Quenching of porphyrin excited states by silver(I) ions and charge separation in bimolecular systems and in macropolycyclic coreceptors

Abstract

Silver(I) ions quench the excited states of free-base octaethylporphyrin (H{sub 2}OEP) 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)-N{sub 2}O{sub 4} macrocycle in acetonitrile, and the complex quenches the porphyrin excited states predominantly through an electron-transfer mechanism, forming separated redox products. Two (18)-N{sub 2}O{sub 4} 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{sup +} at each of the lateral receptors, with cooperativity, in addition to forming a zinc porphyrin complex (2). With Ag{sup +} ions in each (18)-N{sub 2}O{sub 4} the intermediate formation of redox products. The rate of intramolecular charge separation (k > 6 {times} 10{sup 8} s{sup {minus}1}) depended upon the reactants and solvent while charge recombination was slow (k < 10{sup 7} s{sup {minus}1}). The triplet excited states of the porphyrin subunits in 1 and 2 are hardly quenched by bound Ag{sup +} ions due to poor thermodynamic driving forces. Replacing the biphenylmore » strap by a second porphyrin, forming a macropentacyclic ligand (4), induces extensive exciton coupling between the cofacial porphyrin rings. Again, with Ag{sup +} ions in the two lateral receptors available in 4 and its zinc porphyrin complex (5), rapid charge separation is followed by relatively slow recombination.« less