B. MICELLES ANDVESICLES
An interesting approach to have multiple components non-
covalent assembled, based on identical or different metal
complexes, keeping a well-defined structure, is to organize them
in micelles and vesicles. Such self-assembly, in certain solvents,
must be promoted by an amphiphilic nature of the metal complex
which can be achieved by tailoring the ligands coordinated to the
metal ion. A fine-tuning of the ligands could also allow obtaining
the correct geometrical shape to be able to discriminate between
micelles or double layer structures such as vesicles or eventually
linear arrangements such as fibers.
In a general design, the metal complex represents the head
group of the surfactant and often constitutes the polar part, as
most of the metallosurfactants investigated contain charged
complexes. The tails are then the hydrophobic units and can be
appended to the coordinated ligands to have a straight geometri-
cal shape or defining a conical form in the case of divergent or
double chains. The geometry of the assembly will therefore
depend on the relative size ratio chain/metal complex, the length
of the chain and its shape, the number and position on the
bipyridine ligands, as well as the nature of the counter-ion
( 50 – 52 ). However, the rationalization of the assembly is not that
straightforward. In fact, secondary intermolecular interactions
(pp, electrostatic, hydrogen bonds, etc.) as well as solvent and
temperature could play a major role resulting in assemblies of
unpredictable shape and size. Also, so far there are no pre-
dictions or experimental evidences on how to interchange from
one structure to another one in a reversible or desired way. In
simple metallosurfactants, it has been demonstrated by several
groups that they all display the typical surfactant-like behaviors,
such as adsorbing at interfaces, being it polar/apolar (e.g.,
micelles, vesicles and liposomes), solid/liquid (e.g., monolayers),
or liquid/gas (e.g., Langmuir–Blodgett films). Embedding the
typical self-assembling properties of surfactants and the inher-
ent chemical and physical properties of transition metal
complexes could provide a clever and straightforward way for
incorporating specific functionalities to an interface, such as
magnetism, color, or pH sensitivity, and even more importantly,
redox, photophysical, and catalytic properties.
Even though metallosurfactants are scarce compared to their
nonmetallic organic counterparts, there is an increasing interest
due of their multiple applications in fields such as probes in
emulsion ( 53 ), formation of monolayers (52,54– 56 ), thin film
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 59