Molecular mechanisms of C–C-coupling reactions: A microscopic view of on-surface chemical bond formation processes

自下而上构建复杂的功能分子无疑一直是纳米科学研究人员的指导性愿景，因为费曼提出了著名的声明：底部有足够的空间。在过去的几个世纪里，化学家已经为构建分子的各种反应过程建立了坚实的机理模型基础。最近的研究表明，共官能化的AFM-TIPS可以以原子分辨率成像分子结构，并使明确的化学键识别成为可能。从那时起，科学家们可以清楚地分辨出表面合成过程中发生的即使是细微的结构变化，并最终了解不同的反应途径。在表面组装有机(芳香)结构的关键是控制自由基中间体的形成和反应活性。因此，本项目的主要目标是在合适的表面模型反应的基础上研究芳香族自由基的稳定性和反应性。这包括形成(C-离开基团键的断裂)，表面和/或有机中间体的潜在稳定化，以及进一步的反应，如插入反应、迁移或重排反应。这些基本过程是理解反应、优化反应、建立一般规则、如何在表面直接进行有机合成的关键。为了解决这些问题，选择了一个简单的模型反应：将1-苯基萘衍生物环化为荧菲。实现项目目标的一个关键因素是能够跟踪和理解反应途径中的每一个步骤，从脱卤化、中间体形成到扩散。我们结合了用于合成的有机化学家的专业知识，用于成像和操纵金属表面单个有机分子的扫描探针专家，以及用于与从头计算模拟进行比较的理论家。最终目标是揭示具有代表性的有机分子的C-C偶联的完整反应动力学，包括不同的卤化、自由基形成、同分异构体效应，以及了解关于热活化的竞争过程。

The bottom-up construction of complex functional molecules has undoubtedly been a guiding vision of researchers in the nanosciences, ever since Feynman’s famous declaration that “there is plenty of room at the bottom”. In the past centuries chemists have established a solid basis of mechanistic models for the course of various reactions to construct molecules. Recently it was demonstrated that CO-functionalized AFM-tips allow imaging molecular structures with atomic resolution and unambiguous chemical bond identification became possible. Since then scientists can clearly distinguish even subtle structural changes as they are occurring in on-surface synthesis and finally understand different reaction pathways. The key in assembling organic (aromatic) structures on-surface is in controlling the formation and reactivity of radical intermediates. Therefore, the main objective of this project is to study the stability and reactivity of aromatic radicals on the basis of a suitable on-surface model reaction. This involves the formation (cleavage of the C–leaving group bond), potential stabilization by the surface and/or an organic intermediate, and further reactions, such as insertion reactions, migration or rearrangement reactions. These elementary processes are the key to understand the reactions, to optimize them and to establish general rules, how to conduct organic synthesis directly on surfaces. To address these issues a simple model reaction has been chosen: The cyclization of a 1-phenylnaphthalene derivative to fluoranthene. A key factor to achieve the project goals is the ability to follow and understand each individual step in a reaction pathway, from dehalogenation and intermediate-formation to diffusion. We combine the expertise of an organic chemist for synthesis, a scanning probe expert for imaging as well as manipulating individual organic molecules on metal surfaces, as well as a theoretician to compare to ab-initio simulations. The final goal is to unravel the complete reaction dynamics of the C-C coupling for a representative organic molecule, including different halogenations, radical formation, isomeric effects, and understanding competing processes with respect to thermal activation.