two polymorphs and, in particular, to the different local organi-
zation of molecular dipoles.
Interestingly, both of the crystalline phases exhibit intense
photoluminescence (PLQY>0.50), with excited-state lifetimes
in the microsecond regime, suggesting that the emitting excited
state has an^3 MLCT nature. Most likely, the restricted rotation
of the Me 3 Si groups in the crystals is responsible for the
enhancement of the emission with respect to the solution. This
statement is supported by the fact that similar dinuclear
complexes lacking the Me 3 Si group possess such higher emission
quantum yields. This is the first time that intramolecular
motions of nonconjugated rotors quench the emission of a lumi-
nescent compound in solution.
These findings highlight how packing can strongly perturb the
photophysical properties of the molecules even in the absence of
particularly short interactions.
IV. Molecular Systems Based on Aggregates of d^8 Metal ComplexesThe square-planar coordination geometry of d^8 complexes
opens many possibilities for the design of supramolecular
architectures. Their tendency toward aggregation and stacking
is a crucial feature to trigger self-assembling processes. The pref-
erential stacking geometry imposes a structural constraint that
frequently leads to filaments, rods, or needles. Depending on
the balance between stacking tendency and solvent affinity, soft
or crystalline materials can be obtained. In this sense, the selec-
tion of peripheral substituents facilitates a rational design strat-
egy for the manipulation of molecular entities. For instance,
balancing the aggregating nature of the square-planar coordina-
tion complex with solubilizing moieties of variable aspect ratios
and polarities can lead, alternatively, to liquid crystalline
structures ( 209 – 211 ), flexible fibers ( 212 – 216 ), and gelating
filaments ( 217 – 221 ). The substitution symmetry is a further
parameter to be considered for the controlled assembly, while
the intrinsic properties of the monomeric constituents can be
tuned by judicious choice of ligands.
As previously mentioned, the interaction between protruding,
doubly occupied dz^2 orbitals critically affects the photophysical
properties upon aggregate formation. Absorption and emission
of light, excited-state lifetimes, and redox properties are dramat-
ically affected. A particularly interesting feature is represented
by the possibility of tuning the distance between the monomeric
units and, consequently, the degree of electronic coupling
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 73