School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, Jinan, 250100 (P.R. China), Fax: (+86) 531-8856-4464.
Chem Asian J. 2013 Dec;8(12):3164-76. doi: 10.1002/asia.201300872. Epub 2013 Sep 10.
Three series of organoboron-based molecules, including biphenyls 1a-c, diphenylacetylenes 2a-c, and stilbenes 3a-c, in which the electron-accepting boryl and the electron-donating amino groups are introduced at different positions, have been comprehensively investigated to explore the effect of the substitution pattern on the intramolecular charge-transfer emissions. In cyclohexane solution, the change of substitution pattern from p,p' to o,p' by introduction of boryl at the lateral o-position rather than the terminal p-position leads to bathochromism in the absorption and emission spectra. With further variation of the amino position from the terminal p'-position in o,p'-substitution to the lateral o'-position in an o,o'-substitution pattern, a blueshift was observed in the absorption owing to the less-efficient conjugation extension of the amino group as the result of sp(3) hybridization. It is notable that the emission of the three series of molecules changes with completely different trends. Only the emission of the biphenyl is redshifted further from o,p'-substituted 1b to o,o'-substituted 1a, whereas o,o'-substituted diphenylacetylene 2a maintains almost the same spectrum as that of o,p'-substituted diphenylacetylene 2b and the fluorescence of o,o'-substituted stilbene 3a is even blueshifted compared with o,p'-substituted stilbene 3b. As a result, the o,o'-substituted biphenyl 1a shows the longest emission wavelength despite the limited conjugation of the parent biphenyl skeleton. The long emission wavelength of 1a may arise from its extremely twisted structure, which would cause a significant structural relaxation in the exited state. In the solid state, 1a still keeps almost the longest emission wavelength. In addition, its quantum yield is also among the highest. The unusual properties, intense solid-state emission together with long emission wavelength, and particularly large Stokes shift, which are difficult to attain by structural modification of other parent π-conjugated frameworks, have been achieved by the introduction of boryl and amino groups at the o,o'-positions of the biphenyl skeleton.
已经全面研究了包括联苯 1a-c、二苯乙炔 2a-c 和二苯乙烯 3a-c 在内的三组基于有机硼的分子，其中吸电子的硼基和供电子的氨基分别位于不同的位置。通过引入硼基来改变取代模式（从 p,p'变为 o,p'），取代基位于侧 o-位而不是末端 p-位，会导致吸收和发射光谱的红移。进一步将氨基的位置从 o,p'-取代的末端 p'-位变为 o,o'-取代的邻位 o'-位，由于氨基的共轭延伸效率降低，导致吸收发生蓝移，这是 sp(3)杂化的结果。值得注意的是，这三组分子的发射变化趋势完全不同。只有联苯的发射进一步红移，从 o,p'-取代的 1b 变为 o,o'-取代的 1a，而 o,o'-取代的二苯乙炔 2a 保持与 o,p'-取代的二苯乙炔 2b 几乎相同的光谱，并且 o,o'-取代的二苯乙烯 3a 的荧光甚至比 o,p'-取代的二苯乙烯 3b 蓝移。因此，尽管母体联苯骨架的共轭有限，但 o,o'-取代的联苯 1a 表现出最长的发射波长。1a 的长发射波长可能源于其极其扭曲的结构，这会导致激发态发生显著的结构弛豫。在固态中，1a 仍保持几乎最长的发射波长。此外，其量子产率也很高。通过在联苯骨架的 o,o'-位引入硼基和氨基，可以实现这些不寻常的性质，包括强烈的固态发射和长发射波长，以及特别大的斯托克斯位移，这很难通过其他母体 π-共轭骨架的结构修饰来实现。
