Amos Ben-Asuly

Predicting the Cis−Trans Dichloro Configuration of Group 15−16 Chelated Ruthenium Olefin Metathesis Complexes: A DFT and Experimental Study

Gradient-corrected (BP86) and hybrid (M06-L) density functional calculations were used to study the relative stability of cis and trans-dichloro X-chelated benzylidene ruthenium complexes (X = O, S, Se, N, P). Calculations in the gas phase differed from experimental results, predicting the trans-dichloro configuration as being more stable in every case. The addition of Poisson-Boltzmann (PBF) continuum approximation (dichloromethane) corrected the disagreement and afforded energies consistent with experimental results. Novel N, Se, and P chelated ruthenium olefin metathesis complexes were synthesized to evaluate calculation predictions. These findings reinforce the importance of including solvent corrections in DFT calculations of ruthenium metathesis catalysts and predict that stronger sigma donors as chelating atoms tend to electronically promote the unusual and less active cis-dichloro configuration.

Studies on Electronic Effects in O‐, N‐ and S‐Chelated Ruthenium Olefin‐Metathesis Catalysts

A short overview on the structural design of the Hoveyda-Grubbs-type ruthenium initiators chelated through oxygen, nitrogen or sulfur atoms is presented. Our aim was to compare and contrast O-, N- and S-chelated ruthenium complexes to better understand the impact of electron-withdrawing and -donating substituents on the geometry and activity of the ruthenium complexes and to gain further insight into the trans-cis isomerisation process of the S-chelated complexes. To evaluate the different effects of chelating heteroatoms and to probe electronic effects on sulfur- and nitrogen-chelated latent catalysts, we synthesised a series of novel complexes. These catalysts were compared against two well-known oxygen-chelated initiators and a sulfoxide-chelated complex. The structures of the new complexes have been determined by single-crystal X-ray diffraction and analysed to search for correlations between the structural features and activity. The replacement of the oxygen-chelating atom by a sulfur or nitrogen atom resulted in catalysts that were inert at room temperature for typical ring-closing metathesis (RCM) and cross-metathesis reactions and showed catalytic activity only at higher temperatures. Furthermore, one nitrogen-chelated initiator demonstrated thermo-switchable behaviour in RCM reactions, similar to its sulfur-chelated counterparts.

Novel fluorescent stilbene analogs involving a carbazole moiety

Abstract The synthesis of a series of novel efficient TPA chromophores involving a carbazole moiety from N -( p -formyl)phenylcarbazole precursors and their linear absorption and fluorescent properties are described.

2-D effects in the second-order optical nonlinearity of organic molecules incorporating carbazole

Abstract The second order nonlinearity ( β ) of a series of organic π-conjugated molecules involving 1,3 indandione derivatives as an acceptor moiety has been measured. The effect of varying the donor from dialkylamino to N -carbazolyl substituents was examined. Despite the chemically similar nature of the two donors, a drastic reduction of electric field induced second harmonic (EFISH) β values for the molecules involving the N -carbazolyl substituents was detected. Quantum mechanical calculations indicate that the decrease in β values results from the presence of two overlapping transitions that contribute to β with opposite signs. Thus, the charge transfer band gives a positive β zzz contribution along the long molecular axis, while a transition essentially within the carbazole group provides a negative β zxx contribution to β EFISH . Therefore, these molecules can only be described with a two-dimensional model as opposed to the ‘classical’ one-dimensional model for NLO chromophores. Experimental verification of this hypothesis was provided by a combination of two experimental methods, EFISH and hyper Rayleigh scattering (HRS). Each technique probes a different combination of the β tensor elements, so we could deduce the two contributions.

Dibenzo[2,3:5,6]pyrrolizino[1,7-bc]indolo[1,2,3-lm]carbazole: a new electron donor

The new heterocyclic system, dibenzo[2,3:5,6]pyrrolizino[1,7-bc]indolo[1,2,3-lm]carbazole 3, which can be considered as a planar hybrid of carbazole and p-phenylenediamine, can readily be prepared from commercial precursors and possesses high thermal stability and strong electron donor properties comparable to tetra- and pentacene.

Cross-linked ROMP polymers based on odourless dicyclopentadiene derivatives

Hydroxydicyclopentadiene (DCPD-OH) and some ester and ether derivatives were synthesized and used for the first time as ring-opening metathesis polymerization (ROMP) monomers to create cross-linked thermoset polymers with Ru-catalysts. The properties of this new family of polymers were measured mainly by means of TGA, DSC and DMA to compare them with those of polydicylopentadiene (pDCPD). The novel polymers showed thermal properties comparable to those of pDCPD; although glass transition temperatures were mainly reduced by the modifications. More importantly, tuneable physical properties, depending on the substituents attached, were obtained. Thus, a window towards many important applications is opened by making novel thermoset ROMP homopolymers and copolymers from monomers with significantly reduced odour.

Two-photon absorption properties of (N-carbazolyl)-stilbenes

We have studied the two-photon absorption (TPA) and fluorescence properties of a series of phenylpolyenes with N-carbazolyl as donor end-groups in symmetric positions. The symmetric substitution of carbazole end-groups enhances the photostability of the molecules and maintains high TP coefficients. High fluorescence quantum efficiencies have been measured. The results suggest that TP chromophores with carbazole donors are promising materials for applications in two-photon imagin and sensitization. The study covers a broad excitation spectrum from 580nm to 820nm using a picosecond optical parametric source. We take advantage of the correlation between idler and signal waves emerging from the parametric generator and calibrate our results against Rhodamine B for which reliable data is available in the long wave range.