Park Jesse G, Aubrey Michael L, Oktawiec Julia, Chakarawet Khetpakorn, Darago Lucy E, Grandjean Fernande, Long Gary J, Long Jeffrey R
Department of Chemistry , University of California , Berkeley , California 94720 , United States.
Department of Chemistry, Missouri University of Science and Technology , University of Missouri , Rolla , Missouri 65409-0010 , United States.
J Am Chem Soc. 2018 Jul 11;140(27):8526-8534. doi: 10.1021/jacs.8b03696. Epub 2018 Jun 28.
Metal-organic frameworks are of interest for use in a variety of electrochemical and electronic applications, although a detailed understanding of their charge transport behavior, which is of critical importance for enhancing electronic conductivities, remains limited. Herein, we report isolation of the mixed-valence framework materials, Fe(tri)(BF) (tri = 1,2,3-triazolate; x = 0.09, 0.22, and 0.33), obtained from the stoichiometric chemical oxidation of the poorly conductive iron(II) framework Fe(tri), and find that the conductivity increases dramatically with iron oxidation level. Notably, the most oxidized variant, Fe(tri)(BF), displays a room-temperature conductivity of 0.3(1) S/cm, which represents an increase of 8 orders of magnitude from that of the parent material and is one of the highest conductivity values reported among three-dimensional metal-organic frameworks. Detailed characterization of Fe(tri) and the Fe(tri)(BF) materials via powder X-ray diffraction, Mössbauer spectroscopy, and IR and UV-vis-NIR diffuse reflectance spectroscopies reveals that the high conductivity arises from intervalence charge transfer between mixed-valence low-spin Fe centers. Further, Mössbauer spectroscopy indicates the presence of a valence-delocalized Fe species in Fe(tri)(BF) at 290 K, one of the first such observations for a metal-organic framework. The electronic structure of valence-pure Fe(tri) and the charge transport mechanism and electronic structure of mixed-valence Fe(tri)(BF) frameworks are discussed in detail.
金属有机框架因其在各种电化学和电子应用中的用途而受到关注，尽管对其电荷传输行为的详细理解（这对于提高电导率至关重要）仍然有限。在此，我们报告了通过对导电性较差的铁（II）框架Fe（tri）进行化学计量化学氧化而获得的混合价框架材料Fe（tri）（BF）（tri = 1,2,3 - 三唑；x = 0.09、0.22和0.33）的分离，并发现电导率随着铁氧化水平的提高而显著增加。值得注意的是，氧化程度最高的变体Fe（tri）（BF）在室温下的电导率为0.3(1) S/cm，与母体材料相比增加了8个数量级，是三维金属有机框架中报道的最高电导率值之一。通过粉末X射线衍射、穆斯堡尔光谱以及红外和紫外 - 可见 - 近红外漫反射光谱对Fe（tri）和Fe（tri）（BF）材料进行的详细表征表明，高电导率源于混合价低自旋铁中心之间的价间电荷转移。此外，穆斯堡尔光谱表明在290 K时Fe（tri）（BF）中存在价态离域的铁物种，这是对金属有机框架的此类观察中的首次观察之一。详细讨论了价态纯的Fe（tri）的电子结构以及混合价Fe（tri）（BF）框架的电荷传输机制和电子结构。
