Multiresponsive Luminescent Cationic Cyclometalated Gold(III) Amphiphiles and Their Supramolecular Assembly

A new class of amphiphilic tridentate cyclometalated
gold­(III)
complexes has been designed and synthesized as luminescent supramolecular
building blocks. Positively charged trimethylammonium (−CH2NMe3+) containing alkynyl ligands have
been incorporated to introduce the electrostatic interactions. The
X-ray crystal structures of two of the complexes have been determined,
and the existence of π–π interactions between molecules
has been observed. Steady-state and time-resolved absorption and emission
studies have been carried out to investigate the nature of the excited
states. The complexes are found to exhibit self-assembly properties
with the assistance of π–π stacking and hydrophobic
interactions and possibly weak Au···Au interaction,
resulting in notable emergence of low-energy absorption bands and
luminescence changes. The presence of a large hydrophobic moiety is
found to be crucial for the formation of aggregates, especially in
polar media where hydrophobic interactions play an important role.
The nature of the counterion has been shown to have a significant
effect on the extent of self-assembly in different media. Upon aggregation,
nanofibers are formed in polar media, while nanorods are observed
in nonpolar media in one of the representative complexes. Interestingly,
a small modification on the alkynyl ligand resulted in the formation
of nanoribbons instead. Intriguing luminescence mechanochromic properties
have also been observed. This orthogonal and rational molecular design
strategy has been shown to be effective in the construction of gold­(III)-based
smart and multiresponsive materials.