Bhattacharjee Rameswar, Tovar John D, Kertesz Miklos
Department of Chemistry and Institute of Soft Matter, Georgetown University 37th and O Streets, NW Washington DC 20057-1227 USA
Department of Chemistry, and Department of Materials Science and Engineering, Johns Hopkins University 3400 North Charles Street Baltimore MD 21218 USA.
Chem Sci. 2025 Apr 24. doi: 10.1039/d5sc00639b.
Radially π-conjugated macrocycles with mixed aromatic and quinonoid units are considered. As a function of including an increasing number of aromatic units into a ring-like nanohoop with quinonoid units, a transition occurs where the HOMO and LUMO levels cross, leading to a topological transition described for the first time. Such transitions have been seen before in ethynylene-linked oligoacene polymers as a function of the acene size on gold surfaces and in various π-conjugated polymers as a function of external strain, but not in small molecular nanohoops or any other zero-dimensional system. Near the level crossing, the HOMO-LUMO gap becomes very small, offering novel photophysical properties while maintaining extensive delocalization. The open shell character of the rings changes continuously as the composition is gradually changed, switching from a singlet ground state to a triplet, providing a zero-dimensional analogy to topological transitions between a non-trivial to a trivial phase as observed in linear one-dimensional conjugated polymers. The spins of the triplet are localized near the two aromatic-quinonoid connections.
研究了具有混合芳香族和醌型单元的径向π共轭大环。随着在带有醌型单元的环状纳米箍中引入越来越多的芳香族单元，会发生一种转变，即最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）能级交叉，从而首次导致了一种拓扑转变。这种转变之前在乙炔连接的低聚并苯聚合物中作为金表面并苯尺寸的函数被观察到，在各种π共轭聚合物中作为外部应变的函数也被观察到，但在小分子纳米箍或任何其他零维系统中未被观察到。在能级交叉附近，HOMO-LUMO能隙变得非常小，在保持广泛离域的同时提供了新颖的光物理性质。随着组成逐渐变化，环的开壳层性质不断变化，从单重基态转变为三重态，这提供了一种零维类比，类似于在线性一维共轭聚合物中观察到的从非平凡相到平凡相的拓扑转变。三重态的自旋定域在两个芳香族-醌型连接附近。
