Highly Selective On-Surface Dehydrogenative Aromatization of n-Hexyl to Phenyl Substituents.

Abstract

Dehydrogenative aromatization of alkyl substituents represents a powerful approach to aryl-substituted functional molecules. However, the inertness of alkyl groups and the need for harsh reaction conditions accompanied by low product selectivity hamper its widespread applications. Here, we demonstrate the highly selective on-surface thermal aromatization of n-hexyl substituents on polycyclic aromatic hydrocarbons (PAHs) to their phenyl-substituted analogues under mild conditions. After depositing two representative precursor molecules, n-hexyl-substituted hexaphenylbenzene (HPB-Hex) and bianthryl octacarboxylic tetraimide (BATI-Hex), onto a pre-heated Au(111) substrate, dehydroaromatization of the peripheral n-hexyl groups into phenyl rings occurs, following the planarization of hexaphenylbenzene and bianthryl core. This process involves sequential intramolecular C-C bond rotations, dehydrogenation, and cyclodehydrogenation reactions, yielding phenyl-substituted hexabenzocoronene (HBC-Ph) and bisanthene octacarboxylic tetraimides (BSTI-Ph). The reaction sequences were monitored using scanning tunneling microscopy (STM) and bond-resolved non-contact atomic force microscopy (nc-AFM), offering structural proof of both intermediates and final products. These experimental techniques were complemented by density functional theory (DFT) simulations, which facilitated the detection of crucial steps in the conversion of n-hexyl to phenyl groups. Moreover, the effect of alkyl aromatization on the electronic properties of the newly formed aromatic hydrocarbons was elucidated using scanning tunneling spectroscopy (STS).