Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
Nat Chem. 2015 May;7(5):418-22. doi: 10.1038/nchem.2233. Epub 2015 Apr 13.
The reversal of (anti)aromaticity in a molecule's triplet excited state compared with its closed-shell singlet ground state is known as Baird's rule and has attracted the interest of synthetic, physical organic chemists and theorists because of the potential to modulate the fundamental properties of highly conjugated molecules. Here we show that two closely related bis-rhodium hexaphyrins (R26H and R28H) containing [26] and [28] π-electron peripheries, respectively, exhibit properties consistent with Baird's rule. In the ground state, R26H exhibits a sharp Soret-like band and distinct Q-like bands characteristic of an aromatic porphyrinoid, whereas R28H exhibits a broad absorption spectrum without Q-like bands, which is typical of an antiaromatic porphyrinoid. In contrast, the T-T absorption of R26H is broad, weak and featureless, whereas that of R28H displays an intense and sharp Soret-like band. These spectral signatures, in combination with quantum chemical calculations, are in line with qualitative expectations based on Baird's rule.
与分子的单重基态相比，三重激发态中（反）芳构性的反转被称为贝尔德规则，由于有可能调节高度共轭分子的基本性质，它引起了合成、物理有机化学家和理论家的兴趣。在这里，我们表明，两个密切相关的双铑六吡咯（R26H 和 R28H）分别含有[26]和[28]π-电子外围，表现出与贝尔德规则一致的性质。在基态下，R26H 表现出尖锐的 Soret 样带和特征性的 Q 样带，这是芳构化卟啉的特征，而 R28H 则表现出没有 Q 样带的宽吸收光谱，这是反芳构化卟啉的典型特征。相比之下，R26H 的 T-T 吸收较宽、较弱且无特征，而 R28H 的 T-T 吸收则显示出强烈而尖锐的 Soret 样带。这些光谱特征与量子化学计算相结合，与基于贝尔德规则的定性预期一致。
