  Metalated phthalocyanines (Pc's) are robust and versatile molecular
complexes, whose properties can be tuned by changing their functional groups
and central metal atom. The electronic structure of magnesium Pc (MgPc) -
structurally and electronically similar to chlorophyll - adsorbed on the
Ag(100) surface is investigated by low-temperature scanning tunneling
microscopy (STM) and spectroscopy (STS), non-contact atomic force microscopy
(ncAFM) and density functional theory (DFT). Single, isolated MgPc's exhibit a
flat, four-fold rotationally symmetric morphology, with doubly degenerate,
partially populated (due to surface-to-molecule electron transfer) lowest
unoccupied molecular orbitals (LUMOs). In contrast, MgPc's with neighbouring
molecules in proximity undergo a lift of LUMOs degeneracy, with a near-Fermi
local density of states with reduced two-fold rotational symmetry, indicative
of a long-range attractive intermolecular interaction. The latter is assigned
to a surface-mediated two-step electronic hybridization process. First, LUMOs
interact with Ag(100) conduction electrons, forming hybrid molecule-surface
orbitals with enhanced spatial extension. Then, these delocalized
molecule-surface states further hybridize with those of neighbouring molecules.
This work highlights how the electronic structure of molecular adsorbates -
including orbital degeneracies and symmetries - can be significantly altered
via surface-mediated intermolecular hybridization, over extended distances
(beyond 3 nm), having important implications for prospects of molecule-based
solid-state technologies.
