A theoretical chemistry-based strategy for the rational design of new luminescent lanthanide complexes: an approach from a multireference SOC-NEVPT2 method

Preparation of Eu(III) Complexes Containing Maleopimaric Acid Anhydride with Ultra-narrow and Efficient Fluorescence Emission for High Determination on Gallic Acid in Acetonitrile Solution

Two novel Eu(III) complexes constructed by maleopimaric acid anhydride (MPA) and 2,2'-bipyridyl (Bpy) / 1, 10-phenanthroline (Phen), named as MPA-Bpy-Eu / MPA-Phen-Eu, have been synthesized by a one-step precipitation method. FTIR, UV, TG, elemental analysis, XPS, and ESI-MS revealed the successful coordination of Eu(III) ions with MPA and Bpy / Phen through Eu-O and Eu-N bonds, respectively. Introducing MPA into coordinate structure increased the electron cloud density in 3d_5/2 orbit of Eu(III) ions, enhanced the absolute quantum yields of MPA-Bpy-Eu (89.27%) and MPA-Phen-Eu (94.41%), and decreased the full width at half maxima of ⁵D₀ → ⁷F₂ transitions of MPA-Bpy-Eu (2.23 nm) and MPA-Phen-Eu (2.93 nm), respectively. The fluorescence quenching experiments showed that there was a good linear relationship between the relative fluorescence intensity of MPA-Bpy-Eu and gallic acid (GA) concentration in acetonitrile solution in the range of 0 to 8.41 × 10^-2 mM. The results of UV-vis spectra, the fluorescence lifetimes, and the quantum yields demonstrated that the dynamic quenching model played a major role in the quenching process of GA on MPA-Bpy-Eu. The quenching process of GA on MPA-Phen-Eu involved dual fluorescence quenching mechanistic pathways, including static and dynamic models.

Intramolecular Hydrogen Bond in Pyridine Schiff Bases as Ancillary Ligands of Re(I) Complexes Is a Switcher between Visible and NIR Emissions: A Relativistic Quantum Chemistry Study

Rhenium(I) tricarbonyl complexes have been described as suitable fluorophores, particularly for biological applications. fac-[Re(CO)₃(N,N)L]^(0 or 1+) complexes, where N,N is a substituted dinitrogenated ligand (bipyridine or derivatives with relatively small substituents) and L the ancillary ligand [a pyridine Schiff base (PSB) harboring an intramolecular hydrogen bond (IHB)], have presented promissory results concerning their use as fluorophores, especially for walled cells (i.e., bacteria and fungi). In this work, we present a relativistic theoretical analysis of two series of fac-[Re(CO)₃(N,N)PSB]¹⁺ complexes to predict the role of the IHB in the ancillary ligand concerning their photophysical behavior. N,N corresponds to 2,2'-bipyridine (bpy) (series A) or 4,4'-bis(ethoxycarbonyl)-2,2'-bipyridine (deeb) (series B). We found that all the complexes present absorption in the visible light range. In addition, complexes presenting a PSB with an IHB exhibit luminescent emission suitable for biological purposes: large Stokes shift, emission in the range of 600-700 nm, and τ in the order of 10^-2 to 10^-3 s. By contrast, complexes with PSB lacking the IHB show a predicted emission with the lowest triplet excited-state energy entering the NIR region. These results suggest a role of the IHB as an important switcher between visible and NIR emissions in this kind of complexes. Since the PSB can be substituted to modulate the properties of the whole Re(I) complex, it will be interesting to explore whether other substitutions can also affect the photophysical properties, mainly the emission range.

The role of substituted pyridine Schiff bases as ancillary ligands in the optical properties of a new series of fac-rhenium(i) tricarbonyl complexes: a theoretical view

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.

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.