Crown ethers are capable of selectively and efficiently encapsulating guest species. Generally, their encapsulation ability is influenced by the size and structure of the host and guest. Unfortunately, obtaining stable crystals for all the host-guest systems is difficult to achieve. Therefore, researchers have been looking for alternatives and have identified gas phase spectroscopy in a cold environment to host-guest species. High-resolution absorption spectra of molecules and ions can be observed when cooled in the gas phase, free from thermal and solvent effects. Besides, the spectroscopy provides new chemical and physical insights regarding the host-guest species.
A group of researchers at Hiroshima University: Motoki Kida, Mayuko Kubo, Tomoyuki Ujihira, Professor Takayuki Ebata, Professor Manabu Abe and led by Professor Yoshiya Inokuchi investigated the electronic and geometric structures of dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with potassium ion, K+DB21C7 and K+DB24C8. They utilized ultraviolent photodissociation (UVPD) spectroscopy under cold gas-phase conditions to measure the UV spectra of the complexes and compared the results to those previously reported for dibenzo-15-crown-5 and dibenzo-18-crown-6 complexes (K+DB15C5 and K+DB18C6). Furthermore, the effects of conformation and crown size on the electronic interaction in these complexes were also comprehensively investigated. They purposed to use the dibenzo-crown ethers to observe the excimer fluorescence in the solution thus detecting potassium ion encapsulation. The work is published in the journal, Chemphyschem
The authors observed that the UV absorption of K+DB24C8 broadened even in the gas phase under cold conditions, which is due to the interaction of the benzene rings in DB24C8. Consequently, it was easy to observe intramolecular excimer fluorescence of DB24C8 in solution for only K+DB24C8 in methanol by UV irradiation. However, no excimer fluorescence was observed when other alkali metals were used. This was attributed to the short distance between the benzene rings in the K+DB24C8 which results in a strong electronic attraction within the complex.
It was necessary to perform quantum chemical calculations in order to determine the electronic and geometric structures of the complexes based on the UVPD spectra. It was noted that the electronic excitations of the K+DB15C5 and K+DB21C7 complexes are nearly localized in one of the benzene rings. On the other hand, the electronic excitations of K+DB24C8 are delocalized over the two benzene rings in nature. This was the main reason for the conformation and molecular orbitals characteristics of K+DB24C8 thus resulting in the intramolecular excimer formation as well as broad nature of the UVPD spectrum.
Hiroshima University scientists successfully utilized intramolecular excimer fluorescence of dibenzo-crown ethers to develop a simple selective probing of potassium ion in a particular given solution. This is due to the fact that the intramolecular excimer fluorescence of DB24C8 can be detected only with potassium ion among the different alkali metals ions. Therefore, by detection of the excimer fluorescence, DB24C8 can be used as a simple, high-sensitive and effective potassium ion selective probe.
Kida, M., Kubo, M., Ujihira, T., Ebata, T., Abe, M., & Inokuchi, Y. (2018). Selective Probing of Potassium Ion in Solution by Intramolecular Excimer Fluorescence of Dibenzo-Crown Ethers. Chemphyschem, 19(11), 1331-1335.Go To Chemphyschem