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.

Iridium(III)-Incorporating Self-Healing Polysiloxanes as Materials for Light-Emitting Oxygen Sensors

Polymer-metal complexes (PMCs) based on poly(2,2'-bipyridine-4,4'-dicarboxamide-co-polydimethylsiloxanes) with cyclometalated di(2-phenylpyridinato-C2,N')iridium(III) fragments and cross-linked by Zn2+ (Zn[Ir]-BipyPDMSs) or Ir3+ (Ir[Ir]-BipyPDMSs) represent flexible, stretchable, phosphorescent, and self-healing molecular oxygen sensors. PMCs provide strong phosphorescence at λem = 595-605 nm. Zn[Ir]-BipyPDMS with PDMS chain length of Mn = 5000 has the highest quantum yield of 9.3% and is a molecular oxygen sensor at different O2 concentrations (0-100 vol%) compared to Ir[Ir]-BipyPDMSs. A Stern-Volmer constant is determined for Zn[Ir]-BipyPDMS as KSV = 0.014%-1, which is similar to the reported oxygen-sensitive iridium(III) complexes. All synthesized PMCs exhibit high elongation at break (up to 1100%) and self-healing efficiency (up to 99%).

Organometallic Ir(III) complexes: post-synthetic modification, photophysical properties and binuclear complex construction

Two methods of post-synthetic modification (Suzuki coupling and CuAAC click-reaction) were applied to Ir(III) complexes [Ir(C^{^}N)₂N^{^}N]⁺ to provide the second highly selective donor site. One family of functionalized complexes was used to demonstrate the potential of post-synthetic modification for controlled construction of d-d and d-f binuclear complexes. The complexes obtained were characterized by CHN elemental analysis, NMR spectroscopy, ESI mass-spectrometry, FTIR spectroscopy and single crystal X-ray diffraction analysis. By means of XPS and NEXAFS spectroscopy the coordination of diimine donor site to the Ln(III) centre has been definitely confirmed. The photophysical properties of mono- and binuclear complexes were carefully investigated, and the evolution of luminescent characteristics during the formation of a system of connected metallocenters is also discussed. TDDFT calculations were used to describe the luminescence mechanism and to confirm the conclusions made on the basis of experimental data.

Mechanochemical Magnesium-Mediated Minisci C-H Alkylation of Pyrimidines with Alkyl Bromides and Chlorides

A novel method to synthesize 4-alkylpyrimidines by the mechanochemical magnesium-mediated Minisci reaction of pyrimidine derivatives and alkyl halides has been reported. The reaction process operates with a broad substrate scope and excellent regioselectivity under mild conditions with no requirement of transition-metal catalysts, solvents, and inert gas protection. The practicality of this protocol has been demonstrated by the up-scale synthesis, mechanochemical product derivatization, and antimalarial drug pyrimethamine preparation.

Evaluation of the photocatalytic performance of molecularly imprinted S-TiO2 by paper microzones

The paper microzones method (PMZs) is a green chemical method that uses the principle of the three primary colors of red, green and blue (RGB) to detect the water quality of the droplets on white paper. However, this method is rarely used in the performance evaluation of photocatalysts. The paper details the first use of paper microzones utilized in the evaluation of photocatalyst performance. A sol-gel method was used to prepare molecularly imprinted modified TiO₂ photocatalysts for the treatment of different wastewaters, and characterized the catalysts using XRD and several other methods. The reliability of PMZs on the evaluation of photocatalytic activity and selectivity was also analyzed. The following results were obtained: EP-TiO₂ catalysts (EP, ethyl paraben, the imprinting molecule) with different S doping levels were synthesized using a one-step sol-gel method, and the best S doping ratio was found to be n(Ti):n(S) 3:1. S-EP-TiO₂ was found to be 100% anatase and showed excellent photocatalytic performance, while the PMZs method accurately determined changes in RGB levels for the photocatalytic degradation process of pollutants using S-EP-TiO₂ as the photocatalyst. A photocatalytic kinetic analysis showed the PMZs method was quite suitable for the evaluation of photocatalyst activity, but the evaluation of selectivity needs improvement. This method is a promising green chemistry way to evaluate photocatalyst performance and the rapid detection of outdoor sewage water quality.

A Cyclodextrin-Hosted Ir(III) Complex for Ratiometric Mapping of Tumor Hypoxia In Vivo

Hypoxia is considered as a key microenvironmental feature of solid tumors. Luminescent transition metal complexes particularly those based on iridium and ruthenium have shown remarkable potentials for constructing sensitive oxygen-sensing probes due to their unique oxygen quenching pathway. However, the low aqueous solubility of these complexes largely retards their sensing applications in biological media. Moreover, it remains difficult so far to use the existing complexes typically possessing only one luminescent domain to quantitatively detect the intratumoral hypoxia degree. Herein, an Ir(III) complex showing red emissions is designed and synthesized, and innovatively encapsulated within a hydrophobic pocket of Cyanine7-modified cyclodextrin. The Ir(III) complex enables the oxygen detection, while the cyclodextrin is used not only for improving the water solubility and suppressing the luminescence quenching effect of the surrounding aqueous media, but also for carrying Cyanine7 to establish a ratiometric oxygen fluorescence probe. 2D nuclear magnetic resonance is carried out to confirm the host-guest structure. The oxygen-responsive ability of the resulting ratiometric probe is evaluated through in vitro cell and multicellular experiments. Further animal studies about tumor oxygen level mapping demonstrate that the probe can be successfully used for quantitatively visualizing tumor hypoxia in vivo.

Multinuclear 2-(Quinolin-2-yl)quinoxaline-Coordinated Iridium(III) Complexes Tethered by Carbazole Derivatives: Synthesis and Photophysics

Five mono/di/trinuclear iridium(III) complexes (1-5) bearing the carbazole-derivative-tethered 2-(quinolin-2-yl)quinoxaline (quqo) diimine (N^N) ligand were synthesized and characterized. The photophysical properties of these complexes and their corresponding diimine ligands were systematically studied via UV-vis absorption, emission, and transient absorption (TA) spectroscopy and simulated by time-dependent density functional theory. All complexes possessed strong well-resolved absorption bands at <400 nm that have predominant ligand-based ¹π,π* transitions and broad structureless charge-transfer (¹CT) absorption bands at 400-700 nm. The energies or intensities of these ¹CT bands varied pronouncedly when the number of tethered Ir(quqo)(piq)₂⁺ (piq refers to 1-phenylisoquinoline) units, π conjugation of the carbazole derivative linker, or attachment positions on the carbazole linker were altered. All complexes were emissive at room temperature, with 1-3 showing near-IR (NIR) ³MLCT (metal-to-ligand charge-transfer)/³LLCT (ligand-to-ligand charge-transfer) emission at ∼710 nm and 4 and 5 exhibiting red or NIR ³ILCT (intraligand charge-transfer)/³LMCT (ligand-to-metal charge-transfer) emission in CH₂Cl₂. In CH₃CN, 1-3 displayed an additional emission band at ca. 590 nm (³ILCT/³LMCT/³MLCT/³π,π* in nature) in addition to the 710 nm band. The different natures of the emitting states of 1-3 versus those of 4 and 5 also gave rise to different spectral features in their triplet TA spectra. It appears that the parentage and characteristics of the lowest triplet excited states in these complexes are mainly impacted by the π systems of the bridging carbazole derivatives and essentially no interactions among the Ir(quqo)(piq)₂⁺ units. In addition, all of the diimine ligands tethered by the carbazole derivatives displayed a dramatic solvatochromic effect in their emission due to the predominant intramolecular charge-transfer nature of their emitting states. Aggregation-enhanced emission was also observed from the mixed CH₂Cl₂/ethyl acetate or CH₂Cl₂/hexane solutions of these ligands.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

An oxygen-bridged triarylamine polycyclic unit based tris-cyclometalated heteroleptic iridium(iii) complex: correlation between the structure and photophysical properties

A novel oxygen-bridged triarylamine polycyclic unit based tris-cyclometalated heteroleptic iridium(iii) complex (Ir-NO) has been designed and prepared. Similar iridium(iii) complexes (Ir-N and Ir-O) have also been prepared for comparison. The single crystal structure indicates that the oxygen-bridged triarylamine polycyclic unit of Ir-NO shows certain planarity, resulting in obvious rigidity and interesting π-π stacking. Additionally, Ir-NO shows a larger redshift emission (586 nm) and a longer emission lifetime (628 ns) than Ir-N (525 nm, 344 ns) and Ir-O (543 nm, 436 ns), and the phosphorescence color can be tuned from green to orange. Electrochemical experiments and DFT calculations reveal that Ir-NO exhibits a high HOMO energy level and intraligand charge transfer (3ILCT) excited state properties, which is in contrast to the low HOMO energy level and metal-to-ligand charge transfer (3MLCT) excited state properties in Ir-N and Ir-O. This result can be attributed to the strong electron-donating ability of the unit.

Tuning the Photophysical and Excited State Properties of Phosphorescent Iridium(III) Complexes by Polycyclic Unit Substitution

Two novel N-embedded polycyclic units functionalized phosphorescent iridium(III) complexes (Ir-1 and Ir-2) with substituents in different positions have been prepared. Complex Ir-1 bearing the substituent at the 3-position shows a distinct blue shift single-peak emission (524 nm) with a higher luminescence efficiency (ΦPL=42 %) and shorter emission lifetime (τ=282 ns) by comparison with 4-position substitution based complex Ir-2 (ΦPL=23 %, τ=562 ns), which exhibits a dual-peak emission (564 nm and 602 nm), and phosphorescence color can be tuned from green to yellow. In addition, DFT calculations demonstrate that unusual ligand-to-metal charge transfer (3LMCT) excited state property can be found in Ir-2, which is in contrast to metal-to-ligand charge transfer (3MLCT) excited state character in Ir-1. This result can be attribute to strong electron-donating character and 4-position substitution effect of the unit.

A Cu-NHC based phosphorescent binuclear iridium(iii)/copper(i) complex with an unpredictable near-linear two-coordination mode

A novel Cu-NHC based phosphorescent binuclear iridium(iii)/copper(i) complex has been prepared. The copper(i) center adopts an unpredictable near-linear two-coordination mode with short Cu-N and Cu-C bond lengths and two obvious CH-π interactions, which support its stability in the solid state. It also shows better optical properties (higher luminescence efficiency and shorter emission lifetime) in both solution and solid states relative to the model binuclear copper(i)/copper(i) complex, and was successfully applied in solution-processed organic light-emitting diodes.