Department of Chemistry , University of Connecticut , Unit 3060 , Storrs , Connecticut 06269-3060 , United States.
J Phys Chem A. 2019 Aug 29;123(34):7470-7485. doi: 10.1021/acs.jpca.9b05656. Epub 2019 Aug 16.
Advances in the utilization of porphyrinoids for photomedicine, catalysis, and artificial photosynthesis require a fundamental understanding of the relationships between their molecular connectivity and resulting electronic structures. Herein, we analyze how the replacement of two pyrrolic C═C bonds of a porphyrin by two lactone (O═C-O) moieties modulates the ground-state thermodynamic stability and electronic structure of the resulting five possible pyrrole-modified porphyrin isomers. We made these determinations based on density functional theory (DFT) and time-dependent DFT computations of the optical spectra of all regioisomers. We also analyzed the computed magnetically induced currents of their aromatic π-systems. All regioisomers adopt the tautomeric state that maximizes aromaticity, whether or not transannular steric strains are incurred. In all isomers, the O═C-O bonds were found to support a macrocycle diatropic ring current. We attributed this to the delocalization of nonbonding electrons from the ring oxa- and oxo-atoms into the macrocycle. As a consequence of this delocalization, the dilactone regioisomers are as-or even more-aromatic than their hydroporphyrin congeners. The electronic structures follow different trends for the bacteriochlorin- and isobacteriochlorin-type isomers. The presence of either oxo- or oxa-oxygens conjugated with the macrocyclic π-system was found to be the minimal structural requirement for the regioisomers to exhibit distinct electronic properties. Our computational methods and mechanistic insights provide a basis for the systematic exploration of the physicochemical properties of porphyrinoids as a function of the number, relative orientation, and degree of macrocycle-π-conjugation of β-substituents, in general, and for dilactone-based porphyrinic chromophores, in particular.
卟啉衍生物在光医学、催化和人工光合作用中的应用进展需要深入了解其分子连接性与所得电子结构之间的关系。在此，我们分析了卟啉中二吡咯 C═C 键被两个内酯（O═C-O）基团取代后如何调节所得五种可能的吡咯修饰卟啉异构体的基态热力学稳定性和电子结构。我们基于密度泛函理论（DFT）和所有区域异构体光学光谱的时间相关 DFT 计算来确定这些性质。我们还分析了其芳香 π 体系计算得到的磁诱导电流。所有区域异构体均采用最大限度地增加芳香性的互变异构状态，无论是否存在跨环空间位阻。在所有异构体中，O═C-O 键被发现支持大环的双极性环电流。我们将其归因于非键电子从环的氧杂和氧原子离域到大环中。由于这种离域作用，二内酯区域异构体与它们的氢化卟啉同系物一样或甚至更具芳香性。电子结构对于细菌叶绿素和异细菌叶绿素型异构体呈现出不同的趋势。我们发现，无论氧原子还是氧杂原子与大环 π 体系共轭，都成为区域异构体呈现独特电子性质的最小结构要求。我们的计算方法和机理见解为系统地研究卟啉衍生物的物理化学性质提供了基础，其可以作为β取代基的数量、相对取向和大环-π 键合程度的函数，一般来说，也为基于二内酯的卟啉类发色团提供了基础。
