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

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journal contribution
posted on 2017-01-18, 00:00 authored by Yoshiya Inokuchi, Motoki Kida, Takayuki Ebata
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 the C2v point group and exhibits 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 half of that 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.

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