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  and  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.
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This work at Yonsei University was supported by the Samsung Science and Technology Foundation (Project No. SSTF-BA1402-10). The quantum calculations were performed using the supercomputing resources of the Korea Institute of Science and Technology Information (KISTI). The work at Kyoto University was supported financially by the Global Research Laboratory (GRL, 2013K1A1A2A0205183) Program funded by the Ministry of Education, Science and Technology (MEST) of Korea. The authors express their appreciation to R. Herges and J.L. Sessler for valuable discussions.
The authors declare no competing financial interests.
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Sung, Y., Yoon, MC., Lim, J. et al. Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird's rule. Nature Chem 7, 418–422 (2015). https://doi.org/10.1038/nchem.2233
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