Physisorption controls the conformation and density of states of an adsorbed porphyrin

Jarvis, S. P. and Taylor, S. and Baran, J. D. and Thompson, D. and Saywell, A. and Mangham, B. and Champness, N. R. and Larsson, J. A. and Moriarty, P. (2015) Physisorption controls the conformation and density of states of an adsorbed porphyrin. The Journal of Physical Chemistry C, 119 (50). pp. 27982-27994. ISSN 1932-7447

Abstract

Conformational changes caused by adsorption can dramatically affect a molecules properties. Despite extensive study, however, the exact mechanisms underpinning conformational switching are often unclear. Here we show that the conformation of a prototypical flexible molecule, the free-base tetra(4-bromophenyl) porphyrin, adsorbed on Cu(111), depends critically on its precise adsorption site and that, remarkably, large conformational changes are dominated by van der Waals interactions between the molecule and the substrate surface. A combination of scanning probe microscopy, single-molecule manipulation, DFT with dispersion density functional theory, and molecular dynamics simulations show that van der Waals forces drive significant distortions of the molecular architecture so that the porphyrin can adopt one of two low-energy conformations. We find that adsorption driven by van der Waals forces alone is capable of causing large shifts in the molecular density of states, despite the apparent absence of chemical interactions. These findings highlight the essential role that van der Waals forces play in determining key molecular properties.