Usually, absorption and emission of light between states of dif-
ferent multiplicity are spin-forbidden. However, due to the mixed
multiplicity character of the electronic excited states of transi-
tion metal complexes, such processes become, at least, partially
allowed, enabling the fabrication of luminescent species with
long-lived electronic excited states, due to their predominant
triplet character. This opens the possibility for electrical excita-
tion with potentially 100% efficient exciton harvesting:
recombinations of holes and electrons lead to electronic excited,
singlet and triplet, states which, ultimately, lead to the lowest
excited triplet level. Another interesting feature of triplet
emitters is their marked Stokes-shift, which is determined by
the singlet–triplet splitting of the lowest electronic excited sin-
glet and triplet states, as well as by the energy of structural reor-
ganization upon intersystem crossing and subsequent
vibrational relaxation. Moreover, the emitting triplet state can
be of a different origin than the optically excited singlet state,
thus leading to even more pronounced shifts. The major triplet
character of the long-lived, lowest electronic excited states also
renders them prone to quenching by molecular oxygen, which
can be used for sensing or photosensitizing purposes.
The geometry of complexes of groups 7–9 of the transition met-
als, in particular, Re(I), Ru(II), Os(II), and Ir(III), is determined
by the d^6 electronic configuration that leads to octahedral coordi-
nation geometries. The d-orbitals of the central atom can be
roughly viewed as split into two sets of triply and doubly degen-
erate nature, named as t2gand eg, respectively, where the latter
ones are higher in energy, and the splitting is determined by the
ligand field (vide supra). The metallic center is therefore shielded
from the environment, and intermolecular interactions mainly
perturb the organic ligands and, consequently, the ground and
excited electronic states whose energy content they determine.
For the octahedral Ru(II) complexes, and the other d^6 metal ions,
thesLandpLorbitals are completely filled as well as the HOMO
(pM(t2g))^6 is fully occupied and the ground state configuration is
closed shell. The ground state is therefore a singlet, while the
excited states are either singlet or triplet. The polypyridine
complexes, accounted as paradigmatic examples of MLCT states,
involve 4d-orbitals from which electron density is displaced to
the organic ligand. In complexes in which the coordinated ligands
do not possess accessiblepL* orbitals, the lowest excited state is a
MC. This leads to fast radiationless deactivation to the ground
state and/or ligand dissociation reactions. Strongs-donors andp-
acceptors lead to a larger d-orbital splitting and, consequently,
destabilize MC states, making thermal activation less accessible.
52 CRISTIAN A. STRASSERTet al.