Dual Light and Redox Control of NIR Luminescence with Complementary Photochromic and Organometallic Antennae

Photoswitchable luminescent lanthanide complexes controlled and interrogated by four orthogonal wavelengths of light

Optical information storage requires careful control of excitation and emission wavelengths in a reversible and orthogonal manner to enable efficient reading, writing, and erasing of information. Photochromic systems, in which a photoswitch is typcially coupled to an emissive organic fluorophore, have much promise in this regard. However, these suffer from considerable spectral overlap between the switch and fluorophore, such that their emissive and photoswitchable properties are not orthogonal. Here, we overcome this limitation by coupling visible/NIR emissive lanthanide complexes with molecular photoswitches, enabling reversible and orthogonal photoswitching with visible light. Crucially, photoswitching does not lead to sensitised emission from the lanthanide, while excitation of the lanthanide does not induce photoswitching, enabling the state of the system to be probed without perturbation of the switch. This opens up the possibility of developing multi-colour read-write methods for information storage using emissive photoswitches.

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-Photoswitch Friendship: From Photochromic Complexes to Functional Materials

Cooperative metal-photoswitch interfaces comprise an application-driven field which is based on strategic coupling of metal cations and organic photochromic molecules to advance the behavior of both components, resulting in dynamic molecular and material properties controlled through external stimuli. In this Perspective, we highlight the ways in which metal-photoswitch interplay can be utilized as a tool to modulate a system's physicochemical properties and performance in a variety of structural motifs, including discrete molecular complexes or cages, as well as periodic structures such as metal-organic frameworks. This Perspective starts with photochromic molecular complexes as the smallest subunit in which metal-photoswitch interactions can occur, and progresses toward functional materials. In particular, we explore the role of the metal-photoswitch relationship for gaining fundamental knowledge of switchable electronic and magnetic properties, as well as in the design of stimuli-responsive sensors, optically gated memory devices, catalysts, and photodynamic therapeutic agents. The abundance of stimuli-responsive systems in the natural world only foreshadows the creative directions that will uncover the full potential of metal-photoswitch interactions in the coming years.

Heterometallic Molecular Architectures Based on Fluorinated β-Diketone Ligands

This review summarizes the data on the synthesis of coordination compounds containing two or more different metal ions based on fluorinated β-diketonates. Heterometallic systems are of high interest in terms of their potential use in catalysis, medicine and diagnostics, as well as in the development of effective sensor devices and functional materials. Having a rich history in coordination chemistry, fluorinated β-diketones are well-known ligands generating a wide variety of heterometallic complexes. In this context, we focused on both the synthetic approaches to β-dicarbonyl ligands with additional coordination centers and their possible transformations in complexation reactions. The review describes bi- and polynuclear structures in which β-diketones are the key building blocks in the formation of a heterometallic framework, including the examples of both homo- and heteroleptic complexes.

Luminescent Metal Complexes for Bioassays in the Near-Infrared (NIR) Region

Near-infrared (NIR, 700-1700 nm) luminescent imaging is an emerging bioimaging technology with low photon scattering, minimal autofluorescence, deep tissue penetration, and high spatiotemporal resolution that has shown fascinating promise for NIR imaging-guided theranostics. In recent progress, NIR luminescent metal complexes have attracted substantially increased research attention owing to their intrinsic merits, including small size, anti-photobleaching, long lifetime, and metal-centered NIR emission. In the past decade, scientists have contributed to the advancement of NIR metal complexes involving efforts to improve photophysical properties, biocompatibility, specificity, pharmacokinetics, in vivo visualization, and attempts to exploit new ligand platforms. Herein, we summarize recent progress and provide future perspectives for NIR metal complexes, including d-block transition metals and f-block lanthanides (Ln) as NIR optical molecular probes for bioassays.

A visible-light-gated donor–acceptor Stenhouse adduct chemosensor: synthesis, photochromism and naked-eye colorimetric/fluorometric sensing of Al3+ and Zn2+

A visible-light-gated donor–acceptor Stenhouse adduct chemosensor is designed for the colorimetric/fluorometric sensing of Al3+ and Zn2+.

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.

Tuning Excited-State Properties of [2.2]Paracyclophane-Based Antennas to Ensure Efficient Sensitization of Lanthanide Ions or Singlet Oxygen Generation

The multistep synthesis of original antennas incorporating substituted [2.2]paracyclophane (pCp) moieties in the π-conjugated skeleton is described. These antennas, functionalized with an electron donor alkoxy fragment (A¹) or with a fused coumarin derivative (A²), are incorporated in a triazacyclonane macrocyclic ligand L¹ or L², respectively, for the design of Eu(III), Yb(III), and Gd(III) complexes. A combined photophysical/theoretical study reveals that A¹ presents a charge transfer character via through-space paracyclophane conjugation, whereas A² presents only local excited states centered on the coumarin-paracyclophane moiety, strongly favoring triplet state population via intersystem crossing. The resulting complexes EuL¹ and YbL² are fully emissive in red and near-infrared, respectively, whereas the GdL² complex acts as a photosensitizer for the generation of singlet oxygen.

Molecular Engineering onto RuII Bis(1,2-diphenylphosphinoethane) Synthon: Toward an Original Organometallic Gelator

In this article, we report the successful molecular engineering of Ru bis-acetylides that led for the first time to a gelator and more specifically in aromatic solvents. By means of a nonlinear ligand and an extended aromatic platform, the bulky Ru bis-acetylides were able to self-assemble into lamellar structures as evidenced by scanning electron microscopy (SEM) in benzene, toluene, and o- and m-xylene, which in turn induced gelation of the solution with a critical gelation concentration of 30 mg/mL. Nuclear magnetic resonance (NMR), variable temperature (VT)-NMR, and Fourier transform infrared (FT-IR) spectroscopies evidenced that hydrogen bonds are mainly responsible for the self-organization. VT-NMR and small-angle X-ray scattering (SAXS) have also suggested that the pro-ligand and the complex stack in different ways.

Efficient NIR-I fluorescence photoswitching based on giant fluorescence quenching in photochromic nanoparticles

A photoswitchable near-infrared (NIR) fluorescent nanoparticle (NP) was designed and prepared. The NP showed a characteristic AIE property and high-contrast NIR fluorescence photoswitching with full reversibility. Such efficient NIR fluorescence photoswitching originated from the amplified fluorescence quenching mechanism based on intermolecular energy transfer in a densely packed NP state.

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.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

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.

Redox-Modulations of Photophysical and Single-molecule Magnet Properties in Ytterbium Complexes Involving Extended-TTF Triads

The reaction between the 2,2’-benzene-1,4-diylbis(6-hydroxy-4,7-di-tert-butyl-1,3-benzodithiol-2-ylium-5-olate triad (H₂SQ) and the metallo-precursor [Yb(hfac)₃]⋅2H₂O led to the formation of a dinuclear coordination complex of formula [Yb₂(hfac)₆(H₂SQ)]⋅0.5CH₂Cl₂ (H₂SQ-Yb). After chemical oxidation of H₂SQ in 2,2’-cyclohexa-2,5-diene-1,4-diylidenebis(4,7-di-tert-butyl-1,3-benzodithiole-5,6-dione (Q), the latter triad reacted with the [Yb(hfac)₃]⋅2H₂O precursor to give the dinuclear complex of formula [Yb₂(hfac)₆(Q)] (Q-Yb). Both dinuclear compounds have been characterized by X-ray diffraction, DFT optimized structure and electronic absorption spectra. They behaved as field-induced Single-Molecule Magnets (SMMs) nevertheless the chemical oxidation of the semiquinone to quinone moieties accelerated by a factor of five the relaxation time of the magnetization of Q-Yb compared to the one for H₂SQ-Yb. The H₂SQ triad efficiently sensitized the Yb^III luminescence while the chemical oxidation of H₂SQ into Q induced strong modification of the absorption properties and thus a quenching of the Yb^III luminescence for Q-Yb. In other words, both magnetic modulation and luminescence quenching are reached by the oxidation of the protonated semiquinone into quinone.

Multicolour fluorescent “sulfide–sulfone” diarylethenes with high photo-fatigue resistance

Compact, photochromic, reversible and fluorescent: blue and red emission from the “open” and “closed” forms (see picture) and up to 20 000 cycles in organic solvents without exclusion of air–oxygen.

Efficient luminescence control in dithienylethene functionalized cyclen macrocyclic lanthanide complexes

We report the synthesis of an original ligand scaffold based on a dimethyl-cyclen platform Medo2pa with two dithienylethene units attached to each picolinate arm and the corresponding yttrium(iii), europium(iii) and ytterbium(iii) complexes.