Pyridyl-functionalized tripod molecules on Au(111): Interplay between H-bonding and metal coordination

The self-assembly of pyridyl-functionalized triazine (T4PT) was studied on Au(111) using low-temperature scanning tunneling microscopy (STM) under ultra-high vacuum conditions combined with density functional theory (DFT) calculations. In particular, we investigated the effect of temperature on the intermolecular interactions within the assemblies. STM measurements revealed that T4PT molecules form a well-ordered, close-packed structure, with the molecules adopting a planar conformation parallel to the Au surface for coverages < 1 monolayer upon room temperature deposition. The intermolecular interactions stabilizing the self-assembled arrangement are based on a combination of hydrogen bonding and weak van der Waals forces. Upon post-deposition annealing up to 200° C, the assemblies are additionally stabilized by metal-ligand bonding between the pyridyl ligands and native Au adatoms. Further post-deposition annealing at temperatures above 200°C led to the breaking of the N-Au bonds with the molecular assemblies transforming into a second close-packed hydrogen-bonded structure. For temperatures exceeding 230° C, few covalently linked dimers formed, most likely as a result of CH-bond activation. We rationalize the kinetically-driven structure formation by unveiling the interaction strengths of the different bonding motifs using DFT and compare the respective molecular conformations to the structurally similar pyridyl-functionalized benzene (T4PB).本研究在超高真空环境下结合密度泛函理论（DFT）计算，采用低温扫描隧道显微镜（STM），探究了吡啶基功能化三嗪（T4PT）在Au(111)表面的自组装行为。尤为重要的是，本研究考察了温度对组装体内分子间相互作用的影响。STM测试结果表明，在室温沉积且覆盖度低于1个单层的条件下，T4PT分子会形成有序密排结构，分子呈与金表面平行的平面构象。稳定该自组装结构的分子间相互作用，由氢键与弱范德华力共同提供。当沉积后退火温度升至200℃时，吡啶基配体与基底原生金吸附原子之间形成的金属-配体键，会进一步增强组装体的稳定性。当继续将退火温度提升至200℃以上时，N-Au键发生断裂，分子组装体转变为第二种由氢键维系的密排结构。当温度超过230℃时，会形成少量共价连接的二聚体，这大概率是C-H键活化所致。本研究通过DFT计算揭示了不同键合模式的相互作用强度，阐明了动力学驱动的结构形成机制，并将该分子构象与结构相似的吡啶基功能化苯（T4PB）进行了对比。