MOLECULAR MECCANO, 29 - THE SYNTHESIS AND SPECTROSCOPIC PROPERTIES OFA [2]CATENANE INCORPORATING AN ANTHRACENE CHROMOPHORIC UNIT

A modified bis-p-phenylene-34-crown-10 ring in which one of the 1,4-di oxybenzene units has been replaced by a 9,10-dioxyanthracene unit has been employed as a template for the formation of cyclobis(paraquat-p-p henylene). The [2]catenane which results has been shown by H-1 NMR spe ctroscopy to exist in solution exclusively in the translationally isom eric form in which (a) only the 1,4-dioxybenzene ring occupies the cen tral cavity of the tetracationic cyclophane and (b) the crown ether ri ng is prevented from circumrotating through the cyclophane by the larg e 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 methylate d triethylene glycol chains on the 9 and 10 positions of the anthracen e 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 ef ficient (k(q) greater than or equal to 4 x 10(10) s(-1)) energy-transf er process from the 1,4-dioxybenzene to the 9,10-dioxyanthracene type unit. The absorption spectrum of the [2]catenane is noticeably differe nt from the sum of the spectra of its two cyclic components, particula rly as far as the presence of a very broad charge-transfer (CT) band i n the visible spectral region (lambda(max) = 545 nm, epsilon(max) = 61 5 M(-1)cm(-1)) cm is concerned. Comparison with the CT band of a model compound shows that the very broad CT band of the [2]catenane is in f act 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-acc eptor bipyridinium-type units of the cyclobis(paraquat-p-phenylene). T he emission spectrum of the [2]catenane does not show any band because of the quenching action (rate constant k(q) greater than or equal to 5 x 10(10) s(-1)) of the low-energy non-luminescent charge-transfer le vels on the higher energy, potentially luminescent levels of the crown ether.