  We use ab initio simulations to investigate the incorporation of pyridine
molecules (C$_5$H$_5$N) in the van der Waals gaps of Bi$_2$Se$_3$. The
intercalated pyridine molecules increase the separation distance between the
Bi$_2$Se$_3$ quintuple layers (QLs), suppressing the parity inversion of the
electronic states at the $\Gamma$-point. We find that the intercalated region
becomes a trivial insulator. By combining the pristine Bi$_2$Se$_3$ region with
the one intercalated by the molecules, we have a non-trivial/trivial
heterojunction characterized by the presence of (topologically protected)
metallic states at the interfacial region. Next we apply an external
compressive pressure to the system, and the results are (i) a decrease on the
separation distance between the QLs intercalated by pyridine molecules, and
(ii) the metallic states are shifted toward the bulk region, turning the system
back to insulator. That is, through a suitable tuning of the external pressure
in Bi$_2$Se$_3$, intercalated by pyridine molecules, we can control its
topological properties; turning-on and -off the topologically protected
metallic states lying at the non-trivial/trivial interface.
