Complexes of dibenzo-18-crown-6 (DB18C6, host) with water, ammonia, methanol, and acetylene (guest) in supersonic jets have been characterized by laser induced fluorescence (LIF), UV-UV hole-burning (UV-UV HB), and IR-UV double resonance (IR-UV DR) spectroscopy. Firstly, we reinvestigated the conformation of bare DB18C6 (species m1 and m2) and the structure of DB18C6-H(2)O (species a) [R. Kusaka, Y. Inokuchi, T. Ebata, Phys. Chem. Chem. Phys., 2008, 10, 6238] by measuring IR-UV DR spectra in the region of the methylene CH stretching vibrations. The IR spectral feature of the methylene CH stretch of DB18C6-H(2)O is clearly different from those of bare DB18C6 conformers, suggesting that DB18C6 changes its conformation when forming a complex with a water molecule. With the aid of Monte Carlo simulation for extensive conformational search and density functional calculations (M05-2X/6-31+G*), we reassigned species m1 and m2 to conformers having C(1) and C(2) symmetry, respectively. Also, we confirmed the DB18C6 part in species a of DB18C6-H(2)O to be "boat" conformation (C(2v)). Secondly, we identified nine, one, and two species for the DB18C6 complexes with ammonia, methanol, and acetylene, respectively, by the combination of LIF and UV-UV HB spectroscopy. From the IR spectroscopic measurement in the methylene CH stretching region, a similar conformational change was identified in the DB18C6-ammonia complexes, but not in the complexes with methanol or acetylene. The structures of all the complexes were determined by analyzing the electronic transition energies, exciton splitting, and IR spectra in the region of the OH, NH, and CH stretching vibrations. In DB18C6-ammonia complexes, an ammonia molecule is incorporated into the cavity of the boat conformation by forming "bifurcated" and "bidentate" hydrogen-bond (H-bond), similar to the case of the DB18C6-H(2)O complex. On the other hand, in the DB18C6-methanol and -acetylene complexes, methanol and acetylene molecules are simply attached to the C(1) and C(2) conformations, respectively. From the difference of the DB18C6 conformations depending on the type of the guest molecules, it is concluded that DB18C6 distinguishes water and ammonia from methanol and acetylene when it forms complexes, depending on whether guest molecules have an ability to form bidentate H-bonding.

The conformation and complex formation with guest molecules have been investigated for jet-cooled dibenzo-24-crown-8-ether (DB24C8) by laser induced fluorescence (LIF), UV-UV hole-burning (UV-UV HB), and IR-UV double-resonance (IR-UV DR) spectroscopy. The results are compared with the results of dibenzo-18-crown-6-ether (DB18C6) and analyzed by density functional theory (DFT) calculations at the B3LYP/6-31+G* level. Five conformers are identified for DB24C8 under jet-cooled conditions, and the structure of the major isomer is determined to be a boat form, similar to the case of DB18C6. Two molecules (water and methanol) are investigated as the guest species for the encapsulation experiment. DB24C8 hardly encapsulates a water molecule, different from the case of DB18C6 in which a water molecule is efficiently captured. However, it is likely that larger (water)(n) clusters can be captured in the DB24C8 cavity. The different feature in the complex formation between DB24C8 and DB24C8 with the water molecules are attributed to a larger cavity size of DB24C8 than that of DB18C6. For methanol, two kinds of DB18C6-(methanol)(1) isomers are identified by IR-UV DR spectroscopy; one is the "OH...pi" H-bonded isomer, characteristic of methanol with one OH group, and the other is the "OH...O" H-bonded ones. These results indicate the multiple H-bond formation to the oxygen atoms of the ether ring may play an important role in the complexes of DB24C8 or DB18C6 with water.