Phillip J. Wright

Photophysical and Photochemical Trends in Tricarbonyl Rhenium(I) N-Heterocyclic Carbene Complexes

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO)3(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the central imidazole ring is substituted at the N3 atom by a butyl, a phenyl, or a mesityl group and substituted at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the ancillary L ligand alternates between chloro and bromo. For the majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus, the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re-C bond cleaves from excited states of metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the pyrimidyl- and quinoxyl-NHC ligands is proposed.

Modulation of the organelle specificity in Re(I) tetrazolato complexes leads to labeling of lipid droplets

The biological behaviour in terms of cellular incubation and organelle specificity for two complexes of the type fac-[Re(CO)3(phen)L], where phen is 1,10-phenanthroline and L is either 3-pyridyltetrazolate or 4-cyanophenyltetrazolate, are herein investigated. The emission signal detected from the live insect Drosophila and human cell lines, generated by exploiting two-photon excitation at 830 nm to reduce cellular damage and autofluorescence, suggests photophysical properties that are analogous to those measured from dilute solutions, meaning that the complexes remain intact within the cellular environment. Moreover, the rhenium complex linked to 4-cyanophenyltetrazolate shows high specificity for the lipid droplets, whereas the complex bound to 3-pyridyltetrazolate tends to localise within the lysosomes. This differential localisation implies that in these complexes, organelle specificity can be achieved and manipulated by simple functional group transformations thus avoiding more complex bioconjugation strategies. More importantly, these results highlight the first example of phosphorescent labeling of the lipid droplets, whose important cellular functions have been recently highlighted along with the fact that their role in the metabolism of healthy and diseased cells has not been fully elucidated.

Enhanced deep-blue emission from Pt(II) complexes bound to 2-pyridyltetrazolate and an ortho-xylene-linked bis(NHC)cyclophane

The coordination of 2-pyridyltetrazolate and ortho-xylene-linked bis(NHC)cyclophane to Pt(II) yielded a novel complex characterised by enhanced pure deep-blue emission, whose intensity can be modulated via methylation of the tetrazole ring.

Investigating Intracellular Localisation and Cytotoxicity Trends for Neutral and Cationic Iridium Tetrazolato Complexes in Live Cells

A family of five neutral cyclometalated iridium(III) tetrazolato complexes and their methylated cationic analogues have been synthesised and characterised. The complexes are distinguished by variations of the substituents or degree of π conjugation on either the phenylpyridine or tetrazolato ligands. The photophysical properties of these species have been evaluated in organic and aqueous media, revealing predominantly a solvatochromic emission originating from mixed metal-to-ligand and ligand-to-ligand charge transfer excited states of triplet multiplicity. These emissions are characterised by typically long excited-state lifetimes (∼hundreds of ns), and quantum yields around 5-10 % in aqueous media. Methylation of the complexes caused a systematic red-shift of the emission profiles. The behaviour and the effects of the different complexes were then examined in cells. The neutral species localised mostly in the endoplasmic reticulum and lipid droplets, whereas the majority of the cationic complexes localised in the mitochondria. The amount of complexes found within cells does not depend on lipophilicity, which potentially suggests diverse uptake mechanisms. Methylated analogues were found to be more cytotoxic compared to the neutral species, a behaviour that might to be linked to a combination of uptake and intracellular localisation.

One-step assembly of Re(I) tricarbonyl 2-pyridyltetrazolato metallacalix[3]arene with aqua emission and reversible three-electron oxidation

The reaction of 2-pyridyltetrazolate with [Re(CO)5X] (X = Cl, Br) yielded the formation of an unexpected cyclic metallacalix[3]arene, as revealed by X-ray structural studies, characterised by aqua emission and reversible three-electron oxidation.

Rhenium tetrazolato complexes coordinated to thioalkyl-functionalised phenanthroline ligands: synthesis, photophysical characterisation, and incubation in live HeLa cells

Three new complexes of formulation fac-[Re(CO)3(diim)L], where diim is either 1,10-phenanthroline or 1,10-phenanthroline functionalised at position 5 by a thioalkyl chain, and L is either a chloro or aryltetrazolato ancillary ligand, were synthesised and photophysically characterised. The complexes exhibit phosphorescent emission with maxima around 600 nm, originating from triplet metal-to-ligand charge transfer states with partially mixed ligand-to-ligand charge transfer character. The emission is relatively long-lived, within the 200-400 ns range, and with quantum yields of 2-4%. The complexes were trialed as cellular markers in live HeLa cells, along with two previously reported rhenium tetrazolato complexes bound to unsubstituted 1,10-phenanthroline. All five complexes exhibit good cellular uptake and non-specific perinuclear localisation. Upon excitation at 405 nm, the emission from the rhenium complexes could be clearly distinguished from autofluorescence, as demonstrated by spectral detection within the live cells. Four of the complexes did not appear to be toxic, however prolonged excitation could result in membrane blebbing. No major sign of photobleaching was detected upon multiple imaging on the same cell sample.

A molecular probe for the detection of polar lipids in live cells

Lipids have an important role in many aspects of cell biology, including membrane architecture/compartment formation, intracellular traffic, signalling, hormone regulation, inflammation, energy storage and metabolism. Lipid biology is therefore integrally involved in major human diseases, including metabolic disorders, neurodegenerative diseases, obesity, heart disease, immune disorders and cancers, which commonly display altered lipid transport and metabolism. However, the investigation of these important cellular processes has been limited by the availability of specific tools to visualise lipids in live cells. Here we describe the potential for ReZolve-L1™ to localise to intracellular compartments containing polar lipids, such as for example sphingomyelin and phosphatidylethanolamine. In live Drosophila fat body tissue from third instar larvae, ReZolve-L1™ interacted mainly with lipid droplets, including the core region of these organelles. The presence of polar lipids in the core of these lipid droplets was confirmed by Raman mapping and while this was consistent with the distribution of ReZolve-L1™ it did not exclude that the molecular probe might be detecting other lipid species. In response to complete starvation conditions, ReZolve-L1™ was detected mainly in Atg8-GFP autophagic compartments, and showed reduced staining in the lipid droplets of fat body cells. The induction of autophagy by Tor inhibition also increased ReZolve-L1™ detection in autophagic compartments, whereas Atg9 knock down impaired autophagosome formation and altered the distribution of ReZolve-L1™. Finally, during Drosophila metamorphosis fat body tissues showed increased ReZolve-L1™ staining in autophagic compartments at two hours post puparium formation, when compared to earlier developmental time points. We concluded that ReZolve-L1™ is a new live cell imaging tool, which can be used as an imaging reagent for the detection of polar lipids in different intracellular compartments.

Imaging nuclear, endoplasmic reticulum and plasma membrane events in real time.

Live cell imaging can provide important information on cellular dynamics; however, the full utilisation of this technology has been hampered by the limitations of imaging reagents. Metal‐based complexes have the potential to overcome many of the issues common to many current imaging agents. The rhenium (I)‐based complex fac‐[Re(CO)3(1,10‐phenanthroline)(4‐pyridyltetrazolate)], herein referred to as ReZolve‐ER™, shows promise as a live cell imaging agent with rapid cell uptake, low cytotoxicity, resistance to photobleaching and compatibility with multicolour imaging. ReZolve‐ER™ localised to the nuclear membrane/endoplasmic reticulum (ER) and allowed the detection of exocytotic events at the plasma membrane. Thus, we present a new imaging agent for monitoring live cell events in real time, which is ideal for imaging either short‐ or long‐time courses.

Laser-based formation of copper nanoparticles in aqueous solution: Optical properties, particle size distributions, and formation kinetics

We explore the formation kinetics, optical absorption spectra, and particle size distributions of copper nanoparticles (CuNPs) formed by direct laser ablation from the bulk metal via a process we refer to as Laser Ablation Synthesis in Solution (LASiS). Comparisons are made between CuNPs formed in pure water versus those formed in the presence of 1 × 10−4 M solutions of the N-donor ligands 4,4′-bipyridine (4,4′Bipy) and 1H-5-(4-pyridyl)tetrazole (T-4Py). CuNPs formed in pure water and in the presence of 4,4′Bipy display similar UV-visible absorption spectra and very similar particle size distributions. In comparison, CuNPs formed in the presence of T-4Py display significantly different absorption properties, with the surface plasmon resonance transition blue-shifted by ∼55 nm, and a much smaller and narrower particle size distribution compared with the former samples. Based on previous literature reports, it is possible to ascribe these differences to differences in the CuNP surface oxidation states for samples prepared in the presence of T-4Py. However, an analysis of the formation kinetics of all three samples indicates near-identical behaviour.