Enhancing the Luminescence of Eu(III) Complexes with the Ruthenocene Organometallic Unit as Ancillary Ligand

Abstract

Five novel Eu(III)-β-diketonate complexes containing ruthenocene ancillary ligands (1,1′-bis(diphenylphosphoryl)-ruthenocene�RcBPO) were synthesized and characterized. The coordination compounds presented the general formula [Eu(βdik) 3 (RcBPO)], where β-dik stands for 2-thenoyltrifluoroacetonate (tta), 3-benzoyl-1,1,1-trifluoroacetone (btf), 2-dibenzoylmethanate (dbm), 2-acetyl-1,3-indandionate (aind), and 2-benzoyl-1,3-indandionate (bind), and RcBPO stands for 1,1′-bis-(diphenylphosphoryl)ruthenocene. The [Eu(aind) 3 (RcBPO)] complex crystallizes in a monoclinic Cc non-centrosymmetric space group with the europium site environment, assuming a bicapped trigonal prism coordination polyhedron with the symmetry point group close to C 2v . Photoluminescent properties for the solid-state samples were described in terms of excitation, emission, lifetime decay curves, and intrinsic and overall quantum yields. The replacement of the two coordinated H 2 O molecules by the RcBPO ancillary ligand leads to great enhancements of the overall quantum yields (Q Eu L ), with the minimum increment by a factor of 5 for the case of [Eu(btf) 3 (RcBPO)] and the maximum enhancement of 270 times for the case of the [Eu(dbm) 3 (RcBPO)] complex. In addition, theoretical calculations were carried out to model the spectroscopic properties of the investigated compounds. To obtain theoretical Judd−Ofelt parameters (Ω λ , λ = 2, 4, and 6) and intramolecular energy transfer rates, the JOYSpectra web platform was employed using the structure obtained from density functional theory calculations. Hence, a rate equation model provided theoretical overall quantum yields, which are in great agreement with measured data.

Shedding Light on Eu(III) β‐Diketonate Compounds with 1,2‐Bis(diphenylphosphino)ethane Oxide Ligand: an Optical Study

“…As previously reported by Faustino e co‐workers, [18] LMCT state with energy in the range of 20000 to 25000 cm −1 may play a key role in Eu 3+ luminescence quenching, decreasing the population of the metallic center emitting state. In addition, the LMCT state can damage the antenna effect by deactivating either the low‐lying singlet or triplet excited states, as reported in references⋅ [12,17] . Although the factors that result in low‐energy charge transfer states in lanthanide ion complexes are still unclear in the literature, Eu 3+ ‐compounds with diketonate ligands containing the tert‐butyl group generally present LMCT of very low energy likely due to their high electron‐donor capability, as reported in our previous manuscript [10b]…”

“…. [12,17] Although the factors that result in low-energy charge transfer states in lanthanide ion complexes are still unclear in the literature, Eu 3 + -compounds with diketonate ligands containing the tert-butyl group generally present LMCT of very low energy likely due to their high electron-donor capability, as reported in our previous manuscript. [10b] The phosphorescence spectra of Gd 3 + -complexes have been extensively used to mimic the electronic structure of the organic ligand in analogous systems containing Eu 3 + ions (Figure S5).…”

“…While for the other compounds the opposite trend is observed. However, even for the compounds with dbm ( Eu‐3 ), bzac ( Eu‐5 ) and fod ( Eu‐6 ) β‐diketonate ligands, the values of $${{Q}_{Eu}^{Eu}}$$ are still higher than those for the precursor aquo‐complexes [12,27] …”

“…The lanthanide chloride, LnCl 3 .6H 2 O, (Ln: Eu and Gd), were obtained by reaction between the respective lanthanide oxide Ln 2 O 3 and concentrated chloridric acid according to the literature [32] . The complexes of general formula Ln(β‐dik) 3 (H 2 O) 2 and Ln(dbm) 3 (H 2 O) complexes (Ln: Eu 3+ and Gd 3+ while β‐dik: tta, btf, dbm, tfac, bzac and fod) were synthesized according to the procedure reported by Teotonio and co‐workers [12] . The 1,2‐bis(diphenylphosphino)ethane oxide (dppeO 2 ) was synthesized by reacting 1,2‐bis(diphenylphosphino)ethane (dppe) with hydrogen peroxide (H 2 O 2 ).…”

“…Multidentate bis and tris(alkylphosphine) oxides ligands containing aliphatic and aromatic spacer groups have been the subject of intensive studies due to the possibility of forming mononuclear, binuclear complexes, or coordination polymers [11] . Recently, our research group also carried out a systematic study on new highly luminescent Eu 3+ tris‐β‐diketonate complexes containing organometallic units [12] In addition, Hasegawa and co‐workers [13a–e] investigated a series of compounds containing diphenylphosphine oxide derivative ligands as ancillary bidentate ligands, which make a bridge between two tris(hexafluoroacetonate)lanthanide(III) and tris(dipivaloylmethanate)lanthanide(III) complexes. Further, the spectroscopic behavior of the ligand‐metal charge transfer (LMCT) state has also been reported [14a–c] .…”