Synthesis, Self-assembly Behavior And Opto-electric Properties Of Porphyrin Derivatives Tethered By Fullerene C₆₀

Porphyrins have attracted increasing attention due to their electrochemical and photophysical properties, which can be tuned upon insertion of different metal ions and by changing the substituent on the porphyrin core. These unique properties include photo stability, intense photoluminescence, large molar absorption coefficients, good light-harvesting, and high fluorescence quantum yield. Fullerene C6o, on the hand is used as a spherical electron acceptor due to their unique photophysical, electrochemical and chemical properties. Fullerene C60has been shown to reversibly accept up to six electrons in solution and accelerate charge separation and slow down charge recombination versus donor-acceptor (DA) dyads. The most important feature of porphyrins and fullerene C6o is that they spontaneously attract each other mainly through dispersive and donor-acceptor interactions. Therefore, porphyrin as a donor and fullerene C6o as an acceptor seems an ideal combination for application in organic solar cells.This thesis focuses on the synthesis, spectroscopic studies and self-assembly behavior of porphyrin derivatives linked to fullerene C60, which included COOH groups in its fullerene unit so as to promote their anchoring onto TiO2of the dye sensitized solar cells. Since the porphyrin derivatives-fullerene C60dyad molecules can interact with TiO2nanoparticles using the COOH groups in the fullerene unit, the photoinduced electrons produced by the porphyrin unit would be transferred to the fullerene C6o unit and from thence to the nanoporous TiO2. The four compounds in this research are5-(4-hydroxyphenyl)-10,15,20-tri-(4-methylphenyl)-H2porphyrin(1),5-(4-hydroxyphenyl)-10,15,20-tri-(4-methylphenyl)-Zinc-porphyrin (2),4-methylphenyl-H2-porphyrin-fullerene C60-COOH (3) and4-methylphenyl-zinc porphyrin-fullerene C60-COOH (4). Their solution self-assembly has been widely studied, and opto-electric properties have been researched.The synthesis and characterization of a series of compounds in which porphyrin and fullerene C60are coupled were done, in particular, the two series of compounds; free metallization compounds and zinc-metallization compounds. Characterization in solution (1H NMR and13C NMR spectroscopy) and FT-IR spectroscopy proves the identity of these compounds under investigation.The solution self-assembly and opto-electric properties of H2Por-OH (1) and H2Por-C6o-COOH (3) were investigated by UV-Vis absorption spectra, fluorescence spectra, electrochemical measurements and power conversion efficiency. The UV-Vis absorption spectra of H2Por-C60-COOH (3) exhibit a blue shift as compared to those of H2Por-OH (1). The molar absorption coefficients of the two compounds were more than5.5×105M-1cm-1. There was quenching in fluorescence intensity of H2Por-C60-COOH (3) as compared to that of H2Por-OH (1) at the same concentration, suggesting a probable intramolecular electron transfer between porphyrin and fullerene C6o. The electrochemical measurements showed that H2Por-C60-COOH (3) span narrower range (1.60eV) than H2Por-OH (1)(2.55eV), hence the electron transfer in H2Por-C60-COOH (3) is faster than H2Por-OH (1). The power conversion efficiency (η) value was further enhanced by introducing fullerene C60in the porphyrin donor, whereby0.49%for H2Por-C60-COOH (3) and0.18%for H2Por-OH (1), which was about two times larger than that of their corresponding porphyrin donor compound. UV-Vis absorption spectra and fluorescence spectra in single solvent and bi-solvent were used to study self-assembly behavior. Detailed spectral studies reveal that the two compounds showed polarity-dependent, solvent-dependent and temperature-dependent. The results showed that J-type aggregation occurred in chloroform at high concentration and/or at low temperature in toluene solution.Insertion of zinc ions into the core of the porphyrin donor tends to change the opto-electric properties of their precursors. The solution self-assembly and opto-electric properties of ZnPor-OH (2) and ZnPor-C60-COOH (3) were investigated. The UV-Vis absorption spectra of ZnPor-C60-COOH (4) exhibit a blue shift as compared to ZnPor-OH (1). The molar absorption coefficients of the two compounds was more than5.5×105M-1cm-1, hence they all exceed1×105M-1cm-1, which was the highest among the organic compounds. There was quenching in fluorescence intensity of ZnPor-C60-COOH (4) as compared to that of ZnPor-OH (2) under the same concentration, suggesting a probable intramolecular electron transfer between zinc-porphyrin and fullerene C60. The electrochemical measurements showed that ZnPor-C60-COOH (4) span narrower range (1.40eV) than ZnPor-OH (2)(2.30eV), hence the electron transfer in ZnPor-C60-COOH (4) is faster than ZnPor-OH (2). The power conversion efficiency (η) value was further enhanced by introducing fullerene C60in the Zinc-porphyrin donor, whereby0.56%for ZnPor-C60-COOH (4) and0.25%for ZnPor-OH (2), which was about two times larger than that of their corresponding porphyrin donor compound. The UV-Vis spectra and fluorescence spectra in single solvent and bi-solvent were used to study self-assembly behavior. Detailed spectral studies of the two compounds showed polarity-dependent, solvent-dependent and temperature-dependent. The results showed that J-type aggregation occurred in chloroform at high concentration and/or at low temperature in toluene solution.The power conversion efficiency (η) value was further enhanced by introducing fullerene C60in the porphyrin donor, which is about two times larger than that of their corresponding porphyrin donor compounds. The H2Por-C60-COOH (3) gave efficiency (η) of0.49%, while ZnPor-C60-COOH (4) gave efficiency (η) of0.56%, these efficiencies are still low, which may have been caused by the presence of solvent impurity in the dye, dye aggregation or electron recombination with oxidized dye molecules. The COOH group promotes the compound adsorption onto the TiO2yielding higher photocurrent per molecule by photosensitization process, which may probably prove that H2Por-C60-COOH (3) and ZnPor-C60-COOH (4) may have a potentially application in dye-sensitized solar cells.