Comparison of two isostructural organic compounds, one metallic and the other insulating
Comparison of two isostructural organic compounds, one metallic and the other insulating
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DOI：
10.1103/physrevb.22.4960
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发表时间：
1980-11
影响因子：
3.7
通讯作者：
J. Torrance;J. Mayerle;K. Bechgaard;B. Silverman;Y. Tomkiewicz
中科院分区：
文献类型：
--
作者：
J. Torrance;J. Mayerle;K. Bechgaard;B. Silverman;Y. Tomkiewicz
有机给体六亚甲基四硫富瓦烯（HMTTF）与四氰基对苯醌二甲烷（TCNQ）及其四氟衍生物（TCNQF 4）形成电荷转移盐。本文报道了HMTTF-TCNQ F4的制备、结构和导电性能。发现该盐与相应的TCNQ盐是同构的，但在300 K下的电导率低约7个数量级，类似于先前发现的硒衍生物六亚甲基四硒富瓦烯（HMTSF）的类似盐的行为。为了确定电导率差异如此之大的原因，我们彻底检查了HMTTF-TCNQ F 4和HMTTF-TCNQ之间的分子和固态差异。比较了TCNQ F 4−和TCN Q−的超精细耦合常数、分子轨道能级、溶液光谱和电化学性质。对于HMTTF盐，我们比较的结构，电导率，马德隆能量，磁化率，和光谱。可以得出结论，电导率的大的差异是由电荷转移的程度的差异，而不是由分子内库仑排斥U的值的差异（如已建议）。因此，HMTTF-TCNQ是一种混合价金属，其电荷转移程度为ρ= 0.72; TCNQ F 4较强的电子亲和力导致HMTTF-TCNQ F 4中的电荷转移完成（ρ= 1），使其成为莫特绝缘体。此外，明确的证据表明，在TCNQ F 4堆栈上的U不是由HMTTF分子的激子极化率屏蔽。
The organic donor hexamethylenetetrathiafulvalene (HMTTF) forms charge-transfer salts with both tetracyano-p-quinodimethane (TCNQ) and its tetrafluoro derivative (TCNQ F 4). We report here the preparation, structure, and conductivity of HMTTF-TCNQ F 4. This salt is found to be isostructural with the corresponding TCNQ salt, but with a conductivity at 300 K approximately seven orders of magnitude lower, similar to the behavior previously found for the analogous salts of the selenium derivative, hexamethylenetetraselenafulvalene (HMTSF). In order to determine the cause of this large difference in conductivity, we have thoroughly examined the molecular and solid-state differences between HMTTF-TCNQ F 4 and HMTTF-TCNQ. The hyperfine coupling constants, molecular-orbital energy levels, solution optical spectra, and electrochemical properties of TCNQ F 4− and TCN Q− are compared. For the HMTTF salts, we compare the structures, conductivities, Madelung energies, magnetic susceptibilities, and optical spectra. It is concluded that the large difference in conductivity is caused by a difference in the degree of charge transfer, and not by a difference in the value of the intramolecular Coulomb repulsion U (as had been suggested). Thus, HMTTF-TCNQ is a mixed-valence metal with a degree of charge transfer of ρ= 0.72; the stronger electron affinity of TCNQ F 4 causes the charge transfer in HMTTF-TCNQ F 4 to be complete (ρ= 1), making it a Mott insulator. In addition, clear evidence is given that U on the TCNQ F 4 stack is not screened by the excitonic polarizability of the HMTTF molecules.