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Antonio Facchetti

Publications and source records attributed to Antonio Facchetti.

35 records · Page 2Linked to original sources
PubMed2004

Self-assembled electrooptic thin films with remarkably blue-shifted optical absorption based on an X-shaped chromophore.

An "X-shaped" two-dimensional electrooptic (EO) chromophore with extended orthogonal conjugation was designed and synthesized. Self-assembled thin films of this chromophore were fabricated via layer-by-layer chemisorptive siloxane-based self-assembly. The films exhibit a dramatically blue-shifted optical maximum (325 nm) while maintaining a large EO response (chi33(2) approximately 232 pm/V at 1064 nm; r33 approximately 43 pm/V at 1550 nm).

Journal Article↗
PubMed2004

Very large electro-optic responses in H-bonded heteroaromatic films grown by physical vapour deposition.

A crucial goal for modern telecommunications systems is development of very high-speed components for broadband (>100 GHz), all-optical signal/information processing. The core component of such technologies is the electro-optic modulator, which encodes electrical signals onto fibre-optic transmissions. A significant challenge therefore is obtaining materials that have large electro-optic responses and that can be readily fabricated into devices at low cost. We report here on the realization of high-response heteroaromatic organic chromophores that can be straightforwardly self-organized from the vapour phase into intrinsically acentric, high-quality, micrometre-scale films. These pi-conjugated electro-optically active films (with second-order susceptibilities up to approximately 100 pm V(-1)) are thermally stable and conveniently grown by a simple physical vapour deposition process in a few hours. Supramolecular acentricity is achieved without electric field poling, enforced by biomimetic heterocycle-hydroxycarbonyl head-to-tail hydrogen-bonding.

Crystallization↗
PubMed2004

Ion pair first and second acidities of some beta-diketones and aggregation of their lithium and cesium enediolates in THF.

Ion pair pK values were measured for three beta-diketones in THF, 1-3, with lithium and cesium counterions. The results showed variations with concentration indicative of aggregation of the metal enolates to dimers. Similarly, ion pair pK values could be determined for some of these metal enolates going to the corresponding dimetal dienediolates which were also found to form dimers. These equilibria are more complicated to analyze because aggregation affects both sides of the proton transfer equilibria. The results show that all of the species measured exist mostly as dimers at concentrations >0.01 M typical of most organic synthesis reactions and physical measurements. NMR measurements show that the enols of 1 and 2, which can undergo intramolecular hydrogen bonding, predominate in both THF and DMSO solutions, whereas 3, whose enols cannot be so stabilized, is mostly keto in THF but approximately equimolar enol and keto in DMSO. Dimerization of the monolithium salts is rapid on the NMR time scale but that of the dilithium salts is slow.

Anions↗
PubMed2004

Building blocks for n-type molecular and polymeric electronics. Perfluoroalkyl- versus alkyl-functionalized oligothiophenes (nT; n = 2-6). Systematics of thin film microstructure, semiconductor performance, and modeling of majority charge injection in field-effect transistors.

The solid-state properties and FET electrical behavior of several series of alpha,omega- and beta,beta'-fluorocarbon- and alkyl-substituted and unsubstituted oligothiophenes nTs (n = 2-6) are compared and contrasted. The thin films were grown by slow vacuum deposition over a range of substrate temperatures and/or by casting from solution and were investigated by X-ray diffraction and scanning electron microscopy. Our results indicate that vacuum deposition at 60-80 degrees C affords films with remarkably similar microstructures despite the extensive H --> F substitution. Trends in observed d spacing versus molecular core extension provide quantitative information on molecular orientation. Field-effect transistor measurements performed for all systems and having the same device structure, components, and fabrication conditions demonstrate that all nTs functionalized with fluorocarbon chains at the thiophene termini are n-type semiconductors, in contrast to the p-type activity of the remaining systems. One of these systems, alpha,omega-diperfluorohexyl-4T, exhibits a mobility of 0.22 cm2/(V s) and an Ion:Ioff ratio of 10(6), one of the highest so far reported for an n-type organic semiconductor. The effect of substitution regiochemistry on FET majority charge carrier was additionally studied, in the case of a 6T core, by shifting the fluorocarbon substituents from the terminal to the central thiophene units. Finally, we propose a simple theoretical model for electrode/organic interfacial carrier injection. The results suggest why modest substituent-induced changes in the injection barrier can produce working n-type materials.

Journal Article↗
PubMed2004

Building blocks for N-type molecular and polymeric electronics. Perfluoroalkyl- versus alkyl-functionalized oligothiophenes (nTs; n = 2-6). Systematic synthesis, spectroscopy, electrochemistry, and solid-state organization.

The synthesis, comparative physicochemical properties, and solid-state structures of five oligothiophene (nT) series differing in substituent nature and attachment, regiochemistry, and oligothiophene core length (n) are described. These five series include the following 25 compounds: (i) alpha,omega-diperfluorohexyl-nTs 1 (DFH-nTs, n = 2-6), (ii) beta,beta'-diperfluorohexyl-nTs 2 (isoDFH-nTs, n = 2-6), (iii) alpha,omega-dihexyl-nTs 3 (DH-nTs, n = 2-6), (iv) beta,beta'-dihexyl-nTs 4 (isoDH-nTs, n = 2-6), and (v) unsubstituted oligothiophenes 5 (alphanTs, n = 2-6). All new compounds were characterized by elemental analysis, mass spectrometry, and multinuclear NMR spectroscopy. To probe and address quantitatively how the chemistry and regiochemistry of conjugated core substitution affects molecular and solid-state properties, the entire 1-5 series was investigated by differential scanning calorimetry, thermogravimetric analysis, and optical absorption and emission spectroscopies. Single-crystal X-ray diffraction data for several fluorocarbon-substituted oligomers are also presented and compared. The combined analysis of these data indicates that fluorocarbon-substituted nT molecules strongly interact in the condensed state, with unit cell level phase separation between the aromatic core and fluorocarbon chains. Surprisingly, despite these strong intermolecular interactions, high solid-state fluorescence efficiencies are exhibited by the fluorinated derivatives. Insight into the solution molecular geometries and conformational behavior are obtained from analysis of optical and variable-temperature NMR spectra. Finally, cyclic voltammetry data offer a reliable picture of frontier MO energies, which, in combination with DFT computations, provide key information on relationships between oligothiophene substituent effects and electronic response properties.

Journal Article↗
PubMed2003

Vapor phase self-assembly of electrooptic thin films via triple hydrogen bonds.

The donor-acceptor pi-electron chromophore 5-{4-[2-(4,6-diamino-[1,3,5]triazin-2-yl)-vinyl]-benzylidene}-pyrimidine-2,4,6-trione (DTPT) was designed and synthesized. Triple H-bonding interactions between neighboring molecules direct self-assembled chromophore alignment in a head-to-tail orientation using a straightforward vapor phase deposition process. Angle-dependent SHG interference patterns and the quadratic dependence of the 532 nm light output intensity on the thickness of the DTPT films for glass substrates coated on both sides demonstrate high, reproducible film quality and uniformity. XRD also demonstrates long-range order in the film and yields a molecular tilt angle in good agreement with polarized SHG data, clearly showing that out-of-plane ordering of chromophore molecules has been achieved.

Journal Article↗
PubMed2003

Novel heteroaromatic-based multi-branched dyes with enhanced two-photon absorption activity.

The first examples of heterocycle-based multi-branched dyes with efficient two-photon absorption (TPA) activity are reported; the novel chromophores exhibit large TPA cross sections (as high as 1600 x 10(-50) cm4 s photon(-1) molecule(-1), measured with 150 fs laser pulses at 800 nm); a strong cooperative enhancement in the branched systems with respect to the one-dimensional sub-units is found.

Journal Article↗
PubMed2003

Easily processable phenylene-thiophene-based organic field-effect transistors and solution-fabricated nonvolatile transistor memory elements.

The synthesis of a new series of mixed phenylene-thiophene oligomers is reported; 2,5-bis(4-n-hexylphenyl)thiophene (dH-PTP, 1), 5,5'-bis(4-n-hexylphenyl)-2,2'-bithiophene (dH-PTTP, 2), 5,5' '-bis(4-n-hexylphenyl)-2,2':5',2' '-terthiophene (dH-PT(3)P, 3), 5,5' "-bis(4-n-hexylphenyl)-2,2':5',2' ':5' ',2' "-quaterthiophene (dH-PT(4)P, 4), 1,4-bis[5-(4-n-hexylphenyl)-2-thienyl]benzene (dH-PTPTP, 5), and 2,5-bis[4(4'-n-hexylphenyl)phenyl]thiophene (dH-PPTPP, 6) were characterized by (1)H NMR, elemental analysis, UV-visible spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Vacuum-evaporated and solution-cast films were characterized by X-ray diffraction and scanning electron microscopy. All compounds display high p-type carrier mobilities as evaporated (up to 0.09 cm(2)/Vs) and as solution-cast (up to 0.03 cm(2)/Vs) films on both Si/SiO(2) and ITO/GR (glass resin) substrates. The straightforwardly synthesized dH-PTTP (2) displays an unprecedented combination of mobility, on/off ratio, stability, and processability. Both dH-PTTP (2) and dH-PPTPP (6) display a reversible, tunable, and stable memory effect even as solution-cast devices, with turn-on characteristics shifting from accumulation mode to zero or depletion mode after a writing voltage V(w) is applied. The charge storage is distributed over the gate dielectric structure and is concentrated near the dielectric-semiconductor interface, as evidenced by the response of "floating gate" configuration devices. Simple nonvolatile elements have been fabricated by solution-only techniques on ITO substrates using spin-coated glass resin, solution-cast oligomeric semiconductors, and painted graphite paste electrodes.

Journal Article↗
PubMed2003

A distinctive example of the cooperative interplay of structure and environment in tuning of intramolecular charge transfer in second-order nonlinear optical chromophores.

The strongly enhanced cooperative influence of medium polarity and organic structural design on the first hyperpolarizability beta of a novel family of highly polarizable azinium-(CH=CH-thienyl)-dicyanomethanido chromophores 1-3 is described. The dyes can be efficiently synthesized by regioselective protonation/alkylation of the corresponding bidentate anion precursors. Consecutive annelation of the pyridyl ring of 1 (pyridine-->quinoline-->acridine) and medium polarity effects are responsible for an extraordinarily variable range of intramolecular charge transfer (ICT), leading to a large set of pi-electron distribution patterns. Accordingly, systems with remarkably different zwitterionic/quinoid character in the ground and excited states present beta values in a broad range, eventually switching from negative to positive. Our investigation is based on a combination of experimental (UV/Vis spectroscopy, multinuclear NMR spectroscopy, and electrooptical absorption measurements) and computational (ab initio) approaches. It is shown that: 1) beta and mubeta are dramatically influenced, even by orders of magnitude, by a complex, non-monotonic interplay of structure and medium action, which in turn affects molecular ICT and bond length alternation (BLA), 2) the computations, validated by different experimental data, are to be recommended as an extremely useful tool in the search for a greatly improved set of molecular nonlinear optical (NLO) responses (in the case of 1-3 they show that such conditions may be attained only in a narrow and limited range of dielectric constants in which the annelation effect operates most efficiently), and 3) the search for the most favorable molecular NLO response of a highly polarizable chromophore both in solution and in solid matrices should simultaneously take into account not only the molecular design supplemented by annelation effects but also the polarity of the medium.

Journal Article↗
PubMed2002

Metal chelation aptitudes of bis(o-azaheteroaryl)methanes as tuned by heterocycle charge demands.

We describe the synthesis of a number of 1,3-azol-2-yl-, 1,3-benzazol-2-yl-, and azinyl-based bis(o-azaheteroaryl)methanes (LH, L(-) = Het(2)CH(-)) and their coordinating properties toward divalent transition metals (Zn, Cu, Co, Ni, Hg, Pd). This extended investigation includes both symmetrical and unsymmetrical ligands based on several substituted and/or unsubstituted thiazole, benzothiazole, benzoxazole, benzimidazole, pyridine, and quinoline derivatives. Depending on the structure and electron properties of the ligand, a vast set of neutral chelates ML(2) were obtained, where the ligand is present in its carbanionic form L(-). Additionally, we have prepared salt complexes [M(LH)(n)]X(m), where the ligand is present as a neutral system. Neutral chelates were typically obtained by the reaction of the ligand with metal acetates in alcoholic solution; salt complexes were formed by reaction with other metal salts such as chlorides. By exploring the coordinating properties of several bisheteroarylmethane ligands based on heteroaromatics of variable pi-electron structure and substitution pattern, we demonstrate that the formation of neutral chelates is strictly dependent on the electron-withdrawing capacity (charge demand) of the heteroaromatic moiety. The latter primarily dictates the efficiency by which the negative charge of the anionic ligand L(-) is stabilized by delocalization in ML(2) and, therefore, the stability of the chelate itself. On the basis of the large number and the variable nature of the nitrogen ligands used, we confirm the general validity of the charge-demand-dependent formation of chelates. This key factor can therefore be used for the efficient design of new pi-deficient heteroaromatic nitrogen ligands in chelates of great potential in many synthetic, catalytic, and technological fields.

Benzimidazoles↗
PubMed2002

Novel heterocycle-based two-photon absorbing dyes.

[structure: see text]. The synthesis and nonlinear optical characterization of two novel heteroaromatic-based chromophores is described. The new dyes present an A-pi-D-pi-A general framework, where A is a pi-deficient heteroaromatic ring (pyridine, quinoline, benzothiazole) and D a pi-excessive pyrrolyl moiety. Both systems exhibit large two-photon absorption (TPA) values in the femtoseconds regime (TPA cross section as high as 150 x 10(-50) cm(4) s photon(-1) molecule(-1) with 150 fs laser pulses). Their TPA-based optical limiting activity is also shown.

Journal Article↗
PubMed1999

Diheteroarylmethanes. 8.(1) Mapping Charge and Electron-Withdrawing Power of the 1,2,4-Triazol-5-yl Substituent.

Three new triazolyl derivatives, bis(1H-1-phenyl-1,2,4-triazol-5-yl)methane (11), 1H-1-phenyl-5-(beta-styryl)-1,2,4-triazole (12), and 1H-5-benzyl-1-phenyl-1,2,4-triazole (13) have been synthesized and the carbanions 11(-)() and 13(-)() investigated in DMSO by multinuclear NMR spectroscopy. By applying previously proposed and widely used pi-charge/(13)C shift relationships on the spectra of the anions, it is possible to rank the pi electron-withdrawing power of the 1,2,4-triazol-5-yl group in terms of charge demand c(X), a quantity that represents the fraction of pi negative charge delocalized by the heterocyclic ring. Our results indicate that the charge demand c(X) of this heterocycle is considerably greater than that of other 1,3-azoles (2-imidazolyl, 2-oxazolyl, 2-benzoimidazolyl), being close to that of some mono- and diazinyl substituents. A single set of resonances is presented by both carbanions 11(-)() and 13(-)(), thus showing that they exist either as a single geometric isomer species or as a mixture of isomers in a rapid (on the NMR time scale) equilibrium. (13)C and (15)N shift/pi-charge relationships allow accurate pi-charge mapping of carbanionic systems. Our results clearly show that, in the case of benzyl carbanion 13(-)(), all of the three nitrogen atoms are almost equally involved in delocalizing the negative charge. Also, the N-phenyl group contributes to charge delocalization. Anion 11(-)()( )()is the first of the bis(heteroaryl)methyl carbanions that we have studied, in which all of the negative charge originated by deprotonation of the carbon acid 11 is hosted on the nitrogen atoms without any appreciable involvement of the heteroaromatic carbon frame.

Journal Article↗
PubMed1998

8-Purinyl versus 2-Benzimidazolyl Carbanions: Charge Demands of the Heterocycles and Ligand Properties of the Bis(heteroaryl)methanes(1).

Two new purinyl derivatives, 8-benzyl-7-methylpurine (8) and bis(7-methylpurin-8-yl)methane (9), have been synthesized and their corresponding carbanions investigated in DMSO. The application of our previously proposed pi-charge/shift relationships to the (13)C and (15)N shifts of the carbanions has made it possible to map pi-charges and obtain the values of the charge demand of the heterocyclic rings, a resonance index of the pi-electron-withdrawing power of the substituent. The charge demand of the 7-methylpurin-8-yl substituent is found to be the highest among the previously investigated azinyl and azolyl substituents and is comparable with that of the strongest classical electron-withdrawing functions. The replacement of the fused benzene ring in the benzimidazolyl substituent by a pyrimidine ring containing the nitrogen atoms in appropriate positions causes a considerable increase in the electron-withdrawing capacity of the heterocycle. Spectroscopic and reactivity data confirm the strong electron-withdrawing nature of the purinyl ring. The (13)C NMR spectrum shows the existence of the carbanions of both purinyl derivatives as a mixture of geometric isomers, a consequence of the high double-bond character along the bond linking the carbanionic carbon to C(8) of the purinyl ring. Bis(7-methylpurin-8-yl)methane can be easily converted to its stable NH tautomer by means of basic catalysis. It behaves as an "active methylene" compound giving high-yield condensations with electrophiles. Finally, unlike the corresponding benzimidazol-2-yl derivative, bis(7-methylpurin-8-yl)methane reacts with metal acetates to give neutral methanates [ML(2)] (M = Zn, Cu, Co; LH = bis(7-methylpurin-8-yl)methane), where the ligand is present as an anionic system. On the basis of this and previous data, it is concluded that charge demand plays a strategic role in obtaining stable chelates of this sort.

Journal Article↗