D. AGGREGATION-INDUCEDEMISSIONENHANCEMENT INCRYSTALS
It is well known that aggregation of light-emitting compounds,
both organic and organometallic, is usually associated with a
strong quenching of emission efficiency. This effect was first
recognized by Förster and coworker studying the fluorescence
of pyrene( 172 ). They observed a decrease of the emission inten-
sity upon increasing the concentration of pyrene in fluid solution
conditions. Nowadays, it is well established that this phenome-
non is almost ubiquitous and involves many aromatic compounds
as well as transition metal complexes. Most of the technological
applications in which light-emitting molecules are used as
emitters in OLED, biological probes and sensors, require a
rather high local concentration of such molecules and often the
molecules are investigated in solid state. Usually, in these
conditions, a strong aggregation-caused quenching effect takes
place, which in turn represents a strong limitation in the real
world for using these classes of compounds. Finding a way to mit-
igate this detrimental effect, or even take advantage of aggrega-
tion for inducing enhancement of emission, would change the
scenario and even create a new class of emitters. In this respect,
Tang and coworkers first reported in 2001 on an example of
aggregation-induced emission (AIE) (173,174). Recently, they
reported some possible mechanisms for this phenomena and pos-
sible applications of compounds showing AIE effect ( 175 – 178 ).
Most of the reported compounds which show this effect are
organic molecules( 179 – 184 ), where restriction of intramolecular
rotation is generally accounted for being the main cause for AIE
(175,185). The enhancement is therefore mostly related to the
reduction of the nonradiative rate constant rising from the vibra-
tional and rotational mode of the appended groups to the
chromophores, which in the aggregates are strongly prevented.
However, few organometallic species based on Cu(I)( 186 ), Zn
(II) ( 187 ), Al(III) ( 188 ), Pt(II) ( 189 – 193 ), and Ir(III) ( 194 – 196 ) also
display color shift, switch on, or enhancement of their emission
efficiency upon aggregation. In these cases, several different
reasons were considered being involved in the AIE effect, namely
metallophilic interaction as in the cases of square-planar Pt(II)
complexes, giving rise to new excited state (MMLCT) with larger
transition dipole moments associated to increasing radiative rate
constants, orp–pstacking of the coordinated ligand. Less com-
mon are Ir-based complexes, as the octahedral geometry around
the metal center rules out any possible IrIr interaction, thus
the formation of intermolecular metal-to-ligand–ligand charge
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 71