Luminescence properties of some synthetic metallocorrins

Abstract

The luminescence spectra of a series of synthetic metallocorrins, where corrin is 1,2,2,7,7,12,12-heptamethyl-15-cyanocorrin, have been measured. The study has been undertaken to elucidate the mechanism of the photochemical A/D-secocorrin → corrin cyclisation devised by Eschenmoser in the course of the synthesis of vitamin B₁₂. It is found that metal-free, lithium(I), beryllium(II), magnesium(II), zinc(II), and cadmium(II) corrins fluoresce readily, the emission coming from the lowest excited singlet state of the corrin ring. In addition, cadmium(II) corrin emits in the near-i.r. at 11·6 kk at low temperature. This emission is the first location of the triplet state of the corrin chromophore. Nickel(II) and copper(II) corrin show no luminescence whatsoever, whereas a very feeble luminescence of uncertain origin has been seen from dicyanocobaIt(III)–corrin. Dicyanorhodium(III) corrin, palladium(II), and platinum(II) corrin show an intense structured luminescence both in glasses at low temperature and in fluid solution. This luminescence is displaced to low energy of the absorption band of corrin by 3000–6000 cm^–1 and has a lifetime of the order of 10^–6 s. Mainly on account of the typical corrin chromophore stretching frequency seen in the band system it can be assigned to the phosphorescence of the lowest triplet state of the corrin ring, perturbed by the heavy transition-metal ion. These results lead to the conclusion that in the case of nickel(II), copper(II) and, in all probability, dicyanocobalt(III) corrins metal d-states lying below the lowest triplet state of the corrin ring quench the emission of the latter by facilitating rapid radiationless transitions onto the metal ion. On the other hand, in the corrins of palladium, platinum, and rhodium the lowest energy d-states are above the lowest triplet state of corrin and, most likely, above the lowest singlet also, so that the lowest corrin states are still populated after excitation of the ring transitions. These results taken together with those of Eschenmoser reveal a striking parallel between those closed ring metal corrins which display corrin luminescence and those secocorrins which undergo a facile photocyclisation. This leads us to conclude that the presence of low-lying metal d-states quenches both the emission of the corrin chromophore and the photochemical ring closure of secocorrin.