electron oxidation on the heterolytic bond dissociation energies (BDEhet), defined for the processes Fe-X --> Fe+ + X- and Fe-X*+ --> Fe2+ + X-. In this case, the oxidation led to bond strengthening in all cases. The computed BDE values have been analyzed within Ziegler's transition state methodology and decomposed into two components, one electrostatic and one covalent, describing the interaction between the unrelaxed fragments. In all the computed BDEhom and BDEhet values of the [Fe]X models the electrostatic component is important. This helps to understand their respective variations upon oxidation.
ABSTRACT
Electrochemical combination of the magnetic dinuclear anion [MM'(C2O4)(NCS)8](4-) (MM' = Cr(III)Cr(III), Cr(III)Fe(III)) with the ET organic pi-donor (ET = BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) gives rise to two new isostructural molecular hybrid salts ET5[MM'(C2O4)(NCS)8], with MM' = CrCr (1), CrFe (2). The molecular structure of compound 1 has been determined by single crystal X-ray diffraction. The particular arrangement of the organic units consists of an unprecedented two-dimensional organic sublattice nearly similar to that observed in kappa-phase structures. For both compounds, the magnetic susceptibility measurements indicate (i) the ET radicals do not contribute to the magnetic moment probably due to the presence of strong antiferromagnetic interaction between them, and (ii) in the anion, the magnetic coupling is antiferromagnetic for 1 (J = -3.65 cm(-1)) and ferromagnetic for 2 (J = 1.14 cm(-1), J being the parameter of the exchange Hamiltonian H = -2JS1S2). The field dependence of the magnetization of compound 2 at 2.0 K gives further evidence of the S = 4 ground-state arising from the interaction between S = 3/2 Cr(III) and S = 5/2 Fe(III). EPR measurements confirm the nature of the magnetic interactions and the absence of any contribution from the organic part, as observed from the static magnetic measurement. Conductivity measurements and electronic band structure calculations show that both salts are semiconductors with low activation energies.
