<i>ortho</i>-Phenylenes: Unusual Conjugated Oligomers with a Surprisingly Long Effective Conjugation Length

ortho-Phenylenes represent a fundamental but relatively unexplored class of conjugated molecular architecture. We have developed a robust synthetic approach to monodisperse o-phenylene oligomers which we have demonstrated by synthesizing a homologous series up to the dodecamer. The o-phenylenes exhibit complex conformational behavior but are biased toward a specific 2-fold-symmetric conformation which we believe corresponds to a stacked helix. Surprisingly, the series exhibits long-range delocalization, as measured by bathochromic shifts in UV/vis spectra. Although the overall magnitude of the shifts is modest (but comparable to some other classes of conjugated materials), the effective conjugation length of the series is approximately eight repeat units. The oligomers also exhibit an unusual hypsochromic shift in their fluorescence spectra with increasing length. The origin of these trends is discussed in the context of conformational analysis and DFT calculations of the frontier molecular orbitals for the series.邻亚苯基（ortho-Phenylenes）是一类基础但尚未得到充分探索的共轭分子结构类别。我们开发了一种可用于制备单分散邻亚苯基低聚物的稳健合成策略，并通过合成直至十二聚体的同系物系列验证了该策略的有效性。该类邻亚苯基展现出复杂的构象行为，但倾向于形成一种特定的二重对称构象，我们认为该构象对应一种堆叠螺旋结构。令人意外的是，该同系物系列展现出长程离域现象，这可通过紫外-可见（UV/vis）光谱中的红移（bathochromic shift）现象得以证实。尽管这类红移的整体幅度较为适中（但可与部分其他共轭材料类别相媲美），该系列的有效共轭长度约为8个重复单元。随着链长增加，这类低聚物的荧光光谱还呈现出一种反常的蓝移（hypsochromic shift）现象。我们结合构象分析以及该系列低聚物的前线分子轨道密度泛函理论（DFT）计算结果，对上述趋势的起源进行了讨论。