作者:Jian He、Sanyo M. Mathew、Sarah D. Cornett、Stephan C. Grundy、C. Scott Hartley
DOI:10.1039/c2ob07146k
日期:——
ortho-Phenylenes are an emerging class of helical oligomers and polymers. We have synthesized a series of pushâpull-substituted o-phenylene oligomers (dimethylamino/nitro) up to the octamer. Conformational analysis of the hexamer using a combination of low-temperature NMR spectroscopy and ab initio predictions of 1H NMR chemical shifts indicates that, like other o-phenylenes, they exist as compact helices in solution. However, the substituents are found to have a significant effect on their conformational behavior: the nitro-functionalized terminus is 3-fold more likely to twist out of the helix. Protonation of the dimethylamino group favors the helical conformer. UV/vis spectroscopy indicates that the direct charge-transfer interaction between the pushâpull substituents attenuates quickly compared to other conjugated systems, with no significant charge-transfer band for oligomers longer than the trimer. On protonation of the dimethylamino group, significant bathochromic shifts with increasing oligomer length are observed: the effective conjugation length is 9 repeat units, more than twice that of the parent oligomer. This behavior may be rationalized through examination of the frontier molecular orbitals of these compounds, which exhibit greater delocalization after protonation, as shown by DFT calculations.
邻苯基是一类新兴的螺旋低聚物和聚合物。我们合成了一系列推拉取代的邻苯基低聚物(二甲基氨基/硝基),直至八聚体。利用低温核磁共振光谱和 1H NMR 化学位移的 ab initio 预测相结合的方法对六聚体进行的构象分析表明,与其他邻苯基一样,它们在溶液中以紧凑的螺旋形式存在。然而,研究发现取代基对它们的构象行为有显著影响:硝基官能化的末端从螺旋中扭曲出来的可能性要高出 3 倍。二甲基氨基的质子化有利于螺旋构象。紫外/可见光谱显示,与其他共轭体系相比,推拉取代基之间的直接电荷转移相互作用迅速减弱,对于长于三聚体的低聚物来说,没有明显的电荷转移带。当二甲基氨基发生质子化时,随着低聚物长度的增加,会出现明显的浴色偏移:有效共轭长度为 9 个重复单元,是母体低聚物的两倍多。这种行为可以通过研究这些化合物的前沿分子轨道得到合理解释,正如 DFT 计算所显示的那样,质子化后,前沿分子轨道表现出更大的脱位。