meso-Substituted octabromoporphyrins: synthesis, spectroscopy, electrochemistry and electronic structure
作者:Gali Hariprasad、Sanjay Dahal、Bhaskar G. Maiya
DOI:10.1039/dt9960003429
日期:——
The free-base, copper(II) and zinc(II) derivatives of 5,10,15,20-tetraarylporphyrin (aryl = phenyl, 4-methylphenyl or 4-chlorophenyl) and the corresponding brominated 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraarylporphyrin derivatives have been synthesized and their spectral and redox properties compared by UV/VIS, 1H NMR, ESR and cyclic voltammetric methods. Substitution with the electron-withdrawing bromine groups at the pyrrole carbons has a profound influence on the UV/VIS and 1H NMR spectral features and also on the redox potentials of these systems. On the other hand, electron-withdrawing chloro or electron-donating methyl groups at the para positions of the four phenyl rings have only a marginal effect on the spectra and redox potentials of both the brominated and the non-brominated derivatives. The ESR data for the copper(II) derivatives of all these systems reveal that substitution at either the β-pyrrole carbons and/or the para positions of the meso-phenyl groups does not significantly affect the spin-Hamiltonian parameters that describe the metal centre in each case. Collectively, these observations suggest that the highest-occupied (HOMO) and lowest-unoccupied molecular orbitals (LUMO) of the octabromoporphyrins involve the porphyrin π-ring system as is the case with the non-brominated derivatives. Investigations have been carried out to probe the electronic structures of these systems by three different approaches involving spectral and redox potential data as well as AM1 calculations. The results obtained suggest that the electron-withdrawing β-bromine substituents stabilize the LUMOs and, to a lesser degree, the HOMOs and that the extent of these changes can be fine-tuned, in a subtle way, by substituting at the meso-aryl rings of a given porphyrin.
研究人员合成了 5,10,15,20-四芳基
卟啉(芳基=苯基、4-甲基苯基或 4-
氯苯基)的游离基、
铜(II)和
锌(II)衍
生物以及相应的
溴化 2,3,7,8,12,13-四芳基
卟啉衍
生物、合成了相应的
溴化 2,3,7,8,12,13,17,18-八
溴-5,10,15,20-四芳基
卟啉衍
生物,并通过紫外/可见光谱、1H NMR、ESR 和循环伏安法比较了它们的光谱和氧化还原特性。在
吡咯碳原子上取代具有电子吸收性的
溴基团对这些体系的紫外可见光/可见光和 1H NMR 光谱特征以及氧化还原电位都有深远的影响。另一方面,四个苯基环的对位上的取电子
氯基或供电子甲基对
溴化和非
溴化衍
生物的光谱和氧化还原电位的影响微乎其微。所有这些体系的
铜(II)衍
生物的 ESR 数据都表明,β-
吡咯碳原子和/或中苯基对位的取代并不会显著影响描述
金属中心的自旋-哈密顿参数。总之,这些观察结果表明,八
溴卟啉的最高占位(HOMO)和最低未占位分子轨道(LUMO)涉及
卟啉 π 环系统,这与非
溴化衍
生物的情况相同。研究人员采用了三种不同的方法来探究这些系统的电子结构,包括光谱和氧化还原电位数据以及
AM1 计算。研究结果表明,吸电子的 β-
溴取代基能稳定 LUMOs,在较小程度上也能稳定 HOMOs,而且这些变化的程度可以通过在特定
卟啉的中芳基环上进行取代来微调。