Spontaneous-Symmetry-Breaking Charge Separation Induced by Pseudo-Jahn-Teller Distortion in Organic Photovoltaic Material

Takeaki Zaima; Wataru Ota; Naoki Haruta; Motoyuki Uejima; Hideo Ohkita; Tohru Sato

doi:10.1021/acs.jpclett.3c02527

Spontaneous-Symmetry-Breaking Charge Separation Induced by Pseudo-Jahn-Teller Distortion in Organic Photovoltaic Material

J Phys Chem Lett. 2023 Nov 2;14(43):9706-9712. doi: 10.1021/acs.jpclett.3c02527. Epub 2023 Oct 25.

Authors

Takeaki Zaima^{1

2}, Wataru Ota^{1

2}, Naoki Haruta^{1

2}, Motoyuki Uejima³, Hideo Ohkita⁴, Tohru Sato^{1

2}

Affiliations

¹ Fukui Institute for Fundamental Chemistry, Kyoto University, Takano Nishibiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan.
² Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.
³ MOLFEX, Inc., Takano Nishibiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan.
⁴ Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.

PMID: 37877625
DOI: 10.1021/acs.jpclett.3c02527

Abstract

The driving force of charge separation in the initial photovoltaic conversion process is theoretically investigated using ITIC, a nonfullerene acceptor material for organic photovoltaic devices. The density functional theory calculations show that the pseudo-Jahn-Teller (PJT) distortion of the S₁ excimer state induces spontaneous symmetry-breaking charge separation between the identical ITIC molecules even without the asymmetry of the surrounding environment. The strong PJT effect arises from the vibronic coupling between the pseudodegenerate S₁ and S₂ excited states with different irreducible representations (irreps), i.e., A_u for S₁ and A_g for S₂, via the asymmetric vibrational mode with the A_u irrep. The vibrational mode responsible for the spontaneous polarization, which is opposite in one ITIC monomer and the other, is the intramolecular C-C stretching vibration between the core IT and terminal IC units. These results suggest that controlling the PJT effect can improve the charge separation efficiency of the initial photovoltaic conversion process.