Institute of Physical Chemistry, Friedrich-Schiller-University Jena, Helmholtzweg 4, 07743, Jena, Germany.
J Phys Chem A. 2010 Dec 23;114(50):13163-74. doi: 10.1021/jp107007a. Epub 2010 Nov 23.
To gain a deeper understanding of how structural modifications may influence photochemical properties of 4'-phenyl-2,2':6',2''-terpyridines, the investigations presented here focus on electron delocalization in 4'-phenyl-2,2':6',2''-terpyridine derivatives and their Ru(II) and Zn(II) complexes. In those systems of neighboring aromatic rings the considerable torsion between the rings is commonly regarded to be the limiting factor for a well pronounced π-conjugation between the rings. A common approach to improve the π-conjugation is to lower the steric hindrance, thus achieving a more planar geometry. Here, we present a fundamentally different approach towards enhanced π-conjugation by manipulation of the electronic properties of the pyridine-phenyl (py-ph) bond. This is accomplished by introducing various substituents at the phenylene moiety or coordinating the terpyridine moiety to transition metal ions. The electron delocalization was quantified via the DFT-calculated ellipticity in the bond-critical point (BCP) of the py-ph bond. This ellipticity can be modified due to substituents in the para position of phenylene and via the transition metals coordinated to the terpyridine moiety. Changes in electron density distribution induced by the substituents and the metal ions are further studied by means of intermolecular electron density difference plots. It was shown that a NH(2) group in the para position of the phenyl ring as well as the coordination to Ru(II) or Zn(II) ions significantly enhances the π-character of the py-ph bond. Surprisingly, an even higher π-character of the py-ph bond is achieved by introducing additional NH(2) groups in ortho position to the py-ph bond, despite the increased torsion between pyridine and phenylene. The introduction of other substituents (-NO(2), -Br, -CN, -vinyl, -ethynyl) studied within the presented work enables an actuation of the electron delocalization between terpyridine and phenylene. In doing so, the ellipticity is a concise quantity to characterize electron delocalization in the studied systems. Furthermore, the ellipticity in the BCP of the py-ph bond is related to the corresponding geometrical properties (e.g., bond length and dihedral angle) and to the DFT-calculated HOMO and LUMO energies.
为了更深入地了解结构修饰如何影响 4'-苯基-2,2':6',2''-三联吡啶的光化学性质，本研究关注的是 4'-苯基-2,2':6',2''-三联吡啶衍生物及其 Ru(II)和 Zn(II)配合物中的电子离域。在这些相邻芳环体系中，环之间的相当大的扭转通常被认为是限制环之间良好的π共轭的因素。提高π共轭的常用方法是降低空间位阻，从而实现更平面的几何形状。在这里，我们提出了一种通过操纵吡啶-苯基（py-ph）键的电子性质来增强π共轭的根本不同的方法。通过在联苯部分引入各种取代基或将三联吡啶部分配位到过渡金属离子来实现这一点。通过 DFT 计算在 py-ph 键的键临界点（BCP）处的椭圆率来量化电子离域。由于联苯部分的对位取代基以及配位到三联吡啶部分的过渡金属，可以改变这种椭圆率。通过取代基和金属离子诱导的电子密度分布变化进一步通过分子间电子密度差图进行研究。结果表明，苯环对位的 NH(2)基团以及与 Ru(II)或 Zn(II)离子的配位显著增强了 py-ph 键的π特征。令人惊讶的是，通过在 py-ph 键的邻位引入额外的 NH(2)基团，尽管吡啶和联苯之间的扭转增加，但 py-ph 键的π特征甚至更高。在所研究的工作中引入的其他取代基（-NO(2)、-Br、-CN、-vinyl、-ethynyl）能够实现三联吡啶和联苯之间的电子离域的驱动。在这种情况下，椭圆率是一个简洁的量来描述所研究体系中的电子离域。此外，py-ph 键的 BCP 中的椭圆率与相应的几何性质（例如，键长和二面角）以及 DFT 计算的 HOMO 和 LUMO 能量有关。
