Upon photon absorption, π-conjugated organics are apt to undergo ultrafast structural reorganization via electron-vibrational coupling during non-adiabatic transitions. Ultrafast nuclear motions modulate local planarity and quinoid/benzenoid characters within conjugated backbones, which control primary events in the excited states, such as localization, energy transfer, and so on. Femtosecond broadband fluorescence upconversion measurements were conducted to investigate exciton self-trapping and delocalization in cycloparaphenylenes as ultrafast structural reorganizations are achieved via excited-state symmetry-dependent electron-vibrational coupling. By accessing two high-lying excited states, one-photon and two-photon allowed states, a clear discrepancy in the initial time-resolved fluorescence spectra and the temporal dynamics/spectral evolution of fluorescence spectra were monitored. Combined with quantum chemical calculations, a novel insight into the effect of the excited-state symmetry on ultrafast structural reorganization and exciton self-trapping in the emerging class of π-conjugated materials is provided.
Bibliographical noteFunding Information:
The work at Yonsei University was supported by Strategic Research (NRF2016R1E1A1A01943379) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science. The work at Osaka University was supported by Japan Society for the Promotion of Science (JSPS, KAKENHI grants JP18H01943, JP17H05157, and JP26107004), Kyoto Technoscience Center, and the Iketani Science and Technology Foundation. Theoretical calculations were partly performed at the Research Center for Computational Science, Okazaki, Japan. The work at Kyoto University was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI grant no. 18H01962 (E.K.) and no. 16H06352 (S.Y.).
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