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Effects of Substituents on Synthetic Analogs of Chlorophylls. Part 3: The Distinctive Impact of Auxochromes at the 7‐ versus 3‐Positions
Author(s) -
Springer Joseph W.,
Faries Kaitlyn M.,
Diers James R.,
Muthiah Chinnasamy,
Mass Olga,
Kee Hooi Ling,
Kirmaier Christine,
Lindsey Jonathan S.,
Bocian David F.,
Holten Dewey
Publication year - 2012
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2012.01083.x
Subject(s) - substituent , chemistry , chlorin , homo/lumo , photochemistry , excited state , singlet state , spectral line , absorption spectroscopy , chlorophyll a , absorption (acoustics) , stereochemistry , computational chemistry , porphyrin , organic chemistry , molecule , materials science , physics , biochemistry , quantum mechanics , astronomy , nuclear physics , composite material
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.