Acid dissociation of the axial hydroxylgroup of hydroxy(1-pyrenebutoxy)phosphorus(v) porphyrin controls the intramolecular excitation energy transfer

Abstract

Hydroxy(1-pyrenebutoxy)phosphorus(V) porphyrin was synthesized to investigate the competition process between intramolecular excitation energy transfer and electron transfer from the photo-excited pyrenyl group to the porphyrin ring. The absorption spectrum of the phosphorus(V) porphyrin moiety was blue-shifted depending on an increase in pH of the solvent (water–acetonitrile, 90/10, vol%/vol%), by the acid dissociation of the axial hydroxyl group. The acid dissociation constant was obtained from the absorption change and the pK_a is estimated to be 9.5. In the photo-excited state of the pyrene moiety, the electron transfer competes with the energy transfer. The electron transfer is the predominant process (89%) and the energy transfer is suppressed in the hydroxyphosphorus(V) porphyrin with protonated hydroxyl group. The energy transfer yield increased depending on pH in a similar manner to the absorption change and become almost unity (98%) in the completely dissociated form of the hydroxyl group. In conclusion, the axial hydroxyl group connected at the central phosphorus atom of the porphyrin ring can be the switching site of these electron transition processes.