Synthesis And Properties Of Dendrimers With Carbazole Derivatives Branches

Dendrimers are macromolecules that possess precisely defined three-dimensional architecture and monodispersed size with a core surrounded by different layers of dendrons stretching outward. These characteristics are much different from the linear or branched polymers, which suffer from the wide molecular weight distribution so that the batch to batch reproducibility often is a big problem for the property stability. Over the past decade, a number of dendrimers have been prepared by either a convergent or divergent approach for the specific application such as drug delivery, magnetic resonance imaging, homogeneous catalysis, optical data storage, solar cells and organic light-emitting device.The motivation of the present work is to synthesize a series of novel dendrimers from different core and boronic acid-terminated9-phenylcarbazole-based dendrons via Suzuki coupling reaction by the convergent approach. Because of the presence of the torsional angle between the benzene ring and carbazole ring as well as the bending angle of C-N-C bond, the9-phenylcarbazole-based dendrons are expected to become more twisted with the increase of generation, which will significantly influence the conformations and topological structures, and thereby alter the photophysical and electrochemical properties of the dendrimers. The synthesis and structure-property relationships of the dendrimers are investigated in detail. The main contents and results are as follows:Synthesis and properties of starburst dendrimers consisting of triphenylamine core and9-phenylcarbazole-based dendrons:Novel dendrimers consisting of a triphenylamine core and1st to3rd generations of9-phenylcarbazolebased dendrons were synthesized by Suzuki coupling reaction through convergent approach. Their structures were confirmed by two-dimensional correlated H-H COSY and C-H HSQC NMR spectra, MALDI-TOF MS and elemental analysis. The dendrimers exhibit excellent thermal stability with5%weight loss temperatures over540℃. The computer modeling reveals that the dendrons in dendrimers greatly twisted with the generation, leading to the dendrimers decreased crystalline ability. Of interest is the observation that, for an identical dendrimer, the solid film displays the similar UV absorption and luminescence emission profiles to the solution sample, indicating that, after evaporation of solvent, the rigid dendrimer can well maintain its conformational morphology and the aggregation or stacking of the chromophoric groups is significantly inhibited. All the dendrimers can emit intense fluorescence with narrow full width at half maximum (FWHM) around46-50nm. Moreover, with the incremental generation, the quantum efficiencies remarkably increase from64to95%, suggesting that the highly contorted and bulky dendrons effectively decrease energy wastage and non-radiative decay. The synergistic effect of electron-donating triphenylamine core and9-phenylcarbazole-based dendrons results in the HOMO energy level of-5.36eV for the3rd-generation dendrimer, very close to the work function of the ITO/PEDOT electrode (-5.2eV), which characteristic is very advantageous for the hole injection and transport materials.Synthesis and properties of starburst dendrimers consisting of pyrene core and9-phenylcarbazole-based dendrons:Three generations of dendrimers (P1, P2and P3) with a pyrene core and9-phenylcarbazole derivatives dendrons have been designed and synthesized by convergent approach. Their chemical structures were well characterized by means of1H NMR, IR spectroscopy, MALDI-TOF MS and elemental analysis. The studies revealed interesting effects of topological variation with incremental generation number on the thermal, photophysical and electrochemical properties of the dendrimers. The results showed that they displayed excellent thermal stability with5%weight-loss temperature up to610℃and high glasstransition temperature (260℃for P3). The HOMO and energy band gaps of materials could be modulated by changing the generation of the dendrimers. Compared to P1, the higher generation P3had significantly enhanced photoluminescent quantum yield from86.9to99.5%, and exhibited good stability for hole and electron injection. The luminescence spectra of P1showed that, relative to the solution sample, the emission peaks of the solid dendrimer film were apparently broadened and red-shifted, indicating the strong π-π stacking effect between the pyrene moieties. By doping1.5%of the dendrimer in4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), a light-emitting diode device was fabricated in the ITO/NPB/NPB:PI (1.5%)/TPBI/Mg.Ag configuration, which emitted a white color with Commission Internationale de L’Eclairage (CIEx,y) coordinates of (0.29,0.34) and a maximum brightness of1300cdm-2, exhibiting promising potential in white light-emitting diode application.Synthesis and properties of starburst dendrimers consisting of tetraphenylsilane core and9-phenylcarbazole-based dendrons:Three generations of novel dendrimers (Sil, Si2and Si3) with a tetraphenylsilane core and9-phenylcarbazole-based dendrons have been successfully synthesized. These dendrimers possess excellent thermal stability with5%weight-loss temperatures in the range of574to622℃. Compared to the lower generation of dendrimer, Si3exhibits a high glass transition at271℃, indicating that the high content of9-phenylcarbazole results in a dendrimer with significantly enhanced rigidity. The contorted dendrons effectively inhibit the aggregation of conjugative chromophore groups, leading to a fluorescence quantum yield of up to99.1%. Moreover, all the dendrimers have high energy band gaps, a characteristic which is especially desirable for host materials in the fabrication of organic light-emitting devices.