Influence of Outer-Sphere Anions on the Photoluminescence from Samarium(II) Crown Complexes

Abstract

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.

Structural and Spectroscopic Analysis of Ln(II) 18-crown-6 and Benzo-18-crown-6 Complexes (Ln = Sm, Eu, Yb)

“…The broad transitions observed at both temperatures are not uncommon in Sm(II) compounds and are often indicative of 4f → 5d transitions. 30 The new peak that resolves at −180 °C is likely the result of a 4f → 5d transition as well based on intensity and broadness. In comparison to the room temperature solution-phase absorption spectrum for Sm(18-crown-6)I 2 in acetonitrile and Sm(benzo-18-crown-6)I 2 in ethanol (Figure S6), while a similar broad 4f → 5d transition is observed in the visible region, the splitting exhibited in the solid-state absorption spectrum is not present.…”

“…The photoluminescence observed for the 15-crown-5/benzo-15-crown-5 compounds containing non-coordinating iodide ions was dependent on the temperature of the compounds, and this was ultimately attributed to the positioning of the iodide ions in relation to the Sm(II) crown complexes as well as the influences of said ions on the local coordination environment of the Sm(II) metal center. 30 This resulted in a range of observed photoluminescence pathways for these compounds, including a compound with variable 5d → 4f and 4f → 4f photoluminescence. Given this observation, it is anticipated that the Sm(II) 18-crown-6 and benzo-18-crown-6 compounds, where the iodide ions are directly coordinated to the metal center and share similar coordination environments, would also have comparable electronic structures considering that a coordinated iodide ion would be less susceptible to shifting as a function of temperature.…”

“…Recently, our efforts have turned toward synthesizing crown ether complexes containing divalent lanthanides. , As other researchers have noted previously, crown ether molecules are exceptional when it comes to sequestering metal ions in solution, − and the ability to synthesize crown ethers of varying size makes it possible to design complexes best suited for the ionic radii of specific metal cations. , Furthermore, crown ethers (and macrocycles in general) support the stabilization of the divalent lanthanides both in solution and in the solid state. , This has been the inspiration for some of our most recent work with crown ethers such as dibenzo-30-crown-10, 15-crown-5, and benzo-15-crown-5 and their interactions with SmI 2 , predominately in the solid state.…”

“…34,35 Furthermore, crown ethers (and macrocycles in general) support the stabilization of the divalent lanthanides both in solution and in the solid state. 21,36 This has been the inspiration for some of our most recent work with crown ethers such as dibenzo-30crown-10, 29 15-crown-5, and benzo-15-crown-5 30 and their interactions with SmI 2 , predominately in the solid state.…”

Atypical Spectroscopic Behavior in Divalent Lanthanide Dibenzyldiaza-18-crown-6 Complexes (Ln = Sm, Eu, Yb)

“…26−32 Recently, our own efforts have yielded success in obtaining novel crystal structures containing a variety of Ln 2+ complexes with crown ethers commonly used in synthetic chemistry such as 15-crown-5, 18-crown-6, and their benzosubstituted derivatives. 33,34 These compounds synthesized from the direct synthesis of LnI 2 salts and the corresponding crown ether crystallize rapidly and reliably, allowing for analysis of their electronic structure through solid-state spectroscopy. Having established a robust method for the synthesis and crystallization of these sorts of divalent lanthanide complexes, extrapolating this method to more complex and unique crown ethers was necessary to determine the efficacy of this methodology in other systems.…”