Marc Sallé

First signals of electrochemically oxidized species of TTF and TTM-TTF: A study by in situ spectroelectrochemical FTIR and DFT calculations

A first study by in situ FTIR spectroelectrochemistry of TTF and TTM-TTF has been undertaken. The oxidation, in this case, is caused only by electrochemistry, which constitutes a clear advantage over chemical oxidation since no side products are present in the solution. In this context, we obtained the signals of neutral, radical cation, and dication species of TTF and TTM-TTF. The experimental conditions were chosen in order to avoid the possible formation of π-dimer species and to obtain a satisfactory signal-to-noise ratio. A weak signal was detected for TTF and a stronger one for TTM-TTF. The changes induced by the oxidation process in the IR spectra of TTF and TTM-TTF have been analyzed with the aid of B3P86/6-31G** density functional theory (DFT) calculations, which allows for a comprehensive assignment of the bands observed. DFT calculations show that the IR signal associated to the asymmetric stretching of the lateral CC bonds can be used as a structural signature to identify the different oxidation states of TTF and TTM-TTF. The frequency downshift and the intensity increase undergone by the νasym(CC) vibration upon oxidation are shown to be the key factors to understand the evolution of the IR spectra.

Electronic structure and electro-optical coefficients in push-pull chromophores incorporating the 1,3-dithiole-2-ylidene moiety as electron-donating part

We report on experimental and theoretical studies of linear electrooptic (Pockels) coefficients in push-pull chromophores incorporating the 1,3-dithiol-2-ylidene moiety as the electron donating. The proposed theoretical approach is based on molecular dynamics geometry optimization and quantum chemical calculations of the appropriate molecules. Intermolecular interactions are taken into account within a framework of solid state local density approximation. The quantum chemical method is based on a self-consistent norm- conserving nonlocal pseudopotential with orthogonalization to the core pseudo-wavefunctions by linear combination of atomic orbitals. Contribution of the electronic and ionic parts into the output electrooptic tensor component r111 ((lambda) equals 633 nm) is evaluated. An enhancement of heteropolar molecular ionicity leads to an increase of the resulting r111 electrooptic coefficient. Vibration (ionic) and electronic modes are found to be sensitive to substitution of the side chemical groups. The dominant role of some particular chemical fragments dominating in the observed coefficients is really evident.

Theoretical and experimental studies of third-order nonlinear optical susceptibilities of new p-N,N'-dimethylaniline tetrathiafulvalene derivatives

We report large third-order nonlinear optical susceptibilities (chi) <3>ijkl of new tetrathiafulvalene (TTF) derivatives, using the degenerate four wave mixing (DFWM) method. To know the physical origin of their optical nonlinearities, we separate electronic and ionic contributions to nonlinear optical susceptibilities. The electronic contribution to the third-order nonlinear optical susceptibilities of the studied molecules is dominant. From DFWM measurements we also deduce values of the third-order hyperpolarizabilities (chi) which are about 105 greater than the (chi) value for CS2. We have thus shown that the molecules under consideration posses larger third order nonlinear optical susceptibilities compared to the polyazine derivatives, acetylenic analogues of TTF and to the ethylenic TTF derivatives. All theoretical simulations are done within a framework of semi-empirical quantum chemical calculation. A correlation between molecular dipole moments and third order susceptibilities is found.