Nonacene Generated by On-Surface Dehydrogenation

Abstract

The on-surface synthesis of nonacene has been accomplished by dehydrogenation of an air-stable partially saturated precursor, which could be aromatized by using a combined scanning tunneling and atomic force microscope as well as by on-surface annealing. This transformation allowed the in-detail analysis of the electronic properties of nonacene molecules physisorbed on Au(111) by scanning tunneling spectroscopy measurements. The spatial mapping of molecular orbitals was corroborated by density functional theory calculations. Furthermore, the thermally induced dehydrogenation uncovered the isomerization of intermediate dihydrononacene species, which allowed for their in-depth structural and electronic characterization.

Higher Acenes by On‐Surface Dehydrogenation: From Heptacene to Undecacene

“…Figure 1 a shows the typical STM overview with mostly intact tetrahydroacene species, which are recognized in empty state images by their characteristic appearance with two pronounced lobes associated with the presence of four methylene groups per molecule. These lobes are also recognizable in the STM image profiles shown in Figure 1 c. The precursors are preferentially located in the corners of the surface herringbone reconstruction similarly to the previously reported pentacene29 and nonacene27 molecules. With the increase of coverage, the molecules tend to follow the reconstruction pattern settling themselves mostly in‐between the brighter appearing rows of the surface reconstruction.…”

“…A small amount of partially or completely dehydrogenated species (dihydroacenes and acenes, respectively) could also be found at this stage, as exemplified by the molecules in the white and red contours in Figure 1 a. After annealing compounds 1 were converted into acenes as documented in the STM image shown in Figure 1 b, which indicates the disappearance of the bright lobes attributed to the methylene moieties as previously observed for nonacene 27. Vanishing of lobes could also be noted in the purple, grey and yellow profiles visualized in Figure 1 c. Moreover, the behavior of the molecules during STM imaging differs between saturated precursors and parent acenes.…”

“…Increasing the applied bias voltage results in a dehydrogenation process accompanied with the aromatization and planarization of the molecular fragment subjected to manipulation. The threshold voltage for the dehydrogenation depends on the actual structure of the molecule 27. However, we found that raising the bias voltage to 2.5 V was sufficient to carry out the dehydrogenation of all the investigated hydroacene compounds 1 .…”

“…However, their use as direct precursors of the conjugated systems has been typically circumscribed to the preparation of acenes with up to five linearly fused benzene rings. We recently developed a general method for the synthesis of hydroacenes based on the gold(I)‐catalyzed cyclization of aryl‐tethered 1,7‐enynes,26 and successfully used tetrahydrononacene, one of the resulting partially saturated molecules, as a stable precursor for the synthesis and study of the properties of nonacene on a Au(111) surface 27. On the basis of these results, we anticipated the development of a unified approach for the preparation and study of higher acenes on metallic surfaces, which could be extended for the synthesis of yet unknown even higher acenes.…”

“…With the tetrahydroacenes 1 a , b and 1 d , e in hand, we next turned our attention to their application as precursors of the acene series by on‐surface dehydrogenation following a procedure related to the one we previously developed for the preparation of nonacene from 1 c 27. In general, parent acenes could be generated either thermally by annealing of the sample to 520 K for 10 minutes or by the application of the microscope tip 28.…”