Rotaxane Based on Bis-Porphyrin Threaded onto Bis-Viologen Macrocycle

Abstract

Rotaxanes have been prepared using benzenelinked bis-porphyrin or bis-zinc porphyrin as a string component and bis-viologen as a macrocyclic ring component. Absorption and fluorescence spectra of rotaxane formed by complexation of two components have been investigated.Construction of some interlocked molecules, such as pseudorotaxanes, rotaxanes, and catenanes may be considered as one of the most useful tools for preparing the functional synthetic molecular-level nanomachines or switches.1-11 When the proper chemical or physical stimuli are used, their interlocked structures allow tunable translational motion at the nanoscale. The mechanical movements of movable components from the stationary components in molecular machine may be induced in response to some external stimulus such as pH, chemical, electrical energy, or light. In turn, some observable properties such as absorption or luminescence are switched on/off and thereby opening up the way to process information at the molecular level.Rotaxanes 12 consisting of ring and string fragments have been very accessible compounds by the many efficient template synthesis studied since 1980s. The template-directed synthetic strategies reliant on pi-electron donor-acceptor interactions, transition metal complexation, and steric fit/hydrogen bonding have not only made the synthesis of rotaxanes more accessible but have also allowed the preparation of compounds incorporating chemically, electrochemically, and photochemically accessible functionalities. They play a role as building blocks for molecular-scale electronic and photonic devices. Recently, a wide variety of rotaxanes with different strings and macrocyclic rings have been developed for different applications.Among external stimuli activating the molecular machine, light has many advantages such as no generation of waste products, rapid on/off switching, and easy monitoring. 13 In terms of building light-controllable molecular machine, it is interesting to introduce photo-active components such as porphyrin and viologen into the supramolecular interlocked molecules including rotaxanes.Porphyrins are strong visible light absorbing molecules and frequently employed as electron donor in multicomponent electron transfer systems. Benzene is used as a linker as well -electron donor. Positively charged viologen is a typical electron acceptor. Moreover, many positively charged rotaxanes have been prepared by self--stacked interaction between viologen and aromatic compound. [14][15][16] In supramolecule composed of porphyrin and viologen or other electron acceptor, it is expected that visible irradiation leads to the generation of excited porphyrin and then photoinduced electron transfer from excited porphyrin to viologen or other electron acceptor.17,18 Light-induced redox reaction of viologen moiety may apply to derive the lighttriggered translocation of molecular components in rotaxanes.19 While many rotaxanes 20 and pseudorotaxanes 21 containing both porphyrin and viologen have been studied, rotaxanes cons...