Femtosecond time-resolved spectroscopy of photoisomerization of methyl orange in cyclodextrins

The photoisomerization of methyl orange (MO) encapsulated in the cavities of α-, β-, and γ-cyclodextrins (CDs) was measured by the ultrafast transient lens (UTL) method and transient absorption spectroscopy. The signal for free MO was well-fitted to the sum of two exponential functions, except for the component of the optical Kerr effect (OKE), and their time constants were ≲1 (τ1) and ∼10 ps (τ2). The UTL signal of the 1:1 complex, in which one MO molecule was included in one CD molecule, was almost the same as that of free MO. On the other hand, MO in two α-CD molecules showed slower relaxation and considerably lower yield of cis isomer. Thus, there were clear confinement effects when MO was capped at both ends by two CD molecules. The observed changes of ultrafast dynamics and yield of the isomer were explained in terms of CD−MO interactions and a steric effect. In the case of γ-CD, which included two MO molecules as a dimer, these confinement effects were also observed even when each MO was capped on ...