PHOTOCHEMICAL, PHOTOPHYSICAL AND ELECTROCHEMICAL PROPERTIES OF A PHOTOISOMERIZABLE CYCLOPHANE AND ITS [2]CATENANES WITH AROMATIC CROWN-ETHERS

The photochemical, photophysical, and electrochemical behaviour of t-1 ,2-bis(1-benzyl-4-pyridinium)ethylene (t-DBBPE(2+)) (used as model com pound) a cyclophane made of mio t-1,2-bis(4-pyridinium)ethylene (BPE) units (1(4+)), and its catenanes (1BB(4+), 1BN(4+), and 1NN(4+)) with aromatic crown ethers containing two p-dimethoxybenzene units (BB), on e p-dimethoxybenzene and one 1,5-dimethoxynaphthalene unit (BN), and t wo 1,5-dimethoxynaphthalene units (NN) have been investigated in aceto nitrile solution. For both t-DBBPE(2+) and 1(4+), fluorescence and dir ect trans-->cis photoisomerization are prevented by the presence of an intramolecular charge-transfer (CT) excited state close to or below t he 1(pi pi) level, but the triplet-sensitized photoisomerization take s place with high quantum yield. In the 1BB(4+), 1BN(4+), and 1NN(4+) catenanes, also the triplet sensitized photoisomerization is quenched by the presence of lower lying intercomponent CT levels. The intercomp onent CT interaction present in the catenanes affects the reduction po tentials of the t-DBBPE(2+) units of 1(4+). Such an interaction, which plays the role of a brake against the free rotation of the two rings of catenanes, is released upon reduction of the 1(4+) cyclophane.