Speaker
Description
π conjugation – the delocalization of electrons across a network of overlapping p$_z$ orbitals is a foundational concept in organic chemistry, governing molecular stability, reactivity, and functionality across biological and technological domains. Aromaticity arises as a manifestation of this delocalization, where the population of delocalized bonding and antibonding orbitals, in accordance with Hückel’s rule, imparts thermodynamic stability and chemical resilience. Disruptions to π conjugation – through steric twisting, oxidation, or structural distortion – diminish electron delocalization and promote the emergence of localized aromaticity, thereby drastically altering chemical reactivity and stability. Polycyclic aromatic hydrocarbons (PAHs) offer an ideal playground for investigating such effects, as they often tend to self-assemble on surfaces, enabling experimental study using conventional methods of surface science, e.g., low-energy electron diffraction and scanning tunneling microscopy. Photoemission orbital tomography (POT) [1] provides direct experimental access to the frontier orbital structure of PAHs [2] and reveals the nature of their aromaticity [3]. In this study, combining POT with conventional surface-science techniques and density functional theory (DFT) calculations, we show that keto-functionalization of a pentacene (C$_{22}$H$_{14}$) dramatically disrupts its π conjugation. The resulting 6,13-pentacenequinone (C$_{22}$H$_{12}$O$_2$) exhibits an orbital structure reminiscent of naphthalene (C$_{10}$H$_8$). This effect is attributed to a delocalization barrier for π electrons introduced by the keto groups at the molecule’s core, effectively partitioning the molecule into two naphthalene-like segments with localized conjugation. As a consequence, we observe a notable increase in the energy gap between the highest occupied and lowest unoccupied molecular orbitals.
References:
[1] Puschnig et al., Science 326 (2009) 702–706
[2] Lüftner et al., PNAS 111 (2014) 605–610
[3] Haags et al., ACS Nano 14 (2020) 15766–15775