In aromatic systems, π–π interaction plays a central role in determining the stacking geometry and binding strength of molecules and thus a detailed microscopic understanding is highly desirable. Herein, by using scanning tunneling microscopy with submolecular resolution complemented with first-principles calculations based on density functional theory, we report the atomic-scale imaging of π–π interaction in nonplanar phthalocyanine (Pc) bilayers on different substrates, including graphite and Au(111) with weak interaction and Cu(111) with strong binding. We reveal that nonplanar Pc of the second layer on all substrates exhibits an in-plane rotation angle of 15° with a parallel offset of 1.19 Å, which minimizes π–π repulsion. Interestingly, on Cu(111), it is found that the inequivalent charge distribution along with the alternating orientation of Pc molecules in the first layer creates a preferable anchoring site for Pc of the second layer, leading to the assembly of the √2 × √2R45° superstructure, consistent with theoretical calculations showing that π-systems with extra negative charge have weaker interlayer binding energy. Our joint experimental–theoretical efforts provide direct evidence for the most energetically favorable parallel offset π–π stacking and charging effects on the preferential interaction between aromatic rings, which may shed new light on molecular assembly and organic nanoelectronics.

中文翻译：

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非平面酞菁双层中平行偏移π–π堆积的亚分子成像

在芳族体系中，π-π相互作用在确定分子的堆积几何形状和结合强度方面起着核心作用，因此，对微观的详细了解是非常可取的。在本文中，通过使用基于亚分子分辨率的扫描隧道显微镜，并结合基于密度泛函理论的第一性原理计算，我们报道了在包括石墨和金（ 111）的相互作用弱，而Cu（111）的粘合力强。我们发现，所有基板上第二层的非平面Pc均具有15°的面内旋转角，平行偏移为1.19Å，从而使π-π斥力最小。有趣的是，在Cu（111）上，R 45°上层结构，与理论计算一致，表明具有额外负电荷的π系统的层间结合能较弱。我们共同的实验理论工作为芳香族环之间优先相互作用的能量上最有利的平行偏移π-π堆积和带电效应提供了直接证据，这可能为分子组装和有机纳米电子学提供新的思路。