On-surface synthesis of higher acenes beyond undecacene: Evolution of conductance and electronic states

苯是一类具有线性稠合苯环的多环芳烃。它们被广泛认为是很有前途的有机和分子电子学材料。它们的电子结构的性质目前正在积极地讨论，而关于它们的基态的电子组态作为环的数目的函数的基本问题还没有得到回答。目前对并苯的高度兴趣主要是由于预期的自由基性质的增加以及对越来越多的环的光学激发势垒的减少。聚苯项目的建立是为了了解作为线性稠合苯环数目的函数的高烯基态的能隙发展和自由基特性。该项目的目标是在实验上确定表面合成的低分子烯的电子性质和电导，研究电子共振的演化和隧穿电流的衰减。利用低温下的高分辨率扫描隧道显微镜(STM)在单分子水平上验证了化学转化。在聚并苯中，我们的目标是将表面合成的一系列并苯延续到12烯、14烯甚至更长。扫描隧穿光谱将提供电子态的能量位置和空间分布，从而决定电子共振的演化。为了进一步了解新的长刺的开壳特性，我们将结合间隙和电子共振的研究与横向操纵实验相结合。我们将使用所获得的长并三烯分子进行电导测量。通过用扫描隧道显微镜的尖端拉动分子的一端，我们将记录非共振区的隧穿电流，研究电导随分子长度的变化以及共轭效应。此外，我们还将研究衬底在表面合成苯类化合物中的作用，以及通过钝化H层在Si(001)上进行去耦合的效果。

Acenes are a class of polycyclic aromatic hydrocarbons with linearly fused benzene rings. They are widely considered as promising materials for organic and molecular electronics. The nature of their electronic structure is presently actively discussed and fundamental questions regarding the electronic configuration of their ground state as a function of the number of rings are not answered yet. The present high interest in acenes is mainly motivated by the expected increasing radical character as well as the reduced optical excitation barrier for an increasing number of rings. The POLYACENES project is established to understand the energy gap development and the radical character of the ground state of higher acenes as a function of the number of linearly fused benzene rings. The goal of the project is to experimentally determine the electronic properties and the conductance of oligoacenes synthetized on surface, studying the evolution of the electronic resonances and the decay of the tunneling current. High-resolution scanning tunneling microscopy (STM) at low temperature is employed to verify the chemical conversion at a single-molecule level.In POLYACENES, we aim to continue the series of acenes synthetized on surface to 12 cene, 14 cene, and longer. Scanning tunneling spectroscopy will give access to the energy position and spatial distribution of the electronic states, determining the evolution of the electronic resonances. To gain further information on the open shell character of the new long acenes, we will combine the study of gap and electronic resonances with lateral manipulation experiments. We will use the obtained long acene molecules for conductance measurements. By pulling one end of the molecule with the apex of the STM, we will record the tunneling current in non-resonant regime studying the evolution of the conductance with the molecular length and the effect of conjugation. Moreover, we will investigate the role of the substrate in the on-surface synthesis of acenes and the effect of decoupling via a passivating H-layer on Si(001).