Optical properties of synthetic porphyrins bearing or lacking an exo-five-membered ring and a keto carbonyl group on it, both of which are present in naturally occurring chlorophylls

By modifying β-octaethylporphyrin we prepared 131-oxo- and deoxo-porphyrins 1 and 3 possessing an exo-five-membered E-ring, which is a structural requirement of naturally occurring chlorophyllous pigments, and also 131-oxo- and deoxo-porphyrins 2 and 4 lacking the E-ring as reference compounds. Visible absorption spectra of 131-deoxo-porphyrins 3/4 bearing/lacking the E-ring and their zinc complexes showed a relatively small difference, indicating that geometry of the tetrapyrrole unit in 3 was altered by formation of the E-ring but its contribution to the visible spectrum was limited. In contrast, a spectral difference between 131-oxo-porphyrins 1/2 bearing/lacking the E-ring (as well as their zinc complexes) was clearly observed; fixation of 131-oxo group by the E-ring as in 1 resulted in its red-shifted absorption spectrum (ca. 10 nm for each band). These results indicated that introduction of the E-ring to a porphyrin macrocycle slightly affected its optical properties and that a larger effect was observed by fixation of 131-oxo group to a porphyrin π-conjugate system.