Unusual reversibility of molecular break-up of PAHs: the case of pentacene dehydrogenation on Ir(111)

The chemical reactivity of polycyclic aromatic hydrocarbons such as pentacene, a molecule consisting of five linearly-fused benzene rings, has received extensive attention since it is an active semiconducting material with a very high charge-carrier mobility. Pentacene is considered a benchmark organic semiconductor for electronic devices, given its long history as an essential component in molecular and organic electronics. However, the majority of applications of pentacene relies on crystalline frameworks or molecular nanocrystals, whereas the p-conjugated electronic structure with relatively small HOMO–LUMO gap and the relatively high carrier mobility makes pentacene interesting even in the isolated form. Furthermore, there is a high potential for energy gap manipulation upon controlled hydrogen removal, since the bandgap of hydrogenated sp²-hybridized carbon-based compounds tends to become smaller upon dehydrogenation.
In the first step of our study the room temperature adsorption of pentacene on Ir(111) was investigated. Figure 1 reports the C 1s core level measured at the SuperESCA beamline via high-resolution x-ray photoelectron spectroscopy (XPS) after the deposition of 0.75 ML of pentacene. We were able to discriminate the different contributions to the overall spectrum stemming from each C atom of the pentacene molecule. The histogram shows the distribution of the calculated core-electron binding energy obtained via density functional theory. Subsequently, the process of dehydrogenation was investigated. Whilst room temperature adsorption preserves the molecular structure of the five benzene rings and the bonds between carbon and hydrogen atoms, we found that complete C–H molecular break up takes place between 450 K and 550 K, eventually resulting in the formation of small graphene islands at temperatures higher than 800 K. The experiments reveal that the dehydrogenation process is compatible with a double reaction barrier. The first and lower barrier (1.12 eV) is in very good agreement with the results of DFT calculations (1.11 eV), while the second barrier (1.86 eV) that has to be overcome for complete C–H dissociation represents the rate determining step of the overall process.

Figure 1. (a) High-resolution C 1s spectrum obtained with photon energy hν = 400 eV after deposition of 0.75 ML of pentacene. The histogram shows the distribution of the calculated binding energies, and the color scale of the histogram reflects the color scale for the ball model shown in part (b) of the image. (b) Results of the DFT calculations for the C 1s binding energies for all the carbon atoms in the relaxed pentacene molecule.

Figure 2 shows Temperature Programmed-XPS spectra of C 1s during exposure to molecular hydrogen and thermal annealing. Remarkably, the pristine pentacene adlayer after dehydrogenation can be re-hydrogenated by simply cooling the sample in a hydrogen atmosphere (p = 5´10^-7mbar). The detected reversible reaction appears quite special, since the edge carbon atoms do not bind so strongly to Ir atoms due to the geometry of the adsorbed C₂₂ nanocluster.

Figure 1.  (a) Left – A C 1s TP-XPS experiment performed with photon energy hν = 400 eV for 0.3 ML of pentacene at 0.25 K/s, while exposing the system to H2. When the annealing ramp is ended at 520 K and the sample rapidly quenched to 420 K, the C 1s spectra mostly return to the original lineshape (dark gray curve). (b) Several sequential plots of the coverage of pentacene (black) and dissociated pentacene (gray) during 3 dehydrogenation/rehydrogenation cycles, the first of which is shown in (a).

This peculiarity can be exploited to lay the foundations of C-nanoclusters engineering or to produce polyacenes, via on-surface synthesis, as shown for example in the case of nanoacenes. The outcome of this study could help for understanding the fundamental properties of graphene nano-ribbons (GNRs), which so far have been prevalently synthesized in a hydrogen capped form. GNRs may also have interesting higher electron/hole mobilities and better thermal transport when dehydrogenated: hence this reaction also has interesting technological applications related to the potential importance of thermal switching capabilities of nanoribbons. In closing, the process of hydrogenation/dehydrogenation of pentacene has more general implications in heterogeneous catalysis and in astrophysics, since the zig-zag edges in GNRs show strong signatures that have been detected in astronomical infrared spectra. These objects could play, as for other polycyclic aromatic hydrocarbons, an important role in the formation of H₂, the most abundant molecule of the universe. 

This research was conducted by the following research team:

D. Curcio¹, E. Sierda^2,3, M. Pozzo^4,5, L. Bignardi¹, L. Sbuelz¹, P. Lacovig⁶, S. Lizzit⁶, D. Alfè^4,5,7,8and A. Baraldi^1,6,9,

¹ Department of Physics, University of Trieste, Trieste, Italy
² Department of Physics, University of Hamburg, Germany
³ Institute of Physics, Poznan University of Technology, Poznan, Poland 
⁴ Department of Earth Sciences, Thomas Young Center, University College London
⁵ London Centre for Nanotechnology, University College London, United Kingdom
⁶ Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
⁷ Department of Physics and Astronomy, Thomas Young Center, University College London
⁸ Dipartimento di Fisica "Ettore Pancini", Università di Napoli "Federico II", Napoli, Italy
⁹ IOM-CNR, Laboratorio TASC, AREA Science Park, Trieste, Italy

Contact persons:

Alessandro Baraldi, email: abaraldi@units.it