Department of Chemistry, KAIST, Daejeon, 305-701, Republic of Korea.
Dalton Trans. 2014 Apr 7;43(13):4978-85. doi: 10.1039/c3dt52465e.
1,4-Di-(1-Ar-o-carboran-2-yl)benzene (Ar = 3,5-bis(trifluoromethyl)phenyl (1), phenyl (2), 4-n-butylphenyl (3), 4-N,N-dimethylaniline (4)) compounds that are electronically modulated at the C1-position of o-carborane with the electron-withdrawing or -donating aryl groups were prepared and characterized. The X-ray crystal structures of 1, 3, and 4 reveal that the two aryl groups on the C1-carborane carbon atoms are oppositely positioned, featuring overall C2-symmetry, and the C1-C2 bond length of carborane increases with the increasing order of electron-donating effect of an aryl group. UV-vis absorption spectra exhibit small low-energy absorption bands at around 275-300 nm for 1-3 while 4 shows a broad absorption tail at 350-400 nm. Although 1-4 show virtually no emission in solution, an intense aggregation-induced emission over the region ranging from 400-700 nm is observed in the solid state. Importantly, the emission wavelengths of 1-4 exhibit an apparent red-shift upon changing the aryl substituent from the CF3 to the NMe2 group (from 1 to 4). TD-DFT calculations suggest that the low-energy electronic transition is attributed to the intramolecular "through-space" charge transfer between the appended aryl group (HOMO) and the central phenylene ring (LUMO), and the greater change in the HOMO level by the substituent than that in the LUMO is responsible for the emission color tuning.
1,4-双-(1-芳-o-卡硼烷-2-基)苯（Ar = 3,5-双(三氟甲基)苯基(1)、苯基(2)、4-正丁基苯基(3)、4-N,N-二甲基苯胺(4))化合物是通过在 o-卡硼烷的 C1-位置上用吸电子或供电子芳基取代基对电子进行调制而制备和表征的。1、3 和 4 的 X 射线晶体结构表明，C1-卡硼烷碳原子上的两个芳基基团处于相反的位置，具有整体 C2 对称性，并且卡硼烷的 C1-C2 键长随着芳基供电子效应的增加而增加。紫外-可见吸收光谱在 275-300nm 左右显示出小的低能量吸收带，而 4 在 350-400nm 处显示出宽的吸收尾巴。尽管 1-4 在溶液中几乎没有发射，但在固态中观察到强烈的聚集诱导发射，其范围从 400-700nm。重要的是，1-4 的发射波长在改变芳基取代基从 CF3 到 NMe2 基团时（从 1 到 4）明显红移。TD-DFT 计算表明，低能量电子跃迁归因于附加的芳基基团（HOMO）和中心亚苯基环（LUMO）之间的分子内“通过空间”电荷转移，取代基对 HOMO 水平的变化大于对 LUMO 的变化是发射颜色调谐的原因。
