A theoretical protocol to study the sensitization and emission mechanism in lanthanide compounds on the basis of multireference CASSCF/PT2 calculations is proposed and applied to [Eu(NO)(dppz-CN)] and [Eu(NO)(dppz-NO)] compounds synthesized and characterized herein. The method consists of a fragmentation scheme where both the ligand and the lanthanide fragments were calculated separately but at the same level of theory, using ab initio wave-function-based methods which are adequate for the treatment of quasi-degenerate states. This is based on the fact that the absorption is ligand-localized and the emission is europium-centered. This characteristic allowed us to describe the most probable energy transfer pathways that take place in the complexes, which involved an ISC between the S to T ligand states, energy transfer to D in the lanthanide fragment, and further D → F emission. For both compounds, the triplet and D states were determined at the CASPT2 level to be around ∼26000 and ∼22400 cm, respectively. This difference is in the optimal range for the energy transfer process. Finally, the emissive state D was found at ∼18000 cm and the emission bands in the range 550-700 nm, in quite good agreement with the experimental results.
Variations in bonding between trivalent lanthanides and actinides is critical for reprocessing spent nuclear fuel. The ability to tune bonding and the coordination environment in these trivalent systems is a key factor in identifying a solution for separating lanthanides and actinides. Coordination of 4,4′−bipyridine (4,4′−bpy) and trimethylsilylcyclopentadienide (Cp′) to americium introduces unexpectedly ionic Am−N bonding character and unique spectroscopic properties. Here we report the structural characterization of (Cp′3Am)2(μ − 4,4′−bpy) and its lanthanide analogue, (Cp′3Nd)2(μ − 4,4′−bpy), by single-crystal X-ray diffraction. Spectroscopic techniques in both solid and solution phase are performed in conjunction with theoretical calculations to probe the effects the unique coordination environment has on the electronic structure.
This work presents a theoretical protocol to analyze the symmetry effect on the allowed character of the transitions and to estimate the probability of energy transfer in lanthanide(III) complexes. For this purpose, a complete study was performed based on the multireference CASSCF/PT2 technique along with TDDFT, to build the energy level diagrams and determine the spectral overlap integrals, respectively. This approach was applied on a series of LnIII complexes, viz. [LnCl(DMF)(Dpq)]/[Ln(NO)(DMF)(Dpq)], where Ln = Sm, Tb, Er/Eu, Nd and dpq = dipyridoquinoxaline, synthesized and characterized by Patra et al. ( Dalton Trans. 2015 , 44 ( 46 ), 19844 - 19855 ; CrystEngComm 2016 , 18 ( 23 ), 4313 - 4322 ; Inorg. Chim. Acta 2016 , 451 , 73 - 81 ). A fragmentation scheme was applied where both the ligand and the lanthanide fragments were treated separately but at the same level of theory. The symmetry analysis only partially reproduced the expected results, and a more detailed analysis of the crystal field became necessary. On the other hand, the most probable energy transfer pathways that take place in the complexes were elucidated from the energy gaps between the ligand-localized triplet state and the emitting levels of the lanthanide fragments. These gaps, which are related to the energy transfer rate, properly reproduced the trend reported experimentally for the best and worst yields. Finally, the spectral overlap integral was calculated from the emission spectra of the dpq ligand and the absorption spectra of the lanthanide fragment. The obtained values are in good agreement with the quantum yields calculated for the systems. The most remarkable aspect of this protocol was its ability to explain the emission and nonemission of the studied compounds.
A series of coordinatively saturated LnIII complexes: [Ln(R-TPY)(TTA)3] (1–6) were designed and structurally characterized and plausible energy transfer (ET) pathways determined using a theoretical method.
A fragmentation scheme has been used to describe the photophysical phenomena associated with the antenna effect in organometallic lanthanide complexes. The theoretical protocol allows justifying the sensitization pathways.
Three samarium(II) crown ether complexes,
[Sm(15-crown-5)2]I2 (1), [Sm(15-crown-5)2]I2·CH3CN (2), and
[Sm(benzo-15-crown-5)2]I2 (3),
have been prepared via the
reaction of SmI2 with the corresponding crown ether in
either THF or acetonitrile in good to moderate yields. The compounds
have been characterized by single crystal X-ray diffraction and a
variety of spectroscopic techniques. In all cases, the Sm(II) centers
are sandwiched between two crown ether molecules and are bound by
the five etheric oxygen atoms from each crown ether to yield 10-coordinate
environments. Despite the higher symmetry crystal class of 1 (R3c), the samarium center resides
on a general position, whereas in 2 and 3 (both in P21/c) the
metal centers lie upon inversion centers. Moreover, the complexes
in 2 and 3 are approximated well by D
5d
symmetry. The molecule in 1, however, is distorted from idealized D
5d
symmetry, and the crown ethers are
more puckered than observed in 2 and 3.
All three complexes luminesce in the NIR at low temperatures. However,
the nature of the luminescence differs between the three compounds. 1 exhibits broadband photoluminescence at 20 °C but at
low temperatures transitions to narrow peaks. 2 only
exhibits nonradiative decay at 20 °C and at low temperatures
retains a mixture of broadband and fine transitions. Finally, 3 displays broadband luminescence regardless of temperature.
Spin–orbit (SO) CASSCF calculations reveal that the outer-sphere
iodide anions influence whether broadband luminescence from 5d → 4f or fine 4f → 4f transitions occur through the alteration
of symmetry around the metal centers and the nature of the excited
states as a function of temperature.
Lanthanides such as europium with more accessible divalent states are useful for studying redox stability afforded by macrocyclic organic ligands. Substituted cryptands, such as 2.2.2B cryptand, that increase the oxidative stability of divalent europium also provide coordination environments that support synthetic alterations of Eu(II) cryptate complexes. Two single crystal structures were obtained containing nine-coordinate Eu(II) 2.2.2B cryptate complexes that differ by a single coordination site, the occupation of which is dictated by changes in reaction conditions. A crystal structure containing a [Eu(2.2.2B)Cl] + complex is obtained from a methanol−THF solvent mixture, while a methanol−acetonitrile solvent mixture affords a [Eu-(2.2.2B)(CH 3 OH)] 2+ complex. While both crystals exhibit the typical blue emission observed in most Eu(II) containing compounds as a result of 4f 6 5d 1 to 4f 7 transitions, computational results show that the substitution of a Cl − anion in the place of a methanol molecule causes mixing of the 5d excited states in the Eu(II) 2.2.2B cryptate complex. Additionally, magnetism studies reveal the identity of the capping ligand in the Eu(II) 2.2.2B cryptate complex may also lead to exchange between Eu(II) metal centers facilitated by π-stacking interactions within the structure, slightly altering the anticipated magnetic moment. The synthetic control present in these systems makes them interesting candidates for studying less stable divalent lanthanides and the effects of precise modifications of the electronic structures of low valent lanthanide elements.
Controlling the properties of heavy element complexes, such as those containing berkelium, is challenging because relativistic effects, spin-orbit and ligand-field splitting, and complex metal-ligand bonding, all dictate the final electronic states of the molecules. While the first two of these are currently beyond experimental control, covalent M‒L interactions could theoretically be boosted through the employment of chelators with large polarizabilities that substantially shift the electron density in the molecules. This theory is tested by ligating BkIII with 4’-(4-nitrophenyl)-2,2’:6’,2”-terpyridine (terpy*), a ligand with a large dipole. The resultant complex, Bk(terpy*)(NO3)3(H2O)·THF, is benchmarked with its closest electrochemical analog, Ce(terpy*)(NO3)3(H2O)·THF. Here, we show that enhanced Bk‒N interactions with terpy* are observed as predicted. Unexpectedly, induced polarization by terpy* also creates a plane in the molecules wherein the M‒L bonds trans to terpy* are shorter than anticipated. Moreover, these molecules are highly anisotropic and rhombic EPR spectra for the CeIII complex are reported.
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