Coordination Geometrical Effect on Ligand-to-Metal Charge Transfer-Dependent Energy Transfer Processes of Luminescent Eu(III) Complexes

Abstract

Photophysical properties of europium (Eu(III)) complexes are affected by ligand-to-metal charge transfer (LMCT) states. Two luminescent Eu(III) complexes with three tetramethylheptadionates (tmh) and pyridine (py), [Eu(tmh)3(py)1] (seven-coordinated monocapped-octahedral structure) and [Eu(tmh)3(py)2] (eight-coordinated square antiprismatic structure), were synthesized for geometrical-induced LMCT level control. Distances between Eu(III) and oxygen atoms of tmh ligands were estimated using single-crystal X-ray analyses. The contribution percentages of π-4f mixing in HOMO and LUMO at the optimizedstructure in the ground state were calculated using DFT (LC-BLYP). The Eu-O distances and their π-4f mixed orbitals affect the energy level of LMCT states in Eu(III) complexes. The LMCT energy level of eight-coordinated Eu(III) complex was higher than that of sevencoordinated Eu(III) complex. The energy transfer processes between LMCT and Eu(III) ion were investigated using temperature-dependent and time-resolved emission lifetime measurements of 5 D0→ 7 FJ transitions of Eu(III) ions. In this study, the LMCT-dependent energy transfer processes of seven-and eight-coordinated Eu(III) complexes are demonstrated for the first time.

Molecular light-upconversion: we have had a problem! When excited state absorption (ESA) overcomes energy transfer upconversion (ETU) in Cr(iii)/Er(iii) complexes

“…The remaining gap by a factor 100 with respect to the experimental quantum yield ϕ up tot (ETU) = 5.8(6) × 10 −8 is difficult to unambiguously assign, but it could be related to some improved intrinsic erbium-centered quantum yield in going from [GaErGa(bpb-bzimpy) 3 ] 9+ to [CrErCr(bpb-bzimpy) 3 ] 9+ where minor mixing with low-lying Cr-based LMCT states may severely reduce . 43 We conclude that the main upconversion mechanism operating in [CrErCr(bpb-bzimpy) 3 ] 9+ starts with an initial Cr( 2 T 1 ← 4 A 2 ) excitation (718 nm), followed by fast internal conversion to reach the Cr( 2 E) level, from which a Cr( 2 E)-to-Er( 4 I 9/2 ) energy transfer occurs ( W 1 Cr→Er ). The major pathway for superexcitation is associated with an efficient Er( 2 H 11/2 , 4 F 7/2 ← 4 I 13/2 ) absorption of the second photon at 718 nm, followed by internal conversion to Er( 4 S 3/2 ) and ultimate green Er( 4 S 3/2 → 4 I 15/2 ) photoluminescence.…”

“…In this context, the [ErN 9 ] chromophores, produced by the binding of three bulky 2,6-bis(benzimidazol-2-yl)pyridine ligand strands possessing low-lying π* orbitals in [Er(Et-bzimpy) 3 ] 3+ and [GaErGa(bpb-bzimpy) 3 ] 9+ , give the most efficient excited state absorptions with 26 ≤ ε 1→2 ≤ 43 M −1 cm −1 , which are at least one order of magnitude larger than those expected for standard intrashell f–f transitions. Recently, some non-negligible mixing of 4f-metal with ligand π orbitals have been demonstrated to significantly boost the efficiency of energy transfer processes in related europium tris-diketonate complexes, 43 and a similar mechanism might be responsible for this unexpected improvement for molecular upconversion. For testing this hypothesis, the oscillator strengths f ij , which are proportional to the molar absorption coefficient ε , 44 of the electric-dipole (ED), magnetic-dipole (MD) and electric-quadrupole (EQ) contributions to the ligand field Er( 2 S +1 L J ← 4 I 15/2 ) (Table S16 † ) and Er( 2 S +1 L J ← 4 I 13/2 ) (Table S17 † ) transitions intensities have been evaluated from SO-CASSCF calculations (see computational details in the ESI † ).…”

Preparation of photonic molecular trains via soft-crystal polymerization of lanthanide complexes

“…We then attempted in situ transformation from a dinuclear complex to a coordination polymer using an external stimulus. In a previous study, metastable eight-coordinate [Eu(tmh) 3 (py) 2 ] was synthesized via py vaporization of the precursor [Eu 2 (tmh) 6 ] 29 . The Tb(III) coordination polymers also exhibited an eight-coordinate structure.…”

“…Currently, the predominant lanthanide complexes exhibit seven-, eight- or nine-coordinate structures 28 . We reported the formation of seven-coordinate [Eu(tmh) 3 (py) 1 ] (tmh: 2,2,6,6-tetramethyl-3,5-heptanedionato; py: pyridine) and eight-coordinate [Eu(tmh) 3 (py) 2 ] 29 , demonstrating the potentially transformative geometrical coordination structures of crystalline lanthanide complexes. Based on our structural findings, a seven-coordinate dinuclear complex [Ln 2 (tmh) 6 (4,4’-bpy)] (4,4’-bpy: 4,4’-bipyridine) with a simple bidentate 4,4’-bpy bridging ligand was selected for soft-crystal connection using a monodentate py trigger.…”

Nontoxic Tb3+-induced hyaluronic nano-poached egg aggregates for colorimetric and luminescent detection of Fe3+ ions

“…The luminescence properties of THNE are affected by ligand-to-metal charge transfer (LMCT) states. 50 The O atom of hya was coordinated with Tb 3+ to form the Tb–O chemical bond, leading to the formation of the nano-poached egg structures ( Fig. 3f and 4a ).…”