Herzberg–Teller Effect in Single-Crystalline Hexacene at Finite Temperatures

The nuclear dependency of electronic wave function upon optical excitation is encoded in the line shape of light absorption spectrum. Experimentally, we measured the electronic spectra of single-crystalline hexacene at 77, 185, and 293 K using polarized visible–near -IR microscopy. Through a compari...

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Bibliographic Details
Published in:Journal of physical chemistry. C 2022-02, Vol.126 (7), p.3366-3374
Main Authors: Qian, Yuqin, Zhang, Tong, Li, Xia, Rao, Yi, Chen, Hanning
Format: Article
Language:English
Subjects:
C: Energy Conversion and Storage
Chemistry
Materials Science
Science & Technology - Other Topics
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Summary:The nuclear dependency of electronic wave function upon optical excitation is encoded in the line shape of light absorption spectrum. Experimentally, we measured the electronic spectra of single-crystalline hexacene at 77, 185, and 293 K using polarized visible–near -IR microscopy. Through a comparison with our simulated vibrationally resolved absorption spectrum, the Herzberg–Teller (HT) coupling was found to be the dominant contributor to all spectroscopic features, except for the lowest excitation peak when the incident photon is polarized along crystal axis b. With the aim to explore the evolution of the absorption spectrum upon temperature change, the quasi-harmonic approximation was first employed to calculate the thermal expansion coefficient of the hexacene unit cell before its volumes at 77, 185, and 293 K were determined by Helmholtz free energy minimization. Then, at each temperature, the unit cell’s phonon modes were ascertained at the density functional theory (DFT) level while its electronic transition dipole moments and their nuclear gradients were evaluated by quasi-particle GW method in conjunction with the Bethe–Salpeter equation (BSE) approach. Finally, the HT vibronic theory was utilized to afford the full absorption spectrum by considering the Franck–Condon factor (FC), FC/HT interference, and HT coupling. Our proposed computational framework was justified by the satisfactory agreement between our theoretical and experimental spectra, making it a reliable tool to investigate the delicate vibronic coupling by examining temperature-dependent light absorption spectrum.
ISSN:1932-7447
1932-7455
DOI:10.1021/acs.jpcc.1c10253