Controllable Donor-Acceptor Neutral [2]Rotaxanes

摘要

In pursuit of a neutral bistable [2]rotaxane made up of two tetra-arylmethane stoppers-both carrying one isopropyl and two tert-butyl groups located at the para positions on each of three of the four aryl rings-known to permit the slippage of the rt-electron-donating 1,5-dinaphtho[38]crown-10(1/5DNP38C10)at the thermodynamic instigation of pi-electron-accepting recognition sites,in this case,pyromellitic diimide(Pml)and 1,4,5,8-naphthalene-tetracarboxylate diimide(Npl)units separated from each other along the rod section of the rotaxane's dumbbell component,and from the para positions of the fourth aryl group of the two stoppers by pentamethylene chains,a modular approach was employed in the synthesis of the dumbbell-shaped compound NpPmD,as well as of its two degenerate counterparts,one(PmPmD)which contains two Pml units and the other(NpNpD)which contains two Npl units.The bistable [2]rotaxane NpPmR,as well as its two degenerate analogues PmPmR and NpNpR,were obtained from the corresponding dumbbell-shaped compounds NpPmD,PmPmD,and NpNpD and 1/5DNP38C10 by slippage.Dynamic 'H NMR spectroscopy in CD2Cl_2 revealed that shuttling of the 1/5DNP38C10 ring occurs in NpNpR and PmPmR,with activation barriers of 277 K of 14.0 and 10.9 kcalmol~(-1),respectively,reflecting a much more pronounced donor-acceptor stabilizing interaction involving the Npl units over the Pml ones.The photophysical and electrochemical properties of the three neutral [2]rotaxanes and their dumbbell-shaped precursors have also been investigated in CH_2Cl_2.Interactions between 1/5DNP38C10 and Pml and Npl units located within the rod section of the dumbbell components of the [2]ro-taxane give rise to the appearance of charge-transfer bands,the energies of which correlate with the electron-accepting properties of the two diimide moieties.Comparison between the positions of the visible absorption bands in the three [2]rotaxanes shows that,in NpPmR,the major translatio-nal isomer is the one in which 1/5DNP38C10 encircles the Npl unit.Correlations of the reduction potentials for all the compounds studied confirm that,in this non-degenerate [2]ro-taxane,one of the translational isomers predominates.Furthermore,after deac-tivation of the Npl unit by one-electron reduction,the 1/5DNP38C10 macrocy-cle moves to the Pml unit.Li~+ ions have been found to strengthen the interaction between the electron-donating crown ether and the electron-accepting diimide units,particularly the Pml one.Titration experiments show that two Li~+ ions are involved in the strengthening of the donor-acceptor interaction.Addition of Li~+ ions to NpPmR induces the 1/5DNP38C10 macrocycle to move from the Npl to the Pml unit.The Li~+-ion-promoted switching of NpPmR in a 4:1 mixture of CD_2Cl_2 and CD_3COCD_3 has also been shown by 'H NMR spectroscopy to involve the mechanical movement of ' the 1/5DNP38C10 macrocycle from the Npl to the Pml unit,a process that can be reversed by adding an excess of [12]crown-4 to sequester the Li~+ ions.