Unusual reversibility of molecular break-up

Pentacene molecules adsorbed on Ir(111) can undergo a reversible deydrogenation process that can be engineered to produce polyacenes, via on-surface synthesis.

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D. Curcio et al.Chemical Science 12, 170 (2021)
The chemical reactivity of polycyclic aromatic hydrocarbons such as pentacene, a molecule consisting of five linearly-fused benzene rings, has received extensive attention being an active semiconducting material with a very large charge-carrier mobility. Pentacene is indeed considered a benchmark organic semiconductor for electronic devices, given its long history as an essential component in molecular and organic electronics. However, most applications rely on crystalline frameworks or molecular nanocrystals, but the pi-conjugated electronic structure, the relatively small HOMO– LUMO gap and the relatively high carrier mobility make pentacene interesting even in the isolated form. Furthermore, there is a large potential for energy gap manipulation upon controlled hydrogen removal, since the general trend for bandgaps in hydrogenated sp2-hybridized carbon-based compounds is to become smaller upon dehydrogenation.

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) which has to be overcome to reach the complete C–H dissociation represents the rate determining step of the overall process. 
Remarkably the pristine pentacene adlayer after dehydrogenation can be rehydrogenated by simply cooling the sample in a hydrogen atmosphere (P = 5x10-7mbar). The reversible reaction we observed is therefore quite special, since the edge carbon atoms do not bind to Ir atoms so strongly due to the geometry of the adsorbed C22 nanocluster. This peculiarity can be exploited to shed light on how to engineer C-nanoclusters or to produce polyacenes, via on-surface synthesis, as shown for example in the case of nanoacene.
Our findings could be instrumental for the understanding of the fundamental properties of graphene nano-ribbons, that to date have been prevalently synthesized in a hydrogen capped form. GNRs may also have interesting higher electron/hole mobilities and better thermal transport when dehydrogenated, so this reaction also has interesting technological applications given the potential importance of thermal switching capabilities of nanoribbons. Finally, the process of hydrogenation/dehydrogenation of pentacene has important implication in heterogeneous catalysis and in astrophysics since the zigzag edges show strong infrared signatures which have been detected in astronomical spectra and could play, as for other polycyclic aromatic hydrocarbons, an important role in the formation of H2, the most abundant molecule of the universe.

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Unusual reversibility of molecular break-up of PAHs: the case of pentacene dehydrogenation on Ir(111)
D. Curcio, E. sierda, M. Pozzo, L. Bignardi, L. Sbuelz, P. Lacovig, S. Lizzit, D. Alfè and A. Baraldi
Chemical Science 12, 170 (2021)

Selected for the back cover of  Vol.12, Iss. 1
Last Updated on Thursday, 07 January 2021 12:46