There has been fast-growing interest in the exploitation of the photophysical and photochemical properties of luminescent transition metal complexes in biological applications, with a focus on both diagnostic and therapeutic aspects. In particular, the design of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents and anticancer drugs has received considerable attention for a number of reasons. First, most rhenium(i) tricarbonyl polypyridine complexes possess diverse photophysical and photochemical properties through the coordination of functionalized ligands. The typical photophysical properties of the complexes such as large Stokes shifts, long emission lifetimes, and high photostability allow them to serve as attractive candidates for optical imaging. Also, the cellular uptake of the complexes can be readily quantified by atomic absorption spectroscopy and inductively coupled plasma-mass spectrometry. Additionally, owing to the characteristic infrared absorption bands and the isostructural relationship between rhenium and technetium-99m, rhenium(i) tricarbonyl complexes have been exploited as multimodal imaging reagents for vibrational and radio-imaging, respectively. Furthermore, the facile photosensitizing properties and the three carbon monoxide (CO) ligands render rhenium(i) tricarbonyl complexes promising candidates as photodynamic therapy reagents and photoactivatable CO-releasing molecules, respectively, for cancer treatment. In this Perspective, we describe the recent development of luminescent rhenium(i) tricarbonyl polypyridine complexes as cellular imaging reagents, anticancer drugs, and antibacterial agents.

There has been emerging interest in the exploitation of the photophysical and photochemical properties of transition metal complexes for diagnostic and therapeutic applications. In this Perspective, we highlight the major recent advances in the development of luminescent and photofunctional transition metal complexes, in particular, those of rhenium(I), ruthenium(II), osmium(II), iridium(III), and platinum(II), as bioimaging reagents and phototherapeutic agents, with a focus on the molecular design strategies that harness and modulate the interesting photophysical and photochemical behavior of the complexes. We also discuss the current challenges and future outlook of transition metal complexes for both fundamental research and clinical applications.

As an important nuclear substructure, the nucleolus has received increasing attention because of its significant functions in the transcription and processing of ribosomal RNA in eukaryotic cells. In this work, we introduce a proof-of-concept luminescence assay to detect RNA and to accomplish nucleolus imaging with the use of the supramolecular self-assembly of platinum(II) complexes. Noncovalent interactions between platinum(II) complexes and RNA can be induced by the introduction of a guanidinium group into the complexes, and accordingly, a high RNA affinity can be achieved. Interestingly, the aggregation affinities of platinum(II) complexes enable them to display remarkable luminescence turnon upon RNA binding, which is a result of the strengthening of noncovalent Pt(II)•••Pt(II) and π−π stacking interactions. The complexes exhibit not only intriguing spectroscopic changes and luminescence enhancement after RNA binding but also specific nucleolus imaging in cells. As compared to fluorescent dyes, the lowenergy red luminescence and large Stokes shifts of platinum(II) complexes afford a high signal-to-background autofluorescence ratio in nucleolus imaging. Additional properties, including long phosphorescence lifetimes and low cytotoxicity, have endowed the platinum(II) complexes with the potential for biological applications. Also, platinum(II) complexes have been adopted to monitor the dynamics of the nucleolus induced by the addition of RNA synthesis inhibitors. This capability allows the screening of inhibitors and can be advantageous for the development of antineoplastic agents. This work provides a novel strategy for exploring the application of platinum(II) complex-based cell imaging agents based on the mechanism of supramolecular self-assembly. It is envisaged that platinum(II) complexes can be utilized as valuable probes because of the aforementioned appealing advantages.

Amyloid fibrillation has been acknowledged as a hallmark of a number of neurodegenerative ailments such as Alzheimer's disease. Accordingly, efficient detection of amyloid fibrillation will allow for great advances in the field of biomedical applications as well as in achieving early medical diagnosis. In this work, a luminescence assay for the sensitive and specific detection of amyloid fibrillation was developed by using platinum(II) complexes as sensing platforms. Supramolecular self-assembly of platinum(II) complexes was induced upon addition of amyloid, leading to alterations in the spectroscopic and luminescence properties of the complexes. As compared to fluorescent dyes, luminescent platinum-(II) complexes exhibit attractive large Stokes shifts, phosphorescence lifetimes in the microsecond to submicrosecond regime, and low-energy red emission after aggregation, which are advantageous to biological imaging. At the same time, the platinum(II) complex adopted herein was found to have high photostability, high selectivity and specificity, and low cytotoxicity. The proposed design is the very first approach to detect amyloid fibrillation through the supramolecular selfassembly of luminescent platinum(II) complexes.