Direct observation of electron transfer in solids through X-ray crystallography.

Nanoscale electron transfer (ET) in solids is fundamental to the design of multifunctional nanomaterials, yet its process is not fully understood. Herein, through X-ray crystallography, we directly observe solid-state ET via a crystal-to-crystal process. We first demonstrate the creation of a robust and flexible electron acceptor/acceptor (A/A) double-wall nanotube crystal ([(Zn2+)4(LA)4(LA=O)4]n) with a large window (0.90 nm × 0.92 nm) through the one-dimensional porous crystallization of heteroleptic Zn4 metallocycles ((Zn2+)4(LA)4(LA=O)4) with two different acceptor ligands (2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridine (LA) and 2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridin-9(10H)-one (LA=O)) in a slow-oxidation-associated crystallization procedure. We then achieve the bottom-up construction of the electron donor incorporated-A/A nanotube crystal ([(D)2⊂(Zn2+)4(LA)4(LA=O)4]n) through the subsequent absorption of electron donor guests (D = tetrathiafulvalene (TTF) and ferrocene (Fc)). Finally, we remove electrons from the electron donor guests inside the nanotube crystal through facile ET in the solid state to accumulate holes inside the nanotube crystal ([(D•+)2⊂(Zn2+)4(LA)4(LA=O)4]n), where the solid-state ET process (D - e- → D•+) is thus observed directly by X-ray crystallography.

Rapid Synthesis of Chiral Figure-Eight Macrocycles Using a Preorganized Natural Product-Based Scaffold.

Chiral D2 -symmetric figure-eight shaped macrocycles are promising scaffolds for amplifying the chiroptical properties of π-conjugated systems. By harnessing the inherent and adaptable conformational dynamics of a chiral C2 -symmetric bispyrrolidinoindoline (BPI) manifold, we developed an enantio-divergent modular synthetic platform to rapidly generate a diverse range of chiral macrocycles, spanning from 14- to 66-membered rings, eliminating the need for optical resolution. Notably, a 32-membered figure-eight macrocycle showed excellent circularly polarized luminescence (CPL: |glum |=1.1×10-2 ) complemented by a robust emission quantum yield (Φfl =0.74), to achieve outstanding CPL brightness (BCPL : ϵ×Φfl ×|glum |/2=480). Using quadruple Sonogashira couplings, this versatile synthetic platform enables precise adjustments of the angle, distance, and length among intersecting π-conjugated chromophores. Our synthetic strategy offers a streamlined and systematic approach to significantly enhance BCPL values for a variety of chiral D2 -symmetric figure-eight macrocycles.

Optical Resolution of Carboxylic Acid Derivatives of Homoleptic Cyclometalated Iridium(III) Complexes via Diastereomers Formed with Chiral Auxiliaries.

We report on a facile method for the optical resolution of cyclometalated iridium(III) (Ir(III)) complexes via diastereomers formed with chiral auxiliaries. The racemic carboxylic acids of Ir(III) complexes (fac-4 (fac-Ir(ppyCO2H)3 (ppy: 2-phenylpyridine)), fac-6 (fac-Ir(tpyCO2H)3 (tpy: 2-(4'-tolyl)pyridine)), and fac-13 (fac-Ir(mpiqCO2H)3 (mpiq: 1-(4'-methylphenyl)isoquinoline))) were converted into the diastereomers, Δ- and Λ-forms of fac-9 (from fac-6), fac-10 (from fac-4), fac-11 (from fac-6), and fac-14 (from fac-13), respectively, by the condensation with (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-2-aminocyclohexanol. The resulting diastereomers were separated by HPLC (with a nonchiral column) or silica gel column chromatography, and their absolute stereochemistry was determined by X-ray single-crystal structure analysis and CD (circular dichroism) spectra. Spectra of all diastereomers of the Ir(III) complexes are reported. Hydrolysis of the ester moieties of Δ- and Λ-forms of fac-10, fac-11, and fac-14 gave both enantiomers of the corresponding carboxylic acid derivatives in the optically pure forms, Δ-fac and Λ-fac-4, -6, and -13, respectively.

Catalytic electron drives host-guest recognition.

Electron injection is demonstrated to trigger electrocatalytic chain reactions capable of releasing a solvent molecule and forming a redox active guest molecule. One-electron reduction of a hydroxy anthrone derivative (AQH-CH2CN) results in the formation of an anthraquinone radical anion (AQ˙-) and acetonitrile (CH3CN). The resulting fragment of AQ˙- exhibits high stability under mild reducing conditions, and it has enough reducing power to reduce the reactant of AQH-CH2CN. Hence, subsequent electron transfer from AQ˙- to AQH-CH2CN yields the secondary AQ˙- and CH3CN, while the initial AQ˙- is subsequently oxidized to AQ. Overall, the reactants of AQH-CH2CN are completely converted into AQ and CH3CN in sustainable electrocatalytic chain reactions. These electrocatalytic chain reactions are mild and sustainable, successfully achieving catalytic electron-triggered charge-transfer (CT) complex formation. Reactant AQH-CH2CN is non-planar, making it unsuitable for CT interaction with an electron donor host compound (UHAnt2) bearing parallel anthracene tweezers. However, conversion of AQH-CH2CN to planar electron acceptor AQ by the electrocatalytic chain reactions turns on CT interaction, generating a host CT complex with UHAnt2 (AQ ⊂ UHAnt2). Therefore, sustainable electrocatalytic chain reactions can control CT interactions using only a catalytic amount of electrons, ultimately affording a one-electron switch associated with catalytic electron-triggered turn-on molecular recognition.

Zinc-Ion-Stabilized Charge-Transfer Interactions Drive Self-Complementary or Complementary Molecular Recognition.

Here, we show that charge-transfer interactions determine whether donor and acceptor ditopic ligands will associate in a complementary or self-complementary fashion upon metal-ion clipping. Anthracene-based (9,10LD and 1,5LD) and anthraquinone-based (1,5LA) ditopic ligands containing two imidazole side arms as zinc coordination sites were designed. The 9,10LD and 1,5LA systems associated in a complementary fashion (LA/LD/LA) upon clipping by two zinc ions (Zn2+) to form an alternating donor-acceptor assembly [(9,10LD)(1,5LA)2-(Zn2+)2]. However, once the charge-transfer interactions were perturbed by subtle modifications of the imidazole side arms (9,10LD'(S) and 1,5LA'(S)), self-complementary association (LD'/LD'/LD'/LD' and LA'/LA'/LA'/LA') between the donor (9,10LD'(S)) and acceptor (1,5LA'(S)) ligands predominantly occurred to form homoassemblies [(9,10LD'(S))4-(Zn2+)2 and (1,5LA'(S))4-(Zn2+)2]. As in the case of a homochiral pair (9,10LD'(S) and 1,5LA'(S)), self-complementary association (narcissistic self-sorting) occurred in the Zn2+ assembly with heterochiral combinations of the donor and acceptor ligands (9,10LD'(S)/1,5LA'(R) and 9,10LD'(S)/1,5LA'(R)/1,5LA'(R)). Narcissistic self-sorting also took place between the positional isomer of the donor ligands (9,10LD and 1,5LD) to form individual homoligand assemblies [(9,10LD)4-(Zn2+)2 and (1,5LD)4-(Zn2+)2]. Conversely, statistical association took place in the Zn2L4 assembly process of an isomorphous pair of the donor and acceptor ligands (1,5LD and 1,5LA).

Remarkable self-sorting selectivity in covalently linked homochiral and heterochiral pairs driven by Pd2L4 helicate formation.

Imidazole-based ditopic ligands bearing two chiral alkyl groups (LRR, LSS, and LRS) were synthesized. The ligands formed Pd2L4 helicates with palladium ions (Pd2+). Self-sorting occurred between LRR and LRS to form (Pd2+)2(LRR)4 and (Pd2+)2(LRS)4 homoligand assemblies, whereas mixing of LRR and LSS with Pd2+ gave a near statistical mixture.

Dynamic Open Coordination Cage from Nonsymmetrical Imidazole-Pyridine Ditopic Ligands for Turn-On/Off Anion Binding.

This work demonstrates a new nonconventional ligand design, imidazole/pyridine-based nonsymmetrical ditopic ligands (1 and 1S ), to construct a dynamic open coordination cage from nonsymmetrical building blocks. Upon complex formation with Pd2+ at a 1:4 molar ratio, 1 and 1S initially form mononuclear PdL4 complexes (Pd2+ (1)4 and Pd2+ (1S )4 ) without formation of a cage. The PdL4 complexes undergo a stoichiometrically controlled structural transition to Pd2 L4 open cages ((Pd2+ )2 (1)4 and (Pd2+ )2 (1S )4 ) capable of anion binding, leading to turn-on anion binding. The structural transitions between the Pd2 L4 open cage and the PdL4 complex are reversible. Thus, stoichiometric addition (2 equiv) of free 1S to the (Pd2+ )2 (1S )4 open cage holding a guest anion ((Pd2+ )2 (1S )4 ⋅G- ) enables the structural transition to the Pd2+ (1S )4 complex, which does not have a cage and thus causes the release of the guest anion (Pd2+ (1S )4 +G- ).