On-Surface Synthesis of Azobenzene-Linked Porphyrin Derivatives

Area of Science:

• Surface chemistry
• Supramolecular chemistry
• Organic synthesis

Background:

• On-surface synthesis enables bottom-up construction of functional carbon nanostructures.
• The influence of pre-reaction chirality in self-assembled structures on coupling reactions remains poorly understood.

Purpose of the Study:

• Investigate the effect of molecular chirality in self-assembled Pt-porphyrin derivatives on the homocoupling of nitro-phenyl groups on Au(111).
• Determine how different chiral self-assembly structures (linear oligomers of opposing chirality vs. homochiral trimers) influence reaction yields.

Main Methods:

• Low-temperature scanning tunneling microscopy (LT-STM) for molecular imaging and assembly analysis.
• Synthesis of Pt-porphyrin derivatives with varying phenyl substituents (phenyl vs. 3,5-di-tert-butylphenyl).
• Machine-learning-assisted protocol for large-scale statistical analysis of reaction yields.

Main Results:

• Two distinct self-assembled structures were formed: linear oligomers of opposing chirality and discrete homochiral trimers.
• Significantly different reaction yields for azobenzene formation were observed between the two assembly types.
• Homochiral assemblies exhibited lower yields, attributed to the requirement of disassembly before forming heterochiral azobenzene products.

Conclusions:

• Porphyrin chirality plays a crucial role in directing the outcome and efficiency of on-surface azo coupling reactions.
• The pre-assembled structure's chirality dictates the feasibility and yield of forming specific coupled products.
• Understanding these structure-reactivity relationships is key for designing controlled on-surface synthesis strategies.