-
[show abstract]
[hide abstract]
ABSTRACT: The study of charge transport processes through organic molecules by using molecular junctions has generated great attention in the last few years, partially triggered by the possibility of developing molecular electronic devices to be implemented somehow into current silicon-based technology. As experimental tools, a large variety of conceptually and geometrically different metal-molecule(s)-metal junctions has been proposed. While the intrinsic conductivity of a molecule is still elusive, parameters crucial for molecular electronics have been extracted by using a variety of junctions. Significantly, the results extracted from molecular junctions and those obtained by the kinetic approach in supramolecular D-B-A systems are complementary. For the sake of a practical discussion, a distinction is made between "active junctions" and "non-active junctions". Active junctions are those aimed at switching the electrical response by an external stimulus acting "in situ" to modify the electronic structure of the molecular system. Non-active junctions are those aimed at studying different conduction regimes by incorporating molecules of different electronic structures. Depending on their geometry, the junctions can incorporate different numbers of molecules. Large area molecular junctions present two main advantages: (1) a simpler assembly, by requiring less sophisticated fabrication and (2) a higher versatility, relative to single molecule junctions, towards potential applications in organic electronics. The present chapter focuses on the fabrication of a variety of large-area molecular junctions and summarizes and compares the experimental results.
Topics in current chemistry 09/2011; 313:85-119. · 4.29 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Complexation of Zn(II) ions by cyclam cored dendrimers appended with four (G0), eight (G1) and 16 naphthyl chromophores (G2) at the periphery have been investigated in CH₃CN-CH₂Cl₂ 1 : 1 (v/v) solution by absorption and emission, ESI-mass and ¹H NMR spectroscopy. The results obtained can be interpreted by the formation of complexes of 2 : 1 dendrimer to metal stoichiometry, at low metal ion concentration, and 1 : 1 complexes upon further addition of Zn(II) ions, for all the dendrimer generations. Upon addition of a molecular clip C²⁻ consisting of two anthracene sidewalls bridged by a benzene group with two sulfate substituents in the para positions, heteroleptic complexes of general formula [GnZnC] are formed. Interestingly, in these complexes, a very efficient quenching (practically 100%) of the dendrimer naphthyl luminescence and sensitization (ca. 90%) of the clip anthracene emission take place. The complex [G2ZnC] exhibits a very high molar absorption coefficient in the UV spectral region owing to the 16 naphthyl chromophores of the dendrimer and the two anthracene units of the clip (ε = 1.7 × 10⁵ M⁻¹ cm⁻¹ at 263 nm). Furthermore, the excitation energy absorbed by the naphthyl chromophores is efficiently funneled to the two anthracene units of the clip, which emits in the blue spectral region.
Dalton Transactions 02/2011; 40(6):1356-64. · 3.84 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Zn(ii) complexation drives the formation of a light-harvesting antenna constituted by two multicomponent luminescent ligands: a cyclam-cored dendrimer decorated at the periphery with 16 naphthyl units and an anthracene-based molecular clip.
Chemical Communications 05/2010; 46(20):3571-3. · 6.17 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have investigated the self-assembly of three luminescent species in CH(3)CN/CH(2)Cl(2), namely: 1) a polylysin dendrimer (D) composed of 21 aliphatic amide units and 24 green luminescent dansyl chromophores at the periphery, 2) a molecular clip (C) with two blue luminescent anthracene sidewalls and a benzene bridging unit that bears two sulfate groups in the para position, and 3) a near infrared (NIR)-emitting Nd(3+) ion. For purposes of comparison, analogous systems have also been investigated in which Gd(3+) replaced Nd(3+). The dendrimer and the clip can bind Nd(3+) ions with formation of [D.2Nd(3+)] and [C.Nd(3+)] complexes, in which energy transfer from dansyl and, respectively, anthracene to Nd(3+) ion takes place with 65 and 8% efficiency, in air-equilibrated solution. In the case of [C.Nd(3+)], the energy-transfer efficiency is quenched by dioxygen, thereby showing that the energy donor is the lowest triplet excited state of anthracene. In [D.2Nd(3+)] the intrinsic emission efficiency of Nd(3+) is much higher (ca. 5 times) than in [C.Nd(3+)] because of a better protection of the excited lanthanide ion towards nonradiative deactivation caused by interaction with solvent molecules. By mixing solutions of D, Nd(3+), and C with proper concentrations, a supramolecular structure with five components of three different species, [D.2Nd(3+).2C], is formed. The excitation light absorbed by the clips is transferred with 100% efficiency to the dansyl units of the dendrimer and then to the Nd(3+) ions with 65% efficiency either in the presence or absence of dioxygen. These results show that the [D.2Nd(3+).2C] complex is able to efficiently harvest UV light by the 24 dansyl units of the dendrimer and the four anthracene chromophores of the two clips, and efficiently transfer it to the encapsulated Nd(3+) ions that emit in the NIR spectral region.
Chemistry 05/2010; 16(20):6048-55. · 5.93 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A family of poly(propylene amine) dendrimers, decorated at their periphery with 4, 16, and 32 dansyl units and a molecular clip, composed of two anthracene sidewalls and a disulfate benzene bridging unit, show intense UV absorption and strong fluorescence in the visible region when in a CH(3)CN/CH(2)Cl(2) (1:1, v/v) solvent mixture. Both these classes of compounds are good ligands for Zn(II) ions, as demonstrated by the changes in the absorption and fluorescence spectra upon addition of metal ions. These coordinating properties have been exploited in the self-assembly of complex structures in which the interaction between a dansylated dendrimer and anthracene-functionalized clips is mediated by Zn(II) ions. The self-assembly process is reversible and the number of metal ions and molecular clips associated with each dendrimer increases with the generation number. In these adducts, an energy transfer process from the anthracene to yield the fluorescent excited state of dansyl takes place with almost unitary efficiency.
Chemistry 04/2009; 15(32):7876-82. · 5.93 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have synthesized molecular clips 1 comprising (i) two benzo[k]fluoranthene sidewalls and (ii) a dimethylene-connected benzene bridge that carries two acetoxy (1a), hydroxy (1b), or methoxy (1c) substituents in the para position. Their NMR spectra, single-crystal structures, and photophysical (fluorescence intensity, lifetime, depolarization) and electrochemical properties are discussed. For the purpose of comparison, similar compounds (2 and 3) containing only one benzo[k]fluoranthene unit have been prepared and studied. The strongly fluorescent clips 1 form stable complexes with electron-acceptor guests because of a highly negative electrostatic potential on the inner van der Waals surface of their cavity. The complexation constants in chloroform solution for a variety of guests, determined by NMR and fluorescence titration, are much larger than those of the corresponding anthracene and naphthalene clips (4 and 5), particularly in the case of extended aromatic guests. The effect of the substituents in the para position of the benzene spacer unit of clips 1 is discussed on the basis of the host-guest complex structures obtained by X-ray analysis and molecular mechanics simulations. In the case of 9-dicyanomethylene-2,4,7-trinitrofluorene (TNF) guest, complex formation with clip 1a causes dramatic changes in the photophysical and electrochemical properties: (i) a new charge-transfer band at 600 nm arises, (ii) a very efficient quenching of the strong benzo[k]fluoranthene fluorescence takes place, (iii) shifts of both the first oxidation (clip-centered) and reduction (TNF-centered) potentials are observed, and (iv) reversible disassembling of the complex can be obtained by electrochemical stimulation.
The Journal of Organic Chemistry 08/2008; 73(15):5839-51. · 4.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have synthesized a dendrimer (1) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core, appended with four benzyl substituents that carry, in the 3- and 5-positions, a dansyl amide derivative (of type 2), in which the amide hydrogen is replaced by a benzyl unit that carries an oligoethylene glycol chain in the 3- and 5-positions. All together, the dendrimer contains 16 potentially luminescent moieties (eight dansyl- and eight dimethoxybenzene-type units) and three distinct types of multivalent sites that, in principle, can be protonated or coordinated to metal ions (the cyclam nitrogen atoms, the amine moieties of the eight dansyl units, and the 16 oligoethylene glycol chains). We have studied the absorption and luminescence properties of 1, 2, and 3 in acetonitrile and the changes taking place upon titration with acid and a variety of divalent (Co2+, Ni2+, Cu2+, Zn2+), and trivalent (Nd3+, Eu3+, Gd3+) metal ions as triflate and/or nitrate salts. The results obtained show that: 1) double protonation of the cyclam ring takes place before protonation of the dansyl units; 2) the oligoethylene glycol chains do not interfere with protonation of the cyclam core and the dansyl units in the ground state, but affect the luminescence of the protonated dansyl units; 3) the first equivalent of metal ion is coordinated by the cyclam core; 4) the interaction of the resulting cyclam complex with the appended dansyl units depends on the nature of the metal ion; 5) coordination of metal ions by the dansyl units follows at high metal-ion concentrations; 6) the effect of the metal ion depends on the nature of the counterion. This example demonstrates that dendrimers may exhibit complete functionality resulting from the integration of the specific properties of their component units.
Chemistry 01/2007; 12(35):8926-34. · 5.93 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The bis- and tris-pyrylium and thiopyrylium cations 1-4 were prepared in gram scale by heterocyclization of the corresponding bis- and tris-1,5-pentanediones 6 and 8. Their reaction with CD3ONa in CD3OD was studied by 1H NMR at -40 degrees C and at +25 degrees C. At low temperature, kinetically controlled mixtures of 2H and/or 4H adducts were detected, whereas at room temperature the mixtures equilibrated to yield, in all of the cases, the more stable 2H adducts exclusively. A spectrophotometric study of the reactions with sodium methoxide in methanol was carried out at 25.0 degrees C with the aim of determining the stepwise equilibrium constants for the addition of MeO- at the alpha position of the heteroaromatic rings. The obtained equilibrium constants allowed the evaluation of the electronic effects of chalcogenopyrylium and 2H-chalcogenopyran subunits as substituents. Despite the different sensitivity to electronic effects, pyrylium and thiopyrylium rings have a similar electron-withdrawing effect with a sigma(+)p approximately 0.8 and a sigma(+)m approximately 0.5. Apart from the expected importance of the inductive effect due to the positive charge, the difference between these two values remarks the importance of the resonance contribution. In contrast both the neutral 2H-pyranyl and thiopyranyl rings have a negligible effect as substituents, independently of the position, para or meta, they occupy.
The Journal of Organic Chemistry 09/2005; 70(16):6422-8. · 4.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Laccase, a blue copper oxidase, in view of its moderate redox potential can oxidise only phenolic compounds by electron-transfer. However, in the presence of ABTS (2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) as a redox mediator, laccase reacts with the more difficult to oxidise non-phenolic substrates, such as benzyl alcohols. The role of ABTS in these mediated oxidations is investigated. Redox interaction with laccase could produce in situ two reactive intermediates from ABTS, namely ABTS++ or ABTS*+. These species have been independently generated by oxidation with Ce(iv) or Co(iii) salts, respectively, and their efficiency as monoelectronic oxidants tested in a kinetic study towards a series of non-phenolic substrates; a Marcus treatment is provided in the case of ABTS++. On these grounds, intervention of ABTS++ as a reactive intermediate in laccase-ABTS oxidations appears unlikely, because the experimental conditions under which ABTS++ is unambiguously generated, and survives long enough to serve as a diffusible mediator, are too harsh (2 M H2SO4 solution) and incompatible with the operation of the enzyme. Likewise, ABTS*+ seems an intermediate of limited importance in laccase-ABTS oxidations, because this weaker monoelectronic oxidant is unable to react directly with many of the non-phenolic substrates that laccase-ABTS can oxidise. To solve this paradox, it is alternatively suggested that degradation by-products of either ABTS++ or ABTS*+ are formed in situ by hydrolysis during the laccase-ABTS reactions, and may be responsible for the observed oxidation of non-phenolics.
Organic & Biomolecular Chemistry 08/2005; 3(14):2604-14. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A steady-state and laser flash photolysis (LFP) study of the TPPBF(4)-photosensitized oxidation of ring-methoxylated benzyl alcohols has been carried out. Direct evidence on the involvement of intermediate benzyl alcohol radical cations and benzylic cations in these reactions has been provided through LFP experiments. The reactions lead to the formation of products (benzaldehydes, dibenzyl ethers, and diphenylmethanes) whose amounts and distributions are influenced by the number and relative position of the methoxy substituents. This behavior has been rationalized in terms of the interplay between the stabilities of benzyl alcohol radical cations and benzyl cations involved in these processes. A general mechanism for the TPPBF(4)-photosensitized reactions of ring-methoxylated benzyl alcohols has been proposed, where the alpha-OH group of the parent substrate acts as the deprotonating base promoting alpha-C-H deprotonation of the benzyl alcohol radical cation (formed after electron transfer from the benzyl alcohol to TPP) to give a benzyl radical and a protonated benzyl alcohol, precursor of the benzylic cation. This hypothesis is in contrast with previous studies, where formation of the benzyl cation was suggested to occur from the neutral benzyl alcohol through the Lewis acid action of excited TPP(+) (TPP).
The Journal of Organic Chemistry 01/2005; 69(25):8874-85. · 4.45 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The reactivities of aryl and vinyl radicals, two fundamental transient intermediates, have been investigated with respect to two elementary processes: H-atom abstraction and reaction with a nucleophile (Y−, in the SRN1 reaction). The radicals of interest were generated from haloarene or haloethene precursors, either by use of the Bu3SnH/AIBN system or by photostimulated electron transfer from a nucleophile, and the partition of the intermediate radical between competing pathways was investigated. Use both of indirect methods (such as the study of the reaction products in competition experiments; use of a radical-clock probe) and of direct ones (such as the detection of the radicals by flash photolysis experiments) enabled the following rate constants to be obtained (all values in M−1·s−1 at 25 °C). For phenyl-type radicals, the rate constants for H abstraction (kH) from the solvents Me2SO (2.8·106) and CH3CN (6.7·106) and the rate constant for combination with a nucleophile (kY) such as Me3CCOCH2− ion (3.3·109) were determined. For vinyl radical Ph2C=C(·)Ph (7·), the kH values from Me2SO (1.1·105), CH3CN (1.2·105), Bu3SnH (7.5·108), and (Me3Si)3SiH (1.6·109) and the kY values with Me3CCOCH2− (3.9·107), (EtO)2PO− (2.8·106), and PhS− (1.9·107) ions were determined. Semiempirical calculations confirmed a stabilization of radical 7· by the α-Ph substituent (ca. 8 kcal/mol), and provided the BDE of the C−Y bond for the vinylic substitution products of 7·. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)
Annalen der Chemie und Pharmacie 08/2002; 2002(16):2844 - 2854. · 3.10 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We have synthesized the water-soluble molecular clip AC containing two anthracene sidewalls connected by semirigid aliphatic units to a benzene bridge which carries two sodium sulfate substituents. For comparison purposes, the analogous clip NC with naphthalene instead of anthracene sidewalls has been synthesized. In methanol solution both clips exist in their monomeric form, while in aqueous solution the anthracene clips AC self-assemble to give a highly stable dimer with log K(dim) = 5.1 (for the naphthalene clip NC, log K(dim) = 2.5). The (1)H NMR spectra of the dimers have evidenced the interaction of two intertwined clip molecules in a perpendicular arrangement. The non-associated clip AC exhibits a very strong blue fluorescence typical of the anthracene chromophoric unit. The (AC)(2) dimer has a broader absorption spectrum and a much weaker and red shifted emission band. Addition of acetylcholine leads to the disruption of the dimer (AC)(2) and the revival of the very strong fluorescence typical of the monomeric clip AC. Both clips interact with nicotinamide adenine dinucleotide (NAD(+)) in buffered aqueous solution at pH = 7.2. A (1)H NMR analysis shows that the naphthalene clip NC forms a 1 : 1 complex with NAD(+) including the active site of the cofactor inside the clip cavity. In methanol, formation of a stable 1 : 1 adduct between AC and a fluorescent first-generation dendrimer containing four dansyl groups appended to a 1,4-diaminobutane core is driven by acid addition. In such adduct, a very efficient energy transfer takes place from the excited anthracene units of the clip to the dansyl chromophores of the dendrimer. The adduct can be reversibly disassembled by addition of base or of competitive guests.
NEW JOURNAL OF CHEMISTRY. 33(2):397-407.
