The photoisomerization of methyl orange (MO) encapsulated in the cavities of R-, -, 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 j1 (τ 1 ) and ∼10 ps (τ 2 ). 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 R-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 only one side (2:1 complex). These results showed that a strong intermolecular interaction was induced between two MO molecules by confinement in a nanospace and this also hindered the isomerization. In particular, the complex with two γ-CD molecules (2:2 complex) showed significantly slower relaxation than the others, and no cis isomer was formed. It seemed that the intermolecular interaction of two MO molecules was further enhanced by photoexcitation in the 2:2 complex and this resulted in the formation of an aggregate-like intermediate in the γ-CD nanocavity.
The ultrafast photoisomerization dynamics of azocompounds encapsulated in the cavity of α-, β-, and γ-cyclodextrin (CD) was investigated by the ultrafast transient lens method regarding effects of special restriction and intermolecular interactions. As expected, the spatial restriction reduced the yield of photoisomerization, but the effect was not so remarkable, indicating that the host and guest were relatively freely bounded. This effect was more prominent in azo: γ-CD=2:2 system, where the two guest molecules were packed in parallel as a dimer. From the viewpoint of the confined nanospace as a new reaction field, we found that the azo: γ-CD=2:2 system induced a specific intermediate having a long lifetime, which was not observed in free solutions. We also found that the formation of hydrogen-bonding between CD and guest remarkably elongated the trans–cis transformation of guest molecules in Orange II/CD systems.
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