Quasi-Planar π-Extended Cycloparaphenylenes: On-Surface Synthesis, Characterization, and Electronic Properties.

Area of Science:

• Organic Chemistry
• Materials Science
• Surface Science

Background:

• Cycloparaphenylenes (CPPs) are hoop-shaped macrocycles with curved aromatic frameworks and unique optoelectronic properties.
• Achieving planar π-systems in CPPs is challenging due to inherent ring strain.
• Partial planarization can be attained by incorporating rigid bending units for π-extension.

Purpose of the Study:

• To report the on-surface synthesis and characterization of quasi-planar, π-extended [2n]CPPs (n = 8-11).
• To investigate the electronic properties and structural behavior of these molecules on an Au(111) surface.
• To explore the influence of π-extension and substrate-induced flattening on CPP electronic structure.

Main Methods:

• On-surface synthesis using a bottom-up, surface-assisted strategy.
• Characterization via scanning tunneling microscopy/spectroscopy (STM/STS).
• Theoretical analysis using density functional theory (DFT).

Main Results:

• Successful synthesis of quasi-planar π-extended CPPs with perylenyl bridging units on Au(111).
• The n=8 CPP adopted a planar geometry, while larger homologues remained nonplanar but exhibited substrate-induced flattening.
• A progressive reduction in the HOMO-LUMO gap was observed with increasing ring size, contrasting with nonplanar CPPs.
• π-extension led to a reduced gap compared to unsubstituted planar CPPs.
• STM/STS revealed orbital confinement at molecular edges and fjord regions, modulated by perylenyl groups in larger CPPs.

Conclusions:

• The study demonstrates a method for synthesizing strained, quasi-planar π-extended CPPs.
• These molecules offer a platform for studying the relationship between curvature, conjugation, and electronic properties.
• The findings highlight the potential of surface-assisted synthesis for creating novel conjugated macrocycles with tunable electronic behavior.