Determination of deep and shallow levels in conjugated polymers by electrical methods

Conjugated organic semiconductors have been submitted to various electrical measurement techniques in order to reveal information about shallow levels and deep traps in the forbidden gap. The materials consisted of poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV), poly(3-methylthiophene) (PMeT), and alpha-sexithienyl (alpha T6) and the employed techniques were IV, CV, admittance spectroscopy, TSC, capacitance and current transients. (C) 1999 Elsevier Science B.V. All rights reserved.

Electrical study of impurity states in conjugated polymers

Schottky diodes resulting from an intimate contact of aluminum on electro-deposited poly(3-methylthiopene), PMeT, have been studied by admittance spectroscopy, capacitance-voltage and current-voltage measurements, and optically-induced current transients. The loss-tangents show the existence of interface states that can be removed by vacuum annealing, also visible in the transients. Furthermore, the CV curves don't substantiate the idea of movement of the dopant ions.

Raman Spectroscopy and DFT calculations for the Electronic Structure Characterization of Conjugated Polymers

In the last three decades, there has been a broad academic and industrial interest in conjugated polymers as semiconducting materials for organic electronics. Their applications in polymer light-emitting diodes (PLEDs), polymer solar cells (PSCs), and organic field-effect transistors (OFETs) offer opportunities for the resolution of energy issues as well as the development of display and information technologies1. Conjugated polymers provide several advantages including low cost, light weight, good flexibility, as well as solubility which make them readily processed and easily printed, removing the conventional photolithography for patterning2. A large library of polymer semiconductors have been synthesized and investigated with different building blocks, such as acenes or thiophene and derivatives, which have been employed to design new materials according to individual demands for specific applications. To design ideal conjugated polymers for specific applications, some general principles should be taken into account, including (i) side chains (ii) molecular weights, (iii) band gap and HOMO and LUMO energy levels, and (iv) suited morphology.3-6 The aim of this study is to elucidate the impact that substitution exerts on the molecular and electronic structure of π-conjugated polymers with outstanding performances in organic electronic devices. Different configurations of the π-conjugated backbones are analyzed: (i) donor-acceptor configuration, (ii) 1D lineal or 2D branched conjugated backbones, and (iii) encapsulated polymers (see Figure 1). Our combined vibrational spectroscopy and DFT study shows that small changes in the substitution pattern and in the molecular configuration have a strong impact on the electronic characteristics of these polymers. We hope this study can advance useful structure-property relationships of conjugated polymers and guide the design of new materials for organic electronic applications.

Development of docetaxel and alendronate-loaded chitosan-conjugated polylactide-co-glycolide nanoparticles: In vitro characterization in osteosarcoma cells

Purpose: To develop docetaxel (DTX)- and alendronate (ALN)-loaded, chitosan (CS)-conjugated polylactide- co-glycolide (PLGA) nanoparticles (NPs) to increase therapeutic efficacy in osteosarcoma cells. Methods: Drug-loaded PLGA NPs were prepared by nanoprecipitation and chemically conjugated by the carboxylic group of PLGA to the amine-bearing CS polymer. The nanocarrier was characterized by dynamic light scattering, transmission electron microscopy, scanning electron microscopy, and differential scanning calorimetry as well as by in vitro drug release and cell culture studies. Results: NP size was within the tumour targeting range (~200 nm) with an effective positive charge (20 mV), thus increasing cellular uptake efficiency. Morphological analysis revealed clear spherical particles with uniform dispersion. The NPs exhibited identical sustained release kinetics for both DTX and ALN. CS-conjugated PLGA with dual-drug-loaded (DTX and AL) NPs showed typical time-dependent cellular uptake and also displayed superior cytotoxicity in MG-63 cells compared with blank NPs, which were safe and biocompatible. Conclusion: Combined loading of DTX and ALN in NPs increased the therapeutic efficacy of the formulation for osteosarcoma treatment, thus indicating the potential benefit of a combinatorial drug regimen using nanocarriers for effective treatment of osteosarcoma.

A possibility for non-invasive diagnosis of superficial scald in 'Rocha' pear based on chlorophyll a fluorescence, colorimetry, and the relation between alpha-farnesene and conjugated trienols

This study aimed to identify physiological markers in superficially scalded 'Rocha' pear (Pyrus communis L 'Rocha') that would relate to chlorophyll a fluorescence (CF), allowing a non-invasive diagnosis of the disorder. Conditions chosen before shelf life provided two fruit groups with different developing patterns and severity of superficial scald: T fruit fully developed the disorder in storage, while C fruit developed it progressively throughout shelf life. Principal component analysis (PCA) of all the measured variables, and simple linear correlations among several major parameters and scald index (SI)/shelf life showed that scald and ripening/aging were concurring processes, and that it was not possible to isolate a particular variable that could deliver a direct non-invasive diagnosis of the disorder. For both fruit groups the SI resulted from the balance between the reducing power (OD200) and the content of conjugated trienols (CTos) and alpha-farnesene (alpha-Farn) in the fruit peel. At OD200 > 150 there was a linear relationship between CTos and OD200, suggesting that the level of antioxidants was self-adjusted in order to compensate the CTos level. However, at OD200 < 150 this relationship disappeared. A consistent linear relationship between dos and alpha-Farn existed throughout shelf life in both fruit groups, contrarily to the early storage stage, when those compounds do not relate linearly. The CF variables F-0, F-v/F-m, and the colorimetric variables, L* and h degrees were used in multi-linear regressions with other physiological variables. The regressions were made on one of the fruit groups and validated through the other. Reliable regressions to alpha-Farn and CTos were obtained (R approximate to 0.6; rmsec approximate to rmsep). Our results suggest that a model based on CF and colorimetric parameters could be used to diagnose non-invasively both the contents and the relationship between alpha-Farn and CTos and hence the stage of scald development. (C) 2011 Elsevier By. All rights reserved.

Determination of deep and shallow levels in conjugated polymers by electrical methods

Conjugated organic semiconductors have been submitted to various electrical measurement techniques in order to reveal information about shallow levels and deep traps in the forbidden gap. The materials consisted of poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV), poly(3-methylthiophene) (PMeT), and alpha-sexithienyl (alpha T6) and the employed techniques were IV, CV, admittance spectroscopy, TSC, capacitance and current transients. (C) 1999 Elsevier Science B.V. All rights reserved.

Electrical study of impurity states in conjugated polymers

Schottky diodes resulting from an intimate contact of aluminum on electro-deposited poly(3-methylthiopene), PMeT, have been studied by admittance spectroscopy, capacitance-voltage and current-voltage measurements, and optically-induced current transients. The loss-tangents show the existence of interface states that can be removed by vacuum annealing, also visible in the transients. Furthermore, the CV curves don't substantiate the idea of movement of the dopant ions.

Conjugated polymer nanoparticles for biological labeling and delivery

Cancer remains one of the world’s most devastating diseases, with more than 10 million new cases every year. However, traditional treatments have proven insufficient for successful medical management of cancer due to the chemotherapeutics’ difficulty in achieving therapeutic concentrations at the target site, non-specific cytotoxicity to normal tissues, and limited systemic circulation lifetime. Although, a concerted effort has been placed in developing and successfully employing nanoparticle(NP)-based drug delivery vehicles successfully mitigate the physiochemical and pharmacological limitations of chemotherapeutics, work towards controlling the subcellular fate of the carrier, and ultimately its payload, has been limited. Because efficient therapeutic action requires drug delivery to specific organelles, the subcellular barrier remains critical obstacle to maximize the full potential of NP-based delivery vehicles. The aim of my dissertation work is to better understand how NP-delivery vehicles’ structural, chemical, and physical properties affect the internalization method and subcellular localization of the nanocarrier. ^ In this work we explored how side-chain and backbone modifications affect the conjugated polymer nanoparticle (CPN) toxicity and subcellular localization. We discovered how subtle chemical modifications had profound consequences on the polymer’s accumulation inside the cell and cellular retention. We also examined how complexation of CPN with polysaccharides affects uptake efficiency and subcellular localization. ^ This work also presents how changes to CPN backbone biodegradability can significantly affect the subcellular localization of the material. A series of triphenyl phosphonium-containing CPNs were synthesized and the effect of backbone modifications have on the cellular toxicity and intracellular fate of the material. A mitochondrial-specific polymer exhibiting time-dependent release is reported. Finally, we present a novel polymerization technique which allows for the controlled incorporation of electron-accepting benzothiadiazole units onto the polymer chain. This facilitates tuning CPN emission towards red emission. ^ The work presented here, specifically, the effect that side-chain and structure, polysaccharide formulation and CPN degradability have on material’s uptake behavior, can help maximize the full potential of NP-based delivery vehicles for improved chemotherapeutic drug delivery.^

Synthetic approaches to flexible fluorescent conjugated polymers

Conjugated polymers (CPs) are intrinsically fluorescent materials that have been used for various biological applications including imaging, sensing, and delivery of biologically active substances. The synthetic control over flexibility and biodegradability of these materials aids the understanding of the structure-function relationships among the photophysical properties, the self-assembly behaviors of the corresponding conjugated polymer nanoparticles (CPNs), and the cellular behaviors of CPNs, such as toxicity, cellular uptake mechanisms, and sub-cellular localization patterns. ^ Synthetic approaches towards two classes of flexible CPs with well-preserved fluorescent properties are described. The synthesis of flexible poly( p-phenylenebutadiynylene)s (PPBs) uses competing Sonogashira and Glaser coupling reactions and the differences in monomer reactivity to incorporate a small amount (∼10%) of flexible, non-conjugated linkers into the backbone. The reaction conditions provide limited control over the proportion of flexible monomer incorporation. Improved synthetic control was achieved in a series of flexible poly(p-phenyleneethynylene)s (PPEs) using modified Sonogashira conditions. In addition to controlling the degree of flexibility, the linker provides disruption of backbone conjugation that offers control of the length of conjugated segments within the polymer chain. Therefore, such control also results in the modulation of the photophysical properties of the materials. ^ CPNs fabricated from flexible PPBs are non-toxic to cells, and exhibit subcellular localization patterns clearly different from those observed with non-flexible PPE CPNs. The subcellular localization patterns of the flexible PPEs have not yet been determined, due to the toxicity of the materials, most likely related to the side-chain structure used in this series. ^ The study of the effect of CP flexibility on self-assembly reorganization upon polyanion complexation is presented. Owing to its high rigidity and hydrophobicity, the PPB backbone undergoes reorganization more readily than PPE. The effects are enhanced in the presence of the flexible linker, which enables more efficient π-π stacking of the aromatic backbone segments. Flexibility has minimal effects on the self-assembly of PPEs. Understanding the role of flexibility on the biophysical behaviors of CPNs is key to the successful development of novel efficient fluorescent therapeutic delivery vehicles.^

Synthesis and Characterization of Photochromic Copolymers Containing 3-Indolylfulgides/Indolylfulgimides

Fulgides and fulgimides are important organic photochromic compounds and can switch between the open forms and the closed forms with light. The 3-indolylfulgides and 3-indolylfulgimides exhibit promising photochromic properties and have great potential in optical memory devices, optical switches and biosensors. Copolymers containing 3-indolylfulgides/indolylfulgimides synthesized via free radical polymerizations increase conformation changes and allow the photochromic compounds to be uniformly distributed in the polymer matrix. A trifluoromethyl 3-indolylfulgide and two trifluoromethyl 3-indolylfulgimides with one or two polymerizable N-stryryl group(s) were prepared. Copolymerization with methyl methacrylate provided two linear copolymers or a cross-linked copolymer. The properties of the monomeric fulgide/fulgimides and copolymers in toluene or as thin films were characterized. In general, the photochromic monomers and copolymers revealed similar photochromic properties and exhibited good thermal and photochemical stability. All compounds absorb visible light in both open forms and closed forms. The closed form copolymers were more stable than the open form copolymers and showed little or no degradation after 400 h. The photochemical degradation rate was less than 0.03% per cycle. In films, conformational restrictions were observed for the open forms suggesting that the preparation of films from the closed forms is advantageous. Two novel methyl 3-indolylfulgimides with one or two polymerizable N-stryryl group(s) were prepared. Copolymerization of acrylamide with the methyl indolylfulgimides or the trifluoromethyl indolylfulgimides yielded two aqueous soluble linear copolymers and two photochromic hydrogels. The closed form copolymers containing trifluoromethyl indolylfulgimides were hydrolyzed in aqueous solution by replacing the trifluoromethyl group with a carboxylic acid group. The resulting carboxylic copolymers were also photochromic. The copolymers containing methyl fulgimides were stable in aqueous solutions and did not hydrolyze. Both methyl and carboxylic copolymers exhibited good stability in aqueous solutions. In general, the open form copolymers were more stable than the closed form copolymers, and the copolymers revealed better stability in acidic solution than neutral solution. The linear copolymers displayed better photochemical stability in neutral solution and degraded up to 22% after 105 cycles. In contrast, the hydrogels showed enhanced fatigue resistance in acidic condition and underwent up to 60 cycles before degrading 24%.

New functional polythiophene based materials: fine-tuning of photovoltaic or chiroptical properties

There is a remarkable level of interest in the development of π-conjugated polymers (ICPs) which have been employed, thanks to their promising optical and electronic properties, in numerous applications including photovoltaic cells, light emitting diodes and thin-film transistors. Although high power conversion efficiency can be reached using poly(3-alkylthiophenes) (P3ATs) as electron-donating materials in polymeric solar cells of the Bulk-Heterojunction type (BHJ), their relatively large band gap limits the solar spectrum fraction that can be utilized. The research work described in this dissertation thus concerns the synthesis, characterization and study of the optical and photoactivity properties of new organic semiconducting materials based on polythiophenes. In detail, various narrow band gap polymers and copolymers were developed through different approaches and were characterized by several complementary techniques, such as gel permeation chromatography (GPC), NMR spectroscopy, thermal analyses (DSC, TGA), UV-Vis/PL spectroscopy and cyclic voltammetry (CV), in order to investigate their structural and chemical/photophysical properties. Moreover, the polymeric derivatives were tested as active material in air-processed organic solar cells. The activity has also been devoted to investigate the behavior of polythiophenes with chiral side chain, that are fascinating materials capable to assume helix supramolecular structures, exhibiting optical activity in the aggregated state.

Dithienopyrrole-based materials: development of new polymeric derivatives

The possibility to control molar mass and termination of the growing chain is fundamental to create well-defined, reproducible materials. For this reason, in order to apply polydithienopyrrole (PDTP) as organic conjugated polymer, the possibility of controlled polymerization needs to be verified. Another aspect that is still not completely explored is bound to the optical activity of the PDTP, which bearing appropriate substituents may adopt a helical conformation. The configuration of the helix, built up from achiral co-monomers, can be established in an enantiopure way by using only a small percentage of the chiral monomer co-polymerized with achiral co-monomer. The effect, called “sergeants and soldiers effect”, is expressed by the nonlinear increase of the chiral response vs the ratio of the chiral co-monomer used for the polymerization. To date, this effect is still not completely explored for PDTP. In this framework the project will investigate, firstly, the possibility to obtain a controlled polymerization of PDTP. Then, monomers with different side chains and organometallic functions will be screened for a CTCP-type polymerization. Also a Lewis-acid based cationic polymerization will be performed. Moreover the chemical derivatization of dithienopyrrole DTP is explored: the research is going to concern also block copolymers, built up by DTP and monomers of different nature. The research will be extended also to the investigation of optically active derivates of PDTP, using a chiral monomer for the synthesis. The possibility to develop a supramolecular distribution of the polymeric chains, together with the “sergeants and soldiers effect” will be checked investigating a series of polymers with increasing amounts of chiral monomer.

Synthesis of Metal-Binding Ligand-Containing Copolymers, Nanoparticles and Blends

In this thesis we developed three copper-containing systems. Copper shows intriguing abilities in photocatalysis, however, one of the major limitations of many copper complexes is that photochemical properties might be quenched in solution caused by π-interactions between solvent and solute, due to Jahn-Teller distortion in the excited state. As such, we herein seek to synthesise copper heteroleptic complexes that will subsequently be nanoprecipitated with a polymer. This will allow the polymer to encase the complex and prevent the solvent-induced quenching. Subsequently, the preparation of blends of polymer with the aforementioned copper complexes, at different weight ratios is sought. The preparation of the blend is particularly interesting as the catalytic properties are anticipated to be inferior on account of the low surface area. However, owing to the polymer matrix better, mechanical properties are anticipated. The blends can combine the mechanical properties of the polymer and the luminescence of the complex, with the advantage that the polymer matrix can also prevent quenching from oxygen. As final task, we developed a copper-containing monomer. The synthesis of a monomer that contains copper and can be excited under ultraviolet (UV) light is particularly interesting.

Design, synthesis and photovoltaic properties of some thiophene-based conjugated polymers for organic solar cells

The current issue of the resource of energy combined with the tendency to give a green footprint to our lifestyle have prompted the research to focus the attention on alternative sources with great strides in the optimization of polymeric photovoltaic devices. The research work described in this dissertation consists in the study of different semiconducting π-conjugated materials based on polythiophenes (Chapter I). In detail, the GRIM polymerization was deepened defining the synthetic conditions to obtain regioregular poly(3-alkylthiophene) (Chapter II). Since the use of symmetrical monomers functionalized with oxygen atom(s) allows to adopt easy synthesis leading to performing materials, disubstituted poly(3,4-dialkoxythiophene)s were successfully prepared, characterized and tested as photoactive materials in solar cells (Chapter III). A “green” resource of energy should be employed through sustainable devices and, for this purpose, the research work was continued on the synthesis of thiophene derivatives soluble in eco-friendly solvents. To make this possible, the photoactive layer was completely tailored starting from the electron-acceptor material. A fullerene derivative soluble in alcohols was successfully synthetized and adopted for the realization of the new devices (Chapter IV). New water/alcohol soluble electron-donor materials with different functional groups were prepared and their properties were compared (Chapter V). Once found the best ionic functional group, a new double-cable material was synthetized optimizing the surface area between the different materials (Chapter VI). Finally, other water/alcohol soluble materials were synthetized, characterized and used as cathode interlayers in eco-friendly devices (Chapter VII). In this work, all prepared materials were characterized by spectroscopy analyses, gel permeation chromatography and thermal analyses. Cyclic voltammetry, X-ray diffraction, atomic force microscopy and external quantum efficiency were used to investigate some peculiar aspects.

Modelling the influence of diradical character and aggregation on conjugated molecular materials

Molecular materials are made by the assembly of specifically designed molecules to obtain bulk structures with desired solid-state properties, enabling the development of materials with tunable chemical and physical properties. These properties result from the interplay of intra-molecular constituents and weak intermolecular interactions. Thus, small changes in individual molecular and electronic structure can substantially change the properties of the material in bulk. The purpose of this dissertation is, thus, to discuss and to contribute to the structure-property relationships governing the electronic, optical and charge transport properties of organic molecular materials through theoretical and computational studies. In particular, the main focus is on the interplay of intra-molecular properties and inter-molecular interactions in organic molecular materials. In my three-years of research activity, I have focused on three major areas: 1) the investigation of isolated-molecule properties for the class of conjugated chromophores displaying diradical character which are building blocks for promising functional materials; 2) the determination of intra- and intermolecular parameters governing charge transport in molecular materials and, 3) the development and application of diabatization procedures for the analysis of exciton states in molecular aggregates. The properties of diradicaloids are extensively studied both regarding their ground state (diradical character, aromatic vs quinoidal structures, spin dynamics, etc.) and the low-lying singlet excited states including the elusive double-exciton state. The efficiency of charge transport, for specific classes of organic semiconductors (including diradicaloids), is investigated by combining the effects of intra-molecular reorganization energy, inter-molecular electronic coupling and crystal packing. Finally, protocols aimed at unravelling the nature of exciton states are introduced and applied to different molecular aggregates. The role of intermolecular interactions and charge transfer contributions in determining the exciton state character and in modulating the H- to J- aggregation is also highlighted.