Aromaticity reversal in the lowest triplet state, or Baird's rule, has been postulated for the past few decades. Despite numerous theoretical works on aromaticity reversal, experimental study is still at a rudimentary stage. Herein, we investigate the aromaticity reversal in the lowest excited triplet state using a comparable set of - and hexaphyrins by femtosecond time-resolved infrared (IR) spectroscopy. Compared to the relatively simple IR spectra of bis(rhodium) hexaphyrin (R26H), those of bis(rhodium) hexaphyrin (R28H) show complex IR spectra the region for the stretching modes of conjugated rings. Whereas time-resolved IR spectra of R26H in the excited triplet state are dominated by excited state IR absorption peaks, while those of R28H largely show ground state IR bleaching peaks, reflecting the aromaticity reversal in the lowest triplet state. These contrasting IR spectral features serve as new experimental aromaticity indices for Baird's rule.
Bibliographical noteFunding Information:
This work at Yonsei University was supported by Samsung Science and Technology Foundation under Project Number 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 financially supported by the Global Research Laboratory (GRL, 2013K1A1A2A0205183) Program funded by the Ministry of Education, Science and Technology of Korea (MEST). The work at Pusan National University was supported by the National Research Foundation of Korea (NRF) grant funded by the MEST (NRF-2014R1A2A2A01002456).
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