Brückner Christian, Foss Paul C D, Sullivan James O, Pelto Ryan, Zeller Matthias, Birge Robert R, Crundwell Guy
Department of Chemistry, University of Connecticut, Storrs, 06269-3060, USA.
Phys Chem Chem Phys. 2006 May 28;8(20):2402-12. doi: 10.1039/b600010j. Epub 2006 Apr 6.
The UV-Vis and fluorescence spectra of free base and diprotonated meso-tetrathien-2'-ylporphyrins are, when compared to the spectra of meso-tetra-phenyl- or even -thien-3'-ylporphyrins, characterized by surprisingly large red-shifts. A comparison of the optical spectra and the computed rotational barriers for these meso-aryl-substituted porphyrins and a detailed conformational analysis of the single crystal X-ray structure of a diprotonated meso-tetrathien-2'-ylporphyrin suggest that the origin of the altered electronic properties of meso-tetrathien-2'-ylporphyrins are mainly due to the contribution of conformations in which the thienyl groups adopt idealized co-planar arrangements with the porphyrin ring. These conformations allow an efficient extension of the porphyrinic pi-system through conjugation. We synthesized a meso-tetrathien-2'-ylporphyrin with methyl groups in the o-position, thus preventing the formation of conformers with co-planar thienyl groups and a corresponding thien-2'-ylporphyrin with methyl substituents in a distal position that possesses the same steric requirements for thienyl group rotation as the parent compound, to conclusively deduce the influence of the conformers on the electronic structure. A MNDO-PSDCI computation of their optical spectra further supports our key hypothesis. DFT computations of the total energies of the hypothetical diprotonated thien-2'-ylporphyrin conformer with perpendicular thienyl groups and the conformer containing near-co-planar thienyl groups quantify the resonance stabilization energy. Our results support and complement recent photophysical and theoretical studies by Gupta and Ravikanth and Friedlein et al. on thien-2'-yl-substituted core-modified porphyrins and [meso-tetra(5'-bromothien-2'-yl)porphyrinato]Zn(ii), respectively.
与中位四苯基卟啉甚至中位四噻吩-3'-基卟啉的光谱相比，游离碱和二质子化的中位四噻吩-2'-基卟啉的紫外可见光谱和荧光光谱具有惊人的大红移。对这些中位芳基取代卟啉的光谱和计算出的旋转势垒进行比较，并对二质子化的中位四噻吩-2'-基卟啉的单晶X射线结构进行详细的构象分析，结果表明，中位四噻吩-2'-基卟啉电子性质改变的根源主要是由于噻吩基与卟啉环呈理想共平面排列的构象的贡献。这些构象通过共轭作用有效地扩展了卟啉的π体系。我们合成了一种在邻位带有甲基的中位四噻吩-2'-基卟啉，从而防止形成具有共平面噻吩基的构象异构体，以及一种在远端位置带有甲基取代基的相应的噻吩-2'-基卟啉（该甲基取代基对噻吩基旋转具有与母体化合物相同的空间要求），以最终推断构象异构体对电子结构的影响。对它们的光谱进行MNDO-PSDCI计算进一步支持了我们的关键假设。对具有垂直噻吩基的假设二质子化噻吩-2'-基卟啉构象异构体和含有近共平面噻吩基的构象异构体的总能量进行密度泛函理论计算，定量了共振稳定能。我们的结果支持并补充了Gupta和Ravikanth以及Friedlein等人最近分别对噻吩-2'-基取代的核心修饰卟啉和[中位四(5'-溴噻吩-2'-基)卟啉合]锌(ii)所做的光物理和理论研究。
