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In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . Important members of this series are the tetramer (n = 4), the pentamer (n = 5), and the hexamer (n = 6). The term "crown" refers to the resemblance between the structure of a crown ether bound to a cation, and a crown sitting on a person's head. The first number in a crown ether's name refers to the number of atoms in the cycle, and the second number refers to the number of those atoms that are oxygen. Crown ethers are much broader than the oligomers of ethylene oxide; an important group are derived from catechol. Crown ethers strongly bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis. The denticity of the polyether influences the affinity of the crown ether for various cations. For example, 18-crown-6 has high affinity for potassium cation, 15-crown-5 for sodium cation, and 12-crown-4 for lithium cation. The high affinity of 18-crown-6 for potassium ions contributes to its toxicity. The smallest crown ether still capable of binding cations is 8-crown-4, with the largest experimentally confirmed crown ether being 81-crown-27. Crown ethers are not the only macrocyclic ligands that have affinity for the potassium cation. Ionophores such as valinomycin also display a marked preference for the potassium cation over other cations. Crown ethers have been shown to coordinate to Lewis acids through electrostatic, σ-hole (see halogen bond) interactions, between the Lewis basic oxygen atoms of the crown ether and the electrophilic Lewis acid center.

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Multimodal host–guest complexation for efficient and stable perovskite photovoltaics

Michael Graetzel, Shaik Mohammed Zakeeruddin, David Lyndon Emsley, Ursula Röthlisberger, Felix Thomas Eickemeyer, Jovana Milic, Michael Allan Hope, Marko Mladenovic, Ulf Anders Hagfeldt, Hong Zhang, Dan Ren, Zaiwei Wang, Paramvir Ahlawat, Aditya Mishra, Zhaowen Dong, Farzaneh Jahanbakhshi, Anurag Krishna, Jinming Guo, Olivier Ouellette, Zhiwen Zhou, Tzu-Sen Su

Formamidinium lead iodide perovskites are promising light-harvesting materials, yet stabilizing them under operating conditions without compromising optimal optoelectronic properties remains challenging. We report a multimodal host–guest complexation strategy to overcome this challenge using a crown ether, dibenzo-21-crown-7, which acts as a vehicle that assembles at the interface and delivers Cs+ ions into the interior while modulating the material. This provides a local gradient of doping at the nanoscale that assists in photoinduced charge separation while passivating surface and bulk defects, stabilizing the perovskite phase through a synergistic effect of the host, guest, and host–guest complex. The resulting solar cells show power conversion efficiencies exceeding 24% and enhanced operational stability, maintaining over 95% of their performance without encapsulation for 500 h under continuous operation. Moreover, the host contributes to binding lead ions, reducing their environmental impact. This supramolecular strategy illustrates the broad implications of host–guest chemistry in photovoltaics.

2021

UV and IR Spectroscopy of Transition Metal–Crown Ether Complexes in the Gas Phase: Mn2+(benzo-15-crown-5)(H2O)0–2

Thomas Rizzo, Yoshiya Inokuchi

Ultraviolet photodissociation (UVPD) and IR–UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn2+(benzo-15-crown-5), Mn2+(B15C5)(H2O)n (n = 0–2), under cold (∼10 K) gas-phase conditions. Density functional theory (DFT) calculations are also carried out to derive information on the geometric and electronic structures of the complexes from the experimental results. The n = 0 complex shows broad features in the UVPD spectrum, whereas the UV spectra of the n = 1 and 2 complexes exhibit sharp vibronic bands. The IR–UV and DFT results suggest that there is only one isomer each for the n = 1 and 2 complexes in which H2O molecules are directly attached to the Mn2+ ion through Mn2+···OH2 bonds with no intermolecular bond between the water molecules. Time-dependent DFT calculations suggest that the π–π* transition of the B15C5 part is highly mixed with the “ligand to metal charge transfer” transition in the n = 0 complex, which can result in broad features in the UVPD spectrum. In contrast, attachment of H2O molecules to Mn2+(B15C5) suppresses the mixing, providing sharp vibronic bands assignable to the π–π* transition for the n = 1 and 2 complexes. These results indicate that the electronic structure and transition of benzo-crown ether complexes with transition metals are strongly affected by solvation.

2019

Microhydration Effects on the Encapsulation of Potassium Ion by Dibenzo-18-Crown-6

Thomas Rizzo, Yoshiya Inokuchi

We have measured electronic and conformer-specific vibrational spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium ion, K+center dot DB18C6 center dot(H2O)(n) (n = 1-5), in a cold, 22-pole ion trap. We also present for comparison spectra of Rb+center dot DB18C6 center dot(H2O)(3) and Cs+center dot DB18C6 center dot(H2O)(3) complexes. We determine the number and the structure of conformers by analyzing the spectra with the aid of quantum chemical calculations. The K+center dot DB18C6 center dot(H2O)(1) complex has only one conformer under the conditions of our experiment. For K+center dot DB18C6 center dot(H2O)(n) with n = 2 and 3, there are at least two conformers even under the cold conditions, whereas Rb+center dot DB18C6 center dot(H2O)(3) and Cs+center dot DB18C6 center dot(H2O)(3) each exhibit only one isomer. The difference can be explained by the optimum matching in size between the K+ ion and the crown cavity; because the K+ ion can be deeply encapsulated by DB18C6 and the interaction between the K+ ion and the H2O molecules becomes weak, different kinds of hydration geometries can occur for the K+center dot DB18C6 complex, giving multiple conformations in the experiment. For K+center dot DB18C6 center dot(H2O)(n) (n = 4 and 5) complexes, only a single isomer is found. This is attributed to a cooperative effect of the H2O molecules on the hydration of K+center dot DB18C6; the H2O molecules form a ring, which is bound on top of the K+center dot DB18C6 complex. According to the stable structure determined in this study, the K+ ion in the K+center dot DB18C6 center dot(H2O)(n) complexes tends to be pulled largely out from the crown cavity by the H2O molecules with increasing n. Multiple conformations observed for the K+ complexes will have an advantage for the effective capture of the K+ ion over the other alkali metal ions by DB18C6 because of entropic effects on the formation of hydrated complexes.

American Chemical Society2014

Crown ether

Cryptand

In chemistry, cryptands are a family of synthetic, bicyclic and polycyclic, multidentate ligands for a variety of cations. The Nobel Prize for Chemistry in 1987 was given to Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen for their efforts in discovering and determining uses of cryptands and crown ethers, thus launching the now flourishing field of supramolecular chemistry. The term cryptand implies that this ligand binds substrates in a crypt, interring the guest as in a burial.

Macrocycle

Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, mature area of chemistry. The formation of macrocycles by ring-closure is called macrocylization. Pioneering work was reported for studies on terpenoid macrocycles. The central challenge to macrocyclization is that ring-closing reactions do not favor the formation of large rings.

CH-424: Supramolecular chemistry

The course provides an introduction to supramolecular chemistry. In addition, current trends are discussed using recent publications in this area.

Explores crown ethers, complexation with alkali metal ions, anion recognition, and fluorescence sensors.

Explores multivalency, cooperativity, and binding constants in supramolecular chemistry, emphasizing NMR spectroscopy and isothermal titration calorimetry.

Explores supramolecular chemistry, emphasizing metal ion arrays, helicates, and interlocked molecules.