C. DOUBLECOMPLEXSALTS
Iridium(III) complexes have been widely explored in the past
few decades because of their outstanding photophysical pro-
perties and superior photo- and chemical stability ( 123 – 129 ).
Much effort has been devoted to both neutral and cationic lumi-
nescent iridium complexes in which fine-tuning of the energy of
their long-lived excited state can be achieved by proper choice
of the coordinated ligands ( 130 – 136 ).
Work focusing on these compounds was principally prompted by
fundamental studies of excited-state electron and energy transfer
processes( 137 – 139 )as well as their potential use as biological
sensors ( 140 ), organic light-emitting diodes (OLEDs)( 141 – 145 ),
and light-emitting electrochemical cells (LEECs)( 146 – 150 ).
Besides soft structures containing luminescent metal
complexes, strong effort has been devoted to the development of
luminescent porous organometallic frameworks as recently
reviewed by Allendorf and coworkers( 151 ). Also, Kitagawa and
coworkers reviewed on the synthetic strategies and properties
of functional from 1D to 3D coordination networks ( 152 – 154 ).
In these materials, the metal atoms play a fundamental role in
the structure formation as well as in the spectroscopic properties
of the crystalline materials. Several of these structures involve
strong covalent interactions as coordination bonds to a mono-
or multinuclear metal center, which make them more“robust”,
as in metal organic frameworks (155,156), but also examples con-
sisting of weaker and noncovalent interactions are known ( 157 ).
The latter are usually formed from purely organic compounds
linked by hydrogen bonds, and the individual building blocks
are commonly referred to as tectons.
Besides these materials, porous organic–inorganic hybrid mat-
erials made by organosilica were firstly reported by Inagaki and
coworkers ( 158 ), and since then several examples are known, as
reviewed by Fröba and coworkers ( 159 ) which is, however, a topic
outside the aim of this review.
As already mentioned, the supramolecular assemblies can lead
to new properties and photoinduced processes which have been
studied in great details in solution (see previous section). On
the contrary, only few cases of intermolecular photoinduced pro-
cesses, and in particular, energy transfer, involving transition
metal complexes in pure crystalline phases have been reported
and will be now discussed. Here, relative orientation and
distance between interacting species can be more precisely deter-
mined by means of single-crystal X-ray diffractometric analysis.
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 67