Charge-transfer excited state properties of chiral transition metal coordination compounds studied by chiroptical spectroscopy

Molecular Properties of 3d and 4f Coordination Compounds Deciphered by Raman Optical Activity Spectroscopy

Molecular properties of coordination compounds can be efficiently studied by vibrational spectroscopy. The scope of Raman spectroscopy has been greatly enhanced by the introduction of Raman optical activity (ROA) sensitive to chirality. The present review describes some of its recent applications to study the coordination compounds. 3d and 4f metal complexes often absorb the excitation light, or exhibit luminescence. Therefore, effects caused in ROA spectra by electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) must be taken into consideration.In 3d metal complexes ECD and circularly-polarized Raman scattering compete with the resonance ROA (RROA) signal. Pure RROA spectrum can thus be obtained by subtracting the so-called ECD-Raman component. CPL is frequently encountered in 4f systems. While it can mask the ROA spectra, it is useful to study molecular structure. These electronic effects can be reduced by using near-infrared excitation although vibrational ROA signal is much weaker compared to the usual green laser excitation scenario. The ROA methodology is thus complex, but capable of providing unique information about the molecules of interests and their interaction with light.

A Diverse Array of Large Capsules Transform in Response to Stimuli

The allosteric regulation of biomolecules, such as enzymes, enables them to adapt and alter their conformation to fit specific substrates, expressing different functionalities in response to stimuli. Different stimuli can also trigger synthetic coordination cages to change their shape, size, and nuclearity by reconfiguring the dynamic metal-ligand bonds that hold them together. Here we demonstrate an abiological system consisting of different organic subcomponents and Zn^II metal ions, which can respond to simple stimuli in complex ways. A Zn^II₂₀L₁₂ dodecahedron transforms to give a larger Zn^II₃₀L₁₂ icosidodecahedron through subcomponent exchange, as an aldehyde that forms bidentate ligands is displaced in favor of one that forms tridentate ligands together with a penta-amine subcomponent. In the presence of a chiral template guest, the same system that produced the icosidodecahedron instead gives a Zn^II₁₅L₆ truncated rhombohedral architecture through enantioselective self-assembly. Under specific crystallization conditions, a guest induces a further reconfiguration of either the Zn^II₃₀L₁₂ or Zn^II₁₅L₆ cages to yield an unprecedented Zn^II₂₀L₈ pseudo-truncated octahedral structure. The transformation network of these cages shows how large synthetic hosts can undergo structural adaptation through the application of chemical stimuli, opening pathways to broader applications.

Self-Assembled Charge-Transfer Chiral π-Materials: Stimuli-Responsive Circularly Polarized Luminescence and Chiroptical Photothermic Effects

Despite significant achievements in the field of chiroptical organic materials, the full utilization of both the excited state and ground state chiroptical properties in a single supramolecular system is still rarely disclosed. Here, we report that the rational combination of the charge-transfer (CT) interaction with the spacer effect and controlled protonation of π-histidine leads to chiroptical organic π-materials with both circularly polarized luminescence (CPL) and the supramolecular chirality-directed chiroptical photothermic effect. Three pyrene-conjugated histidine derivatives with varied acyl linkers (PyHis, PyC1His, and PyC3His) were designed to coassemble with electron-deficient 1,2,4,5-tetracyanobenzene (TCNB), leading to the formation of supramolecular CT complexes with intense orange to red CPL depending on the linker length. The linker length also affected the protonation-induced CPL responsiveness of the corresponding CT assemblies. Upon protonation of the histidine moiety, PyC3His/TCNB CT assemblies exhibited an inverted CPL signal, while PyHis/TCNB pairs gave quenched CPL due to the disassembly. The protonation-controlled PyC3His/TCNB CT assemblies at varied pH values showed different chiroptical photothermic effects (CPEs) for the same incident chiral light despite the molecular chirality of PyC3His remaining unchanged, supporting an interesting supramolecular chirality-directed photothermic effect.

Subtle Stereochemical Effects Influence Binding and Purification Abilities of an FeII4L4 Cage

A tetrahedral Fe^II₄L₄ cage assembled from the coordination of triangular chiral, face-capping ligands to iron(II). This cage exists as two diastereomers in solution, which differ in the stereochemistry of their metal vertices, but share the same point chirality of the ligand. The equilibrium between these cage diastereomers was subtly perturbed by guest binding. This perturbation from equilibrium correlated with the size and shape fit of the guest within the host; insight as to the interplay between stereochemistry and fit was provided by atomistic well-tempered metadynamics simulations. The understanding thus gained as to the stereochemical impact on guest binding enabled the design of a straightforward process for the resolution of the enantiomers of a racemic guest.

Charge transfer in metal-organic frameworks

Metal-organic frameworks (MOFs, also known as porous coordination polymers or PCPs) are a novel class of crystalline porous material. The tailorable porous structure, in terms of size, geometry and function, has attracted the attention of researchers across all disciplines of materials science. One of the many exciting aspects of MOFs is that through directional and reversible coordination bonding, organic linkers (chromophores with metal-coordinating functional groups) and metal ions (and clusters) can be spatially organized in a preconceived geometry. The well-defined spatial geometry of the metals and linkers is very advantageous for optoelectronic functions (solar cells, light-emitting diodes, photocatalysts) of the materials. This feature article evaluates the scope of charge transfer (CT) interactions in MOFs, involving the organic linkers and metal ion or cluster components. Irrespective of the type (size, shape, electronic property) of organic chromophores involved, MOFs provide an insightful path to design and make the CT process efficient. The selected examples of MOFs with CT characteristics do not only illustrate the design principles but render a pathway towards understanding the complex photophysical processes and implementing those for future optoelectronic and catalytic applications.

Induced CPL-Active Materials Based on Chiral Supramolecular Co-Assemblies

Circularly polarized luminescence (CPL) has attracted much interest due to its potential applications on chiral photonic techniques and optoelectronic materials science. As known, dissymmetry factor (g_em ) of CPL is one essential factor for evaluating the features of CPL-active materials. Much attention has focused on how to increase the g_em value, which is one of the most important issues for CPL practical applications. Recently, more and more works have demonstrated that chiral supramolecular could provide the significant strategy to improve the g_em value through the orderly helical superstructure of chiral building blocks. Normally, this kind of chiral supramolecular assembly process can be accompanied by chirality transfer and induction mechanism, which can promote the amplification effect on the induced CPL of achiral dyes. In this review, we fully summarized recent advances on the induced CPL-active materials of chiral supramolecular co-assemblies, their applications in circularly polarized organic light-emitting diodes (CP-OLEDs) and current challenges.

ECD exciton chirality method today: a modern tool for determining absolute configurations

The application of the exciton chirality method (ECM) to interpret electronic circular dichroism (ECD) spectra is a well‐established and still popular approach to assign the absolute configuration (AC) of natural products, chiral organic compounds, and organometallic species. The method applies to compounds containing at least two chromophores with electric dipole allowed transitions (e.g., π–π* transitions). The exciton chirality rule correlates the sign of an exciton couplet (two ECD bands with opposite sign and similar intensity) with the overall molecular stereochemistry, including the AC. A correct application of the ECM requires three main prerequisites: (a) the knowledge of the molecular conformation, (b) the knowledge of the directions of the electric transition moments (TDMs), and (c) the assumption that the exciton coupling mechanism must be the major source of the observed ECD signals. All these prerequisites can be easily verified by means of quantum‐mechanical (QM) calculations. In the present review, we shortly introduce the general principles that underpin the use of the ECM for configurational assignments and survey its applications, both classic ones and some reported in the recent literature. Based on these examples, we will stress the advantages of the ECM but also the key requisites for its correct application. Additionally, we will discuss the dependence of the couplet sign on geometrical parameters (angles α,β,γ between TDMs), which can be helpful for discerning the sign of exciton chirality in ambiguous situations. Finally, we will present a molecular orbital (MO) description of the exciton coupling phenomenon.

Cyclodextrins with Multiple Pyrenyl Groups: An Approach to Organic Molecules Exhibiting Bright Excimer Circularly Polarized Luminescence

Circularly polarized luminescence (CPL) has attracted much attention because of its potential for electronic and biological applications. Herein, we report a simple and effective approach to organic molecules exhibiting bright CPL by combining a chiral cyclic molecular scaffold and multiple excimer-enabling moieties. An a-cyclodextrin ( CyD ) scaffold was modified with six pyrenyl groups to obtain pyrene-cyclodextrins ( PCD s) in a one-step synthesis from commercially available compounds. The PCDs exhibited high molar extinction coefficients (ε ~ 10 5 M -1 cm -1 ), polarized emission with a good dissymmetry factor (| g lum | ~ 10 -2 ), and quantum yield ( Φ f ~ 0.5). Owing to the excellent photophysical properties of the PCDs, the circularly polarized luminescence brightness ( B CPL ) reached 340 M -1 cm -1 . Comprehensive photophysical and chiroptical studies of the PCD s with only five pyrene units and with linkers of various lengths connecting the CyD with the pyrene units revealed that the formation of a pyrene excimer in a spatially crowded environment is crucial for CPL anisotropy. This study paves the way for the development of bright CPL organic molecules.

Enantiomeric MOF Crystals Using Helical Channels as Palettes with Bright White Circularly Polarized Luminescence

The host-guest chemistry of metal-organic frameworks (MOFs) has enabled the derivation of numerous new functionalities. However, intrinsically chiral MOFs (CMOFs) with helical channels have not been used to realize crystalline circularly polarized luminescence (CPL) materials. Herein, enantiomeric pairs of MOF crystals are reported, where achiral fluorophores adhere to the inner surface of helical channels via biology-like H-bonds and hence inherit the helicity of the host MOFs, eventually amplifying the luminescence dissymmetry factor (g_lum ) of the host l/d-CMOF (±1.50 × 10^-3 ) to a maximum of ±0.0115 for the composite l/d-CMOF⊃fluorophores. l/d-CMOF⊃fluorophores in pairs generate bright color-tunable CPL and almost ideal white CPL (0.33, 0.32) with a record-high photoluminescence quantum yield of ≈30%, which are further assembled into a white circularly polarized light-emitting diode. The present strategy opens a new avenue for propagating the chirality of MOFs to realize universal chiroptical materials.

Boosting Circularly Polarized Luminescence of Organic Conjugated Systems via Twisted Intramolecular Charge Transfer

Realizing a high luminescence dissymmetry factor (g _lum) is a paramount yet challenging issue in the research field of circularly polarized luminescence (CPL). Here, we reported a novel set of organic conjugated systems with twisted intramolecular charge transfer (TICT) characteristics based on conjugated o-carborane-binaphthyl dyads composing of binaphthyl units as chiral electron donors and o-carborane units as achiral electron acceptors, demonstrating intense CPL with large g _lum values. Interestingly, single-crystalline o-1 exhibited a high-level brightness and a large g _lum factor as high as +0.13, whereas single-crystalline o-2 processed a relatively low brightness with a decreased g _lum value to -0.04. The significant diversity of CPL-active properties was triggered by the selective introduction of o-carborane units onto the binaphthyl units. Benefiting from the large magnetic dipole transition moments in TICT states, the CPL activity of TICT o-carborane-based materials exhibited amplified circular polarization. This study provides an efficient molecular engineering strategy for the rational design and development of highly efficient CPL-active materials.

Electronic strain effect on Eu(iii) complexes for enhanced circularly polarized luminescence

The structural and electronic strain of ligands promotes the enhancement of the magnitude of circularly polarized luminescence in chiral Eu(iii) complexes.

Broad-Range Spectral Analysis for Chiral Metal Coordination Compounds: (Chiro)optical Superspectrum of Cobalt(II) Complexes

Chiroptical broad-range spectral analysis extending from UV to mid-IR was employed to study a family of Co(II) N-(1-(aryl)ethyl)salicylaldiminato Schiff base complexes with pseudotetrahedral geometry associated with chirality-at-metal of the Δ/Λ type. While common chiral organic compounds have well-separated absorption and circular dichroism spectra (CD) in the UV/vis and IR regions, chiral Co(II) complexes feature an almost unique continuum of absorption and CD bands, which cover in sequence the UV, visible, near-IR (NIR), and IR regions of the electromagnetic spectrum. They can be collected in a single (chiro)optical superspectrum ranging from the UV (230 nm, 5.4 eV) to the mid-IR (1000 cm^-1, 0.12 eV), which offers a fingerprint of the structure and stereochemistry of the metal complexes. Each region of the superspectrum contributes to one piece of information: the NIR-CD region, in combination with TDDFT calculations, allows a reliable assignment of the metal-centered chirality; the UV-CD region facilitates the analysis of the Δ/Λ diastereomeric equilibrium in solution; and the IR-VCD region contains a combination of low-lying metal-centered electronic states (LLES) and ligand-centered vibrations and displays characteristically enhanced and monosignate VCD bands. Circular dichroism in the NIR and IR regions is crucial to reveal the presence of d-d transitions of the Co(II) core which, due to the electric-dipole forbidden character, would be otherwise overlooked in the corresponding absorption spectra.

Revision of the Absolute Configuration of Preussilides A-F Established by the Exciton Chirality Method

In a recent contribution to this Journal (J. Nat. Prod. 2017, 80, 1531-1540), Noumeur et al. reported the isolation and structure elucidation of six novel polyketides named preussilides A-F, endowed with remarkable antiproliferative activity. The absolute configuration of the new compounds was established mainly by analyzing exciton-coupled electronic circular dichroism (ECD) spectra. However, the application of the exciton chirality method (ECM) was incorrect, because the chirality defined by transition moments was assigned in a wrong way. A correct application of the ECM, substantiated by time-dependent density functional theory (TDDFT) calculations of ECD spectra, led us to revise the absolute configuration of Preussilides A-F. A brief discussion on the criteria required for a correct application of the ECM is also presented.

Homochiral luminescent lanthanide dinuclear complexes derived from a chiral carboxylate

A family of chiral Ln₂ clusters based on a chiral monocarboxylate ligand has been successfully constructed via a diffusion method, and Eu, Tb and Dy analogues display good luminescent properties.

Diastereoselective synthesis of O symmetric heterometallic cubic cages

Enantiopure chiral cubic cages have been diastereoselectively synthesized, showing CD enhancement as a result of configuration rigidity.

The application of chiroptical spectroscopy (circular dichroism) in quantifying binding events in lanthanide directed synthesis of chiral luminescent self-assembly structures

The binding of asymmetrical and optically pure tridentate ligands containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions was studied in CH₃CN.

The binding of asymmetrical and optically pure tridentate ligands (L = 1(S) and 1(R)) containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions (M = La(iii) and Eu(iii)) was studied in CH₃CN, showing the successive formation of M:L, M:L₂ and M:L₃ stoichiometric species in solution. The europium complexes EuL₃ were also synthesised, structurally characterised and their photophysical properties probed in CH₃OH and CH₃CN. The changes in the chiroptical properties of both 1(S) and 1(R) were used (by circular dichroism (CD) spectroscopy) to monitor the formation of these chiral self-assemblies in solution. While circularly polarised luminescence (CPL) showed the formation of Eu(1(S))₃ and Eu(1(R))₃ as enantiomers, with high luminescence dissymmetry factors (g_lum), fitting the CD changes allowed for binding constants to be determined that were comparable to those seen in the analyses of absorbance and luminescence changes.

Programmed stereoselective assembly of DNA-binding helical metallopeptides

We have applied solid phase peptide synthesis methods for the construction of peptide ligands that coordinate Fe(ii) ions and fold into chiral peptide helicates that show great affinity and chiral selectivity for three-way DNA junctions and promising cell-internalization properties.

Acid-induced formation of hydrogen-bonded double helix based on chiral polyphenyl-bridged bis(2,2′-bipyridine) ligands

Chiral polyphenyl-bridged bis(2,2′-bipyridine) ligands L2–4 give intense CD signals when protonated with tetrachloroferric acid or perchloric acid. X-ray crystal structure of [(L2)₂H₂](FeCl₄)₂ shows that a double-stranded helix stabilized by both interior and exterior hydrogen bonding is formed. Theoretical DFT calculations show that such stabilization exists in solution.

Sm and Eu(iii) lanthanide triple helicate cages based on N,N′-methylene-bis(pyridin-4-one)

Sm and Eu(iii) lanthanide triple helicate cages have two distinct metal environments, with the Sm(iii) complex exhibiting homochirality and an increase in sensitization.

Efficient long-range stereochemical communication and cooperative effects in self-assembled Fe4L6 cages

A series of large, optically active Fe(4)L(6) cages was prepared from linear 5,5'-bis(2-formylpyridines) incorporating varying numbers (n = 0-3) of oligo-p-xylene spacers, chiral amines, and Fe(II). When a cage was constructed from the ligand bridged by one p-xylene spacer (n = 1) and a bulky chiral amine, both a homochiral Fe(2)L(3) helicate and Fe(4)L(6) cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe(4)L(6) cage was observed. In the case of larger cages (n = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands' geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands.