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

The 3-acetyl groups of synthetic methyl pyropheophorbides were rotated around the 3–31 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 π-conjugation to affect the redmost Qy band, which has been observed in natural photosynthetic antenna systems, bacteriochlorophyll-a molecules in oligopeptides.