Effect of Single-Atom Defects on Metal Surfaces on Porphycene Tautomerization

Intramolecular hydrogen transfer reactions have been proposed as a molecular switch in integrated nanodevices. For this potential to be realized in practical applications, precise control of the direction of the reaction is required. Here, we investigate the effect of the local environment on porphycene tautomerization using dispersion-inclusive density functional theory. In each investigated system, we consider porphycene adjacent to a single-atom surface adduct (adatom) on an otherwise pristine metal surface, while the surface type, degree of coverage, and distance between the molecule and adatom are varied. Our calculations show that on the Cu(110) surface, reaction directionality is achieved only under low-coverage conditions, while at high coverage, adatom-molecule interactions are overwhelmed by interactions with neighboring molecules. On Cu(111), the only distance-related effect on the tautomerization reaction occurred when porphycene was adsorbed directly above the Cu adatom. Based on our calculations, we attribute the observed changes in the reaction landscape to alterations in the HOMO-1 orbital order and d-band center of the molecule when complexed to the altered surface.