Department of Chemistry, Washington University, St. Louis, MO, USA.
Photochem Photobiol. 2012 May-Jun;88(3):651-74. doi: 10.1111/j.1751-1097.2012.01083.x. Epub 2012 Feb 17.
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.
评估取代基对叶绿素光谱的影响对于深入了解光合作用过程至关重要。叶绿素 a 和 b 仅在 7-取代基（甲基与甲酰基）的性质上有所不同，而叶绿素 a 和 d 仅在 3-取代基（乙烯基与甲酰基）上有所不同，但具有明显的长波长吸收最大值：665（a）、646（b）和 692nm（d）。在此，研究了含有乙炔基、乙酰基、甲酰基和其他取代基的合成叶绿素和 13（1）-氧代卟啉在 3-、7-和/或 13-位的光谱、单重激发态衰减特性和 DFT 计算结果。用 Gouterman 的四轨道模型很好地解释了取代基对吸收光谱的影响。主要发现是：（1）在 7-位与 3-或 13-位取代基的助色效应的巨大差异主要源于对 LUMO+1 和 LUMO 的相对影响；（2）7-或 8-位的甲酰基有效地“卟啉化”了叶绿素；（3）取代基效应的顺序为乙烯基<乙炔基<乙酰基<甲酰基。因此，光谱性质受特定位置的取代基对叶绿素大环四个前线 MO 的电子效应的复杂相互作用的控制。
