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 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.