title: Anomalous cage effect of the excited state dynamics of catechol in the 18-crown-6-catechol host-guest complex.
abstract: We determined the number of isomers and their structures for the 18-crown-6 (18C6)-catechol host-guest complex, and examined the effect of the complex formation on the S1 ((1)ππ*) dynamics of catechol under a supersonically cooled gas phase condition and in cyclohexane solution at room temperature. In the gas phase experiment, UV-UV hole-burning spectra of the 18C6-catechol 1:1 complex indicate that there are three stable isomers. For bare catechol, it has been reported that two adjacent OH groups have an intramolecular hydrogen (H) bond. The IR-UV double resonance spectra show two types of isomers in the 18C6-catechol 1:1 complex; one of the three 18C6-catechol 1:1 isomers has the intramolecular H-bond between the two OH groups, while in the other two isomers the intramolecular H-bond is broken and the two OH groups are H-bonded to oxygen atoms of 18C6. The complex formation with 18C6 substantially elongates the S1 lifetime from 7 ps for bare catechol and 2.0 ns for the catechol-H2O complex to 10.3 ns for the 18C6-catechol 1:1 complex. Density functional theory calculations of the 18C6-catechol 1:1 complex suggest that this elongation is attributed to a larger energy gap between the S1 ((1)ππ*) and (1)πσ* states than that of bare catechol or the catechol-H2O complex. In cyclohexane solution, the enhancement of the fluorescence intensity of catechol was found by adding 18C6, due to the formation of the 18C6-catechol complex in solution, and the complex has a longer S1 lifetime than that of catechol monomer. From the concentration dependence of the fluorescence intensity, we estimated the equilibrium constant K for the 18C6 + catechol ⇄ 18C6-catechol reaction. The obtained value (log K = 2.3) in cyclohexane is comparable to those for alkali metal ions or other molecular ions, indicating that 18C6 efficiently captures catechol in solution. Therefore, 18C6 can be used as a sensitive sensor of catechol derivatives in solution with its high ability of fluorescence enhancement.
authors.affiliation: Department of Chemistry, Graduate School of Science, Hiroshima University , Higashi-Hiroshima 739-8526, Japan.
authors.forename: Fumiya
authors.initials: F
authors.lastname: Morishima
authors.forename: Ryoji
authors.initials: R
authors.lastname: Kusaka
authors.forename: Yoshiya
authors.initials: Y
authors.lastname: Inokuchi
authors.forename: Takeharu
authors.initials: T
authors.lastname: Haino
authors.forename: Takayuki
authors.initials: T
authors.lastname: Ebata
mesh_terms: D002396:Catechols / Q000737:chemistry*; D043844:Crown Ethers / Q000737:chemistry*; D006860:Hydrogen Bonding; D007536:Isomerism; D008958:Models, Molecular; D008968:Molecular Conformation; D013050:Spectrometry, Fluorescence
chemical_list: D002396:Catechols; D043844:Crown Ethers; C015762:18-crown-6; C034221:catechol