QUENCHING OF PORPHYRIN EXCITED-STATES BY SILVER(I) IONS AND CHARGE SEPARATION IN BIMOLECULAR SYSTEMS AND IN MACROPOLYCYCLIC CORECEPTORS

Silver(I) ions quench the excited states of free-base octaethylporphyrin (H2OEP) and its zinc complex (ZnOEP) in acetonitrile. Redox products are observed only from the singlet excited state of ZnOEP; in all other cases the quenching mechanism involves catalyzed intersystem crossing due to spin-orbit coupling. Silver(I) ions complex readily with an [18]-N2O4 macrocycle in acetonitrile, and the complex quenches the porphyrin excited states predominantly through an electron-transfer mechanism, forming separated redox products. Two [18]-N2O4 receptor molecules were covalently bound to opposite sides of the porphyrin ring and connected together via a biphenyl strap to give a macrotetracyclic ligand (1) capable of binding Ag+ at each of the lateral receptors, with cooperativity, in addition to forming a zinc porphyrin complex (2). With Ag+ ions in each [18]-N2O4 receptor, fluorescence from the porphyrin subunits in 1 and 2 was quenched (>85%), and flash photolysis studies showed the intermediate formation of redox products. The rate of intramolecular charge separation (k > 6 × 108 s-1) depended upon the reactants and solvent while charge recombination was slow (k < 107 s-1). The triplet excited states of the porphyrin subunits in 1 and 2 are hardly quenched by bound Ag+ ions due to poor thermodynamic driving forces. Replacing the biphenyl strap by a second porphyrin, forming a macropentacyclic ligand (4), induces extensive exciton coupling between the cofacial porphyrin rings. Again, with Ag+ ions in the two lateral receptors available in 4 and its zinc porphyrin complex (5), rapid charge separation is followed by relatively slow recombination. © 1990 American Chemical Society.