The synthesis and spectroscopic properties of a [2]catenane incorporating an anthracene chromophoric unit

Abstract

A modified bis-p-phenylene-34-crown-10 ring in which one of the 1,4-dioxybenzene units has been replaced by a 9,10-dioxyanthracene unit has been employed as a template for the formation of cyclobis(paraquat-p-phenylene). The [2]catenane which results has been shown by 1H NMR spectroscopy to exist in solution exclusively in the translationally isomeric form in which (a) only the 1,4-dioxybenzene ring occupies the central cavity of the tetracationic cyclophane and (b) the crown ether ring is prevented from circumrotating through the cyclophane by the large 9,10-dioxyanthracene unit. The absorption spectrum and luminescence properties of this new [2]catenane and of its crown ether component in its free state have been investigated and compared with those of 1,4-dimethoxybenzene and a model anthracene derivative, carrying methylated triethytlene glycol chains on the 9 and 10 positions of the anthracene ring. While the absorption spectrum of the crown ether is the sum of the spectra of the two component chromophoric moieties, its emission spectrum shows only the fluorescence band of the 9,10-dioxyanthracene-type unit The excitation spectrum shows that the disappearance of the 1,4-dioxybenzene type emission in the crown ether is due to a very efficient (kq ≥ 4 × 1010 s-1) energy-transfer process from the 1,4-dioxybenzene to the 9,10-dioxyanthracene type unit. The absorption spectrum of the [2]catenane is noticeably different from the sum of the spectra of its two cyclic components, particularly as far as the presence of a very broad charge-transfer (CT) band in the visible spectral region (λmax = 545 nm, εmax = 615 M-1 cm-1) is concerned. Comparison with the CT band of a model compound shows that the very broad CT band of the [2]catenane is in fact the result of two component bands originating from the interaction of the two different electron-donor units (1,4-dioxybenzene and 9,10-dioxyanthracene type) present in the crown ether with the electron-acceptor bipyridinium-type units of the cyclobis(paraquat-p-phenylene). The emission spectrum of the [2]catenane does not show any band because of the quenching action (rate constant kq ≥ 5 × 1010 s-1) of the low-energy non-luminescent charge-transfer levels on the higher energy, potentially luminescent levels of the crown ether.