Diverse Coordination Numbers and Geometries in Pyridyl Adducts of Lanthanide(III) Complexes Based on β -Diketonate

: Ten mononuclear rare earth complexes of formula [La(btfa) 3 (H 2 O) 2 ] ( 1 ), [La(btfa) 3 (4,4 (cid:48) Mt 2 bipy)] ( 2 ), [La(btfa) 3 (4,4 (cid:48) 371 nm for both compounds yielded 1.04% for 9 and up to 34.56% for 10


Introduction
The coordination chemistry of lanthanides has been the subject of extensive studies over the last two decades as these compounds revealed unusual physicochemical characteristics including fluorescent and potent magnetic properties because of their unique 4f electrons [1][2][3][4]. Many of the lanthanide compounds have been used in technological devices such as smartphones, solar cells, solid-state lasers emitting in the UV, visible, or near-infrared (NIR) regions, optical glasses, batteries, increasing the memory storage of computers [1,2,[5][6][7][8][9][10][11][12] as well as chiral sensing of biomolecules [13]. The small ionic size of lanthanide ions makes them have the ability to replace metal ions inside protein complex such as calcium [14]. Lanthanide-tagged proteins are valuable for investigating protein structure, function, and dynamics [14,15]. Additionally, lanthanides, especially, La 3+ and Gd 3+ block different types of calcium, potassium, and sodium channels in human and animal neurons [16]. The lanthanide(III) ions or their compounds binding to DNA and cleavage DNA is a growing topic to understand mutations that lead to cancer and treatment of this disease [17,18]. Furthermore, the in vitro and vitro cytotoxic activities of some Ln(III) complexes especially, La(III) and Ce(III), which showed very promising anti-tumor activity [19,20].
The general electronic configurations [Xe] 4f 0−14 of lanthanide ions (Ln(III)) generate a variety of electronic energy levels [1][2][3]12]. The electronic transitions within the 4f orbitals are shielded by the filled 5s 2 5p 6 subshells, and as a result they are becoming less sensitive to the chemical environments around Ln(III) ions. Consequently, each lanthanide ion exhibits narrow and characteristic 4f-4f transitions except La 3+ (4f 0 ) and Lu 3+ (4f 14 ). The 4f-4f transitions in Ln(III) complexes are Laporte forbidden leading to weak light absorption [2,4] and hence the process of direct excitation of metal electrons is very inefficient. However, the most prominent feature in the lanthanide compounds is through the "antenna effect" in which the ligand or linker is used for the excitation process followed by energy transfer to the lanthanide centers, from which the emission occurs and, in this case, the forbidden 4f -transitions can be fairly enhanced, i.e., partly circumvented [1][2][3]12]. Charge-transfer, CT luminescence is generated from an allowed transition from the charge-transfer excited state to the ground state(s). Two types of charge transfers are found in lanthanide complexes: the L→MCT (the electronic transition from an organic linker-localized orbital to a metalcentered orbital) and M→LCT (the electronic transition from a metal-centered orbital to an organic linker-localized orbital). In some cases, the two processes L→MCT and M→LCT luminescence may occur with ligand-based luminescence.

Description of the Crystal Structures 1-10
Partially labelled molecular plots and coordination figures of the title complexes 1-10 are presented in Figures 1-3 and main bond parameters are summarized in Table S1 Table 1). The aromatic ring systems in these complexes are involved in numerous π···π ring···ring and C-H/F··ring interactions, which further stabilize the packing of the mononuclear complexes (Tables S2-S11).  (9) and Europium (10) Complexes

Luminescence Emission of Terbium
The photoluminescence of 9 and 10 was measured in the solid state at room temperature. For Compound 9 ( Figure 4) the spectrum shows an intense and broad band with two peak maxima at 300 and 371 nm corresponding to the π→π* transitions of 4,4 -Me 2 bipy and btfa − ligands, respectively [36]. The excitation wavelength for the emission spectrum (λ em ) is 371 nm. The first emission peak at 492 nm could be attributed to 5 D 4 → 7 F 6 transition, whereas the most intense band located at 543 nm is attributed to 5 D 4 → 7 F 5 transition. These were followed by another three peaks at 593, 614 and 650 nm, which could be assigned to 5 D 4 → 7 F 4 , 5 D 4 → 7 F 3 and 5 D 4 → 7 F 2 transitions, respectively. Moreover, the very weak bands observed around 700 nm can be assigned to the 5 D 4 → 7 F 1 and 5 D 4 → 7 F 0 transitions [37][38][39]. For compound 10 ( Figure 5), the excitation spectrum monitored at 613 nm ( 5 D 0 → 7 F 2 ) reveals an intense broad band at 371 nm due to the π→π* transition from the coordinated btfa − ligands to the Eu(III) ion. In addition, the narrow, which was observed at 464 nm is assigned to 7 F 0 → 5 D 2 f-f transition from the central europium ion [40]. Emission spectrum of 10 ( Figure 5) recorded at excitation wavelength λ ex = 371 nm exhibits the characteristic bands that arise from Eu(III) f-f transitions. At 580 nm ( Figure 5, inset left), 5 D 0 → 7 F 0 transition appeared as the weakest intense peak. Furthermore, the pure magnetic dipole transition 5 D 0 → 7 F 1 at 591 nm, in which its intensity is practically independent of the Eu 3+ environment, is divided into two components due to crystal field splitting of the 7 F 1 level [41]. the emission spectra of 10 is revealing four main bands at 591 ( 5 D 0 → 7 F 1 ), 613 ( 5 D 0 → 7 F 2 ), 656 ( 5 D 0 → 7 F 3 ) and 704 ( 5 D 0 → 7 F 4 ) [31,40,42]. The emission peak centered at 613 nm, which is the strongest peak and corresponds to the hypersensitive band 5 D 0 → 7 F 2 is the one responsible for the observed emission color of the sensitized europium compound. The intensity ratio between 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 is 5.45 is indicating that the emission color is shifted to the orange range and the Eu(III) ion is not placed in a position with inversion symmetry according to the coordination geometry obtained from the SHAPE measurement (TDD8-D 2 d) [43]. In addition, the emission spectrum was recorded at λ ex = 464 nm ( 5 D 2 ← 7 F 0 ) ( Figure S21). The luminescence spectrum showed the same characteristic bands from the europium ion when excited at the longer wavelength with a measured luminescence Quantum Yield (QY) in the solid state of 14.82% proposing the possible use of the compound for biomedical applications. Emission time decay (τ obs) monitored at 546 and 613 nm for 9 and 10, respectively have been measured in solid state. For both compounds the experimental data fits monoexponentially indicating a single coordination sphere around the lanthanide ion ( Figure 6). In order to characterize the sensitization efficiency of the btfa ligand to the excited state of the lanthanide ions, the Overall Quantum Yield (φ TOT) have been measured at the excitation wavelength of 371 nm for both compounds yielding 1.04% for 9 and up to 34.56% for 10 ( Table 2).  The measured QY at 371 nm of the Terbium compound (9) is relatively low, indicating a poor efficiency of the sensitization effect due to a potential overlap between the ligand triplet state and the terbium 5 D 4 emission resonance level that leads to back energy transfer [44][45][46]. On the other hand, the sensitization efficiency of the btfa ligand to Eu 3+ transfer (η sens ) for 10 is 78.2% η sens. This reflects the efficiency in which the energy is transferred from the excited states of the ligand to the excited states of the lanthanide metal and is defined as η sens = φ TOT /φ Ln . The calculated Eu 3+ intrinsic quantum yield (φ Ln ) for compound 10 is 44.19%. Since the excitation band corresponding to the 5 L 6 ← 7 F 0 f-f centered excitation transition, around~395 nm, is not seen in the excitation spectra, φ Ln could not be measured in order to compare it with the calculated one [44]. The φ Ln is calculated by φ Ln = τ obs /τ rad , where τ rad is the radiative lifetime that is referred to the lifetime of an emissive compound in the absence of non-radiative processes and this value is 2.229 ms for 10. Due to Eu 3+ pure magnetic dipole character of the 5 D 0 → 7 F 1 transition, τ rad can be calculated using a simpler equation: where I TOT /I MD is the ratio of integrated area of 5 D 0 emission bands from the corrected emission spectra to the pure magnetic dipole 5 D 0 → 7 F 1 integrated emission band and A MD,0 and n are a constant equal to 14.65 s −1 and the refractive index in where the sample is measured (1.517 for solid state), respectively, [40,47]. The obtained results agree with other Eu(III) [31,32,36,48] and Tb(III) [49,50] β-diketonate compounds.

Single Crystal X-ray Diffraction Analysis
Single-crystal data of 1-8 complexes were measured on an APEX II CCD diffractometer (Bruker-AXS; Madison, WI, USA) and those of 9 and 10 on a D8 Venture (Bruker-AXS, Madison, WI, USA). Table S12 summarizes crystallographic data, intensity data collection, and structure refinement specifications. Data collections were performed at 100(2) K with Mo-Kα radiation (λ = 0.71073 Å); computer programs APEX and SADABS [51,52] were used for data reduction, LP, and absorption corrections. The program library SHELX [53,54] was used for solution (direct methods) and refinement (full-matrix least-squares methods on F 2 ). Anisotropic displacement parameters were applied to all non-hydrogen atoms. H atoms (Uiso) were obtained from difference Fourier maps. Additional software: Mercury [55] and PLATON [56]. CCDC deposition numbers: CCDC 2099334-CCDC 2,099,343 for 1-10, respectively.

Luminescence Measurements
Solid state fluorescence spectra of compounds 9 and 10 were recorded on a Horiba Jobin Yvon SPEX Nanolog fluorescence spectrophotometer (Fluorolog-3 v3.2, HORIBA Jovin Yvon, Cedex, France) equipped with a three slit double grating excitation and emission monochromator with dispersions of 2.1 nm/mm (1200 grooves/mm) at room temperature. The steady-state luminescence was excited by unpolarized light from a 450 W xenon CW lamp and detected at an angle of 22.5 • for solid state measurement by a redsensitive Hamamatsu R928 photomultiplier tube. The instrument was adjusted to obtain the highest Background-to-noise ratio. Spectra were corrected for both the excitation source light intensity variation (lamp and grating) and the emission spectral response (detector and grating).
The excited state decay curves were measured in the same instrument in the phosphorescence mode using a 450 W xenon pulsed lamp (λ = 371 nm, 1.5 ns pulse). The measured decays were analyzed using the Origin software package. Both decay curves fitted monoexponentially: . The fit quality was determined by the χ 2 method of Pearson. Absolute Quantum Yield measurements were acquired in the G8 Quantum Integrating Sphere from GMP with an interior reflective coating made of Spectralon ® (Zürich, Switzerland). Then, the Φ TOT was calculated following Equation (2): where L a is the calculated area of the outgoing amount of light without interaction with a sample (blank) at the used λ exc and L c after interaction with the sample. E c referees to the calculated area from the emission spectrum of the sample and E c (blank) from the emission spectrum of the Blank.