Proton nuclear magnetic resonance of photoporphyrins; assignments and aggregation behaviour

Abstract

The ¹H n.m.r. signals of 2-devinyl-1,2-dihydro-1-hydroxy-2-(2-oxoethylidene)protoporphyrin IX and 4-devinyl-3,4-dihydro-3-hydroxy-4-(2-oxoethylidene)protoporphyrin IX (photoporphyrin A and B) have been assigned to specific pyrrolic substituents by observation of nuclear Overhauser enhancements. N.m.r. and visible spectra were concentration-dependent in some solvents (chloroform and methylene dichloride) but not in others (acetone and methanol) that could act as hydrogen-bond acceptors. The effects may be due to aggregation promoted by hydrogen bonding that is more extensive than the usual porphyrin π–π stacking.