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 (¹ππ*) 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 (¹ππ*) and ¹πσ* 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 <i>K</i> for the 18C6 + catechol ⇄ 18C6–catechol reaction. The obtained value (log <i>K</i> = 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