Molecular Recognition of Indole and Naphthalene Carboxylic Acids with Cyclic and Acyclic Bipyridinium Hosts Based on Charge Transfer Absorption

Abstract

Three types of cyclic bisbipyridinium host compounds (Cpp, Cpm, Cmm), each being structural isomers containing two bipyridinium units bridged by a p- or m-xylene unit at N-position of the bipyridine, and two acyclic hosts of their analogues (ACp, ACm) have been prepared to investigate their molecular-recognition abilities for indole and naphthalene carboxylic acid derivatives by charge-transfer absorption in acetonitrile. When the solution contained the bisbipyridinium host and the guest, charge-transfer absorption was observed in the visible region with three different modes, in which the absorption maximum was observed at around 450 nm, 530 nm, and no maximum appeared. Such development of the charge transfer absorption is associated with the combination of a bisbipyridinium host and a guest. By using dual-wavelength analysis at 450 and 530 nm, it was found that ACp and ACm recognized 2-naphthalene carboxylic acid derivatives rather than the isomer with 1-substituted ones. Since the ester derivatives of the naphthalein carboxylic acid guest exhibited no response based on charge transfer, the carboxylic group would play an important role for the recognition system.