Synthesis And Properties Of A Series Of Meso-tetra(4-alkylamidophenyl) Porphyrin Liquid Crystals And Their Metal Complexes

Porphyrins and metalloporphyrins have received extraordinary attention in recent years for using these compounds in photodynamic therapy, optoelectronic devices, sensors, molecular logic devices, and artificial solar energy harvesting and storage schemes. The porphyrin skeleton has an extendedπ-conjugation system, leading to a wide range visible light absorptions and p-type properties as an electronic system. Thus, the control of orientation of porphyrin chromophores can play a very important role.Porphyrin liquid crystal is an important member of the liquid crystals family. The first liquid crystalline porphyrin derivative was reported by Goodby et al. in 1980. Since then, many mesogenic porphyrins have been synthesized and their mesomorphicproperties were investigated. But up to now, most of the side chains of TPP derivative liquid crystals are alkyl, alkoxy, or acyloxy group, those bearing the alkylamido group as side chains porphyrin liquid crystals have been reported little. Whether the alkylamidophenylporphyrins show liquid crystal properties or not and what kind of liquid crystals they are asise our interest. So a series of 5,10,15,20-tetra(p-alkylamidophenyl)porphyrin ligands and their metal complexes were synthesized and their liquid crystal properties were studied.In this paper, six alkylamidophenylporphyrin ligands and their sixteen transition metal complexes were synthesized and characterized by means of elemental analysis, MS, UV-Vis, IR and 1H NMR.. Furthermore, we have investigated these compounds by Resonance Raman spectra, cyclic voltammetry, thermal studies and fluorescence spectroscopy. These studies will provide wider ground for choice and application of the materials. Our work includes the following aspects:1. A seires of 5,10,15,20-tetra(p-alkylamidophenyl)porphyrin ligands and their sixteen transition-metal complexes were synthesized. All of them have been characterized, assigned and analyzed by elemental analysis, MS, UV-Vis, IR and 1H NMR. 2. The electrochemical, photochemical, photophysical and thermal properties of the compounds have been analyzed by cyclic voltammetry, Raman spectra, fluorescence spectra, TG/DTA, SPS and FISPS. The effects of chain length on their properties have also been studied.(1) Electrochemical properties: The spectrum shapes of Ni, Cu, Zn porphyrin complexes are similar to the porphyrin ligands, which show the redox of the porphyrin ring. The cyclic voltammograms of Mn and Fe porphyrin complexes are different from the ligands and other transition metal complexes, which showed not only the redox of the porphyrin ring, but also the redox of the metal ion. Different chain length has little influence on the redox property of the porphyrin because their substituent constants of the alkylamido groups are almost identical and the size of porphyrin has little effect on the redox potentials. The complexes are easier to reduce than the free-base porphyrin.(2) Fluorescence emission spectra: for the porphyrin ligands and Zn porphyrin complexes with different length of side chains, the fluorescence intensity and quantum yield changed rulelessly. The fluorescence intensities of other porphyrin complexes are much weaker than that of the porphyrin ligands and Zn porphyrin complexes. The result is same with the reference and confirmed that the transitional metal on centre of pophyrin ring quench the fluorescence of porphyrin(3) Raman spectra: the 1554 cm-1 band of porphyrin ligands is assigned to Cβ?Cβstretchν2, which is up-shifted to 1570 cm-1 in Mn complexes and down-shifted to the 1545 cm-1 in Zn complexes. The band at about 1370 cm-1 of porphyrin ligands is another important structure sensitive band, which is assigned to theν4,ν(Cα-N) /ν(CαCβ). Theν4 mode redshift to 1369 cm-1 of Mn complexes and buleshift to 1339 cm-1 because the effect of the metal ion. The 390 cm-1 band of metal complexes are assigned to the vibration of M?N. The band at about 365 cm-1 of Mn complexes is assigned to the vibration of N?Mn. In addition, the chain length has little influence on the Raman spectra of the porphyrin.(4) The surface photovoltage property: By SPS and FISPS, the photophysical properties of porphyrins were studied. The results show that porphyrins are p-type semiconductor. The band of SPS represents the transition ofπ→π*. The SPS of all the free-base porphyrins and their complexes are similar with their corresponding UV-Vis spectrum.(5) Thermal studies: Porphyrin ligands and Mn complexes are stable up to nearly 300 oC, and the decomposition of the compounds is a continuous process. They were decomposed completely in the range of 300?600 oC, involved the loss of side chains and the collapse of the whole porphyrin framework. 3. The liquid crystal properties were investigated by DSC, POM and XRD and the results show that only the porphyrin ligands with long side chains (more than 12 carbon atoms) show liquid crystalline behavior, and they exhibit a high phase transition temperature and a broad mesophase temperature span. The liquid crystals show a discotic lamellar structure. While the other porphyrin ligands and all the metal porphyrin complexes are not liquid crystals.