between the interacting metal orbitals. The introduction of bridg-
ing or spacing ligands can tune the color of dimeric Pt(II)
complexes: bringing them closer causes a red-shifted absorption
and emission, while widely spaced units resemble the monomeric
species. It has been shown that they can form aggregates or even
excimers, causing shifts in the emitted wavelengths and affecting
the photoluminescence quantum yields(222,223). These tunable
properties and the fact that platinum complexes emit from a trip-
let state have been exploited in several optoelectronic
applications ( 224 – 232 ). Even though these aggregates can be
exploited for the construction of white organic light-emitting
diodes ( 233 – 236 ), it also constitutes a disadvantage for techno-
logical applications where color purity is desirable. Therefore,
controlling the aggregation and being able to predict and design
appropriate compounds with the desired properties are an
important step to fully exploit their potential for technological
purpose. We will highlight the most important achievement in
this area and try to correlate the chemical structures with the
observed photophysical behavior. We will discuss not only 1D,
2D and 3D architectures but also crystalline systems. An inter-
esting feature of the crystals is represented by vapochromism:
exposure to different solvents, which occupy cavities within the
framework, causes the unit cell to“breathe,”thus shifting the
distance between the metal centers. This variable interaction
can be employed for sensing purposes, due to the switchable
emission of the frameworks ( 237 – 244 ).
A. 1D ARRAYS
Terpyridine, N^N^N ligands( 245 – 249 ) and their N^C^N and
N^N^C analogues ( 250 – 257 ) have been successfully coordinated
to Pt(II), leading to neutral, mono- or doubly charged species,
which in some cases display bright luminescence, both in
degassed fluid solutions and frozen matrices. In particular, it
has been shown by Cheet al. that they can form supramolecular
structures, such as nanowires, nanosheets, and polymeric
mesophases, with interesting optical properties ( 214 – 216,258).
As an example, cationic cyclometalated/terpyridyl platinum(II)
complexes bearing arylisocyanide/arylacetylide ligands possessing
highly emissive triplet MLCT or MMLCT excited states in solution
and in the solid state( 214 ) have been investigated. The molecular
aggregation through PtPt or ligand–ligand interactions is signif-
icant for these two classes of planar platinum(II) complexes, and
as they are robust toward moisture, air, and light irradiation
74 CRISTIAN A. STRASSERTet al.