functionalities of the single subentities(3,4). Nowadays, one of
the most striking goals is to provide building blocks that can be
rationally mixed and matched to have functional supramolecular
architecture and materials with properties which can be
modified upon an external stimulus. Adaptive materials ( 5 ),
self-healing systems ( 6 ), and dynamic species ( 7 ), able to reorga-
nize their structure as a response to an input, are the next gener-
ation of organized molecular systems which started to appear in
the literature.
By mimicking biological systems, it is possible nowadays to
create species able to act as (supra)molecular sensors, as well
as to develop catalysts which undergo allosteric control( 8 ), or
produce by light molecular movements ( 9 ). Moreover, biological
systems make strong use of the concept ofmultivalency, which
is based on recognition as well as on relatively weak and non-
covalent interactions, as hydrophobicity and hydrophilicity
(10,11), host–guest chemistry (12,13), and H-bonding (14,15).
Thus, taking inspiration from nature offers the possibility to
deliberately design cooperative synthetic systems, blossoming
novel, and fascinating scientific scenarios in both fundamental
and applied research. A more ambitious goal is not to mimic
nature but to provide simpler artificial systems which can
replace natural ones.
In many cases, in nature, the assemblies are constituted of
organic chromophores. Much less common is to encounter
assemblies of organometallic species or hybrid organic–inorganic
arrays. Organometallic complexes display a variety of outstand-
ing characteristics, among which are photophysical, catalytic,
redox, and magnetic properties. Combination of different metal
complexes, possessing complementary properties, for example,
emission colors, absorption, ability to accept and donate energy
and charge, can lead to white-light emission, light-harvesting
systems, photoinduced energy and charge-transfer (CT)
processes, and photocatalysis.
Herein, we will discuss several approaches that have led from
molecular entities to supramolecular soft and hard systems. In
particular, we will show how the molecular structure can be
modified to induce the controlled self-assembly of transition metal
complexes into sophisticated photoactive arrays with unusual pro-
perties derived from the structure of the metal complexes and
their intermolecular interactions in the ground and/or excited
electronic states within the assemblies. We will start with a sur-
vey of the photophysical properties of selected transition metal
complexes, followed by an overview of the aggregation mechanism
they can undergo to. We will focus our attention on soft assemblies
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 49