Terpyridyl-Imidazole Based Ligand Coordinated to Ln(Hexafluoroacetyl acetonate)3 Core: Synthesis, Structural Characterization, Luminescence Properties, and Thermosensing Behaviors in Solution and PMMA Film

A new array of ternary lanthanide complexes of the form [Ln(hfa)3(tpy-HImzphen)] have been synthesized and thoroughly characterized wherein Ln = LaIII (1), EuIII (2), SmIII (3), and TbIII (4); hfa = hexafluoroacetylacetonate; and tpy-HImzphen = 2-(4-[2,2':6',2″]terpyridin-4'-yl-phenyl)-1H-phenanthro[9,10-d]imidazole. Incorporation of tpy-HImzphen onto the Ln-hfa moiety induced a bathochromic shift of the absorption window of the complexes into the visible region. Extensive investigations of the luminescence characteristics have been conducted both at RT and at 77 K to understand the deactivation pathways of the complexes. Both steady-state and time-resolved emission spectral behaviors indicate four distinctive behaviors upon incorporation of tpy-HImzphen onto the lanthanide core, viz., a huge red-shift of the ligand-centered peak for LaIII; almost complete energy transfer for EuIII; very little energy transfer for SmIII, while reverse energy transfer in the case of TbIII. In addition, the EuIII-complex exhibits its excellence in luminescence thermometry in the solution state as well as in poly(methyl methacrylate) (PMMA) thin films. The thermosensitive luminescence response in solution was further utilized to mimic set-reset flip-flop logic operation. A plausible energy transfer scheme has been devised to explain dissimilar luminescence behaviors in the complexes. The role of LMCT was also considered for the observed thermosensing property of the Eu(III) complex.

Ytterbium(III) Complex with Photochromic Ruthenium(II) Acetylide Ligand: All Visible Light Photoswitching of NIR Luminescence

We report a ruthenium(II) bisacetylide complex bearing a photochromic dithienylethene (DTE) acetylide arm and a coordinating bipyridyl on the trans acetylide unit. Its coordination with Yb(TTA)₃ centers (TTA = 2-thenoyltrifluoroacetonate) produces a bimetallic complex in which the dithienylethene isomerization is triggered by both ultraviolet (UV) light absorbed by the DTE unit and 450 nm excitation in a transition of the organometallic moiety. The redox behavior arising from the ruthenium(II) bisacetylide system is fully investigated by cyclic voltammetry and spectroelectrochemistry, revealing a lack of stability of the DTE-closed oxidized state preventing effective redox luminescence switching. On the other hand, the photoswitching of ytterbium(III) near-infrared (NIR) emission triggered by the photochromic reaction is fully operational. The electronic structure of this complex in its different states characterized by strong electronic coupling between the DTE and the ruthenium(II)-based moieties leading to metal-assisted photochromic behavior were rationalized with the help of time-dependent density functional theory (TD-DFT) calculations.

Metal complexes bearing photochromic ligands: photocontrol of functions and processes

Metal complexes associated with photochromic molecules are attractive platforms to achieve smart light-switching materials with innovative and exciting properties due to specific optical, electronic, magnetic or catalytic features of metal complexes and by perturbing the excited-state properties of both components to generate new reactivity and photochemical properties. In this overview, we focus on selected achievements in key domains dealing with optical, redox, magnetic properties, as well as application in catalysis or supramolecular chemistry. We also try to point out scientific challenges that are still faced for future developments and applications.

Diastereoselective self-assembly of a triple-stranded europium helicate with light modulated chiroptical properties

Chiroptical photoswitches are of increasing interest for their potential in advanced information technologies. Herein, an achiral bis-β-diketonate ligand (o-L) with a photoresponsive diarylethene moiety as a linker was designed, which co-assembled with Eu³⁺ ions and R- and S-bis(diphenylphosphoryl)-1,10-binaphthyl (R/S-BINAPO) as chiral ancillaries to form dinuclear triple-stranded helicates, [Eu₂(o-L)₃(R/S-BINAPO)₂]. The helicates in the enantiopure form were confirmed by ¹H, ¹⁹F, ³¹P NMR and DOSY NMR analyses. Furthermore, the mirror-image CD and CPL spectra also demonstrate the existence of stable ground- and excited-state chiralities in solution. When exposed to alternate ultraviolet and visible light, the helicates showed reversible color variations from colorless to purple, followed by the presence of light-triggered quadruple optical and chiroptical outputs, named CD, PL, CPL and g_lum switches. With these light-modulated optical outputs, the possibility for the fabrication of IMPLICATION and INHIBIT logic gates was discussed.

Metal-directed synthesis of quadruple-stranded helical Eu(III) molecular switch: a significant improvement in photocyclization quantum yield

The self-assembly of a quadruple-stranded Eu(iii) helicate induces the conformation transformation of a DAE-based photochromic ligand from parallel to antiparallel, which brings a significant improvement in the photocyclization quantum yield (Φo-c) as compared with the free ligand. Furthermore, the photocontrolled open- and closed-rings of the ligand realized a reversible modulation toward Eu3+ center emission.