SYNTHESIS, STRUCTURE AND PHOTOPHYSICS OF NEUTRAL PI-ASSOCIATED [2]CATENANES

The supramolecular utility of a neutral re-stacking system of aromatic donors and accepters is illustrated with the syntheses of three neutr al [2]catenanes. These templated syntheses are based on the oxidative dimerisation of electron-deficient aromatic diimide derivatives, equip ped with terminal acetylene functions, in the presence of a preformed crown macrocycle containing complementary electron-rich aromatic dieth ers. X-ray crystallographic study of one of the catenanes and a precat enane crown/diimide inclusion complex reveals the ordered donor-accept or stacking responsible for templating the catenane syntheses and subs equently organising the packing of the interlocked molecules in the so lid state. NMR investigation of the catenanes reveals a dominant dynam ic process at room temperature involving the sweeping of the crown mac rocycle around the periphery of the interlocked structures; NOESY exch ange spectra reveal a higher-energy process involving the revolution o f the crown macrocycle through the centre of the second ring. This com bined dynamic picture has been used to support the observed photophysi cal behaviour. Room-temperature fluorescence of the catenanes is only partially quenched by neighbouring chromophores, implying some mobilit y in the constituent rings; for one of the catenanes fluorescence quen ching data enables an estimate to be made of the ratio of translationa l isomers present in solution. In contrast, low-temperature (77 K) flu orescence is totally quenched as a consequence of freezing all dynamic movement. Collectively, the results presented in this work lend suppo rt to the assertion that the robust, neutral building blocks employed in the catenane syntheses are ideal vehicles for the development of co mplex [n]catenane syntheses.