Π-extended Systems Research Articles

Pyrimidine Ring as Building Block for the Synthesis of Functionalized ?-Conjugated Materials

In the past two decades, π-conjugated organic compounds have attracted much attention due to their potential applications in various fields such as liquid crystals, light-emitting, and nonlinear optical materials and self assembling. Introduction of heteroaryl moieties into π-extended systems are particularly useful where electron transfer is necessary and is a way to modify and sometimes enhance properties of advanced “electro-optic”. In π-conjugated materials, azaheterocycles such as pyrimidine, which owing to its aromaticity, its strong electron-withdrawing character, its high dipolar interaction and its pH sensibility, presents the requested characteristics to be incorporated in organic materials in view of numerous and various applications. Such molecular architectures and their applications are reported in this review. Keywords: Pyrimidine, π-conjugated-materials, liquid crystals, non-linear optic, luminescence, charge transport, supramolecular assemblies

Synthesis and physico-chemical properties of highly conjugated azo-aromatic systems. 4-(azulen-1-yl)-pyridines with mono- and bis azo-aromatic moieties at C3-position of azulene

Highly conjugated azo-aromatic systems have been prepared in high to moderate yields by linking mono- and bis-azo aromatic fragments to 4-(Rn-azulen-1-yl)-2,6-dimethyl-pyridine. The synthesized π-extended systems have been studied by NMR spectroscopy, UV–Vis and electrochemistry. Systematic increase of the conjugation along the azobenzene skeleton has affected the spectral properties of the azophenyl substituted 4-(azulen-1-yl)-pyridine. The synthesized compounds exhibit a bathochromic shift of the visible absorption maxima with the increase of the conjugating skeleton and introduction of an electron-withdrawing group. The electrochemical behavior revealed a high stability toward oxidation owing to the higher polarization induced by the azulenylpyridine moiety.

New Highly Stable Metallabenzenes via Nucleophilic Aromatic Substitution Reaction

Treatment of the ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)]Cl (1) with excess 8-hydroxyquinoline in the presence of CH(3)COONa under air atmosphere produced the S(N)Ar product [(C(9) H(6)NO)Ru{CHC(PPh(3))CHC(PPh(3))C}(C(9)H(6)NO)(PPh(3))]Cl(2) (3). Ruthenabenzene 3 could be stable in the solution of weak alkali or weak acid. However, reaction of 3 with NaOH afforded a 7:1 mixture of ruthenabenzenes [(C(9)H(6)NO)Ru{CHC(PPh(3))CHCHC}(C(9)H(6)NO)(PPh(3))]Cl (4) and [(C(9)H(6)NO)Ru{CHCHCHC(PPh(3))C}(C(9)H(6)NO)(PPh(3))]Cl (5), presumably involving a P-C bond cleavage of the metallacycle. Complex 3 was also reactive to HCl, which results in a transformation of 3 to ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))C}Cl(2)(C(9)H(6)NO)(PPh(3))]Cl (6) in high yield. Thermal stability tests showed that ruthenabenzenes 4, 5, and 6 have remarkable thermal stability both in solid state and in solution under air atmosphere. Ruthenabenzenes 4 and 5 were found to be fluorescent in common solvents and have spectral behaviors comparable to those organic multicyclic compounds containing large π-extended systems.

ChemInform Abstract: Synthesis and Nitrosation Reactions of π-Extended 1,3-Dithiol-2-ylidene Systems.

New 1,3-dithiol-2-ylidene derivatives, notably π-extended systems, have been synthesised by Wittig reactions of phosphorane and phosphonate ester derivatives of 1,3-dithiole with activated ketones and α,β-unsaturated ketones. Nitrosation reactions of a range of these π-extended systems, results in the formation of nitroalkenes, via unstable nitrosoalkene intermediates, which, in general, could not be isolated. The X-ray crystal structure of 1-(4,5-dicarbomethoxy-1,3-dithiol-2-ylidene)-1-cyano-1-phenyl-methane reveals a small degree of intramolecular electron transfer from the dithiole ring to the conjugated cyano group.

New strategies and building blocks for functionalised 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene derivatives, including pyrrolo-annelated derivatives and pi-extended systems with intramolecular charge-transfer.

A range of new functionalised 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (TTFAQ) derivatives have been synthesised from the key di(halomethyl) building blocks, 10-[4,5-bis(bromomethyl)-1,3-dithiol-2-ylidene]-anthracene-9(10H)-one 10, 10-[4,5-bis(chloromethyl)-1,3-dithiol-2-ylidene]anthracene-9(10H)-one 11 and 9-[4,5-bis(chloromethyl)-1,3-dithiol-2-ylidene]-10-[4,5-bis(hexylsulfanyl)- 1,3-dithiol-2-ylidene]-9,10-dihydroanthracene 18. A Diels-Alder strategy comprising trapping of the transient exocyclic diene 19, which is derived from 18, with 1,4-naphthoquinone leads to the aromatised TTFAQ anthraquinone system 21. Horner-Wadsworth-Emmons olefination of 21 with the anion generated from reagent 22 gave the fused bis(TTFAQ) structure 23. Pyrrolo-annelated derivatives 30-34 have been obtained in a sequence of reactions from compound 10. Mono-formylation of the pyrrole ring of 32 and 33 under Vilsmeier conditions gave 35 and 36 which upon reaction with 2,4,5,7-tetranitrofluorene gave the donor-pi-acceptor diads 38 and 39. Cyclic voltammetry (CV) in solution for all the TTFAQ derivatives shows the typical quasi-reversible two-electron oxidation wave of the TTFAQ core at potentials which vary slightly depending on the substituents. For example, the value of Eox is raised by the electron withdrawing anthraquinone and tetranitrofluorene units of 21 and 38, respectively. The CV of the conjugated TTFAQ dimer 23 showed two, two-electron oxidation waves corresponding to the sequential formation of 23(2+) and 23(4+) (delta Eox = 130 mV) providing evidence for a significant intramolecular electronic interaction, i.e. the dication 23(2+) acts as a conjugated donor-pi-acceptor diad, thereby raising the oxidation potential of its partner TTFAQ unit. Spectroelectrochemical studies on 23 support this explanation. A strong intramolecular charge transfer band at lambda max 538 nm is seen in the UV-Vis spectra of the TTFAQ-pi-tetranitrofluorene diads 38 and 39. The X-ray crystal structures are reported for compounds 30, 33 and 34. The pyrrolo-TTFAQ moiety adopts a saddle-shape with the central ring of the dihydroanthracene moiety folded along the C(9) ... C(10) vector in each case. Significant intermolecular interactions are observed in the structures.

Synthesis and nitrosation reactions of π-extended 1,3-dithiol-2-ylidene systems

New 1,3-dithiol-2-ylidene derivatives, notably π-extended systems, have been synthesised by Wittig reactions of phosphorane and phosphonate ester derivatives of 1,3-dithiole with activated ketones and α,β-unsaturated ketones. Nitrosation reactions of a range of these π-extended systems, results in the formation of nitroalkenes, via unstable nitrosoalkene intermediates, which, in general, could not be isolated. The X-ray crystal structure of 1-(4,5-dicarbomethoxy-1,3-dithiol-2-ylidene)-1-cyano-1-phenyl-methane reveals a small degree of intramolecular electron transfer from the dithiole ring to the conjugated cyano group.

Synthesis, electrochemical properties and effect of substituents on π-extended TCNQ and DCNQI systems

The synthesis of novel π-extended TCNQ and DCNQI derivatives from the corresponding 1,4-anthraquinones is described. The cyclic voltammetric data reveal that both TCNQ-type and DCNQI-type acceptors present two one-electron reduction waves to the corresponding anion-radical and dianion. These novel 2,3-fused TCNQ and DCNQI π-extended systems are, according to the electrochemical values, better acceptors than the previously reported 2,3–5,6-fused TCNQ and DCNQI molecules. A good linear correlation between the first reduction potential of the substituted DCNQI-type derivatives versus the Hammett's σ3 constant has been found.

Synthese, electrochemical properties and crystal structure of tetracyano- p-quinodimethane (TCNQ) derivatives with π-extended systems containing a sulfur atom

Two novel TCNQ derivatives ( 7 and 8) with π-extended conjugation containing a sulfur atom have been synthesized from the corresponding quinones by reaction with Lehnert's reagent. The X-ray structural study of 7 shows a highly distorted non-planar molecule and cyclic voltammetric studies reveal a two-electron single-wave reduction to the dianion.