Synthesis and Photochemical Properties of 12-Substituted versus 13-Substituted Chlorins

Understanding the effects of substituents on natural photosynthetic pigments is essential for the rational design of artificial photosynthetic systems. The long-wavelength absorption of chlorins derives from a transition that encompasses rings A and C, which includes the 2,3- and 12,13-positions, respectively. Chlorophylls bear a 3-vinyl group and a 13-keto group, as well as a full complement of substituents at the other β-pyrrole sites. Prior studies of sparsely substituted synthetic chlorins to probe the effects of substituents yielded 3,13-substituted chlorins that contain a geminal dimethyl group in the pyrroline ring (for stability) and a mesityl group at the 10-position. Attempts to prepare analogous chlorins lacking the 10-mesityl substituent encountered unexpected difficulties during construction of the Eastern half precursor (8,9-dibromo-1-formyldipyrromethane) to the 13-bromochlorin. Direct bromination of 1-formyldipyrromethane with 2 mol equiv of NBS at −78 °C led to an isomeric mixture of the desired 8,9-dibromodipyrromethane (minor) and the unexpected 7,9-dibromodipyrromethane (major). Hence, a new rational route was developed for the synthesis of 8,9-dibromo-1-formyldipyrromethane that entailed (i) InCl₃-catalyzed condensation of 4-bromo-2-(hydroxymethyl)pyrrole and pyrrole to give the 8-bromodipyrromethane, (ii) 1-formylation, and (iii) 9-bromination. Two new substituted chlorins carrying auxochromes at the 3- and 13-positions were synthesized. The photophysical and redox properties of the 13-substituted chlorins were compared with those of isomeric 12-substituted chlorins, synthesized previously via a 7,9-dibromo-1-formyldipyrromethane. Such studies (static absorption and fluorescence spectroscopy, time-resolved fluorescence spectroscopy, electrochemistry of the zinc chelates, and density functional theoretical calculations) reveal only very slight differences between the isomeric 12- and 13-substituted chlorins.