Estrutura e propriedades ópticas de difenilfosfinatos de terras raras
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Data
2018-07-27
Autores
Bim, Luís Felipe Bricks [UNESP]
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Editor
Universidade Estadual Paulista (Unesp)
Resumo
A busca por compostos poliméricos altamente porosos e cristalinos tornou-se uma realidade em pesquisas ao redor do mundo e tem despertado, nas últimas três décadas, um interesse crescente não só da comunidade científica como de diversas áreas da tecnologia, devido às suas estruturas supramoleculares e topologias estruturais atraentes e potencialidades para aplicações tecnológicas inovadoras de alto valor agregado. Contudo, sínteses de precipitação por via úmida ou por métodos hidro/solvotérmicos podem suscitar a formação de redes de coordenação porosas altamente insolúveis, o que dificulta a obtenção de monocristais passíveis de serem facilmente caracterizados por difração de raios X. Difenilfosfinatos de terras taras – [TR(dpp)3]n , sintetizados pela reação direta entre um cloreto de terra rara (TRCl3) e o ácido difenilfosfínico (Hdpp), são exemplos autênticos que integram esta classe de materiais altamente insolúveis e que não produzem monocristais. O objetivo central deste trabalho foi sintetizar compostos de coordenação envolvendo cátions trivalentes de terras raras e o ligante organofosforado difenilfosfinato através de 4 diferentes metodologias de precipitação por via úmida – síntese de precipitação por via úmida (PVU); assistida por ultrassom (U-PVU); assistida por modulador (M-PVU); com tratamento solvotérmico (S-PVU) – e caracterizá-los, investigando e determinando suas propriedades estruturais e espectroscópicas. Tais metodologias permitiram a obtenção de polímeros de coordenação nanoestruturados 1D1, isoestruturais, com partículas em formato de nanobastões e/ou nanofios, nos quais coexistem os modos de coordenação ponte e quelato e com rendimentos globais de reação sempre superiores a 90%. Ademais, a concepção desses [TR(dpp)3]n como estruturas oligoméricas com, ao menos, três unidades de repetição, foi pela primeira vez evidenciada. Dados provenientes das análises elementar dos [TR(dpp)3]n, aliados aos das titrimetrias de complexação, permitiram confirmar a proporção metal:ligante 1:3 como a envolvida na formação dos compostos da série dos difenilfosfinatos de terras raras. As metodologias U-PVU, M-PVU e S-PVU propiciaram a nucleação homogênea e um maior controle da taxa de recristalização para a preparação dos [TR(dpp)3]n, levando a um aumento da cristalinidade dos materiais e uma melhoria em suas qualidades morfológicas. De fato, as imagens de MEV-FEG revelaram controle da forma e tamanho das partículas dos [TR(dpp)3]n em função da metodologia empregada. Assim, modificações na cinética de precipitação e na taxa de formação das fases cristalinas levaram a distribuição de íons TR3+ preferencialmente em sítios de alta simetria (SAS). De todas as metodologias testadas, a que melhor ordenou a estrutura dos [Eu(dpp)3]n foi a S-PVU, reduzindo significativamente a contribuição dos sítios de baixa simetria para a emissão. De maneira inédita, a existência de mais de um tipo de sítio de simetria ocupado pelo íon Eu3+ em uma matriz pura de [Eu(dpp)3]n também foi reportada. Por fim, a emissão característica do íon Er3+, à temperatura ambiente, no composto [Er(dpp)3]n permitiu pressupor que difenilfosfinatos de terras raras sejam matrizes poliméricas pouco flexíveis, as quais não sofrem grandes influências de relaxações por vibração.
The search for highly porous and crystalline polymer compounds has become relevant in the research around the world and has generated, in the last three decades, a growing interest not only in the scientific community but also in several areas of technology. Main reasons for this are due to their supramolecular structures, aesthetically attractive structural topologies and potentialities for innovative technological applications of high added value. However, obtaining RE3+MOFs by wet route precipitation or hydro/solvothermal methods may lead to the formation of highly insoluble porous coordination networks, which makes it difficult to obtain single-crystals that can be easily characterized by X-ray diffraction. Rare earth diphenylphosphinates – [RE(dpp)3]n, synthesized by direct reaction between a rare earth chloride (RECl3) and diphenylphosphinic acid (Hdpp), are authentic examples that integrate this class of materials presenting very low solubilities that do not produce single crystals. The main objective of this work was to synthesize coordination compounds involving trivalent rare earth cations and the organophosphorous ligand diphenylphosphinate through 4 different wet route precipitation methodologies – wet route (WRS), ultrasound-assisted wet route (U-WRS), modulator-assisted wet route (M-WRS) and wet route synthesis with solvothermal treatment (S-WRS) – and to characterize them by investigating and determining their structural and spectroscopic properties. Such methodologies allowed the obtaining of nanostructured 1D1 isostructural coordination polymers with nanorod and/or nanowire shaped particles, in which bridge and chelate coordination modes coexist and with overall reaction yields always higher than 90%. In addition, the design of these [RE(dpp)3]n as oligomeric structures with at least three replicate units was first evidenced. Data from the elemental analyzes of [RE(dpp)3]n, together with complexation titrimetries results, allowed us to establish the 1:3 metal:ligand ratio as involved in the formation of [RE(dpp)3]n compounds series. U-WRS, M-WRS and S-WRS methodologies provided a homogeneous nucleation and a controlled recrystallization rate for [RE(dpp)3]n preparations, leading to a significant increase in the crystallinity of the material and an improvement in its morphological quality. In fact, SEM-FEG images revealed control of [RE(dpp)3]n shape and particle size n as a function of the methodology employed. Thereby, changes in precipitation kinetics and crystalline phases formation rate generated a distribution of RE3+ ions in higher symmetry sites. Of all the methodologies tested, the one that best ordered the structure of [RE(dpp)3]n was the S-PVU, significantly reducing the contribution of the less symmetrical site to the emission. In an unprecedented way, the existence of more than one type of symmetry site occupied by Eu3+ ion in a [Eu(dpp)3]n pure matrix has also been reported. Finally, the characteristic emission of the Er3+ ion in the compound [Er(dpp)3]n, at room temperature, has allowed us to assume that rare earth diphenylphosphinates are poorly flexible polymer matrices, which are not influenced by vibrational relaxations.
The search for highly porous and crystalline polymer compounds has become relevant in the research around the world and has generated, in the last three decades, a growing interest not only in the scientific community but also in several areas of technology. Main reasons for this are due to their supramolecular structures, aesthetically attractive structural topologies and potentialities for innovative technological applications of high added value. However, obtaining RE3+MOFs by wet route precipitation or hydro/solvothermal methods may lead to the formation of highly insoluble porous coordination networks, which makes it difficult to obtain single-crystals that can be easily characterized by X-ray diffraction. Rare earth diphenylphosphinates – [RE(dpp)3]n, synthesized by direct reaction between a rare earth chloride (RECl3) and diphenylphosphinic acid (Hdpp), are authentic examples that integrate this class of materials presenting very low solubilities that do not produce single crystals. The main objective of this work was to synthesize coordination compounds involving trivalent rare earth cations and the organophosphorous ligand diphenylphosphinate through 4 different wet route precipitation methodologies – wet route (WRS), ultrasound-assisted wet route (U-WRS), modulator-assisted wet route (M-WRS) and wet route synthesis with solvothermal treatment (S-WRS) – and to characterize them by investigating and determining their structural and spectroscopic properties. Such methodologies allowed the obtaining of nanostructured 1D1 isostructural coordination polymers with nanorod and/or nanowire shaped particles, in which bridge and chelate coordination modes coexist and with overall reaction yields always higher than 90%. In addition, the design of these [RE(dpp)3]n as oligomeric structures with at least three replicate units was first evidenced. Data from the elemental analyzes of [RE(dpp)3]n, together with complexation titrimetries results, allowed us to establish the 1:3 metal:ligand ratio as involved in the formation of [RE(dpp)3]n compounds series. U-WRS, M-WRS and S-WRS methodologies provided a homogeneous nucleation and a controlled recrystallization rate for [RE(dpp)3]n preparations, leading to a significant increase in the crystallinity of the material and an improvement in its morphological quality. In fact, SEM-FEG images revealed control of [RE(dpp)3]n shape and particle size n as a function of the methodology employed. Thereby, changes in precipitation kinetics and crystalline phases formation rate generated a distribution of RE3+ ions in higher symmetry sites. Of all the methodologies tested, the one that best ordered the structure of [RE(dpp)3]n was the S-PVU, significantly reducing the contribution of the less symmetrical site to the emission. In an unprecedented way, the existence of more than one type of symmetry site occupied by Eu3+ ion in a [Eu(dpp)3]n pure matrix has also been reported. Finally, the characteristic emission of the Er3+ ion in the compound [Er(dpp)3]n, at room temperature, has allowed us to assume that rare earth diphenylphosphinates are poorly flexible polymer matrices, which are not influenced by vibrational relaxations.
Descrição
Palavras-chave
Polímeros de coordenação, Materiais nanoestruturados, Nanofios, Espectroscopia de luminescência, Metais de terras raras, Coordination polymers, Nanostructured materials, Nanowires, Luminescence spectroscopy, Rare earth metals