density shift, they can be considered either as metal-to-ligand
charge-transfer (MLCT) or ligand-to-metal charge-transfer
(LMCT) states. Within the orbital approximation, rough elec-
tronic wave functions are constructed as linear combinations of
basis functions centered on the atomic components, which result
in MOs. At this point, the electron spin has not been considered
so far. Within the orbital approximation, the excited electronic
states involve half-filled orbitals occupied by unpaired electrons.
In most cases, purely singlet or triplet wave functions can be con-
structed as zero-order approximations, which might be fair
enough for organic chromophores constituted preponderantly by
light atoms. However, relativistic effects due to acceleration of
electrons in the immediacy of the elevated nuclear charges of
heavy atoms originate a perturbation that, once again, leads to
mixing of zero-order wave functions. Representation of electronic
states as purely singlet or triplet in nature breaks down and
gives way to mixing, by spin–orbit coupling, of electronic wave
functions with defined spin. The spin–orbit coupling constant,
which determines the magnitude of the perturbation, roughly
scales as Z^4 , where Z refers to the nuclear charge. The perturba-
tion is the strongest for Au, Re, Os, and Pt.
The properties of photoactive transition metal complexes, such
as those described herein, are fundamentally marked by
spin–orbit coupling. In metal complexes usually fast intersystem
crossing from the optical excited state, mainly of singlet charac-
ter, to an isoenergetic state, which is predominantly triplet in
nature is observed. This radiationless process is followed by fast
relaxation into the lowest excited state with preponderant triplet
character. From there, emission of light, radiationless deactiva-
tion, electron or energy transfer, and chemical reactions can
occur. Radiationless deactivation pathways, especially for ruthe-
nium complexes, are often privileged by thermally accessible MC
states, which provide a crossing point with the potential energy
surface of the ground electronic state, due to their distorted
nuclear configuration favored by placement of electrons in unoc-
cupied, antibonding d-orbitals. MC states can be pushed up in
energy by the introduction of strong sigma-donating ligands that
locate a negative charge density on unoccupied d-orbitals of the
metallic center, as well as by the binding of effectivep-accepting
units that stabilize the occupied d-orbitals of the central atom.
Further, the energy level of the lowest electronic excited states
can be tuned by a judicious choice of the ligands’“chemical struc-
ture” which can critically stabilize or destabilize them with
respect to the ground electronic state.
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 51