Luminescent ruthenium(II) complex bearing bipyridine and N-heterocyclic carbene-based C∧N∧C pincer ligand for live-cell imaging of endocytosis

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

Luminescence imaging is a powerful and versatile technique for investigating cell physiology and pathology in living systems, making significant contributions to life science research and clinical diagnosis. In recent years, luminescent transition metal complexes have gained significant attention for diagnostic and therapeutic applications due to their unique photophysical and photochemical properties. In this Review, we provide a comprehensive overview of the recent development of luminescent transition metal complexes for bioimaging and biosensing applications, with a focus on transition metal centers with a d6, d8, and d10 electronic configuration. We elucidate the structure-property relationships of luminescent transition metal complexes, exploring how their structural characteristics can be manipulated to control their biological behavior such as cellular uptake, localization, biocompatibility, pharmacokinetics, and biodistribution. Furthermore, we introduce the various design strategies that leverage the interesting photophysical properties of luminescent transition metal complexes for a wide variety of biological applications, including autofluorescence-free imaging, multimodal imaging, organelle imaging, biological sensing, microenvironment monitoring, bioorthogonal labeling, bacterial imaging, and cell viability assessment. Finally, we provide insights into the challenges and perspectives of luminescent transition metal complexes for bioimaging and biosensing applications, as well as their use in disease diagnosis and treatment evaluation.

Long-lived MLCT states for Ru(ii) complexes of ferrocene-appended 2,2'-bipyridines

In this study, we present two ruthenium(ii) diimine complexes appended with ferrocene which show metal to ligand charge transfer 3MLCT emission lifetimes around 630 ns. We also present a similar complex with two ferrocene units which has decreased emission. These complexes have been studied by electrochemical, electronic absorption, and Raman, resonance Raman and transient resonance Raman means, coupled with density functional theoretical approaches. For these systems, the optical spectra are dominated by a low energy ruthenium(ii) MLCT transition; which can be modulated by the presence of pendant ferrocene units and the extent of conjugation of the ferrocenyl bipyridine backbone. Tuning of the lowest energy transition in terms of intensity (4 to 18 × 10-3 M-1 cm-1) and energy (535 to 563 nm) was achieved by these means.

Study of a Miniaturizable System for Optical Sensing Application to Human Cells

Conventional approaches to human intracellular optical sensing, generally, require dedicated laboratories with bulky detection systems. They are performed by cell labeling procedures based on the use of fluorophores that are, mostly, phototoxic, invasive, bleached in case of prolonged light exposures, which require carriers and/or structural modifications for the cellular uptake. These issues, together with the sensitivity of the eukaryotic cell model, could be problematic towards the development of a robust sensing system suitable for biomedical screening. In this work, we studied a sensing system resulting from the combination of the commercial tris(2,2’bipyridyl)ruthenium(II) fluorophore, for cell labeling, with a potentially miniaturizable optical system composed by a laser source and a photomultiplier tube, for the fluorescence analysis.

Reaction of FcC[triple bond, length as m-dash]CC(O)R (Fc = ferrocenyl) with Ru3(CO)12 leading to unexpected nitro-group reduced ruthenoles and 1,2-CO-inserted triruthenium clusters

The reaction of Ru3(CO)12 with ferrocene-containing alkynyl ketones FcC[triple bond, length as m-dash]CC(O)R (Fc = ferrocenyl; R = Ph (1); 2-thienyl (2); 4-CH3O-Ph (3); 4-NH2-Ph (4); 4-NO2-Ph (5); ferrocenyl (6)) proceeds in toluene with the formation of triruthenium clusters (1a-6a), ruthenoles (1b-5b, 5c and 1d-5d) and unexpected 1,2-CO-inserted triruthenium clusters (1c-4c). 1a-6a were isolated from the reaction of Ru3(CO)12 with one equivalent of 1-6, respectively. Ruthenoles 1b-5b, 5c and 1d-5d were collected by adding 1-5 to the corresponding 1a-5a in a molar ratio of 1 : 1, respectively. Unexpectedly, the nitro group in one of the two phenyl rings in both 5c and 5d molecules was reduced to an amino group, while their ruthenole skeletons are retained. When 1-4 were added to the corresponding 1a-4a in a molar ratio of 1 : 1, respectively, the unusual triruthenium clusters (1c-4c) were isolated, involving 1,2-insertion of a terminal coordinated carbonyl between two C[triple bond, length as m-dash]C units of the ynone molecules. No reaction between 6a and 6 was observed. And the familiar cyclotrimerization products were not found. All new compounds were characterized by NMR, FT-IR, and MS-ESI and most of them were structurally confirmed by single crystal X-ray diffraction. The results suggested that the ferrocenyl groups in the 1,3-ynones exhibit strong electron and steric effects on the reaction process and product distribution during their reactions with Ru3(CO)12.

Fluorescent antitumor titanium(iv) salen complexes for cell imaging

First live cell imaging using florescent salen Ti(iv) complexes, which are cytotoxic and inactive, both entering the cell but with different subcellular accumulations.

π-Expansive Heteroleptic Ruthenium(II) Complexes as Reverse Saturable Absorbers and Photosensitizers for Photodynamic Therapy

Five heteroleptic tris-diimine ruthenium(II) complexes [RuL(N^N)₂](PF₆)₂ (where L is 3,8-di(benzothiazolylfluorenyl)-1,10-phenanthroline and N^N is 2,2'-bipyridine (bpy) (1), 1,10-phenanthroline (phen) (2), 1,4,8,9-tetraazatriphenylene (tatp) (3), dipyrido[3,2-a:2',3'-c]phenazine (dppz) (4), or benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn) (5), respectively) were synthesized. The influence of π-conjugation of the ancillary ligands (N^N) on the photophysical properties of the complexes was investigated by spectroscopic methods and simulated by density functional theory (DFT) and time-dependent DFT. Their ground-state absorption spectra were characterized by intense absorption bands below 350 nm (ligand L localized ¹π,π* transitions) and a featureless band centered at ∼410 nm (intraligand charge transfer (¹ILCT)/¹π,π* transitions with minor contribution from metal-to-ligand charge transfer (¹MLCT) transition). For complexes 4 and 5 with dppz and dppn ligands, respectively, broad but very weak absorption (ε < 800 M^-1 cm^-1) was present from 600 to 850 nm, likely emanating from the spin-forbidden transitions to the triplet excited states. All five complexes showed red-orange phosphorescence at room temperature in CH₂Cl₂ solution with decreased lifetimes and emission quantum yields, as the π-conjugation of the ancillary ligands increased. Transient absorption (TA) profiles were probed in acetonitrile solutions at room temperature for all of the complexes. Except for complex 5 (which showed dppn-localized ³π,π* absorption with a long lifetime of 41.2 μs), complexes 1-4 displayed similar TA spectral features but with much shorter triplet lifetimes (1-2 μs). Reverse saturable absorption (RSA) was demonstrated for the complexes at 532 nm using 4.1 ns laser pulses, and the strength of RSA decreased in the order: 2 ≥ 1 ≈ 5 > 3 > 4. Complex 5 is particularly attractive as a broadband reverse saturable absorber due to its wide optical window (430-850 nm) and long-lived triplet lifetime in addition to its strong RSA at 532 nm. Complexes 1-5 were also probed as photosensitizing agents for in vitro photodynamic therapy (PDT). Most of them showed a PDT effect, and 5 emerged as the most potent complex with red light (EC₅₀ = 10 μM) and was highly photoselective for melanoma cells (selectivity factor, SF = 13). Complexes 1-5 were readily taken up by cells and tracked by their intracellular luminescence before and after a light treatment. Diagnostic intracellular luminescence increased with increased π-conjugation of the ancillary N^N ligands despite diminishing cell-free phosphorescence in that order. All of the complexes penetrated the nucleus and caused DNA condensation in cell-free conditions in a concentration-dependent manner, which was not influenced by the identity of N^N ligands. Although the mechanism for photobiological activity was not established, complexes 1-5 were shown to exhibit potential as theranostic agents. Together the RSA and PDT studies indicate that developing new agents with long intrinsic triplet lifetimes, high yields for triplet formation, and broad ground-state absorption to near-infrared (NIR) in tandem is a viable approach to identifying promising agents for these applications.

Luminescent Iridium(III) Complexes Supported by N-Heterocyclic Carbene-based C^C^C-Pincer Ligands and Aromatic Diimines

Iridium(III) hydrido complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1-butylimidazolin-2-ylidene)phenyl anion (C¹^C^C¹) or 1,3-bis(3-butylbenzimidazolin-2-ylidene)phenyl anion (C²^C^C²) and aromatic diimine (2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 4,4′-dimethyl-2,2′-bipyridine (Me₂bpy), or dipyrido-[3,2-f:2′,3′-h]-quinoxaline (dpq)) in the form of [Ir(C^C^C)(N^N)(H)]⁺ have been prepared. Crystal structures for these complexes show that the Ir–C_NHC distances are 2.043(5)–2.056(5) Å. The hydride chemical shifts for complexes bearing C¹^C^C¹ (−20.6 to −20.3 ppm) are more upfield than those with C²^C^C² (−19.5 and −19.2 ppm), revealing that C¹^C^C¹ is a better electron donor than C²^C^C². Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (λ = 340–530 nm (ε ≤ 10³ dm³ mol⁻¹ cm⁻¹)) originate from a d_π(Ir^III) → π*(N^N) metal-to-ligand charge transfer transition, where the d_π(Ir^III) level contain significant contribution from the C^C^C ligands. All these complexes are emissive in the yellow-spectral region (553–604 nm in CH₃CN and CH₂Cl₂) upon photo-excitation with quantum yields of 10⁻³–10⁻¹.