Publicação
Innovative π-conjugated heterocyclic systems as optical chemosensors and two-photon absorbing (TPA) chromophores: design, synthesis, characterization and development for applications
| Resumo: | Molecular receptors are chemical species designed to achieve a high degree of complementarity with a selected guest. When synthetic receptors are coupled with certain signaling units that are capable of changing one or more physical properties upon receptor-guest interaction (e.g. color, fluorescence, redox potential), a molecular sensor is obtained. In this field, it is especially appealing to employ optical outputs such as changes in color and/or fluorescence that allow the use of lowcost and widely available instrumentation. In recent years there has been considerable effort dedicated to design, synthesize and apply novel molecular probes that selectively and specifically respond to the presence of specific analytes (ions and neutral molecules). Optical response (fluorescence/color) allows the highest sensitivity (up to detection of single molecules) in sensory applications. Recently, increasing interest in the design of NLO chromophores for two photon absorption (TPA) applications has been motivated by the numerous and varied applications of TPA phenomenon in fields such as materials science (3D data storage and microfabrication, optical power limiting), biology and medicine (fluorescence imaging, two-photon microscopy (TPM), photodynamic therapy). For in vivo imaging, two-photon processes allow finer resolution and in-depth tissue penetration, with reduced cell damage. High two-photon excited fluorescence (TPEF) quantum yields, good photostability, adequate solubility, and optimized response wavelengths are also required in fields such as fluorescence microscopy or upconversion lasing. Incorporation of heterocycles into π-conjugated systems allows the fine tuning of electrooptical properties and often results in strong fluorescence, mandatory for certain TPA-based applications. The work presented in this thesis reports the synthesis and characterisation of heterocyclic compounds as well as the evaluation of their photophysical properties having in mind their potential application as colorimetric and/or fluorimetric chemosensors for anions and cations and as TPA fluorophores. Various heterocyclic compounds with improved functionalities, such as fluorescence and the ability to recognize anions and cations, were synthesized from easily obtained precursors and low cost commercially available reagents. Different combinations of donor and/or acceptor groups and π-conjugated bridges were studied in order to obtain more efficient optical systems. New unnatural amino acids were also obtained by synthetic manipulation in the side chain of natural amino acids with the incorporation of sulfur, nitrogen and oxygen heterocycles and their chemosensory ability evaluated in the presence of cations. The heterocycles used in the present work were thiophene, furan, imidazole, benzoxazole and benzothiazole, and with these π-conjugated systems of heterocyclic nature, the primary aim was to improve the photophysical properties of the resulting systems, and to optimize the recognition of the target analytes by introducing additional binding sites in the overall structure and the TPA capacity. All heterocyclic compounds were completely characterized by the usual characterization techniques (proton and carbon nuclear magnetic resonance spectroscopy, UV-vis absorption and fluorescence spectroscopy, infrared spectroscopy and mass spectrometry and/or elemental analysis). The photophysical properties of the synthesized compounds were evaluated in organic solvents of different character and in aqueous mixtures, and in some cases, related to the values of * proposed by Kamlet and Taft. In general, the novel compounds displayed high fluorescence quantum yields, making them good candidates for application as fluorimetric chemosensors and as efficient TPA fluorophores. In order to evaluate the sensing ability of the compounds towards organic and inorganic anions (AcO-, F-, Cl-, Br-, CN-, NO3-, BzO-, H2PO4- and HSO4-) and alkaline, alkaline-earth and transition metal cations (Na+, Cu2+, Ca2+, Co2+, Pd2+, Ni2+, Hg2+, Zn2+, Fe2+, Fe3+ and Cr3+) spectrophotometric/fluorimetric titrations were carried in acetonitrile and aqueous mixtures. Studies have shown that these compounds can be used as chemical sensors for AcO-, F-, CN-, BzO-, H2PO4 -, Cu2+, Ni2+, Hg2+ and Fe3+. The association constants and the stoichiometry of the formed complexes were calculated by HypSpec software and the stoichiometry was confirmed by Job’s plots. The binding process of the ions was also studied using 1H NMR spectroscopy. The TPA study for some heterocyclic derivatives, in ethanol, revealed that compounds functionalized with strong acceptor groups (CN, NO2) and/or containing a bithienyl spacer exhibited the highest TPA cross-section (σ2) values. |
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| Autores principais: | Ferreira, Rosa Cristina Moutinho |
| Assunto: | Ciências Naturais::Ciências Químicas |
| Ano: | 2017 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso restrito |
| Instituição associada: | Universidade do Minho |
| Idioma: | inglês |
| Origem: | RepositóriUM - Universidade do Minho |
| Resumo: | Molecular receptors are chemical species designed to achieve a high degree of complementarity with a selected guest. When synthetic receptors are coupled with certain signaling units that are capable of changing one or more physical properties upon receptor-guest interaction (e.g. color, fluorescence, redox potential), a molecular sensor is obtained. In this field, it is especially appealing to employ optical outputs such as changes in color and/or fluorescence that allow the use of lowcost and widely available instrumentation. In recent years there has been considerable effort dedicated to design, synthesize and apply novel molecular probes that selectively and specifically respond to the presence of specific analytes (ions and neutral molecules). Optical response (fluorescence/color) allows the highest sensitivity (up to detection of single molecules) in sensory applications. Recently, increasing interest in the design of NLO chromophores for two photon absorption (TPA) applications has been motivated by the numerous and varied applications of TPA phenomenon in fields such as materials science (3D data storage and microfabrication, optical power limiting), biology and medicine (fluorescence imaging, two-photon microscopy (TPM), photodynamic therapy). For in vivo imaging, two-photon processes allow finer resolution and in-depth tissue penetration, with reduced cell damage. High two-photon excited fluorescence (TPEF) quantum yields, good photostability, adequate solubility, and optimized response wavelengths are also required in fields such as fluorescence microscopy or upconversion lasing. Incorporation of heterocycles into π-conjugated systems allows the fine tuning of electrooptical properties and often results in strong fluorescence, mandatory for certain TPA-based applications. The work presented in this thesis reports the synthesis and characterisation of heterocyclic compounds as well as the evaluation of their photophysical properties having in mind their potential application as colorimetric and/or fluorimetric chemosensors for anions and cations and as TPA fluorophores. Various heterocyclic compounds with improved functionalities, such as fluorescence and the ability to recognize anions and cations, were synthesized from easily obtained precursors and low cost commercially available reagents. Different combinations of donor and/or acceptor groups and π-conjugated bridges were studied in order to obtain more efficient optical systems. New unnatural amino acids were also obtained by synthetic manipulation in the side chain of natural amino acids with the incorporation of sulfur, nitrogen and oxygen heterocycles and their chemosensory ability evaluated in the presence of cations. The heterocycles used in the present work were thiophene, furan, imidazole, benzoxazole and benzothiazole, and with these π-conjugated systems of heterocyclic nature, the primary aim was to improve the photophysical properties of the resulting systems, and to optimize the recognition of the target analytes by introducing additional binding sites in the overall structure and the TPA capacity. All heterocyclic compounds were completely characterized by the usual characterization techniques (proton and carbon nuclear magnetic resonance spectroscopy, UV-vis absorption and fluorescence spectroscopy, infrared spectroscopy and mass spectrometry and/or elemental analysis). The photophysical properties of the synthesized compounds were evaluated in organic solvents of different character and in aqueous mixtures, and in some cases, related to the values of * proposed by Kamlet and Taft. In general, the novel compounds displayed high fluorescence quantum yields, making them good candidates for application as fluorimetric chemosensors and as efficient TPA fluorophores. In order to evaluate the sensing ability of the compounds towards organic and inorganic anions (AcO-, F-, Cl-, Br-, CN-, NO3-, BzO-, H2PO4- and HSO4-) and alkaline, alkaline-earth and transition metal cations (Na+, Cu2+, Ca2+, Co2+, Pd2+, Ni2+, Hg2+, Zn2+, Fe2+, Fe3+ and Cr3+) spectrophotometric/fluorimetric titrations were carried in acetonitrile and aqueous mixtures. Studies have shown that these compounds can be used as chemical sensors for AcO-, F-, CN-, BzO-, H2PO4 -, Cu2+, Ni2+, Hg2+ and Fe3+. The association constants and the stoichiometry of the formed complexes were calculated by HypSpec software and the stoichiometry was confirmed by Job’s plots. The binding process of the ions was also studied using 1H NMR spectroscopy. The TPA study for some heterocyclic derivatives, in ethanol, revealed that compounds functionalized with strong acceptor groups (CN, NO2) and/or containing a bithienyl spacer exhibited the highest TPA cross-section (σ2) values. |
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