quantum yield, which is attributed to the isolation of the lumi-
nescent chelate from the oxygen dissolved in the liquid phase.
These findings consistently indicate that introduction of metal
chelates into the gel is one of the most effective strategies toward
a variety of photo- and electrochemical nanomaterials.
A last example refers to the complexes described by Yamet al.
( 220 ). They have synthesized a series of alkynylplatinum(II)
terpyridyl complexes and investigated their electrochemical,
photophysical, and luminescence properties. Some of the
complexes were found to exhibit stable thermotropic metallogels
in organic solvents and were morphologically characterized by
transmission electron microscopy and scanning electron micros-
copy (Fig. 25). One of them showed drastic color changes from a
deep-purple gel to an orange sol during the gel-to-sol phase tran-
sition. In the variable temperature UV–vis absorption study of a
DMSO gel, the MMLCT absorption shoulder at 580 nm dis-
appears completely above the sol–gel phase-transition tempera-
ture (Tgel), indicating that PtPt and p–p interactions are
involved in the process of metallogelation and are completely
destroyed in the sol form at elevated temperature. This is further
supported by the observation of the complete switching off of the
(^3) MMLCT emission at 780 nm at T
gel. The DMSO metallogels
also displayed different colors, with different UV–vis and emis-
sion spectral traces depending on the nature of the counter
anions, which governs the degree of aggregation and the extent
of PtPt andp–pinteractions in the gel phases, which results
in the formation of different supramolecular architectures.
The systematic study of these alkynylplatinum (II) terpyridyl
complexes has led to a better understanding of the factors
that direct the gel-formation properties. Longer hydrocarbon
chains are found to enhance solubility in most common organic
solvents hindering the formation of stable metallogels, whereas
bulky tert-butyl substituents on the terpyridyl ligand forms
less stable metallogels, as reflected by its higher critical
gelation concentration and lower sol–gel transition temperature
than the unsubstituted terpyridyl analogue. Metal–metal
andp–pinteractions have also been established to play impor-
tant roles in the stabilization of these metallogels. This
demonstrates the subtle interplay of factors influencing the
formation and stability of these metallogels and shows that
with modifications of the ligands as well as variation of the
organic solvents, the electronic absorption, and luminescence
properties could readily be tuned.
88 CRISTIAN A. STRASSERTet al.