Our interest not only in luminescent gels but also in the use of
metal complexes to monitor the gelation process started in 2007
( 273 ). In these studies, gels were prepared by using alternatively
a carboxylate-based aliphatic gelator bearing adamantyl sub-
stituents, or an anisole-substituted oxalamide derivative. The
cohesion between the small molecules assembling into filaments
was given by hydrogen bonds, while the balance between the
hydrophobic or hydrophilic backbone bearing adamantane or
anisole moieties, respectively, determined the gelating ability
either in DMF or water. The gels were doped with an hemicaged
Eu(III) complex, which was retained in the cavities containing
the entrapped solvent, which was monitored by the excited-state
lifetime of the lanthanide complex. The red-emitting unit effec-
tively acted as a sensor of the solvent environment within the
gel. However, the self-assembly process yielding the hydrogel
was nicely probed with the aid of a Ru(II) complex, which is not
luminescent in aqueous environments. The luminescence was
turned on upon gelation, due to the fact that the organometallic
species intercalated into the hydrophobic pockets of the network,
thus allowing a highly sensitive monitoring of the self-assembly.
These findings constitute a basis for the use of luminescent
probes, able to change their properties upon solidification of the
systems, for the monitoring of bond forming and breaking in soft
materials. To make use of the luminescence switching effect
upon self-assembly, we developed a Pt(II) complex that by itself
is nonluminescent, but is able to stack into luminescent gelating
nanofibers ( 217 ). The neutral, soluble Pt(II) coordination com-
pound bears a dianionic tridentate, terpyridine-like ligand. The
coordination of an alkyl-pyridine ancillary moiety to the 2,6-bis-
tetrazolyl-pyridine-based complex enhanced the solution
processability.
The Pt(II) gelator is nonemissive in diluted solution. However,
in frozen CH 2 Cl 2 matrix at 77 K and in thin films, it displays a
bright unstructured luminescence centered at 570 nm and also
shows an intense absorption band around 420 nm, a feature that
is not present in solution at room temperature. The
photoluminescence quantum yield reaches up to 87%, and the
emission spectra do not depend on the excitation wavelength.
These results are particularly remarkable considering that Pt
(II) complexes usually show rather low-emission intensity and
strongly concentration-dependent emission due to aggregate or
excimer formation in the solid state and in thin film. The
photophysical characteristics unambiguously point toward
aggregation processes in the ground state that lead to excited
(^3) MMLCT states, facilitated by the interaction between the axial
PHOTOPHYSICS OF MOLECULAR ASSEMBLIES 83