Synthesis, Characterization And Photophyscial Properties Of Monodisperse Conjugated Oligocarbazoles

The design and optic-electronic properties of novelπ-conjugated systems have attracted increasing attention, since functional molecular materials have been proved their high potential applications in solar cell, organic light-emitting diodes (OLEDs), sensors, field-effect transistors, frequency converters, etc. As a result of its outstanding intense luminescence and high-mobility hole-transporting properties, carbazole has been used as a functional building block in the fabrication of organic photoconductors, NLO materials, and photorefractive materials. Chemically, carbazole can be easily functionalized at its 3-, 6-, or 9-positions and covalently linked to other molecular moieties, so it is not only a potential AB2 monomer for the construction of dendrimers, but also a potential AB monomer for the construction of linear oligomers.Herein, we have designed, synthesized and characterized a series of linear, star-shaped and dendritic carbazole-based oligomers, which were functionalized with porphyrin, subporphyrin, phosphorus(V) porphyrin and C60, to study their photoinduced intramolecular energy-transfer and electron-transfer processes, and intense luminescence properties. Some creative results have been obtained, and the main results are outlined as following:(1) Versatile synthetic routes to subporphyrins with dendritic carbazole arms T(Cz-Gn)SubPs (n = 1-3) by one-pot two-step reactions from pyridine-tri(pyrrol-1-yl)borane and the corresponding aldehydes have been explored. The intramolecular energy-transfer from the carbazole dendron to the subporphyrin core occurs with a high efficiency, which decreases with increasing generation of the dendron, in accord with the F?rster mechanism of energy transfer. The energy transfer in the higher generation dendritic subporphyrins occurs in several transfer steps. In addition, the light-harvesting abilities of these compounds increase with increasing generation, so the T(Cz-Gn)SubPs are efficient light harvesters. Meanwhile, these dendritic macromolecules emit intense yellow-green light and may be good candidates for photonic devices.(2) We have synthesized three dendritic carbazole-based porphyrins T(Cz-Gn)Ps (n = 0-2) with photoinduced intramolecular energy-transfer properties. The efficiency of the energy transfer (ΦET) decreases with the generation because the energy-transfer process followed the F?rster mechanism, and that the distance between the donor (carbazole dendron) and the acceptor (porphyrin core) might play an important role in the energy transfer process. The light-harvesting abilities of these compounds increase with increasing generation number, so the T(Cz-Gn)Ps are outstanding light-harvesting antennas. Meanwhile, these dendritic macromolecules could emit intense red light with high fluorescence quantum yields (0.199-0.215) which is around twice of that of TPP, and so might be good candidates for photonic devices.(3) For the first time, three axial phosphorus(V) porphyrins with dendritic carbazoles [P(TPP)(O-Gn-Cz)2]Cl (n = 1-3) have been designed and synthesized. These dendrimers have good solubility and well film-formation, which facilitate their applications in the construction of devices. The axial dendritic substituents can efficiently suppress co-facialπ-πintermolecular interactions, creating effective site-isolation effect. Their non-fluorescence caused by the photoinduced electron transfer may lead to potential applications as photovoltaic materials in solar cells. The IPCE spectra of these dendrimers demonstrate that the structure of the dendrimers could significantly affect their photovoltaic response to the visible light, and at the same absorption their photovoltaic response ability is in the order as G2 > G3 > G1.(4) A facile approach to the synthesis of a series of carbazole-based dendrons with a aldehyde and the corresponding [60]fullerene adducts have been developed. Photoinduced intramolecular electron transfer in these donor-acceptor systems was confirmed by steady-state fluorescent spectroscopy, and the electron transfer could be switched depending on excitation wavelength in toluene. In THF, a moderate polar solvent, the photoinduced electron-transfer via the excited states of C60 was not evidently detected in Cz-G1-C60 due to the lack of the electron-donating ability. With the increasing generation, the electron-donating ability increased, and the photoinduced intramolecular electron transfer can be observed in Cz-G2-C60 and Cz-G3-C60. But the photoinduced intramolecular electron transfer of Cz-G2-C60 was more efficient than Cz-G3-C60 because of the shorter distance between the perphery carbazole and C60 unit. These dendrimers may have potential applications as photovoltaic materials in solar cells.(5) For the first time, we have established a facile synthetic strategy for a series of novel monodisperse well-defined linear oligocarbazoles linked through 3,9-positions (OCAns), and incorporate them as arms into porphyrin core to construct novel nanosized star-shaped porphyrins T(OCAn)Ps (n = 1-5). The diameter of compound T(OCA6)P is 7.4 nm, to the best of our knowledge, which represents one of the largest known conjugated star-shaped systems. Because the emission band of monodisperse oligocarbazoles overlaps the absorption of the porphyrin, efficient photoinduced intramolecular energy transfer occurs from oligocarbazole arms to the porphyrin core. The energy transfer efficiency decreases with increasing length of the OCAns arms, although the light-harvesting ability of T(OCAn)Ps increases with increasing number of carbazole units due to F?rster energy-transfer mechanism. We estimate the longest distance for F?rster energy transfer is ca. 3 nm in such systems. Meanwhile, the obtained star-shaped porphyrins could emit intense red light with high fluorescence quantum yields, so they might be good candidates for photonic devices.