Professor LI Bin and his team at State Key Laboratory of Luminescence and Applications in CIOMP have developed red emissive 2D Cationic Covalent Organic Frameworks (COFs) with tunable porosity and pore sizes at nanoscale. This is the ﬁrst time that the stable cationic crystalline frameworks allowed for the fabrication of a series of charged COFs through ion exchange processes, which have potential applications in the fields of organic semiconductor devices and optoelectronic sensors. This work was recently published in Journal of the American Chemical Society (J. Am. Chem. Soc., 2016, DOI: 10.1021/jacs.5b13490). The first author is assistant professor Heping Ma. This work is supported by NSFC.
Two dimensional (2D) materials are emerging materials with many extraordinary properties. Especially in 2004, the discovery of atomic-scale graphene greatly promoted the development of two-dimensional materials, so that it quickly became the leading edge of material research. However, because graphene has almost no band gap, it has limited its application in semiconductor devices. Compared with graphene, two-dimensional semiconductor materials with natural band gap have great potential applications in microelectronics, photovoltaics and optoelectronics. Benefits of their plenty of organic light-emitting functional groups, adjustable photoelectron transfer rate and excellent photoelectric conversion properties, the 2D porous organic crystal materials have potential applications in the photoelectric sensor, organic semiconductor, field effect transistor and organic solar cells.
Although a few ionic amorphous porous organic materials with ion exchange properties exhibit promising applications in proton-conducting, it is still a challenge for crystalline Covalent Organic Frameworks (COFs) as a platform to regulate the conduction of protons or electrons. Constructing sufficiently stable charged COFs would open a door for these well-defined crystalline porous networks for a wide variety of applications, for example, as ion exchangers, ion conductors, solid electrolytes, and solid catalysts.
In the current study, Prof. Li’s team have designed and synthesized a cationic covalent organic framework with high thermal and chemical stability by combining a cationic monomer, ethidium bromide (EB), with 1,3,5-triformylphloroglucinol (TFP) in Schiff base reactions. The stable porous cationic organic framework can be able to load polyoxometalate anions so as to form novel proton-conducting materials.
After cooperation with prof. ZHU Guangshan and prof.ZANG Hongying in Northeast Normal University, they introduced keggin polyoxometalate (POM) anions into this porous cationic framework, which can greatly enhance the proton conductivity of ionic COF-based material. The hydrogen bond exchange between a keto-enamine group and the adsorbed H2O can form H3O+, which acts as the proton source for proton conduction. The increased proton conductivity of POM doped COF can be attributed to the formation of water clusters around POM anions.
Hydrophilic POM anions interacting with water molecules may provide interconnected hydrogen bonding networks within the 1D channel. The isotope-effect experiment and hydrogen pumping test proved the proton-conducting nature. EB-COF:POM shows the best proton conductivity at room temperature among reported COF materials. These properties suggest the cationic open COF framework can be an effective platform for ion exchange and proton conduction research. Researchers believe that the two dimensional ionic COFs are suitable for organic semiconductor devices and optoelectronic sensors research in the near future.
(a) Schematic Representation of the Synthesis of EB-COF:Br: (b) Top Views and (c) Side Views of the offset AA Stacking Structure of the EB-COF:Br
(Image by Dr.Heping Ma)
(a) Schematic of POM doping in COF. (b) Figure under the daylight and UV light of EB monomer. (Image by Dr.Heping Ma)
Proton conductivity of (a) EB-COF:Br and (b) EB-COF:POM in 97% RH condition. (c) RH dependence of the proton conductivity (σ) for COFs at 298 K. (d) Arrhenius-type plot of proton conductivity of EB-COF:PW 12 at various temperatures under ∼ 53% RH condition. (Image by Dr.Heping Ma)
Article link: http://pubs.acs.org/doi/abs/10.1021/jacs.5b13490
Prof.LI Bin, Changchun Institute of Optics, Fine Mechanics and Physics