Photochemistry of Rhenium(I) Diimine Tricarbonyl Complexes in Biological Applications

Abstract

Luminescent rhenium complexes continue to be the focus of growing scientific interest for catalytic, diagnostic and therapeutic applications, with emphasis on the development of their photophysical and photochemical properties. In this short review, we explore such properties with a focus on the biological applications of the molecules. We discuss the importance of the ligand choice to the contribution and their involvement towards the most significant electronic transitions of the metal species and what strategies are used to exploit the potential of the molecules in medicinal applications. We begin by detailing the photophysics of the molecules; we then describe the three most common photoreactions of rhenium complexes as photosensitizers in H2 production, photocatalysts in CO2 reduction and photochemical ligand substitution. In the last part, we describe their applications as luminescent cellular probes and how photochemical ligand substitution is utilized in the development of photoactive carbon monoxide-releasing molecules as anticancer and antimicrobial agents.

Rhenium fac-Tricarbonyl Bisimine Chalcogenide Complexes: Synthesis, Photophysical Studies, and Confocal and Time-Resolved Cell Microscopy

“…1,2 These originate from their electronic and photophysical properties that can be easily modified/tuned by structural variations around the diimine ligand N∧N and the axial ligand X. 3 Such complexes can thus display reactivity as catalysts (photo(electro)reduction of CO 2 , for example), 4−6 in photodynamic therapy, 7 or as photo-COreleasing molecules, 8,9 for example. Inert rhenium tricarbonyl complexes also find interesting applications in bioimaging.…”

“…Rhenium­(I) fac -tricarbonyl complexes [Re­(N∧N)­(CO) 3 X] n + are an interesting class of organometallic complexes with many different potential applications. , These originate from their electronic and photophysical properties that can be easily modified/tuned by structural variations around the diimine ligand N∧N and the axial ligand X . Such complexes can thus display reactivity as catalysts (photo­(electro)­reduction of CO 2 , for example), − in photodynamic therapy, or as photo-CO-releasing molecules, , for example. Inert rhenium tricarbonyl complexes also find interesting applications in bioimaging. − Such complexes are usually stable in biological media with low toxicity and can be imaged with different modalities: X-ray fluorescence (XRF) imaging of the rhenium, − infrared imaging of the M-CO bond vibrations that lie in the transparency window of biological media, − and luminescence imaging. , Their photophysical properties mainly derive from 3 MLCT excited states and comprise low quantum yield, large Stokes shifts, and long emission lifetimes …”

Synthesis and Photophysical Evaluation of 3,3’‐Nitrogen Bis‐Substituted fac‐[Re(CO)3(Diimine)Br] Complexes

“…Generally, fac ‐[Re(CO) 3 (diimine)X] complexes rely on three main derivatization strategies for system optimization. Modification of the diimine ligand allows tailoring of the photophysical behavior of the complex, since the LUMO is largely located on the diimine ligand [4,17,18] . The HOMO, on the other hand, is generally impacted by the rhenium d‐orbitals and the carbonyl ligands.…”

“…The HOMO, on the other hand, is generally impacted by the rhenium d‐orbitals and the carbonyl ligands. Finally, substitution of the axial halide by a neutral ligand leads to water soluble [Re(CO) 3 (diimine)L] + complexes [17] . Strong field ligands like thiocyanate suppress ligand exchange with solvents [2–4,19] …”

Etalon-Assisted Study of the Strong CO Ligand Vibrations of the fac-[Re(CO)3(bpy)(CH3CN)]+ Octahedral Complex

“…There have been a number of cavity–vibration studies on octahedral complexes with CO, CN, and NO ligands because the vibrations are so strong and isolated from other fundamental vibrations, including liquid Fe­(CO) 5 , W­(CO) 6 in hexane, − [Fe­(CN) 5 (NO)] 2– in methanol, ,, [Fe­(CN) 6 ] 4 – in water (as low as 0.015 M), [Co­(CN) 6 ] 3– , and related compounds . Transition-metal complexes of the type fac -[Re­(CO) 3 (pp) (L)] n + [“pp” = 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) derivatives; “L” = monodentate ligand; n = 0, 1] offer similar advantages and have additional practical interest as catalysts for CO 2 reduction, − electron transfer photosensitizers for solar energy conversion, ,, photolytic CO sources for anti-cancer therapy, ,− and small molecule sensors. , Consequently, the ability to alter the nature of the CO vibration via coupling to optical modes of etalons has the potential to affect reactions involving the CO group as has previously been demonstrated for organic reactions involving coupled carbonyl groups. − …”