Excited state dynamics in organic solids and aggregates

　Vibronic coupling plays a pivotal role in excited state dynamics in both organic solids and aggregates. We have studied the optical responses of a single crystal of rubrene, which is known for its excellent carrier mobility, and demonstrated that ultrafast singlet fission (SF) occurs in its photoexcited state. We observed a coherent phonon signal in the pump-probe traces, indicating possible vibrational modes that drive SF. With the help of state of the art quantum chemical calculations, we proposed a novel SF mechanism wherein an intermolecular mode with a symmetry-breaking character turns on coupling, which enables ultrafast SF to emerge.

　Vibronic structures in the optical absorption spectra of organic aggregates can be used to extract the excitonic interactions in those aggregates. We have demonstrated that this theoretical scheme can also be applied to the electronic absorption bands of charged states (holes injected via field effect). Our analysis revealed a systematic variation of the charge transfer integral, with the aggregate structure being modified by the introduction of alkyl substituents in thin films of Dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophen. Furthermore, a time-resolved emission study showed that these structural changes also modify the exciton coherence size

References

1. "Coherent singlet fission activated by symmetry breaking", K. Miyata, Y. Kurashige, K. Watanabe, T. Sugimoto, S. Takahashi, S. Tanaka, J. Takeya, T. Yanai, and Y. Matsumoto Nature Chemistry, 9, 983–989 (2017).

2. “Microscopic hole-transfer efficiency in organic thin-film transistors studied with charge-modulation spectroscopy”, Kiyoshi Miyata, Shunsuke Tanaka, Yuuta Ishino, Kazuya Watanabe, Takafumi Uemura, Jun Takeya, Toshiki Sugimoto, and Yoshiyasu Matsumoto, Phys. Rev. B, 91, 195306 (2015).

3. "Enhancement of the Exciton Coherence Size in Organic Semiconductor by Alkyl Chain Substitution", Shunsuke Tanaka, Kiyoshi Miyata, Toshiki Sugimoto, Kazuya Watanabe, Takafumi Uemura, Jun Takeya, and Yoshiyasu Matsumoto J. Phys. Chem. C, 120, 7941–7948 (2016).

4. "Decoupling from a Thermal Bath via Molecular Polariton Formation", Shota Takahashi, and Kazuya Watanabe, J. Phys. Chem. Lett. 11, 1349-1356 (2020).

5. "Anomalous Temperature Dependence of Exciton Spectral Diffusion in Tetracene Thin Film" , Tatsuya Yoshida, Kazuya Watanabe, Marin Petrovic, and Marko Kraji, J. Phys. Chem. Lett. 11, 5248-5254 (2020).

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