Rotational deviation of 3-acetyl group from cyclic tetrapyrrole pi-plane in synthetic bacteriochlorophyll-a analogs by 20-substitution.

The 3-acetyl groups of synthetic methyl pyropheophorbides were rotated around the 3-3(1) bond and the rotational conformers were obtained in a dichloromethane solution of 20-bromo- and methyl-substituted compounds, based on their electronic and vibrational absorption spectra. Such a rotational deviation of the 3-acetyl group from the cyclic tetrapyrrole plane induced less pi-conjugation to affect the redmost Q(y) band, which has been observed in natural photosynthetic antenna systems, bacteriochlorophyll-a molecules in oligopeptides.