Pedro Paulo Ferreira da Rosa

Seven‐coordinate Tb^III complexes with strong luminescence and thermosensing properties are reported. Mononuclear [Tb(tmh)₃(PEB)] [tmh: 2,2,6,6‐tetramethyl‐3,5‐heptanedione, PEB: (diphenylphosphoryl)ethynyl]benzene and dinuclear [Tb₂(tmh)₆(m‐BPEB)] [m‐BPEB: 1,3‐bis(diphenylphosphoryl)ethynyl]benzene were characterized by single‐crystal X‐ray analysis. The quantum yields of [Tb(tmh)₃(PEB)] and [Tb₂(tmh)₆(m‐BPEB)] were estimated to be 71 and 39 %, respectively. Thermosensing properties are evaluated by temperature‐dependent emission lifetime measurements (Arrhenius analysis), which are affected by the presence of ligand‐to‐ligand charge transfer (LLCT) bands. The LLCT bands are confirmed by DFT calculations.

Novel Eu(III) coordination polymers with furan-based bridging ligands [Eu(hfa)₃(Cy)]_n and [Eu(hfa)₃(Tol)]_n (hfa: hexafluoroacetylacetonato, Cy: 2,5-bis(dicyclohexylphosphoryl)furan), Tol: 2,5-bis(di-p-tolylphosphoryl)furan) are reported. The rigidity of assembly steric structures was controlled by intermolecular interactions through the side groups in bridging ligands. They exhibited one of the best performances (thermal stability above 320 °C and external photoluminescence quantum yields of up to 71%) among reported lanthanide(III) compounds. The triboluminescence activity was demonstrated to be dependent on the mechanical stability of the coordination polymers, which was proportional to the number of hydrogen atoms in the side groups. The second example of a large TL/PL spectral difference in [Tb,Eu(hfa)₃(Tol)]_n also revealed discrete photophysical processes under the conditions of grinding and UV irradiation.

The steric structures of luminescent compounds play a key role in controlling the thermal and optical properties. In order to improve the thermal properties of a strong luminescent Eu(III) coordination polymer [Eu(hfa)₃(dpedot)]_n (hfa: hexafluoroacetylacetonate, dpedot: 2,5-bis(diphenylphosphoryl)-3,4-ethylenedioxythiophene) in our previous study, a dithiane hexyl ring was introduced instead of a dioxane one. The prepared [Eu(hfa)₃(dpedtt)]_n (dpedtt: 2,5-bis(diphenylphosphoryl)-3,4-ethylenedithiothiophene) exhibited thermal stability by suppressing side group decomposition. The dpedtt ligand showed a smaller dipole moment than that of dpedot, and [Eu(hfa)₃(dpedtt)]_n formed less twisted and densely packed polymer chains, resulting in excellent photophysical properties (quantum yield > 60%).

According to the crystal field theory, the forbidden selection rules in lanthanide’s 4f-4f transition can be relaxed by highly asymmetric structures around the lanthanide ion. Their radiative rates can be increased, facilitating for strong luminescent complex. Geometries of seven-coordination structures are more asymmetric than those of most common eight-coordination ones. In this review, luminescent seven-coordinated lanthanide complexes were introduced. Their photophysical properties are dependent on antenna molecules, ancillary ligands and surrounding media. Photophysical properties and applications using seven-coordinate lanthanide complexes are also presented in this review.

Herein, the π–f orbital interaction depending on the coordination geometry in the Eu(III) complex is demonstrated. Thermal analysis and computational calculations showed the phase transition of the Eu(III) complex based on the change in the coordination geometry. A red-shifted LMCT band and radiative rate changes associated with the phase transition were found in the Eu(III) complex.

A design for an effective molecular luminescent thermometer based on long-range electronic coupling in lanthanide coordination polymers is proposed. The coordination polymers are composed of lanthanide ions Eu(III) and Gd(III), three anionic ligands (hexafluoroacetylacetonate), and a chrysene-based phosphine oxide bridges (6,12-bis(diphenylphosphoryl)chrysene). The zig-zag orientation of the single polymer chains induces the formation of packed coordination structures containing multiple sites for CH-F intermolecular interactions, resulting in thermal stability above 350 ˚C. The electronic coupling is controlled by changing the concentration of the Gd(III) ion in the Eu(III)-Gd(III) polymer. The emission quantum yield and the maximum relative temperature sensitivity (Sm) of emission lifetimes for the Eu(III)-Gd(III) polymer (Eu:Gd = 1:1, Φ_tot = 52%, Sm = 3.73 % K^-1) were higher than those for the pure Eu(III) coordination polymer (Φ_tot = 36%, Sm = 2.70 % K^-1), respectively. Enhanced temperature sensing properties are caused by control of long-range electronic coupling based on phosphine oxide with chrysene framework.

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 optimized structure 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 an eight-coordinated Eu(III) complex was higher than that of a seven-coordinated 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 ⁵D₀ → ⁷F_J 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.

A long-lived near-infrared Nd(III) emission is demonstrated using a Tb(III) donor. The observed emission lifetime of 290 μs at 1057 nm for a Tb(III)–Nd(III) dinuclear complex is attributed to the long-lived Tb(III) donor and the appropriate spacing between the lanthanide ions. This design strategy leads to novel lanthanide photophysics.

In this study, we demonstrated a two-legs standing-up molecular design method for mono-chromatic and bright red luminescent Ln(III)-silica nanomaterials. A novel Eu(III)-silica hybrid nanoparticle using double binding TPPO-Si(OEt)₃ (TPPO: triphenyl phosphine oxide) linker were developed. The TPPO-Si(OEt)₃ was determined by ¹H, ³¹P, ²⁹Si NMR spectroscopy and single-crystal X-ray analysis. Luminescent Eu(hfa)₃ and Eu(tfc)₃ parts (hfa: hexafluoroacetylacetonate, tfc: 3-(trifluoromethylhydroxymethylene)camphorate) were fixed on the TPPO-Si(OEt)₃ modified silica nanoparticles, producing in Eu(hfa)₃(TPPO-Si)₂-SiO₂ and Eu(tfc)₃(TPPO-Si)₂-SiO₂, respectively. The Eu(hfa)₃(TPPO-Si)₂-SiO₂ exhibited higher intrinsic luminescence quantum yield (93%) and longer emission lifetime (0.98 ms), which is much larger than those of previously reported Eu(III)-based hybrid materials. Eu(tfc)₃(TPPO-Si)₂-SiO₂ also showed extra-large intrinsic emission quantum yield (54%), although the emission quantum yield for the precursor Eu(tfc)₃(TPPO-Si(OEt)₃)₂ was found to be 39%. These results confirmed that the TPPO-Si(OEt)₃ linker is a promising candidate for development of Eu(III)-based luminescent materials.

Transition metal complexes provide photofunctional properties through the charge transfer excited states of their metal ion and organic ligand components. Recently, there are increasing reports on the charge transfer excited states of the ligand (π)- and 4f-orbitals of lanthanide complexes, where the latter are shielded by filled 5s² and 5p⁶ orbitals. This area of research is relatively unestablished; thus, the study of photo-excited organic–lanthanide charge transfer would lead to the construction of next-generation photofunctional metal complexes. In this review, we summarize the latest research progress in photofunctional materials using the charge transfer excited states of lanthanide complexes, and discuss the photophysical/theoretical analyses of these charge transfer excited states