Hierarchical Emergence and Dynamic Control of Chirality in a Photoresponsive Dinuclear Complex

Magneto-Structural Correlations in Coordination Polymers Based on Formate Ligand and Transition Metal Cations

We present five three-dimensional (3D) coordination polymers (CPs) based on the formate ligand, [NaM(HCOO)3(H2O)]n with M = Co2+ and Ni2+ and [KM(HCOO)3]n with M = Mn2+, Co2+, and Ni2+, introducing three new nuclear structures with the P21 space group for [NaCo(HCOO)3(H2O)]n and [NaNi(HCOO)3(H2O)]n, and P6322 Sohncke SG with chiral nuclear structure for [KNi(HCOO)3]n, along two centric C2/c isomorphs [KMn(HCOO)3]n and [KCo(HCOO)3]n. Magnetic measurements indicate that antiferromagnetic interactions predominate in the five CPs, with averaged antiferromagnetic zJ/kB mean values from -1.18 to -94.9 K. Moreover, magnetic long-range order (LRO) at low temperatures is evidenced by the magnetic susceptibility and heat capacity measurements. Furthermore, single-crystal and powder neutron diffraction experiments were performed to elucidate the magnetic structure, confirming the antiferromagnetic ordering with possible spin canting, thus understanding these systems' magnetic exchange pathway topology.

Amino-Acid-Induced Circularly Polarized Luminescence of Octahedral Lanthanide Cage

Chiral metal organic cage compounds with excellent circularly polarized luminescent performance have broad application prospects in many fields. Herein, two lanthanide complexes with luminescent properties in the form of racemic hexagonal octahedral cages were synthesized using a tri (β-diketone) ligand. Eu6(C21H6F15O6)8(H2O)6 exhibited red light emission with high quantum yields of 61 %. Then, we successfully synthesized lanthanide compounds with circularly polarized luminescent activity by chiral induction of the racemic mixture with arginine. The luminescence asymmetry factor was 0.53, and the circularly polarized luminescent merit figure reached 0.323. High-performance circularly polarized luminescent of racemic cage like compound systems have achieved using cheap and readily available chiral amino acid molecules through chiral induction.

Substituent alkyl-chain-dependent supramolecular chirality, tunable chiroptical property, and dye adsorption in azobenzene-glutamide-amphiphile based hydrogel

Controlling the supramolecular chirality of a self-assembly system by molecular structure design and external stimuli in aqueous solution is significant but challenging. Here, we design and synthesize several glutamide-azobenzene-based amphiphiles with different length alkyl chains. The amphiphiles can form self-assemblies in aqueous solution and show CD signals. As the number of the alkyl chain of amphiphiles increases, the CD signals of the assemblies can be enhanced. However, the long alkyl chains conversely restrict the isomerization of the azobenzene and the corresponding chiroptical property. Moreover, the alkyl length can determine the nanostructure of the assemblies and exert critical influence on the dye adsorption efficiency. This work exhibits some insights into the tunable chiroptical property of the self-assembly by delicate molecular design and external stimuli, and emphasizes the molecular structure can determine the corresponding application.

In Situ Regulation of Microphase Separation-Recognized Circularly Polarized Luminescence via Photoexcitation-Induced Molecular Aggregation

Circularly polarized luminescence (CPL) has attracted great interest owing to its extensive optical information and chiral structural dependence. However, rationally regulating solid-phase CPL signals remains difficult because of the close packing of molecules in solid-state materials and the lack of structural visualization. In this work, we proposed a microphase-separation-recognized CPL regulation strategy via coassembly of a hexathiobenzene-based luminophore and chiral block copolymer (cBCP) with in situ photocontrollability. As a consequence to the continuous increase in the luminophore-to-cBCP ratio, the CPL signal of the supramolecular system exhibited an increasing trend until a critical point. Then, further increasing the ratio stretched the helical pitch of cBCP, which led to CPL reduction. With the photoexcitation-induced molecular aggregation of the luminophore, which was implemented using in situ photoirradiation, the helical pitch was retracted along with the restoration of the CPL signal. These processes were fully recognized and monitored by the microphase-separated nanomorphological change of the coassembled system, which indicated that such a structural contrast could be an effective method for rationally regulating the supramolecular chiropticity of solid-state materials.

Anion-Driven Circularly Polarized Luminescence Inversion of Unsymmetrical Europium(III) Complexes for Target Identifiable Sensing

Anion-responsive sign inversion of circularly polarized luminescence (CPL) was successfully achieved by N3O6-type nona-coordinated europium(III) (Eu3+) complexes [(R)-1 and (S)-1] composed of a less-hindered unsymmetrical N3-tridentate ligand (a chiral bis(oxazoline) ligand) and three O2-chelating (β-diketonate) ligands. Here, (R)-1 exhibited a positive CPL signal (IL - IR > 0) at the 5D0 → 7F1 transition of Eu3+, which can be changed to a negative sign (i.e., IL - IR > 0 → IL - IR < 0) by the coordination of trifluoroacetic anions (CF3COO-) to the Eu3+ center. However, (R)-1 preserved the original positive CPL signal (i.e., IL - IR > 0 → IL - IR > 0) in the presence of a wide range of competing anions (Cl-, Br-, I-, BF4-, ClO4-, ReO4-, PF6-, OTf-, and SbF6-). Thus, (R)-1 acts as a smart target identifiable probe, where the CPL measurement (IL - IR) can distinguish the signals from the competing anions (i.e., IL - IR < 0 vs IL - IR > 0) and eliminate the background emission (i.e., IL - IR = 0) from the background emitter (achiral luminescent compounds). The presented approach is also promising in terms of bio-inspired optical methodology because it enables nature's developed chiral sensitivity to use circularly polarized light for object identification (i.e., IL - IR = 0 vs | IL - IR | > 0).

Solvent-Modulated Chiral Self-Assembly: Selective Formation of Helical Nanotubes, Nanotwists, and Energy Transfer

As the medium for self-assembly processes, solvents strongly influence the supramolecular assemblies via specific solute-solvent interactions, which may result in effective modulation of properties, self-assembled nanostructures, and functions through varying the solvent. Here, two kinds of pyridine-cyanostilbene functionalized chiral amphiphiles (l/d-PyPhG and l-PyG) were designed, and their self-assembly behaviors in different solvents were investigated. It was found that both amphiphiles formed gels in dimethyl sulfoxide (DMSO) and self-assembled into right-handed nanotwists, while they formed suspensions in ethanol consisting of left-handed nanotubes. Although the molecular chirality in the compounds remained unchanged in the two solvents, the nanoassemblies showed opposite handedness at the nanoscale together with opposite circular dichroism (CD) and circularly polarized luminescence (CPL) signals. Furthermore, when the amphiphiles were co-assembled with an achiral dye, it was found that efficient energy transfer took place in the systems composed of nanotubes rather than those composed of nanotwists. Therefore, by assembling molecules with the same molecular chirality in different solvents, a selective formation of helical nanotubes or nanotwists and the regulation of handedness as well as energy transfer efficiency were achieved.

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.

Structure Determination of Europium Complexes in Solution Using Crystal-Field Splitting of the Narrow f-f Emission Lines

Nine nona-coordinated Eu(III) complexes (1-9) studied here have three unsymmetric β-diketonate ligands and one chiral Ph-Pybox ligand, which can produce eight possible coordination isomers, depending on the position of the three unsymmetric β-diketonate ligands. Substituents on the β-diketonate ligands cause a rational structural rearrangement upon crystallization. Substituents with higher polarity, including -CN, -F, -Cl, -Br, -OMe, and -OEt, employ intercomplex hydrogen bonding to generate an association complex through structural rearrangement upon crystallization. Substituents with lower polarity, including -CF3, -SMe, and -Me, cause the most energetically favorable isomer to crystallize directly from solution. These two crystal structures exhibit well-resolved f-f emission lines with characteristic Stark splitting structures. This work revealed that the configuration of the Eu(III) complexes in solution can be determined by systematic comparison of their Stark splitting structures to those obtained from the solid phase using density functional theory (DFT)-based predictions combined with circular dichroism data.

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.

Switchable Circularly Polarized Luminescence in Supramolecular Gels through Photomodulated FRET

While the tremendous deal of efforts has been dedicated to the design and fabrication of materials with circularly polarized luminescence (CPL), the development of the chiroptical switch between different CPL signals is one of the important routes toward its application. Here, we prepared a supramolecular gel from the coassemblies containing a chiral gelator (9-fluoren-methoxycarbonyl-functionalized glutamate derivatives, FLG), a fluorescent molecule [(rhodamine B, RhB) or (2',7'-dichlorofluorescein sodium salt, DCF)], and a photochromic molecule [1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene, DAE], thus constructing photomodulated switchable CPL soft materials. It was found that FLG could form supramolecular gel in ethanol and self-assemble into left-handed twisted nanostructures. During the formation of a co-gel with RhB (or DCF) and DAE, the chirality of FLG could be effectively transferred to both the fluorescent and photochromic components, which induced them with chiroptical properties including CPL and circular dichroism (CD). DAE undergoes a reversible transition between the achromatous open state and the dark purple closed state in the co-gel under alternating irradiation with UV and visible light. During such a process, an intermolecular Förster resonance energy transfer (FRET) behavior from fluorescent RhB to ring-closed DAE caused the emission quenching of RhB, which led to CPL silence of RhB in the co-gel. Subsequent irradiation with visible light caused the restoration of the emission and CPL activity with the restored open state. These changes could be repeated many times upon alternate UV and visible irradiation. Therefore, a reversible CPL switch was fabricated in supramolecular gels through the photomodulated FRET process.

Structural variation of self-assembled lanthanide supramolecular complexes induced by reaction conditions

The structural variation of self-assembled lanthanide supramolecular complexes which can be induced by different factors such as concentration, anion and solvent, cationic radii, stoichiometric ratio and light.

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.

Histidine Proton Shuttle-Initiated Switchable Inversion of Circularly Polarized Luminescence

Switchable inversion of the sign of circularly polarized luminescence (CPL) in chiral supramolecular systems has gained remarkable interest because of its role in understanding the chirality-switching phenomena in biological systems and developing smart chiral luminescent materials. Herein, inspired by the histidine proton shuttle in natural enzymes, we synthesized a histidine π-gel (PyC₃H) and realized reversible inversion of supramolecular chirality and CPL by receiving and then transferring a proton. It was found that in the course of histidine protonation by adding an external proton source, the transcription of intrinsic molecular chirality of PyC₃H to the supramolecular level biased, achieving dynamic control over the PyC₃H gel with left-handed CPL inversed into the right-handed one. The mechanism study revealed that the supramolecular chirality and CPL inversion are mainly affected by the cooperation adjustment of hydrogen bonds and π-π stacking upon histidine protonation and deprotonation, which causes the re-orientations of pyrene chromophores. This work sets up an alternative effective method to fabricate tunable CPL-active materials while using the same chiral small molecules, which provides a new insight into developing bio-inspired switchable supramolecular materials.

Steric and Electronic Control of Chiral Eu(III) Complexes for Effective Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) is characterized by the differential emission of right and left circularly polarized light by a chiral molecule. This mini-review describes the recent developments in chiral trivalent europium (Eu(III)) complexes with effective CPL. CPL has many potential applications in security tags, lasers, and three-dimensional organic electroluminescence devices, which is one of the most intensely investigated topics in molecular luminophores. Eu(III) complexes have attracted considerable attention as effective CPL luminophores for the above-mentioned applications. In this review, recent studies on the Eu(III) CPL, including the steric (dimer, tetramer, aggregates, and coordination polymers) and electronic control (mononuclear) of Eu(III) complexes for the construction of a luminophore with effective CPL, are discussed. The characteristic CPL applications employing the chiral mononuclear Eu(III) complexes are also described. Chiral Eu(III) complexes with well-designed organic ligands can result in the establishment of new research areas in the fields of photochemistry and materials science.

Supramolecular chiroptical switches

Recent progress in chiroptical switches including on/off, amplification, and inversion of the chiral signals such as ECD and CPL in supramolecular assemblies is shown.

Chirality transfer between hexaazamacrocycles in heterodinuclear rare earth complexes

Both the chiral hexaazamacrocyle L1 based on trans-1,2-diaminocyclohexane and the achiral hexaazamacrocyle L2 based on ethylenediamine form lanthanide(iii) dinuclear μ-hydroxo bridged complexes which have been characterized by NMR and CD spectroscopy. The homodinuclear complexes of the type [Ln₂(L1)₂(μ-OH)₂](NO₃)₄ (Ln = Nd^III, Eu^III, Tb^III and Yb^III) have been synthesized in the enantiopure form and the X-ray crystal structures of Nd^III, Eu^III and Yb^III derivatives have been determined. The heterodinuclear cationic complexes [Ln(L1)Ln'(L2)(μ-OH)₂X₂]ⁿ⁺ have been generated and characterized in solution by using the mononuclear complexes of L1 and L2 as substrates. While the formation of [LnLn'(L1)₂(μ-OH)₂X₂]ⁿ⁺ dinuclear complexes is accompanied by chiral narcissistic self-sorting, the formation of [Ln(L1)Ln'(L2)(μ-OH)₂X₂]ⁿ⁺ dinuclear complexes is accompanied by the sizable sociable self-sorting of macrocyclic units. The homodinuclear complexes [Y₂(L1)₂(μ-OH)₂X₂]ⁿ⁺ and [Ln₂(L2)₂(μ-OH)₂X₂]ⁿ⁺ (Ln = Dy^III, Pr^III and Nd^III) are CD silent in the visible region due to the lack of f-f transitions and the presence of an achiral ligand, respectively. In contrast, the heterodinuclear [Y(L1^S)Ln(L2)(μ-OH)₂X₂]ⁿ⁺ complexes give rise to CD signals arising from the f-f transitions because of the chirality transfer from the L1 macrocyclic unit to the L2 macrocyclic unit.