The Eu(III) Ion as Luminescent Probe: Investigation of the Metal‐Ion Sites in a Dicyclohexyl‐18‐crown‐6 Complex

Abstract

The metal ion sites of the 3 : 2 complex between europium nitrate and the A-isomer of dicyclohexyl-18-crown-6, [Eu(NO,),(DCI 8C6)],[Eu(N03),], have been probed by high-resolution excitation and emission spectra at 296 and 77 K. The [Eu(N0,),l2-anion gives rise to a luminescent spectrum dominated by the SD0+7F, transition. The crystal-field splitting of the 7F, levels is close to that observed for (Phe,As), [Eu(NO,),], pointing to a structurally similar pentakis(nitrat0) species. The CDo+7Fo excitation spectrum of the two crystallographically independent complex cations displays five maxima. A detailed analysis of the corresponding and selectively excited emission spectra leads to the following conclusions. Well differentiated spectra are assigned to different conformations of the complex cation, in which half of the ligand atoms, including 0-atoms, present large thermal motions. The other spectra are very similar and arise from slightly unequivalent [Eu(NO,) [7]. In particular, the Eu(II1) ion displays an intense luminescence from a long-lived excited state, 'Do which, along with its ground-state 'F,, is not split by ligand-field effects. High-resolution, laser-excited excitation and emission spectra of Eu(II1)-containing compounds allow the following information to be extracted from a detailed analysis: i) the number of chemically different metal-ion sites in the material; ii) the total formal charge of the ligating groups directly bound to the Eu(II1) ion, and iii) the local symmetry of these sites. Moreover, the number of bonded H,O molecules and the distance between metal ion sites may be estimated from life-time measurements [8]. We have recently shown that indeed the Eu(II1) ion behaves as an extremely sensitive probe responding to faint chemical differences, revealing slightly unequivalent moieties present in molecular solids [9]. Such a detailed analysis is not amenable to X-ray or neutron-diffraction studies which yield an averaged image of the unit-cell content, whereas the faster time scale of luminescence experiments provides an instantaneous representation of it.In continuation of our interest in the relationship between the structure of Eu(II1)-containing materials and their luminescent properties [ 10-121, we report here a photophysical study of the 3 : 2 complex between europium nitrate and the A-isomer of dicyclo-

Structural and Luminescence Study of the 3:2 Complex between Europium Nitrate and the B Isomer of Dicyclohexyl‐18‐crown‐6

“…These spectra are typical of low-symmetry species, as indicated by the number of components of each 5D, + 7F, transition and by the relative intensities of these transitions ( Table 3 ) . The P-type spectra are dominated by a strong 'Do + 7F, transition, as already reported for other pentakis(nitrat0) complexes [5] [19]. The C-type spectra have a less intense hypersensitive transition and are similar to those described for bis(nitrat0) complexes with crown ethers [5-81; the 'Do + 7Fl transition contains more than 25 + 1 components, pointing to these spectra as being the superposition of two or three different spectra.…”

“…If this were true, isomer A would be barred, since, for instance, the energies of the ,Do + 'Fa transitions corresponding to C, and C, fall outside the range reported for the compound with the A isomer. Moreover, the crystal field levels reported in Table 5 do not correspond to those reported for the A isomer [5].…”

“…Other differences between the two complex cations are observed upon examination of the Eu-coordination shell. The mean Eu-O(ether) bond lengths are 2.56(8) and 2.55 (5) A for Eu(1) and Eu(2), respectively, but the six bonds are not equivalent. The Eu-0 distances range from 2.50 to 2.71 A for Eu(1) and from 2.48 to 2.63 AforEu (2).…”

“…The situation for [Eu(N0,),I2-is relatively simple in that the two spectra Pa and P, are similar and can be obtained free from interferences due to other species. Spectrum Pa arises from a major species with low symmetry, the crystal field levels of which are reported in Table 4, together with the corresponding data for the pentakis-(nitrato) species found in the compound with the A isomer, P(LA), [5] and in the tetraphenylarsonium salt (PAS) [19]. The barycenters of the 'F, levels are similar in all these compounds, e.g.…”

“…. " (7) 2.92(1) (2) 5-6 82.5(5) 7-8 74.6(4) 7-9 85.6(5) 8-9 70.8 (5) ") Atoms defining the plane are indicated by *.…”