Mixed-valence state of symmetric diruthenium complexes: synthesis, characterization, and electron transfer investigation

Complexes of the type {[(pyS)Ru(NH3)(4)](2)-mu-L}(n), where pyS = 4-mercaptopyridine, L = 4,4'-dithiodipyridine (pySSpy), pyrazine (pz) and 1,4-dicyanobenzene (DCB), and n = +4 and +5 for fully reduced and mixed-valence complexes, respectively, were synthesized and characterized. Electrochemical data showed that there is electron communication between the metal centers with comproportionation constants of 33.2, 1.30 x 10(8) and 5.56 x 10(5) for L = pySSpy, pz and DCB, respectively. It was also observed that the electronic coupling between the metal centers is affected by the p-back-bonding interaction toward the pyS ligand. Raman spectroscopy showed a dependence of the intensity of the vibrational modes on the exciting radiations giving support to the assignments of the electronic transitions. The degree of electron communication between the metal centers through the bridging ligands suggests that these systems can be molecular wire materials.CNPqCNPqFAPESPFAPESPFUNCAP [PRONEM PRN-0040-00065.01.00/10, 10582696-0]FUNCAPCAPESCAPE

Spectroscopic characterization of hybrid materials of polyaniline and hexaniobate prepared by different routes

Esta tese versa sobre a caracterização espectroscópica de materiais híbridos orgânico-inorgânicos de polianilina (PANI), um polímero condutor, e hexaniobato, um óxido semicondutor. Os materiais híbridos foram preparados por diferentes rotas, empregando-se nanopartículas de hexaniobato esfoliado (nanofolhas ou nanoscrolls), e PANI previamente preparada (rotas ex-situ), ou através da polimerização da anilina na presença do hexaniobato (rotas in-situ). As principais técnicas de caracterização utilizadas foram as espectroscopias Raman ressonante (RR) e de ressonância paramagnética eletrônica (EPR), microscopia eletrônica de varredura (SEM) e análise termogravimétrica acoplada a espectrometria de massas (TGA-MS). No entanto, outras técnicas como espectroscopias eletrônica no UV-VIS-NIR e vibracional no infravermelho, e difratometria de raios X foram muito importantes para complementar a caracterização e suportar os resultados. Observou-se que a PANI pode ser adsorvida sobre os nanoscrolls de hexaniobato através de diferentes rotas de preparação, como por montagem de filmes layer-by-layer, mistura das dispersões dos componentes, ou polimerização in-situ da anilina na presença das nanopartículas de hexaniobato. De acordo com os dados espectroscópicos, estas morfologias favorecem maiores graus de dopagem (protonação) do polímero em relação à polianilina pura (ou prístina), associados a fortes interações entre a polianilina e o hexaniobato, que apresenta alta acidez superficial. Os dados espectroscópicos revelaram que a deslocalização eletrônica dos pólarons e as populações relativas dos pólarons e bipólarons da polianilina são afetadas na presença do hexaniobato, indicando que a interação entre os componentes também pode induzir mudanças conformacionais das cadeias poliméricas. Através da espectroscopia RR empregando diferentes radiações excitantes, foi possível identificar os segmentos cromofóricos formados durante a oligo/polimerização da anilina em meios de diferentes pHs, distintos do empregado na síntese convencional da polianilina (ácido clorídrico 1 mol L-1). O produto majoritário é constituído de unidades 1,4-benzoquinona-monoimina acopladas a unidades aminofenil, compondo estruturas complexas denominadas \"adutos de anilina\". Além destas estruturas, a espectroscopia RR possibilitou a identificação de segmentos tipo-fenazina e segmentos polarônicos da PANI como produtos minoritários. Quando a reação é realizada na presença de hexaniobato, ainda há formação majoritária de adutos de anilina, mas há formação de maiores quantidades de segmentos polarônicos da polianilina. Estes resultados foram associados à alta acidez superficial do hexaniobato, mostrando novamente a importância do componente inorgânico para a estrutura dos materiais híbridos. Os estudos das propriedades eletrocrômicas dos filmes híbridos layer-by-layer apresentaram acentuada variação de coloração em diferentes regiões do espectro visível e eficiência eletrocrômica superior às dos filmes constituídos por hexaniobato ou PANI, indicando a características promissoras para utilização desses filmes híbridos em dispositivos eletrocrômicos. A melhoria das propriedades térmicas dos materiais híbridos foi comprovada pelos dados de TGA-MS e espectroscopia Raman, que mostraram claramente que a PANI na presença do hexaniobato permanece dopada após tratamento térmico (150 °C por 90 min), enquanto que polímero prístino sofre desdopagem. Estas características são consideradas importantes para o desenvolvimento de dispositivos que envolvam processamento térmico dos materiais.This thesis presents spectroscopic characterization of organic-inorganic hybrid materials of polyaniline (PANI), a conducting polymer, and hexaniobate, a semiconducting metal oxide. The hybrid materials were prepared by different routes, which used nanoparticles of exfoliated hexaniobate (nanosheets or nanoscrolls), and PANI previously prepared (ex-situ routes), or by aniline polymerization in the presence of hexaniobate (in-situ routes). The main characterization techniques were resonance Raman (RR) and electron paramagnetic resonance (EPR) spectroscopies, scanning electron microscopy (SEM) and mass spectrometry coupled-thermogravimetric analysis (TGA-MS). However, other techniques such as electronic UV-VIS-NIR and vibrational infrared spectroscopies, and X-ray diffractometry were also employed to support the characterization and results. Scanning electron microscopy showed that PANI is adsorbed on the hexaniobate nanoscrolls in the hybrid materials prepared by different routes, such as layer-by-layer assembly, mixture of dispersions of the components, or in-situ polymerization of aniline in the presence of the hexaniobate nanoparticles. According to spectroscopic data, such morphologies promote higher doping degrees (protonation) of the polymer comparing with pure PANI (pristine PANI), associated to strong interactions between PANI and hexaniobate, which presents high acidic surfaces. The spectroscopic data also revealed that the electronic delocalization of the polaronic segments, and the relative populations of polarons and bipolarons are affected in the presence of hexaniobate, indicating that the interactions between the components may also induce conformational changes on the polymeric chains. By RR spectroscopy using different exciting radiations, it was possible to identify the chromophoric segments produced during aniline oligo/polymerization in media of different pHs than that used in conventional procedures for preparation of PANI (hydrochloric acid, 1 mol L-1). It was shown that the major product is 1,4-benzoquinone-monoimine moieties coupled to aminophenyl moieties, resulting in complex structures named aniline adducts. Moreover, RR spectroscopy was very useful for the identification of phenazine-type segments and polaron segments as minor reaction products. When aniline-APS reactions are performed in the presence of hexaniobate, \"aniline adducts\" are also the major products, but RR spectra showed that there is higher formation of polaron segments of PANI. Such results were associated to the high acidic surface of hexaniobate, showing once again the important role of the inorganic component to determine the structure of the hybrid materials. The investigation on the electrochromic properties of the layer-by-layer hybrid films showed intense color variation at different regions of the visible spectrum, and higher electrochromic efficiencies comparing with the films composed but only hexaniobate or PANI, indicating the promising features for development of electrochromic devices. The improvement of thermal properties of the hybrid materials was verified by TGA-MS data and Raman spectra, which clearly showed that PANI in the presence of hexaniobate remains in the doped form after thermal treatment (150 °C for 90 min), while the pristine polymer undergoes a dedoping process. These features are considered important for the development of devices which depend on the thermal processing of the materials

Aniline-1,4-benzoquinone as a model system for the characterization of products from aniline oligomerization in low acidic media

Increase in the pH medium of aniline polymerization is used for giving products of different morphologies, which are often wrongly attributed to PANI chains. Infrared and Raman spectroscopic data, supported by quantum chemical calculations, show that aniline-1,4-benzoquinone (AnBzq) is a model system for the characterization of the products of aniline oligomerization in low acidic media. The Raman spectra excited at different laser lines reveal the bichromophoric nature of AnBzq, whose absorption bands at 550 nm and 440 nm can be attributed to pi-pi* transitions of the delocalized benzoquinone and amino-phenyl moieties, respectively. (C) 2012 Elsevier B.V. All rights reserved.Brazilian agency CNPqBrazilian agency CNPqBrazilian agency FAPESPBrazilian agency FAPES

Spectroscopic Study on the Structural Differences of Thermally Induced Cross-Linking Segments in Emeraldine Salt and Base Forms of Polyaniline

This paper reports the spectroscopic study on the structural differences of thermally induced cross-linking segments in polyaniline in its emeraldine salt (PANI-ES) and base (PANI-EB) forms. Casting films of PANI-ES (ES-film) and PANI-EB (EB-film) were prepared and heated at 150 degrees C under atmospheric air for 30 min. Raman spectra excited at 632.8 nm of heated ES-film presented the characteristic bands of phenazine-like structures at 1638, 1392, and 575 cm(-1), whereas EB-film showed lower relative intensities for these bands. The lower content of phenazine-like segments in heated EB-film is related to residual polaronic segments from preparation procedures, as revealed by Raman. This statement was confirmed by a sequence of thermal and doping experiments in both films. Quantum-chemical calculations by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) showed that the phenazine-like structure presents the intense Raman band at 1350 cm(-1) due to heterocycle breathing mode, and the non-phenazine-like structure (substituted hydrophenazine-type) presents higher energy for HOMO-LUMO transition, indicating the lack of conjugation in the heterocycle compared with the phenazine-like structure. According to experimental and theoretical data reported here, it is proposed that only thermally treated PANI-ES presents phenazine-like rings, whereas PANI-EB presents heterocyclic non-aromatic structures.CNPqCNPqFAPESPFAPES