  We show theoretically that carriers confined in semiconductor colloidal
nanoplatelets (NPLs) sense the presence of neighbor, cofacially stacked NPLs in
their energy spectrum. When approaching identical NPLs, the otherwise
degenerate energy levels redshift and split, forming (for large stacks)
minibands of several meV width. Unlike in epitaxial structures, the molecular
behavior does not result from quantum tunneling but from changes in the
dielectric confinement. The associated excitonic absorption spectrum shows a
rich structure of bright and dark states, whose optical activity and
multiplicity can be understood from reflection symmetry and Coulomb tunneling.
We predict spectroscopic signatures which should confirm the formation of
molecular states, whose practical realization would pave the way to the
development of nanocrystal chemistry based on NPLs.
