Colour change and luminescence enhancement in a cholesterol-based terpyridyl platinum metallogel via sonication

Two organometallic terpyridyl platinum gelators containing hydrophobic cholesterol with or without large steric hindrance were synthesized and characterized. The complex with three bulky tert-butyl groups (1a) can form a stable gel in several kinds of solvent, such as alcohol, ethyl acetate and toluene with ultrasound irradiation. These sonicated gels display different colour and emission properties compared with their related solutions and precipitates. The intensity of the photoluminescence in gel or solid state of 1a is obviously enhanced by aggregation. The morphology, as well as the surface wettability of 1a can also be reversibly adjusted by sonication and heating. However, the complex without three bulky tert-butyl groups (1b) has poorer solubility and gelation capability, and lower luminescence quantum yield due to strong intermolecular pi-pi stacking and metal-metal interaction. The spectral and structural variation was characterized by UV-visible and fluorescence spectra, SEM and TEM images, and XRD analysis. The mechanism of the molecularly self-assembled pattern indicates that the large steric hindrance is the reason for the different gelation capabilities and emission properties of the two complexes. Sonication may change the molecular conformation of 1a and promote the ionic dipolar interaction and hydrophobic interactions for gelation, and thus switch the optical properties of the complex in the aggregated state.