Photophysical and Biological Properties of Iridium Tetrazolato Complexes Functionalised with Fatty Acid Chains

Abstract

Five cyclometalated Ir(III) tetrazolato complexes functionalised with fatty acid chains (octanoic, palmitic, stearic, palmitoleic, and oleic) have been synthesised. The fatty acids were chosen to evaluate the potential effect of the length and degree of unsaturation on the biological properties of the complexes for use as cellular imaging agents. The complexes were analysed in both organic and aqueous media to determine if the presence and nature of the fatty acid chains had a significant effect on their photophysical properties. The complexes display green–yellow emission in dichloromethane solutions with relatively long excited state decays, within the range 360–393 ns, and quantum yields between 5.4% and 6.7% (from degassed solutions). Temperature-dependent photophysical studies suggest that the emitting excited states of the complexes might be quenched by the thermal population of dark states. In water, the quantum yields drop within the range of 0.5%–2.4%, and the photophysical measurements are influenced by the variable degrees of aggregation. In general, the entire series displayed low cytotoxicity and relatively high photostability, which are favourable attributes in the design of cellular imaging agents. Images of live HeLa cells were obtained for all the complexes, but those functionalised with palmitic and stearic acids had limitations due the lower solubility conferred by the saturated aliphatic chains. The complexes were mainly detected within the endoplasmic reticulum.

Shining New Light on Biological Systems: Luminescent Transition Metal Complexes for Bioimaging and Biosensing Applications

“…286 Massi and co-workers have studied the photophysical and biological behavior of neutral iridium(III) tetrazolato complexes modified with a fatty acid of different chain lengths and saturation (131) (Chart 61). 287 The length and degree of unsaturation of the fatty acid chains do not significantly impact the photophysical properties, lipophilicity, and cytotoxic activity of the conjugates. Upon cellular internalization, conjugates 131a−131c reside in the ER of live HeLa cells.…”

Strategies to promote permeation and vectorization, and reduce cytotoxicity of metal complex luminophores for bioimaging and intracellular sensing

“…In general, increasing the lipophilicity of the metal through modification of the ligand or applying a lipophilic pendant, has been successful approach to promoting metal complex uptake, drawbacks of this approach are that such complexes can have low water solubility and so require organic solvent for dissolution. 54 As commonly used solvents in imaging, DMSO and ethanol increase membrane permeability, it can be difficult to assess the true permeability of probes that are pre-dissolved in such solvent. Furthermore, although by no means a universal effect, increasing lipophilicity can promote dark cytotoxicity, often because the mitochondria is targeted.…”