If compared with their Ru(II) analogues, Os(II) complexes dis-
play smaller gaps between the ground and the excited electronic
states, as the relevant 5d-orbitals lie higher in energy, thus low-
ering the energy of the MLCT state. This leads to red-shifted
emissions and shortened excited-state lifetimes, due to a faster
nonradiative deactivation, which can be rationalized in terms of
the energy gap law ( 25 – 28 ). If compared with Ru(II), non-
radiative MC states of Os(II) complexes are relatively less ther-
mally accessible due to the lower energy content of the MLCT
states. Further, the splitting induced by a given ligand field is
more pronounced for the diffuse 5d-orbitals of Os(II) than for
the 4d-orbitals of Ru(II), which further increases the gap
between the emissive MLCT and the nonradiative MC states.
In the case of Rhenium, Re(I) complexes require the introduc-
tion of even strongerp-accepting ligands (typically CO or CN)
to stabilize their high lying 5d-orbitals. This is necessary to push
up in energy the MLCT states, which otherwise possess a very
low-energy content leading to extremely low-emission efficiencies
according to the energy gap law. Further, strong field ligands
induce a larger splitting of d-orbitals, thus making nonradiative
MC states less accessible and favoring radiative processes.
Luminescent Ir(III) complexes are often comprising
cyclometalating ligands, and even though they possess the same
electronic configuration of the Ru(II) and Os(II), the nature of
their excited state is less defined. In fact for many complexes,
the lowest emitting excited state is a mixed MLCT and LC,
which is reflected in longer excited-state lifetimes and often
structured emission. This mixed nature is well shown by the dif-
ferent interplay between the krand knr. For pure^3 MLCT, a
larger kr is usually observed than for a pure^3 LC states.
A remarkable behavior is observed for the excited-state lifetime,
as even for larger contribution of the^3 LC states, a short lifetime
(few microseconds) is recorded which is related to the heavy atom
effect which induce a strong triplet-singlet mixing. Another
interesting feature of the Ir complexes is their much higher sen-
sitivity toward modifications of the frequently used
cyclometalating ligands. Such properties allow a very fine-tuning
of the excited-state energies, and the emission maxima can vary
from UV to near IR.
In the case of Pt(II), however, the d^8 configuration leads to
square-planar coordination geometries. A particular feature of
these species is the doubly occupied dz^2 orbitals, which are nor-
mal to the coordination plane. This geometry facilitates the stac-
king of molecular units into dimers, oligomers, or columnar
arrays in which the protruding dz^2 orbitals can interact, leading
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 53