Low‐lying LUMO Boosts Conductance in Antiaromatic Dibenzopentalene Versus Aromatic Analogues

• Published in: Chemistry : a European journal 2024-07, Vol.30 (40), p.e202400935-n/a
• Main Authors: Schmidt, Maximilian, Abellán Vicente, Lydia, González, M. Teresa, Zotti, Linda A., Esser, Birgit, Leary, Edmund
• Format: Article
• Language: English
• Subjects: Anthracene; antiaromaticity; Bonding strength; break junction; Chemical bonds; Conductance; dibenzopentalene; Electric potential; Electron transport; Fluorine; pyridyl anchor groups; single-molecule conductance; Spectroscopy; Voltage
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202400935

Antiaromaticity is a fundamental concept in chemistry, but the study of molecular wires incorporating antiaromatic units is limited. Despite initial predictions, very few studies show that antiaromaticity has a beneficial effect on electron transport. Dibenzo[a,e]pentalene (DBP) is a stable structure that displays appreciable antiaromaticity within the five‐membered rings of the pentalene core. We have investigated derivatives of DBP furnished with pyridyl (Py) and F4‐pyridyl (PyF4) anchor groups, and compared the conductance with purely aromatic phenyl and anthracene analogues. We find that the low‐bias conductance of DBP‐Py is approximately 60 % larger than that of the anthracene analogue Anth‐Py and 250 % larger compared to the phenyl derivative Ph‐Py. This is due to a better alignment of the LUMO with the gold Fermi level, which we confirm by conductance‐voltage spectroscopy where the conductance of DBP‐Py shows the greatest voltage‐dependence. The F4‐pyridyl compounds, which have lower LUMO energies compared to the pyridyl analogues, did not, however, form detectable molecular junctions. The strongly electron‐withdrawing fluorine atoms reduce the donor capability of the nitrogen lone‐pair to the point where stable N−Au bonds no longer form. Dibenzopentalene paired with pyridyl anchor groups is shown to conduct better than aromatic analogues containing phenyl or anthracene inside single‐molecule junctions. Compared to thioanisole anchors, pyridyls provide better molecular orbital level‐Fermi level alignment. The result is a significant conductance‐voltage response especially for a sub‐2 nm long molecule. The conductance increases nearly ten‐fold over a 1.5 V range.