Measuring the mechanical properties of molecular conformers.

S P Jarvis,J A Larsson,S Taylor,J D Baran,P Moriarty,N R Champness

doi:10.1038/ncomms9338

Abstract

Scanning probe-actuated single molecule manipulation has proven to be an exceptionally powerful tool for the systematic atomic-scale interrogation of molecular adsorbates. To date, however, the extent to which molecular conformation affects the force required to push or pull a single molecule has not been explored. Here we probe the mechanochemical response of two tetra(4-bromophenyl)porphyrin conformers using non-contact atomic force microscopy where we find a large difference between the lateral forces required for manipulation. Remarkably, despite sharing very similar adsorption characteristics, variations in the potential energy surface are capable of prohibiting probe-induced positioning of one conformer, while simultaneously permitting manipulation of the alternative conformational form. Our results are interpreted in the context of dispersion-corrected density functional theory calculations which reveal significant differences in the diffusion barriers for each conformer. These results demonstrate that conformational variation significantly modifies the mechanical response of even simple porpyhrins, potentially affecting many other flexible molecules.

Highlights

Scanning probe-actuated single molecule manipulation has proven to be an exceptionally powerful tool for the systematic atomic-scale interrogation of molecular adsorbates
When two molecules of the same chemical composition differ in structure via rotations around intramolecular bonds—that is, the breaking and reforming of bonds is not required to superimpose the two structures—we describe the molecules as conformers[1]
We show that the lateral forces required for manipulation have a strong dependence on conformation type, even though each conformer shares almost identical adsorption characteristics

Summary

Measuring the mechanical properties of molecular conformers

We probe the mechanochemical response of two tetra(4-bromophenyl)porphyrin conformers using non-contact atomic force microscopy where we find a large difference between the lateral forces required for manipulation. Our results are interpreted in the context of dispersion-corrected density functional theory calculations which reveal significant differences in the diffusion barriers for each conformer These results demonstrate that conformational variation significantly modifies the mechanical response of even simple porpyhrins, potentially affecting many other flexible molecules. Scanning probe studies of molecules capable of adopting multiple conformations, have been much fewer in number[15,16,17,18], and there has not been an attempt to date to elucidate the differences in the response of different conformers to tip-induced forces. Nudged elastic band (NEB) calculations using van der Waals density functional theory (vdW-DFT) reveal that despite a similar adsorption energy for each conformer, the energy barriers for diffusion increase by over 50% for one conformational form

Results

Manipulation point

Author contributions