Orientational dynamics of a charge transfer complex in cyclodextrin cavity as receptor

Abstract

This paper reports the structure and dynamics of a twisted intermolecular charge transfer molecule 2-(4-(dimethylamino) styryl)-1-methylpyridinium iodide (o-DASPMI) included inside α-, β- and γ-cyclodextrin, investigated by using steady state and time-resolved emission spectroscopy and also theoretical modeling. A nice 1 : 1 inclusion complex with β-CD in the excited state could be found with the dimethylamino group of the molecule sticking out as revealed from steady state and time-resolved emission. The inclusion complex has a longer decay time compared to that in neat water. Time-resolved anisotropy decay has been used to study the rotational dynamics of the molecule inside cyclodextrin cavity. The average angular structure of the inclusion complex as found from semiempirical PM3 calculations corroborates excellently the experimental results of angular orientation in β-CD. The minimum energy of the complex is found to be nearly 5 Å in the length of the molecule with the dimethylamino part sticking out in the bulk water. Hydrogen bonding at the rim hinders the inclusion complex of o-DASPMI in γ-CD and instead it produces association at the rim. Hydrogen bond breaker urea breaks the bonding of o-DASPMI with the rim of γ-CD and the formation of inclusion complex with γ-CD ensues.