Abstract
Over the last few years, luminescent Re(i) tricarbonyl complexes have been increasingly proposed as fluorophores suitable for fluorescence microscopy to visualize biological structures and cells. In this sense, incorporating an asymmetrical pyridine Schiff base (PSB) as the ancillary ligand strongly modifies the staining and luminescent properties of Re(i) tricarbonyl complexes. In this work, we analyzed two series of Re(i) tricarbonyl complexes with their respective PSB ligands: (1) fac-[Re(CO)3(2,2′-bpy)(PSB)]1+ and (2) fac-[Re(CO)3(4,4′-bis(ethoxycarbonyl)-2,2′-bpy)(PSB)]1+, where the PSB exhibits substitutions at positions 4 or 6 in the phenolic ring with methyl or halogen substituents. Thus, we performed computational relativistic DFT and TDDFT studies to determine their optical properties. The ten complexes analyzed showed absorption in the visible light range. Furthermore, our analyses, including zero-field splitting (ZFS), allowed us to determine that the low-lying excited state locates below the 3LLCT states. Interestingly, seven of the ten analyzed complexes, whose corresponding PSB harbors an intramolecular hydrogen bond (IHB), exhibited luminescent emission that could be suitable for biological purposes: large Stokes shift, emission in the range 600–700 nm and τ in the order of 10−2 to 10−3 s. Conversely, the three complexes lacking the IHB due to two halogen substituents in the corresponding PSB showed a predicted emission with the lowest triplet excited state energy entering the NIR region. The main differences in the complexes' photophysical behavior have been explained by the energy gap law and time-resolved luminescence. These results emphasize the importance of choosing suitable substituents at the 4 and 6 positions in the phenolic ring of the PSB, which determine the presence of the IHB since they modulate the luminescence properties of the Re(i) core. Therefore, this study could predict Re(i) tricarbonyl complexes' properties, considering the desired emission features for biological and other applications.
Highlights
It has been stated that fac-Re(I)(CO)3(2,20-bpy)(Br) can stain yeasts, whereas fac-Re(I)(CO)3(4,40bis(ethoxycarbonyl)-2,20-bpy)(Br) is unable to stain the same cells under similar experimental condition.[19,37]
Relativistic density functional theory (DFT) and Time-dependent density functional theory (TDDFT) methods were used to investigate the effect on electronic structures in two series of Re(I) tricarbonyl complexes
The electronic structure analysis showed that R3, R7 and R9 emit in the NIR region with a low emission probability from the second excited triplet state
Summary
Over the last few years, luminescent Re(I) tricarbonyl complexes have been increasingly proposed as uorophores suitable for uorescence microscopy to visualize biological structures and cells.[15,27,28,29] Their chemical stability,[30] enhanced photostabilities (leading to lower photobleaching),[31] and a relatively good cellular uptake[15,29,32,33] represent attractive features. When to Re(I) complexes harboring the same ancillary ligand (Br: bromide) where compared, it has been shown that facRe(I)(CO)3(2,20-bpy)(Br) presents a maximum absorption at 383 nm,[34] whereas fac-Re(I)(CO)3(4,40-bis(ethoxycarbonyl)-2,20bpy)(Br) exhibits maximum absorption at 419 nm.[35,36] In addition, it has been stated that fac-Re(I)(CO)3(2,20-bpy)(Br) can stain yeasts (eukaryotic walled cells), whereas fac-Re(I)(CO)3(4,40bis(ethoxycarbonyl)-2,20-bpy)(Br) is unable to stain the same cells under similar experimental condition.[19,37] All this evidence shows that relatively small substitution can affect both photophysical and staining properties.[19]
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