  The breaking of a chemical bond is fundamental in most chemical reactions. To
understand chemical processes in heterogeneous catalysis or on-surface
polymerization the study of bond dissociation in molecules adsorbed on
crystalline surfaces is advantageous. Single molecule studies of bond breaking
can give details of the dissociation dynamics, which are challenging to obtain
in mole-scale ensemble experiments. Bond breaking in single adsorbed molecules
can be triggered using the energy of the tunnelling electrons in a scanning
tunnelling microscope (STM) at selected positions to investigate the
dissociation dynamics. Single bond dissociation dynamics has been deeply
investigated only in small molecules, but not in larger molecules that exhibit
distinct rotational degrees of freedom. Here, we use low temperature (7 K) STM
to dissociate a single bromine atom from an elongated molecule
(dibromo-terfluorene) adsorbed on a Ag(111) surface. This rod-like molecule
allows to clearly identify not only displacement of the reaction fragments, but
also their rotation. The results show that the molecular fragment binds to the
nearest silver atom and only further rotation is allowed. Moreover, the
excitation responsible for the bond breaking can propagate through the
molecular backbone to dissociate a bromine atom that is not located at the
pulse position. These results show the important role of the metal substrate in
conditioning the bond dissociation dynamics. Our results might allow to improve
the control of the synthesis of 2D materials and targeted engineering of
molecular architectures.
