Impact of Connectivity on the Electronic Structure of N-Heterotriangulenes

N-heterotriangulenes (N-HTAs) are promising organ-ic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the structural and the electronic properties of two different N-HTAs, N-HTA 550 and N-HTA 557, the latter containing an additional 7-membered ring, adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono-and multilayer, both N-HTAs adopt a planar adsorption geometry with the molecular backbone oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) are assigned. The optical gap (S-0 -> S-1 transition) is found to be 3.4 eV for N-HTA 550 and 2.5 eV for N-HTA 557. Thus, the introduction of the -C=C- double bond in N-HTA 557 resulted in a pronounced decrease of the optical gap size by 0.9 eV due to the larger pi-conjugated electron system compared to N-HTA 550. Structural variations or substitution patterns in N-HTAs foster the opportunity for tailoring their electronic properties.

Automated summary

In this study, the structural and electronic properties of N-HTA 550 and N-HTA 557 adsorbed on Au(111) were investigated using experimental and computational methods. The results showed that both N-HTAs adopt a planar adsorption geometry, and N-HTA 557 has a smaller optical gap due to its larger pi-conjugated electron system.