Geometric and Electronic Structures of Dibenzo-15-Crown-5 Complexes with Alkali Metal Ions Studied by UV Photodissociation and UV-UV Hole-Burning Spectroscopy

We measure UV photodissociation (UVPD) and UV-UV hole-burning (HB) spectra of dibenzo-15-crown-5 (DB15C5) complexes with alkali metal ions, M+•DB15C5 (M = Li, Na, K, Rb, and Cs), under cold (~10 K) conditions in the gas phase. The UV-UV HB spectra of the M+•DB15C5 (M = K, Rb, and Cs) complexes indicate that there is one dominant conformation for each complex except the Na+•DB15C5 complex, which has two conformers with a comparable abundance ratio. It was previously reported that the M+•(benzo-15-crown-5) (M+•B15C5, M = K, Rb, and Cs) complexes each have three conformers. Thus, the attachment of one additional benzene ring to the crown cavity of benzo-15-crown-5 reduces conformational flexibility, giving one dominant conformation for the M+•DB15C5 (M = K, Rb, and Cs) complexes. In the UVPD spectra of the K+•DB15C5, Rb+•DB15C5, and Cs+•DB15C5 complexes, the S1–S0 and S2–S0 transitions are observed independently at different positions with different vibronic structures. The spectral features are substantially different from those of the K+•(dibenzo-18-crown-6) (K+•DB18C6) complex, which belongs to C2v point group and exhibits the exciton splitting with an interval of 2.7 cm–1. The experimental and theoretical results suggest that in the M+•DB15C5 complexes the two benzene rings are not symmetrically equivalent with each other, and the S1–S0 and S2–S0 electronic excitations are almost localized in one of the benzene rings. The electronic interaction energy between the two benzene chromophores is compared between the K+•DB15C5 and K+•DB18C6 complexes by quantum chemical calculations. The interaction energy of the K+•DB15C5 complex is estimated to be less than a half of the K+•DB18C6 complex (~30 cm–1) due to less suitable relative angles between the transition dipole moments of the two benzene chromophores in K+•DB15C5.This work was partly supported by JSPS KAKENHI Grant Number 16H04098

Conformation of Alkali Metal Ion−Benzo-12-Crown‑4 Complexes Investigated by UV Photodissociation and UV−UV Hole-Burning Spectroscopy

We measure UV photodissociation (UVPD) spectra of benzo-12-crown-4 (B12C4) complexes with alkali metal ions, M+·B12C4 (M = Li, Na, K, Rb, and Cs), in the 36300−37600 cm−1 region. Thanks to the cooling of ions to ∼10 K, all the M+·B12C4 complexes show sharp vibronic bands in this region. For UV−UV hole-burning (HB) spectroscopy, we first check if our experimental system works well by observing UV−UV HB spectra of the K+ complex with benzo-18-crown-6 (B18C6), K+·B18C6. In the UV−UV HB spectra of the K+·B18C6 complex, gain signals are also observed; these are due to vibrationally hot K+·B18C6 complex produced by the UV excitation of cold K+·B18C6 complex. Then we apply UV−UV HB spectroscopy to the M+·B12C4 complexes, and only one conformer is found for each complex except for the Li+ complex, which has two conformers. The vibronic structure around the origin band of the UVPD spectra is quite similar for all the complexes, indicating close resemblance of the complex structure. The most stable structures calculated for the M+·B12C4 (M = Li, Na, K, Rb, and Cs) complexes also have a similar conformation among them, which coincides with the UVPD results. In these conformers the metal ions are too big to be included in the B12C4 cavity, even for the Li+ ion. In solution, it was reported that 12-crown-4 (12C4) shows the preference of Na+ ion among alkali metal ions. From the similarity of the structure for the M+·B12C4 complexes, it is suggested that the solvation of free metal ions, not of the M+·12C4 complexes, may lead to the selectivity of Na+ ion for 12C4 in solution.This work was partly supported by JSPS KAKENHI Grant Number 16H04098

Selective Probing of Potassium Ion in Solution by Intramolecular Excimer Fluorescence of Dibenzo-Crown Ethers

Observation of an excimer fluorescence in solution is proposed for detecting the encapsulation of potassium ion as opposed to other alkali ions by dibenzo-crown ethers. The scheme has been validated by ultraviolet photodissociation (UVPD) spectroscopy of dibenzo-21-crown-7 and dibenzo-24-crown-8 complexes with potassium ion, K+·DB21C7 and K+·DB24C8, performed under cold (~10 K) conditions in the gas phase and by quantum chemical calculations of the geometry and electronic structures of the complexes. Calculations suggest formation of a closely spaced excimer structure of benzene rings only for the K+·DB24C8. Interaction of the rings may lead to lifetime broadening in UV absorption, which is experimentally observed in the gas phase, indeed, only for this cold complex. Consistently, intramolecular excimer fluorescence of DB24C8 in solution is observed only for K+·DB24C8. The excimer fluorescence is not observed with other alkali metal ions. The detection of such intramolecular excimer fluorescence therefore can, potentially, serve as a simple, background-fee, selective probe of potassium ion in solution.This work was partly supported by JSPS KAKENHI Grant Numbers JP17H0302200 (MA) and JP16H04098 (YI)

Ultraviolet Photodissociation Spectroscopy of Cold K+•Calix[4]arene Complex in the Gas Phase

The cooling of ionic species in the gas phase greatly simplifies the UV spectrum, which is of special importance to study the electronic and geometric structures of large systems, such as bio-related molecules and host-guest complexes. Many efforts have been devoted to achieving the ion cooling with a cold quadrupole Paul ion trap (QIT), but one problem was insufficient cooling of ions (up to ~30 K) in the QIT. In this study, we construct a mass spectrometer for ultraviolet photodissociation (UVPD) spectroscopy of gas-phase cold ions. The instrument consists of an electrospray ion source, a QIT cooled with a He cryostat, and a time-of-flight mass spectrometer. Giving a great care for the cooling condition, we can achieve ~10 K for the vibrational temperature of ions in the QIT, which is estimated from UVPD spectra of the benzo-18-crown-6 (B18C6) complex with potassium ion, K+•B18C6. Using this setup, we measure a UVPD spectrum of cold calix[4]arene (C4A) complex with potassium ion, K+•C4A. The spectrum shows a very weak band and a strong one at 36018 and 36156 cm–1, respectively, accompanied by many sharp vibronic bands in the 36000–36600 cm–1 region. In the geometry optimization of the K+•C4A complex, we obtain three stable isomers: one endo and two exo forms. On the basis of the total energy and UV spectral patterns predicted by density functional theory calculations, we attribute the structure of the K+•C4A complex to the endo isomer (C2 symmetry), in which the K+ ion is located inside the cup of C4A. The vibronic bands of K+•C4A at 36018 and 36156 cm–1 are assigned to the S1(A)–S0(A) and S2(B)–S0(A) transitions of the endo isomer, respectively.This work is partly supported by the Japan Society for the Promotion of Science (JSPS) through the program “Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation”

極低温・気相分光法によるホスト－ゲスト錯イオンの立体構造と電子状態に関する研究

内容の要約広島大学(Hiroshima University)博士(理学)Doctor of Sciencedoctora

金属イオン－カリックスアレーン錯体の極低温気相レーザー分光

第9回分子科学討論会, 2015年9月16日-19日, 東京工業大学大岡山キャンパス(東京), 1A1