Dibenzo-18-crown-6 Research Articles

Directional Ring Translocation in a pH‐ and Redox‐Driven Tristable [2]Rotaxane

AbstractWe describe the synthesis and characterization of a [2]rotaxane comprising a dibenzo‐24‐crown‐8 (DB24C8) macrocyclic component and a thread containing three recognition sites: ammonium (AmH+), bipyridinium (Bpy2+) and triazolium (Trz+). AmH+ and Bpy2+ are responsive to fully orthogonal stimuli, pH and electrochemical, which allows to precisely control the directional translation of the macrocycle along the axle. A better understanding of the processes driving the operation of the system was obtained thanks to an in‐depth thermodynamic characterization. Orthogonal stimuli responsive tristable rotaxanes represent the starting point for the creation of linear motors and the development of molecular logic gates.

Directional Ring Translocation in a pH- and Redox-Driven Tristable [2]Rotaxane.

We describe the synthesis and characterization of a [2]rotaxane comprising a dibenzo-24-crown-8 (DB24C8) macrocyclic component and a thread containing three recognition sites: ammonium (AmH+), bipyridinium (Bpy2+) and triazolium (Trz+). AmH+ and Bpy2+ are responsive to fully orthogonal stimuli, pH and electrochemical, which allows to precisely control the directional translation of the macrocycle along the axle. A better understanding of the processes driving the operation of the system was obtained thanks to an in-depth thermodynamic characterization. Orthogonal stimuli responsive tristable rotaxanes represent the starting point for the creation of linear motors and the development of molecular logic gates.

Conformation and Photodissociation Process of Benzo-15-Crown-5 and Benzo-18-Crown-6 Complexes with Ammonium Ions Investigated by Cold UV and IR Spectroscopy in the Gas Phase.

We examined the conformation of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with ammonium ions, NH4+, CH3NH3+ (MeNH3+), CH3CH2NH3+ (EtNH3+), and CH3CH2CH2NH3+ (PrNH3+), using cold UV and IR spectroscopy in the gas phase. We measured the UV photodissociation (UVPD) spectra of the ammonium complexes and compared them with those of the K+(B15C5) and K+(B18C6) complexes in order to identify the conformation on the basis of the band position. The number of possible conformations for the ammonium complexes of B15C5 is limited compared with alkali metal ions with similar ionic radii. The NH4+(B15C5), MeNH3+(B15C5), and EtNH3+(B15C5) complexes show two conformers, whereas the K+(B15C5) complex has three stable conformers. In the case of the PrNH3+(B15C5) complex, one conformer was found predominantly in the UVPD spectrum. The ammonium complexes of B15C5 prefer to adopt crown conformations with large dihedral angles on the C-O-C-C atoms around the benzene moiety. In the case of the ammonium complexes of B18C6, two or three conformers were found in the UVPD spectra. One conformation of the B18C6 complexes is similar to that of the K+(B18C6) complex, which has a planar form on the C-O-C-C atoms around the benzene moiety. The other but dominant conformations of the ammonium complexes could be attributed to those with large C-O-C-C dihedral angles. These conformational findings for the ammonium complexes suggest that the benzo-crown ethers tend to adopt nonplanar conformations around the benzene moiety to encapsulate the ammonium ions. The IR-UV double-resonance (DR) spectra of the B15C5 and B18C6 complexes were compared to those of benzo-12-crown-4 (B12C4) and dibenzo-18-crown-6 (DB18C6) complexes. The N-H···O hydrogen bond (H-bond) is weaker with increasing ring size from B12C4 to B18C6, although the calculated binding energy is smaller for B12C4 than for B18C6. This result indicates that cooperative H-bonds with three N-H groups can strengthen the intermolecular bond between the ammonium ions and B18C6. The difference in the conformational preference between the ammonium and K+ complexes is attributed to directed N-H···O H-bonds in the ammonium complexes. Proton transfer and dissociation of the crown ring were also observed for the photoexcitation of the NH4+(B15C5) and NH4+(B18C6) complexes.

Synthesis of cationic lanthanoid tetraphenylborate crown ether complexes

The efficacy of Me3NHBPh4 as a protolytic agent in the generation of reactive lanthanoid cations has been further explored. Reaction of [Nd(BH4)3(thf)4] with Me3NHBPh4 in a 1:3 mole ratio, and dibenzo-18-crown-6 (DB18C6) in thf did not yield [Nd(thf)n][BPh4]3, but [Nd(BH4)2(DB18C6)][BPh4]⋅2THF (1), a complex primed for further functionalisation, was isolated. The structure has an eight coordinate Nd atom with two trans tridentate BH4− ligands. Redox transmetallation/protolysis (RTP) reactions between Yb metal, PhHgC6F5, and Me3NHBPh4 in the presence of 18-crown-6 (18C6) or DB18C6 in MeCN or in thf followed by work up with MeCN yields [Yb(CE)(MeCN)3][BPh4]2⋅nMeCN (CE = 18C6, n = 0 (2), and DB18C6 n = 3 (3)), the structures of which have nine coordinate Yb atoms. Two pairs of MeCN ligands are transoid, whereas the third pair have a small cis NYbN angle. In the absence of a crown ether ligand, the RTP reaction in MeCN yields the known [Yb(MeCN)8] [BPh4]2.

Perfect confinement of crown ethers in MOF membrane for complete dehydration and fast transport of monovalent ions.

Fast transport of monovalent ions is imperative in selective monovalent ion separation based on membranes. Here, we report the in situ growth of crown ether@UiO-66 membranes at a mild condition, where dibenzo-18-crown-6 (DB18C6) or dibenzo-15-crown-5 is perfectly confined in the UiO-66 cavity. Crown ether@UiO-66 membranes exhibit enhanced monovalent ion transport rates and mono-/divalent ion selectivity, due to the combination of size sieving and interaction screening effects toward the complete monovalent ion dehydration. Specifically, the DB18C6@UiO-66 membrane shows a permeation rate (e.g., K+) of 1.2 mol per square meter per hour and a mono-/divalent ion selectivity (e.g., K+/Mg2+) of 57. Theoretical calculations and simulations illustrate that, presumably, ions are completely dehydrated while transporting through the DB18C6@UiO-66 cavity with a lower energy barrier than that of the UiO-66 cavity. This work provides a strategy to develop efficient ion separation membranes via integrating size sieving and interaction screening and to illuminate the effect of ion dehydration on fast ion transport.

Crown-Assisted CsCu2I3 Growth and Trap Passivation for Perovskite Light-Emitting Diodes.

Copper (Cu)-based perovskites are promising for lead-free perovskite light-emitting diodes (PeLEDs). However, it remains a significant challenge to achieve high performance devices due to the nonradiative loss caused by the disordered crystallization and lack of passivation. Crown ethers are known to form host-guest complexes by the interaction between C-O-C groups and certain cations, and 18-crown-6 (18C6) with an appropriate complementary size can interact with Cs+ and Cu+ cations. Herein, we studied the interaction between CsCu2I3 and two crowns with the same cyclic size, 18C6 and dibenzo-18-crown-6 (D18C6). Particularly, D18C6 can reduce the nonradiative recombination rate of CsCu2I3 film by passivating the defects and optimizing the film morphology effectively. The room mean square (RMS) decreased from 5.06 to 2.95 nm, and the PLQY was promoted from 4.71% to 19.9%. Besides, D18C6 can also decrease the barrier of hole injection. The PeLEDs based on D18C6-modified CsCu2I3 realized noticeable improvement with a maximum luminance and EQE of 583 cd/m2 and 0.662%, respectively.

A Crown-Ether-Based Elastomer Bearing Loop Structures with Dissipating Characteristics and Enhanced Mechanical Performance.

Loops are prevalent topological structures in cross-linked polymer networks, resulting from the folding of polymer chains back onto themselves. Traditionally, they have been considered as defects that compromise the mechanical properties of the network, leading to extensive efforts in synthesis to prevent their formation. In this study, we introduce the inclusion of cyclic dibenzo-24-crown-8 (DB24C8) moieties within the polymer network strands to form CCNs, and surprisingly, these loops enhance the mechanical performances of the network, leading to tough elastomers. The toughening effect can be attributed to the unique cyclic structure of DB24C8. The relatively small size and the presence of rigid phenyl rings provide the loops with relatively stable conformations, allowing for substantial energy dissipation upon the application of force. Furthermore, the DB24C8 rings possess a broad range of potential conformations, imparting the materials with exceptional elasticity. The synergistic combination of these two features effectively toughens the materials, resulting in a remarkable 66-fold increase in toughness compared to the control sample of covalent networks. Moreover, the mechanical properties, particularly the recovery performance of the network, can be effectively tuned by introducing guests to bind with DB24C8, such as potassium ions and secondary ammonium salts.

A Crown‐Ether‐Based Elastomer Bearing Loop Structures with Dissipating Characteristics and Enhanced Mechanical Performance

AbstractLoops are prevalent topological structures in cross‐linked polymer networks, resulting from the folding of polymer chains back onto themselves. Traditionally, they have been considered as defects that compromise the mechanical properties of the network, leading to extensive efforts in synthesis to prevent their formation. In this study, we introduce the inclusion of cyclic dibenzo‐24‐crown‐8 (DB24C8) moieties within the polymer network strands to form CCNs, and surprisingly, these loops enhance the mechanical performances of the network, leading to tough elastomers. The toughening effect can be attributed to the unique cyclic structure of DB24C8. The relatively small size and the presence of rigid phenyl rings provide the loops with relatively stable conformations, allowing for substantial energy dissipation upon the application of force. Furthermore, the DB24C8 rings possess a broad range of potential conformations, imparting the materials with exceptional elasticity. The synergistic combination of these two features effectively toughens the materials, resulting in a remarkable 66‐fold increase in toughness compared to the control sample of covalent networks. Moreover, the mechanical properties, particularly the recovery performance of the network, can be effectively tuned by introducing guests to bind with DB24C8, such as potassium ions and secondary ammonium salts.

Moment Analysis Method for Measurement of Reaction Equilibrium and Rate Constants by Using High-Performance Liquid Chromatography.

The moment analysis method was developed for the determination of association equilibrium constant (KA) and association (ka) and dissociation (kd) rate constants of intermolecular interactions between solute and ligand molecules. They are accurately determined by using moment equations from elution peak profiles because they are measured by using high-performance liquid chromatography (HPLC) under preferable conditions that neither immobilization nor chemical modification (i.e., fluorescence labeling) of solute and ligand molecules is required. To demonstrate the effectiveness of the method, it was applied to the inclusion complex formation system between dibenzo-18-crown-6 (DB18C6) and alkaline earth metal cations, i.e., Mg2+, Ca2+, and Sr2+, as a concrete example. Because the diameter of the three metal cations is smaller than that of the inner cavity of DB18C6, the values of KA, ka, and kd were analytically determined by assuming the stoichiometry of 1:1 between DB18C6 and the metal cation. They reflected the influence of the difference in the size between the inner cavity of DB18C6 and the metal cations on the inclusion complex formation. It seems that the moment analysis method based on HPLC separation is effective for the multifaceted analysis of chemical reactions because some characteristics of the method are different from those of other conventional methods. It must contribute to the dissemination of an opportunity for the study of chemical reactions to many researchers because of the versatility of HPLC.

Characterization by Gel Permeation Chromatography of the Molecular Weight of Supramolecular Polymers Generated by Forming Polyrotaxanes through the Introduction of External Stoppers.

Supramolecular polymers find wide applications across diverse domains, and the molecular weight exerts a critical influence on their applicability. Consequently, the measurement of molecular weight for supramolecular polymers assumes paramount significance. Gel Permeation Chromatography (GPC) requiring low-concentration condition is a common characterization employed for molecular weight determination, which is not suitable for supramolecular polymers possessing concentration-independence property. Here, to break this threshold, we synthesized M1 embodying dibenzo-24-crown-8 (DB24C8) moiety as well as dibenzylammonium salt (DBA) group, which was capable of self-assembling into supramolecular polymers terminated with aldehyde groups at its end. Upon the addition of (4- (1,2,2-Triphenylvinyl) phenyl) methylamine (TPE-NH2), supramolecular polymers underwent a transition into polyrotaxanes, for which it was led by the generation of imine bonds. By virtue of GPC, the molecular weight of polyrotaxanes was obtained, then it was available to gain the molecular weight of supramolecular polymers with the help of transformation efficiency.

Extraction of Na+ ions by dibenzo-18-crown-6 in biphasic aqueous systems of ionic liquid and chloroform

The molecular dynamics simulations were carried out to explore the extraction of Na+ ions in the ionic liquid (IL)–water, chloroform–water biphasic systems containing the crown ether: dibenzo-18-crown-6 (DB18C6) to compare organic solvents such as chloroform to ionic liquid (IL) as receiving solvent phase. The free DB18C6 was found to be highly interacted with IL as compared to chloroform. The Na+–DB18C6 complex interacts preferentially with IL at the interface. The complex has higher interaction energies with IL (-47.04 kcal/mol) as compared to chloroform (-0.11 kcal/mol). The large sharp peak height for the radial distribution function in Na+–DB18C6 complex suggests high stability and strong interaction. At the IL–water interface, it was observed that the Na+–DB18C6 complex had a lower self-diffusion coefficient (0.538 × 10−9 m2/s) than other species. The spatial distribution functions further indicated that the surfaces of Na+ were very closely distributed around the active sides of crown ether in the IL–water system whereas, in chloroform–water systems, it was distributed very far from the crown ether. The present results may help us to understand the role of solvents in heavy metal ion extraction and also in the future design of crown ethers and solvents.

Separation of an ε-poly-L-lysine derivative by solvent extraction under a controlled interfacial potential difference.

This paper describes the availability of a 1,2-dichloroethane (DCE)-water (W) interfacial system under a controlled interfacial potential difference for the separation of polycationic species. The system was applied to the production of polyethylene glycol-modified ε-poly-L-lysine (PEG-εPL). PEG-εPL is produced by a fermentation process, and the crude product contains a significant amount of non-modified εPL, which is hardly separated by conventional chromatographic techniques. Both εPL species exist in fully protonated forms under certain acidic conditions, and an extractant, dibenzo-18-crown-6 (DB18C6), associates with their ammonium groups to stabilize the polycations in DCE. Despite the polydispersity of the samples, the εPL and crude PEG-εPL give well-defined cyclic voltammetric waves due to the DB18C6-assisted transfer of the polycations at the polarizable DB18C6 (DCE) | (W, pH ~ 3) interface with midpoint potentials useful for a rough prediction of ion partition equilibria. Thus, the partition experiment was performed using the DB18C6, Bu4N[(CF3SO2)2N] (DCE) | crude PEG-εPL, Li[(CF3SO2)2N] (W, pH ~ 3) interfacial system, of which the potential difference was controlled to enable selective extraction of polycationic PEG-εPL by partition of the [(CF3SO2)2N]- ion. The extract could be collected from the DCE phase and was found to consist of highly purified PEG-εPL.

Photosalience and Thermal Phase Transitions of Azobenzene- and Crown Ether-Based Complexes in Polymorphic Crystals.

Stimuli-responsive molecular crystals have attracted considerable attention as promising smart materials with applications in various fields such as sensing, actuation, and optoelectronics. Understanding the structure-mechanical property relationships, however, remains largely unexplored when it comes to functionalizing these organic crystals. Here, we report three polymorphic crystals (Forms A, B, and C) formed by the non-threaded complexation of a dibenzo[18]crown-6 (DB18C6) ether ring and an azobenzene-based ammonium cation, each exhibiting distinct thermal phase transitions, photoinduced deformations, and mechanical behavior. Structural changes on going from Form A to Form B and from Form C to Form B during heating and cooling, respectively, are observed by single-crystal X-ray crystallography. Form A shows photoinduced reversible bending, whereas Form B exhibits isotropic expansion. Form C displays uniaxial negative expansion with a remarkable increase of 44% in thickness under photoirradiation. Force measurements and nanoindentation reveal that the soft crystals of Form A with a low elastic modulus demonstrate a significant photoresponse, attributed to the non-threaded molecular structure, which permits flexibility of the azobenzene unit. This work represents a significant advance in the understanding of the correlation between structure-thermomechanical and structure-photomechanical properties necessary for the development of multi-stimulus-responsive materials with tailored properties.

An Interlocked Figure‐of‐Eight Molecular Shuttle

AbstractHere is reported the synthesis and characterization of an interlocked figure‐of‐eight rotaxane molecular shuttle from a dibenzo‐24‐crown‐8 (DB24C8) derivative. This latter bears two molecular chains, whose extremities are able to react together by click chemistry. One of the two substituting chain holds an ammonium function aimed at driving the self‐entanglement through the complexation of the DB24C8 moiety. In the targeted figure‐of‐eight rotaxane, shuttling of the DB24C8 along the threaded axle from the best ammonium station to the weaker binding site triazolium was performed through deprotonation or deprotonation‐then‐carbamoylation of the ammonium. This way, two discrete co‐conformational states were obtained, in which the folding and size of the two loops could be changed.

An Interlocked Figure-of-Eight Molecular Shuttle.

Here is reported the synthesis and characterization of an interlocked figure-of-eight rotaxane molecular shuttle from a dibenzo-24-crown-8 (DB24C8) derivative. This latter bears two molecular chains, whose extremities are able to react together by click chemistry. One of the two substituting chain holds an ammonium function aimed at driving the self-entanglement through the complexation of the DB24C8 moiety. In the targeted figure-of-eight rotaxane, shuttling of the DB24C8 along the threaded axle from the best ammonium station to the weaker binding site triazolium was performed through deprotonation or deprotonation-then-carbamoylation of the ammonium. This way, two discrete co-conformational states were obtained, in which the folding and size of the two loops could be changed.

Ionophore-Based Nanosphere Emulsion Incorporating Ion-Exchanger for Picogram Potentiometric Determination of HCV Drug (Daclatasvir) in Pharmaceutical Formulations and Body Fluids

Daclatasvir dihydrochloride (DAC) is a drug used to treat hepatitis C virus (HCV) infection. In this study, an ionophore-based nanosphere emulsion was made of tricresyl phosphate (TCP) as the oil phase that is dispersed in water using Pluronic F-127 as an emulsifying agent. The nanospheres, consisting of the oil phase TCP, were doped with sodium tetraphenyl borate (Na-TPB) as a cation-exchanger and dibenzo-18-Crown-6 (DB18C6) as an ionophore (chelating agent) for DAC. The nanosphere emulsion was employed as a titrant in the complexometric titration of DAC (the analyte), and the DAC-selective electrode (ISE) was used as an indicator electrode to detect the endpoint. In the sample solution, DAC2+ ions diffused into the emulsified nanospheres, replaced Na+ from the ion exchanger (Na-TPB), and bonded to the ionophore (DB18C6). The DAC-selective nanospheres were successfully utilized to determine DAC in various samples, including standard solutions, commercial tablets (Daclavirocyrl®), serum, and urine. The method exhibited a linear dynamic range of 81.18 µg/mL to 81.18 pg/mL (10−4 to 10−10 M), achieved high recovery values ranging from 99.4% to 106.5%, and displayed excellent selectivity over similar interfering species (sofosbuvir and ledipasvir). The proposed method offers a new approach to determine the drug species (neutral, anionic, and cationic) without the requirement of water-soluble ligands or pH control.

Free Energy Calculations and Conformational Analysis of Dibenzo-30-crown-10 with Sm2+, Eu2+, and Three Halide Salts in THF Using the AMOEBA Force Field.

Crown ether complexes have been tailored for use in industrial separations of lanthanides (Ln) as a part of rare earth mining and refining. Dibenzo-30-crown-10 (DB30C10) is one of the most efficient complexants for the separation of rare earth mixtures based on the cation size. To understand the origin of this complexation, molecular dynamics (MD) simulations of DB30C10 have been performed using different combinations of divalent Sm and Eu and three halide salts Cl-, Br-, and I- in tetrahydrofuran (THF) solvent. DB30C10 was parameterized here for the polarizable atomic multipole optimized energetics for biomolecular simulation (AMOEBA) force field, and the existing parameters of THF, Sm2+, and Eu2+ were employed from our previous efforts. The large conformational fluctuations present in the DB30C10 systems were found to be dependent both on the identity of the lanthanide and halide complexes. For Cl- and Br- systems, there were no observed conformational changes at 200 ns, while in I- systems, there were two conformational changes with Sm2+ and one with Eu2+ within that same timeframe. In SmI2-DB30C10, there were three stages of conformational changes. In the first stage, the molecule is unfolded, in the second stage, the molecule is partly folded, and finally, in the third stage, the molecule is completely folded. Lastly, the Gibbs binding free energies of DB30C10 with SmBr2 and EuBr2 have been computed, which resulted in nearly identical ΔGcomp values for each lanthanide with Sm2+ being slightly more favorable. Considering the folding mechanism of the SmI2 system with DB30C10, the Gibbs binding free energies of DB30C10 and dicyclohexano-18-crown-6 (DCH18C6) with SmI2 were calculated separately and compared to probe their complexation affinities, in which the former was found to be more favorable.

Role of Dibenzo Crown Additive for Improving the Stability of Inorganic Perovskite Solar Cells.

Photovoltaics are being transformed by perovskite solar cells. The power conversion efficiency of these solar cells has increased significantly, and even higher efficiencies are possible. The scientific community has gained much attention due to perovskites' potential. Herein, the electron-only devices were prepared by spin-coating and introducing the organic molecule dibenzo-18-crown-6 (DC) to CsPbI2Br perovskite precursor solution. The current-voltage (I-V) and J-V curves were measured. The morphologies and elemental composition information of the samples were obtained by SEM, XRD, XPS, Raman, and photoluminescence (PL) spectroscopies. The distinct impact of organic DC molecules on the phase, morphology, and optical properties of perovskite films are examined and interpreted with experimental results. The efficiency of the photovoltaic device in the control group is 9.76%, and the device efficiency gradually increases with the increase of DC concentration. When the concentration is 0.3%, the device efficiency is the best, reaching 11.57%, short-circuit current is 14.01 mA/cm2, the open circuit voltage is 1.19 V, and the fill factor is 0.7. The presence of DC molecules effectively controlled the perovskite crystallization process by inhibiting the in-situ generations of impurity phases and minimizing the defect density of the film.

DIBENZO-18-CROWN-6/POLYCAPROLACTONE COMPOSITE NANOFIBERS FOR SELECTIVE ADSORPTION OF CATIONS

Electrospun nanofibers are attractive alternatives to traditional adsorbents due to their high surface-to-volume ratio, porosity, and loading capacity. Functionalization of nanofibers with macrocycles can contribute to further enhancement in selective adsorption of ions. In this study, polycaprolactone (PCL) nanofibers were functionalized with a crown ether, namely dibenzo-18-crown-6 (DB18C6), and the potential of the resultant electrospun PCL/DB18C6 nanofibers for selective ion adsorption, particularly the selective recovery for K+, was investigated. The morphology, chemical structure and thermal properties of PCL/DB18C6 nanofibers were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The average diameter of PCL/DB18C6 nanofibers was 529±196 nm. Single-ion adsorption experiments indicated that the adsorption capacity for K+ ion was 137 mg·g−1 and the selectivity sequence was found as K+>Ca2+>Na+. The adsorption selectivity coefficients of K+/Ca2+ and K+/Na+ were calculated respectively as 1.37 and 4.28 for single ion experiments and as 1.13 and 5.11 for mixed ion adsorptions which illustrate that the difference between the adsorption capacities of K+ and Ca2+ decreased in mixed ion adsorption experiments. Overall results demonstrate that the electrospun PCL/DB18C6 nanofibers are amenable for use as polymer adsorbents for the selective ion recovery.

Cryogenic ion mobility-mass spectrometry for the study of conformations of host-guest complexes

Variable-temperature cryogenic ion mobility-mass spectrometry (VT-Cryo-IM-MS) was developed to examine the conformations of host-guest complexes. The VT-Cryo-IM-MS consists of an electrospray ionization source, a quadrupole ion trap, a cryogenic ion drift tube, and a time-of-flight mass spectrometer. The ion drift tube was cooled to 83 K with liquid nitrogen. Two sets of ion funnels are installed at the source and the drift tube to focus the ion beam. We used Li+-encapsulated fullerene (Li+@C60) as a reference ion to examine the ion-mobility resolution of the VT-Cryo-IM-MS apparatus because this ion has a rigid structure with a single conformer. The temperature dependence of collision cross section (CCS) of Li+@C60 and the resolution of the apparatus were determined at each temperature. We examined the conformations of host-guest complexes of a series of metal ions (M = Ca2+, Na+, Ag+, K+, Rb+, and Cs+) with dibenzo-24-crown-8 (DB24C8) by Cryo-IM-MS. All CCS distributions were fitted with two Gaussian functions which are assigned to closed and open conformers. These closed and open conformers have different distances between the two benzene rings of DB24C8. The abundance ratio of closed conformers for these M(DB24C8) complex ions depends on the guest M ion with an order, Ca2+ < Na+ < Ag+ < Cs+ < Rb+ < K+. The stability of the closed conformer of K+(DB24C8) originates from the benzene ring positional relationship which is quite close to that of the free benzene dimer. This special benzene dimer-like conformation produces the strong π-π interaction between the benzene rings of K+(DB24C8).