title: Effects of substituents on synthetic analogs of chlorophylls. Part 3: The distinctive impact of auxochromes at the 7- versus 3-positions.
abstract: Assessing the effects of substituents on the spectra of chlorophylls is essential for gaining a deep understanding of photosynthetic processes. Chlorophyll a and b differ solely in the nature of the 7-substituent (methyl versus formyl), whereas chlorophyll a and d differ solely in the 3-substituent (vinyl versus formyl), yet have distinct long-wavelength absorption maxima: 665 (a) 646 (b) and 692 nm (d). Herein, the spectra, singlet excited-state decay characteristics, and results from DFT calculations are examined for synthetic chlorins and 13(1)-oxophorbines that contain ethynyl, acetyl, formyl and other groups at the 3-, 7- and/or 13-positions. Substituent effects on the absorption spectra are well accounted for using Gouterman's four-orbital model. Key findings are that (1) the dramatic difference in auxochromic effects of a given substituent at the 7- versus 3- or 13-positions primarily derives from relative effects on the LUMO+1 and LUMO; (2) formyl at the 7- or 8-position effectively "porphyrinizes" the chlorin and (3) the substituent effect increases in the order of vinyl < ethynyl < acetyl < formyl. Thus, the spectral properties are governed by an intricate interplay of electronic effects of substituents at particular sites on the four frontier MOs of the chlorin macrocycle.
authors.affiliation: Department of Chemistry, Washington University, St. Louis, MO, USA.
authors.forename: Joseph W
authors.initials: JW
authors.lastname: Springer
authors.forename: Kaitlyn M
authors.initials: KM
authors.lastname: Faries
authors.forename: James R
authors.initials: JR
authors.lastname: Diers
authors.forename: Chinnasamy
authors.initials: C
authors.lastname: Muthiah
authors.forename: Olga
authors.initials: O
authors.lastname: Mass
authors.forename: Hooi Ling
authors.initials: HL
authors.lastname: Kee
authors.forename: Christine
authors.initials: C
authors.lastname: Kirmaier
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Lindsey
authors.forename: David F
authors.initials: DF
authors.lastname: Bocian
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
mesh_terms: D002734:Chlorophyll / Q000031:analogs & derivatives / Q000138:chemical synthesis / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D013050:Spectrometry, Fluorescence; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization
chemical_list: D002734:Chlorophyll