We report UV photodissociation (UVPD) spectra of dibenzo-18-crown-6 (DB18C6) complexes with alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) in a cold, 22-pole ion trap. All the complexes show a number of vibronically resolved bands, but the vibronic structure changes drastically in the series between Na+ and K+. The Li+ and Na+ complexes show extensive and intense activity of low-frequency vibrations, indicating that the β carbon atoms are largely displaced from the plane of the phenyl rings. For the K+, Rb+, and Cs+ complexes, such a long progression is not observed, suggesting that the β carbon atoms are almost in-plane of the phenyl rings. To hold Li+ and Na+ ions, DB18C6 shrinks the ether ring to fit the cavity size to the diameter of Li+ and Na+. In the K+, Rb+, and Cs+ complexes, the DB18C6 component opens the cavity the most by locating the β carbon atoms in the plane of the benzene rings, giving a boat-type open conformation. The K+ ion is effectively captured in the hole of the open conformer thanks to the optimum matching of the cavity size and ion diameter. The Rb+ and Cs+ ions are thought to be on the ether ring because the ion is too large to enter the cavity of the open conformer.