Centre for Advanced Electron Spin Resonance (CAESR), University of Oxford , South Parks Road, Oxford OX1 3QR, United Kingdom.
Department of Chemistry, Duke University , French Family Science Center, 124 Science Drive, Durham, North Carolina 27708, United States.
J Am Chem Soc. 2017 Apr 19;139(15):5301-5304. doi: 10.1021/jacs.7b01204. Epub 2017 Apr 4.
The influence of electronic symmetry on triplet state delocalization in linear zinc porphyrin oligomers is explored by electron paramagnetic resonance techniques. Using a combination of transient continuous wave and pulse electron nuclear double resonance spectroscopies, it is demonstrated experimentally that complete triplet state delocalization requires the chemical equivalence of all porphyrin units. These results are supported by density functional theory calculations, showing uneven delocalization in a porphyrin dimer in which a terminal ethynyl group renders the two porphyrin units inequivalent. When the conjugation length of the molecule is further increased upon addition of a second terminal ethynyl group that restores the symmetry of the system, the triplet state is again found to be completely delocalized. The observations suggest that electronic symmetry is of greater importance for triplet state delocalization than other frequently invoked factors such as conformational rigidity or fundamental length-scale limitations.
通过电子顺磁共振技术研究了电子对称对线性锌卟啉寡聚物三重态离域的影响。实验采用瞬态连续波和脉冲电子-核双共振光谱学相结合的方法，证明了完全的三重态离域需要所有卟啉单元的化学等价性。这些结果得到了密度泛函理论计算的支持，表明在一个末端乙炔基使两个卟啉单元不等价的卟啉二聚体中存在不均匀的离域。当在第二个末端乙炔基上进一步增加分子的共轭长度以恢复系统的对称性时，三重态再次被发现完全离域。这些观察结果表明，电子对称性对于三重态离域的重要性大于其他经常被提及的因素，如构象刚性或基本长度尺度限制。
