SPECTRAL PROPERTIES OF CIS-METALLOOCTAETHYLCHLORIN AND TRANS-METALLOOCTAETHYLCHLORIN - EFFECTS OF PYRROLINE RING STEREOCHEMISTRY AND MACROCYCLIC CONFORMATION

We recently observed that differences in the structure of nonconjugated substituents on the pyrroline ring of metallochlorins had a dramatic effect on the resonance Raman (RR) spectral frequencies. To explore this novel finding, we have compared the spectral properties of the cis and trans stereoisomers of planar copper(II) octaethylchlorin [Cu(OEC)] and of S4-ruffled nickel(II) octaethylchlorin [Ni(OEC)], where only the configuration of the nonconjugated substituents is varied. Study of these complexes addresses both stereochemistry at the pyrroline ring and the macrocyclic conformation of hydroporphyrins as modulators of the spectral properties. (1) Electronic absorption transitions of the cis-M(OEC) complexes are red-shifted from those of trans-M(OEC), whereas the Soret/Qy intensity ratio of S4-ruffied Ni(OEC) complexes is less than that for the planar Cu(OEC) complexes. (2) The infrared frequencies of Cb-H deformation modes of cis-M(OEC) are higher than those of trans-M(OEC). (3) The RR band at ~ 745 cm−1 is lower in frequency for cis-M(OEC) than for trans-M(OEC), whereas this band exhibits increased intensity for S4-ruffled Ni(OEC) vs planar Cu(OEC) complexes. (4) Extension of the core size/RR frequency correlation of metalloporphyrins to the -chlorins indicates that cis-M(OEC) complexes are smaller in core size than trans-M(OEC), whereas the Ni(OEC) complexes are smaller in core size than Cu(OEC). These data demonstrate that structural variations on the pyrroline ring of chlorins have a marked effect on the spectral properties and that macrocyclic conformation also has a significant influence. Thus, apparently localized changes in the macrocyclic structure strongly perturb the overall properties of chlorins and should be considered in spectral analyses of novel biological and model hydroporphyrins. © 1990, American Chemical Society. All rights reserved.