Search

Poly(vinyl alcohol) nanofibrous mats containing ad hoc synthesized gold nanostructures were prepared via a single-step electrospinning procedure and investigated as a novel composite platform with several potential applications. Specifically, the effect of differently shaped and sized gold nanostructures on the resulting mat physical-chemical properties was investigated. In detail, nearly spherical nanoparticles and nanorods were first synthesized through a chemical reduction of gold precursors in water by using (hexadecyl)trimethylammonium bromide as the stabilizing agent. These nanostructures were then dispersed in poly(vinyl alcohol) aqueous solutions to prepare nanofibrous mats, which were then stabilized via a humble thermal treatment able to enhance their thermal stability and water resistance. Remarkably, the nanostructure type was proven to influence the mesh morphology, with the small spherical nanoparticles and the large nanorods leading to thinner well defined or bigger defect-rich nanofibers, respectively. Finally, the good mechanical properties shown by the prepared composite mats suggest their ease of handleability thereby opening new perspective applications.

Alginate-based hydrogels can be obtained by the simple addition of bivalent ions to aqueous polysaccharide solutions without the use of toxic chemicals; however, the hydrogels prepared by this way result inhomogeneous and irreproducible due to the extremely fast crosslinking reaction. To overcome such limitations, in the present work a novel preparation approach is proposed and optimized; the method is based on the use of agar moulds rich of bivalent ions as diffusing system to modulate the release of Ca 2+ , Sr 2+ , and Ba 2+ cations in order to guarantee a better control of the crosslinking process. A full factorial experimental design was applied to assess the effects and the mutual interactions of three selected parameters (sodium alginate concentration, crosslinking agent concentration within the agar moulds, crosslinking time) on the gelation process. The swelling and mechanical properties of the prepared hydrogels were investigated both in deionized water and in a saline environment; furthermore, the hydrogels were characterized by means of thermogravimetric analysis and field-emission scanning electron microscopy. The obtained results demonstrated that both the experimental variables and the environmental conditions have a significant influence on the structure and the performances of the resulting hydrogels. In particular, the use of high polymer concentration and crosslinking agent amount leads to strong hydrogels corresponding to a superior crosslinking degree; more in detail, Ba 2+ ions show a great affinity for alginate leading to the hydrogels with the highest mechanical properties. Moreover, the ion-exchange occurring in saline environment seems to strongly reduce the effective crosslinking degree of alginate hydrogels with a consequential loss of mechanical properties and stability. The collected data were finally elaborated through statistical analysis combined with response surface methodology to develop theoretical models successfully able to correlate the crosslinking degree to the experimental conditions, thus opening the way of designing and fabricating alginate-based hydrogels with tailored morphology and mechanical properties.

Thin films of a series of symmetrical 2,5-bis(dodecanoxy)phenyleneethynylenes, bearing a butadiyne central group and 4, 6 and 8 phenyleneethynylene moieties, were prepared by spin coating. In general, narrower and red shifted peaks are observed in both absorption and emission spectra with respect to the solutions previously reported, however, the fluorescence quantum yield was quenched. All of these features are explained with the planarization of the conjugated chain, and intra/intermolecular interactions, as supported by spectroscopic aggregation studies and by atomic STM and microscopic AFM characterization. By cathodoluminescence spectroscopy, hot charged states are found that may also affect the fluorescence and electroluminescence properties. From the series, the latter were studied for the hexamer 6DAc for which an oLED with configuration ITO/PEDOT:PSS/6DAc/Al was constructed in ambient conditions, giving a threshold voltage of 11 V and quantum yield of 0.031 ph/e%, of the same order of other phenyleneethynylenes or phenylenebutadyines devices.

Noble metal anisotropic nanostructures have achieved a growing interest in both academic and industrial domains mostly because of their shape-dependent plasmonic properties in the near-infrared region. In this paper, gold and gold-silver anisotropic nanostructures were synthesized in very high shape-yields through a wet, seed-mediated approach based on the use of nearly spherical silver nanoparticles as seeds and chitosan as stabilizing agent. Two chitosans of different origin and molecular properties were selected for the synthetic pathway, leading to the formation of variously sized and shaped end products. In detail, quite homogeneous nanoplatelets of about 25-nm size and 7-nm thickness or nearly spherical, highly porous nanocages of about 50-nm size were obtained, depending on the type of polysaccharide employed. The shape transition towards anisotropic morphologies occurred through a slow, spontaneous process, in which the chitosan nature seemed to play a key role. As expected, both nanoplatelets and nanocages exhibit shape-dependent plasmonic features and surface properties tunable for a variety of application fields. To prove this point, the nanostructures were successfully post-functionalized with poly(10,12-pentacosadiynoic acid) (PCDA), a carboxylic-endowed diacetylene able to anchor on noble metal substrates, to obtain versatile, chromic platforms suitable for sensing and spectroscopic purposes.

In this work triangular gold nanoplates (AuNTPs) are prepared by spontaneous conversion of spherical-like gold precursors induced by the shape-directing properties of the diacetylene N,N,N-trimethylpentacosa-10,12-diyn-1-ammonium bromide (PCD_ABr), employed as capping agent. In detail, thin AuNTPs with edge length below 200 nm are obtained by incubating premade isotropic gold nanoparticles protected with chitosan (Chit@AuNPs) in PCD_ABr solutions through a standard ligand-exchange reaction (LER) carried out in water at room temperature. Control of the AuNTPs size as well as the reaction shape-yield up to 65% is achieved by simply varying the reagents ratio in the incubation mixture. Because of the high anisotropy of the flat nanoparticles, the AuNTPs-containing hydrosols show intense, well-defined absorptions in the NIR spectral region, which make them excellent candidates as biosensors also for in vivo assays with biological tissues and fluids. When subjected to UV light, both triangular and nearly spherical PCD_ABr-coated gold nanoparticles exhibit fluorescent properties and Raman features typical of disordered, sp2-based carbon nanostructures. The photogeneration of graphite-like moieties within the diacetylene shell instead of expected polydiacetylene skeleton is most likely due to poor alignment of the monomer chains on the gold surfaces and seems to be favored on isotropic nanoparticles.

Spectroscopic and microscopic measurements show that 10,12-pentacosadiynoic acid is able to densely adsorb on the surface of silver nanoparticles by properly selecting the synthesis procedure. Its subsequent polymerization by UV irradiation under controlled photochemical conditions produces the blue form of poly(pentacosa-10,12-diynoic acid). The resulting nanohybrids are very stable in aqueous suspension and exhibit a bilayered, core–shell architecture. We propose a model of the polyPCDA self-assembled on silver nanoparticles and we evaluate the surface coverage density.

A series of fulleropyrrolidines-conjugated bridge-ferrocene or triazene oligomers were selectively synthesized by the Sonogashira reaction by applying the step-by-step approach. The bridge is constituted by 1, 2 and 3 benzoateethynylene units (BzE) and bears triazene (Et2N3) or ferrocene as terminal groups affording the C60-2PEN3 and C60-3PEN3 dyads and C60-1PEFe, C60-2PEFe and C60-3PEFe triads. DQF-COSY, HETCOR, 1H and 13C NMR and the MALDI-TOF characterization clearly confirmed the expected molecular structure. The absorption spectra of the fulleropyrrolidine oligomers do not match the sum of the individual spectra: N-methylfulleropyrrolidine (NMF) and BzEs, suggesting electronic interaction between the two moieties in the ground state. The fluorescence of the BzE is strongly quenched after functionalization with NMF, which could be indicative of energy or electron transfer from the triazene or ferrocene as electron donor to the fulleropyrrolidine electron acceptor through the π-bridge. The latter process was confirmed by cyclic voltammetry. The strength of the electron-accepting group gets to increase anodically the oxidation potential, or decrease cathodically the reduction potential in the order C60-pyrrolidine > benzoate. The character of the HOMO in the series is defined by the electron-donating ferrocene or triazene moiety, whereas the character of the LUMO is mainly determined by the electron-accepting group and is further supported by theoretical calculations. Photovoltaic devices presented low efficiencies, due to the absorption range of the oligomers being out of the maximum solar irradiance and the inhomogeneous organization in the films.

The synthesis of a 2,5-bis(dodecanoxy)phenyleneethynylene-butadiyne series with 2, 4, 6 and 8 phenyl rings is reported. Sonogashira coupling reaction, rather than the Glaser/Eglinton/Hay reactions classically used for butadiyne formation was applied. The molecular structures of all compounds were confirmed by 1H, DEPT-135, APT 13C, MALDI-TOF, FTIR and FT-Raman analyses. The linear and nonlinear optical properties were studied in solution by UV/Vis, static and time-resolved fluorescence, and by two-photon absorption (2PA) spectroscopy. With the exception of the dimer, for which intersystem crossing is very favoured due to the low energy gap between the singlet and triplet states as theoretically predicted, the other oligomers present high fluorescence quantum yields (0.77–0.82) and large cross-sections (up to 5000 GM for the octamer) that could be applied in multiphoton microscopy or nonlinear optics.

The charge transport properties of organic semiconductors are one of the foremost limiting factors in technological applications of these materials, which are becoming important competitors with respect to the inorganic semiconductors. In fact, conjugated organic molecules are used at present as active materials in different types of devices. For this reason, the theoretical study of the electron and hole mobility, carried out in order to give hints for the design of new molecules or for the optimization of their supramolecular organization, is a task of great interest. Here, we present the results of a quantum chemical study, in the framework of the Marcus and density functional theories, on the effects of terminal groups (when they directly interact with the pi-conjugated system of the organic semiconductors) on the charge carriers mobility of organic semiconductors. In particular, using a representative oligomer of poly(para-phenylenevinylene) as a model system, we have found that strong effects on the predicted values of the intramolecular transfer integrals as well as on their dependence on the supramolecular organizations occur, when the vinyl moiety (as ending group) is taken into account.

A flexible way of fabricating core-shell noble metal-organic nanohybrids with tailored chemical and spectroscopic properties is proposed here. The synthetic protocol consists of a multi-step procedure able to guarantee acceptable reproducibility of core size and shape as well as control of the organic outer layer. The proposed method highlights limitations in obtaining highly controllable products, although the heterogeneity degree of the nanostructures is in line with that expected from bottom-up approaches in solution. Selective functionalization of the nanohybrids with properly-substituted fluorescent dyes under variable experimental conditions allowed the preparation of composite systems of tunable spectroscopic properties to be employed as nanoprobes in sensing or photonic applications. To this end, preliminary investigation on embedding the nanohybrids in compatible polymeric matrices is also reported.

Worldwide polychlorinated biphenyls (PCBs) pollution is due to complex mixtures with high number of congeners, making the determination of total PCBs in the environment an open challenge. Because the bulk of PCBs production was made of Aroclor mixtures, this analysis is usually faced by the empirical mixture identification via visual inspection of the chromatogram. However, the identification reliability is questionable, as patterns in real samples are strongly affected by the frequent occurrence of more than one mixture. Our approach is based on the determination of a limited number of congeners chosen to enable objective criteria for Aroclor identification, summing up the advantages of congener-specific analysis with the ones of total PCBs determination. A quantitative relationship is established between congeners and any single mixture, or mixtures combination, leading to the identification of the actual contamination composition. The approach, due to its generality, allows the use of different sets of congeners and any technical mixture, including the non-Aroclor ones. The results confirm that PCB environmental pollution in northern Italy is based on Aroclor. Our methodology represents an important tool to understand the source and fate of the PCBs contamination.

We report on the synthesis of a trimer, pentamer, heptamer and a polymer of the (benzoate)ethynylene type bearing highly polar diethylene glycol methyl ethers as side chains. All of the materials were characterized by 1H, DEPT-135, APT, 13C NMR, UV-Vis, static and dynamic fluorescence spectroscopy. The X-ray scattering patterns of the pentamer and heptamer revealed a Smectic C lamellar order of molecules, while that of the polymer was of the Nematic type; however, any known texture reported in the literature was observed by POM. The general term to classify the mesomorphic behaviour of these materials is Sanidic liquid crystals (LCs), which is consistent with their molecular shape assembly resembling bricks or board-like structures. The distance within the lamellae of 2.1, 2.3 and 2.0 nm for the pentamer, heptamer and polymer, respectively, is almost twice the distance of a single molecule in its more extended conformation of 1.48 nm. Therefore, the model of organization is consistent with a supramolecular assembly in bilayers, as was found in other (alkyl)benzoateethynylenes, where the side chains are oriented in the opposite way, in a back-to-back “comb-like” style. The strong π–π interaction that governs these materials was evidenced by the repeated distance of 0.35 nm found by both X-ray scattering and HRTEM microscopy, which corresponds to the distance between conjugated chains. Photophysical properties are discussed as a function of the conjugation length and of the iodine or hydrogen terminal groups and are supported by theoretical calculations.

Herein, a theoretical design addressed to the synthesis of new conjugated co-polymers for organic photovoltaic devices based on tetrazine, benzothiadiazole, dithiophene and thiophene moieties has been applied. This theoretical investigation aims to explore which combination of these molecules could be the most promising for photovoltaic applications, by studying their intrinsic electronic properties. In particular, the simulated values of the band gap energy and the open circuit voltage are the parameters to be taken into account for the selection of potential materials for solar cell devices. The results shows that the most suitable combination, giving rise to low band gaps, higher absorption coefficients and with an interesting offset of the HOMO and LUMO energy levels, is the bridge-linked copolymer which alternates the benzodithiophene and the benzothiadiazole moieties, likely because of their push–pull effect.

The collective electronic oscillator (CEO) method allows to analyze the electronic excitations in molecular systems. Using this technique the excitations are directly connected to the corresponding one-electron transition density matrices, obtained as eigenmodes of the time dependent Hartree–Fock (TDHF) Liouville operator, which represent motions of electron–hole (e–h) pairs. Two-dimensional plots of the density matrices establish the connection between the electronic spectra and the electronic rearrangement due to the photon absorption. In this paper, we use the CEO procedure to study the nature of the electronic excitations on three series of one-dimensional porphyrin systems, showing through the analysis of the CEO eigenmodes how they can be described in terms of Frenkel or Wannier–Mott excitons.

The collective electronic oscillator (CEO) approach recently proposed by Mukamel, which is based on an INDO/S Hamiltonian in the framework of the time-dependent Hartree–Fock approximation, is used to investigate the effect of the supramolecular architecture on the photogenerated excitations in different aggregates of a large-sized oligodiacetylene. The method gives the transition energies and the corresponding one-electron transition density matrices directly, which allows one to analyze, in terms of electron–hole motions in real space, the electronic redistribution upon photon absorption. Electronic excitations depend strongly on the arrangement of the oligomer molecules in the cluster, appreciable interchain effects being obtained even when the arrangement of the oligomer molecules corresponds to a minimum interchain interaction. Interchain interactions give rise to new transitions whose oscillator strengths depend on the nature of the aggregate (neutral or charged), and which can significantly modify the optical properties of the isolated molecular system.

Polydiacetylenes (PDAs) self-assembled onto silver nanoparticles (AgNPs) show superior performance relative to conventionally-structured PDAs because their unique optical properties are combined with the peculiar features of the nanosized noble metal cores. The present paper presents a study of the experimental factors influencing the photopolymerization of the carboxy-terminated 10,12-pentacosadiynoic acid (PCDA) monomer anchored to silver nanoparticles in an aqueous solution. Specific spectroscopic parameters have been identified that characterize the process both qualitatively and quantitatively, thus allowing the comparison of the polymerization yield carried out under different experimental conditions. On this basis, a well-defined synthetic protocol able to maximize the chromophoric potential of these novel optical transducers, as well as to give reproducible results, is proposed and tested on differently-stabilized and/or aged silver nanoparticles used as substrates for the PCDA chemisorption. The highly-conjugated blue polyPCDA has considerable potential in the optoelectronic, photovoltaic and sensing fields.

A simple method is reported allowing the fast evaluation of the athermal limit that is implicitly present within the set of equations recently proposed by Jonah, Brostow and Hess, dealing with the anisotropic-isotropic equilibria of single monodisperse or nearly monodisperse semirigid liquid polymers, including polymer liquid crystals. We have found an athermal limit dependent on the degree of rigidity 0 of the given semirigid liquid polymer (θ being the fraction of its monomeric units belonging to rigid rodlike sequences within each semiflexible polymeric chain) and marked off by critical lengths η al of the rigid sequences, giving rise to non-finite asymptotic values of the anisotropic-isotropic temperatures.

Long-range dispersion and induction coefficients for the homodimers of Li2, Na2, K2 are evaluated in the LALBM scheme using explicit formulae presented previously and numerical values of moments, polarizabilities and dispersion constants obtained from 2-term reduced spectra of the Kaiserslautern group. While induction coefficients are 5–8 times smaller than dispersion, induction anisotropies are always larger than the corresponding dispersion anisotropies for all homodimers.

The carbazolyldiacetylenes show very different fluorescence behaviors depending on how the carbazolyl (Cz) moiety is connected to the diacetylenic backbone. In fact, when Cz is attached to the backbone through one or more CH2 spacer(s) fluorescence properties are observed which are closely related to those of the fluorescent Cz group. On the contrary, almost no fluorescence is observed when Cz is directly attached to the backbone. In this paper, these phenomena are studied quantum chemically and an explanation for the observed facts is provided. The relevant excited states are studied in detail, showing that in the lowest singlet excited state a considerable intramolecular charge transfer occurs from Cz to the backbone when they are directly bonded.

The C 8 and C 10 long-range induction coefficients for two like centrosymmetric linear molecules are derived explicitly using the irreducible properties of spherical tensor operators. An application to the H 2 –H 2 system shows that whereas the isotropic induction coefficients are 50–100 times smaller than dispersion, the induction anisotropies γ n are always larger than the corresponding dispersion anisotropies.

The real spherical tensor theory of long-range intermolecular coefficients developed in previous papers is applied to derive explicit formulae for the first three dispersion coefficients for like centrosymmetric linear molecules. The expansion of angledependent coefficients in associated Legendre polynomials allows one to identify the isotropic and anisotropic components of the dispersion interaction in terms of London dispersion constants, the treatment of higher coefficients being simplified by the coupling of the elementary (1, 1′)-polarizations to resultant angular momenta LA and LB onto each molecule. The contributions from all coupling schemes are given explicitly for C6, C8, c10, and numerical results are presented for H2H2 using two-term reduced spectra values from the Kaiserlautern group.

An accurate evaluation of C 6 dispersion coefficients for long-range interactions involving H, He, H 2 + and H 2 has been performed using the London formula in terms of ab initio pseudostate decomposition of the static dipole polarizabilities of the individual atoms and molecules.

The pseudostate decomposition of accurate Kolos-Wolniewicz static dipole polarizabilities for ground-state H 2 allows an extremely accurate evaluation of C 6 dispersion coefficients for (H 2 ) 2 .

Recently new molecular systems based on the pyrene moiety were developed for photovoltaic applications. Here we present the results of a quantum chemical study focused on the effects induced by some different substituents on the electronic properties of pyrene, to obtain general hints for the molecular design of new pyrene-based systems. In particular, a series of electron-donating (hydroxy, amino, acetylamino) and electron-withdrawing (cyano, carbamoyl, formyl, ethynyl, ethenyl) groups were considered. Furthermore, in addition to the single pyrene molecule, two pyrene units linked by ethenylene, ethynylene, 2,5-thienylene, and ethynylene-p-phenylene containing chains of different lengths were taken into account. For all of the model structures presented, the ground state geometries have been optimized using the density functional approach, while the vertical transition energies were calculated using the time-dependent density functional theory. We will show that the tuning of the lowest electronic excitation energy (i.e., the HOMO-LUMO energy gap) as well as the localization of the spatial distributions of the frontier molecular orbitals (i.e., the nature of the electron-hole pair, generated by photon absorption) can be obtained through the analysis of the pyrene frontier molecular orbitals. This approach allows to evaluate the most suitable position of the substituents on the pyrene moiety giving rise to enhanced electronic effects also in function of their electronic nature. In this way, pyrene-structures with tailored electronic properties could be modeled. Our screening shows that promising candidates for photovoltaic applications could be molecular structures formed by two pyrene units joined/linked by a short conjugated bridge containing double or triple bonds (henceforth pyrene-linked dimers). As far as the single pyrene units are considered, the most significant reduction of the transition energy of the lowest optical electronic excitation is obtained with disubstituted pyrenes with push-pull character.

The photopolymerization study of novel diacetylene-decorated platforms obtained by self-assembling of properlyfunctionalized monomers onto pre-existing silver nanoparticles is reported. For this purpose, differently-terminated diacetylenes of general formula X-(CH 2) m-C?C- C?C-(CH 2) 8-COOH have been synthesized and tested. Qualitative and quantitative evaluation of the blue polymer photogeneration, performed by means of spectroscopic techniques, is reported and discussed in order to select the best performing photoresponsive nanohybrids to be employed in sensoristics as well as in optoelectronic and biomedical applications.

We have determined the photophysical properties of [Eu(C12H8N2)2](NO3)3, (EuPhen), a complex which is very promising for photonic and optoelectronic applications, because of its easy synthetic procedure and high thermal stability (up to 300 °C) combined with large sensitization efficiency and good emission quantum yield. Available experimental absorption and emission data have been analyzed by using Judd-Ofelt analysis. Moreover, semi-empirical calculations have been used to determine the structure of the complex and to interpret the convoluted shape of the absorption spectrum. © 2009 Elsevier B.V. All rights reserved.

The formation of polaron and/or bipolaron species upon chemical doping and photo- or electro-chemical generation is a scientific and technological problem which is still object of discussion. Here we use a simple electrostatic model to study the role of the interchain interaction in stabilizing or destabilizing the bipolaronic structure, which is usually neglected in theoretical studies. The results show that, as a consequence of the interchain interactions in the bulk system, a bipolaron generated on a single oligomer chain more than 12 units long will soon split into two polarons on adjacent chains, this process being maximized for specific interchain distances.

The stability of lanthanide complexes and the efficiency of the energy transfer process, which makes these molecules interesting materials for technological applications, are correlated to the chemical environment surrounding the metal ion. In particular the efficiency depends on the relative position of the antenna (the ligand moiety that acts as photon absorption center) and the lanthanide ion (the emitting center), while the stability of the complex is correlated to the strength of the coordination between the rare earth and the ligands. For these reasons, knowledge of the structural properties of the complex is an interesting task to achieve. Since a large number of ligand structures hold the carboxylate group (COO - ), which is used as an anchor for binding the antennae to the lanthanide ion, in this work we will show how the vibrational shifts of this group, induced by the interactions between the carboxylate moiety and the metal center of the lanthanide complex, can be used for obtaining in a simple way information on the structure of the chemical environment surrounding the lanthanide ion. Lanthanide complexes have raised increasing interest for their potential applications in several research areas such as in the biomedical field, where use of these complexes as chemosensors for in vivo applications has been recently described, 1 and in the technological field, where the design of efficient lightemitting devices and their use in optical technology has been reported in several papers. 2,3 The physicochemical property of lanthanide complexes that is the basis for developing efficient light-conversion molecular devices is the luminescence intensity. This effect originates from the intramolecular energy transfer from one excited ligand to the f-electronic structure of the metal ion, and is critically dependent on the molecular geometry of the ligands aggregate as well as on the chemical nature of the single aggregate unit. In particular, the ligand should be able to promote the energy transfer of the optical excitation and, through suitable functional groups, to efficiently saturate the lanthanide coordination sphere and make a strong binding interaction with the rare earth ion. The saturation of the sphere should minimize the quenching of the complex luminescence that may occur due to the interactions with the solvent, while a strong binding should increase the complex stability. In turn, an increased complex stability implies an increased processability of the material, an important factor for its technological applications. One suitable functional group to this end is the carboxylate moiety that is present in several ligand structures. Moreover, due to the increasing interest in the molecular design of hetero- or homometallic multinuclear complexes (which are promising materials for practical applications due to their photophysical and magnetic properties 4 ) the control of the metal-ligand coordination type represents an important task to achieve. Ligands designed with different carboxylate clamps can be fully coordinated to the same metal ion or, due to the presence of competitive ligands and of steric hindrance, only one COO - group of the clamp at a time may be bonded to the metal center. In the latter case the rest of the carboxylate moieties remain free to coordinate to other metal ions of the same or of a different chemical nature.

Long neutral and doubly charged oligodiacetylenes (with a maximum of 30 repeat units) as well as some of their aggregates have been studied in a semiempirical framework. By analyzing the plots of the one-electron ground-state density matrices, we have shown that the 12-units long oligomer is the borderline between two different behaviors of the electronic structure of the isolated charged systems. For oligomers shorter than this limit the bipolaronic structure is the dominant one, while for larger oligomers the polaron-pair structure is preferred. The major conclusion of this work concerns the effects of the interchain interactions in bipolaron systems, which are usually neglected in theoretical works. It is predicted that as a consequence of the latter in the bulk system a bipolaron generated on a single polymer chain will soon split into two polarons on adjacent chains.

A series of neodymium complexes based on (alpha-oligothiophen-2-yl)(2-hydroxy-3-methoxyphenyl)methanone moiety was prepared and studied by FTIR, NMR, optical spectroscopies. As a function of the thiophene ring number in the ligand, a variation of both stoichiometry and optical properties of the complex are observed. The solid-state ligand structure indicates a close packing and a stressed situation in enolate residues, i.e. non-coplanarity between thiophene and phenyl moieties. An un-precedent stoichiometry of complexes was observed, namely two metal and five ligands, according to both the synthesis conditions and the encumbrance of ligand with respect to ionic radius of Nd3+. This fact is attributed to the contribution to the coordination of the methoxy group. Luminescence and lifetime determination are discussed in the light of the above factors. (C) 2007 Elsevier B.V. All rights reserved.

In this paper the quantum chemical semiempirical procedure recently proposed by us to predict ground- and excited-state geometries of lanthanide complexes, the pseudo coordination centre method (PCC), is preliminarily compared with the semiempirical sparkle model for the calculation of lanthanide complexes (SMLC). Contrary to the SMLC method, where the rare-earth ion is replaced by a reparameterized sparkle atom, in our approach we replace it with a metal ion which is already present in the chosen semiempirical parameterization. This implies that in the optimization of the geometry of the complexes a different weight is implicitly given to the complex region including the rare-earth ion and its neighbour atoms with respect to the region of the ligands aggregate. As a consequence our approach is expected to reproduce better than the SMLC one the geometry of the ligands aggregate embedded in the complex, while the contrary happens for the coordination distances.

Lanthanide ions emitting in the near-infrared (NIR) region possess an intrinsically small molar absorption coefficient in the ultraviolet (UV)–vis–NIR spectrum, which is unfavourable for pumping efficiency. On the contrary, using organic lanthanide complexes it is possible to populate the excited state levels of the emitting ion through an efficient intramolecular energy transfer from the optically excited ligands, which act as light-harvesting antennae. With the aim of studying and maximizing the transfer to lanthanide metals, we have synthesized oligothiophene and thiophenefluorene ligands bearing carboxylate clamps able to complex erbium and other lanthanide 3 + ions. The complexes of {4′-(hydroxycarbonyl)methyl-[2,2′;5′,2″]terthiophen-3′-yl}acetic acid and 9-(hydroxycarbonyl)-methyl-2,7-dithien-2-yl-[fluoren-9-yl-]acetic acid with Er 3+ and different ancillary ligands have been prepared and their optical properties were carefully studied. Moreover, relaxation dynamics measurements have been carried out on all complexes in order to determine emission lifetimes, which result to be of the order of magnitude 2 μs. Quantum chemical calculations have been performed to explain optical absorption data in terms of different coordination types. The complexes containing phenanthroline/pyridine are modelled by adding to the dianion of the ligand one univalent/divalent counterion. The absorption spectra computed in this way are in close agreement with experiment, and the univalent→divalent theoretical wavelength shift goes in the right direction. The addition of a counterion has an even bigger effect on the triplet states, and hence on their matching with the emitting states of the ion.

We discuss the effect of the interchain interactions on the electronic properties of a "dimer", named (CHD(4))(2), built from two molecules of a fully carbazolyl-substituted oligodiacetylene containing four repeating units. Each carbazolyl moiety is connected to its respective backbone through a methylene spacer, and the excitations of the neutral and doubly positively charged dimer are obtained using the CIS (configuration interaction including singles) and CEO (collective electronic oscillator) models. The separation distance between the backbones is fixed to a value that could possibly imply a very weak, if any, interchain interaction between the oligomers. In the charged dimer, where we have not been able to perform CEO calculations, it can be expected from previous results that the CIS method will behave satisfactorily. Contrary to the neutral case and surprisingly enough, the simulated photoinduced absorption (PA) spectra indicate a substituent-induced strong interaction between the oligomers, due to the very large amount of excess charge on the distorted oligomer which is spread over its carbazolyl moieties. Broadening of the lowest energy band due to the latter interaction improves the agreement between the theoretical and the experimental PA spectrum for the corresponding polymer. The through-space charge transfer directly connecting the charged with the neutral molecule could supply an efficient channel for charge transport in substituted polydiacetylenes.

We are studying how the electronic properties of an aggregate, built with conjugated oligomers carrying bulk substituents, are affected by intermolecular interactions. In this paper we apply the CEO (Collective Electronic Oscillator) method, on the basis of the semiempirical INDO/S Hamiltonian, to compute the electronic density matrix modifications following the photon absorption in a doubly charged cluster of two units of a fully carbazolyl-substituted oligodiacetylene tetramer, taken as a model system. The picture that had emerged from our previous calculations based on the less sophisticated CIS (Configuration Interaction including Singles) approach is seen to be confirmed. Despite the large separation between the backbones, a through-space charge transfer occurs between the two oligomers due to the fact that the excess charge, contrary to what is generally believed, is not localized on the conjugated backbone, but is spread out over the carbazolyl moieties of the charged molecule. Consideration of this kind of interaction improves the theoretical results obtained for the isolated charged oligomer chain, and aids in better explaining some features of the experimental photoinduced spectra of the corresponding polymer. (c) 2005 Elsevier B.V. All rights reserved.

We use a simple quantum chemical semiempirical procedure to study the electronic properties of organic-lanthanide complexes, taking as a model system Er(8-hydroxyquinolinate)3. Among the problems inherent to such a study is the fact that the lanthanide ion has never been parametrized in any of the standard semiempirical Hamiltonians. To overcome this difficulty, the lanthanide ion is replaced by a different but somewhat similar parametrized ion, or merely by a point charge. Good agreement with experiment, where available, is obtained, particularly in the former case. In fact, the electronic properties of the complex (apart from the emission properties) are seen to be scarcely affected by the nature of the lanthanide ion itself, but the core interactions between the metal ion and the ligand units play a relevant role, also in the calculation of the excitation energies. In particular, the ordering and separation of both singlet and triplet excited states are affected. The main conclusion is that to describe in detail the mechanism of the energy-transfer process occurring in the complex it is essential to take into account the geometry relaxation effects in the excited states.

Absorption, photoluminescence, and photoinduced absorption (PIA) spectra of films of red phase polycarbazolyldiacetylenes (polyCzDAs) carrying selected acyl (polyDPCHD) or alkyl (polyDCHD-HS) groups are reported. The absorption spectrum of polyDCHD-HS, which is almost independent of the temperature down to 20 K, shows an excitonic absorption followed by an inhomogeneously broadened vibronic progression. Very different is instead the case of polyDPCHD whose thin films show a very sharp excitonic transition and a vibronic progression with a lower electron-phonon (el-ph) coupling with respect to that of polyDCHD-HS. These peculiar characteristics allow the detection of additional features in the spectra, which are assigned on the basis of quantum chemical calculations to string modes of the backbone. As far as thick films of polyDPCHD are concerned, broad electronic spectra are observed. Raman spectroscopy studies suggest that this broadening is due to the presence of two forms of the polymer. Unlike other polydiacetylenes (PDA's) including polyDCHD-HS films, polyDPCHD films are also strongly luminescent, thus suggesting a different ordering of the dipole forbidden and allowed states. For these materials, the photoinduced absorption spectra are dominated by triplet excitons. Effects of different forms in acyl substituted polymers are observed also in the PIA spectra. The rise and decay of the triplet signal for polyDCHD-HS are accounted for through a dynamical model, which includes saturation effects in the monomolecular decay regime. A more complex kinetics is instead observed for polyDPCHD. From the study of the kinetics of the PIA spectra both the triplet generation efficiency and the density of traps are evaluated.

Absorption, photoluminescence and photoinduced absorption spectra of polycarbazolyldiacetylenes (polyCzDAs) carrying selected acyl (polyDPCHD) or alkyl (polyDCHD-HS) groups are reported. The absorption spectrum of polyDCHD-HS films, which is almost independent of the temperature, shows an excitonic absorption followed by an inhomogeneously broadened vibronic progression. Very different is instead the case of polyDPCHD whose thin films show a very sharp excitonic transition and a vibronic progression with a lower electron-phonon (el-ph) coupling with respect to that of polyDCHD-HS. Unlike other polydiacetylenes (PDAs), the polyCzDAs which exhibit narrow spectral features and reduced el-ph coupling, such as polyDPCHD films and polyDCHD-HS in benzene solutions, are luminescent thus suggesting a different ordering of the dipole forbidden and allowed states. This interpretation is confirmed by two-photon spectroscopic data. The ordering of the Ag and Bu excited states also affects the generation of triplet excitons, which are the main long-living excited states of PDAs. The rise and decay of the triplet signal for polyDCHD-HS are accounted for through a dynamical model, based on a monomolecular decay regime including saturation effects. A more complex kinetics is instead observed for polyDPCHD. From the study of the kinetics of the PIA spectra both the triplet generation efficiency and the density of traps are evaluated.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper, we present the results of a theoretical study of the electronic and optical excitations of isolated carbazolyl-substituted oligodiacetylenes using the collective electronic oscillator (CEO) method. Within this technique the excitations are directly connected to the corresponding one-electron transition density matrices, obtained as eigenmodes of the time dependent Hartree–Fock (TDHF) Liouville operator, which represent motions of electron/hole pairs. The two-dimensional plot of the transition density matrix (CEO mode) establishes the physical connection between the electronic spectra and the electronic rearrangement due to the photon absorption. Consequently this representation allows for an intuitive interpretation of the optical spectra in terms of the chemical structure, and gives a useful tool for the design of molecules showing the required physical properties.

In this work we report the synthesis and photophysical properties of a novel ligand and its lanthanide complexes based on macrocycle 1,4.7,10-tetraazacyclododecane-1,4,7-triacetic acid bearing 8-hydroxyquinoline as antenna. Visible and IR sensitized emission is obtained because of the efficient energy transfer from the antenna to the emitting ion. The interaction between ions and antenna is accounted for by suitable quantum chemical calculations. The good stability of the complexes and their solubility in organic solvents make them good candidates for the fabrication of plastic waveguides and optical amplifiers. (C) 2004 Elsevier B.V All rights reserved.

We report on the absorption, photoluminescence and photoinduced absorption of polydiacetylenes (PDAs) having in the side groups the photoconductive carbazolyl rings substituted by acyl CO(CH 2 ) n CH 3 chains ( n =10, polyDLCHD; n =14, polyDPCHD). A novel feature in the absorption spectra is detected and assigned on the basis of theoretical calculation including the electron correlation. Different features are observed in the photoexcitations spectra of thin and thick samples and a preliminary interpretation is proposed.

Absorption and photoinduced absorption (PIA) spectra of thin films of polycarbazolyldiacetylenes carrying alkyl (polyDCHD-HS) or acyl (polyDPCHD) substitutions are reported. The absorption spectra are dominated by an excitonic absorption followed by a vibronic progression, and are quite insensitive to the temperature. The PIA spectra show the typical spectral feature of a triplet excitation whose rise and decay times are different. The PIA signal shows a strong sublinear pumping intensity dependence, which also affects the rise time.

We have performed a quantum chemical study on neutral and doubly charged oligodiacetylene clusters, using a recent time-dependent Hartree-Fock (TDHF) approach based on an INDO/S Hamiltonian. Our goal is to achieve information on the effect of the supramolecular architecture on the nature of the photoexcitations and on the non-linear optical properties of these systems, which are of relevant interest for their potential applications in photonics. Considering aggregates of three oligodiacetylene molecules with 15 repeat units, results for two geometrical configurations as models for maximal and minimal interchain interactions are reported.

We have performed theoretical calculations on the singly and doubly charged species of the lowest unsubstituted oligodiacetylenes, with the scope of achieving information on the charged photoexcitations of polydiacetylenes. On the basis of the AM1-optimized geometries of the radical ions and of the dications we have computed their low-energy vertical transitions, using the INDO-SCI method in order to take into account the electron correlation. Extrapolated polymer results are compared with the photoinduced absorption spectra.

We report results of steady-state photoinduced absorption experiments on PDA-4BCMU grown and oriented on silica substrate. A high degree of axial order, checked by linear and nonlinear optical dichroism and correlated with X-ray measurements was found. Polycrystalline films were obtained with a preferential orientation of crystallites. Unlike the solution cast films exhibiting the so called `red form' only, the deposited films are blue thus showing a better (pi) -electron conjugation than in the former case due to a better planarity of polymer chains. The films were used to perform steady-state photoinduced absorption experiments. Three photoinduced absorption bands at 0.82, 0.95, and 1.39 eV were observed. The 0.82 and 0.95 eV absorption features are assigned to charged states while the 1.39 eV peak is originated by triplet excitons. The photogeneration in PDA- 4BCMU of both charged and neutral states is discussed and compared with the results obtained in polydiacetylenes having carbazolyl groups as substituents. The relative weight of one excited species over the other one is dictated by the molecular and supramolecular organization of the polymer.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Convergence of dipole pseudospectra for the 2-electron model systems He and H 2 is studied for Riley–Dalgarno- and Kolos–Wolniewicz-type wavefunctions explicitly including electron correlation in terms of positive powers of the interelectronic distance. 40-term pseudospectra for He and 34-term for H 2 give almost a 4-figure accuracy for the static polarizabilities of the monomers and the resulting C 6 dispersion coefficients of the dimers.