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 the C_2v 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.