Dinuclear Ruthenium(II) Complexes as Two-Photon, Time-Resolved Emission Microscopy Probes for Cellular DNA

A Ruthenium(II) Polypyridyl Complex Disrupts Actin Cytoskeleton Assembly and Blocks Cytokinesis

The dinuclear Ru^II complex [(Ru(phen)₂)₂(tpphz)]⁴⁺ (phen=1,10‐phenanthroline, tpphz=tetrapyridophenazine) “RuRuPhen” blocks the transformation of G‐actin monomers to F‐actin filaments with no disassembly of pre‐formed F‐actin. Molecular docking studies indicate multiple RuRuPhen molecules bind to the surface of G‐actin but not the binding pockets of established actin polymerisation inhibitors. In cells, addition of RuRuPhen causes rapid disruption to actin stress fibre organisation, compromising actomyosin contractility and cell motility; due to this effect RuRuPhen interferes with late‐stage cytokinesis. Immunofluorescent microscopy reveals that RuRuPhen causes cytokinetic abscission failure by interfering with endosomal sorting complexes required for transport (ESCRT) complex recruitment.

A Ruthenium(II) Polypyridyl Complex Disrupts Actin Cytoskeleton Assembly and Blocks Cytokinesis

The dinuclear RuII complex [(Ru(phen)2)2(tpphz)]4+ (phen=1,10-phenanthroline, tpphz=tetrapyridophenazine) "RuRuPhen" blocks the transformation of G-actin monomers to F-actin filaments with no disassembly of pre-formed F-actin. Molecular docking studies indicate multiple RuRuPhen molecules bind to the surface of G-actin but not the binding pockets of established actin polymerisation inhibitors. In cells, addition of RuRuPhen causes rapid disruption to actin stress fibre organisation, compromising actomyosin contractility and cell motility; due to this effect RuRuPhen interferes with late-stage cytokinesis. Immunofluorescent microscopy reveals that RuRuPhen causes cytokinetic abscission failure by interfering with endosomal sorting complexes required for transport (ESCRT) complex recruitment.

Iridium(III) Sensitisers and Energy Upconversion: The Influence of Ligand Structure upon TTA-UC Performance

Six substituted ligands based upon 2-(naphthalen-1-yl)quinoline-4-carboxylate and 2-(naphthalen-2-yl)quinoline-4-carboxylate have been synthesised in two steps from a range of commercially available isatin derivatives. These species are shown to be effective cyclometallating ligands for Ir^III , yielding complexes of the form [Ir(C^N)₂ (bipy)]PF₆ (where C^N=cyclometallating ligand; bipy=2,2'-bipyridine). X-ray crystallographic studies on three examples demonstrate that the complexes adopt a distorted octahedral geometry wherein a cis-C,C and trans-N,N coordination mode is observed. Intraligand torsional distortions are evident in all cases. The Ir^III complexes display photoluminescence in the red part of the visible region (668-693 nm), which is modestly tuneable through the ligand structure. The triplet lifetimes of the complexes are clearly influenced by the precise structure of the ligand in each case. Supporting computational (DFT) studies suggest that the differences in observed triplet lifetime are likely due to differing admixtures of ligand-centred versus MLCT character instilled by the facets of the ligand structure. Triplet-triplet annihilation upconversion (TTA-UC) measurements demonstrate that the complexes based upon the 1-naphthyl derived ligands are viable photosensitisers with upconversion quantum efficiencies of 1.6-6.7 %.

Dual-Phase Thermally Activated Delayed Fluorescence Luminogens: A Material for Time-Resolved Imaging Independent of Probe Pretreatment and Probe Concentration

Developing luminescent probes with long lifetime and high emission efficiency is essential for time-resolved imaging. However, the practical applications usually suffer from emission quenching of traditional luminogens in aggregated states, or from weak emission of aggregation-induced emission type luminogens in monomeric states. Herein, we overcome this dilemma by a rigid-and-flexible alternation design in donor-acceptor-donor skeletons, to achieve a thermally activated delayed fluorescence luminogen with high emission efficiency both in the monomeric state (quantum yield up to 35.3 %) and in the aggregated state (quantum yield up to 30.8 %). Such a dual-phase strong and long-lived emission allows a time-resolved luminescence imaging, with an efficiency independent of probe pretreatment and probe concentration. The findings open opportunities for developing luminescent probes with a usage in larger temporal and spatial scales.

The development of anticancer ruthenium(ii) complexes: from single molecule compounds to nanomaterials

Cancer is rapidly becoming the top killer in the world. Most of the FDA approved anticancer drugs are organic molecules, while metallodrugs are very scarce. The advent of the first metal based therapeutic agent, cisplatin, launched a new era in the application of transition metal complexes for therapeutic design. Due to their unique and versatile biochemical properties, ruthenium-based compounds have emerged as promising anti-cancer agents that serve as alternatives to cisplatin and its derivertives. Ruthenium(iii) complexes have successfully been used in clinical research and their mechanisms of anticancer action have been reported in large volumes over the past few decades. Ruthenium(ii) complexes have also attracted significant attention as anticancer candidates; however, only a few of them have been reported comprehensively. In this review, we discuss the development of ruthenium(ii) complexes as anticancer candidates and biocatalysts, including arene ruthenium complexes, polypyridyl ruthenium complexes, and ruthenium nanomaterial complexes. This review focuses on the likely mechanisms of action of ruthenium(ii)-based anticancer drugs and the relationship between their chemical structures and biological properties. This review also highlights the catalytic activity and the photoinduced activation of ruthenium(ii) complexes, their targeted delivery, and their activity in nanomaterial systems.

Photochemically active DNA-intercalating ruthenium and related complexes - insights by combining crystallography and transient spectroscopy

Recent research on the study of the interaction of ruthenium polypyridyl compounds and defined sequence nucleic acids is reviewed. Particular emphasis is paid to complexes [Ru(LL)2(Int)]2+ containing potentially intercalating ligands (Int) such as dipyridophenazine (dppz), which are known to display light-switching or photo-oxidising behaviour, depending on the nature of the ancillary ligands. X-ray crystallography has made a key contribution to our understanding, and the first complete survey of structural results is presented. These include sequence, enantiomeric, substituent and structural specificities. The use of ultrafast transient spectroscopic methods to probe the ultrafast processes for complexes such as [Ru(TAP)2(dppz)]2+ and [Ru(phen)2(dppz)]2+ when bound to mixed sequence oligonucleotides are reviewed with particular attention being paid to the complementary advantages of transient (visible) absorption and time-resolved (mid) infra-red techniques to probe spectral changes in the metal complex and in the nucleic acid. The observed photophysical properties are considered in light of the structural information obtained from X-ray crystallography. In solution, metal complexes can be expected to bind at more than one DNA step, so that a perfect correlation of the photophysical properties and factors such as the orientation or penetration of the ligand into the intercalation pocket should not be expected. This difficulty can be obviated by carrying out TRIR studies in the crystals. Dppz complexes also undergo insertion, especially with mismatched sequences. Future areas for study such as those involving non-canonical forms of DNA, such as G-quadruplexes or i-motifs are also briefly considered.

Precision targeted ruthenium(ii) luminophores; highly effective probes for cell imaging by stimulated emission depletion (STED) microscopy

Fluorescence microscopy has undergone a dramatic evolution over the past two decades with development of super-resolution far-field microscopy methods that break the light diffraction limited resolution of conventional microscopy, offering unprecedented opportunity to interrogate cellular processes at the nanoscale. However, these methods make special demands of the luminescent agents used for contrast and development of probes suited to super-resolution fluorescent methods is still relatively in its infancy. In spite of their many photophysical advantages, metal complex luminophores have not yet been considered as probes in this regard, where to date, only organic fluorophores have been applied. Here, we report the first examples of metal complex luminophores applied as probes for use in stimulated emission depletion (STED) microscopy. Exemplified with endoplasmic reticulum and nuclear targeting complexes we demonstrate that luminescent Ru(ii) polypyridyl complexes can, through signal peptide targeting, be precisely and selectively delivered to key cell organelles without the need for membrane permeabilization, to give high quality STED images of these organelles. Detailed features of the tubular ER structure are revealed and in the case of the nuclear targeting probe we exploit the molecular light switch properties of a dipyrido[3,2-a:2',3'-c]phenazine containing complex which emits only on DNA/RNA binding to give outstanding STED contrast and resolution of the chromosomes within the nucleus. Comparing performance with a member of the AlexaFluor family commonly recommended for STED, we find that the performance of the ruthenium complexes is superior across both CW and gated STED microscopy methods in terms of image resolution and photostability. The large Stokes shifts of the Ru probes permit excellent matching of the stimulating depletion laser with their emission whilst avoiding anti-Stokes excitation. Their long lifetimes make them particularly amenable to gated STED, giving a much wider window for gating than traditional probes. Our findings indicate that ruthenium polypyridyl peptide targeted probes are a powerful new partner to STED microscopy, opening up new approaches to probe design for STED microscopy.

Serum Albumin Binding Inhibits Nuclear Uptake of Luminescent Metal-Complex-Based DNA Imaging Probes

The DNA binding and cellular localization properties of a new luminescent heterobimetallic Ir(III) Ru(II) tetrapyridophenazine complex are reported. Surprisingly, in standard cell media, in which its tetracationic, isostructural Ru(II) Ru(II) analogue is localized in the nucleus, the new tricationic complex is poorly taken up by live cells and demonstrates no nuclear staining. Consequent cell-free studies reveal that the Ir(III) Ru(II) complex binds bovine serum albumin, BSA, in Sudlow's Site I with a similar increase in emission and binding affinity to that observed with DNA. Contrastingly, in serum-free conditions the complex is rapidly internalized by live cells, where it localizes in cell nuclei and functions as a DNA imaging agent. The absence of serum proteins also greatly alters the cytotoxicity of the complex, where high levels of oncosis/necrosis are observed due to this enhanced uptake. This suggests that simply increasing the lipophilicity of a DNA imaging probe to enhance cellular uptake can be counterproductive as, due to increased binding to serum albumin protein, this strategy can actually disrupt nuclear targeting.

Combined two-photon excitation and d→f energy transfer in a water-soluble Ir(III)/Eu(III) dyad: two luminescence components from one molecule for cellular imaging

The first example of cell imaging using two independent emission components from a dinuclear d/f complex is reported. A water-stable, cell-permeable Ir(III) /Eu(III) dyad undergoes partial Ir→Eu energy transfer following two-photon excitation of the Ir unit at 780 nm. Excitation in the near-IR region generated simultaneously green Ir-based emission and red Eu-based emission from the same probe. The orders-of-magnitude difference in their timescales (Ir ca. μs; Eu ca. 0.5 ms) allowed them to be identified by time-gated detection. Phosphorescence lifetime imaging microscopy (PLIM) allowed the lifetime of the Ir-based emission to be measured in different parts of the cell. At the same time, the cells are simultaneously imaged by using the Eu-based emission component at longer timescales. This new approach to cellular imaging by using dual d/f emitters should therefore enable autofluorescence-free sensing of two different analytes, independently, simultaneously and in the same regions of a cell.

RutheniumII complexes bearing fused polycyclic ligands: from fundamental aspects to potential applications

In this review, we first discuss the photophysics reported in the literature for mononuclear ruthenium complexes bearing ligands with extended aromaticity such as dipyrido[3,2-a:2',3'-c]phenazine (DPPZ), tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]-phenazine (TPPHZ),  tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]acridine (TPAC), 1,10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene (PHEHAT) 9,11,20,22-tetraaza- tetrapyrido[3,2-a:2',3'-c:3'',2''-l:2''',3'''-n]pentacene (TATPP), etc. Photophysical properties of binuclear and polynuclear complexes based on these extended ligands are then reported. We finally develop the use of binuclear complexes with extended π-systems for applications such as photocatalysis.