title: Synthesis and photochemical properties of 12-substituted versus 13-substituted chlorins.
abstract: 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 beta-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 degrees 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) InCl(3)-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.
authors.affiliation: Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA.
authors.forename: Olga
authors.initials: O
authors.lastname: Mass
authors.forename: Marcin
authors.initials: M
authors.lastname: Ptaszek
authors.forename: Masahiko
authors.initials: M
authors.lastname: Taniguchi
authors.forename: James R
authors.initials: JR
authors.lastname: Diers
authors.forename: Hooi Ling
authors.initials: HL
authors.lastname: Kee
authors.forename: David F
authors.initials: DF
authors.lastname: Bocian
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Lindsey
mesh_terms: D003198:Computer Simulation; D004563:Electrochemistry; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry; D010084:Oxidation-Reduction; D010777:Photochemistry; D011166:Porphyrins / Q000138:chemical synthesis* / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D009942:Organometallic Compounds; D011166:Porphyrins; C006969:chlorin