Schiff-base [4]helicene Zn(II) complexes as chiral emitters

Aggregation-induced enhanced fluorescence emission of chiral Zn(II) complexes coordinated by Schiff-base type binaphthyl ligands

A pair of novel chiral Zn(II) complexes coordinated by Schiff-base type ligands derived from BINOL (1,1'-bi-2-naphthol), R-/S-Zn, were synthesized. X-ray crystallography revealed the presence of two crystallographically independent complexes; one has a distorted trigonal-bipyramidal structure coordinated by two binaphthyl ligands and one disordered methanol molecule (molecule A), while the other has a distorted tetrahedral structure coordinated by two binaphthyl ligands (molecule B). Numerous CH⋯π and CH⋯O interactions were identified, contributing to the formation of a 3-dimensional rigid network structure. Both R-/S-Zn exhibited fluorescence in both CH2Cl2 solutions and powder samples, with the photoluminescence quantum yields (PLQYs) of powder samples being twice as large as those in solutions, indicating aggregation-induced enhanced emission (AIEE). The AIEE properties were attributed to the restraint of the molecular motion arising from the 3-dimensional intermolecular interactions. CD and CPL spectra were observed for R-/S-Zn in both solutions and powders. The dissymmetry factors, gabs and gCPL values, were within the order of 10-3 to 10-4 magnitudes, comparable to those reported for chiral Zn(II) complexes in previous studies.

Circularly polarized blue fluorescence based on chiral heteroleptic six-coordinate bis-pyrazolonate-Zn2+ complexes

Applying molecular design to chiral organo-Zn2+ complexes, a new pair of chiral heteroleptic bis-pyrazolonate-Zn2+ enantiomers [Zn(PMBP)2(1R,2R-Chxn)] (R,R-Zn2+; HPMBP = 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and 1R,2R-Chxn = (1R,2R)-cyclohexane-1,2-diamine) and [Zn(PMBP)2(1S,2S-Chxn)] (S,S-Zn2+; 1S,2S-Chxn = (1S,2S)-cyclohexane-1,2-diamine) have been synthesized and characterized in terms of photophysical and thermodynamic properties. In addition to a small Flack parameter (0.05(3)) associated with the solid-state elucidation of S,S-Zn2+, the circular dichroism (CD) and circularly polarized light (CPL) spectra for the chiral Zn2+ enantiomers show perfect mirror symmetry, establishing that the enantiopure 1,2-diamines successfully induce the optical isomerism of R,R-Zn2+ and S,S-Zn2+. As a result of the combined strong chiral induction capability of chiral 1,2-diamines and excellent photophysical properties of the pyrazolone ligand (PMBP)-, the two Zn2+ enantiomers exhibit high-quality pure blue fluorescence (ΦPL = 9-10%) and significant CPL activity (|glum| = 0.0065-0.0068). The heteroleptic strategy adopted in this study offers a new route to develop high-performance chiroptical luminophores.

Observation of Chiral Channels in Helical Covalent Organic Frameworks

Unraveling the mechanism of chirality transfer across length scales is crucial to the rational development of functional materials with hierarchical chirality. The key obstacle is the lack of structural information, especially at the mesoscopic level. We report herein the structural identification of helical covalent organic frameworks (heliCOFs) with hierarchical chirality, which integrate molecular chirality, channel chirality, and morphology chirality into one crystalline entity. Specifically, benefiting from the highly ordered structure of heliCOFs, the existence of chiral channels at the mesoscopic level has been confirmed by electron crystallography, and the handedness of these chiral channels has been directly determined through the stereopair imaging technique. Accordingly, the chirality transfer in heliCOFs from microscopic to macroscopic levels could be rationalized with a layer-rotating model that has been supported by both crystal structure analysis and theoretical calculations. Observation of chiral channels in heliCOFs not only provides unprecedented data for the understanding of the chirality transfer process but also sheds new light on the rational construction of highly ordered polymeric materials with hierarchical chirality.

Circularly Polarized Luminescence from Schiff-base [4]Helicene Boron Complexes

Boron complexes with Schiff-base [4]helicene ligands were synthesized. These complexes were characterized by NMR spectroscopy and their helical molecular structures were unequivocally established by X-ray diffraction (XRD) analysis. The helical boron complexes exhibited efficient photoluminescence under UV irradiation, and the circularly polarized luminescence (CPL) properties were investigated for optically pure samples. Density functional theory (DFT) calculations were conducted to further understand their photophysical properties including chiroptical responses.

Efficient emission of Zn(II) and Cd(II) complexes with nopinane-annelated 4,5-diazafluorene and 4,5-diazafluoren-9-one ligands: how slight structural modification alters fluorescence mechanism

Zinc(II) and cadmium(II) chlorido complexes with an N,N-chelating nopinane-annelated 4,5-diazafluoren-9-one ligand (LO) were synthesized. While the zinc(II) complex is mononuclear and adopts a tetrahedral ZnN₂Cl₂ coordination geometry, its cadmium(II) analogue features a 1D polymeric structure due to the bridging coordination of chlorido ligands with Cd²⁺ ions having an octahedral CdN₂Cl₄ coordination geometry. The photophysical properties of the oxygen-containing LO ligand and its zinc(II) and cadmium(II) complexes were studied in solution and in the solid state and matched against the properties of its oxygen-free 4,5-diazafluorene congener L and its complexes of the same metal ions. Comprehensive experimental and theoretical studies revealed the impact of the oxygen atom in the ligand core on the luminescence of the ligands and the complexes. For the oxygen-free L ligand and L-based complexes, the structural differences between the S₀ and S₁ geometries are small, which leads to fluorescence with extraordinarily small Stokes shifts. The emission of these compounds is of locally excited character for L and of mixed locally excited + ligand-to-halide charge transfer character for the L-based complexes. The introduction of the oxygen atom in the ligand core results in a drastic red-shift of the emission band due to short-range charge transfer. The differences between the S₀ and S₁ geometries are much more pronounced for LO and LO-based compounds than those of their oxygen-free analogues, leading to an order of magnitude larger Stokes shifts. On going from solution to the solid state, LO and its complexes exhibit aggregation-induced emission (AIE) behaviour with photoluminescence quantum yields (PLQYs) reaching tens of percent.

Sign control of circularly polarized luminescence of chiral Schiff-base Zn(II) complexes through coordination geometry changes

The handedness of circularly polarized luminescence (CPL) of Zn(II) complexes with a chiral Schiff-base ligand can be controlled by coordination geometry changes in solution and in the solid state. The relationships between molecular structure and chiroptical properties were discussed with the results of X-ray diffraction analysis and DFT calculations.

[4]Helicene based anions in electrocrystallization with tetrachalcogenafulvalene donors

Electrocrystallization is an ubiquitous tool for the assembly of ions formed in situ from electroactive precursors into ordered crystalline macroscopic assemblies. Using tetrachalcogeno-fulvalene derivatives, many conducting and superconducting materials have...