Probing vibronic coupling of a transiently charged state of a single molecule through subnanometer resolved electroluminescence imaging
Authors: Tian Xiao-Jun , Kong Fan-Fang , Jing Shi-Hao , Yu Yun-Jie , Zhang Yao , Zhang Yang , Dong Zhen-Chao ,   Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China,   School of Physical Sciences, University of Science and Technology of China, Hefei 230026, China
Published: January 2022

The intramolecular vibronic coupling has a great effect on molecular electronic transitions and associated spectral characteristics, which is a central topic in the study of molecular spectroscopy. In this paper, we investigate the vibronic coupling of a transiently charged state within a single 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) molecule in real space by imaging the spatial distribution of single-molecule electroluminescence via highly localized excitation of tunneling electrons in a plasmonic nanocavity. The electron injections from a scanning tunneling microscope tip into a PTCDA molecule on a silver-supported ultrathin salt layer produce a transient doubly charged molecular anion that emits vibrationally resolved fluorescence. The sub-molecular resolved spectroscopic imaging for the –2 valence transiently charged state shows a two-spot pattern along the molecular short axis for the purely electronic 0-0 transition. However, the observed two-spot orientation for certain anti-symmetric vibronic-state imaging is found to be evidently different from the purely electronic 0-0 transition, rotating 90°, which reflects the change in the transition dipole orientation from along the molecular short axis to the long axis. Such a change directly reveals the occurrence of strong vibronic coupling associated with a large Herzberg-Teller (HT) contribution, which goes beyond the conventional Franck-Condon (FC) picture. Combined with theoretical calculations, the anti-symmetric vibration is found to have a strong dynamic disturbance to the transition density of purely electronic transitions, especially those atoms with large transition densities, which induces a strong transition charge oscillation along the long axis of the molecule and thus leads to a transition dipole along the long axis of the molecule. On the other hand, for vibronic emissions associated with the totally symmetric molecular vibration (such as the v<sub>1</sub> (A<sub>g</sub>) mode described above), the observed two-spot orientation in the vibronic-state imaging pattern is found to be the same as the purely electronic 0-0 transition, which directly reveals its FC-dominated nature. Notably, the vibration-induced emission associated with HT-dominated contributions (such as the v<sub>2</sub> (B<sub>3g</sub>) mode) is often discussed in the literature by using an intensity borrowing mechanism via the state mixing with other high-lying eigenstates. In the present work, the v<sub>2</sub>-vibration with B<sub>3g</sub> symmetry is likely to modulate the zero-order electronic wavefunction of the S<sub>1</sub> state in a way to best resemble that of the S<sub>2</sub> state (<i>i.e.</i>, induce efficient mixing of the electronic excited state S<sub>1</sub> with the electronic excited state S<sub>2</sub>), so that the v<sub>2</sub>-vibration induced emission seems to borrow intensities from neighboring S<sub>2</sub>→S<sub>0</sub> transitions. Our results provide a new route for the real-space understanding of the microscopic picture for the vibronic coupling within a single molecule in a transiently charged state.