Francesco Naso

Fluorinated organic materials for electronic and optoelectronic applications: the role of the fluorine atom

In this article we highlight, by means of selected examples drawn from work performed in our or other laboratories, the features of some classes of fluorinated conjugated materials and their use in electronic devices such as electroluminescent diodes or field effect transistors. A variety of fluorinated conjugated systems, either molecular or polymeric, such as poly(phenylenevinylene)s, poly(phenyleneethynylene)s, polythiophenes, polyphenylenes, are dealt with. Attention is also focused on a different class of electroluminescent compounds, represented by the cyclometalated iridium complexes with various forms (mer and fac). In particular, fluorine atoms lower both the HOMO and LUMO energy levels. Consequently, the electron injection is made easier, the materials display a greater resistance against the degradative oxidation processes and organic n-type or ambipolar semiconducting materials may result. Moreover, the C-H...F interactions play an important role in the solid state supramolecular organization, originating a typical pi-stack arrangement which enhances the charge carrier mobility.

A sensitivity-enhanced field-effect chiral sensor

Organic thin-film transistor sensors have been recently attracting the attention of the plastic electronics community for their potential exploitation in novel sensing platforms. Specificity and sensitivity are however still open issues: in this respect chiral discrimination-being a scientific and technological achievement in itself--is indeed one of the most challenging sensor bench-tests. So far, conducting-polymer solid-state chiral detection has been carried out at part-per-thousand concentration levels. Here, a novel chiral bilayer organic thin-film transistor gas sensor--comprising an outermost layer with built-in enantioselective properties-is demonstrated to show field-effect amplified sensitivity that enables differential detection of optical isomers in the tens-of-parts-per-million concentration range. The ad-hoc-designed organic semiconductor endowed with chiral side groups, the bilayer structure and the thin-film transistor transducer provide a significant step forward in the development of a high-performance and versatile sensing platform compatible with flexible organic electronic technologies.

Blue emitting iridium complexes : synthesis, photophysics and phosphorescent devices

Homoleptic Ir(Fnppy)3 and heteroleptic (Fnppy)2Ir(acac) complexes (n = 3: F3ppy = 2-(3′,4′,6′-trifluorophenyl)pyridine; n = 4: F4ppy = 2-(3′,4′,5′,6′-tetrafluorophenyl)pyridine; acac = acetylacetonate) have been synthesized and their spectroscopic properties investigated. The homoleptic complexes exist as two stereoisomers, facial (fac) and meridional (mer), that have been isolated and fully characterized. Their electrochemical and photophysical properties have been studied both in solution and in the solid state and electroluminescent devices have been fabricated. The emissive layers in devices have been obtained mixing the iridium complexes with a PVK [poly(9-vinylcarbazole)] host matrix, in the presence of the electron carrier Bu-PBD [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole]. The application of a voltage (5.0–6.5 V) between the electrodes of devices leads to electro-generated blue luminescence which has similar energy to the solution emissions. Interestingly, the stability of the devices made with the homoleptic fluorinated iridium complexes strongly depends on the stereochemistry of these phosphors and high (up to 5.5%) external quantum efficiencies for the fac complexes are measured.

Substituent-dependence of the optical nonlinearities in poly(2,5-dialkoxy-p-phenylenevinylene) polymers investigated by the Z-scan technique

Abstract We investigated the relationship existing between the third-order nonlinear optical (NLO) properties and the chemical structure of some poly(2,5-dialkoxy-1,4-phenylenevinylene) polymers. The latter have similar degrees of polymerization and polydispersity indexes and differ only for the structure of the side groups in the 2,5 positions of the phenylene rings. The two-photon absorption coefficient and the nonlinear refractive index have been measured using the picosecond Z -scan technique. Experimental results show that all the investigated organic molecules exhibit a small Im χ (3) and a large, negative Re χ (3) at λ =1064 nm. A considerable dependence of the third-order NLO properties on the steric effect of the substituents has been observed.

Synthesis of conjugated oligomers and polymers: the organometallic way

Results of special interest in the field of organometallic chemistry directed towards the synthesis of conjugated oligomers and polymers are reviewed. Methodologies widely employed in the synthesis of well-defined molecules and based on the transition metal catalyzed coupling reactions of various organometallic (B, Sn, Si, Mg and Zn) reagents have been more recently extended to build up poly- or oligo-meric conjugated molecular architectures. Although the focus of the present work is on transition metal cross-coupling or homo-coupling processes, other useful organometallic routes, such as metathesis reactions and acetylene polymerization, have also been dealt with. Owing to their higher versatility with respect to the non-organometallic processes commonly used for the synthesis of these materials, the approaches presented allow the realization of more complex molecular structures, which are nowadays required in many electronic and optoelectronic applications. The use of organometallic strategies has been discussed from a synthetic organic chemists point of view, and advantages and limitations of the different methodologies reviewed have been highlighted. Some attention has also been paid to the properties of the resulting materials and to their dependence on the procedure followed and the framework obtained.

Use of readily available chiral compounds related to the betti base in the enantioselective addition of diethylzinc to aryl aldehydes

Abstract Readily available members of the family of chiral non-racemic aminonaphthols related to the Betti base 1 were tested as complexing agents in the catalytic enantioselective addition of diethylzinc to aryl aldehydes. The use of these bases gave high ee values (up to > 99%). The highest ee values were obtained with the tertiary aminonaphthol 2. An important role was played by the solvent. The effect of the nature and the position of the substituents on the aromatic ring of the aldehyde was also investigated.

THE BETTI BASE : ABSOLUTE CONFIGURATION AND ROUTES TO A FAMILY OF RELATED CHIRAL NONRACEMIC BASES

Abstract A detailed protocol for the resolution of the Betti base [i.e. 1-(α-aminobenzyl)-2-naphthol] was investigated and the absolute configuration of the two isomers was established by means of an X-ray diffractometric study. A series of optically active derivatives was also prepared. The list includes the N-benzyl- and the N,N,O-trimethyl derivatives. The N,N-dimethyl derivative was prepared in racemic form by means of the Betti reaction and was resolved into the two enantiomers with an extremely easy and efficient procedure. The O-methyl derivative was prepared in a racemic form and subjected to resolution. The configuration of each base was determined by correlation with the configuration of the Betti amine.

Stereospecific synthesis of olefins through sequential cross-coupling reactions

A conceptually straightforward and useful procedure leading to (I)- or (g)-olefins is presented. The method is based upon the r eaction of the readily available (I)- or (E)-l-bromo-2-phenylthioethene (1 or 2) with Grignard reagents in the presence of Ni(II1 or Pd(I1) catalysts. Other ca- talysts, 5.2. iron(II1) derivatives, and other organometallics, 2.2. zinc reagents or copper derivatives, can also be successfully used during the substitution of the halogen. The stereospecificit y iss99% in the case of (El-isomers and slightly lower in the case of (I)-isomers. Enantioselecti- vTty is achieved by using chiral ligands. By appropriate choice of the reagent, alkyl- or a ryl-substituted ethenes, 1,3- or 1 ,6-dienes1 and con- jugated enynes can be prepared. Ally1 and alkadienyl silanes can also be obtained. The reaction of the latter compounds with palladium chloride leads to conjugated tetraenes. Oienyl bromides or ketones are prepared by reaction of the same compounds with aluminum chloride and cyanogen bromide or acyl chloride respectively. The versatility of this method has been demonstrated by the stereospecific synthesis of insect pheromones and a number of key synthetic intermediates. logeno-2-phenylethenes and 1 -halogeno-2-phenylsulphonylethenes. The two systems showed a different behaviour. A stereochemical pathway of retention of configuration was observed for the styrene system, whereas a variable stereochemical course depending upon the configuration of the substrate, the nature of the leaving group, and the type of cuprate was valid for the sulphones. Furthermore, a marked leaving group effect (Br>Cl>F) was observed for the first system whereas the halogeno-sulphones were found to be much less sensitive to the change of the leaving group (Br-Cl-F). These and other results were explained in terms of two main mechanistic pathways with blurred boundaries: a concerted substitution of the halogen should occur in the styrene system whereas the halogeno-sulphones should follow the more common addition-elimina tion mechanism (ref.

High third-order nonlinear optical susceptibility in new fluorinated poly(p-phenylenevinylene) copolymers measured with the Z-scan technique

The third-order nonlinear optical properties of a series of copoly(2, 3, 5, 6-tetrafluoro-1, 4-phenylenevinylene-2, 5-dioctyloxy-1, 4-phenylenevinylene) that contain variable ratios of two differently substituted monomers have been studied in chloroform solutions at l=1064 nm by the picosecond Z-scan technique. Nonlinear refractive index n(2) of the samples investigated has been found to be negative, and a strong dependence of its magnitude on the copolymers composition has been observed. The highest third-order nonlinear optical susceptibility, |x((3))|=(6 +/- 2)x 10(-10) esu, was measured for a copolymer obtained by reaction of equimolar quantitites of the parent monomers.

A straightforward route to polyenylsilanes by palladium- or nickel-catalyzed cross-coupling reactions

Abstract An efficient conversion of bis-silylated dienes and trienes into polyenylsilanes by cross-coupling reactions is described. The aryl substituted polyenes are synthesized by ipso-borodesilylation with BCl3 followed by Pd-catalyzed coupling reactions of the resulting boron derivative with aryl halides, whereas the synthesis of alkyl substituted polyenes requires the conversion of the boron intermediate into iodo-derivative and the subsequent coupling with alkyl Grignard reagents in the presence of a Ni(II) catalyst.